chicane67
08-16-2004, 11:23 AM
ddennis68
Registered User
Posts: 264
(7/25/04 3:04 pm)
Reply 3-link
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Now that I've gotten everybody fired up about the way-to-anal-about-grammar-and-spelling-crowd over at c-c.com I have a question about some of the things I have been looking into over there. I'd ask them but they won't allow me into their little group. I don't have a rustang nor do I want one (even thought the 04 Mach 1 looks good), a few of the fox chassis guys were removing one of the UCA from the rear and adding a PHB, effectivly creating a 3-link. Is there anyone who has attempted this on an "A" body chassis and what were the results? I really like the 3-link (not everything over at c-c is that bad) and the reason I am leaning toward that over the truck-arm now is the weight factor that I had not thought of before. I was thinking some nice adjustable links with johnny-joints to eliminate any deflection from the rear, a good PHB, and some bracing to the rear frame section. I know, I know-not another list with a what do you think at the end, this is actually a well researched and thought out plan of attack with no bolt-on package available anyway. Just looking for input in case there is something I missed. Thanks-
Dennis-
check out progress of Bondobucket
Mean 69
Registered User
Posts: 81
(7/25/04 3:25 pm)
Reply No!!!!
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Dennis, don't do it. Removing one of the upper links will reduce the bind associated with the splayed upper control arms, but it is NOT the right way to do a three link.
Take a look at Evolution Motorsports, they are doing a four-to-three link conversion for the late model guys. I don't really like the setup, but it is intended to be a bolt in affair. Good guys too, I like them, and they are knowingly making compromises to satisfy a particular market niche.
By removing one of the upper control arms, you lose the bind, but gain some nasties in return. The upper arms are obviously slanted relative to the vehicle centerline. With a three link, the upper link is responsible for reacting to axle torque (acceleration, or braking), "only." These forces are longitudinal, but the remaining link would not be, inducing a big time bending tendancy on the link. Not good, especially for the stamped stuff that GM used, and even worse considering the fact that GM put TWO of those little suckers up there to react to torque, but now the load would be handled by only one: not good.
This does not even come close to addressing the kinematics of the system, most notably the "anti's," as in squat and lift.
All that said, you have an advantage over the leaf spring guys, there is attachement potential on top of the dif' from the upper mounts (again, reference EVM for ideas). With a 12 bolt leaf differential, making a suitable upper mount looks a bit challenging at first glance.
Norm Peterson's late model 'bu uses the same basic rear suspension approach, I would guess that he has thought of this/talked to others that have tried, hopefully he'll drop into this thread. My opinion, nothing good can come of that attempt. (By the way, not sure if you mentioned it, but the upper pair is responsible for lateral location, so you would NEED!!!!! a different means of locating the axle laterally, Panhard or Watt's, for instance).
Mark
ddennis68
Registered User
Posts: 266
(7/25/04 4:29 pm)
Reply Re: No!!!!
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I'm sorry, I thought I had stated that I would be running a PHB in place of the UCA's for lateral positioning. Also , the UCA's are not really that canted-I think the UCA mounts where they are in conjunction with a solid ended UCA would provide more than adequite strength for the differential "twist" during throttle/brake. I would NEVER even consider doing this with the stamped factory peices-I would use some heavy tube style arms with heim ends or johnny joints. I have looked at some the factory attempts at a 3-link type suspension and my ideas are much stronger than say the Jeep which doesnt even positively mount the 3rd link to the housing-it has a ball joint type mount running from the center of the housing to both the R & L sides of the rear frame with rubber bushings attaching. The mount is constructed out of 1/4 stamped steel.
Dennis-
check out progress of Bondobucket
o1mrquick
Registered User
Posts: 2036
(7/25/04 8:25 pm)
Reply 3 link revisited?
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Hey Dennis, when I was building my GTO I was playin with ideas of installing a bolt on 3 link that uses most of the stock UCA mounting holes. I had a bracket that bolted to the mounts then spanned across the frame that had a kick down for the upper link and dampner mount.The LCA mounts were extended foreward to utilize a 26" johnny joint equipt 1 1/2" DOM LCA. I sent the plans to a guy and he said it would work out nicely.The adjusable PHB mounts at rear with custom bracket that would also mount the LCA which was also adjustable.I later found that the stock 4 link design is more than enough for the way I drive. It might give you some good ideas. C-yaaa Vince
ddennis68
Registered User
Posts: 268
(7/25/04 8:47 pm)
Reply Re: 3 link revisited?
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Thanks Vince-still have those plans by chance?
Dennis-
check out progress of Bondobucket
MarkM68
Registered User
Posts: 1617
(7/26/04 6:41 am)
Reply 3 link
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I'm really considering doing this on my '66. Since I'm going to mini-tub the car using the stock frame. I'd just remove the uca crossmember all together, and use a more solid c/m.
mean69, do you have a contact email?
katz
Unregistered User
(7/26/04 7:55 am)
Reply re
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Here's a pic of revised 3-link/PHB on my Riviera (I'd call it project Banana Boat for now b/c of its ugly yellow paint job). The car originally came with 3-link/PHB from factory, but the geometry is crappy at best especially with 4" lowered ride height. Factory LCA packaging is similar to A/G-body sprayed 4-links.
New geometry is pretty good considering I retained all factory chassis mounting points. I know it is similar to what Mean69 is doing and he's planning to market his as a kit, so I won't go into further details. But I'll tell you it is very stable, and traction during out-of-corner acceleration is phenominal. You wouldn't believe you're driving a 4200-lb full size car. Ride quality is a bit stiffer and rod ends transmits more vibrations, but it's still pretty good overall.
Mean 69
Registered User
Posts: 82
(7/26/04 9:26 am)
Reply Yeah Baby!
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Now that's what I'm talking about! Very cool, Katz, we'd expect nothing less from you! One of the things that is pretty common to do with the upper link is to offset it as shown, relative to vehicle centerline. It is pretty well documented that if you place it appropriately, you can compensate for axle torque under acceleration and have a very neutral response under throttle as a result. The downside (reported) is that under braking, the rear will react assymetrically, causing a twitching effect. I have never driven a car in this orientation, and it clearly is used on many other cars (FFR, for instance), so I question how big of a deal this is. Can you feel any funny business, Katz, under braking?
Does anyone have a similar picture for an early A-body? I had one for years, but never did much with the rear suspension (other than a home-built set of traction bars ala Comp. Engineering, very effective for straight line). I can't recall too well how things are configured.
MarkM: I'll drop you an e-mail.
Mark (M also, interestingly...)
o1mrquick
Registered User
Posts: 2038
(7/26/04 10:54 am)
Reply nice riv
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Hey Katz, looking good. I plan on doing the same...can't say more either,but did you have to move them rear ward also? and what do you think about moving the front mounts outwards a bit to get the LCA abit more parallel? Just some funny thoughts I had layin on my back one day. now you got me goin. take er easy ayh C-yaaa vince
Norm Peterson
Registered User
Posts: 158
(7/26/04 10:54 am)
Reply Re: No!!!!
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The so-called "Poor Man's 3-Link" was tried, and mostly abandoned by the Mustangers for the reasons that Mark mentions. The whole idea of a 3-link is to get each of the various links to locate along only one principal vehicle axis. If you rod-end the still-splayed 3rd link, bending in the UCA won't be an issue (zero end moments and no intermediate lateral loads equals zero bending everywhere). Leave them softish OE rubber and there will be some bending, and it will be about double what it was in either UCA when both UCA's were in place. But the lateral component of the PM3L and the now-necessary PHB or WL will get into some unsymmetrical dispute over the lateral loads. Enough to affect the axle roll steer and pinion angle, I think.
As a side note of sorts, you probably hear more about Fox and later Mustang owners completely revising their rear suspension configurations than 3rd and 4th gen F-body folks in part because the torque arm-equipped F-cars didn't need nearly as much help for extreme cornering applications.
I also know specifically of a G-body 3-link under development by another member of this site.
Dennis - last I knew, new membership at cc had been re-opened and has remained so at least into July. However, I do seem to recall that registration attempts from certain ISP's were/are specifically rejected (aol might be one of them, but see the Wiki to be sure).
Norm
'79 Malibu (too unstock for Street Mod)
o1mrquick
Registered User
Posts: 2039
(7/26/04 10:56 am)
Reply upper link bracket
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Hey dennis i'll get those plans out to you asap im not gonna be at the shop till Sunday so i'll draw something up for you.Just to give you a couple ideas. Vince
katz
Unregistered User
(7/26/04 11:44 am)
Reply re
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Mark,
Thanks. The UCA is offset from the factory on this car. This being my daily driver, I have to keep the downtime minimum so I decided to re-use factory chassis mount locations. If I had more time and money, I'd probably done 9" with central UCA. The offset has been decreased by 0.53", however, by mounting a narrow-width rod end to the inboard side of factory brackets (the factory bushing measures 3.275").
I haven't experienced any pull or twitchness under braking, but that's probably b/c I can't brake hard enough with factory 4-wheel drums Unfortunately, I won't be able to do disc conversion for a while.
Vince,
I moved back UCA mount on HSG quite a bit. New UCA is 5" longer than stock. I had to put 20-degree bend in it to clear axle housing at full bump, but it should hold up fine with my current setup (about 350hp at flywheel and street radials).
Decreasing the converging angle of LCAs is a good idea. On my setup, the roll axis flips towards oversteer direction on 2" rebound travel when PHB is mounted in bottom two holes (but not by much) due to the large skew. With annual Bonnville trip nearing, I couldn't afford parts to build new LCAs, and as you know, I already spent too much time building the exhaust for this car and I can't move the LCA chassis mounts outboard w/o re-routing the pipes. If I ever decide to convert it to coil overs, then I'll probably make LCAs with rod end on each end and decrease the skew within the existing mounts.
The stock geometry @ 4" lower height was terrible. SVSA length was 40.2" with 14.6% roll understeer. The car used to wander a lot on freeway, which I thought was due to excessive bumpsteer. Now the car tracks pretty straight. It's quite likely that excessive rollsteer and loose PHB bushings were causing the problem.
ddennis68
Registered User
Posts: 270
(7/26/04 12:32 pm)
Reply Norm
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So Norm, I am guessing the biggest issue then is the that the 3rd link is not parallel to vehicle centerline. If the upper crossmember were removed and another built that would provide a different angle would the angle of the LCA as they are become an issue? Also, it is my understanding that the longer the better with the LCA. Since it is an El Camino and intrusion into the cabin area is not an issue, should the 3rd link be extended as well, I know that the 3rd link needs to be 65% of the LCA but is there a limit to how long it should be? Are you suggesting NOT to run solid ends on the UCA(3rd link)? Lastly, as I understand it the 3rd link should run from the diff housing DOWN to the crossmember, this in contrast to how the factory built the half-ass 4-link currently in place. I also understand that adjustments to that angle make a significant difference in how the rear suspension responds (often used to tighten/loosen the rear-end), is there a formula for figuring out a starting point for this angle?
Thanks for the enrollement tip, maybe I'll try again. I'm sure the "big meanie heads" would love some fresh meat to tear up.
Edited by: ddennis68 at: 7/26/04 12:37 pm
Norm Peterson
Registered User
Posts: 159
(7/26/04 2:32 pm)
Reply Re: Norm
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Quote:
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So Norm, I am guessing the biggest issue then is the that the 3rd link is not parallel to vehicle centerline. If the upper crossmember were removed and another built that would provide a different angle would the angle of the LCA as they are become an issue?
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LCA skew already is an issue, as the plan view intersection point of the LCA's goes to defining the axle's own roll axis (in conjunction with the PHB/WL/whatever). There's probably some optimum range of skew angle for the LCA's too.
Zero skew (parallel) LCA's have their virtual intersection point located at infinity, hence the axle's own roll axis is always parallel to the LCA's. 3rd & 4th gen F-bodies use this geometry. As the chassis rises into rebound to put the pivot at its end of the LCA above the axle pivot, you move into vehicle oversteer on a one-to-one basis with the side view LCA inclination. So a 2" chassis rise above level for 20" long (plan view parallel) LCA's gives 10% roll oversteer (neglecting any effects due to changes in chassis rake). Here's where the LCA length comes in most strongly - make them 25" long and that roll steer figure drops to 8%.
Most recent OE converging (triangulated) 4-link LCA's are splayed at something like 18* or so. The ones those shown in katz's picture above look to be more like 30* (correction requested). A little converging skew angle in the LCA's toward the chassis ends takes away some of the sensitivity of rollsteer % to ride height, which is arguably a good thing.
To put some numbers on the steer percentages, I have a fairly old paper written by a person then employed at Ford that gets into this a bit and ultimately concludes that 7% roll understeer adversely affects slalom performance. With my G-body being somewhere in the 7% - 8% range empty but with a light trunk load and being a bit clumsy through Solo II slaloms (me in it, otherwise empty) I'm inclined to agree.
Quote:
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Are you suggesting NOT to run solid ends on the UCA(3rd link)?
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Not at all. That was to illustrate that bending stresses in the UCA can be eliminated even if the UCA was to be left splayed. That part is fine with solid ends. Not so benign is the lateral force component of the UCA splay, as it and the PHB's axial load will then have different ideas concerning axle movement during roll to accommodate the various arcs. The result is bind and/or *unexpected* axle movements, and gets worse as the link end connections are made more rigid along the 3rd link's axis. Make the UCA parallel to centerline with rod ends and it won't be fighting with the PHB. But make the UCA bushings unmodified one-piece poly and you'll introduce bind even through a parallel UCA.
Quote:
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Lastly, as I understand it the 3rd link should run from the diff housing DOWN to the crossmember, this in contrast to how the factory built the half-ass 4-link currently in place. I also understand that adjustments to that angle make a significant difference in how the rear suspension responds (often used to tighten/loosen the rear-end), is there a formula for figuring out a starting point for this angle?
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With a 3-link, the side view inclination of the 3rd link only affects anti-squat. In the OE 4-link it also affects the axle's own roll axis somewhat.
In the 3-link/PHB setup, the construction line for axle steer runs between the center of the PHB and the intersection point of the LCA's projected into side view. Statically, anyway.
Norm
'79 Malibu (too unstock for Street Mod)
ddennis68
Registered User
Posts: 271
(7/26/04 2:56 pm)
Reply Re: Norm
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-hanging my head in shame-
Norm, I am embarrassed to say I barely understand half of what is stated above, altough it sounds good. I am running out right now to buy "Suspension Design for Dummies" or what ever suspension related book I can get my hands on at the local bookstore-then I'll order everything on the net I can find. Scary, I've been a journeyman tech for over a decade and I am considered the suspension/alignment specialist at most of the dealerships I have worked at. I guess I am good at fooling alot of people because apperently I don't know @#%$, which doesn't say much for those I have had to help over the years. Thank you for taking the time to help me sort this out. After I educate myself alittle perhaps I'll get back with everybody on what I decide to do. Thanks again-
Dennis-
check out progress of Bondobucket
o1mrquick
Registered User
Posts: 2041
(7/26/04 4:13 pm)
Reply hey now
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Don't feel bad Den, I was in your shoes about a year ago but after lots of reading and advice by several members here I have just now tighten my grasp of this stuff.Several books recommended by Norm,Katz and Jon have opened my eyes and it is actually very interesting.Definately buy the books and your brain will follow. Vince
katz
Unregistered User
(7/26/04 4:28 pm)
Reply re
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First of all, correction on what I said:
Decreased converging angle will actually increase roll oversteer % on rebound travel on my setup. If LCAs' sideview angle is relatively steep (I'm talking about 3~4 degrees here) at ride height, then large skew seems to help decrease the effect. But as the skew angle increases, LCAs would be subjected to increased bending force under acceleration and effective length of LCA decreases. So there definetely is a point of diminishing return. RC height is also a factor here. The combination of low RC height and steep LCA angle on my suspension is what's causing the roll axis flip.
If LCAs are close to level at ride height and/or RC is high, smaller converging angle may be more desirable.
Norm,
The skew angle of my LCAs are about 42.6 degrees (21.3 degrees per side). It was probably done so due to X-shaped or Hour-glass shaped frame. LCAs are 24" long, but the effective length is more like 23" due to large skew.
As Norm said, converging angle of UCAs/LCAs define roll axis & RC height on sprayed 4-links. My guess is GM pointed up the UCAs towards front in order to reduce RC height and roll steer. But this isn't really desirable for traction as it locates IC behind the rear axle, which theoretically will have negative anti-squat (or pro-squat). If you understand this, you'll understand why AME's tri. 4-bar is designed the way it is, unlike typical street rod tri. 4-bars on market. I did the best within parameters given by my bosses.
The beauty of 3-link is that, one thing can be adjusted w/o causing drastic effects on something else. I can change IC and SVSA length w/o changing RC height and roll steer, for example. This simply isn't possible with sprayed 4-links.
Dennis,
As you found out, engineering that goes into seemingly simple link suspension is actually pretty damn complex, and this is only a part of the game. Once you design and build suspension with good geometry, you'll have to go through tuning springs/shocks/anti-roll bars/tires etc. so everything works in harmony. Gather info from whatever sources (credible sources) you can find. CC.com is VERY good. Don't let "beginner" thing bother you and keep you from learning. Pay close attention to posts by ROUSHGTR, Norm, Mark (Mean 69), Preston, and many other very sharp folks. Read everything, then re-read everything again. Repeat if necessary. Then you'll know what to do. Good luck to you.
streetfytr68
Registered User
Posts: 1223
(7/26/04 4:42 pm)
Reply Re: hey now
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I'm with you Dennis--but I'm getting there. No that I have settled on a ride height, I started by plotting out my rear axle in relation to my rear frame rails (kick up) to get a sense of where the LCA pickup points might be located (at both ends) From there you can conceptualize whether or not the LCA's will be parallel or angled. (I think?) Get the LCA's figured out first and then you can build the third link to suit. In your case the LCA's are carved in stone (or are they??) So you can measure their length and inclination and then cut a great big hole in the bed of your truck to figure out the 3rd link.
I'm thinking about a Watts link to avoid the exhaust packaging problems associated with a PHB on a 1st gen F-Body (Much easier on that big ol' Riv or an Elco). I have eliminated my back seat so the 3rd link can be located as needed.
Front suspension is done (or close). So once I get out west, I want to try Chicane's leaf springs. Only reason why I'll yank the leaf springs is because I want to cut the crusty NY floors out. At that point I'm sure I'll get carried away and go 3 Link. Only after that upgrade will I cease to consider myself entry-level.
/Steevo
p.s. Vince Asaro has been their done that with his full frame Elco. A de-coupled torque-arm (too clunky) replaced with a 3-link. The car is radically different than stock but, he should be able to provide some valuable driving feedback if you can get a hold of him.
www.lateral-g.net
MarcusUSA1
Registered User
Posts: 141
(7/26/04 6:37 pm)
Reply Re: hey now
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The technical aspects have been pretty well covered but I`ll add a little practical info. We`ve (Savitske Classic & Custom/ SC&C) been developing a 3 link package for G body (and later on A body) rear suspensions. We`ve intentionally kept it simple to keep the price down. The LCAs remain where they are,in the factory mounts. The UCAs come off and a mount is bolted in between the factory UCA mounting "ears" on the rear. It`s centered on the diff (see above for reasons). The frame side is mounted to a fabbed mount welded into the stock crossmember,which is reinforced. The mounts for both ends of the 3rd link have several mounting holes to allow a lot of adjustment for antisquat. The weld in PHB bar mounts also have several holes to allow RC height adjustment. These are basically out of the box stock cars parts. It`s all mounted in a caged `87 Cutlass with C-5 based front suspension,full cage,LS1/6 speed and adj. QA1 shocks. We`ve learned some interesting things since installing the new 3 link/PHB out back. The car has a totally different feel now. It used have a vague, twitchy feeling in the rear probably caused by the non linear binding of the factory C4L rear suspension. The rear end now feels planted solid as a rock. It`s very confidence inspiring in the driver`s seat. You can drive the car harder and faster with less butterflys in your stomach. That`s saying a lot because this car had very high cornering limits before. Not very scientific I know but this is what we`re really after isn`t it? It was designed in Suspension Analyzer and on stacks of spread sheets but that`s what it boils down to. The adjustable antisquat has worked out well too. We`ve run as much as 200% antisquat with no rear brake lockup problems (even in the rain). The car hooks up HARD now. Big improvement,it used to have both traction and wheel hop problems. That said the much lower rear RC (9"-12" vs. 18" stock) is causing more understeer at the limit. It wants more rear spring rate and more rear anti roll bar. We`ve been running 167lb/in rears with a 7/8"OD rear bar. We`ve got 250lb/in springs on (back) order and we`re going to a fabbed decoupled (from the LCAs) rear swaybar. The stock G and A style rear bars have a much lower rate than you might think due to the huge length of the arms (LCAs in this case). We`re going tubular with splined arms so it`ll be lighter than stock. The torsion bar is a custom order piece we`re still waiting for but even without it the 3 link/PHB has proven itself enough to keep going with the R&D process. I`ll post an update when we get the complete setup in place and on the road. Mark SC&C
o1mrquick
Registered User
Posts: 2043
(7/26/04 6:42 pm)
Reply 3link
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Hey Marcus would you beable to post pictures of your 3 link design, would love to see what you have come up with. Thanks Vince
ddennis68
Registered User
Posts: 272
(7/26/04 6:44 pm)
Reply Re: hey now
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Thanks Marcus, kats?, Mark, Steve, Norm, Vince, and anybody else I left off my list! I went by every book store and auto part store in town-no luck so I bit the bullet and ordered:
"Engineer to Win" by Carroll Smith
"Race Car Engineering and Mechanics" by Paul Valkenburg
"Chassis Engineering/Chassis Design, Building & Tuning for High Performance Handling" by Herb Adams
On Amazon-give me a few weeks to receive and then digest the vast information before I come up with anymore ass-clown type ideas-see I do learn things over at c-c.com
Dennis-
check out progress of Bondobucket
Edited by: ddennis68 at: 7/26/04 6:45 pm
Registered User
Posts: 264
(7/25/04 3:04 pm)
Reply 3-link
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Now that I've gotten everybody fired up about the way-to-anal-about-grammar-and-spelling-crowd over at c-c.com I have a question about some of the things I have been looking into over there. I'd ask them but they won't allow me into their little group. I don't have a rustang nor do I want one (even thought the 04 Mach 1 looks good), a few of the fox chassis guys were removing one of the UCA from the rear and adding a PHB, effectivly creating a 3-link. Is there anyone who has attempted this on an "A" body chassis and what were the results? I really like the 3-link (not everything over at c-c is that bad) and the reason I am leaning toward that over the truck-arm now is the weight factor that I had not thought of before. I was thinking some nice adjustable links with johnny-joints to eliminate any deflection from the rear, a good PHB, and some bracing to the rear frame section. I know, I know-not another list with a what do you think at the end, this is actually a well researched and thought out plan of attack with no bolt-on package available anyway. Just looking for input in case there is something I missed. Thanks-
Dennis-
check out progress of Bondobucket
Mean 69
Registered User
Posts: 81
(7/25/04 3:25 pm)
Reply No!!!!
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Dennis, don't do it. Removing one of the upper links will reduce the bind associated with the splayed upper control arms, but it is NOT the right way to do a three link.
Take a look at Evolution Motorsports, they are doing a four-to-three link conversion for the late model guys. I don't really like the setup, but it is intended to be a bolt in affair. Good guys too, I like them, and they are knowingly making compromises to satisfy a particular market niche.
By removing one of the upper control arms, you lose the bind, but gain some nasties in return. The upper arms are obviously slanted relative to the vehicle centerline. With a three link, the upper link is responsible for reacting to axle torque (acceleration, or braking), "only." These forces are longitudinal, but the remaining link would not be, inducing a big time bending tendancy on the link. Not good, especially for the stamped stuff that GM used, and even worse considering the fact that GM put TWO of those little suckers up there to react to torque, but now the load would be handled by only one: not good.
This does not even come close to addressing the kinematics of the system, most notably the "anti's," as in squat and lift.
All that said, you have an advantage over the leaf spring guys, there is attachement potential on top of the dif' from the upper mounts (again, reference EVM for ideas). With a 12 bolt leaf differential, making a suitable upper mount looks a bit challenging at first glance.
Norm Peterson's late model 'bu uses the same basic rear suspension approach, I would guess that he has thought of this/talked to others that have tried, hopefully he'll drop into this thread. My opinion, nothing good can come of that attempt. (By the way, not sure if you mentioned it, but the upper pair is responsible for lateral location, so you would NEED!!!!! a different means of locating the axle laterally, Panhard or Watt's, for instance).
Mark
ddennis68
Registered User
Posts: 266
(7/25/04 4:29 pm)
Reply Re: No!!!!
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I'm sorry, I thought I had stated that I would be running a PHB in place of the UCA's for lateral positioning. Also , the UCA's are not really that canted-I think the UCA mounts where they are in conjunction with a solid ended UCA would provide more than adequite strength for the differential "twist" during throttle/brake. I would NEVER even consider doing this with the stamped factory peices-I would use some heavy tube style arms with heim ends or johnny joints. I have looked at some the factory attempts at a 3-link type suspension and my ideas are much stronger than say the Jeep which doesnt even positively mount the 3rd link to the housing-it has a ball joint type mount running from the center of the housing to both the R & L sides of the rear frame with rubber bushings attaching. The mount is constructed out of 1/4 stamped steel.
Dennis-
check out progress of Bondobucket
o1mrquick
Registered User
Posts: 2036
(7/25/04 8:25 pm)
Reply 3 link revisited?
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Hey Dennis, when I was building my GTO I was playin with ideas of installing a bolt on 3 link that uses most of the stock UCA mounting holes. I had a bracket that bolted to the mounts then spanned across the frame that had a kick down for the upper link and dampner mount.The LCA mounts were extended foreward to utilize a 26" johnny joint equipt 1 1/2" DOM LCA. I sent the plans to a guy and he said it would work out nicely.The adjusable PHB mounts at rear with custom bracket that would also mount the LCA which was also adjustable.I later found that the stock 4 link design is more than enough for the way I drive. It might give you some good ideas. C-yaaa Vince
ddennis68
Registered User
Posts: 268
(7/25/04 8:47 pm)
Reply Re: 3 link revisited?
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Thanks Vince-still have those plans by chance?
Dennis-
check out progress of Bondobucket
MarkM68
Registered User
Posts: 1617
(7/26/04 6:41 am)
Reply 3 link
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I'm really considering doing this on my '66. Since I'm going to mini-tub the car using the stock frame. I'd just remove the uca crossmember all together, and use a more solid c/m.
mean69, do you have a contact email?
katz
Unregistered User
(7/26/04 7:55 am)
Reply re
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Here's a pic of revised 3-link/PHB on my Riviera (I'd call it project Banana Boat for now b/c of its ugly yellow paint job). The car originally came with 3-link/PHB from factory, but the geometry is crappy at best especially with 4" lowered ride height. Factory LCA packaging is similar to A/G-body sprayed 4-links.
New geometry is pretty good considering I retained all factory chassis mounting points. I know it is similar to what Mean69 is doing and he's planning to market his as a kit, so I won't go into further details. But I'll tell you it is very stable, and traction during out-of-corner acceleration is phenominal. You wouldn't believe you're driving a 4200-lb full size car. Ride quality is a bit stiffer and rod ends transmits more vibrations, but it's still pretty good overall.
Mean 69
Registered User
Posts: 82
(7/26/04 9:26 am)
Reply Yeah Baby!
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Now that's what I'm talking about! Very cool, Katz, we'd expect nothing less from you! One of the things that is pretty common to do with the upper link is to offset it as shown, relative to vehicle centerline. It is pretty well documented that if you place it appropriately, you can compensate for axle torque under acceleration and have a very neutral response under throttle as a result. The downside (reported) is that under braking, the rear will react assymetrically, causing a twitching effect. I have never driven a car in this orientation, and it clearly is used on many other cars (FFR, for instance), so I question how big of a deal this is. Can you feel any funny business, Katz, under braking?
Does anyone have a similar picture for an early A-body? I had one for years, but never did much with the rear suspension (other than a home-built set of traction bars ala Comp. Engineering, very effective for straight line). I can't recall too well how things are configured.
MarkM: I'll drop you an e-mail.
Mark (M also, interestingly...)
o1mrquick
Registered User
Posts: 2038
(7/26/04 10:54 am)
Reply nice riv
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Hey Katz, looking good. I plan on doing the same...can't say more either,but did you have to move them rear ward also? and what do you think about moving the front mounts outwards a bit to get the LCA abit more parallel? Just some funny thoughts I had layin on my back one day. now you got me goin. take er easy ayh C-yaaa vince
Norm Peterson
Registered User
Posts: 158
(7/26/04 10:54 am)
Reply Re: No!!!!
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The so-called "Poor Man's 3-Link" was tried, and mostly abandoned by the Mustangers for the reasons that Mark mentions. The whole idea of a 3-link is to get each of the various links to locate along only one principal vehicle axis. If you rod-end the still-splayed 3rd link, bending in the UCA won't be an issue (zero end moments and no intermediate lateral loads equals zero bending everywhere). Leave them softish OE rubber and there will be some bending, and it will be about double what it was in either UCA when both UCA's were in place. But the lateral component of the PM3L and the now-necessary PHB or WL will get into some unsymmetrical dispute over the lateral loads. Enough to affect the axle roll steer and pinion angle, I think.
As a side note of sorts, you probably hear more about Fox and later Mustang owners completely revising their rear suspension configurations than 3rd and 4th gen F-body folks in part because the torque arm-equipped F-cars didn't need nearly as much help for extreme cornering applications.
I also know specifically of a G-body 3-link under development by another member of this site.
Dennis - last I knew, new membership at cc had been re-opened and has remained so at least into July. However, I do seem to recall that registration attempts from certain ISP's were/are specifically rejected (aol might be one of them, but see the Wiki to be sure).
Norm
'79 Malibu (too unstock for Street Mod)
o1mrquick
Registered User
Posts: 2039
(7/26/04 10:56 am)
Reply upper link bracket
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Hey dennis i'll get those plans out to you asap im not gonna be at the shop till Sunday so i'll draw something up for you.Just to give you a couple ideas. Vince
katz
Unregistered User
(7/26/04 11:44 am)
Reply re
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Mark,
Thanks. The UCA is offset from the factory on this car. This being my daily driver, I have to keep the downtime minimum so I decided to re-use factory chassis mount locations. If I had more time and money, I'd probably done 9" with central UCA. The offset has been decreased by 0.53", however, by mounting a narrow-width rod end to the inboard side of factory brackets (the factory bushing measures 3.275").
I haven't experienced any pull or twitchness under braking, but that's probably b/c I can't brake hard enough with factory 4-wheel drums Unfortunately, I won't be able to do disc conversion for a while.
Vince,
I moved back UCA mount on HSG quite a bit. New UCA is 5" longer than stock. I had to put 20-degree bend in it to clear axle housing at full bump, but it should hold up fine with my current setup (about 350hp at flywheel and street radials).
Decreasing the converging angle of LCAs is a good idea. On my setup, the roll axis flips towards oversteer direction on 2" rebound travel when PHB is mounted in bottom two holes (but not by much) due to the large skew. With annual Bonnville trip nearing, I couldn't afford parts to build new LCAs, and as you know, I already spent too much time building the exhaust for this car and I can't move the LCA chassis mounts outboard w/o re-routing the pipes. If I ever decide to convert it to coil overs, then I'll probably make LCAs with rod end on each end and decrease the skew within the existing mounts.
The stock geometry @ 4" lower height was terrible. SVSA length was 40.2" with 14.6% roll understeer. The car used to wander a lot on freeway, which I thought was due to excessive bumpsteer. Now the car tracks pretty straight. It's quite likely that excessive rollsteer and loose PHB bushings were causing the problem.
ddennis68
Registered User
Posts: 270
(7/26/04 12:32 pm)
Reply Norm
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So Norm, I am guessing the biggest issue then is the that the 3rd link is not parallel to vehicle centerline. If the upper crossmember were removed and another built that would provide a different angle would the angle of the LCA as they are become an issue? Also, it is my understanding that the longer the better with the LCA. Since it is an El Camino and intrusion into the cabin area is not an issue, should the 3rd link be extended as well, I know that the 3rd link needs to be 65% of the LCA but is there a limit to how long it should be? Are you suggesting NOT to run solid ends on the UCA(3rd link)? Lastly, as I understand it the 3rd link should run from the diff housing DOWN to the crossmember, this in contrast to how the factory built the half-ass 4-link currently in place. I also understand that adjustments to that angle make a significant difference in how the rear suspension responds (often used to tighten/loosen the rear-end), is there a formula for figuring out a starting point for this angle?
Thanks for the enrollement tip, maybe I'll try again. I'm sure the "big meanie heads" would love some fresh meat to tear up.
Edited by: ddennis68 at: 7/26/04 12:37 pm
Norm Peterson
Registered User
Posts: 159
(7/26/04 2:32 pm)
Reply Re: Norm
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Quote:
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So Norm, I am guessing the biggest issue then is the that the 3rd link is not parallel to vehicle centerline. If the upper crossmember were removed and another built that would provide a different angle would the angle of the LCA as they are become an issue?
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LCA skew already is an issue, as the plan view intersection point of the LCA's goes to defining the axle's own roll axis (in conjunction with the PHB/WL/whatever). There's probably some optimum range of skew angle for the LCA's too.
Zero skew (parallel) LCA's have their virtual intersection point located at infinity, hence the axle's own roll axis is always parallel to the LCA's. 3rd & 4th gen F-bodies use this geometry. As the chassis rises into rebound to put the pivot at its end of the LCA above the axle pivot, you move into vehicle oversteer on a one-to-one basis with the side view LCA inclination. So a 2" chassis rise above level for 20" long (plan view parallel) LCA's gives 10% roll oversteer (neglecting any effects due to changes in chassis rake). Here's where the LCA length comes in most strongly - make them 25" long and that roll steer figure drops to 8%.
Most recent OE converging (triangulated) 4-link LCA's are splayed at something like 18* or so. The ones those shown in katz's picture above look to be more like 30* (correction requested). A little converging skew angle in the LCA's toward the chassis ends takes away some of the sensitivity of rollsteer % to ride height, which is arguably a good thing.
To put some numbers on the steer percentages, I have a fairly old paper written by a person then employed at Ford that gets into this a bit and ultimately concludes that 7% roll understeer adversely affects slalom performance. With my G-body being somewhere in the 7% - 8% range empty but with a light trunk load and being a bit clumsy through Solo II slaloms (me in it, otherwise empty) I'm inclined to agree.
Quote:
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Are you suggesting NOT to run solid ends on the UCA(3rd link)?
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Not at all. That was to illustrate that bending stresses in the UCA can be eliminated even if the UCA was to be left splayed. That part is fine with solid ends. Not so benign is the lateral force component of the UCA splay, as it and the PHB's axial load will then have different ideas concerning axle movement during roll to accommodate the various arcs. The result is bind and/or *unexpected* axle movements, and gets worse as the link end connections are made more rigid along the 3rd link's axis. Make the UCA parallel to centerline with rod ends and it won't be fighting with the PHB. But make the UCA bushings unmodified one-piece poly and you'll introduce bind even through a parallel UCA.
Quote:
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Lastly, as I understand it the 3rd link should run from the diff housing DOWN to the crossmember, this in contrast to how the factory built the half-ass 4-link currently in place. I also understand that adjustments to that angle make a significant difference in how the rear suspension responds (often used to tighten/loosen the rear-end), is there a formula for figuring out a starting point for this angle?
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With a 3-link, the side view inclination of the 3rd link only affects anti-squat. In the OE 4-link it also affects the axle's own roll axis somewhat.
In the 3-link/PHB setup, the construction line for axle steer runs between the center of the PHB and the intersection point of the LCA's projected into side view. Statically, anyway.
Norm
'79 Malibu (too unstock for Street Mod)
ddennis68
Registered User
Posts: 271
(7/26/04 2:56 pm)
Reply Re: Norm
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-hanging my head in shame-
Norm, I am embarrassed to say I barely understand half of what is stated above, altough it sounds good. I am running out right now to buy "Suspension Design for Dummies" or what ever suspension related book I can get my hands on at the local bookstore-then I'll order everything on the net I can find. Scary, I've been a journeyman tech for over a decade and I am considered the suspension/alignment specialist at most of the dealerships I have worked at. I guess I am good at fooling alot of people because apperently I don't know @#%$, which doesn't say much for those I have had to help over the years. Thank you for taking the time to help me sort this out. After I educate myself alittle perhaps I'll get back with everybody on what I decide to do. Thanks again-
Dennis-
check out progress of Bondobucket
o1mrquick
Registered User
Posts: 2041
(7/26/04 4:13 pm)
Reply hey now
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Don't feel bad Den, I was in your shoes about a year ago but after lots of reading and advice by several members here I have just now tighten my grasp of this stuff.Several books recommended by Norm,Katz and Jon have opened my eyes and it is actually very interesting.Definately buy the books and your brain will follow. Vince
katz
Unregistered User
(7/26/04 4:28 pm)
Reply re
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First of all, correction on what I said:
Decreased converging angle will actually increase roll oversteer % on rebound travel on my setup. If LCAs' sideview angle is relatively steep (I'm talking about 3~4 degrees here) at ride height, then large skew seems to help decrease the effect. But as the skew angle increases, LCAs would be subjected to increased bending force under acceleration and effective length of LCA decreases. So there definetely is a point of diminishing return. RC height is also a factor here. The combination of low RC height and steep LCA angle on my suspension is what's causing the roll axis flip.
If LCAs are close to level at ride height and/or RC is high, smaller converging angle may be more desirable.
Norm,
The skew angle of my LCAs are about 42.6 degrees (21.3 degrees per side). It was probably done so due to X-shaped or Hour-glass shaped frame. LCAs are 24" long, but the effective length is more like 23" due to large skew.
As Norm said, converging angle of UCAs/LCAs define roll axis & RC height on sprayed 4-links. My guess is GM pointed up the UCAs towards front in order to reduce RC height and roll steer. But this isn't really desirable for traction as it locates IC behind the rear axle, which theoretically will have negative anti-squat (or pro-squat). If you understand this, you'll understand why AME's tri. 4-bar is designed the way it is, unlike typical street rod tri. 4-bars on market. I did the best within parameters given by my bosses.
The beauty of 3-link is that, one thing can be adjusted w/o causing drastic effects on something else. I can change IC and SVSA length w/o changing RC height and roll steer, for example. This simply isn't possible with sprayed 4-links.
Dennis,
As you found out, engineering that goes into seemingly simple link suspension is actually pretty damn complex, and this is only a part of the game. Once you design and build suspension with good geometry, you'll have to go through tuning springs/shocks/anti-roll bars/tires etc. so everything works in harmony. Gather info from whatever sources (credible sources) you can find. CC.com is VERY good. Don't let "beginner" thing bother you and keep you from learning. Pay close attention to posts by ROUSHGTR, Norm, Mark (Mean 69), Preston, and many other very sharp folks. Read everything, then re-read everything again. Repeat if necessary. Then you'll know what to do. Good luck to you.
streetfytr68
Registered User
Posts: 1223
(7/26/04 4:42 pm)
Reply Re: hey now
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I'm with you Dennis--but I'm getting there. No that I have settled on a ride height, I started by plotting out my rear axle in relation to my rear frame rails (kick up) to get a sense of where the LCA pickup points might be located (at both ends) From there you can conceptualize whether or not the LCA's will be parallel or angled. (I think?) Get the LCA's figured out first and then you can build the third link to suit. In your case the LCA's are carved in stone (or are they??) So you can measure their length and inclination and then cut a great big hole in the bed of your truck to figure out the 3rd link.
I'm thinking about a Watts link to avoid the exhaust packaging problems associated with a PHB on a 1st gen F-Body (Much easier on that big ol' Riv or an Elco). I have eliminated my back seat so the 3rd link can be located as needed.
Front suspension is done (or close). So once I get out west, I want to try Chicane's leaf springs. Only reason why I'll yank the leaf springs is because I want to cut the crusty NY floors out. At that point I'm sure I'll get carried away and go 3 Link. Only after that upgrade will I cease to consider myself entry-level.
/Steevo
p.s. Vince Asaro has been their done that with his full frame Elco. A de-coupled torque-arm (too clunky) replaced with a 3-link. The car is radically different than stock but, he should be able to provide some valuable driving feedback if you can get a hold of him.
www.lateral-g.net
MarcusUSA1
Registered User
Posts: 141
(7/26/04 6:37 pm)
Reply Re: hey now
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The technical aspects have been pretty well covered but I`ll add a little practical info. We`ve (Savitske Classic & Custom/ SC&C) been developing a 3 link package for G body (and later on A body) rear suspensions. We`ve intentionally kept it simple to keep the price down. The LCAs remain where they are,in the factory mounts. The UCAs come off and a mount is bolted in between the factory UCA mounting "ears" on the rear. It`s centered on the diff (see above for reasons). The frame side is mounted to a fabbed mount welded into the stock crossmember,which is reinforced. The mounts for both ends of the 3rd link have several mounting holes to allow a lot of adjustment for antisquat. The weld in PHB bar mounts also have several holes to allow RC height adjustment. These are basically out of the box stock cars parts. It`s all mounted in a caged `87 Cutlass with C-5 based front suspension,full cage,LS1/6 speed and adj. QA1 shocks. We`ve learned some interesting things since installing the new 3 link/PHB out back. The car has a totally different feel now. It used have a vague, twitchy feeling in the rear probably caused by the non linear binding of the factory C4L rear suspension. The rear end now feels planted solid as a rock. It`s very confidence inspiring in the driver`s seat. You can drive the car harder and faster with less butterflys in your stomach. That`s saying a lot because this car had very high cornering limits before. Not very scientific I know but this is what we`re really after isn`t it? It was designed in Suspension Analyzer and on stacks of spread sheets but that`s what it boils down to. The adjustable antisquat has worked out well too. We`ve run as much as 200% antisquat with no rear brake lockup problems (even in the rain). The car hooks up HARD now. Big improvement,it used to have both traction and wheel hop problems. That said the much lower rear RC (9"-12" vs. 18" stock) is causing more understeer at the limit. It wants more rear spring rate and more rear anti roll bar. We`ve been running 167lb/in rears with a 7/8"OD rear bar. We`ve got 250lb/in springs on (back) order and we`re going to a fabbed decoupled (from the LCAs) rear swaybar. The stock G and A style rear bars have a much lower rate than you might think due to the huge length of the arms (LCAs in this case). We`re going tubular with splined arms so it`ll be lighter than stock. The torsion bar is a custom order piece we`re still waiting for but even without it the 3 link/PHB has proven itself enough to keep going with the R&D process. I`ll post an update when we get the complete setup in place and on the road. Mark SC&C
o1mrquick
Registered User
Posts: 2043
(7/26/04 6:42 pm)
Reply 3link
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Hey Marcus would you beable to post pictures of your 3 link design, would love to see what you have come up with. Thanks Vince
ddennis68
Registered User
Posts: 272
(7/26/04 6:44 pm)
Reply Re: hey now
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Thanks Marcus, kats?, Mark, Steve, Norm, Vince, and anybody else I left off my list! I went by every book store and auto part store in town-no luck so I bit the bullet and ordered:
"Engineer to Win" by Carroll Smith
"Race Car Engineering and Mechanics" by Paul Valkenburg
"Chassis Engineering/Chassis Design, Building & Tuning for High Performance Handling" by Herb Adams
On Amazon-give me a few weeks to receive and then digest the vast information before I come up with anymore ass-clown type ideas-see I do learn things over at c-c.com
Dennis-
check out progress of Bondobucket
Edited by: ddennis68 at: 7/26/04 6:45 pm