Three link and four link geometry

[Probird]

Ive been setting up my 3-link that is going in a 70 nova. I was wondering if it is OK to put the third link that is normally located above the axle, on the bottom of the axle? Just trying to figure out a way of lengthening my third link without having to get rid of my back seat. If I can put the link on the bottom of the axle I can lengthen it quite a bit. Now I'm talking about putting it just under the axle about 1" inch or around there.

[speedy79]

Check this thread out: https://www.pro-touring.com/showthread.php?85354-Rainy-Day-suspension-basics-what-IS-the-deal-with-torque-arms

A torque arm might be a good option.

[Norm Peterson]

Ive been setting up my 3-link that is going in a 70 nova. I was wondering if it is OK to put the third link that is normally located above the axle, on the bottom of the axle? Just trying to figure out a way of lengthening my third link without having to get rid of my back seat. If I can put the link on the bottom of the axle I can lengthen it quite a bit. Now I'm talking about putting it just under the axle about 1" inch or around there.
A 3-link or 4-link suspension needs to have enough vertical separation between the lowers and the uppers to effectively resist the axle housing rotating about the axle shaft centers. You need this separation to hold the angular change down to reasonable limits - and to hold the link forces down to something sane as well.

Versions of the 3-link have been produced that run the upper link to a bracket that extends upward and backward from the passenger side axle tube. One of the reasons for doing this is to lengthen the upper link. If that's your main reason, the tendency to equalize the tire loading under acceleration that also results from the asymmetry comes as a side benefit.


If you absolutely must keep all of the suspension arms and links down below axle center height, a torque arm certainly is a viable option - ground clearance permitting. Although there is a weight penalty involved, it's less of a factor in a car at the typical Pro-Touring weight than it would be in, say, an MGB.


Norm

[Probird]

Thanks Norm, that's what I thought and why I asked. Just trying to save some money and avoid the torque arm. I've already considered the BMR torque arm and is most likely the one I will go with. I already have the upper link offset towards the rear of the car but at my ride height the coilover cross member and the third link bracket are only 2” from hitting each other. I would have to redue my cross member and box a section of the trunk. Factory Five makes a pretty slick bracket but its only wide enough for a heim joint and I want to run a roto-joint.

[Norm Peterson]

If you're willing to run heavier wall tubing for your upper link, it doesn't have to be straight.

Perhaps a 'bent' upper link would provide a little more clearance?

Perhaps the C/O crossmember could be notched and compensated for by locally piggybacking a same-size tube on top of the crossmember the way it now sits? With enough overlap the C/O crossmember shiouldn't be adversely affected - or just run the piggybacked tube full width. I don't see this being out in the middle of the trunk floor . . . so the visual penalty if you consider it that should be minor.


Norm

[Probird]

I think the bent upper link is the ticket. It would allow me to get the clearance I want and at the same time I could legthen the upper link even more. That's a great idea, thanks Norm. Now I just have to find a place that can bend some tubing for me. Would 1-1/2" .120 wall be big enough?

[Norm Peterson]

It's probably in the ballpark, although the thickness might be better chosen to fit whatever adapter you need for your rod ends/Johnny joints/etc. It's not like with only a foot or so of tubing you'd be adding enough sprung or unsprung mass to really matter in a P-T car. An all-out race car might be different - and you might not even have this clearance problem.

If memory serves, the Evolution Motorsport tri-link used a bent upper link. This item was discussed fairly extensively over on corner-carvers, and there may be some information at lateral-g or elsewhere. I know of a thread over on the Corral, where some guy running a serious drag race car had some problems, and there might be some information there. I'd post the information directly, except that I can't get to any of those sites from the office computer any more.

What I could get to was this (http://www.miracerros.com/mustang/t_suspension2.htm).


Norm

[Probird]

That is some great information, thanks for the link. I really like the bolt on bracket that Steeda uses on there 5 link system. I am going to call them tomorrow and see if they would be willing to sell me the bracket, hope so. With that bracket and a bent upper link, problem solved. Thanks again Norm!

[MonzaRacer]

Why not do something similar to the old Ridetech upper link for the RoadGrater system, it was 2 piviots at rear cover and single up front. Use some 2x2 or 2x3 wall tube steel and integrate the third link into the trans tunnel area. Straight links and add in a Fays2 watts link and your good too go.

[Probird]

That doesn't sound like a bad idea either, got any pics? I'm more of a visual type of guy. I currently have a 12 bolt in the car that's from an early A-body, so I already have a place to mount the upper link. I would just need to build a bracket to connect to the original pick up points on the pumpkin, that should be relatively easy. The 12 bolt came with the car and I have had a real problem with cutting the ears off of the pumpkin. I would really hate to destroy the OEM rear. I figured that one day I would just sell it to someone who is looking for an original for some sort of restoration and get something else.

[JRouche]

If you're willing to run heavier wall tubing for your upper link, it doesn't have to be straight.

Perhaps a 'bent' upper link would provide a little more clearance?
Norm

Im really liking this idea. So often I think suspension links should be parallel with the plain that its mostly loaded in. Why? Cause I dont have any engineering skills and cant think out of the box.

I play it safe being a non-educated guy and use straight bars. But after you said it yeah, I dont see any reason why a "bent" rod would not work. But then the skills come up. Tube diameter and the proper bend. Engineers?! LOL They really can fit a larger square into a smaller round hole, they just know how to do it.

So I imagine the bend radius has just as much importance as the material and wall thickness?

Thats a project Id LOVE to see. A rear suspension with ALL the links kinda "bent" around all the main parts to make a very tight fit and still have some heavy load capability. With close fitting but safe tolerances.

Most cars I see have straight links, mine included. I have a parallel four link, watts link and a roll bar. ALL straight links. Its all stuffed into a small area and looks tight, but could be tighter with bent rods. 16 rod ends, 16 balls rolling around in the rod ends when in travel. ITS BUSY!!.

But with the proper engineering knowledge I could see how the systems could use some of the wasted space and make room for other stuff, like exhaust. Im battling the rear suspension for space right now to get the exhaust out to the back instead of before the tire.

Bent tie rods should be considered more I think. Straight tie rods almost seem like a lack of engineering if they arent used more. Cause really, they have been in place for what? Ever....

I would imagine as cars get smaller and space is more of a consideration bent tie rods will be as common as the fuel we use to power our cars. And Im not talking about sheetmetal control arms that are already in place that have odd shapes. But smaller arms that are tubes. Metallurgy will more than likely have to improve before we see suspensions that look like last nights pasta Im sure. But sure enough. There will be a day when you look under a car and dont see one straight link. It will look like some of the great headers we see with all the winding tubes. Snakes nest... JR

[Norm Peterson]

A straight link is still best, if for no other reason than it's more rigid that way and the control over the geometry is that much better.

Cliff's Notes version: flexibility in a bent round tube under a bending moment load is a function of the tube diameter and thickness, and the radius that it is being bent around. In any given tubing diameter and thickness, a tight bend is more flexible than a larger radius bend, and this extra flexibility leads to higher local stresses within the bend. This is somewhat separate from the fact that a straight tube under axial loading doesn't even try to bend in the first place (unless it buckles in compression, anyway). For the purposes of this discussion, the tendency of a tensile load to straighten out the bend does in fact create a "bending moment" within the bend, because the bend is offset from the line of action of the forces.


It's certainly possible to design around some amount of component flexibility - that's exactly what you're getting into once you're designing around compliant suspension bushings. Adding flexible "hard components" to the mix would just be something else to consider from both strength and deflection points of view.


Norm

[Probird]

Haven't had much time to play with the rear suspension. This weekend is when I can get back at it and I plan on sticking with the 3-link. I've been using Billy Shopes on line suspension calculator but recently it hasn't been working right, keep getting different results or just doesn't give me anything at all. I have a question for Norm. What is a good suspension calculator that I can use to help me graph my setup? Billys setup just spits out numbers with no pretty pictures but I'm sure it works just fine. I figure a picture is worth a thousand words and ideas. I'm willing to do whatever it takes to make it happen and work right, bent links and all.

[Norm Peterson]

I've seen a couple of spreadsheets that plot the geometry as you'd see it in cutaway plan and side views of the car.

But the only ones I know of that put up plots of what happens to anti-squat and roll steer as you change the ride height are the ones I've been tinkering with over the years. I imagine that other people have done something similar, just that I haven't happened to run across them yet. It's not that big of a step to go from displaying the A/S and rollsteer at one data point as numbers to plotting a curve through the results you'd get at 15 or 20 different ride heights starting from some specific pivot point measurements or design locations.


It's been a learning process at this end to develop them, and I get to thank this forum and the several current rear suspension threads for prodding me into going through them yet another time, finding and fixing a couple of potential divide-by-zero situations, and putting sheets for some torque arm configurations together.


Norm

[Bryce]

I built my own spreadsheet that reverse calculates suspension pick up points based on desired A/S and IC and one pickup point per link. It took me a few hours to develope but is not user friendly so I would hate to post and add confusion to someone.

There was a good calculator on a offroad website. I will google it and post if I find it.

[Bryce]

try this

http://www.pirate4x4.com/forum/showthread.php?t=204893

[MrQuick]

heres a good one.... right click and save as...... its a read only file
http://mysite.verizon.net/triaged/files/3LinkV1.0bBETA.zip

[Norm Peterson]

try this

http://www.pirate4x4.com/forum/showthread.php?t=204893

I've seen both of the calculators in that link and I have an older version of the 3-link one. I can open the 3-link one OK, but my copy of MS Office seems to think there's a risk problem with the 4-link sheet.


Norm

[Probird]

That's exactly what I was looking for, thanks Norm. Everything opens just fine on my computer both the 3 and 4 link.

[Norm Peterson]

I think the credit belongs to Bryce.

I'm sure I could have opened the 4-link one, but after dealing with an issue on my laptop early last summer where I ended up having to reformat the hard drive and re-install everything I'm not nearly as willing to accept that sort of risk. I may have it on the office computer anyway.


Norm

[Bryce]

I think the credit belongs to Bryce.
Norm

HAHA, thanks Norm.

[Probird]

My apologies Bryce, I replied from my phone and it just opened at Norms reply. Don't I feel like an idiot!!!:hand: Now that I have had some time to play with the calculator, how much anti-squat do you guys think is ideal. I was thinking somewhere around 60-80% but was unsure if the 80% is to much. I plan on having a total of three holes for the forward mount to give me some adjustability but I want to set it at what is hopefully, ideal. If such a thing even exists. All replies are welcomed, again sorry Bryce.

Henry