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boodlefoof
08-15-2006, 07:41 AM
Hey guys. Sorry for another long post (I've gotta break that habit) but I could use some help. :hammer:

I've been pouring over old threads for awhile both here and on CC.com. I think the Satchell link design for my Datsun 240-Z project is coming along well, but I would like any input you might have.


The car: 92'' wheelbase, 62'' track width both front and rear, and I am guessing it will come out to about 2000 - 2200 pounds when done. Front suspension has an RCH of 3''.


The Satchell link (current design):

RCH is 10.9'' in its present iteration
Upper link axle housing mount = 16.5'' high (3.5'' over axle CL)
Upper link frame mount = 15.5 high''
Upper link down angle from rear to front is 1.7*
Upper links are parallel in plan view and 34'' long
Lower link axle housing mount = 8'' (5'' below axle CL)
Lower link frame mount = 9.75''
Lower link up angle from rear to front is 3.5*
Lower links are at a 45* inward angle and are 28.3'' long
Side view swing arm length is 72.5''
Anti-squat is 101%

There appears to be mild understeer, but I am unsure of how to calculate a percentage. For example, in a 3* roll, the axle housing has moved about .17'' (forward on the outside axle relative to the roll, rearward on the inner axle relative to the roll). By 5* of roll the axle has moved .3'' at either end.


The questions...:

1. I'm unsure whether this is or isn't too much roll understeer... based on what I've read, I'm shooting for around 2-3% roll understeer. I can easily increase or decrease the understeer by adjusting the angle of the upper arms in side view.

2. How high of a RRCH would be acceptable? It seems that I've read posts describing anything from 8''-15''. There is lots of info talking about FRCH, but RRCH seems more of an enigma.

At present, the lower arm axle mount point only gives me about 2'' - 2.5'' of clearance from the flat belly pan design, thus limiting my suspension droop to 2'' (as would be regulated by axle bump stops). I would like to raise the lower arms an inch to give myself some extra clearance, but doing so raises RRCH to about 11.6''.

On a side note... man this Suspension Analyzer program gets buggy when dealing with the rear suspension! :dunno:

Thanks for any help all.

Norm Peterson
08-15-2006, 09:09 AM
I'll have to toss this at a 3D-ish 4-link spreadsheet when I get home (in lieu of better information at that time, I'll make an educated guess as to the lateral separation of the chassis/axle pivots of the lowers being 13" & 53" respectively).

I should think that the inclination of those plan-view-parallel uppers would set the rollsteer value at the tangent of whatever side view angle they were at, in this case tan(1.7) = 0.0297, or about 3% rollsteer. That your axle end motions suggest about 11% rollsteer leads me to suspect that some plan view convergence between the uppers is unintentionally present in the model.

FWIW, the 3.5 inclination of the lowers appears to be as viewing the lowers in their true length. In side view projection, that 1.75" difference in pivot heights and the 20" projected link length make for an apparent angle of 5, which is the more useful angle for working up the SVIC coordinates (3.5 up-angle used directly in SVSA calculations will give you a 93.5" SVIC X-coordinate rather than 72.5").


Norm

boodlefoof
08-15-2006, 10:26 AM
Thanks Norm. I appreciate it. If you (or anybody else) is interested in the exact coordinates I'm working with... they are as follows...

X,Y,Z => X = outboard from center, Y = height, Z = forward (positive) or backward (negative) from axle centerline. Both sides are symmetrical.

Upper link frame = 23, 15.5, 33
Upper link axle = 23, 16.5, -1
Lower link frame = 5, 9.75, 20
Lower link axle = 25, 8, 0
Tire is 26'' tall.

According to the program, this makes my IC height 14.34, at 72.45'' forward. It is also what calculated my side view link angles (1.7* upper and 3.5* lower).

Unless the program is doing something wrong, there is no convergence in plan view on the upper arms. I had considered putting some convergence in to help lower the RCH (just 2 inches inboard at the frame reduces RCH about 1/3''), but cockpit space is already going to be at a premium. I'd prefer to keep theim parallel.

Norm Peterson
08-15-2006, 04:40 PM
Different coordinate system
X = longitudinal, positive forward, zero at rear axle rest position
Y = lateral
Z = vertical

Note the tiny plan view skew in the uppers - that's only to sidestep a divide-by-zero problem.

I settled on an 18" CG height after an initial guess plus one fully defined configuration.


4Link11
Filename/Descr. boodlefoof 240Z Satchell

INPUTS
LCAchassisX 20.00 in
LCAchassisY 5.00 in
LCAchassisZ 9.75 in
LCAaxleX 0.00 in
LCAaxleY 25.00 in
LCAaxleZ 8.00 in
CGHeight 18.00 in
Wheelbase 92.00 in
RideHeightChange 0.00 in
LCAchassisZChange 0.00 in
LCAaxleZChange 0.00 in
RearTireRollingRadius 13.00 in
UCAchassisX 33.00 in
UCAchassisY 23.00 in
UCAchassisZ 15.50 in
UCAaxleX -1.00 in
UCAaxleY 23.01 in
UCAaxleZ 16.50 in

CALCULATED PARAMETERS
LCAlength 28.34 in
LCAplanviewlength 28.28 in
LCAsideviewlength 20.08 in
LCAslope 0.0875 in / in
LCAconvX 25.00 in
LCAconvY 0.00 in
LCAconvZ 10.19 in
UCA/LCA overall 1.200
UCAlength 34.01 in
UCAplanviewlength 34.00 in
UCAsideviewlength 34.01 in
UCAslope -0.0294 in / in
UCAconvX 78233.00 in
UCAconvY 0.00 in
UCAconvZ -2284.50 in
UCA/LCA planview 1.202
UCA/LCAsideview 1.694

OUTPUT (at static ride height)
AntiSquatX 72.45 in
ROLLCTRHEIGHT 10.921 in
AXLESTEER 2.93 %
ANTISQUAT 101.16 %
AntiSquatZ 14.34 in
RateOfRCHChange 0.51 in / in
RateOfAxleSteerChange 2.95 %/in
RateOfAntiSquatChange -29.99 %/in


Anti-squat and roll steer are not fixed parameters, BTW. In your geometry, anti-squat percentage drops and roll steer increases as rear ride height decreases. I see about 10% roll steer (vehicle understeer direction) showing up at a decrease of about 2.5" in rear ride height. Vehicle pitch is not considered. I have plots of both.


If rollsteer really is closer to 11% at static ride height, I'd like to see the explanation.


Norm

boodlefoof
08-15-2006, 06:03 PM
Thanks Norm! You're the man! I am still learning, and this is exactly why I wanted some opinions on the design before I went out and put it together. I'm not entirely sure what is or isn't considered to be ideal yet.

The outputs I calculated are the same, I just can't (or haven't figured out how to) plot the graphs as you have, or roll steer as a percentage. The roll figures I mentioned before are in a roll with no dive. I just told it to calculate axle location change at X* roll and it spit out those numbers.

The questions that pop out at me now are these:

First, some observations about roll steer. I'm still a little confused about the roll steer curve and its effects on driving. Based on the graph, yes there is about 3% at ride height and it gets up to about 10% at 2.5'' bump. I believe the calculations are correct. So, questions...

1. Is such a roll steer curve acceptable?

I'm sure that slowing the roll steer gain in bump is a good idea, but I'm having trouble imagining a scenario where I'm in that great a rear bump while cornering... at least not with the anti-squat I am trying to build in. On the other hand, I'd hate to have enough anti-squat to lift the car enough to put me in positive roll steer coming out of a turn! Of course, the lift in the car will be limited via bump-stops as I mentioned (to probably 2-3'').

2. Will roll steer in bump have any impact on driving in a straight line? It seems that going to 2.5'' bump could only happen on major accelleration (or if I'm on a bumpy road). It seems that if roll steer is changing that much in straight bump, effecting both wheels evenly, it won't affect performance. Please, correct me if I am wrong.

3. Any tips for changing things to slow the bump steer curve? I'll toy with it some more tonight and see what I can come up with.

Second, does that anti-squat curve look good? Again, I'm not sure what is or isn't good, so is 30% per inch is ok? Again, I'll toy around with it tonight and see if I can reduce the loss while maintaining my other variables.

Third, back to the RCH. Should I aim to lower it from 10.9'' or would raising it slightly (say, up to 12'') cause problems?

Thanks again.

boodlefoof
08-16-2006, 06:34 AM
Well, here is some more info to add into the mix. Last night I indeed spent some time tinkering with the suspension and I think it has improved. Let me know if you all agree.

I made the upper arms level in side view rather than angling down slightly towards the front. I have read on CC.com that this will give more neutral roll steer. Additionally, I raised the lower arms slightly and increased their angularity slightly. This slowed down roll steer change. I figured out the graph function and compared this to the previous design above. Whereas the previous design had axle lead moving .17'' at 3* roll and .30'' at 5* roll, the figures are down to .11'' at 3* roll and .18'' at 5* roll.

The anti-squat curve looks to be right about the same slope, but I've increased the AS at ride height to 119%.

In playing with the design I've found that shortening the upper arms will flatten the AS curve (slower AS loss in bump), but at the same time this speeds up the roll steer curve. Also, I've always heard that as a rule of thumb the links should be as long as possible, so I feel kind of strange trying to shorten them.

However, if I shorten the upper arms to 21'', I see a significantly better AS curve with only a very small roll steer change... which puzzles me.

This new test also raises RRCH to 11.31''

Here are the figures (with original long upper arms).

Upper link frame = 23, 16.5, 33
Upper link axle = 23, 16.5, -1
Lower link frame = 5, 10.75, 20
Lower link axle = 25, 8.5, 0

Norm Peterson
08-17-2006, 09:01 AM
. . . but I'm having trouble imagining a scenario where I'm in that great a rear bump while cornering... at least not with the anti-squat I am trying to build in. On the other hand, I'd hate to have enough anti-squat to lift the car enough to put me in positive roll steer coming out of a turn!I'd be careful about building in too much A/S unless drag racing is in this car's future. Anti-squat's evil twin is anti-lift, which can result in brake hop or force some sacrifice in rear braking capability as a band-aid. 50% or so seems to be more where cars set up with handling as the #1 priority seem to fall.

There's a separate line of thought regarding A/S, at least as it applies to auto-x - specifically A-Mod (the home of some wild one-off specials). That's to intentionally use relatively little of it. The reasoning is that if you simply let the rear squat a bit, the rear roll center will drop, as will rear lateral load transfer. Translation: more grip becomes available for accelerating off the corners as the rear squats because the front is being forced to do more of the work resulting from the lateral forces.



2. Will roll steer in bump have any impact on driving in a straight line? It seems that going to 2.5'' bump could only happen on major accelleration (or if I'm on a bumpy road). It seems that if roll steer is changing that much in straight bump, effecting both wheels evenly, it won't affect performance.No. But if you encounter bumps while cornering - even perfectly timed two-wheel bumps - the rear axle will steer back and forth slightly. That probably does more to provide a vague or uneasy feeling back to the driver than it does to actually reduce measurable performance. Then again, lack of confidence in the car will tend to make you drive it slower. You can estimate rearward load transfer and compute the amount of spring compression/squat that would occur, and then reduce that using the A/S% to see just what you need.

Simple Example: If you have 2.5" of bump travel available, 2000 lbs of sprung weight, 1g acceleration, and a 18.4" sprung CG height, that's 400 lbs of extra rear force (200 per side) under acceleration. Given a medium-ish 100#/in rear wheel rate, that would be 2" squat with zero A/S. 50% A/S means only 1" of squat, leaving 1.5" of bump travel remaining.



3. Any tips for changing things to slow the bump steer curve? I'll toy with it some more tonight and see what I can come up with.Roll steer gain in a Satchell link is dictated by the side view length of the uppers. The value with the car at rest is then defined by their inclination.


Second, does that anti-squat curve look good? Again, I'm not sure what is or isn't good, so is 30% per inch is ok? Again, I'll toy around with it tonight and see if I can reduce the loss while maintaining my other variables.My thoughts are that it falls off a bit quickly, and that you might have traction right off that goes away on you. Stiffer springs than you might otherwise fit could be a workable band-aid.


Third, back to the RCH. Should I aim to lower it from 10.9'' or would raising it slightly (say, up to 12'') cause problems?I would definitely not raise it. Elsewhere in the world of modifying stick axle suspensions is a growing trend toward bringing that RC down. Jon A (of corner-carvers and FRRAX) has posted some fairly detailed evaluations of his own experiences. And that's in a 101" wheelbase car. I suspect that there's a range of vehicle roll axis inclination over which the car works best, and that for pavement and turning both ways it's small, maybe within something like 3 or so. The car that I own that has a nearly horizontal vehicle roll axis feels far more nimble than either of the cars that have markedly steeper ones - and that car has, relatively speaking, the smallest tires and narrowest wheels of the bunch.


[Edit] - Anti-squat curves are generally "Lazy S" shaped, which depends on the relative side view inclinations of the uppers and lowers. The plot looks really weird when the curve reverses itself twice within the range of suspension motion, but at least the absolute values don't change all that much under those circumstances.


Norm

boodlefoof
08-17-2006, 09:26 AM
Thanks again for your insights Norm.

I've been playing with my design more and have found something interesting that seems to go against conventional wisdom... In fact, I think I might have to add these observations to the "Really Long Trailing Arms" thread over at CC.com.

Typically, you hear that the longer the links, the better. Many people seem to think this will help to keep RRCH from moving, thus making the car more predictable. So, I had been designing to get the longest links possible. On the bottom side, this comes out to the roughly 28'' links. On the upper side, the spaceframe design really will acommodate anything up to 60'' at the extreme. Currently, I had been playing with a roughly 30-35'' upper link (as this is a convenient mount point on the frame and a link this length won't need to be as beefy as a longer one).

The thought occurred to me though that perhaps there is an optimum upper link length relative to the lower length, much like in a double A-arm front suspension. I thought "Perhaps going with too long an upper would actually be detrimental to the design?"

So, as I tried to flatten the AS curve I found that I could do so by shortening the upper link as I mentioned earlier in this thread. I had been looking primarily at the AS curve and the understeer curve, trying to reduce AS loss while not increasing roll steer too much.

Then, I happened to look at what was happening to RCH depending on upper arm length. I found that there does appear to be an ideal upper length relative to the lowers for the purpose of stabilizing RCH migration in bump. It appears to be around 21.5'' on my setup.

Here are the numbers, which I think are pretty interesting... RCH change does the following in bump all else being equal:

Ride height / 30'' upper arm RCH / 21.5'' upper arm RCH

0'' bump / 11.30 / 11.31
1'' bump / 10.90 / 11.30
2'' bump / 10.50 / 11.30
3'' bump / 10.10 / 11.31
4'' bump / 9.71 / 11.35

The same thing happens in lift - the long arms gain roughly .4'' RCH per inch lift while the shorter arms don't change RCH by more than .04'' over the entire range of motion!

Additionally, with the shorter arms AS loses about 20% per inch of bump as opposed to 30% per inch with the longer arms.

Meanwhile, roll steer with the short arms moves from 0% at ride height to 7.9% at 4'' of bump, while figures for the long arms are a range of 0% to 5.2%.

It seems odd that the shorter arm actually appears to be a better setup on paper??? What do you think?

g-roadster
08-17-2006, 01:15 PM
Norm: Interesting discussion on the Satchell Link. Thanks for your input. A couple of points about the Satchell Link: The roll center is determined by the intersection of the two lower links, If you want to minimize roll center migration relative to the frame bring these inner pivots as close together as possible. Making a large A frame with a ball joint would be a solution for minimum roll center migration but would increase stress on the single ball joint rather than two and reduce your alignment adjustment possibilities. Roll steer is controlled by the upper links, adjusting the length and inclination affects roll steer. You hinted at this in your discussion.

I am designing the g-roadster with a Satchell link rear suspension. So far I have only laid this out graphically. I feel comfortable doing this as I have designed everything from large space craft to small optical systems from the drafting board. But I do realize the limitations of graphical only design. What program are you using for your suspension analysis? I will need to get a program, or access to one, to refine my front and rear suspension designs. Recommendations appreciated.

Norm, if it wouldn't be too much trouble could you do computer run for my Satchell link design?

Not sure what: RideHeightChange, LCAchassisZchange, LCAaxleZChange are in your inputs.

boodlefoof
08-17-2006, 02:12 PM
g-roadster,

Thanks for the point about the lower arm attachment points. I tinkered with bringing the lower link frame attachment points closer together. I think I've got them about as close as I can to clearance the driveshaft with some comfort room. I did find though that bringing them closer together also increases the slope of the roll steer curve.

I'm using "Suspension Analyzer" from Performance Trends. If you google it, you'll find their website. They allow a free 10 day demo download that allows you to use all functions of the program. Otherwise, purchase price is something like $350! It will give you the outputs for your design based basically just upon the XYZ mount points for your trailing arms.

Norm Peterson
08-17-2006, 04:20 PM
RideHeightChange allows you to input a ride height change due to a change in vehicle loading or a spring height change for a quick ,if slightly dirty, set of results.

LCAchassisZchange is simply a lowering (or raising) of the chassis side LCA pivots relative to some baseline value, and perhaps useful if you have multi-hole or otherwise adjustable chassis side pivot pickups. It's just another way of modifying the LCAchassisZ data that could provide a hint about what you were thinking when you look at the sheet in a year or whenever. Also slightly approximate.

Ditto for LCAaxleZChange, which is where one would enter changes in LCA attachment elevation at the axle brackets. AKA relocation or anti-squat brackets.

P.S. One can perform moderately sophisticated geometry and engineering analyses using Excel or similar spreadsheet software once the necessary equations have been worked out.

P.P.S It's not much trouble at all to run a few of these every so often. After all, there are less than 20 separate inputs (symmetry has been assumed). But I'd rather handle this sort of consulting via e-mail, as the amount of allotted attachment space is limited.


Norm

boodlefoof
08-28-2006, 07:11 PM
For those interested, I have built the Satchell link! I slotted the mount bracket holes to allow for adjustability of the height of each mount point. That way, I can test-drive the car with the suspension set up for different roll-steer, anti-squat and RCH and see what works best for me.

Pics are up at: http://www.geocities.com/boodlefoof/Project_Racer-Zsuspension6.html

jaybee
09-09-2006, 01:14 PM
Interesting but I have to confess I really don't understand. To me it looks like an upside down triangulated four link, how does that make it behave differently?

Norm Peterson
09-09-2006, 05:58 PM
(1) The geometric roll center is generally several inches lower, meaning that the rear spring/sta-bar/shock package will differ. It also migrates a bit more in the vertical direction (not necessarily a bad thing, BTW). This is probably the largest difference.

(2) Wheel torque reactions are not carried through the axle tubes and into/through the pumpkin. Axle ends are better-supported longitudinally.

(3) The roll steer vs ride height curve is different.


Norm

FoxGranadaChuck
11-09-2006, 07:08 AM
I am bumping this thread wondering what it would take to adapt a Satchell link rear suspension to an AMC Pacer.

I was just musing this the other day after I had just watched an episode of Sports Car Revolution. One of the hosts on that show had driven a highly modified AMC Pacer. My imagination got to thinking about a Pro-Touring Pacer. My thinking is that a forward-thinking car (in the Seventies) should have a forward-thinking rear suspension design (such as the Satchell link).

boodlefoof
11-10-2006, 07:33 PM
FoxGranadaChuck,

Well, the Satchell packages pretty nicely, so I don't see why you couldn't adapt it to the Pacer. Pardon my ignorance, but the Pacer is RWD correct? It would just be a matter of building the frame structure to support the links.

pav8427
11-16-2006, 03:56 AM
Norm,
Not to hijack, but you mentioned Excel spreadsheets, and I was wondering if there might be some out there to download so a person could play with different set ups to narrow down a design?
I have a few; one for IFS, 3-link and 4-link, but would like to evaluate from different angles prior to getting a Performance Trends package.

Doug

FoxGranadaChuck
11-16-2006, 12:03 PM
FoxGranadaChuck,

Well, the Satchell packages pretty nicely, so I don't see why you couldn't adapt it to the Pacer. Pardon my ignorance, but the Pacer is RWD correct? It would just be a matter of building the frame structure to support the links.

Yes, the Pacer is indeed RWD. I was thinking of fitting a Mopar 8 3/4 rearend housing with the Satchell Link fittings on a conceptual level.

FoxGranadaChuck
11-16-2006, 12:05 PM
By the way, exactly HOW do you set up a Satchell Link suspension?! I know what it looks like. I was just wondering how you set it up for a particular car.

Norm Peterson
11-17-2006, 04:51 AM
Norm,
Not to hijack, but you mentioned Excel spreadsheets, and I was wondering if there might be some out there to download so a person could play with different set ups to narrow down a design?
I have a few; one for IFS, 3-link and 4-link, but would like to evaluate from different angles prior to getting a Performance Trends package.

DougThe only other sheet that I know of that's available for download is Greg Locock's "SLARCK". I have several sheets in some state of off and on development (functional but mostly not in very presentable/user-friendly format and lacking the 'HELP' page that I've started to include).


Norm

MonzaRacer
11-17-2006, 03:38 PM
Ok I understand the design, But as for a person who actually drove a simialr set up dont forget that while you design the"most perfect" set up dont forget(talking as an alignment tech) dont forget that while the link designs are very close you still have to dial in alignment specs to get the most benefit from it.
I asked a friend who had a simiar set up on a Sunbeam Tiger (after rebuild it was readended) and he spent 3 weeks on a 4 wheel alignment machine then back to track/road for driving tests. He had some great twisties around his house and was doing late night bonzai runs to test it.
while he thought he had a bad set up he found it really came out better than he thought after he figured out that it required different specs side to side to deal with his 347 in Windsor engine over the 289 that came "factory".
Good luck and after I get his extension program(puts me under blanket license) for advance suspension design (very similar to what Ford/GM/Chryco used to use for designing) if we tried to buy it it would take 3 servers and about 80k to purchase but his company bought the older set up from GM design group and let him add users for help on thier designs (they are actually using it to design some suspension systems for boat engine mounting but system was used to hundreds of inputs so the 20 or so they have is not exactly taxing it too hard)i hope my computer will work ok as its not very fast(new dual core/dual processor AMD Raid 5 setup coming in spring woohoo) oh well goodluck and If it models out for me Ill repost for you,
Lee

boodlefoof
11-23-2006, 09:55 AM
As for setting it up for a particular car, I guess it depends on what you want the car to do. I have made all of my mounting points fully adjustable about 2-3'' vertically. I plan on starting my baseline with the upper arms parallel to the ground and the lower arms set up for 100% anti-squat. If I remember right that gives me an RCH around 10''. I can lower the RCH down a bit at the expense of anti-squat. That might work out well since this will be a light car with short wheelbase (92'') and a pretty even weight distribution. Then I will tune with an adjustable sway bar.

911industries
12-20-2006, 08:52 PM
Im getting ready to set up a satchell link on my 61 falcon this weekend. Ive read everything I can get my hands on to try and fully understand this suspension design. I was wondering if any of you could help me with the important things to watch for.

What Ive got so far is long links(around 20"), 45 degree angle on lower links. Shoot for 100% anti squat.

My questions are what is and ideal and an acceptable RCH? What about roll understeer? Where do I want that?

boodlefoof
12-21-2006, 06:42 AM
911industries, welcome to the board!

Before you start welding, you may want to download the free 10 day trial of Suspension Analyzer from Performance Trends and map out your design.

Depending on the relative length of your links, roll understeer may be a bit high. Also, the measurement of understeer is dynamic (changing with roll). At ride height, I designed mine to have zero understeer, but it gains a little (I forget how many degrees now) understeer per degree of roll.

Acceptable RCH is also debatable. Lower RCH usually involves lower AS. Since there are so many factors at play, I set up my mounting points such that I have a lot of adjustability to move RCH from about 8'' on the low end to 11.5'' on the high end. AS goes down as RCH gets lower.

If you haven't seen it already, go over to corner-carvers.com. There is a long thread regarding the Satchell link. Search the suspension forum and you will find it.

Good luck!

911industries
12-21-2006, 02:17 PM
Thanks for the help. Where can I get or download the suspension analyzer?

boodlefoof
12-22-2006, 07:57 AM
http://www.performancetrends.com/download.htm#sa

jerome
12-23-2006, 09:22 PM
how does one make suspension mounts fully adjustable with a range of heights. i know the jaguar on corner carvers has it too. is it just a elongated hole that you put a bolt through? it doesnt seem you could generate enough clamping force to suspend a car like that.

Jerome

boodlefoof
12-24-2006, 11:59 AM
Jerome,

Yes, you can elongate the bolt holes to gain the adjustability. Nearly all of the forces acting upon the rod-ends is going to be forward/backward forces. Since the holes are elongated in the upward/downward direction, it shouldn't be much of an issue.

If it does turn out to be a problem, one could use washers with an offset hole (much like in the C5/C6 suspension). It is difficult to describe how this works, but if you get a chance to take a look at the Corvette suspension it will make sense.

911industries
12-31-2006, 05:19 PM
Ive figured out where everything is sitting on my car and I plugged in where I want my links. I cant figure out what to look at to find my roll understeer. Also when I roll the car in the view mode it says I have convergence but doesnt say anything about fixing it.

my specs right now are x,y,z

ulf 30,15,22
ula 30,15.5,0
llf 2.5,11,16
lla 18.5,10,0

with 26" tires

If you can help me with figuring this out it would be great.

boodlefoof
12-31-2006, 07:28 PM
To find the understeer, take the tangent of the upper arm angle (vertically from front to rear).

What kind of output figures is your design giving you? You really want to pay attention not only to the static figures, but the change in figures in bump and roll. I found using the graph feature very useful to compare setups.

911industries
02-24-2009, 11:54 PM
I cant believe its been two years since I posted. I got my car on the road a few months ago. I had no sway bars at first and the fender skirts on my falcon made the tires rub bad. I did a quick fix with a bronco sway bar I had laying around which works but isnt right.

After thinking that my design was flawed for awhile I've realized that the satchell link has great possibilities and I cant wait to start fine tuning.

Im just wanting to thank you guys for your help and to let you know that its because of you that I went through with this suspension.

terryr
02-26-2009, 01:46 PM
Neat. I'd like to do a Satchell Link one day. Will mufflers and tailpipes fit in there?

911industries
05-06-2009, 12:27 AM
Yeah, exhaust was easy because I put the mufflers behind the axle. Those are old pics. I'll post some new ones soon. I found out the hard way that the crossmember needs to be braced. I broke it in two and tore up my floor. My problem I havent figured out yet is body roll. For some reason I thought a low roll center meant less body roll, I think its the opposite. This thing feels like a lifted truck when I yank on the wheel.

Norm Peterson
05-06-2009, 03:18 AM
Yeah, exhaust was easy because I put the mufflers behind the axle. Those are old pics. I'll post some new ones soon. I found out the hard way that the crossmember needs to be braced. I broke it in two and tore up my floor.
After you design the geometry to suit your purposes (and probably tweak it a bit so it'll fit), you still have to make everything strong enough to resist the loads (which can get pretty big).


My problem I havent figured out yet is body roll. For some reason I thought a low roll center meant less body roll, I think its the opposite. This thing feels like a lifted truck when I yank on the wheel.
All else equal, low roll centers do mean you'll get more roll than you would with high roll centers.

Having a low roll center means that the car's mass (simplistically, up there at the CG) has greater leverage against the suspension than it would if the roll center was higher. So you either live with more roll - or choose stiffer springs and sta-bars to limit roll elastically.


Norm

911industries
05-06-2009, 11:51 PM
I thought I had some pics of the new braces but apparently I was in a hurry. Ill get some this weekend, I hope I will be able to work on my car. In the books I was reading they never metioned the differences is low and high roll centers. They kept on talking about how you determine where it is and I guess I naturally thought that lower was better. Ooops.

In some way a low roll center would be better because you can use your sway bar to tune it better. If you have a high roll center and very little body roll then minor changes will make a big difference. If your body rolls like a boat then you dont have to be so accurate. Thats just my justification so I dont have to say that I really screwed up.

These pics are just before Kool april nites. Me and my coworker drove down there and back. Around 800 miles round trip. If you look on the left weld on the top of the tunnel you can see the gap from the break.
The second shot is my buddy Jeremiah helping with the interior. Last is me just getting a good hump on with one fine car.

Norm Peterson
05-07-2009, 02:45 AM
In some way a low roll center would be better because you can use your sway bar to tune it better. If you have a high roll center and very little body roll then minor changes will make a big difference.
That's pretty much the way it works. Springs and bars (and shocks) can only tune the handling based on the amount of roll that occurs. With a high RC and not much roll, it takes much greater changes in the spring and bar stiffnesses to achieve a given handling change. When the suspension gets really stiff, you start losing a lot more of your tuning efforts to chassis flex unless you go back and stiffen that, too (not as easy as you might think, BTW). Never mind what happens to the ride.


Norm

iou1choke
05-07-2009, 07:03 PM
911,
It appears that you are using a heim type joint on one end and solid bushing on the other (?). May I ask what type of bushings you decided to use?
I also notice you have the heims on the forward bar mounts. I was under the impression that they should normally be used on the axle end of the bars. Was wondering if you had a particular reason for this orientation?
Also curious if you determined which directional forces seemed to have the most effect on the damages to your x-member? How did your rod ends hold up?
What do you think of your roll steer characteristics? Does it respond how you thought it would from a design standpoint?

Nice to see a satchell link. I, coincidentally, am in the middle stages of fabbing and installing this link set up in my vehicle.

Rhino
05-12-2009, 08:23 PM
Small world. I'm in the planning stages of a Satchell myself. It's nice to see others trying it as well. Up until last week I hadn't found many online real-world tests. I'd be interested in seeing the issues with the crossmember as well.

Norm Peterson
05-13-2009, 03:48 AM
I found out the hard way that the crossmember needs to be braced. I broke it in two and tore up my floor.
I think the crossmember needs at minimum some fore/aft bracing at each of those LCA chassis side pickups, and the other ends need to be tied off to something more substantial than just being bolted to the middle of some sheetmetal. Not having this bracing means that the longitudinal components of the LCA forces are being resisted by the crossmember in torsion.

The inside "corners" of the existing crossmember at the top of the tunnel should probably be filled in with a plate formed to a radius, as sharp corners = fatigue "hot spots".

I'd also consider adding some lateral bracing from the LCA brackets if you can fit it underneath. Or some sort of transverse structure above the floor and solidly welded to it from the tunnel to the sills if you can't (given the roll bar, I don't see the back seat area ever being occupied). Any bending loads that you can get off the crossmember where it goes up and over the tunnel can only help things.


Norm

911industries
06-25-2009, 11:50 PM
Sorry for the delay guys. My car kinda took dump and life stepped in to say my time needs to be spent on other projects. The car problem is burning coolant which took out the O2 sensor which the computer didnt like. Basically I need to tear out the engine to see if its the heads or head gaskets.

Anyways back to the subject.

-Heims and bushings. I cant remember where I got the bushings but if you like certain size tube and call up energy suspension with some numbers Im pretty sure they can find a bushing that will work. I put the heims on the body side so they will see less road debris and last longer. I dont think that putting them on the axle would make them work any better. The axle and links roll the same no matter where they are restricted from. The roll restriction is the bushings.

-crossmember destruction. I've talked to some guru's about that and basically it was do to the POWER. Ha, ha, ha. After looking at all the aspects of the failure the desire for the axle to roll under hard launches makes the most sense. When you launch the lower arms which are those want to push forward and thats exactly what they did.

I had room to put a 45 degree tube above my exhaust from the LCA to a roll cage crossmember. That seems to have done the trick but I havent had much rough testing lately.

-Roll steer. Because I dont have a rear swaybar and a bad front swaybar its hard to tell. The thing I can say is that this car is scary at 140! Why you ask? It has more to go and it feels like Im going 60. Another fun fact. Bad hood hinges and high speeds will actually make your hood lift up and really make pucker.

Even though I roll like a boat the car still takes the line I choose. I was playing with an Audi R8(I think) and he never pulled me even in 30 mile curves with 80 on my speedo.

-Crossmember construction. I also added some diamond shaped plates over my weld points. Those ar supposed to help distribute the load away from the weak point just outside of the main weld. Thats the thing with a weld is; it is strong and so is the tube but the weak point is th hot spot created in between the two.

911industries
01-18-2010, 11:51 PM
Here are some pics of my problem and the solution. I've done a couple of drags since I installed the diagonal braces and everything is looking good.

The first shot you can see my crossmember with the break on the left side of center. If you look real hard you can also see how it buckled the floor pan on the left hand side. My idea was that the floor pan would help with the structure. Turns out it helped until the crossmember broke.

Second shot is inside where the rear seat normally is. You can see my diamond gausetts. Also behind the crossmember is a crack in the floor that was also caused from the broken crossmember.

Third shot you can see the diagonal braces. Outside where they connect to the frame rail are connectors to the rocker and roll cage. One thing that isnt pictured is another diamond gausett I put spanning the two center welds. I actually broke it a second time after I installed just the braces.

sik68
01-21-2010, 01:09 PM
Sorry, I have nothing to contribute...but everytime I see this thread I think of this:

https://static1.pt-content.com/images/noimg.gif

911industries
04-24-2011, 06:31 PM
Okay guys I'm back again. I'm pulling getting ready to pull out the drawing board and do some redesigning. Since I last posted I've moved across the country, over to Savannah, GA. I finally got around to installing a custom sway bar which made a huge difference. Problem is I'm afraid to abuse the car since that crossmember keeps cracking. I installed gaussets from the diagonal bars spanning the link mounts.

Since I have done so much strengthening of the crossmember and it keeps failing I think my design is flawed. An important thing I think is that I back braced the axle which increased stiffness. I think that my design binds the suspension when the body rolls. I have tried to test it with the coilovers off and it seems to roll freely to a certain degree.

I'm just so stumped on why I can't make it work. If anyone knows a suspension guru let me know. Otherwise once I solve this problem I will surely post it.

Suspension numbers: X-left right Y-front rear Z-up down Zero is center vehicle on the ground

X,Y,Z
ULF- 15,22,15
ULA- 15,0,15.5
LLF- 2.5,16,11.5
LLA- 18.5,0,10

Norm Peterson
04-25-2011, 08:10 AM
The floor pan buckled because without (more) fore/aft stiffening it can't carry the loads that you are developing during a dragrace launch. Once the floor buckles elastically, the crossmember has to carry essentially everything, which it apparently isn't able to do. Once the crossmember cracks or otherwise gives, the floorpan buckle is likely to become inelastic, or permanent. Like I mentioned in post #39, you need more fore/aft stiffening, and it needs to tie in to more than just the new crossmember.

A Satchell link, like any other triangulated/converging 4-link, does not inherently "bind", at least not over relatively small deflections, as long as all eight of the link pivots are sphericals of some sort. As soon as you make any of the end connections "cylindrical" where the pivoting is forced to be about an axis that is a fixed line in space (the bolt) rather than a point that can rotate freely in any direction (ball & socket), the stiffness of the cylindrical bushing material starts adding stiffness that resists this motion.

Off the wall question - are you pulling the LF tire on your drag race starts? If so, the car starts rolling over much faster in response to the engine torque reaction, and it all piles up out back (since the front end is no longer resisting much roll).


Norm

Norm Peterson
05-03-2011, 04:32 PM
The first part is some failure thoughts based on having a little more information - the buckling in the floor tells you that the chassis side attachments for the lower links are moving too much under the acceleration load.

The second part is about four link suspensions not having to "bind" as long as the end connections are sphericals (at least for small movements in bump or roll).


Norm

911industries
05-08-2011, 10:42 AM
Okay, I've done some testing and I think I've figured it out this time. Just to clarify things, the floor pan is not part of the structure, it just happens that its in the way. The crossmember is 2x2x.120 tube and where it had to be forced me to put it through the floor. I welded the floor to it to just hold the floor in place. In post #41 you can see the diagonal braces that are welded in to solve all the for/aft forces. Those braces go from the frame(same size as crossmember) to the crossmember.

Now onto what I've found. I did another check to find bind and couldn't find any. With nothing but suspension links attached the axle articulated nice and freely. Bind is out of the question.

I had a helper take a couple pry bars and put a load on the axle to simulate the rotational forces from acceleration and deceleration. Heres where I found the suprise. The fore/aft forces on the four link get transferred into vertical loads on the crossmember. Basically when the frame mounts stop the longitude forces they then transfer into vertical forces. On top of that the crossmember is broke from braking forces and not acceleration. That is probably why I didn't figure this out earlier. It was crazy to watch a braking force(pinion trying to look at the ground) actually push the crossmember down towards the ground. It moved a lot.

Here is my solution. I'm essentially going to create a structural driveshaft loop. Right now the crossmember goes up and over the driveshaft with multiple welds. I want to use 1 3/4 x .120 mandrel bent tube to connect just outside the lower frame links. I have some more testing to do so I can figure out what kinda strength increase I will get. I got some pictures I'll post in a few.

911industries
05-08-2011, 10:59 AM
Here are my test subjects, some good ol balsa wood. My plan is to support them from the highlighted areas and load them in the direction of the arrows. I'm trying to locate a scale to measure the breaking points. I will use the straight piece as a constant and then compare against it. The one in the middle is essentially what my crossmember looks like now. The bottom is what it will look like. The black sharpie lines represent where my lower frame links are.

I think this will give me a good idea if I will be able to fix the problem. Let me know what you guys think