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I thought I would post this here where the suspension guy's hang out.
I have been looking at the rear suspension on my 1988 C4, trying to find ways to increase traction from a stop and pulling from a slow corner.
Right now anything above a mild roll into the throttle will cause excessive wheel spin (my car's details are at the end of the post).
Now to the question about Anti-Squat on a C4.
I plotted out the rear fourlink from an 1984, scaled from a side layout posted by someone awhile ago, in a fourlink program, shown below.
Also from some quick measurements of my 1988.
My only real experience with rear suspensions is from drag racing four links, where anti-squat was maybe +130% to 0 to -80% or so, and used to tune the launch.
Why is the C4 so high? 310% to 350% ??? seams awful high.
Do I have this figured wrong somehow?
Rear trailing arm brackets, with a choice of holes, look pretty easy to fab up.
I seam to remember that higher anti-squat hits the tires harder, lower hits softer.
What might I gain and loose by moving the IC down a bit?
It looks like they made a small change from 1984 to 1988, if my measurements are right, towards less Anti Squat.
Anyway, any comments would be appreciated
!988 C4, automatic, supercharged ZZ4 crate engine (about 390 hp @ rear wheels), TPIS Tpi manifold and runners, 3.54 rear, 2000 stall converter, 315-35-17 BFG KD rear tires, stock Bilstein shocks, poly bushing all front and rear suspension and swaybars, Z52 springs and bars, think that's all the info
My assessment is that the anti squat is built into the C4 Corvette IRS 1) to prevent using up the available suspension travel on hard cornering under power and 2) to minimize the camber and toe change caused by the same suspension travel. One of the first mods I've made to both the 86 and 90 C4's I've built was to install the DRM trailing arm brackets to reduce the anti squat. If I could find another set I'd put them on my 93. FWIW.
But remember that as you lower the car the amount of anti-squat goes down, so a it will be better if you drop the rear ride height....
There is still plenty of weight transfer to get the rear end to hook back up with a light squeeze on the gas in the lower gears, I don't know if a lot more would help. We did install the later production brackets on our BSP car (updating) to improve the 84 that we were running and it did make the car more drivable. That said, we were running out of travel in the end, so having more anti-squat might have aggravated the bottoming and we may have had to raise the ride height to keep it off of the bump stops.......
It is a trade off, since you want to get the car as low as you can to get the rear roll center down as much as possible.....
Also remember that the total weight transfer to the rear tires is a function of forward acceleration. The amount of anti-squat simply controls how it gets there. With anti-squat it goes thru the links, with less anti-squat it goes thru the springs and shocks, so that is what eats up the available travel. The forces on the rear tires are the same, but with it going thru the springs and shocks you can control better the transient effects and how the forces are managed.
FWIW, I think you are using the wrong model for calculating anti-squat %.
The correct model for an IRS measures the SVIC length and height from the CENTER of
the wheel, rather than from the contact point on the ground. The difference is: for an IRS
the chassis is reacting drive torque rather than the axle.
If you change the design basis to an IRS rather than a 4-link live axle your numbers
should fall into a more normal range.
See the Milliken/MillikenVehicle Dynamics text (Chapter 17) for the force diagrams.
There is also quite a bit of suspension discussion on the CornerCarvers.com site.
My assessment is that the anti squat is built into the C4 Corvette IRS 1) to prevent using up the available suspension travel on hard cornering under power and 2) to minimize the camber and toe change caused by the same suspension travel. One of the first mods I've made to both the 86 and 90 C4's I've built was to install the DRM trailing arm brackets to reduce the anti squat. If I could find another set I'd put them on my 93. FWIW.
Larry
Will the early C4 anti-squat relocator brackets not work on the late cars?
Don't know about DRM or the others, the GM parts look similar to each other, but the holes for the control arms are lower, and it all just bolted on.... Since we were in BSP we could update and backdate with OE parts, but couldn't use aftermarket parts so that is what we did.
Maybe Randy can shed some light on if theirs are different than the later model OE parts or are the same...
Thanks all, for the responses.
"Also remember that the total weight transfer to the rear tires is a function of forward acceleration. The amount of anti-squat simply controls how it gets there. With anti-squat it goes thru the links, with less anti-squat it goes thru the springs and shocks, so that is what eats up the available travel. The forces on the rear tires are the same, but with it going thru the springs and shocks you can control better the transient effects and how the forces are managed." Solofast
Yes, that helps me understand why less anti-squat "hit the tires softer".
I didn't want to say it, but it does look like the DRM bracket is, lets say "similar" to the 1988 and later ones. It looks just like the stock ones on my car. I'll try to scale from the picture and compare them.
Very little available information about antisquat, even less on anti-dive of a four link (on the net anyway, I've orderd some books).
Visualizing the forces, under heavy braking, of the rear links "Instant Center" (high anti-squat design) pulling back above the CG. Wouldn't this tend to drop the rear of the car, during hard braking, and help resist nose dive?
Maybe another reason for high anti squat in the C4?
Does this makes sense then:
High (above zero) Anti-Squat values: Raises the rear and plant the tires under accel, and drops the rear under braking resisting nose dive.
Low (below zero) Anti-Squat vales: Lower the rear and lifts the tires under accel, and raises the rear under braking.
"FWIW, I think you are using the wrong model for calculating anti-squat %." DOCTOR J
I've been using the four link model in "performance trends" software. I assumed the C4 rear was a classic four link. I'll look into what your saying, and see what comes up ... thanks
My assessment is that the anti squat is built into the C4 Corvette IRS 1) to prevent using up the available suspension travel on hard cornering under power and 2) to minimize the camber and toe change caused by the same suspension travel. One of the first mods I've made to both the 86 and 90 C4's I've built was to install the DRM trailing arm brackets to reduce the anti squat. If I could find another set I'd put them on my 93. FWIW.
Very little available information about antisquat, even less on anti-dive of a four link (on the net anyway, I've orderd some books).
Visualizing the forces, under heavy braking, of the rear links "Instant Center" (high anti-squat design) pulling back above the CG. Wouldn't this tend to drop the rear of the car, during hard braking, and help resist nose dive?
Maybe another reason for high anti squat in the C4?
Does this makes sense then:
High (above zero) Anti-Squat values: Raises the rear and plant the tires under accel, and drops the rear under braking resisting nose dive.
Low (below zero) Anti-Squat vales: Lower the rear and lifts the tires under accel, and raises the rear under braking.
That is correct. Anti-squat is also anti-lift under braking conditions, at least on live axles. I'm not as familiar with the dynamics of this on an IRS as I am on live axle suspensions, but on live axle suspensions too much anti-squat can easily cause axle hop under braking as a result of too much anti-lift. I'm sure this still applies on IRS, but to what extent I don't know. Unsprung mass is a lot less, so I'm sure that in itself changes things. Brake bias, grip, vehicle weight bias, etc will also affect this.
Yes, DOCTOR J, I've done some more searching and your absolutely right.
My gut feeling was something (in my figuring) was really off.
I also found this "corvette tech spec" page. http://corvetteobsession.homestead.c...etteSpecs.html
If you page down to suspension, it say's 1987 antisquat is 62%.
"Provision for acceleration squat control: Rear suspension geometry control arms positioned to produce 62% anti-squat"
I'm going to abandon the suspension programs and do a 3D model in Solid works, to look at this more clearly, and calculate the angles easier.
Thanks to all for getting me back on the right path
Well, did some "crude" scaling on the 84 layout, and this is what I get.
I'm using 15" for the CG hieght, from a earlier CF thread.
Looks reasonable?
I'll do a more accurate model of this and my 1988 later.
Thanks again 2 all
That last diagram looks much better, but I think there are still a couple of problems:
1. Where Milliken uses the dimension 'L' (lower case) it's for the total wheelbase.
That's going to change the calculation results.
2. I've seen that CG height of 15" before, but have not personally verified it. IMHO
it looks awfully optimistic - a value of 18-20" for a street car would make more sense,
but that's only an opinion.
Also, I have a copy of that 'Corvette Spec' book, and I'd take its 'facts' with a grain
of salt. I don't know how reilable it is - some of the suspension numbers don't seem
reasonable at first blush, and it's not clear to me where they originate.
FWIW, I have DRM trailing arm brackets on my '91 - purchased years ago. They
do in fact move the SVSA dimension out and down, but I don't have the bracket
measurements at hand.
I'd suggest getting hold of a thorough suspension text and reviewing the free-body
diagrams for IRS anti-squat. Would like to give you more help, but I have some other
projects pressing before winter. You're on the right track, keep after it.
Oh ... to learn in front of the whole world
Thanks DrJ.
I did just that already.
Ordered about $160 bucks worth of books, Race Car Vehicle Dynamics by Milliken / Milliken is one of them.
I'll get to the bottom of this, with an accurate model, after some more reading.
Maybe repost with the differences between the pre/post 1987.
In the parts books, the trailing arms are all the same 84-96, but the brackets are different 84-87 and 87-96, some confusion over the support knuckles (I'll keep digging).
This is a winter project also.
thanks again
Don't know if anyone else will find this interesting, but I thought I would complete my results.
I measured out the rear of my 1988 today (not an easy task ) and started laying out in a 3D Solid Model.
Just have the trailing arms in for now, to figure Anti-Squat, and it looks like this, so far.
If all my information is right, it looks like the 1984 rear is set up about neutral (no squat/no rise), and my 1988 has about 63% Anti-Squat (37% below neutral) which means it should be squatting down pretty far on launch (time for a video recorder).
It's funny that my original question, about too much Anti-Squat, should now be "Why so little Anti-Squat?"
I'm starting to see, from reading, that it's difficult to get much Anti-Squat with an IRS rear.
It's still not clear to me if more or less is better for initial traction.
It appears that very small changes, in the front hole locations, have a very large effect on the IC position.
I think fabricating brackets, with a series of holes that allow adjusting from a little over 100% (neutral) down to about 40%, would be pretty easy.
Winter is moving in to fast around here, probably be next spring before I design, fabricate, and post any results, if anyone has interest.
I'm surprised that someone hasn't done this already, at least that I can find.
Maybe the drag race guy's.
11-01-2005, 06:08 PM
Anti Squat on a C4 - Why so much?
vetracr is correct
) and started laying out in a 3D Solid Model.
