Drag racing four-link design

[gwitman]

I'm new to this forum, and am hoping somebody can help me solve a couple mysteries about how to set up a four-link for drag racing. I've reviewed several topics in this forum and some books so I am familiar with how to calculate the IC location and anti-squat percentage, but I haven't found much good info on what those numbers should be. Intuitively, I'm thinking that I should put the IC more or less under the CG and go for 100%-150% anti-squat while keeping the links as parallel as possible as viewed from the side. Does that sound about right? I presume there is some disadvantage to using a really extreme amount of anti-squat, can someone elaborate on this?
I need to use the stock mounting locations for the front of the links, but can vary the height of the rear mounting points. Given a choice between raising the rear mount of the upper link or lowering the mount of the lower link, is one option advantageous (assuming similar IC and anti-squat for both configurations)? The lower links are substantially longer, something like 21" vs. 13" for the uppers. The upper links will be adustable to allow pinion angle and chassis preload to be set.
The vehicle is an '81 Chevy Mailbu that weighs 2800 lbs and has around 550 bhp in bracket race trim and 800 bhp in match race trim. It is always trailered to the dragstrip so I am not concerned about its behavior on an uneven road surface. If anyone has any advice on how to get started on this design, I'd really appreciate it!

Thanks in advance,
George

 

[GregLocock]

Excessive anti squat will, in my opinion, tend to reduce the vertical compliance of the suspension, so it will be less effective at coping with a rough surface. You will have to decide how much to use, but 100% seems like a good place to start, or else copy someone else. That's what we do usually.

All things being equal I'd move the lower mounts down - less likely to hit the body, and because the link is longer you will get smaller errors. Often you need to move both hardpoints. Also consider the true geometry once you have applied the traction loads - there may be enough compression in the bushes or mounting points themselves to upset your calculations.

Are the lower arms parallel in plan view? Are the upper arms parallel, or is it the V arrangement?



Cheers

Greg Locock

 

[gwitman]

Greg, neither set of control arms is parallel in plan view. The lowers are at about a 10 degree included angle and mount near the hub, and the uppers attach to ears on the top of the rear end housing and angle out toward the sides of the car at around a 70-80 degree included angle. I was planning on maintaining these angles to avoid having to use a panhard bar.

Regarding the excessive anti-squat question, in order to achieve a lot of anti-squat you need to use relatively short links and angle them towards each other severely. It occured to me that this could cause a drastic change in pinion angle if the body squats or raises on the launch. It's a lot like the camber change that occurs on an unequal-length A-arm front suspension in bump. So I think I'll shoot for a conservative 100% for now and put big tires on the car.

A final question occurred to me. I was intending to use stock style mounts for the control arms, which have a pin connection at each end of the link. In order to stiffen things up I was going to use aluminum bushings instead of the soft rubber found in the stock arms. But then I realized that the suspension is going to bind up if the car rolls at all. I can see how this would make for a chassis-tuning nightmare and a lot of broken parts. So would it be sufficient to use a spherical rod end at one end of each link, or do I need to use polyurethane bushings instead of aluminum?

Thanks for the response. I welcome any other opinions. Still not sure where the optimum IC location is supposed to be so if anyone has experience with that, please let me know.

Cheers,
George

 

[GregLocock]

If you give me your hardpoint locations (X Y Z) I'll check the thing will not lock up for you with rod ends. (It will have to be in two weeks, i'm off on holiday). Most supensions I work with would not move with rod ends throughout, far less aluminium 'bushes'. Race car suspensions usually do, obviously.

Given a free hand I'd use polyurethane or silicon rubber bushes at one end of each arm and rod ends at the other. Preferably in-line spherical joints, not tie rod ends.

Our production beam axle manages to get 100% anti squat without nodding the pinion too much, I'll have a look at the geometry and try and see how.

Cheers

Greg Locock

 

[gwitman]

Thanks Greg, I'll measure up those locations as soon as my friend with an identical car comes back from vacation (the race car is in pieces right now with the body off the frame, can't measure it). I won't be building the suspension for a couple weeks anyway.

 

[patprimmer]

I just finished building a street class drag car. It's a rear wheel drive Ford Capri (british origin) from the late 70's with a blown injected SBC on methanol. we have only made 3 passes so far, but run 9.1 at 105 MPH, with the brakes on well before the line.

It has a ladder bar rear end, but the guy who fixed it up for us was building at least 3 cars with 4 links at the time, and has done many others in the past, including a few national champions (that is Australian national champions).

He always uses in line spherical joint rod ends, as polyurethane bushes move to much under the thrust generated by serious drag race slicks. As the links tend to be close together for tyre clearance of very wide tyres, a small variation in deflection from side to side can cause quite a lot of rear axle steer.

If you give Greg the drawing and dimensions, I am sure he has the facilities and knowledge to run the numbers.

We just crashed our car from tyre shake. There are some really violent forces at work as you launch a really serious drag car. I just can't see PU bushes standing up. Regards
pat

 

[BillyShope]

You said that you wanted to have no squat, the IC below the CG, and nearly horizontal links. These are mutually exclusive goals.

First, let's locate the no squat/no rise line: This line passes through the rear tire patch, when viewed from the side, and through the intersection of two other lines, one a horizontal line through the CG and the other a vertical line through the front tire patch. If the IC is anywhere on this line, the car will neither squat nor rise. If above, it will rise. If below, it will squat. So, you can have long, nearly parallel (or parallel, but NOT horizontal), links which will provide rise, but the IC will not be below the CG. Can't think of a good reason to keep it below the CG, for that matter.

Hope this has been of some help.