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Wheelhop !!!

Old 04-09-06, 11:57 AM
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Wheelhop !!!

Patient: '85 GSL. Used for bracket racing, Rallycrossing, and general backroads antics.

One thing that I've never been able to determine or locate info on is the root cause of wheelhop. Any insights?

My '80 had a very (very) minor wheelhop problem. The '85 was okay until middle of last race season when the wheelhop got rather intense. It sort-of got better with stiffer springs, and later with the 3 link. That was on 205/60-13s.

This past week I was driving it around with my summer tires (also the drag tires) which are 195/50-15 Yok ES100s and the wheelhop is atrocious. Any and all wheelspin is accompanied by horrible shuddering wheelhop. Tire pressure doesn't seem to affect it, but for what it is worth I am normally running 30-35psi in the rear tires.

I know putting solid bushings in a FWD all but eliminates wheelhop due to eliminating drivetrain windup. I haven't installed the torque strap in the RX-7 yet, but I'm still somewhat doubtful that it will do any good as the shuddering is definitely in the rear of the car. The rear bushings are really stiff, and the upper link brackets are beefy enough that a nuke wouldn't budge 'em. Going to rod-ends would be a major step up cost-wise, and I'd want to reinforce the brackets if I did so, so I'm loath to do it if it's not going to make much of a difference either.

Likewise, I haven't done anything with the shocks yet because I am not sure in which direction I'd need to go. Stiffer would *seem* to be key, but that would probably hurt traction coming out of corners, and what is intuitively obvious isn't always correct.
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Old 04-09-06, 01:49 PM
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When you intalled the 3rd link, did you check and set the pinion angle?
I don't remember, but aren't you running a high(er) than average ride hieght for the rally-x stuff?

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Old 04-09-06, 03:55 PM
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Not being involved in drag racing or rallycross(and all of our roads are straight around here) I can't comment from direct experience. My first guess is going to be shocks. If you have shocks on the car that are designed for the FB they are probably valved to control some pretty soft springs. You have roughly 200lbs of rear end/wheel weight to control with some shocks that are being overpowered by the stiffer springs already. Even with the adjustable shocks for the car that are in the $100 ea. range they only adjust one side of the equation, I think rebound usually.
As for the concern of coming out of the corner hard, if your wheels are coming off the ground your losing acceleration right now.
If you want some shocks tested and want to send them down here I can throw them on my shock dyno and give you a graph of what you have.
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Old 04-09-06, 08:33 PM
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The wheelhop doesn't seem to be affected much by spring rate. If anything, it's slightly *better* with the 200lb springs as opposed to the stock springs (roughly same ride height). I switch back and forth between the two as whim dictates, nice thing about this car is that changing springs is a 5 minute job.

I guess my question would be... is wheelhop in a solid axle RWD more a function of drivetrain windup/release (which rod-ends would help) or is it a function of the axle oscillating between the suspension springs and the tire sidewall "springs" (which rod-ends would hurt?)

Trying to figure cause and effect here... the wheelhop problems really started when the upper link mounts started cracking free. Naturally I figured the hop was due to drivetrain windup. But when they persisted with the 3 link (which again is *really* beefy, it's all .125" boxed and tied in to largish areas of (reinforced) floor and axle) I started to wonder if it was more of a "tramping" motion, because with the 3-link and with the upper links semi-mounted the axle is much more free to articulate, with no geometrical bind to turn the bushings into dampers.

If that makes any sense.

I would take someone along to carefully videotape the car, so I could see what is going on, but frankly wheelhop scares me, it breaks parts *really* fast.

Last edited by peejay; 04-09-06 at 08:36 PM.
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Old 04-09-06, 09:16 PM
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I'll ask a guy in the shop next to mine that has a dragster with a 500+ cu.in. engine what he thinks tomorrow. I think its the rear end bouncing around on soft shocks and springs.
Consider how much you feel through the chassis when its happening. It probably feels pretty violent. The parts breakage is most likely from the traction/no traction bounce and the forces the rear end housing is trying to send through the chassis. When the wheel spins up because its off the ground then comes back down and is slowed down to the speed the car is going the rear end is trying to twist hard. The same with hitting the brakes but opposite forces.
Also think of the car being upside down and the springs are supporting the rearend, what frequency is the softer spring going to bounce at compared to the stiffer set? How far will each spring move?(softer=more)
When Jim Susko and I were working together everybody said the highest spring rate you could put in the back of an FB was 175lb/in. The smooth road course set-up we ended up with had 300lb/in springs, and even on the bumpy tracks we used 250's.
I'd say try some stiffer springs and shocks, of course to stiff will upset things as well.
One thing that a video would answer is if the rearend is bouncing as one or rocking back and forth from tire to tire.
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Old 04-09-06, 10:20 PM
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Originally Posted by jgrewe
When Jim Susko and I were working together everybody said the highest spring rate you could put in the back of an FB was 175lb/in. The smooth road course set-up we ended up with had 300lb/in springs, and even on the bumpy tracks we used 250's.
I'd say try some stiffer springs and shocks, of course to stiff will upset things as well.
The wheelhop seems lessened (or at least less violent) with the 200lb springs. But it's still awful. I am thinking maybe it's because there's just less traction with them

The car still understeers pretty well with 200lb in the rear, and in the front, and no sway bars. I'd rather use soft springs in the rear to get some bite back, though, but I don't want to trash driveshafts to do it.

One thing that a video would answer is if the rearend is bouncing as one or rocking back and forth from tire to tire.
Yeah. Unfortunately, the last time I had a friend take video of the car, he kept the camera zoomed out all the way so I couldn't see what the suspension was doing Maybe I'll just stick my in-car camera (Harbor Freight security camera) to the undercarriage and videotape that.
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Old 04-09-06, 11:20 PM
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Harbor Freight Dataq! I just noticed Cleveland, Ohio? You aren't trying to make this thing work at Nelson's Ledges are you? That sort of counted as Rallycross when I ran there about 13 years ago, I broke a camber plate at that track!
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Old 04-10-06, 12:14 AM
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Naw. Nelson is booooring, they repaved most of it! I didn't think it was very rough beforehand either, certainly nowhere near as bad as the streets, highways, and backroads I bomb down. There aren't even any curbs to hop or ditches to cut or potholes for that all important "realignment on the fly"!

Well, except for that one time, when I crossed a ditch at 60+mph on the infield... it was either drive across the infield or start a tankslapper - with a truck close behind me. The front suspension (stock 1980) compressed so much that the 205/60s on 13x7s grabbed the fender lips and pulled them down. That incident was my first clue that the rear suspension geometry sucked rocks The guy watching from behind in the Lightning thought it was cool the way I gathered it up instead of trying to keep it on the pavement. <peewee> I *meant* to do that... </peewee>

I'm not *too* much of a fan of roadcourse work anymore. It doesn't get my jollies. I'm not happy unless I can feel the tires grinding out from under me, and on pavement that gets expensive fast and you can't run for more than a handful of laps before the tires need to cool off. I play on dirt now, and bracket race to get the engine set up.

I habitually buy shock absorbers when they're on sale, so that's probably the first step I will take. Doesn't cost much if ya already have the parts. First I have to do something about the fuel pump that died on Friday, and before I do *that* I need to figure out if I *really* want to convert to EFI sometime in the future.

Last edited by peejay; 04-10-06 at 12:18 AM.
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