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I cannot imagine how an engine mount would cause a loss of wheel control on an IRS car
The motor/trans is mounted on rubber. The motor and trans can twist with tq reaction. Break the tires loose, the the motor is "wound up" in it's mounts from the tq required to break traction, traction is broken, tq falls, motor "unwinds", tire grab, process repeats itself.
I don't think it's likely...but given my knowledge of Matt's rear suspension, his maintenance of it and C4's unlikeliness to wheel hop in general... that is a place I'd be inclined to have a look at.
Stay tuned, guys. It'll be another day or two before I can get back underneath it. But all of you have helped me put together a checklist of things to inspect.
Well, I figured it out. And I'm ashamed. I've been staring at the problem all along, and it's one of those things that's so obvious you just don't notice it:
The lower spring isolators are completely MIA! This is a VB&P spring and it had their urethane isolators installed (but on a stock mounting setup, not their fancy dual-mount bracket). My guess is that the isolators deteriorated due to age and crumbled enough come all the way out. Note that the lowers would be considerably more exposed to road salt and grime, and maybe most importantly exhaust heat. The good news is that these are easily obtained from VB&P.
It's horrifying to realize I was street driving and autocrossing with this defect! I had already needed to replace the urethane bushings for the bolts at the spring ends when I first got the car. But I've never removed this spring before and frankly didn't realize these isolators were urethane - I just assumed they were stock rubber ones without paying much attention to them.
One lesson is to inspect urethane parts periodically, especially if they are exposed to road conditions or engine/exhaust heat. Another is the importance of a systematic diagnostic process. Thanks to all the input here, I had made a list of all possible things to check. C409 had mentioned to verify the integrity of the spring, and I added "and the spring mount" to that checklist item. That led immediately to seeing the problem. I never got the point of mounting an action camera under the car. But if I had missed this issue while the car was on jack stands, making a video like Tom's would have easily and quickly demonstrated the problem. It was equally valuable to hear from so many that C4s just don't axle-hop when everything is working. So again, thanks to all who chimed in.
In the interest of summing up this thread's info for anyone else who may have to diagnose a wheel hop problem with a C4, here is the checklist of things to inspect:
Rear shocks.
Trailing arm bushings or rod ends and their brackets - both the pivot bolts and the three bolts that attach each bracket to the frame.
Camber rod bushings or rod ends, and bracket to batwing.
Toe links and its center bracket at the batwing.
Bolts that attach all suspension links to uprights (trailing arms, camber rods, and toe links).
Upright itself, for cracks in any arm attachment points.
Rear wheel bearings.
All U-joints, especially for the half shaft (they also serve as the upper lateral links for the suspension).
Rear spring, center spring mount (including shims and isolators), and outer spring bolts and bushings.
Batwing bushings and attachment to differential housing.
Steel tabs that batwing bolts to on frame (make sure spot welds are intact).
C-beam bolts to diff housing and transmission tailshaft.
Motor mounts.
I may also do some kind of heat insulation wrap around the exhaust pipes at that location. I don't actually know how hot the pipes get that far back, I'll have to use my IR thermometer after an autocross run to get an idea.
So it appears the 'bonding agent' by the vendor to VBP failed miserably at 3 of the 4 mounting cushions. Did you ever check the rear for insulators and shims previously. Hard to believe that you never confirmed the shim stack at the spring mount. It is what it is now and you hint VBP sells the cushions. I've never looked.
That I'd think also created the 'clunk' at drive-away! Seems odd that it wasn't maybe a more pronounced 'clunk' than you mentioned.
So it appears the 'bonding agent' by the vendor to VBP failed miserably at 3 of the 4 mounting cushions.
Yes, but I'm not 100% sure the bottom cushions are supposed to be bonded. I have another VB&P rear sport spring in the garage (lower rate) and it only has the top mounts on it, too. But they are bonded on strongly. I'm guessing the loose top mount you see in my pic finally came loose after getting hammered too much after the lowers came out. I'm going to call VB&P Monday morning, and I will ask them if the lowers should be bonded on (by me). Yes, they sell a kit of four of these for $19, part# P1-401.
Did you ever check the rear for insulators and shims previously. Hard to believe that you never confirmed the shim stack at the spring mount.
Never did, but I'm sure they were good two years ago when I first started autocrossing the car. It never used to axle hop back then. It only started that last season.
That I'd think also created the 'clunk' at drive-away! Seems odd that it wasn't maybe a more pronounced 'clunk' than you mentioned.
I'm sure it did. I agree: I'd have expected more noise. But as I think about it, the car would still sit on the upper cushions at rest, and the only time it would have come off would be during cornering and maybe braking. Those are both relatively slow movements, so the noise wasn't too extreme. I'm sure it was noisy during the wheel hop, but there was so much other noise at that time that I wouldn't have noticed it.
You'll soon have it behind you!
Well, it's on the rear spring, so it's always behind me...
Originally Posted by Tom400CFI
I may have to withhold "maintenance credit" in your next thread!
Yes, but I'm not 100% sure the bottom cushions are supposed to be bonded. I have another VB&P rear sport spring in the garage (lower rate) and it only has the top mounts on it, too. But they are bonded on strongly. I'm guessing the loose top mount you see in my pic finally came loose after getting hammered too much after the lowers came out. I'm going to call VB&P Monday morning, and I will ask them if the lowers should be bonded on (by me). Yes, they sell a kit of four of these for $19, part# P1-401.
I've had 1 of their springs and it had 4 bonded cushions. It was many years ago and at the time I believe it was what they marketed as 'sport'.
The way that the rear spring is shimmed and clamped it would seem that 'bonding' of the cushion to the spring is critical. I'm sure an interesting conversation with them.
Who do you suppose does the spring fabrication for them? I avoided them for parts in the past just because they never wanted to discuss vendor.
****How about if the rear 'shim stack' was never correctly established and the clamps fitted? If you alter the 'shim stack' significantly from OE spec the clamps need to be sometimes fitted by adjusting height to clamp properly. Loose very soon a shim vibrates out, gets looser another and then another. Finally it's just the cushions rocking. I can't imagine that if the VBP cushions are near OE quality that HEAT had anything to do with the failure.
I have a little more follow-up info, which may be useful for others who use VBP springs. The previous owner of the car, who purchased and installed this spring and one other (lower rate) VBP Sport rear spring, tells me that both were sent with only the upper isolator pads bonded to the spring. The lowers were not bonded. However, all four pads have two little raised circles (like the top of a Lego block) that lock into corresponding holes on the carrier pieces. So once they are tightened down, they cannot move or come out. I have a theory about why the lowers weren't bonded, but more on that in a minute.
I called VB&P and talked to a tech named Gary. He advised that they bond the pads to the spring with 3M Window Weld, which is used for bonding windshields into car frames. The shear strength on this stuff is significant when you look at the spec sheet, and that would be the main stress on these pads. So that makes sense. He also advised me to bond the lower pads as well as the top (recall that one top pad had come loose, but was still there). The two springs I have are at least 12 years old, I think, so at some point they've started bonding all four pads to the spring. So that's what I did: using the one remaining bonded pad and the receiver brackets as locating guides, I glued the pads on and clamped them down in the brackets. I'm letting it cure now.
I've been mulling over why those lower blocks came apart/out. I am pretty sure it was mostly due to exhaust heat. The pipes come close to them, underneath. Whereas the factory rubber, metal, and paper shims probably hold up to the heat a little better (and in a stock car with smaller pipes and less power there is also less heat), the urethane may be breaking down over time. It's probably worst after track or autocross runs when the exhaust is hot and the car gets parked, and the heat waves just rise up. It's not melting them or anything like that - these aren't the headers, after all. But after years of this they probably break down. It's not a matter of quality, but just that urethane is more susceptible to heat than metal, paper, and rubber. Also, the OE rubber is probably insulated from the heat by the paper shims that stack with the metal shims.
So I wonder if at one time the lowers weren't bonded because that made it easier to replace them from time to time? I don't know, so I may try to fab up a little heat shield to help the ones I just bonded to the bottom of the spring hold up longer. If not, it's no big deal to pop old ones off with a chisel and replace them.
Originally Posted by WVZR-1
****How about if the rear 'shim stack' was never correctly established and the clamps fitted? If you alter the 'shim stack' significantly from OE spec the clamps need to be sometimes fitted by adjusting height to clamp properly. Loose very soon a shim vibrates out, gets looser another and then another. Finally it's just the cushions rocking.
There shouldn't be any shim stack with the VB&P setup. Each isolator pad is sized to take the place of the entire OE shim stack plus the OE rubber pad. I'm very sure the previous owner installed it correctly. That said, once this adhesive is cured I'm going to double-check that the clamp is indeed clamping down tightly on the pad and spring. If it's not, I can make shims easily enough. I don't think I'll have to, though - it looks and feels okay right now.
There shouldn't be any shim stack with the VB&P setup. Each isolator pad is sized to take the place of the entire OE shim stack plus the OE rubber pad. I'm very sure the previous owner installed it correctly. That said, once this adhesive is cured I'm going to double-check that the clamp is indeed clamping down tightly on the pad and spring. If it's not, I can make shims easily enough. I don't think I'll have to, though - it looks and feels okay right now.
If it were lacking and you wanted to avoid the OEM shims the clamp can be clearanced to provide accurate clamping.
A fellow here was installing a D44 for a friend and commented specifically when doing the install about making sure the clamp height was confirmed.
One more follow-up. The poly blocks by themselves did provide a tight clamping force for the spring. No shims were required.
I attended an autocross today, and with cold temps there was plenty of opportunity for wheelspin, both at the starting line and many other places on course. No wheel hop at all, ever, under any condition. So that was the one and only problem, and it's now cured. A related improvement is that the rear grip in corners and transitions is more linear now. That is to be expected. Spring rate in squat or lift (left and right rear corners both moving the same direction) would have maintained proper spring rate even with the lower pads missing. But in lean (each side moving opposite to the other), I would have had zero roll rate from the spring until the slack of the missing lower pads was taken up: probably an inch or more of wheel travel. So basically, I had a binary rear roll rate witht he first half of roll being resisted only by the swaybar, and the second half being resisted by both the bar and the spring. It's happier now, and so am I.
One more follow-up. The poly blocks by themselves did provide a tight clamping force for the spring. No shims were required.
I attended an autocross today, and with cold temps there was plenty of opportunity for wheelspin, both at the starting line and many other places on course. No wheel hop at all, ever, under any condition. So that was the one and only problem, and it's now cured. A related improvement is that the rear grip in corners and transitions is more linear now. That is to be expected. Spring rate in squat or lift (left and right rear corners both moving the same direction) would have maintained proper spring rate even with the lower pads missing. But in lean (each side moving opposite to the other), I would have had zero roll rate from the spring until the slack of the missing lower pads was taken up: probably an inch or more of wheel travel. So basically, I had a binary rear roll rate witht he first half of roll being resisted only by the swaybar, and the second half being resisted by both the bar and the spring. It's happier now, and so am I.
03-06-2018, 05:01 PM
