Vibration/Wheel hop

racecougar

Registered User
Here's the deal guys/gals:

My 94 Splitport Tbird has a few issues that need to be worked out, two of which I'm discussing here. It has a slight vibration when coasting or decelerating from speeds of approximately 70-85 MPH on the highway. It's pretty much only when you let off the gas pedal at those speeds. The car also has a bit of wheelhop that needs to be cured.

The car does have new U-joints, the FRPP centersection, Airlift Airbags in both springs, and a pair of 255 Yokohama's on the rear. As soon as I get a chance, I plan to replace the original rear shocks to help with the wheelhop, and try rotating the driveshaft on the pinion 180 degrees to see if the vibration changes.

What I'd like to know is whether or not I should try poly mounts for the front of the centersection if rotating the driveshaft and new shocks don't help. Is there anything else I'm missing?

Thanks,
Rod
 
New shocks won't help wheel hop. The wheel hop is caused by front to back movement of the axle, not up and down. The up and down is a result of the front to back movement. The shocks will help to dampen the hop, but won't help eliminate it.

Not sure about the vibration. That sounds a lot like a u-joint issue.
 
By front to back movement of the rear end, are you referring to the centersection trying to rotate (changing the pinion angle)? I don't see how or why the rear end would move front to back.

-Rod
 
Not the center section. The center section is decoupled from the wheels and can impart no force on them. I'm talking about the actual wheel assembly is moving forward to backwards in an arc that creates wheel hop.

It is related to a lack of load asorbing material in our 5 speed drivelines. As power is applied to the wheel hub, the tire rotates forward. This forward rotation is hampered by the fact the wheel is coupled to the knuckle, and the knuclke is coupled to the subframe, and the subframe is coupled to the body. Thus there is mass resisting the change in state from stationary to forward motion.

This mass provides resistance even while more power is applied to rotate the tires. As the tire attempts to overcome this mass, it rolls forward. Rubber bushings in the knuckle compress, storing some energy, then the rubber bushings attaching the knuckle to the subframe compresses, storing some energy, then the bushings from the subframe to the body compress, storing some energy until you get to a point where there is sufficient traction to apply additional power to move the mass, or traction is insufficient.

Insufficient traction is the beginning of wheel hop, as the tire looses traction, the stored energy in the bushings fling the wheel assembly backwards (the wheel has no traction to resist this motion) as they give up their stored energy trying to return to their natural state. As the stored energy disapates the tire is able to achieve traction again and when it does it grabs hard, as for a millisecond the driveline was free spinning, shocking the driveline, and starting the whole process over again.

This looks like wheel hop, but people have used witness bars, and slow speed video to see the forward to back motion of the tire. This process happens on Automatics as well, but not to the extend as on 5speeds, as it is the solid coupling of the clutch/flywheel combination that allows for the non-dampened torque application to the tires which reach and reachieve their limits violently. On an auto, you have the slop of the torque converter to dampen these forces in the driveline, smoothing out the application of torque to the wheels.

Due to the violence of the motions and power involved, the stock shocks are easily overwhelmed in trying to dampen the forces. Thus adding air bags with sufficient pressure can help to keep the tire on the ground and reducing it's ability to loose traction. But the final solution is to isolate the wheel assembly from moving back and forth.

Using Poly Uerethane bushings helps, as they compress much less, thus can store much less energy than rubber. But they also can impact the suspesion design and it's stock ability to leverage rubber bushings to allow for some automatic based on the road when under way.

MannySC feels he may have the real solution (people have done air bags, all polly bushings, beefier sway bars and still had some wheel hop) in that the rubber bushings isolating the subframe from the unibody. In his attempts, he added to the rubber bushings solid washers to eliminate their ability to move. He found it completely eliminated wheel hop on his car. It also dramatically increased road vibrations and harshess due to the effectively solid mounting of the subassembly. Last word was he is trying some Poly uerethane in place of the rubber bushings there to see how that works.

A long story, but that's the theory of what is going on with wheel hop.
 
How about this??

By putting a bar from the sub frame to the body on each side keeping the subframe from moving front to back, would this do the trick?? Someone say YES and then I will share what I have been working on.....Rich........Mike you have mail......
 
Got that worked i think??

I have been working on rear end stuff for many years now.. If you can make a 427 Fairlane leave the line at 6500 rpm's and go straight and true, you can do almost anything.. The same for a large block Mustang, with the same results... Once you determine what the movement is, then it is eazy to fix the trouble... Yes I may be talking a bit on the "BIG" side here, but I have had a lot of time going down the track with a shifter in my hand.. This is why I asked about this some time ago... Let me do some fabbing and get some pic's for you to look at and you can give me your idea on it....Rich..... Dave if you want to take this off line and get in deep on this feel free to e-mail me ....Rich
 
You got mail too.

I would think that the key would be to attach the links in a way that allows all motion in the subframe except for fore and aft motion. This should dramatically limit road harsness to the uni-body as the majority of the designed forces on the subfame bushings should be up and down, not front to back.

Again, it's all theory until someone fails. ;) Again and again.

One possibility here could be something a little more fancy. An electro hydraulic system that uses hydraulic cylinders and some electronic valves. Open the orifice and the units give and take like normal. Close the orifice and the cylinders become solid. Yep, adds weight, that's why I'm just throwing it out there.

Here might be the key though. Poly bushings on the links should provide sufficient isolation from harshness yet not provide the energy storage that the subframe rubber bushings have. Remove that energy storage and perhaps you've just solved the wheel hop issue.
 
Along that same line

Hey,
along that same line my 90SC was making a popping noize when I went over bumps and stuff mech got under there and determined it was the rear upper/outer control arm bushings that were shot....he called Ford and they wouldn't sell him just the bushings they wanted to sell him the whole assembly at around 500 bucks a pop,,,,he finally found somewhere to get the bushings...but is this a common thing on these cars...and what the heck is Ford doing...it's like having to buy new rims when your tires wear out:mad:
Rick in FL:cool:
 
Any pictures of the quad shock setup? I couldn't find any. Does it add a shock as a trailing link to control fore/aft motion of the hub?

Thats the type of fix I always thought off, until Mannysc reported success with isolating the subframe. I feel the isolation of the subframe would be much less complex than trying to isolate the hub without getting in the way of suspension movement.
 
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