Rear end hop

[gramicci101]

Originally Posted by sprintamx

This describes some of the particular effects I feel when the rear is "displaced": slop in the drivetrain. There's only so much I can ask shocks, springs and a sway to do if the diff isn't well isolated and/or the subframe has too much movement.

Yep! That was my first order of business; upgraded drivetrain bushings. I want my power to turn the wheels, not deform a bushing. All I need is a set of engine mounts and I'll have the whole set. They're a little more expensive though...

[sprintamx]

Any opinions / evidence on whether the tranny mounts and/or motor mounts add to this issue? I suspect not, but it will be nice to confirm. I've installed much stiffer mounts in other platforms to limit drivetrain movement under heavy deceleration, but not rear end hop.

[gramicci101]

The tranny mounts and motor mounts won't have any effect on wheel hop. Transmission movement is isolated from the rear diff by virtue of the center joint in the 2 piece driveshaft. The slip yoke on the transmission side absorbs any movement, and the center joint is securely mounted to the transmission tunnel.

I was honestly surprised that these cars have wheel hop at all. Normally that's a solid axle/leaf spring thing where the axle rotates on its axis under high torque and you get spring wrap. When the springs hit their limit and snap back to their designed shape it causes the wheels to lose traction and skip. That's why you see high horsepower rear leaf spring cars like older mustangs and camaros with bars bolted under the leaves or an aftermarket ladder bar; they're trying to control spring wrap. Cars with independent suspension don't normally have this problem because of all the locating bars and coils instead of leaves.

With our car, the bushings are soft enough to allow the diff to rotate up and down in the subframe, which effectively causes the same problem. The diff carrier bushings or diff braces like Cusco and Beatrush solve a lot of that problem and the rear subframe bushings solve most of what's left. There is still flex in the individual bushings of the various suspension arms, but not to the extent that the diff has. And Energy Suspension makes a kit to replace all of the rear suspension bushings, it's for sale on @FT-86 SpeedFactory. I don't need to go that far down the rabbit hole, personally. If this was my track car, then I would.

[sprintamx]

Mentioning the live rear axle comparison is ironic. I've been thinking that the car is behaving worse than a contemporary Mustang; probably because its so much lighter in the rear and therefore won't be as damped and restrained by the mass. What a comparison to make with an IRS car!

[gramicci101]

Do the diff inserts and subframe inserts and it'll change your perspective completely. Once the hard parts stop flopping around and the suspension can actually function like it's supposed to, it feels a lot better.

[diss7]

Have only skim read thread, but too much emphasis is being placed on the diff inserts and subframe bushings.

My subframe and diff inserts are solid mounted and I can still get wheel hop.

Check your rear shocks aren't set too hard. My car, with rear dampers set to full hard for drifting gets awful wheel hop if just dumping clutch in a straight line in 1st. Again, that's with solid mounted diff and sf. But if I soften the rear dampers 6 clicks back to my street setting, it doesn't wheel hop.

I'm not saying that solid mounting or whiteline bushings don't help to reduce wheel hop. I'm saying it could just be that your shocks are set too hard.

[NLSP]

I think OP has stock suspension seeing how he was wondering if changing shocks & springs would help. I'm on stock suspension as well.

[sprintamx]

Originally Posted by NLSP

I think OP has stock suspension seeing how he was wondering if changing shocks & springs would help. I'm on stock suspension as well.

I'm actually on Ground Control's Koni package; rears set on softest setting; 350 lb springs.

[sprintamx]

Quick update for those interested:


Installed the Whiteline differential and rear sub frame bushing insert kits. Relatively straightforward install if you have access to a lift and reasonable tools.


Marked improvement with tightening up the rear end dynamics. Car is now nice and tight in the right way: driveline and chassis "feel" like they are much more integrated. Good improvement with keeping everything together over bumps. Very minimal increase in NVH; but I think you have to be searching for it to notice the slight noise increase.


I also tightened the Konis to their middle setting for compression / rebound.


Bottom line: this was a successful mod and solved the particular issue I wanted to address.


Thanks to all who made the suggestions.

[Poodles]

If anything, you want SOFTER suspension to absorb the bumps and keep planted. Skipping over bumps in a corner is usually from unsettling the chassis instead of that wheel moving without unsettling the chassis. Lowering unsprung mass will also help this a bit.


That, or you add more sprung mass, but that's counter productive.

[sprintamx]

As a general proposition, "softer" may keep wheels better planted, depending on many variables and, of course, what is meant by "softer" (compression, rebound, spring rate, etc). In my case, I recognized that there was significant enough flex and other movement in the rear that I wanted to reduce or eliminate. I also condluded that I likely did not have enough rebound damping dialed into the rear struts. They're single-adjustable so more rebound means more compression too; another compromise.

Anyway, the ride is firmer than stock but not harsher. Rather, it's smoother in precisely the way I intended. That's not always the case with suspension and chassis mods, but thankfully it was the right move this time.

I won't turn this into another suspension / chassis tuning thread. Suffice to say that one aspect of the stock architecture that I did not like has now been resolved.