Thinking about rear shock settings - FWD

[Loren]

Trying to work out the proper shock settings for the Mirage. (yeah, I know, my setup is pure compromise... nothing is proper... just looking for what will work best given the hardware that I have to work with)

My approach to rear suspension on a FWD car has always been pretty basic. Add lots of roll stiffness, mostly with a rear swaybar. Shocks that aren't floppy. Tire pressure that keeps the sidewalls from scrubbing. That's about it. Camber settings can help even out rear tire wear, and maybe optimize contact patch... but, honestly, I've never worried about it. All the magic happens at the front of the car. (besides, there is no rear camber adjustment on this car) Rear toe out can be interesting to play with on a FWD car to help the car turn. But, that's not the problem we're dealing with today. (and there is no rear toe adjustment on this car)

The problem: When the suspension is fully loaded under acceleration, the inside front wheel hops. It's annoying, it's not effectively putting down power, and I'm afraid it's going to break the car.

Here's a photo, that is interesting:


It's probably not "full tilt", but it's a pretty hard transition that would have a lot of FWD cars lifting a rear wheel. Note that it is not doing that.

So far, I've set the front shocks as soft as they will go. And that helped a lot. The rear shocks are presently set near soft, as well. (I don't usually go for a big disparity in front/rear shock settings)

What I'm thinking is... lateral weight transfer. And, maybe going stiffer (a lot stiffer) with the rear shocks could help.

Why?

1. Slowing the rebound of the inside rear tire will make that tire lift more readily, which transfers more weight to the front. (mostly to the outside front, but some of that will be distributed to the inside front where it is needed)
2. Controlling the movement of the outside rear tire might prevent some of the "rocking" action that is probably happening when the outside front starts to wheel hop. Maybe it will help keep that wheel planted?

That's my thought for the moment. I won't get a chance to test the theory until the next Open event.

Anybody got any additional ideas?

[Jamie]

What I'm thinking is... lateral weight transfer. And, maybe going stiffer (a lot stiffer) with the rear shocks could help.

Why?

1. Slowing the rebound of the inside rear tire will make that tire lift more readily, which transfers more weight to the front. (mostly to the outside front, but some of that will be distributed to the inside front where it is needed).

And it might be worth playing with the front/rear differential in tire pressure. There are two approaches to this:
1. Set the fronts to where they won't roll over, then set the rears higher -- start 10 psi higher and work upward. Theory here is the stiffer rear sidewall increases the rear spring rate, and also may reduce the rear contact path a little.
2. Set the fronts to where they won't roll over, then set the rears lower -- start with 10 psi lower, and work downward. Theory here is the softer sidewall allows the contact patch to squirm and reach a higher slip angle than the fronts, so the back washes out first.

I've seen different cars favor one or the other. The A1 Rabbit I had liked higher pressures in the back. My Preludes always favored lower pressures in the back -- as much as 20 psi lower.

[Loren]

Artificially reducing rear grip with over- or under-inflated tires is a technique to induce "rotation", which isn't really my driving style, nor is it likely to help the wheel hop problem.

Or... If it did help, it would be helping by reducing overall grip, which isn't what I want. This car is a one trick pony, grip is all its got going for it!

If I was chasing "optimum feel" and the ability to slide the rear of the car around, playing with rear tire pressure could help. I'm not sure it's the solution the front wheel hop, though.

I really need to find some time to experiment at a Classic event.

[twistedwankel]

Looks to me like soft front settings are the way to go in front heavy cars (or run Koni yellows) in the front only. Plant that front outside tire.

Possibly wear ruby red (toed) driving shoes?

[Loren]

Can't adjust spring rates in stock. They're already soft, though.

I'd consider making a smaller front swaybar if it weren't shaped like a pretzel.

[twistedwankel]

What would happen if you unbolted the front bar on one side just for a test?

[Loren]

I could try it. But, I'm almost certain that it would be too "sloppy" and I'd hate it. It would probably also "flop" when the rear wheel lifted, like an early iteration of the FASTiva did. With insufficient front roll stiffness, as soon as the rear wheel comes up, all roll stiffness goes to the front and "scary" body roll ensues.

I need to look and see how hard it would be to fabricate an "extension" to the swaybar. Just a half inch or so on each end. Something that could bolt onto the existing bar, and the existing end links. That would make the front bar softer. And that could help.

[oregonmon]

What would happen if you unbolted the front bar on one side just for a test?

One of the easiest and no harm, no foul tests to try.

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[Loren]

What would happen if you unbolted the front bar on one side just for a test?

One of the easiest and no harm, no foul tests to try.

True.

[Native]

Just be sure the loose arm clears all. I recall an event when someone tried that, can't recall who, and the loose bar end cut up the cv boot as things moved around...

[Loren]

Just be sure the loose arm clears all. I recall an event when someone tried that, can't recall who, and the loose bar end cut up the cv boot as things moved around...

[Loren]

Tire pressures. Spring rates. Sway bars. Good stuff, to be sure. But...

Anybody got any insight on the actual topic of this thread?

[twistedwankel]

I would start with full stiff. Car should turn in nicely and be great for breaking up kidney stones if you forget and leave it that way on the way home.

[Rpwolf]

what spring rates came with the coilovers vs stock spring rates?
what kind of adjustability do the coilovers have?
(compression, rebound, preload? does height adjustment affect suspension travel?)

maybe you just need stiff motor mounts but going to guess your front rebound isn't fast enough, try increasing rear compression

[Loren]

I'm staying in stock. Using the stock spring rates and stock ride height. Most of your questions are irrelevant due to that. Spring rates are what they are. Ride height is what it is. Even the shock length and preload is pretty much "stuck" because I'm totally at the max height of the shock body to get the droop travel required for the stock spring length. (more about that in the "build thread" if you're intersted in specifics)

Shocks aren't well documented. Pretty sure they are monotube, which I believe means the adjustment affects both compression and rebound. But, there's only one adjustment, regardless.

My thought was that more rear rebound damping would help to at least stabilize things. I *want* the inside rear tire to unload, and when it is unloaded, I want it to stay that way. I think part of what's happening is a rapid unload/load cycle on that tire, which changes the loading on the front tires.

Stiffer engine mounts and maybe even stiffer suspension bushings could help. Of course, none of that is available, so would be custom.

Adding a stiffer engine torque strut did help, but not as much as softening the front shocks did.

[Rpwolf]

I know you want to stay in stock, I ask because if the coilover springs were much stiffer than the oem, no amount of adjusting will tune it out.

for any fluid filled mount, drill a hole to drain the fluid and fill it with window weld. cheap mod.

btw, mono tube is just the construction type (there are twin tube and others), you just haplen to have a single adjust.

I'm not sure what you mean by droop travel, but if preload is adjustable, increase the rear.

[Loren]

Coilover springs were 2 & 3 kg/mm. Not all that stiff as far as coilover kits go, but it's a lightweight car. Don't know what the stock spring rates are. And I know "perfection" is not achievable here. Just trying to work around the "problems" as they arise.

Shock guts... sort of like transmission guts. I understand them if I sit and think about it for a while. It's just not something I need to know most of the time, so it doesn't stay fresh in my mind.

[Loren]

Mentioned this in the other thread, but I did experiment with going really stiff on the rear shocks today. Surprisingly, not as big of a difference as I expected going from almost full soft to almost full stiff. But, I think it was an improvement. Wheel hop can still be induced, but seems to be a little easier to stay out of. Autocross will be the true test.

[twistedwankel]

Your shocks are dual acting then not just in rebound. You still have kidney stones.

[mymomswagon]

noob autocrosser here but to the extent one can "street test", my car feels much better after switching to much firmer (but only 1/2 turn from full soft) rear Koni yellows. I'm tempted to set them firmer.

also tried packing bump stops on those shocks to near zero clearance and liked it even more.