UrS4

Hopefully I can articulate my question well enough to have the experts chime in and point me in the right direction.

My car has a stock suspension and running the stock Pirelli 235/55/17 tires.

The rear of the car feels very "loose" to me. It has a lot of lateral left to right movement when going around turns or making slight quick steering corrections. I invision it like when two people are in a horse costume, one in the front and one in the back. They walk okay but not always in synch and look like they are disconnected. Going in a straight line under hard accelleration I feel the rear end taking an ever so slightly shift to the right and then back in line.

What is controlling the rear axle and keeping it tracking straight? LCA and UCA?

What keeps the rear end under the center of the car when going around a corner? PHB and Sway bar?

Will better shocks and or lowering/stiffer springs help?

How much of this side to side and disconnection I feel in the rear end coming from the tall side wall on the stock tires?

Thanks in advance to SamStrano, Rodeoflyer, Norm, etc. etc. infinitum.

RodeoFlyer

Technically.......the answer to all of those questions is yes LOL


Primarily the phb controls the axle laterally. Most of what you feel is the 55 series tires. The unfortunate part is that if you go with lower profile tires you need to lower the car to fill in the newly created gap. To a lesser degree 18's would help but then you have the added mass so everything is a compromise really.

Sam Strano

I'd start with shocks. Better damping will slow the roll and pitch rate down a lot, which makes the car more stable. A swaybar deals with the amount ofbody roll. A PHB deals with lateral loads and keeps the body located in relation to the axle. Shocks deal with the rate of movement. These cars are not super heavily damped on the low-speed side of things, and you tend to get little wiggles when the mass of the car just isn't well controlled by the dampers.

Perfect example. Customer installed Koni's on his Shelby today. Started full soft. Described the car's changing direction as very spongy (despite the bigger bars and heavier and lower springs). As he upped the front damping the car turned in much better, but then he found the rear to be out of sorts with the front, as you'd expect by adding damping at one end only. He then added some rear shock to his liking (not the same amount as the front). Dampers effect turn-in by slowing roll rate as you add rebound, or speeding roll rate up as you take force away. If you can add rebound overall, the effect is a quicker reacting car that many folks claim rolls less (not true, but the roll is slowed so much it feels like the car rolls less). With adjustable shocks you have the ability to tune this rate and the car's reaction, and you can tune the front and the rear so you get a balance you like.

Shocks (generically) are the KEY to a good suspension, above all. If you want to add springs either at the same time or later, fine. Worst case is you have the dampers in place you need to control the springs. I will, with little exception, always start with dampers. The Koni's alone made my car so much more enjoyable because it just so much more stable and nimble than it was with stock Ford dampers. The FRPP dampers are overdone, particularly on the rear in rebound and we almost immeditaly changed those out on the Shelby we run, first for D-specs now for Koni's.

UrS4

Okay,

So if I was content for now with the ride height, a profound change in stability in the corners and a more connected feel can be had with the following:

1) Adj shocks
2) PHB with rod ends
3) Lower sidewall height

The next step would be to change the understeering nature of the car with a good sway bar front and rear or at least go with adj front links like you have shown in another thread.

Sound like positive steps forward without too much compromise on ride quality, correct?

Sam Strano

Yes.

But the tires are not really a huge problem. I know, it's not cool to say the 235/55-17's aren't terrible.... Yes you can do better. But they aren't at all super-flimsy. Case in point, I can take my car, not add any air to the front tires and run a autox course and NOT roll them over onto the sidewalls like I can with most any other street tire, and even some R-comps. Further, the shorter the sidewall, the harder the car rides. Tires are, indeed, parts of the suspension. They deal withimpacts as well.

Keep in mind that I'm privy to driving a lot of different cars. I've driven 360 Challenge Stadales, 911 GT2's, Vipers (including two Chysler test mules), GM factory Solstices. I was contacted in a mostly unofficial manner by a project engineer on the new Camaro about what it might take for that car to be an good autox car vs. the S197. I've driven more cars that you canshake a stick at, of all types over my 11 years of teaching driving schools. I'm pretty damned picky about my stuff, I really am. My Z28 is completely prepped for ESP and I've won 2 Championships in it (Nationals, a few more Pro Solo titles too). I don't use ballistic spring rates, I have some custom parts like bushings. The car makes 321 RWHP on a lower reading Mustang dyno, and gets 28 mpg on the road (when I drive it). I autox a Shelby now, won Nationals going away in that, and there were 9 others in the field that weren't close and even completely outran an entire faster class @ Nationals.

My GT is mostly stock. I do race it in a stock class where I can't do much at all. Same class in fact the Shelby runs in. Nationally the Shelby is slightly quicker in competition (and every time I've run back to back tests the result is never more than .5 over a entire course). The GT is WAY more enjoyable to me on the street. It doesn't beat me up, it's not too stiff, I don't worry about dragging the nose on things..... My GT has the following done: Koni's, adjustable swaybar links, and alignment, an exhaust, a drop-in filter. That's it. Yes, I could do a LOT more to it.... but as I drive it, I ask myself for the way I use that car, what don't I like about it? I come up with very little.

I'm fond of saying, parts are medicines they cure ailments. What ailment are we trying to fix? Pick the part most suitable for that, and see what happens. Multiple "meds" for one ailment only serves to muddy the waters, and from a testing and tuning standpoint you only make one change at a time.

Are the Pirelli's perfect? No, but aside from lacking huge amounts of outright grip, they aren't that bad IMHO.

UrS4

I was suprised on the road course with the stock pirellis as they wore great over the five sessions, much better than a set of Bridgestone Potenza RE750 I ran on my old Audi and that tire was a summer oriented tire.

SlideWRX

I'd go with poly ends on the phb. Rather,I already have. Rod ends are needed for a race car. Poly bushings will be plenty stiff.

Tom

Sam Strano

I'll have to respectfully disagree with the notion that poly is plenty stiff vs. a rod-end. I tested on my old '91 Camaro once, and a tubular poly-bushed PHB allowed as much tire rub as the stock PHB did (not stock wheels), a rod-ended one did not.

Urethane is not rock hard, it will cold flow and flex. That doesn't mean you should rod-end the whole car, but there are places to consider it. There is a HUGE amount of lateral load going through the PHB's and that only increases with bigger wheels and tires, stickier tires, and performance uses. Think about a car pulling even .80g and consider what the car weighs....

If you want a compromise and are worried about terrible noise (and you shouldn't be), then go poly/rod and at least minimize the movement.

Norm Peterson

Probably less than you suspect. Tire carcass construction has a lot to do with it, but sincethis is anunknown quantity to most everybodywho doesn't design tires for a living I can't evaluate it either.But you can get a little handle on it by looking at how much actual sidewall "bulge" you have, and you can estimate that from the nominal dimensions and wheel width. Call that a really rough estimate of how much lateral tire deformation comes easily and nothing more. That said, the 235/55-17 is mounted on a wheel of fairly generous width (8"), so you don't have a lot of sidewall "bulge" that goes away under moderate cornering loads.

Like I said, it's really crude, but let's say that you've got 0.73" of bulge. 1" or higher is heavily skewed toward ride comfort, and anything under about 0.6-ish" is biased primarily toward cornering performance. Much below about 0.5" and you probably wouldn't sense any sidewall flex at all until the tire has hardly any more grip left to give you.

Comparisons: My thumbnail car started life at just over 1.0", but is now just under 0.6" (very similar to the GT's 235/50 on 8.5"). Another car from a previous time in my life was in the low 0.4's and was a daily-driver if a bit stiffish - about where the GT500 front wheel combination lives (255/45 on 9.5").


Shifting gears a bit . . .

How many miles are on those shocks, and how hard has life been to them in general terms of road roughness and other abuse? I also suspect that more of the movement that you are sensing is coming from quicker (and slightly greater) roll motions. Even hard launches cause the car to "roll" slightly in response to engine/powertrain torque reaction.


Norm

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UrS4

Norm,

Tirerack.com says the section width on the 235/55/17 Pirelli P Zero M&S is 9.2" Rim in 8" wide so there is 1.2" or 0.6" bulge on each side of the rim. Although it looks like it has more than a half inch of bulge. I keep the tires at 41psi which is more than the traditional 32-36psi most other people run, so I don't know if that makes less bulge or not.

Car has just over 20K miles with 1 two day road course event and the rest a combo of hwy and city, probably more city than hwy. How long do the stock shocks typically last?

It sure feels like the rear end is moving more than a 1/2 inch but maybe I slide around in the seat a little two and that adds to the feeling of the rear end moving on me.