RKD in OKC

Okay...94 GTS with Devek suspension setup, ie., Springs, Koni reds (adjustable), big blue front sway bar and bigger rear swaybar. I put 8 and 10 18s on the car, Carrera Classic IIIs with Michelin Pilot Sport Ribs. The car was fairly well balanced steady state cornering with slight understeer that could be overcome with throttle oversteer. However the Pilot sports were worn past the wear bars and getting slower.

Went from nearly bald 225/285 Michelin PS to 235/275 Dunlop Direnza Sport Z1 Star Spec. Since PS had a little understeer I thought I would take out some understeer by going to 235vs225 front and 275vs285 rear. But Nooo. The 235s in front understeer much worse! AND the 275 rear Dunlops are so much more sticky I can't power oversteer. The tires stick really well straight line, can no longer spin the rears with throttle, and brakes much better, BUT the understeer slowed me down a full second per lap. I tried adding more air to the fronts but it didn't help at all.

It's kinda weird. About half way thru a 180, just pushing around the corner the fronts will just grab and turn in more. Think I just need to scrub in the tires more to get rid of the mold release? The tires only have a little over 100 miles on them.

IcemanG17

scrubbing in the tires is a good idea.....

do you have the stock front sway bar? If so take off the heavier Devek bar and put the softer stock bar on....that should make the car turn better.... Or even disconnect it all together to see how it feels?

mark kibort

first of all, a 235 is a tire that goes on a 2700lb BMW or porsche, NOT a 3500lb car! have you ever seen a Corvette, from say, 10 years ago? what tire do they have up front. Dont blame the suspension, or tire brand, scrubbing, etc.

you spend alll that time and money on a GTS with a expensive suspension, new tires, swaybares, etc, and you put an 8" rim up front? thats what is on my '99 BMW 328i.

Get some 9" rims up front and put on a 255, min up front. loosen the front swaybar up front and or reduce the compression of the shocks up front , and reduce the rebound in the rear.

oversteer is annoying, but it can be solved. Hang in there , you will figure it out.

never go smaller in tire to solve a handling issue. also, you obviously went to a lesser tire , or a tire that worked better in shaved mode. full tread tires will do this sometimes. As a last note, what is your rear toe set at? if you have too much, it will not allow for poweron oversteer. You probably have seen my oversteer video from 07, i didnt lose much time, but it was not fun to drive!! fixed it with downforce up front.
mk

RKD in OKC

The suspension is what came on the GTS when I got it. I like the wheels and understeer was only under acceleration in tighter corners and manageable with the pilot sports.

When I did a 100ft radius skid pad under slight acceleration with PS it was very balanced and came loose at both ends when it reached the limit.

I haven't found how to adjust the front Koni Reds yet, but had already adjusted the rear rebound to full soft and love the turn-in initial rear rotation with a 55 bar brake bias.

With the dunlops doing the skid pad thing it really surprised me when it understeered. Even with the understeer the dunlops did tuck in the front and grab when I lifted. I was hoping that with the 235s it would oversteer just a bit and I could tune it back to balanced to carve off a second or two from my times assuming the fronts were limiting overall corner speed.

The Dunlops have been getting 1st place in SCCA autocross in street tire classes and a local guy in a S2000 on the Dunlops has been matching times with the guys with full race compound.

I don't remember the numbers and can't find my printout just this minute, but alignment is within factory specs with camber at the max limits and toe at the minimum both front and rear. It is also at factory ride height, I don't want to scrap off my front lip on the streets around here.

I will have to monkey with that big blue bar on the front and find out how to firm up the rebound on those front Koni reds.

On a side note, I was asked to have my GTS represent at the PCA Parade Historical and Heritage Display.

dprantl

What tire pressure are you running? You mentioned you tried more. I would try less.

Dan
'91 928GT S/C 475hp/460lb.ft

pcar928fan

READ the thread and KEEP UP... ALL HE DID WAS CHANGE THE TIRES TO A NEW and different brand...with a size change that should have lessened understeer.

ew928

Are the Michelins still around so you can measure tread widths.
Different brands/tires have different contact patches even at the same sizes.
Too late to measure tire width mounted and contact patches and compare.

Curious how the narrower spec rear tire grips better than the Michelin.
I've got narrow rims and wide tires and that seems to curl the outer treads block up and away from the ground so I have really round tire shoulder tread blocks.

worf928

On balance I agree with (Holy ****... I'm actually writing this..) Kibort. If you want to smack understear on the head with a big hammer go to 255s on the front with a wider wheel.

Now, as to the problem with more understeer going from 225/285 to 235/275 with a tire make/model change a couple of things come to mind:
- tire width to wheel width ratio change convolved with the tire make/model change must be changing how the sidewall works relative to the contact patch for both transient and steady-state.
- the tire width change may have effected the alignment just enough that it isn't right for the new combination. For street driving it wouldn't be an issue, but since you are observing the handling behavior at the limit, a small change in alignment might be noticeable to you.

GlenL

+1.

Soften the end that isn't gripping enough. That goes for swaybar settings, springs and tire pressures. Or tighten the sticky end.

Getting tire pressures right is always a tough one. Varies so much on car set-up, driving style, ambient conditions and, especially, the tires themselves.

I like low tire pressures that doesn't let the tires roll over too much. Chalk 'em and drive 'em. Add pressure to get balance and keep the sides clean. And also check for even wear across each tire surface. Hard to get right.

mark kibort

Many did this in World Challenge GT. Often times a tire with less grip can certainly cause understeer. It did for about half the pro teams. what a lot of them did was what I suggested, so YOU try and keep up!!
Welcome to racing. sometimes (often) one change creates all sorts of other issues, that can be fixed with some changing of the many elements on the race car. I dont care what he did, he has oversteer now. He has to address it. It either was caused by the tires, or other things have changed. just because you go down in rear tire size, doesnt mean you get less grip. (or up a tire size) 235 vs 225 is a very subtle change as is 275 to 265. compound, tread design, pressure, side wall stifiness (are you keeping up?) tempurature, road conditions, all can create all sorts of different results. Heck, Ill admit, I have a hard time keeping up too!

as a side note, i went to 305s up front from 275s and the car is quicker, but technically has more oversteer. why? still figuring that out. Lots of issues, some could be the compound, or the way the tire moves. Im going to do more testing with both the 275s and 305s up front this next race weekend at Thunderhill and really see the difference in time and feel with back to back full races running each different set up.

mk

RKD in OKC

Pressures...
Started at 36 cold. Then aired them from 40 to 44 hot. bled back down to 38 hot.
Michelins are gone, didn't think to measure anything.

Talked to my suspension setup guy this morning...
With the stickier rear tires I am not getting the rear wheel spin with throttle application I was with the PS. This extra traction is transferring more weight off the front causing the understeer (especially with the 928's slow to settle suspension). Suggested more rear spring.

Jadz928

I'm not a tire wizard here but would different traction grades make a difference?

It appears the Dunlop's are AA and the Mich's A

Traction Grades Asphalt g force Concrete g force
AA Above 0.54 0.41
A Above 0.47 0.35

dprantl

Those are some really high pressures! I've found that for a given brand and size of tire, the optimal pressure depends greatly on the ratio of wheel width to tire width. The wider the wheel in relation to the tire, the lower pressure you should run. For example, my 235/40-18 tires on an 8.5" rim would wear on the outsides with pressures above 28psi *hot*. Those same tires on an 8" rim were running well on 32psi. Try the latter pressure on the fronts and see if front traction improves. Also use a infrared thermometer right after a run to check the temps at the middle and the edges of the tire to see how consistent the temps are.

Dan
'91 928GT S/C 475hp/460lb.ft

RyanPerrella

I would love to know when the last time BMW made a car that weighed only 2700 lbs. Only thing i can think of is MAYBE a stripped out E30 M3. That was like 1988 though

This notion that the 928 is heavy in todays marketplace is fiction, most economy cars are over 3,000lbs and most of your sport coupes M3's Infinity G37 and the like are in the 3700-3800 range.

THE 928 CAN NO LONGER BE CALLED A PORKER! !

mark kibort

a stock 993 was 3050lbs

a stock e30 was 2700lbs and thats what i was kind of pointing too.

bottomline, a performance car with 235s up front and 400hp is kind of a joke when the weights are over 3500lbs.