dprantl

You will find if you want to go that wide in the front you *will* need custom wheels to do it without major body mods. A cheap 9.5" or 10" wide 5x130 wheel with a 70mm offset or higher does not exist.

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

Mogans

OK. That's what I've got, just a set of replicas. So I think that really gets to the heart of my question about feasibility. Thanks for all your thoughts everyone.

largecar379

agreed.....none of this is cheap.





--Russ

mark kibort

Yes, Hypercoils and konis.

to run 8" of back spacing on a 9.5" rim will allow 275s on the front with only rolling the front fender lip. I wouldnt go much larger or farther inward. (done that with 8.5" back spacing rims and the same tire. Things get hit, right up to the abs wires and the inner wheel well covers. with the 8.5" backspacing, the 275s were tight, a little to tight even for the street driving I do. However since it was ok on the track, i thought that the 305s on 10" rims with equal back spacing would make a lot of sense. the problem was, there was still not enough room on the fender lip to do it without the mods i did. 9" back spacing would work on a 10" rim, but that would be even worse on the inside.
275s are plenty big on the front, and 285s fit in a pinch. on the rear 305s will fit with only a bat roll. actually, without spacers they will fit, but rub on the rear lower control arm. 1/4" spacer cures this. (but requires more stretching of the fender) Scot runs this with no rubbing issues. My 84 was set up this way, the 79 was as well as the Holbert car

mk

333pg333

Why would the contact space be shorter just because it's wider? Perhaps in a % ratio length vs width, but I'm not sure why eg a 8" tire has a longer contact patch than a 10"?

Also, what do you guys do if you have understeer on turn in? I'm talking scrubbing, squealing from the front tires on the track?

Greg Phillips

The contact patch is a function of the air pressure and weight of the car. With same car (and pressure) and a wider tire the contact patch will change shape to a wider but shorter dimension.
Simplified: 16 sq. in. patch with a 4 inch wide tire = 4x4
8 inch wide tire would give a 2 inch = 8x2.

If your car is understeering, you need more traction on the front wheels. The easiest way to move weight forward (more grip) is to lift off the gas or brake to slow down and unwind the wheel to stop the front tires from scrubbing and regain traction.
But it is hard to convince yourself that if the car is not turning you will improve the situation by turning the steering wheel less
Like turning into a skid, unwinding the steering wheel is not intuitive but correct.

Greg Phillips
1986.5 928S

Greg Phillips

The closest thing you can get from Porsche are the rear winter wheels that have a 70 mm offset. The 16 inch wheels are 9 inch wide but the 17" are only 8 inch wide.
I now have a set of 4 of the 9x16 70 mm offset wheels, but unfortunately only Hoosier makes the 275/45/16 tires. Other options are 255/50/16 or 245/45/16 on all 4 wheels, This allows you to swap the wheels front to back and side to side to help equalize the wear.
I also have a set of Kinesis 10x17" wheels that have about an 80 mm offset and allow use of 275/40/17 tires all the way around, but I needed to have stops placed on the steering rack to prevent hitting the DEVEK sway bar.

Greg Phillips
1986.5 928 S track car

Benton

Mark:
If all you did was add .5" backspacing, then your front track is actually .5" narrower since tire width is irrelevant in track width. I think you would benefit quite a bit from increasing the front track and widening those wheels. You have widened your rear track quite a bit from factory; however, with the huge backspacing on your front wheels, you actually have a slightly narrower front track from factory (less than .5", but still less).

I've looked up the specs of many professional factory-backed race cars and their track widths, and most of them have the front and rear tracks within 1" of each other. I'm setting my car with the rear track about 1" wider I think, but I lost the sheet of paper I had it written on.

Aston DBRS9 - F 1635mm, R 1635mm
Corvette C6R - F 62.2, R 63.1 running 12.5" front wheels and 13" rear (290 F and 310 R tires supposedly)

Lost the other ones I found, but the track width was usually within 1". What are your rear wheel sizes?

On the new Z06 the front track is wider than the rear, as was the 928 stock (I think).

333pg333

Hmm, okay so if the weight is constant, the patch is redistributed? Ok, again, I sorta agree, but I'm sure it's a proven fact so I'll run with it.
As for the understeer issue, I'm not talking about push understeer in this case. Moreso, hairpins and having to wait and wait and wait until I can re-apply the power. It's a lack of turn in and you can hear the wheels scrubbing away. It's not a matter of lift off-tuck it in. That's easy. It's a matter of losing time on slow to mid speed corners. I can double shuffle, heel and toe, left foot trail brake etc...this is not the issue. It's just dumb lack of turn in. Now that's why I'm worried about going up to 10" front and rear. I should state that I'm in a track focused 951, not a 928 but unlike others I'm prepared to learn wherever I can. If someone's done something that I can get an education from, I'm happy to listen.
I can increase toe out and this does help, but I've also heard that this is counterproductive under hard braking?

Benton

There's so many things that could be the issue there. It could be the tire size, your car's setup (toe/camber), shocks/springs/sways can have an effect, you could be taking the corner entry too hot or turning too early (just because the car won't turn in doesn't necessarily mean it's the car's fault).

Generally speaking, if you soften the front of the car in relation to the rear (springs, shock adjustments, swaybar), widen the track, lower the tire pressures, lower the ride height, widen the tires, toe out the tires more (be careful here, it depends on your setup now), add camber--at the cost of possibly burning the inside edge (camber is always a compromise, and it depends on what tires you are running), and many more things, then you will add front grip.

mark kibort

go check out my video. I probably have many more hours of racing on RA1s than most anyone and what i have found is that with my suspension set up and driving style, turn in has never been an issue. medium and high speed turns are near perfect, with recently the medium speed turns being a little bit of an issue. (75mph + is what i call medium speed) . Technique is important, as i can get any car to push at some speed and radiused turn.
A brake straight and then trail brake technique will transfer more weight to the front tires and allow you to set the car into the beginning of the turn. you also scrub speed this way, as the turn in itself allows for some speed to be bled off too. Then, comes the constant radius turn. is the front holding? can you go any faster, are they squealing, does letting the turn out stop the problem. A push is an interesting problem. there are plenty of turns where i know the exact speed my car can go around under normal conditions. when it cant, its obvious i have an issue. too much wing in the rear, not enough splitter, too high of pressure, track condition, tires old, etc etc.

For example, if i am to run a lap at 1 second below my best, its very easy to make the car handle like a dream. its that last second that tells where the out of balance will be. will the rear step out on turn in. (are you turning in too quickly?), can the brakes slow to the required turn in speed?, if its balanced, you get into a 4 wheel slide. how does this feel? is it sliding too much or just enough. (analogous to a drag launch. just enough slip to maximize acceleration). So many factors.

I have two very contrasting videos on the web. same lap time, very different car behavior. check it out. one is the Randy pobst race where he is in the BMW GS Grand Am car and im in my 928 with the new wing with too much of it. later video could be the last race at laguna in '08. watch the difference in the steering wheel. It is quite common to have the same cornering speed in a pushy car as a non pushy car, but you loose a lot of control and will eventually burn up your tires in a longer race.

The ways to fix push has been mentioned. bigger tire, lower pressures, softer bar, softer compression on shocks, less rebound in the rear, wider track, and then a bunch of driving style changes. I remember going for a ride in a car that was supposed to have a wild amount of oversteer. when i drove it , it was perfect. (for me anyway) all style differences.

dont focus on toe out, i agree, its counter productive. Ive found that it only hurts you and rarely helps. (unless it is excessively toed in). I drove the devek racer with .25" of toe out. it was totally undriveable. Mine is 1/8" toed in and rememeber, this changes a lot under braking especially without a bump steer kit!

Mk

mark kibort

Laguna race (new motor) with little push 1:38 lap time

http://www.youtube.com/watch?v=zxImi48nv-A

That is a little unfair, so this a laguna race in June 08 without the new motor to compare below

Laguna no push 1:39.0 lap time
http://www.youtube.com/watch?v=WvH6Yh540eE

Laguna race with big push 1:39.0 lap time

http://www.youtube.com/watch?v=GRG6Y791iBE

333pg333

Thanks guys and Mark I'll check out these videos. I realise there is not one magic fix, and I've had the car go better on some occasions. Looks like it's vital to document all your track settings every time. I'm not up to your level in racing or setting up. I will look to get some professional help I think. There are so many variables. Here's a short video of my last trackday. The track was greasy, I didn't have the worst turn in but it wasn't great. The rears were a bit slippy in some exits too, plus spongy brakes. Oh, beat my best time by a second too. Weird? http://www.youtube.com/watch?v=JygHyzK4ZKc
My setup is: Toyo R888 f 18x9" 255/35 r 18x10 285/30. KW '2 way race' custom suspension f 616lb/in r 708lb/in effective with T-bar. Almost all rubber bushings replaced by Racers Edge solids. 968cs sways front and rear. GTS brakes front, S4 rear 928 proportioning valve. Car is still used on road so it's not really light. Probably 3100lbs with driver/fuel.

mark kibort

nice video.

Quick analysis. Your first few turns, you turn in a little too late. after the bridge, you do much better and you can see how you are gaining on the turbo 911. you get better and on the 3rd lap, you are ok, but the turn in still for several turns can be earlier. earlier turn in and a little brake as well

Your car seems pretty neutral, unlike the turbo who has a lot of oversteer, but much of that is driver induced as well.

What was up with the standing start with only 2 cars?
by the way, when you scrub the tires, it doesnt do you any good to scrub them off line. you pick up a lot of stuff that way. So , is that a format for a 2 car race/time trial event? very cool if so. they are allowing street cars to run this format with minimal safety equip?

nice run! oh yeah, hands on the sterring wheel! before the bridge you move them around a bit, after the bridge you do better. overall, nice job.

Mk

333pg333

Thanks Mark. It was a hard day at the office with the track pretty greasy. You had to wait quite a while before being able to re-apply power. The 2nd turn leading to under the bridge is beyond 90 degrees and that's why people tend to take it very late. If I took it much earlier I'd get thrown out too wide onto the marbles. As it is I was getting close as observed. The turn leading onto the bridge is really bad. You can't tell much in the video but it is really bumpy and I'm inbetween gears. Although 2nd feels better I will stick with 3rd the next time. As for the hands, must be the angle of the camera? They're on there all the time except for gear changes. I don't always keep them at 10 and 2 though. I sometimes have to brace myself in the seat and the hands get involved moving on the wheel, but I will sort that out.
thanks for watching.
Patrick