Tim Pruitt

I have the 928S Flat Dish wheels 8 x 16 with 225/50/16 Hoosier tires currently mounted on the rear of my 924S race car. I want to run a lower profile tire but, do not want to use 15" wheels and Hoosier does not have a lower profile tire in the 16" diameter.

Do you think that the 996/Boxster S wheels 8.5 x 17 with Hoosier 225/40/17 tires will fit on the rear of a 924S?

Lemming

It will be a tight fit, even after you flare the wheel wells. If I get a chance, I'll toss a cupII from my 968 on the 924S track car tonight and see how it fits.

Lemming

Nothing like testing to determine what works. Below are two photos of my 924S with CupII wheels (9") running 255/40/17s. I have my fenders flared quite a bit to accomodate early offset fuchs. And now that I see the fit with the 17's, you have me thinking!

Sorry about the crappy photos, but the cell phone was handy, the camera wasn't.

chrisc

Martin at J-3 has a killer set of 8 & 9's but I believe they are 15's. He keeps trying to tempt me but they cost more than the darn car. Says they will fit the 924S - mag cookie cutters from the Firehawk (I think) days. Very light.

J Silverman

Just out of curiosity, why dont you want to run 15s?

chrisc

Tim's car is a lot faster than mine but even at that I noticed less top end speed on the long straightaway at RA when I run the 15's vs 16's.

Tim Pruitt

Jason,

I currently use 225/50/16, Tire Dia is 24.7".

I want to try something with a lower sidewall height without giving up too much on the diameter. 225/40/17 Tire Dia is 23.8".

225/45/15 tire dia is 22.9".

J Silverman

Ive only driven RA once, and I wasnt all that fast there (2:00 lap times) but I still had plenty of RPM left at the end of the back straight, so I would think that the shorter gearing you can get with the 15s would be an advantage.

The 225 45 15 is what I was thinking would be best for these cars.

Lemming

Do you have data to confirm this?

I run 15's with an S2 tranny and my top speed is 148 at 6500 rpm. I don't quite get there at RA, but it's close. Not sure that going from 15 to 16 would make much difference for me, and I have more HP and am lighter. Of course, YMMV.

924RACR

Yeah, I was gonna say - I hope you're either getting this data from GPS or a tire-corrected data system...

I do run 225/45/15's, though I have to cram them on a 15x6" rim per the rules, which does impede their effectiveness.

As for Road Atlanta and gearing... I was there first time last Nov, and figured out that for my car, with stock gearing and the aforementioned Hoosiers, I don't even need 5th gear! And that's with the 4.111 rear-end, mind you. Of course, I can only hit 110 or so down the back straight.

OTOH, sounds like you've got some work to do there... I got 'er down to a 1:47 and change.

One more thing to note. I'm not sure how you're measuring tire diameter. The correct way is to install the tires on the car, air up to hot pressures. Then make a chalk mark on the tire and road, roll the car forward 10 revolutions (or as many as possible), make another chalk mark, and measure the resultant distance. Obviously divide by 10, do a little arithmetic, and you have the actual loaded rolling radius of the tire. The only error at this point will be due to dynamic effects, which aren't easy to measure without the right equipment.

Obviously, if you've got GPS-based DAS, then you don't have to worry so much about it...

Tim Pruitt

Vaughn,

Thanks for your feedback and insight. My best lap at Road Atlanta was during qualifying last year at the PCA Club Race, 1:44.718.

Over the winter I had my suspension completely rebuilt, new front sway bar, new springs, new shocks, new alignment and new corner balancing setup. Last weekend at the NASA race was my first time on track with the new setup. We only had 1 clear dry day on track and that was for qualifying and the sunday race.

Even with the 225/50/16 tires I change into 5th gear on the back straight and the front straight. I do this to give the motor a break. I know that I could easily keep it in 4th if I must. I will try this during club race practice sessions and see if it makes a difference.

I really need to purchase some type of data acquisition system so that I can see some data instead of relying on the seat of my pants. Any suggestions?

My GOAL is to break the 1:44 barrier this year.

Lemming

I think one of the biggest problems with 15's is the fact that you can't get enough tire width. Hoosier does make a 245/15 but the sidewall is 50 Has anyone tried this size, if so, what are you thoughts? Now, if I had some 9" wide rims, the 275/35/15 might be fun!

Tim, what's the problem of going to 5th on the two straights? Also, <1:44 will be smoking for a 924S, I hope you make it

M758

At the Las Vegas combined infield/oval track a 924S in 944 spec trim ran back to back using 225/50 16 and 225/50 15 tires. The 15's were 2 seconds a lap faster. The lower gearing from the smaller diameter is a big help.

The highest top speed I see on a local track is Cal Speedway. I get the car to 5800 RPM in 5th (0.829) on my 225/50 R15 Tires just before I hit the banking. Once in the banking the car does not accelerate any more due to aero drag and the effect of the high bank. In a draft I can hit 6000 RPM in 5th and this is about 132 mph. I can rev to 6400 rpm if needed. I don't know that with the HP level I have I will ever run out of revs(stock limit) on any track with my 225/50 R15's.

924RACR

Sweet! That's more like it! If you get hooked up with a data system and learn how to crunch the data, you should be able to find that time. As for what type to get - the GPS-based systems really seem to be the best way to go these days - I know a number of people who use the Racepak, I think, and seem to like it. Me, I have an older, simpler AIM system that's sensor-based, and do love it, but it does have it's limitations too.

I've posted them before, but here again are some tips etc on using a DAS for lap time improvement...
http://www.waterfordhills.com/dapres.ppt - Powerpoint presentation
http://www.waterfordhills.com/dapresnotes.doc - notes for the above

It also helps to have local resources and other users for your system if possible...