Running on 15's

[mohaughn]

Is there anybody on here that has worked with different 15" wheels to find an optimum setup for lightweight wheels/tires?

Currently running a 16.8# wheel that is 17x8.5 with a 235x45 17. I'm looking to go back to a 15x7 or 15x8 wheel with a 225x45 15.

I figure I should be able to save about 3-5# minimum per corner, as well as I'm going back to an oem sliding caliper with probably 328 rotors and improved rotor cooling.

In looking around at lightweight 15" wheels, I see several options for the E30 M3, with et20 offset. I see a lot of information on here about running et20 wheels with 17" rims, but not so much about the smaller 15" wheels. My rear fenders were already rolled to handle 245 tires on 17s. If the tires fit and don't rub, the extra 20mm of track width should help handling. Anybody have any experience with this?

I have coilovers and 3 degrees camber up front, and 2 in the rear.

[mohaughn]

hehe.. No one? I guess I'll buck the trend and go with smaller wheels.

Getting the hookup from another member on 328 calipers/carriers, and some SSR comp wheels from a racer on bimmerforums. So I should know soon how much of a difference this makes on the track.

Of course, doing a lightweight flywheel, and the IE rear camber/toe kit, so more time from those mods.. Hoping to be able to compete for 1-3 in NASA-FL TTD. And have a lot of points to spare to ramp up and compete for the top spot..

Run the fastest lap so far at Sebring at a 2:41.9, and that was on 60% tires, but very very cold weather for Florida. TTD record is a 2:37 or so, trying to get there. Not sure if it will happen or not, but have to have goals..

[pdxmotorhead]

Weight is not that much of an issue for these cars(In my opinion). It helps acceleration to go lighter but other than that...

Its all about the tire you choose to run, Bigger rims allow more room for brakes but if you don't need the space why run bigger, 15 inch tires are a bunch cheaper than 17" tires...

Most race classes don't allow upsizing wheels in SCCA so I've most run 15 or 16" depending on the car. and its age...

Dave

[mohaughn]

Yeah, going headlong into the 15s. I really don't think the my ti needs a lot of brakes.

At the last event a few weeks ago i almost ripped off my front bumper getting the car off the trailer. i had to run without the stock brake cooling "vents." One side of the car, the current bbks were very hot.

But I see plenty of faster e30 and miatas running with far less brake rotor than I have. Most ITA and almost all ITR cars are faster than my car, and not running huge brakes.

328 rotors with 2.5" brake ducts run into the center of the rotor should be sufficient. So I can't really find a reason not to at least try smaller rotors and tires.

SSRs are on their way, and a new set of 225x50 r888s are ordered. I was going to try the BFG r1, but they are backordered for a few months in that size. SSRs I've seen weights between 9 and 11 pounds. The set i'm getting have a pair of hoosier slicks on them and weight 28#. I'm thinking 30# with the r888 shaved. Current wheels are probably 40#.

I've seen a e46 m3 rotor weight of 16 or 17 for a two piece design, and in the 23# range for oem style. Shipping weight on 328 rotors is 14.5# a rotor. I need to actually weight them as their could be weight "discounts" in the shipping weight. I'm hoping for 15# of rotating mass reduction on each front corner. It could be more, but i'm not going to be overly optimistic. There is also the drop in ride height without any big changes in suspension geometry. i'm not planning on dropping my coilover ride height. It will be a minor drop if I do drop it.

I've used some online final gear calculators and it could be as much as a .5 increase in rear gear by going with the shorter tire. Combine that with a new JB racing lightweight flywheel, and I hope I have some new found acceleration.

Worst case, if I go slower, i sell everything, and go back to the 17s. Not planning on selling my current k1s and nt01s until I know if I like the car on 15s. Having just driven the car and maintenance for the less 18 months, it is kind of fun to have a project on it again.

[Bluebimma]

No rolling whatsoever, i put 16x8 ET23 fendo/style 5 with a 205/50/16. I lowered the car to 23" from ground to upper fender opening which is about 2.5" off the ground. The rears are also not rolled and had no rubbing from either cases. With a roll, a 225/45-40/16 would easily fit in there.

[mohaughn]

I'd never run that much camber in the rear. Also wouldn't drop the rear that much, so it would probably just mean more room as it wouldn't be as tucked. I talked to a guy that runs a spec e36 and he uses an e39 tire without any issues. So I may end up testing which setup is better on my car, and then running the slower rim/tire combo as a rain setup instead.

Keep in mind that a 225 hoosier or r1 r-comp is as wide as a 235, maybe a 240 regular street tire.

[Bluebimma]

Was just throwing it out there for reference. Ive run many suspension and wheel setups, just try to help others out.

[mohaughn]

Thanks. The alignment sheet is definitely useful. Knowing how low you need to get the rear end to get -3 camber gives me a good idea of where my wheels would sit if I went with a 20et.

I'm having the rear camber/toe kit installed today and will be able to test our the ET38 wheels with a 225 r-comp. That should give me a really good idea of how much room I have to spare to try an ET20 rim.

[NiteOp]

Hey mohaughn did you ever do some testing with this setup? i have some 15x8 et20 now and I would like to have something to reference to. I thought you may see an improvement in raising the roll center over high offset wheels. thanks, I'm hoping to see some track days this coming year.

[mohaughn]

No. Never wanted to buy the et20 wheels. Waiting for the 949 Rev8 wheels to be released so I can get a nice wide wheel in the right offset and then use spacers to play with offset if I feel the need. Right now I'm planning on running the SSR comps with a 205/50/15 SM6 hoosier next year. The 225 hoosiers are just to wide to get a good fit on the SSR 15x7 wheels. The 225x50 R888 pinched on the 15x7 wheel was also not as fast as a 235x40 on a 17x8.5 wheel, not by much though. The 225x50 had very little gearing increase over the 235x40-17. The Ti really needs the rear end ratio difference provided by running a smaller sidewall tire. Used Hoosier/GAC take-offs from the RSR minis(225x50-15) on the 15x7 wheel were just as fast initially as the 235 x45-17 r888's. Should have been faster if not pinched so bad on the SSR wheels. New SM6 205s should be the fastest combo I've tried so far mainly due to the compound being so much better than the r888 and the 205 SM6 having the best fit on the wheels that I have and a smaller sidewall than the 225x50. Once I get a wider wheel, I can go back to running the 225 with a 40 sidewall.

[NiteOp]

I see. i like the 225/45/15 and the 225/50/15 size and my brother runs the 235/50. those led me to the 15x8. I didnt know there was gunna be a 949 Rev8 oh well I'm very happy with these. lots of room for coilovers and 225-235. I will probably bounce back and forth between taller sidewalls and stiffer springs, and shorter side sidewalls and softer springs. The 235/40/17 is another possibility but would require another wheel setup so maybe next year, 2013. Thanks for the follow up.
Carson

[mohaughn]

If you are running e39 wheels, just make sure you are running a hucentric ring and you should be fine. I think a 15x8 would be a good width for a 225x45 street tire or NT01/R888.

[NiteOp]

I ordered the '90 M3 wheels from tirerack so they are 72.5mm centerbore. Thanks, I hope to run the RS3 and/or R1R in 225/45 for autox (STX class) and r888 on Fontana track days.