Design from tires up (what size?)

I have this BMW 2002, and I have many concerns, but let's start with one which I gather is among the prime starting places.

I'm doing my own subframe and front suspension. Depending on how frazzled I am after that, I may either redo the rear as well, or decide that the stock arrangement's okay.

I'm planning on flares, so width and size get some additional room.

The question is, what size tire has (and will continue to have) good availability of both performance street tires and R-comps?

I have a set of 225/50-15 Kumho Ecsta V710s. I'm not dumb enough to build a car around a set of used tires I happen to have, but it does seem like a good guess, but the street selection didn't look good on tirerack.com. Seemed like maybe it was slightly better for 225/45-15, which is also probably a better size for the 2002.

I'm not going to be completely pragmatic; I think 2002s tend to look funny on wheels larger than 15". That alone may mean I'm doomed to scavenging tires not to far into the car's future...

Rough build and usage seems pertinent. This'll be a street-driven/autocross/occasional track use car, with (probably) a turbocharged M42. So, quite a bit of power for a 2200lb car, but a long way from insane.

In reply to ransom: 205/50R15 is a very common size and the 225/45R15 size is very similar.

Toyo has 225/40R14 UHP tires, but they aren't as good as what you can get in the 15" sizes listed above.

For the street I would run a 205/55r16 front 225/50r16 rear. Available in very good grip or cheap driver options and are pretty common.

225/45-15 is a pretty rare size. Your choices are:

• Hankook R-S3
• Toyo R1R

And that's it for street tires. Both excellent tires, but that's not a big range to choose from. You can get R-compounds as well.

205/50-15 is definitely a lot easier to find.

RS3s are on backorder for quite awhile. Ask me how i know.

I was NOT pleased to have to go with 205/50-15 Star Specs.

For some reason even though I read BMW 2002 I had 3 series in my head.

Disregard.

Totally IMHO, but then, you're asking

Start out with deciding how tall and wide of a tire you can use. That will be dictated by the chassis/body, even if you are adding on flares. Next up, you don't say what your suspension plans are -- stock or modified or ??? Suspension will dictate the wheel you can use (ie., width, offset, diameter to clear brakes). Once you figure out those, then it's time to go tire and wheel shopping!

It's cheap, no this is not a canoe, but I'd suggest you get a Percy's wheelrite and use it to start figuring out some of this stuff. I got one and it paid for itself on my TVR build.

Hope this helps -- you have interesting times ahead of you with projects like this

orphancars wrote: Totally IMHO, but then, you're asking

Indeed I am!

Start out with deciding how tall and wide of a tire you can use. That will be dictated by the chassis/body, even if you are adding on flares. Next up, you don't say what your suspension plans are -- stock or modified or ???

I did, too!

ransom wrote: I'm doing my own subframe and front suspension. Depending on how frazzled I am after that, I may either redo the rear as well, or decide that the stock arrangement's okay.

orphancars wrote: It's cheap, no this is not a canoe, but I'd suggest you get a Percy's wheelrite and use it to start figuring out some of this stuff. I got one and it paid for itself on my TVR build.

Interesting. But that looks like it would let me measure stuff that's already there. I'm starting with the tire question so I can design the stuff that doesn't exist yet... For the immediate questions, I'm sure it's a good idea to bolt up the 225/50-15s I have now (with fender removed and see where it shows up now and note the clearance issues with items that can't be moved in the subframe do-over. Smaller tires than that already scuff the firewall, but I'm planning to squeak the tires forward a little bit to help with the 2002's nose-heaviness...

i'd recommend some performance-focused selection criteria, such as:

required rolling radius to achieve desired overall gearing (vehicle speed @ RPM in each gear);

required wheel size to package desired brakes;

required size to carry the expected loads;

and then the aesthetic considerations such as will it look dorktacular with a 45-series tire on a 15" rim?

but that's just me...

Are you designing everything? Fabbing uprights from steel or starting with something else?

I'd still stand by what I said first -- get your hard points figured out. If you do have something (anything!) they by all means bolt that up and start measuring. Use what is already there as a reference point and then start figuring out what changes from there (ie., track width, if you move everything forward, etc). That will help get your hard points decided.

For shopping around for tire sizes/availability, I've had good luck with tirerack and 1010tires.com. 1010tires has a few calculators on their site that will be helpful.

Good luck with the project. I found the front suspension to take the most time on my TVR build. Actually, figuring out the geometry and hard points was easy (narrowed C4 suspension with a custom crossmember). The thing that took the most time and was the most frustrating was getting the flares in the right position so that nothing rubbed at full compression and full lock left/right. Seemed like once I got something that worked, it looked like crap at normal ride height.....it took several sessions to get everything where it needed to be.

I am running 225/45R13 R888s on the 2002 project for track/autocross use until I get it sorted out. They're closer to the original size and will fit fine under the IE turbo flares.

+1 to everything AngryCorvair sez!

These are central questions. I probably could've ballparked something to set constraints before asking for prognostications on what will continue to be available in the tire department. I've got some thoughts for all those questions (but am more than happy to learn if I've got bad assumptions, etc).

AngryCorvair wrote: i'd recommend some performance-focused selection criteria, such as: required rolling radius to achieve desired overall gearing (vehicle speed @ RPM in each gear);

I guess the big concern here will probably be getting 2nd as well-placed as possible to cover most autox conditions. Autox is the only time the stopwatch will count... There are a decent range of rearend ratios, but I will take your advice and do the math on a few tires sizes with the commonly available ratios... Anybody compiled statistics on autocross speed ranges?

required wheel size to package desired brakes;

Semi-secondary. Brakes that fit inside 15s should be overkill for autox, and at least sufficient for track days (it would be annoying to have to ease up due to faded brakes, but I won't be roadracing, and a 2300lb car with 250hp shouldn't tax breaks that size, I hope).

required size to carry the expected loads;

I'm a little torn here. It does seem like all the tire in the world is in use in the SM cars, and I'm going to wind up in XP or a modified class... Actually, I think we've just hit my facepalm moment (and it's a doozy): I need to figure out whether I'm worried about R-comps or slicks...

and then the aesthetic considerations such as will it look dorktacular with a 45-series tire on a 15" rim? but that's just me...

I know it sounded dumb to chuck that aesthetic consideration out there so early in the process. And it's not etched in stone; however, I think the physical limitations of the car are such that by the time we get to a 16" wheel, the car's going to be forced to sit too high. Even the big widths in 15" are getting pretty tall for this car, though availability and brake fitment get harder below that, too

It's a little bit of a funny situation, because I've decided what I really want to build, and now that I've roughed that in, I'm willing to scale the SCCA class chart 'til I find what allows that architecture, then pin down the details to not exit that class on a minor decision. Even though competition is in this car's future, that role is still "for fun". In reality, I don't think there's a good niche in SCCA autocross for a significantly re-engineered street car if you're worried about beyond the local/regional level. As soon as you start moving hard points, having windows and a passenger seat become liabilities. I plan to move my "serious" competitive jones to spec miata once I have this car "done"(ish).

In any case, it was certainly an oversight on my part to ask this question before pinning whether I'm using R-comp street tires or slicks for autox.

AngryCorvair wrote: and then the aesthetic considerations such as will it look dorktacular with a 45-series tire on a 15" rim?

I'll bet a 225/45-15 will look a whole lot like a 205/50-15 from the side You know better than to treat the aspect ratio as a standalone number!

If you are looking at 23" diameter tires such as these, then you're looking at Miata tires. This I know :) Expect the 205/50-15 to stick around for a while, especially in R comp sizes as it's a pretty popular race size. The problem is that to get any wider in that diameter, you start having real availability problems. You can package a set of brakes for a 250 hp, 2300 lb car inside a 15" wheel - again with the Miatas.

In reply to Keith:

Given that I'm chucking the front subframe and starting over on a small, light car, you can be sure I've been giving a lot more than the tires from a Miata a lot of consideration

I sure would like to get more than a 205 under it, though... Oh well, as noted above, I'm deficient on my problem definition at this point.

If you're willing to go with the R compounds, you can get the 225/45-15 in the Nitto NT-01, Toyo R888 and the Toyo RA1. Plus slicks. Toyo offers the R1R in something odd like a 245/35-17, but you need a 17x9 wheel and I've never seen one in your 4x100 bolt pattern. Plus it would probably look pretty odd on a 2002. And of course, there are the 275/35-15 Hoosier slicks...

Point being, there are a lot of cars hitting the track every weekend with a 23" tire. I think we'll see the performance options remaining for a while.

If you can step up to a 25" tire diameter, all sorts of options open up. But I don't know if that's too big for a 2002.