[judec]

to the OP: if U abstract from some of the BS posted here, U'll know that U can track you 19". Anyway, if U mean it seriously, get some light 17" or 18" dedicated for track use.
Pro's:
- 17, 18 or 19" tires of the same width and similar outter diameter weight the same (or almost, not important difference)
- 17&18" tires are cheaper and there is more of them to choose from. Also U dont have to make daily driver related compromises with a dedicated set of track wheels/tires.
- 17&18" wheels weight less than 19" made by the same technology. Pls dont compare 19" lightweight forged wheels to 17" stock for example. apples to apples. And U can feel the saving in unsprung weight as U get more advanced. Your car will feel it too
- 17&18" rims will be more protected against damage. Also replacing the damaged wheels is usually cheaper.
Contras:
- 17&18" won't look that fancy on the home-track-home route. But who cares?
- U need to store another set of wheels, swap the sets when needed etc. I admit using a "universal" set of 19" is more comfortable.
- additional one-time cost of the dedicated set. But if U track regularly (what U don't), a dedicated set will be cheaper on the long run. Just bend your nice 19" BBS rim once on a track and U'll agree.

[heavychevy]

Originally Posted by judec

well, not sure I understand U, chevy. Maybe it's because I assume wheel diameter and tire outter diameter are two different things. Also preferably U keep your tire diameter constant and as close to stock as possible. Also we abstract from changing tire width, it's also constant (lets say 245). So how will the contact patch increase by changing from 17" to 19" wheels?

I didnt make the rules man, you make larger diameter wheel and you are able to keep the stiffness of the sidewall while continuing to stretch the width of the wheel. The overall same height on the same tire with a 19inch wheel will have much less sidewall flex than a 17 inch wheel same width with the same line of tire and same overall height. So you have to keep the sizing down on the 17 to keep the structural integrity of the sidewall the same.

Take the R6 for example. You cant even acheive the same overall diameter with a 17 inch wheel, the sizes range from 23.8-25.9 overall diameter. In the 19's it ranges from 25.6-28.7, a HUGE gap. So there is no way you can get the same amount of contact patch with the 17inch wheel even with the same width because stretching the tire too far will compromise the sidewall stiffness capabilities.

So it doesnt matter which you call it, wheel or tire diameter, if the wheels are the same width, chances are you cant get the same height in performance tires with a 17 inch wheel. diameter and width are what increase contact patch.

I dont agree that the diameter should stay stock, if you car is modded, ride height, gearing, camber or anything of that nature, all of the stock systems are compromised anyways, they do need to stay proportional, but this is all case related, depends on the car. But upgrading to 19's isnt going to kill anything, especially in a non AWD car.


However monetarily, it is beneficial to go with 18's or 17's, no doubt about it, over the long run you spend much less money on tires, and replacement wheels. But breaking wheels at the track is not something that will happen often if you have a strong set of wheels.

[heavychevy]

Hey guys, there just happens to be an article on this very thing in the most recent GRM (Grassroot Motorsports) d(magazine for NASA SE Members) and it's using a new 325i I think.

However the results are with soft sidewall/hard compound tires, so they are a bit different, and the tires they used with the 18's they tried are really like rubber bands, but were still faster than the 16's, but the 17's allowed the best combination of braking and lapping. And it's because it has the best amount of sidewall, not too much, not too little. So if your car can fit enough sidewall on a bigger tire, but I doubt that's the case on a 3 series of any type. It's not going to be the fastest with REALLY low profile tires, but it wont be the slowest either, and we are talking about about 8/10ths of a lap.


I would say since you have 19's already, the short term less expensive route is to slap the 19's on there, and save up for some extra wheels in the meantime, or even use the 17's. You didnt mention competition so this should be about seat time, not lap times, and if you dont have a data logger, it wont matter much anyways.

[S62PWR]

+1 what Judec says.

[cgnahc]

Thanks heavychevy. I just got my grassroot magazine and will read up. I may just pop about $700 and get 161 with RFTs and run those on the track, like I was before...or just drop about $500 and get four regular 17" tires.

[S4to335]

I got my copy too...gonna actually read it.

[judec]

could some of U guys scan and post it?

[S4to335]

I dont have a scanner..I wonder if Grassroots is on the web too? Just checked..its there..but not ALL the articles are posted on the web:

http://grassrootsmotorsports.com/

Sorry

[Hegemony]

Have to say that grassroots is the only car magazine that I actually bother to read. Every time I open Roundel, I find myself wondering why a few minutes later, and I stopped reading the major publications (C&D, R&T, etc.) years ago.

[cgnahc]

OK, very exited. Laguna Seca is my next track event with NASA. Jan 20. Sunday. Running 19".