DavesZ#3

iTrader: (26)

As I have gotten more serious into autocrossing this year, I had this master plan in mind for next year. I was going to get some big Falken RT615's to go on stock wheels (18x8") to use strictly for autocross.

I've seen a lot of posts here and on the autocross forums by a lot of you whose opinions and suggestions I've come to respect and trust. The general theme seemed to be "use the widest tire you can fit". I've seen a lot of setups with 275s and 285s on 8" wide wheels. It all makes sense, more rubber, more traction.

Lately, there have been some threads that are making me re-think the whole "wider is better" theory.

https://my350z.com/forum/wheels-and-tires/231823-03-track-model-wheel-question.html

https://my350z.com/forum/wheels-and-tires/214370-reccomended-tire-widths-for-stock-rims.html#post2743745

https://my350z.com/forum/wheels-and-tires/236618-widest-tire-for-an-8-5-rim.html

The Cliff Notes version of those threads goes something like this: Wide tires on narrow rims cause the side wall to be compressed and the surface of the tire to "bow up". That results in the contact patch being reduced, so you really don't get all the advantages of the wide tire. Also, sidewalls aren't as stiff when compressed so handling suffers.

Originally I thought of going with 265/35 or 275/35 tires but now wonder if I should go with 255/40 as it's only slightly larger than the recommended tire size (245/40). Is it really worth it to have the extra 10 or 20mm of tire width? (Actually, the tread width of the 265 and 275 are listed as 10", the 255 is 9.4" so I'd be giving up 0.6" of tread. And it may not be that much if you take into account the distorted sidewall/contact patch as a result of squeezing the 265/275 tire on the 8" wheel).

I could also save about $280 on a set of 4 255/40s as opposed to the 265/35s.

And to throw another twist into the whole mix - I've done a couple of HPDEs this year and plan on doing more next year. While I thought of using the autocross wheels/tires on the track too, I'm starting to think that's not such a great idea either. It looks like what is good for autoX is not so good for the track, even at the HPDE level. Am I off base there?

Thanks guys!

PDX_Racer

iTrader: (2)

Personally, I'm a bit nervous about the 245s at the limit, since with the sidewall flex, it doesn't leave a whole lot of space between the now-bare rim and the pavement. Hit a bump, and the rims are history.

What a lot of people seem to forget is that tires "self-center" on the rims, but that process takes a bit of time, so you give up some instantaneous transition performance for more ultimate grip. You can go too far both ways.

tomsn16

iTrader: (2)

Here's the setup for the 2nd place BS car(by o.100 sec) at the SCCA Solo Nationals this year.You decide.
2005 Track Model
285/30/18 Kumho 710 on OEM Ray's
CT-1 front swaybar,35mm solid
Koni DA's
1/4" front spacers
Muffler delete pipe

Fingers

iTrader: (6)

Looks like everything has its limits of diminishing returns. It would be interesting if you could go to the track with a pyrometer to measure tire contact patch and check out other peoples set ups. I've never tried auto x, but it would make sense that your tires will be asked to do alot more at the track and have a chance to get hot.

Fingers

iTrader: (6)

285/30 that smaller side wall may be the secret to getting that to work.

baileyrx

iTrader: (16)

275 all the way around is probably the way to go.
a couple of my 350z auto cross buddies run this set up & like it.
also they pretty good for hsde also

DavesZ#3

iTrader: (26)

I assume you are talking about tracking the car, not autocrossing, right. 245's shouldn't be a problem since that's what the OEM tires are on the rear.

DavesZ#3

iTrader: (26)

Yeap, that is one of the setups I've seen. Similar to what Christoc ran on his.

My question is really "is the usable contact patch of the 285/30 much more than the 255/40 or the 265/35?". There's no doubt that 285/30 works, but does something else work nearly as good?

DavesZ#3

iTrader: (26)

Actually, I would love to be able to get a hold of one of each size and get them mounted then measure the true physical contact patch.

Eagle1

iTrader: (2)

I think the choice of your tire will be as much if not more important than the actual size when it comes to AutoX. A lot of tires will take time to heat up before they really start to grip (like almost any DOT-R), so those are going to be out of the question no matter what size you can mount on the wheel and stuff under there. You need something that by acclaim grips cold for the time it takes to get through your tossing and turning right off the line. Then go to your sizing and fitment of that rubber to get the best contact patch for cornering.

I do not think it is going to be the same for sizing or rubber compound for the HPDE track car, but will defer to those more knowledgeable as I am not an autoX guy. My car runs 285/35 in front and 315/30 in the rear on Nitto NT-01 tires, with 3.5 degrees negative camber F. And it makes one heck of a difference in braking and turning, as well as hooking up the power. Depending on weather conditions and track surface, however, it takes at least one lap and sometimes more to heat them up before you can begin to push the adhesion.

It would seem that a stiff sidewall, and especially one that has a strong modulus (a fast, "spring back" from inertial rollover distortion in hard cornering) is what you are looking for as much as footprint, because the inertial distortion will lift the inside edge off and actually decrease the available working footprint and cause the car to lose adhesion at a certain point......that feeling of cornering like heck and then suddenly it starts getting away from you while everything otherwise on the suspension is working great (the car itself is stabilized and not in uncontrolled roll over on the springs and your anti-roll bars are keeping your inside tires gripping).

Anyway, choice of rubber here is critical, and it is very unlikely that what works for drift, autoX, road racing, and street comfort are going to be the same tire.

drivesolo

iTrader: (6)

275/35-18 on all four corners (18x8 wheels) is what I use to run in Stock class. Works great, lets you even out the tire wear since the front shoulders take more of a beating than the rears. But I would be reluctant to use the same set of tires for a track day. ABS will be slightly compromised as a result of running a square setup, but more significantly, the bulging sidewalls do not handle long sweeping corners at high speeds very well. A slightly 'stretched' set of tires is general preffered from what I understand, so wider wheels would be called for. For track days I use 275/35-18 fronts on a 18x9.5 wheel and 275/40-18 rear on a 18x10.5 wheel.

betamotorsports

iTrader: (1)

A customer of mine and I did a test of a very similar scenario on his DSP E36 BMW. 245s and 285s on 9" wide wheels. Our test was more focused on the additional weight and OD issues. Both tires were V710s and both sets of tires were new and heat cycled by Tire Tack. The 285s were over a second faster per run (average of the middle 5 test runs), faster then the 245s.

tomsn16

iTrader: (2)

The 18 month development of our '05 Track included testing (in order) 245/35/18 Kumho 710, 275/35/18 Hoosier A5 and A6 and 285/30/18 710's.The entire setup suddenly "liteup" with the addition of the 285 710's (the result I guess of a slightly larger patch and the lower sidewall).
Keep in mind, all our development was for autox only but the 285 provided a leap forward for the single purpose.Side benefit is fantastic wear(100+ runs) which we never encountered before with the no front camber Z.
Bottom line re your question....for our car nothing else worked nearly as good as the 285 710.

DavesZ#3

iTrader: (26)

One limitation that I have is that I can't get R compound tires like the Hoosiers or Kumhos. I need to be able to drive the car to and from events - some over several hundred miles away with the potential of rain.

While I would love to try the 285/30 Kumho's - that will have to wait till 2008.

drivesolo

iTrader: (6)

If that's the case, you might be better off in a Street Tire class. It would be difficult to be competative in a non-Street Tire class if you're not running a Hoosier or Kumho.

DavesZ#3

iTrader: (26)

The Z doesn't qualify for a ST class. I know I won't be as competitive as those on R-compounds. The Falkens are the next step up over my stock Potenzas. Hopefully next year's project will be a trailer with some R-compound tires.

drivesolo

iTrader: (6)

???

I'm taking it that your planning to run in a series that does not relate back to SCCA PAX indexing?

DavesZ#3

iTrader: (26)

I plan on running in BS. When I said "the Z doesn't qualify", I meant the Z in general, not mine specifically.

Kolia

iTrader: (2)

Are you AutXing to seriously win? Or for fun ?

HPDE is definitely just for fun. And contrairy to most belief, widder does not equal more fun!

I run 255/275 for HPDE. And I'm considering going to 255/255 to up the fun factor. Fun for me being going to the tires' lateral grip treshold. That for me is fun. It's slower of course. I'd carry more speed with 275/275, but going to the limit then ups the ante.

Finding a balanced setup is more important than shooting for the maximum performance. I'm guessing your day job isn't racing cars so relax and enjoy the ride !

My 2 cents...

MoodDude

And going wider will slow you down in the straights, since your rolling resistance will be greater. Plus, you unsprung weight will be greater, thus there is a limit to how wide will actually pay dividens in lap times. A great way to test out the theroy is to test and tune.
I buy used tires to see which ones will yield the lowest lap times! You can usually get a set of 4 used R-compound tires shipped to your house for the cost of one new tire before taxes and shipping, and they come already shaved - heat cycled - and with over 1/2 the tread still left.
My recommendation is to test out some used tires before you spend big money on new and see for your self which ones work for you and your car!
Try John Berget for some great deals on used race tires:
http://www.jbracingtires.net/