Tire Width?

[wootwoot]

Perhaps someone can educate me on tire width?

I have noticed that most autoX guys are running a wide tire, in the realm of 245 or 255 all around. Several of the track day/hpde/road race guys seem to be opting for a smaller tire, 225 or 235, and getting great results with that set up. As I understand it, the wider tires get you more grip at the cost of more weight. More grip means stiffer springs/bars to combat roll (I think).

So why the difference? And what is a good all-around width for performance? More grip? Less rotational mass? So confusing...

[renfield90]

I'm going to guess the track day guys are running with NASA - they are probably either doing HPDE (which is non-competitive, no "need" to spring for a bigger/pricier tire) or they're doing TT in which case the points system will assign points for going over a certain size tire.

All other things being equal, I don't know anyone who would turn down an extra 20mm of tire. The extra grip more than outweighs the miniscule weight addition (we're talking a couple pounds here).

[wootwoot]

Interesting. I remember someone asked robispec about his tire choice for his time attack car. I think he is on 225's and he said the smaller/lighter set up proved to be faster for him. Perhaps he'll chime in soon.

[coyote]

Tyres are like anything else, they can operate under a range of conditions but there are optimal conditions under which they perform at their best.

My 86 has 245 road tyres and 215 slicks. Why? The slicks need heat to work and on a car of this weight and power 245 slicks will never get up to temp.

[sw20kosh]

CSGmike also runs slim tires.

Robispec uses 255's and 245's as well.

I hope they can chime in.

[Huehuecoyotl]

235 for street in 40 series Direzza
hoosier in 225/40 as a dedicated track only-have yet to try this
215 for tracks in 45 series DOT-R for driving to track(R1/R888/Yoko)

245+ I dont know its needed except for looks, I think I prefer the 235 in a 40
others preferences will vary, surely some guys do great on the 245s

I can get the 215s to hook up fine by running front ride height a litte higher, seems to help the weight xfer back better on downhill twisties

[xwd]

Depends on the rims you are using and the specific tire. Most of the autocross guys using 245/255 street tires are using 17x9 rims which 245/255 fit fine on. It's been shown fairly often using a wide tire is faster in autocross than using a narrow tire, even if you lose some steering feel. That's as long as you can get heat in the tires, you can certainly go too big.

Hoosier race tires have extremely stiff sidewalls and can be mounted on narrow rims. I use 245/40s on 17x7 rims, that's the best tire size to use for this car for "stock" autocross class. People in other classes run 275s on a 7" rim or 295s on a 8" rim. 275s won't fit on this car in the front, I tried, and using a staggered setup doesn't work well.

[Areddi]

I got an additional set of stock wheels on the cheap and put 225/45 ZII's on them for autocross. If I weren't on a budget, I probably would have gone with a x8 wheel and a 235 or a 245. That being said, the 225 is fantastic, with loads of grip.

I think anything you pick in the 225-245 range is going to be good. Skinnier is lighter and heats up faster, but has a slightly smaller contact patch. Does it really matter for the average weekend racer? Probably not much.

[GTM_Challenge]

We will be trying many different combinations in the coming season. Unfortunately, they will all be at different times of the season, so ambient as well as track temperatures will be vastly different throughout the year.

In my Miata track experience, I've found a 225 on a 15x9 to be the fastest setup on a course that relies heavily on handling vs. power. On power courses, a 205 is faster than a 225 on a Miata.

My stock miata on 225 RS3s and 15x9 and coilovers (all else stock with full interior, A/C, etc.) ran a half second slower than the Spec Miata record at Gingerman.

Some drivers rely more heavily on a tires grip than others do. For instance in the s2000s, myself and my friend both agree that my driving style tends to take advantage of tire grip more than his does. We're both fast and he won two national titles this year with NASA. But our driving styles are vastly different with mine relying more on the grip of the tires - which is also more dangerous at the limit.

[SubieNate]

Paging @robispec, @CSG Mike, and @CSG David.

[CSG Mike]

Easy answer.

AutoX focuses primarily on low speed turns and agility. A wider tire facilitates faster turning.

The faster turning from a wider tires also translates onto a track. HOWEVER, the wider tire also introduces more rolling resistance as well as more rotational inertia. At every road course we've been on, the additional power put to the ground from having a skinnier tire has been more than worth the loss in raw cornering speed.

In autoX, you don't spend nearly as much time accelerating as you do on a track.

We started with skinny tires, and worked our way up, and found 225 to generally be fastest for us. Robispec also started with wider tires, and went even wider, before going narrower.
@Austin: You have to tiptoe around the rear end and putting down power on a s2k (more so on my s2k than yours), which is why the S2k benefits from a wider tire. It also has a fatter powerband, and SIGNIFICANTLY more aggressive gearing.

For everyone else: 4th gear in the BRZ has the approximately the same mechanical torque output as 6th gear on the s2k (6th on the s2k is slightly less)

[simpleisbest]

Originally Posted by dwx

I use 245/40s on 17x7 rims, that's the best tire size to use for this car for "stock" autocross class.

245/40 street tire? If so, what brand? Thanks!

[xwd]

Originally Posted by simpleisbest

245/40 street tire? If so, what brand? Thanks!

They are Hoosier race tires, not street tires. People run wider tires in autocross because it increases grip. Guys used to run 275/35/15 Hoosiers on ITR's 6" rim in the front and as a result could generate enough front end grip to lift a rear tire in the air. Those tires really became difference makers when courses had long sweepers, in the transition stuff the 225s were better. That's a very unique case using very grippy stiff sidewall autocross tires though.

[robispec]

Originally Posted by renfield90

I'm going to guess the track day guys are running with NASA - they are probably either doing HPDE (which is non-competitive, no "need" to spring for a bigger/pricier tire) or they're doing TT in which case the points system will assign points for going over a certain size tire.

All other things being equal, I don't know anyone who would turn down an extra 20mm of tire. The extra grip more than outweighs the miniscule weight addition (we're talking a couple pounds here).

Time attack or fastest lap has different requirements..from road racing. Wider doesnt have any more grip/surface/psi pressure. area what it does have is the ability to spread/manage heat. Thats why for one fast lap as long as you dont overheat the tire in one lap SMALLER IS FASTER.
That being said this chassis "uses" tires better than almost any chassis I have worked on.