Tire Width?
 

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...

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).

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.

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.

CSGmike also runs slim tires.

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

I hope they can chime in.

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

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.

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.

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.

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

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)

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.

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.

Maybe I'm misunderstanding you, but geometry and physics say you're wrong - a wider tire has more surface area, and assuming the coefficient of friction remains the same, will generate more grip at optimal operating temperature.

What you say is absolutely true for a competition where you have maybe one warm up lap - the skinnier tire will be in its happy place and the wider tire...not so much. What I would then suggest is a wide tire with a softer tire compound (i.e. with a lower operating temperature range). Depending on tire rules for your series, this may be difficult or even impossible to accomplish.

more surface area = lower presure/area = lower friction at = coefficient of friction.
and the coefficient of friction term is also dimentionless when solved for...Ever wonder how a good bicycle racer going downhill in the tight twisty mountain roads on LESS THAN TWO SQUARE INCHES of contact patch can pull away from the motorcycles and other suport vehicles? Paul Handley "the racing and high performance tire" explains this fact in great detail...area controls heat it is the only method to do this. Heat builds up over time...

Im running 245 RS3's on a set of wheels and 225 BF goodrich R comps on another set. I would not go wider than 235 for a bolton on car.

It's all about finding the right balance for the fastest lap time, and right now, that balance happens to be at a relatively skinny tire.

Have you guys tested 215 street tires on 17x8 rims? I'm curious if going down one more size will decrease your lap times even more. :popcorn:

Yup. ;)

We've been juggling 215, 225, and 235 tires lately.

Im Trying 205 Stickies this weekend...

Sat or Sun? I may be there for the other class we discussed :D

Both days instructing.

both days!

gahhhhhh okay.

If the friction coefficient was constant, a 165 wide tire would have the same grip as a 345 wide tire. But the friction coefficient is obviously dependent on the surface pressure (weight of the car divided by the contact patch) and the temperature reached with that surface pressure. (And anything below or above the optimal temperature reduces the friction coefficient.)

At the Autocrosses I participate almost all people in my class run on narrower tires than I (235) (mostly Honda Integra DC2 and Renault Clio RS/Sport).
All races this year have been pretty cold and even wet and they were all won by people on DC2's on just 205 and 195 wide tires. People here run on Toyo R888 or Yokohama A048 (street tire class). I think that these type of tires need possibly more surface pressure in order to obtain enough heat (than the ones people typically use in the US or maybe people racing in the US are simply exposed to much warmer weather).

Also, I talked to a guy with a Celica GT-Four who won in his class and he ran on 225 wide tires with a tire pressure of only 27 psi (cold).

As a autocrosser...and As per the rules for my class, I am allowed a 9 in wheel as well as a tire width up to a 265. I have both 245 and 255 on two different sets of wheels. There are a number official reasons that we choose those sizes...weight, grip, performance and gearing...since most of our stuff is in 2nd gear. So we look for a tire size that will help with that. Additionally, we want fhe widest tire so it has the most lateral grip, so in the case of autocross, the heavy transitions on a wider tire help with heat management...as a thinner tire will heat up more, loosing valuable grip..this is the autocrosser perspective. I don't find that I lose feeling at all, matter of fact its enhanced. This is for a nationally campaigned car in STX.

Bill

That's a good point. Maybe because Autocrosses (in the US) are usually mostly driven in 2nd gear (more torque) and since there are more turns/time, tires are getting hotter and wider tires are an advantage.

robispec, why did you choose to run 225 wide tires on a 17x8 and not a 17x9 rim?

They're lighter :)

At least, that was our reason.

Interesting thread.

I came from the world of autocross where our (my brother and I) main car needed the widest possible tire. Now that I'll be doing HPDE's and the like, I naturally assumed that this need of wide tires would translate to big tracks. Obviously it doesn't and its better for me as skinnier tires tend to be more affordable than wider tires.

I've seen a LOT of autocross guys assume that everything from AutoX carries over to the track, because the general attitude at AutoX is that if you can AutoX, you can do anything.

Now, granted, I've AutoX'd exactly once, but from that experience, I did gather that the only thing that transfers over is your comfort level with the car. Both setups and skillsets are somewhat mutually exclusive.

I've taken some national level autox guys around a track and they freak out at the speeds, while I've ridden with them at an AutoX and was blown away by how quickly an AutoX car transitions.

Very true. Although I would like to say that I know that autox experience doesnt necessarily translate to big track skills, hence why I signed up for the beginner groups in my first two HPDE's.

With autox, I noticed that people tend to be more abrupt and less smooth, but that's because the driver needs the car to rotate and transition quicker. I've been watching big track videos and I'm amazed at the smoothness of the inputs.

i run 235/40/17 on 17x8.5's for autox and the track, might start looking into wider wheels and tires now.

So technically speaking, at the same inflation PSI, tire size only changes the shape of the contact patch right? because physically at 35 psi you need x amount of surface area to react the weight of the car.

So is it the reduction in tire deflection that reduces heat generation when going to a wider tire, or are people reducing inflation pressure to get the same amount of deflection but more usable surface area? Since getting that larger surface area on a smaller tire could result in uneven wear and strange feel because the tire is underinflated?

Cheers
Nathan

There is a saying: A "Good" autocrosser can make a "Decent" roadracer
However, A "Good" Roadracer doesn't make a "Decent" Autocrosser.. Basically the principles of Autocross translates easier going into Roadracing...and Not the other way around.. :burnrubber:
I personally do HPDE's as well.. but I know from experience there are different aspects to each.. and have to respect the differences. If not you end up in the fence..

Bill

The engineering side of you is coming out again.

The interaction between cornering force causing deflection, sidewall resistance, how the tire pressure supports the sidewall, etc all play together.

I can go on for hours....


In short, stretched = better response, higher risk of debeading, and possibly larger contact patch (with proper setup)

I guess mostly what I'm asking, is if you have a 245 and a 215 on the same car and same PSI, the amount of force being put into each square inch of tire (statically at least) that is contacting the ground is the same. So does the bigger tire get less heat because it needs to change shape less to keep that amount of surface area (Wider, narrower contact patch), does it naturally deflect less because it's wider (more stable shape?), or is there something else going on?

If you were to take a "stamp" of the tire's contact patch, would the actual surface area indeed be the same if both were at 35 psi, or does the construction of the tire and sidewall stiffness come into play as well, making a different size tire with the same weight on it and the same inflation pressure actually have a different contact patch surface area?

Nathan

Sidewall deflection will be dependent on the height of the sidewall, and how it's angled depending on the tire and rim width

Tread deflection may be lessened; it depends on the tread pattern of the two different sizes. The blocks aren't identical on all sizes.

The larger tire has a larger mass to sink away heat with.

Contact patch will vary with alignment (and the above mentioned tire width/rim width)...

Andy Hollis, a multiple national champion autocrosser has found that 225 RS3's and Rivals on 15x9's were the fastest setup on his One Lap CRX and his STC Civic on track.

Also, RPF1's have a 0.3 lb of a difference in weight for 17x8 vs 17x9.