What tire.. Width?

With tires being the second best upgrade (tightening the nut behind the wheel being best) how does one decide on width? I ask this in the hope that when someone is searching for race tire information this boards sales would be of help.

I know the easy answer is, that it depends. Yes, but on what? Regardless of compound, budget or rule mandates ( Which seems to be the main focus of "what tire"treads), is widest best? Is it the average speed on course? Length of the event? Weight of the car? Aerodynamic considerations?

"And remember this is for posterity; so please be honest."

I convert the wheel width to mm's and try to keep the tire width close to that. This is for general use, for competition use there may be some advantages to deviating.

Rules first, they usually dictate wheel width instead of tire width, then the physical constraints of the car body. If you're looking for lap times then wider is (almost) always faster. The odd case being super sticky race tires on very low power cars. Keeping in mind that a ton of grip is likely to lead to chassis/suspension changes. If you're talking road tires then the world is different. Or can be. Super wide tires will sometimes grab at imperfections in the road making driving a tiring task. Depending on the car, wide tires often mean larger diameter wheels which reduces side wall which can reduce compliance and comfort. 

So yes, it depends. 

Unfortunately, it depends. That is FAR too broad of a question to get any kind of answer, because everything mentioned does impact the choice. Even the spring rate you're running.

I dunno, if I can find a 12" wheel that would fit I'd put a 275/60 on it....

Great thinkers in all disciplines seek the single unifying answer. I will take the cowards way out and the answer is five, sort of a Hitch Hikers Guide to the Galaxy way out.

I race two race cars.

 Tires for the Formula 500 basically come in one size from Hoosier, there are two compounds and most people including myself use the softest compound.

Tires for the Datsun come in multiple sizes. The car is underpowered  so I use narrowest tire possible because the narrow tire has less rolling resistance, less circumference (lower gearing) and weighs less. All of that helps with acceleration. 

Now when I say narrowest possible the operative word is possible. For example I've run a 185 width tire (Showroom Stock C Miata) in qualifying but then used a 195 for the race. The narrow tire was only faster for a few laps, after that it overheated.

The type of event will factor in as well; for autocross, were even relatively low powered cars can get wheel spin I might opt for a wider tire. I do this on the F500 but for the front tires, the car has a kart axle (no diff) so they tend to understeer. For road racing F500s use a 6" front tire and 7.5" rear but for autocross you use 7.5 fronts and rears to combat the oversteer. Back to why I said "might use" a wider tire for autocross. The narrow tire will heat up faster and actually get up to operating temp sooner which could net a faster time. Again you'd need to test.

I picked up running as little tire as possible while race 125cc GP bikes, the bike would be way more settled on a wider tire but slower. As a rider you simply cope with the bike being really edgy (at least if your fast). 

For trackdays for an average track day driver I'd likely recommend / err on the side of a slightly wider (10mm) tire; your not racing and the extra 2-3 tenths won't matter, the cooler tire would likely last longer and the car would be a little easier to drive.  Really edgy set ups tend to be fast but they won't be fun for most drivers, you want user friendly at a track day. 

My typical automotive formula is to add ~15mm of stickier tire, better brake pads, lower ~25mm, and a dash of as much cheap hp is handy.  Combine and beat vigorously or until check engine light warms to red.

If you run in the Street class in The SCCA, Then the width question is easy to answer.

In reply to Tom1200 :

Good points all but you failed to mention the track.  Elkhart Lake is a 4 mile long track with three seriously long straights.  
Because of that I will trade Dunlop's 6:50's for 6:00's  ( 5.3 inch wide tread versus 5.1 wide)   The cornering speed loss is more than made up for by the straight line speed gain. 
Brainerd is a 3.1 mile track with one mile &1/2 long "straight"  ( actually it has a banked turn that if you're up to it can be taken flat out ) yes it does require a big pair of brass clankers.  Plus one 1/2 mile straight. It too rewards lower drag over cornering power. 

Short answer: testing required.

Grip vs drag. Heat capacity vs vehicles ability to impart heat. Track layout and weather conditions play a part too.

It's a simple question that doesn't have a simple correct answer, lots of nuance.

The rolling resistance gains on most of the cars I've run (100-120hp cars) are in the neighborhood of 2-3 tenths. Over a 10 lap club race that's a lot. 

I've driven a couple of 750hp cars and I'm pretty sure rolling resistance isn't an issue for them.

I touched on it a little bit but the driver is really going to matter, the driver has to be comfortable with the car to get the most out of it lap after lap.

Assuming you had your choice of any wheel width and clearance, the answer would be the proper width that gives you the fore/aft traction you need while maintaining the side-to-side traction you need.

As a very general rule, the size of the contact area doesn't change much.  Skinny tires have a long, skinny longitudinal contact area, and wide tires have a wide contact area.  A wide contact area tends to break traction under power and braking easier than a long skinny one... to a degree.  Conversely, a wide contact area is generally more stable side to side in the twisties.

Another HUGE factor is sidewall height.  My Impala SS always had somewhere between 245mm and 255mm section widths.  I had 245/35-20 and I also had 255/50-17.  While not an apples-to-apples comparison (the 17s were Kumho ASX and the 20s were Pirelli P-zero neros) The 20s were terrible at everything.  It was very hard to get forward traction, and (although possibly offering more peak lateral grip) the difference between sticking and "holy E36 M3 I'm headed for that wall" was nearly undetectable.  Taller sidewalls tend to offer more progressive loss of lateral traction for people like me who are noobs.  I'm sure the Stig could have shaved off a few tenths on a lap with the 20s, but I would have hit so many cones and been butt-first too many times to count.

I am not limited by class but by what fits so I go as big as possible. Whatever I can do to get the most grip that doesn't go away. My racecar takes some weird size/offset wheels so currently I'm limited to a 225/50r16. I would like to step up to a bigger wheel and wider tire but that would require custom wheels, or adapters and I'm not spending that kinda money right now.

Thanks to all that have responded. Feel free to add. As some who likes to research thoroughly, I hope this helps when the time comes.

This came about because of the "cheap brake" thread. And remembering something from a "tuner shootout". I believe it was John Heinracy(sp?) saying he won because the other cars had too much tire and brakes. He was driving a Chevy Cobalt, I think. Coupled with braking that heats your tires, more than swerving all over the track.

What about a square set up over staggered?

BarryNorman said:

 

What about a square set up over staggered?

Whats your tire loading scenario?

Rear heavy car with suspension set up to load the rears?   should work well

Front heavy musclecar with little weight in the back?  uhhh goofy and testing needed. I suppose if you have oodles of power and drive with the throttle you could make it work for you as you would be loading them. That said, off throttle understeer city. 

Again, you ask a one sentence question that a proper answer would be a book. 

It does depend on a few things, but in general wider = better.

Read up on Terry Fair building up his 2011 Mustang GT into a formidable track car that set more than a dozen track records out in Texas.  It started off not too impressive, then he realized that wider tires were always better.  The gain in overall grip always resulted in lower lap times, even on tracks with long straights.  His final tire setup was a 335/18F 345/18R.  What he learned on that car he's applied to all of his race cars, and basically just makes sure he stuffs the widest tire physically possible on every race car he has.  And it always works well.

BarryNorman said:

I believe it was John Heinracy(sp?) saying he won because the other cars had too much tire and brakes. He was driving a Chevy Cobalt, I think. Coupled with braking that heats your tires, more than swerving all over the track.

What about a square set up over staggered?

A stock powered Cobalt (not SS) is likely to be one of the outlier cases where you can over-sticky a car. Hard to say what the too much brakes thing is other than rotating mass and maybe needing to run bigger/heavier wheels to deal with bigger brakes. They did fairly extensive testing with the B-spec race car field when that was up and coming and figured out that they could put too sticky a tire on the car for long tracks. Again, an outlier case and if you're not literally the slowest car out there it's unlikely to apply to you. 

Square vs. staggered has a lot of variables. My advice to people is generally to run square if it's what your car came with, especially while learning, if for no other reason it lets you rotate tires front to back for longer life. That saves money and saving money gets you more track time and it's track time that makes you faster. 

I've never owned a car powerful enough for staggered to make sense :(

Apexcarver said:

 

Again, you ask a one sentence question that a proper answer would be a book. 

Winner winner chicken dinner

This is the correct answer for this thread: YES.

Curtis73 (Forum Supporter) said: .. A wide contact area tends to break traction under power and braking easier than a long skinny one... to a degree.  Conversely, a wide contact area is generally more stable side to side in the twisties...

I question this.  While I've never done a scientiffic test this is inconsistent with my experience and I can't wrap my head around why this might be the case.  I would think that all else being equal (which is unlikely to be the case) it woudn't matter which way the contact patch was oriented.

ProDarwin said:

I've never owned a car powerful enough for staggered to make sense :(

I have and I still bought track wheels that reduced it back to square because $$ and because it made the most sense for One Lap. 

BarryNorman said:

What about a square set up over staggered?

I really like the RT615k for being a non-directional tire. It's not as fast or quite as durable as other tires, but it lets me move any wheel to any other place on the car. That's especially nice when you race a track that kills one tire(CMP front left) but you want to have the ability to replace a different tire just in case.

So in my unique case I like a square, non-directional setup.

If you are not sure about the tire size specifications, you should probably buy new tires the same size as the old tires you are using.