SL vs. XL tires for autocross

Test...don't believe other people.

One thing I've done in the past is to buy two different sized tires in pairs.  Mount F/R pairs.  Run some.  Dismount and switch the other way.  Run some more. 

It will be pretty clear which is better based on the balance change of the car.  And only cost half as much as buying two full sets plus wheels.

Further, since it's FWD you can then just put the less-good setup on the rear.  Won't hurt your times one bit.

mrmattolsen said:

In reply to Andy Hollis :

Thanks, this is a great tip!

Should the SL or XL rating be considered at all for this?

The only time I consider the load rating is when a specific tire is available in both versions for the same size.  And that's kinda rare.

I'm not going to say 'no' but I'd expect it to be pretty far down on the list of importance. If you have two tires that are nominally identical (same model and size) other than the load rating, then absolutely yes. But in this case I'd guess the nearly inch diameter difference between the two 17s, or the less width but shorter sidewall and in between OD of the 18 are much bigger factors than any effect from the load rating itself. Also when comparing across significantly different sizes especially different wheel diameters, I'm not sure there's much information to be extracted from the load rating in itself, but I'm not a tire engineer so I could be off base there.

Keep in mind that the sidewalls aren't stiffer because they support more weight, they are stiffer because they have to hold higher pressures.  I doubt you'll notice much if any difference because you would run the same pressure regardless of SL or XL

Dang, I don't really know what happened and how I deleted my posts lol. But thanks y'all. Very good points and you're making a lot of sense. I'm going to pretty much ignore the load rating, and prioritize the right size for grip and responsiveness.

Yep size should come first. If there were two tires that were identical except for load rating, I'd expect the one with the higher load rating to be slightly faster overall because of the stiffer sidewalls, those do tend to help with cornering grip and feel regardless of pressure.

Interesting related story, I've had tires of the same size and wildly different load ratings on my Samurai, and when "airing down" the difference was massive. While I would run the lower load tires in the low 20s for street or mixed use and air down to around 10-15psi for arena events to get lots of grip in the mud pits etc, on the higher load tires I had to run them in the mid-teens for street or mixed use and air down to something like 4-8psi to get the same kind of compliance, low enough that if I wasn't running a squeezed setup I would've had to seriously worry about debeading.

In reply to GameboyRMH :

Wouldn't stiffer sidewalls be slower, all else kept the same?  More rolling resistance.

I know, I know... how long IS that piece of string, anyway?

Pete. (l33t FS) said:

In reply to GameboyRMH :

Wouldn't stiffer sidewalls be slower, all else kept the same?  More rolling resistance.

I know, I know... how long IS that piece of string, anyway?

On a straight enough course, maybe.  But once cornering behavior is factored in, maybe not.  And once pressure is optimized for each tire, how much difference does sidewall stiffness alone actually make for rolling resistance? 

In reply to Pete. (l33t FS) :

Maybe, but they'd also deform less. More importantly for autox, stiffer tires usually means better transient response, which is super important. However some of the best tires feel pretty squishy to me. It probably just comes down to what works best with a particular car setup and maybe even driver preference.