Tire rack test: effect of wheel width on lap times

[Obioban]

Not the result I would have predicted. Turns out stretched is fast.

[terra]

Huh. The more you know. I’ve already been tempted to downsize from my 245/275 setup to 225/255 or 235/265 next time I’m due for tires.

Would also be curious to see some testing on different wheel diameters. Perhaps that smaller sidewall would actually be beneficial.

[usdmej]

this seems more a case of people "over tiring" their wheels more than anything else

a 245 tire on a 9" wheel is hardly "stretched"

[Tbonem3]

Stance bois grinning ear to ear rn

[Obioban]

Huh. The more you know. I’ve already been tempted to downsize from my 245/275 setup to 225/255 or 235/265 next time I’m due for tires.

Would also be curious to see some testing on different wheel diameters. Perhaps that smaller sidewall would actually be beneficial.

https://www.caranddriver.com/features/a15130598/upsized-wheels-tires/

[bigjae46]

Slightly stretched supposedly optimizes the contact patch.

But here's my theory. A narrower tire allows the tire to get into its optimal range of slip angle which ultimately yields more grip. From my observations as an instructor, many drivers correct when the tires are starting to get to its optimal slip angle. The best way I can describe the feeling is the car will feel loose and about to lose control. But there is a little more and this is when being smooth on the controls comes into play. If I haven't driven for a month or so, it takes me a session to get comfortable with that sensation again. Then I'll spend the rest of the first day smoothing out my inputs so I can get closer to the optimal slip angle for longer. The goal is by Sun I have far fewer steering and throttle corrections through the turns.

A wider tire will give a more planted and confident feeling but it won't have that upper end grip that you get when the tire is slipping - that's where the time difference is. So more so with novice drivers, they like the wider tire because there's more average grip and aren't really wanting to or able to optimize the tire's maximum grip that comes with generating slip in the tire.

[Sergmann]

My previous track car was a turbo Miata. I ran 205 width tires on a 9" wheel. This is standard miata stuff as there has been testing and lap time confirmation that its faster with some stretch. When I got the M and began a wheel hunt my gut was to run 275's on a 10.5" wheel. I had a thread on the m3forum asking about any experience with wider wheels. It was generally believed that a 9.5" wheel was optimal for a 275 tire on our cars. I never went any further with it. Basically to the miata crowd this has been known for years.

[bigjae46]

My previous track car was a turbo Miata. I ran 205 width tires on a 9" wheel. This is standard miata stuff as there has been testing and lap time confirmation that it’s faster with some stretch. When I got the M and began a wheel hunt my gut was to run 275's on a 10.5" wheel. I had a thread on the m3forum asking about any experience with wider wheels. It was generally believed that a 9.5" wheel was optimal for a 275 tire on our cars. I never went any further with it. Basically to the miata crowd this has been known for years.

I run a 275/40R17 on a 10” wheel. It’s about neutral. I think a 9.5” wheel would be a little narrow.

[lcrain]

I found this to be an interesting test. I understand the merits of testing a setup to the fastest lap time but wonder which would produce the faster average lap time over an extended session (like a 30 min race). I also wonder if the same applies for super grippy tires, like a slick or a hoosier, rather than a 200tw street tire.

I have been running 315 hoosier a7 on an 18x11 with the goal of having the grippiest tire afforded by my class and enough surface area as to not overheat them. A skinnier a7 might yield a faster single lap time but in my experience they won't last more than a few hot laps before melting.

I am glad tire rack does tests like this, pretty cool stuff. Maybe one day I will buy a set of skinnier a7, like a 285 or 295 and run them on the 18x11 to see for myself.

[bigjae46]

I found this to be an interesting test. I understand the merits of testing a setup to the fastest lap time but wonder which would produce the faster average lap time over an extended session (like a 30 min race). I also wonder if the same applies for super grippy tires, like a slick or a hoosier, rather than a 200tw street tire.

I have been running 315 hoosier a7 on an 18x11 with the goal of having the grippiest tire afforded by my class and enough surface area as to not overheat them. A skinnier a7 might yield a faster single lap time but in my experience they won't last more than a few hot laps before melting.

I am glad tire rack does tests like this, pretty cool stuff. Maybe one day I will buy a set of skinnier a7, like a 285 or 295 and run them on the 18x11 to see for myself.

I'm sure thermal capacity is the tradeoff.

I tried a 245/40R17 BFG R1 which was stretched onto a 10" wheel. It wasn't civic territory but the tread width was a little narrower than the wheel. I was hoping that I could run a 3.73 diff and these smaller diameter tires and get to essentially a 3.91 with 275/40R17s, significantly shorter gearing.

It definitely felt faster on acceleration but the tires would fall off pretty quickly. The rears went pretty quickly and then the front fell off. I'm sure the smaller OD of the tire contributed in addition to the stretch to the degradation. Likely there is a sweet spot depending on the usage. The 275/40R17s have never really greased up like that but I also run lower pressures than most - 23psi cold/30psi hot. Summer, I'm down to about 21psi cold - 29 psi hot. Not sure what I was running with the 245s, typically shoot for 30psi hot as a starting point.

[Tbonem3]

I run a 275/40R17 on a 10” wheel. It’s about neutral. I think a 9.5” wheel would be a little narrow.

Agreed, I don't ever remember 275 being optimal for 9.5", but rather 10", and usable on 10.5". 265 on 9.5" is what almost everyone runs. The popular ps4s (street), and many 200tw tires run wide I believe, so you can get away even more with a size down (i.e., 265 on 10" and 275 on 10.5")

[///Mangler]

isn't it as simple as the tires on wheels with a little 'stretch' just make it a stiffer side wall, vs the same tire on a thinner wheel making a softer side wall?


T

[elbert]

isn't it as simple as the tires on wheels with a little 'stretch' just make it a stiffer side wall, vs the same tire on a thinner wheel making a softer side wall?

The wheel width also affects the shape of the contact patch.

[Obioban]

I'm sure thermal capacity is the tradeoff.

I tried a 245/40R17 BFG R1 which was stretched onto a 10" wheel. It wasn't civic territory but the tread width was a little narrower than the wheel. I was hoping that I could run a 3.73 diff and these smaller diameter tires and get to essentially a 3.91 with 275/40R17s, significantly shorter gearing.

It definitely felt faster on acceleration but the tires would fall off pretty quickly. The rears went pretty quickly and then the front fell off. I'm sure the smaller OD of the tire contributed in addition to the stretch to the degradation. Likely there is a sweet spot depending on the usage. The 275/40R17s have never really greased up like that but I also run lower pressures than most - 23psi cold/30psi hot. Summer, I'm down to about 21psi cold - 29 psi hot. Not sure what I was running with the 245s, typically shoot for 30psi hot as a starting point.

100% yes-- wider tires have more thermal capacity, so for a track session the results of this test are far less meaningful than for one hot lap.

I just thought it was interesting that it is faster over the course of one lap.

... though you could go with a wider wheel to get both :P

Not gonna lie-- I've long since not selected my track tires by lap time. I select them by price, life span, not overheating, and ease of fitment.

[bigjae46]

Not gonna lie-- I've long since not selected my track tires by lap time. I select them by price, life span, not overheating, and ease of fitment.

I've ran NT01s and Toyo RRs for the last 11 years for those reasons. I love the challenge of setting lap times comparable to others with grippier tires.