My suspension is set up where I use 100% of the bump travel available before the tire hits metal on the inner fender both front and rear using 225/45/15. I can squeak by with a 225/50/15 if I accept a bit of rubbing. a 235/50/15 aunt going to work right without some more modifications like limiting bump travel or tubbing out the fender wells.

I have my car setup and run 275/35/15’s for autocross with a slight bit of rubbing on the inboard side so I can make big width fit but the big thing is diameter needs to be really close to stock to work on a properly set up Miata that retains original sheet metal inner fender. This narrows the tire selection pool a lot. Need like a 40 aspect ratio or less to go wider.

Bob

 

Tested myself Back to Back with the same tires mounted on a track. Better feel at the limit and more grip mounted on the 9", dropped a tad over a second.

Now if your only conscerned about the street, the lightest wheels you can get makes the car feel zippy and the narrower rims give a smoother ride.

Bob

 

Stopwatches don't lie, dude.

 

I've driven a 225 and a 205 on an 8 and the 205 felt much better. The slight stretch is how its supposed to be done and you can see that executed on any and every big-budget racecar. The 225 on the 9 feels incredible and makes the car feel precise.

 

I went from low tread (but still fast) ra1s on 15x8s to new nt01s on 15x9s. The 9s were noticeably grippier. Not the most controlled experiment, but enough to convince me that 9s are faster and now I don't bother using the 8s on the track anymore.

 

Same here. I switched from 15x8 to 15x9 this season and they are much nicer, faster turn-in and more grip.

 

The biggest thing I noticed was turn-in crispness. The 9s have so much more feel and feedback than the 8s did.

 

I feel like the guy with a small ***** in a gang bang scene.

225 on my 7" wheels were much gripper than the 185 stock tires obviously, but it was noticeably squishier, if that makes sense. It lost some of it's immediate turn in crispness.

 

I codrove a big-boost M45 at an autocross like 3 weeks ago, and since I'm an elitist ***** we swapped his 205/55-14 NT-01s on 14x6s for a set of 9s and 225s. His comment after his first run was just "wow".

 

Back when i ran 99 wheels with 205 NT-01's it was pretty easy to put that tire on the dime. I had a revellation after about 4-track days on the 9's....I wasn't using the tire. The limit on 9's compared to skinnies is so far out that I couldn't really comprehend the limit. You I have to really scare the **** out of myself to "use" the 9's.

 

The big difference that I notice between an 8” and a 9” with the 225’s is like in say a 90mph sweeping turn. Where your sphincter is puckered to begin with. Even worse on a 7”. I think 205 is as wide as should be mounted on a 7”.

With the narrower wheel you feel the tires outer edge kind of roll under and dig in with the slip angle changing less with cornering force changes as you near the traction limit then as you go over the limit it kind of pops loose and when you bring it back in it snaps back. The behavior is more non linear. With the wider wheels the slip angle to cornering force relationship near the traction is much more linear and you can drive more smoothly at and even slightly over the limit. The increase in ability to get everything out of the tire is as much to do with the lower lap times as the increase in grip levels.

Bob

 

Got sidewall stability?
I'd imagine it's the same difference I felt between a 235/40 and 215-40 tires on a 17x7.5 wheel. Though the 235 felt like it had more available grip, steering input and at the limit driving felt a bit more muted.

 

My suspension setup won't work with tires that big of diameter. Id have to remove at least 1/2" of bump travel probably 3/4".

Bob

 

on a 85 sec race track, gained a consistent 1.5-2 sec going from a 15x7 to 15X9. everything else constant. 225 R888's.

 

That's one of the nice things about setting up the Catfish; our "stock" setup is 245/35's on 17x9's. Although there's limited rubber at that size (Toyo R1R), the word that does come to mind is "wow" after you've had a chance to sample the cornering grip. Tires are at 20psi.

 

20psi hot? car weight?

 

20psi cold with a 1,550lb car. I'm pretty confident 18-19psi would be even better for track days.

The Thunderhill car weighed more than this with a full cage and started at 26psi. They quickly found out that the car was using only a fraction of its tire. I think they carefully dropped 5-6psi by the end of the race, which is a huge margin.

We ran the car with a naked frame through an autocross course with some 949 15x9's, and the tire tread did not start to roll to the edge until 21psi, with national champion auto crossers driving. The vid is on the website.

It's takes some talent and patience to be able to make suspension, spring rates, camber, sway bars and tire pressure all work together.

 

We do this thread every six months. You want a slight stretch, makes the car feel great and handle sharply. There is a reason this is done on real racecars.

 

All I've heard so far is, wider wheels are faster! Well, they are. The slight stretch increases cornering stiffness of the tire. If you look at the tire slip angle curve below (slip angle on x-axis, lateral force on y-axis), there is the region that is linear (from 0 to a some small slip angle). Being able to see the values on the graph doesn't matter, because depending on the tire size/compound/etc could range from 2 degrees to 13. Cornering stiffness measures the slope of that linear portion as it crosses the graphs origin. Increasing cornering stiffness may not increase peak force (though sometimes it does!), but more importantly it makes the tire build force faster (increasing response), and overall nets a greater area under the curve.

 

Wow