SCCA Solo C Street Discussion

[Kido1986]

40s will have less sidewall, which means less sidewall flex. Also, 40s are close to stock height, 45s are a bit taller than stock, so longer gearing.

[DylanFRS]

Originally Posted by Kido1986

45s are a bit taller than stock, so longer gearing.

May help?

In my mind it seems like the higher gearing might help these cars on certain courses but I do realize that it doesn't always work that way.

Thanks for your input Kido. Not a lot of other people feel like sharing I guess.

[Kido1986]

I thought that for a long time. Nearly bought 18s to run 225/45/18s (would bump 2nd gear to 63.x MPH) until someone did the math over on RRAX and it showed me that I was very wrong.

Plus shorter sidewall is less squirm and more instantaneous responce. The math was done solely a theoretical level and assuming it takes the same time to hit rev limiter on stock size vs 225/45/18s (which it obviously takes longer), the math STILL didn't work out in the taller's favor. Once you through in the longer acceleration times, on the average 60 second course, you're losing well over .25 of a second (the math was done on my Florida State Championship event's run if you care to look up that Sebring video)

[Sccabrz192]

Originally Posted by Kido1986

I thought that for a long time. Nearly bought 18s to run 225/45/18s (would bump 2nd gear to 63.x MPH) until someone did the math over on RRAX and it showed me that I was very wrong.

Plus shorter sidewall is less squirm and more instantaneous responce. The math was done solely a theoretical level and assuming it takes the same time to hit rev limiter on stock size vs 225/45/18s (which it obviously takes longer), the math STILL didn't work out in the taller's favor. Once you through in the longer acceleration times, on the average 60 second course, you're losing well over .25 of a second (the math was done on my Florida State Championship event's run if you care to look up that Sebring video)

One thing about numbers is that you can make them say what you want... I'm not sold just on theoretic gearing alone that a 235/45 is not advantageous compared to other options. The taller 45 sidewall is designed for a 7.5 wheel width, whereas the 40 is designed for an 8, with the sidewall more pinched you are adversely affecting the dynamics of the tire in the shoulder/transition area.

[cy1clown]

I was told once that the biggest savings with a tall gear is if you can avoid an extra shift during the run. Don't know if this is true but did come from a reliable person with much more experience than me.

[piknockout]

It all depends on how much power you have to turn those taller tires/wheels. I went to 18's on my STU STi to get the extra 2mph at the top end and it often kept me from having to shift to 3rd on many occasions. But it has 325wtq and short gearing anyways, acceleration wasn't really hurt.

With the Twins, acceleration is already lacking and going to a taller gear with all of that lack of torque and a torque dip is just going to destroy the acceleration to the point that you won't need third gear...but that's because you can't get up to speed.

We were talking in John's thread and I think that a shorter tire is a better ticket than a taller one and just get used to shifting into 3rd and back down into 2nd. The benefit is that you gain time coming out of every singler corner which I think would more than make up for the occasional third gear shift.

[Sccabrz192]

the shift argument is valid, but is also very course dependent. If you have to shift even with the taller gear, you are no longer in a very beneficial position.

[Sccabrz192]

Originally Posted by piknockout

It all depends on how much power you have to turn those taller tires/wheels. I went to 18's on my STU STi to get the extra 2mph at the top end and it often kept me from having to shift to 3rd on many occasions. But it has 325wtq and short gearing anyways, acceleration wasn't really hurt.

With the Twins, acceleration is already lacking and going to a taller gear with all of that lack of torque and a torque dip is just going to destroy the acceleration to the point that you won't need third gear...but that's because you can't get up to speed.

We were talking in John's thread and I think that a shorter tire is a better ticket than a taller one and just get used to shifting into 3rd and back down into 2nd. The benefit is that you gain time coming out of every singler corner which I think would more than make up for the occasional third gear shift.

This is true, but if you are on a power course against Z cars, you are already at a large disadvantage. If you are on a speed maintenance course with a lot of finesse speed modulation, you may be in a better situation with the taller tire and the acceleration factor is less.

[Kido1986]

There definitely are trade offs and in a world where I am rich, Id have a set of 18x7 +42s made with 225/45/18s for those courses to go along with my 17x7s with 235/40/17s for an acceleration course.

Weighing the pros and cons, I felt the 235/40 would have more advantageous, more often, so I went that route.

Realistically, if I was rich I'd just spend the $9-12k for a full STX prep for this thing where the BRZ stands a better chance hahah. Or just build a killer *SP car out of the most ridiculously stupid car I could find

[Wepeel]

Do C Street twins really run into hitting the limiter that often? Locally I would think it doesn't happen frequently, but I don't know about nationals-style courses.

Like piknockout mentioned, everywhere else on the course where you are accelerating you are (in theory) going to be faster running the shorter tire. It will also keep the RPMs up a little higher per a given cornering speed which would put you in a better position in the torque dip. I'd be really interested in seeing data on how long you need to be hitting the limiter before the taller tire is faster.

[AustinTX]

With the pro season about to start - How are you guys launching the car? I'm starting around 3k with a decent amount of wheelspin and then a torque dip bog once the tires hook up. I tried launching higher, but the wheelspin seems excessive and possibly slower. This is on the 235/40/17's, so I can imagine it would be a bit worse on taller tires.

[Kido1986]

I find myself between 4-4500 on our concrete and better pavement sites. I come off the clutch quickly, spin just a little but keep the RPMs out of the dip

[Biggins]

Originally Posted by Wepeel

Do C Street twins really run into hitting the limiter that often? Locally I would think it doesn't happen frequently, but I don't know about nationals-style courses.

Like piknockout mentioned, everywhere else on the course where you are accelerating you are (in theory) going to be faster running the shorter tire. It will also keep the RPMs up a little higher per a given cornering speed which would put you in a better position in the torque dip. I'd be really interested in seeing data on how long you need to be hitting the limiter before the taller tire is faster.

Last year, I hit the limiter on 7-8 of the 30ish event courses I did running 215/45/17 ZIIs. This includes 2 of 4 National event courses I ran. The problem I ran into was that the car really dies when it hits the limiter rather than at least staying consistent like on other cars I've driven. You can't really "ride the limiter" for another second like I have on previous autocross cars.

As far as launching, 3500-4200 is my preferred range, but I'm never on concrete.

[Sccabrz192]

Originally Posted by Wepeel

Do C Street twins really run into hitting the limiter that often? Locally I would think it doesn't happen frequently, but I don't know about nationals-style courses.

Like piknockout mentioned, everywhere else on the course where you are accelerating you are (in theory) going to be faster running the shorter tire. It will also keep the RPMs up a little higher per a given cornering speed which would put you in a better position in the torque dip. I'd be really interested in seeing data on how long you need to be hitting the limiter before the taller tire is faster.

The few times I ran CS on 245/40 hoosiers I was riding the limiter all the freaking time, without the lateral grip on street tires, we'll see.

Agree with Kido though, everyone just has to pick a starting point and work from there. I chose 235/45, he chose 235/40. I may very well switch as this season gets underway as I learn more. If I had the wallet for it, I'd have a set of everything in every size I can fit on a 16-18" 7" wide wheel, but I don't.