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drag racing the short bus
 
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17-inch Rota wheels - some questions about going up two sizes from 15-inch Fuchs...

I know there are some owners of these wheels out there. I started reading a couple newer threads, and must say, they are very nice wheels. I just have a few questions that hopefully I can get answers to in regard to increasing my wheel/tire size from 15 x7 and 8 Fuchs with 205-50-15/225-50-15 tires, to 17x7.5/17x9 with 225-45-17/255-40-17 tires. My car has SC/Carrera flares.

1) After mounting 17-inch wheels and tires from my previous 15-inch wheels/tires, will I experience a decrease in straight line acceleration? What differences will I notice in my gearing?

2) I run a small ring and pinion (7.31). Should there be a concern that the larger wheel/tires might stress the 7.31 ring and pinion?

3) How might 17-inch wheels/tires effect my braking? Can my brakes handle them? Will my stopping power increase or decrease because of the increased tire widths?

4) Aside from camber changes - if needed - would I have to perform any other suspension modifications because of the larger wheels and tires? Should I reinforce the sway bar mounts? Recalibrate the shocks? I imagine my ride will be more taunt, which I'm not at all opposed to.

Thanks for any help that can be provided toward these questions.
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Old 10-23-2007, 10:29 AM
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What I've noticed in going this same route (used to run 225/50-15s in the rear and now have 275/40-17s) is primarily two things.

1. The increase in diameter will alter the gearing slightly
2. The increased rotational moment of inertia will rear it's ugly head

Both of these conspired to make my car feel more sluggish. The benefits were greatly increased traction. All in all, the larger tires and wheels resulted in lower lap times, so it was a good thing to do.

The difference in gearing is directly proportional to the change in diameter of the tires you choose. Within reason, I would not expect the ring and pinion loads to vary enough to cause a concern unless the tire size you chose was much larger than 25 inches or so (which, obviously a 255/40-17 isn't).

As for braking, I didn't notice a real change in performance other than the increased traction. I have stock Carrera brakes on my car and they are fine.
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Old 10-23-2007, 10:51 AM
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dd--
I didn't notice what wheel you are now using, but I assume they're OEM Fuchs. I went from 16's to 17's on my 87. Was running 205/225's x 55/50 and went to 205x50x17/ 255x40x17 (not Rota's but I've ordered a set as a spare). 225's on the front would not fit my front with wheels and I'll maybe try some 215s with Rota's although 225's seem to fit fine. Since the rear diameter was the nearly same I didn't notice any decrease in acceleration. With the wider profile, the butt seems to really stick better. With the lower profile front, less sidewall flex seems to make the front stick better as well. Moreso with 215s/225s I would guess. Turn in was slightly heavier, but once the car took a "set" it was more planted.

FWIW here's a convenient conversion calculator from the Miata race site so you can compare diameters from old to new/front to rear. http://www.miata.net/garage/tirecalc.html I noticed the difference is the 255's are an inch or so taller so it may affect you some.

As far as increased loads on your diff I wouldn't worry about that unless you're doing a bunch of drg race starts...the 255 meat on the rear seems to eleminate wheel spin on agressive starts but more importantly coming off slow speed turns.

Like IROC I've had no problems with brakes or braking...in fact it seems to have improved with more meat/lower sidewalls. I didn't have to make any camber changes for clearance or handling. I think whatever tradeoffs you might, have the gains you'll see, if for no other reason more tire options in 17", will be well worth it.

IROC--
I love your car...first time I saw it was on the GOGAR/BHT site. I'm in Miss and would love to see it in the flesh as I pass thru b'ham quite often. You planning to run any events at Little Talladega in the next few months?

Last edited by Dueller; 10-23-2007 at 12:02 PM..
Old 10-23-2007, 11:51 AM
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Yes, one reason I'm considering this upgrade is because of wheelspin. Sucks when the meat can't keep up with the horsepower, but there it is.

In the rear, I'm going from 15x8-inch Fuchs to 17x9-inch something or the other - most probably Rotas. The rear tires will increase from 225-50-15 to 255-40-17.
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Old 10-23-2007, 12:09 PM
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Here's a comparison of some different tires, the spec trans is an abortion 915/44 w/ 7:31 instead of 8:31 but you get the idea of what happens when different tires are used.
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Old 10-23-2007, 12:11 PM
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Quote:
Originally Posted by Dueller View Post
IROC--
I love your car...first time I saw it was on the GOGAR/BHT site. I'm in Miss and would love to see it in the flesh as I pass thru b'ham quite often. You planning to run any events at Little Talladega in the next few months?
If somebody will organize one - I'll be there! I was going to try and do to the Smoky Mtn Region's event there last weekend, but was out of town on vacation. I've been going to TGPR since 1993 and still love it...
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Old 10-23-2007, 12:22 PM
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Quote:
Originally Posted by dd74 View Post
Yes, one reason I'm considering this upgrade is because of wheelspin. Sucks when the meat can't keep up with the horsepower, but there it is. ...
Heavier wheels certainly will take more power to get the extra weight spinning.

Have you considered the weight distribution of the car? .. how about stickier tires?
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Last edited by island911; 10-23-2007 at 01:04 PM..
Old 10-23-2007, 12:27 PM
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also worth mentioning; stiffer t-bars don't help the back hook-up. (does it have over-sized t-bars?)
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Old 10-23-2007, 01:07 PM
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I went up 3 wheel sizes, from a 15 to an 18. All I can say is why did I wait so long, what a difference! The tire hieght only increased 1" so speed through increased effective gearing ratio wasn't noticeably altered (at my power level).
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Old 10-23-2007, 02:44 PM
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drag racing the short bus
 
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Quote:
Originally Posted by island911 View Post
Heavier wheels certainly will take more power to get the extra weight spinning.
Interesting you say this. Tyson Schmidt stuck 17-inch Lindsays on his old car, and perceived, as I remember, very little power loss. Who's to say? Some can't tell the difference between 20 or 30 horsepower difference. Still, for me, it is a concern.

Quote:
Originally Posted by island911 View Post
Have you considered the weight distribution of the car? .. how about stickier tires?
Weight distribution interests me. But if anything, as most of the weight is in the back, I would think the 911 is already engineered for as much rear-bias traction as possible. Tires, yes, I've considered them. I've even considered 45 series rear tires in the back, but those would be more prone to spin than 50 series AFAIK. Stickier tires are an option, though I'd really hate to burn up race-track capable street tires every six months. I'm still not sure about an limited slip transaxle, and how that might help against wheel spin. Maybe someone can chime in on that one.

Quote:
Originally Posted by island911 View Post
also worth mentioning; stiffer t-bars don't help the back hook-up. (does it have over-sized t-bars?)
My rears are 28mm. My car's handling is very sharp, so I'm not going to alter the torsions any time soon.
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Old 10-23-2007, 03:41 PM
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i went from 16s (7/8) to 17 (8/9.5). Was running 225/50/16 and 245/45/16, now 235/45/17 and 255/40/17 (RA-1s). I can't really comment on the quickness as we did a bunch of other stuff at the same time (t-bars, etc). I don't think the diameters are that different, and I'm not getting a ton more rubber on the ground. *But* the rubber I'm using is more stretched on the rim and that I think does help significantly, at least with the RA-1s. I always felt like 225 on a 7" rim had too much flex (same with 245 on an 8). The 255 on the 9.5 seems to be stiffer and more predictable.

I can say that my lap times are significantly lower now, but too many variables have changed including suspension, weight, and most importantly, the nut behind the wheel seems to have gotten a little smoother.
Old 10-23-2007, 11:01 PM
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what do bigger wheels/tires do to lap times? -worth a read.
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Old 10-23-2007, 11:55 PM
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Thanks, guys. Good information so far.

Glenn - you're right. That is an interesting read. Thank you.
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Old 10-24-2007, 08:40 AM
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I have basically the same question - but specifically for AX'ing. I recently bought some 7x8x15 fuchs and planned to use them as dedicated AX wheels. BUT, I'm limited to 225s front and rear for any AX tire now as 15" tire selection dwindles. I've done 4 AXs so far, and plan to do 12 or so next year as I am hooked! I won't be doing track events, just AX.

Question: specifically for AX, will the weight advantage (lightest rotational mass) and small diameter (gearing) of 7x8x15 Fuchs with 225s be "faster" on a AX course than, say the 7.5x9x17 Rota Fox wheels (heavier and taller) but with wider tires with 225-45-17/255-40-17 255 (and better tire selection)....

what say ye?
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Old 11-04-2007, 05:14 AM
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What would be better on an AX track, a go-kart or a 4x4?

The only "performance" reason to go to a taller and wider tire is because the soft rubber (needed for max grip) is overheating.
That is, the faster you go (w/ max load) the faster energy is put into the tire. A wider taller tire has many disadvantages, as you know (add 'bad scrub-radius' to the list). The advantage is in thermal management on long fast tracks ...but even there, aero concerns start to push back.

AX, unlike an hour+ long race, is not a "steady-state" high energy event. ...it is a relatively slow blast for a few minutes and done.
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Last edited by island911; 11-04-2007 at 06:44 AM..
Old 11-04-2007, 06:35 AM
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pardon if i'm too dense, but to clarify, are you thinking the 7x8x15 fuchs with 225s will be better AX wheels/tire setup than the bigger/heavier rotas with wider tires??

just wanted to know if anyone would expect one to be likely faster in
AX than the other with a 2650lb Carrera.
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Old 11-04-2007, 06:55 AM
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I think that the smaller wheels have a strong advantage. The key is finding soft/grippy tires.
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Old 11-04-2007, 07:00 AM
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I did not read all the responses to the original question, but I am changing from 15" fuchs to 17" rota's and had some of the same questions and this http://www.miata.net/garage/tirecalc.html Helped me with some of the answers.
What you will find is that you can go up 2" in rim size and not change the outside diameter of the tire.

right now I'm waiting for "my ship to come in"
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Old 11-04-2007, 08:20 AM
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OK I see now that the link i posted had already been posted, I told you I DID NOT READ MUCH
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Old 11-04-2007, 08:23 AM
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ah yes, I see with going from 225/45/15s to 255/40/17s, the diameter goes up 2 inches total, so i effectively would raise my ride hight by 1".

For AX important effects @ wheels/tires = ride height (COG), rotational mass, widest stickest tires you can get, among others I'm sure

so If i went with 17 Rotas vs 15 fuchs for AX, I would lose 1" ride height (aka higher COG), have worse hole shot from bigger diameter gearing, and also Rotas would be heavier than my 8" x 15 fuchs by Xlbs, thus rotational mass.

so the only advantage to the rotas at AX would be wider tires...225 vs 255 = 30mm each tire, so say 2" total more rubber contact patch put down at the rear wheels - which would be nice.

While wider contact patch would be better for AX for sure, I'm guessing COG raised 1", worse gearing, and higher rotatational mass might negate some/all of the extra rubber the rotas would provide for AX....??? But I really don't know what I'm talking about either, which is why I'm asking
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Old 11-04-2007, 09:01 AM
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