17" Tires
05-01-2002, 12:19 AM
#1
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17" Tires
I'm sure this has been discussed often before, but not in the short time I've been a member, so I thought I'd open another discussion on ideal 17" tire sizes.
I'm currently using 225/45-17 front and 255/40-17 rear on 7.5" and 9" wheels. The car feels well balanced on the track, but I'm thinking of putting on bigger tires - like 245/45 front and 275/40 rear, possibly A032R for combined use or V700 for track only.
What size 17" tires are people using? Any fitment or other problems with the 245/275 setup? Does a slightly taller tire like the 275 affect acceleration noticeably?
Thanks,
Gary
'89 951
I'm currently using 225/45-17 front and 255/40-17 rear on 7.5" and 9" wheels. The car feels well balanced on the track, but I'm thinking of putting on bigger tires - like 245/45 front and 275/40 rear, possibly A032R for combined use or V700 for track only.
What size 17" tires are people using? Any fitment or other problems with the 245/275 setup? Does a slightly taller tire like the 275 affect acceleration noticeably?
Thanks,
Gary
'89 951
05-01-2002, 03:46 AM
#3
Race Director
Yeah, considering that the Turbo uses a 225mm tire on an 8" rear and the TurboS has a 245mm tire on 9" rear wheel, you'd want wider rims, especially the front with shorter sidewalls from the 17" +1 upgrade. I'm running 255/40-17 and 275/40-17 A032Rs with 17x10.5/11.5" wheels. Without tensioned support on sidewalls, you actually won't get much benefits from running a wider tire.
05-01-2002, 09:17 AM
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Danno,
What you say is correct but is it worth going up to a 275 for just a slight benefit, as 032's do have stiff sidewalls, and second, whe does a 255 size look very much like a 245 size while a 275 size look much bigger than a 255 size?
What you say is correct but is it worth going up to a 275 for just a slight benefit, as 032's do have stiff sidewalls, and second, whe does a 255 size look very much like a 245 size while a 275 size look much bigger than a 255 size?
05-01-2002, 09:43 AM
#5
Race Director
Well, the 275mm tires look so much bigger because they have 3x the increase. Going from 245->255 is only 10mm, while 245->275 is 30mm. Also the 275mm is 1" taller in diameter as well, giving the tire more tire surface-area to look at, giving it the impresion of being bigger. By comparison, a 285/30-18 tire on the 996TT simply looks whimpy because the sidewalls are so short.
Also going up to a 275mm rear on 9" rim wouldn't give that much of a benefit. Partly because you'll have more lateral-flexing of the tread before the sidewalls undergo tension (compared to same brand and model in 255mm size). Also partly because it would throw off your F/R balance (more understeer). Putting a 255/40-17 front on a 7.5" rim to try an balance the 275mm rear would be an even bigger crime!
Our cars come set up with too much understeer as it is and exacerbating that won't help.
Here's a tire & rim chart I made a couple of years ago:
As you can see in the progression from bottom-to-top, as you get into higher and higher performing cars, the rims get wider and wider compared to the tire-width. And these are only street cars. Looking at the maximum-performance oriented stock-based racecars like SpeedvisionGT, BTTC, Trans-Am, IMSA-GTO/GTU they frequently run rims that are wider than the tire. That's to pre-stretch the sidewalls so that under cornering, there's no lateral movement of the tread at all!
Here's a good comparison. Last January, I switched rims at Laguna Seca between run groups. I went from the stock Fuchs 16x7/8" on my 951 to a set of HRE 16x10.25" wheels I bought off another racer. With the same set of tires the next time out, I almost ran off the INSIDE of turn 1 because there was so much more grip and the steering response was much, much more immediate (wheels didn't have to wait until tread flexed sideways enough to pull on sidewalls).
So I assert that the rim-width makes a bigger difference in cornering-power than the tire does. That may also be why the SCCA/POC/PCA rule books only look at rim-widths in assigning classes and points.
Also going up to a 275mm rear on 9" rim wouldn't give that much of a benefit. Partly because you'll have more lateral-flexing of the tread before the sidewalls undergo tension (compared to same brand and model in 255mm size). Also partly because it would throw off your F/R balance (more understeer). Putting a 255/40-17 front on a 7.5" rim to try an balance the 275mm rear would be an even bigger crime!
Our cars come set up with too much understeer as it is and exacerbating that won't help.
Here's a tire & rim chart I made a couple of years ago:
As you can see in the progression from bottom-to-top, as you get into higher and higher performing cars, the rims get wider and wider compared to the tire-width. And these are only street cars. Looking at the maximum-performance oriented stock-based racecars like SpeedvisionGT, BTTC, Trans-Am, IMSA-GTO/GTU they frequently run rims that are wider than the tire. That's to pre-stretch the sidewalls so that under cornering, there's no lateral movement of the tread at all!
Here's a good comparison. Last January, I switched rims at Laguna Seca between run groups. I went from the stock Fuchs 16x7/8" on my 951 to a set of HRE 16x10.25" wheels I bought off another racer. With the same set of tires the next time out, I almost ran off the INSIDE of turn 1 because there was so much more grip and the steering response was much, much more immediate (wheels didn't have to wait until tread flexed sideways enough to pull on sidewalls).
So I assert that the rim-width makes a bigger difference in cornering-power than the tire does. That may also be why the SCCA/POC/PCA rule books only look at rim-widths in assigning classes and points.
05-01-2002, 12:23 PM
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Well I think everyone would agree with you Danno, that the wider the rim for a particular tire size the less sidewall flex and better steering response and grip. But many SCCA racers, because of the rim size restrictions, still use excessively wide tires on their rims, like 255's on 7.5" rims.
While I'd really rather have wider wheels, I got a good deal on a set of Kinesis wheels in the 7.5" and 9" sizes that I couldn't pass up. The PO was running 245 and 275 Hoosiers (which run wide anyway) on his 968 for autocross (probably due to rim width restrictions for his class).
I was happy with the balance of my car the one time I've had it on the track so far. The suspension is stock Turbo S. Understeer was minimal, but then again, I'm accustomed to driving nose-heavy Audi quattros on the track. I also wouldn't want the gearing any taller as there were a couple very tight turns where I was not where I wanted to be in the boost exiting in second gear (ok, I probably need to work at carrying more speed through those turns)
Anyway, maybe I should stay with the 225/255 setup. Some tires, like the S03's, run very wide in that size. But I'm considering using either a combination tire like the A032R or a track tire like the V700. What is your impression of the A032R on the street, Danno? Is it really excessively loud or stiff? I know the V700's will be faster on the track, but they're not really streetable.
Btw, I got some feedback on alignment settings and I think I'm going to go with
Front: -2 or -2.5 deg camber
+3 deg caster (or max possible)
0 toe
Rear: -2 deg camber
0 toe
Sound reasonable?
Thanks,
Gary
89 951
While I'd really rather have wider wheels, I got a good deal on a set of Kinesis wheels in the 7.5" and 9" sizes that I couldn't pass up. The PO was running 245 and 275 Hoosiers (which run wide anyway) on his 968 for autocross (probably due to rim width restrictions for his class).
I was happy with the balance of my car the one time I've had it on the track so far. The suspension is stock Turbo S. Understeer was minimal, but then again, I'm accustomed to driving nose-heavy Audi quattros on the track. I also wouldn't want the gearing any taller as there were a couple very tight turns where I was not where I wanted to be in the boost exiting in second gear (ok, I probably need to work at carrying more speed through those turns)
Anyway, maybe I should stay with the 225/255 setup. Some tires, like the S03's, run very wide in that size. But I'm considering using either a combination tire like the A032R or a track tire like the V700. What is your impression of the A032R on the street, Danno? Is it really excessively loud or stiff? I know the V700's will be faster on the track, but they're not really streetable.
Btw, I got some feedback on alignment settings and I think I'm going to go with
Front: -2 or -2.5 deg camber
+3 deg caster (or max possible)
0 toe
Rear: -2 deg camber
0 toe
Sound reasonable?
Thanks,
Gary
89 951
05-01-2002, 09:27 PM
#7
Race Director
"Anyway, maybe I should stay with the 225/255 setup. "
Yeah, that would probably be your best bet. Spending more on the larger tires won't give muc of an improvement on those rims. Plus they'll be about 1-lbs lighter each as well (A032R in 275mm is 26.5lb, 255mm is 25.5lb).
"What is your impression of the A032R on the street, Danno? Is it really excessively loud or stiff?"
They seem to work OK. Not too stiff compared to my street wheels. Do notice the quicker 'thumps' times with the lighter weight (race wheels w/tires weigh 28lbs less than my street wheels). They DO make a lot of noise though, like monster-truck tires. But I usually blast stereo anyway, so I don't notice it too much.
The A032Rs would be a good all-around compromise for both track and street. Both the A032Rs and V700s are way, way better than ANY street tire, like 4-6 seconds/lap. But the difference between the A032R and the V700 is only about 1-second. Plus you can drive the A032R in the rain.
Yeah, that would probably be your best bet. Spending more on the larger tires won't give muc of an improvement on those rims. Plus they'll be about 1-lbs lighter each as well (A032R in 275mm is 26.5lb, 255mm is 25.5lb).
"What is your impression of the A032R on the street, Danno? Is it really excessively loud or stiff?"
They seem to work OK. Not too stiff compared to my street wheels. Do notice the quicker 'thumps' times with the lighter weight (race wheels w/tires weigh 28lbs less than my street wheels). They DO make a lot of noise though, like monster-truck tires. But I usually blast stereo anyway, so I don't notice it too much.
The A032Rs would be a good all-around compromise for both track and street. Both the A032Rs and V700s are way, way better than ANY street tire, like 4-6 seconds/lap. But the difference between the A032R and the V700 is only about 1-second. Plus you can drive the A032R in the rain.
