Difference between a 6" and 7" wide wheel?

G

gran racing

Super Moderator
Allright, I'm not looking for the answer of 1" or even in other units of measurement. How much of a performance / handling difference is there between a car that is using 6" wide wheels versus 7" wide wheels? I understand that the contact patch is bigger, but am curious just how much 1" really makes.

On a related topic, what is the purpose of ITC and ITB use 6" versus 7" wide rims? (I am not looking trying to get this rule changed) Is it there because the ITB & ITC cars were the founding cars and ran 6" wide rims, then SCCA wanted to make sure everyone wouldn't have to go out and buy 7" rims? Just wondering...

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Dave Gran
NER #13 ITA
'87 Honda Prelude
 
For years and years and years I ran 5" rims on my Honda with 185-60/13's. Four years ago I finally bought a set of 6" rims.

Night and day difference, even with the same size tire. The car was able to make much better use of the contact patch.

Other things to keep in mind...your 5"rims are probably OEM, where as your 6" rims will more than likely true race rims and will be a lot lighter, significantly reducing both the unsprung and rotating mass.

Also, the guys who mount tires won't threaten to kill you any more
smile.gif
 
Years ago, we did some back to back testing. On a 5.5" rim width we had been running 175/70 series tires. So we mounted up some 185s of the smae overall diameter on seperate wheels and went out and swapped back and forth a bunch. Both drivers were slower on the 185s.

We concluded that it was the way the rim positioned the contact patch. The 175, while smaller, wasn't pinched on the rim, and put more rubber down effectively.

That said, that experiment was with one brand, one model and every brands and model has different construction and may respond differently.

Nothing like back to back testing to know for sure, but I would suspect that the tires you've been using may not work well on a narrower rim.

You will see theoretical gains in less rotational inertia, so better accel and decel, as well as less unsprung weight, so marginally better behavior over bumps. Also, slightly less rolloing resistance means better trap speeds. But all these factors will be slight.

But the loss of contact patch will slow your through the corners, and the tradoff will likely result in slower lap times.

WAG???? 1.5 -2.5% increase in lap times, IF you had optimized your tire/wheel/chassis setup, and all other things being equal. Varies by track.

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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]
 
I'm not really talking about OEM wheels versus a 6" or 7" width. I am curious on the difference between ITB & ITC max width of 6" versus ITA & ITS 7" width - all using race wheels and about the same weight.

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Dave Gran
NER #13 ITA
'87 Honda Prelude
 
I've tried both and my answer is that it depends. I've run Hoosier 205/60-13 RS303 on 6" wide wheels which both looked and felt like the tire was too wide for the rim. I've run those tires on 7" rims and they feel so much better.

I've also run Goodyear 205/60-13 GSCS on 6" rims and they worked great. I haven't tried them on 7" but the Goodyears aren't as wide as the 205 Hoosiers and they look good on the 6" rim width.

BTW, My best time on the Goodyears on 6" rims is almost 1 sec/lap faster than the Hoosiers on 6" rims. I don't have times for the Hoosiers on 7" rims but the car felt comparable to the Goodyears on 6".

Joe Craven
37 ITB Ford Capri
 
The history is that way back when, A, B, and C all ran on 6" wheels. ITS was allowed 7" wheels. I THINK that you'll find that the 7" allowance in ITA happened when the 12a RX7 got moved there from S - in the late '80s.

Remember that when the IT rules were first drafted, we thought that the 14" wheels on the '84 GTI were state of the art. The Omni came out with 15's and we were all amazed.

A 13x6 wheel was HUGE compared to the 4.5" or 5" steelies that we were taking off of pretty much any ITC car and most of the B options...

Kirk
 
Ahhh. Thank guys. Those are the answers I was looking for. Except for the Good Year tire being faster than the Hoosiers. Hmmm. Good Year? Guess we'll just have to wait and see.

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Dave Gran
NER #13 ITA
'87 Honda Prelude
 
Originally posted by gran racing:
Except for the Good Year tire being faster than the Hoosiers. Hmmm. Good Year? Guess we'll just have to wait and see.
Click to expand...

Yup, Goodyear. The top guys in NER in ITS run 'em because they are the fastest we have tested on.

AB

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Andy Bettencourt
06 ITS RX-7
FlatOut Motorsports
New England Region
www.flatout-motorsports.com
 
I've been a Goodyear GS-CS fan since they were introduced in, what, 1992? We were sponsored by them when we were running in SSB and there wasn't a faster tire out there. I'm confident that GY has improved the tire over the last 10 years...

The ONLY reason I'm running Hoosiers instead of Goodyears is because they do not make a tire in my size (50-series 14"). If SCCA approves 15" wheels in IT then I'm there...
 
Here's the available list from Roger Kraus
http://www.rogerkrausracing.com/GYDOT.html

I'd be interested in hearing which is faster on a 14x6. 195 or 225? I ran 195s on my 14x6" wheels on my 83 GTI and my car stuck as good as the other ITB cars which were running 225/15 (Volvo 142e) and 205/13 (BMW) Goodyear tires. If my GTI had some more power, I'd be racing it in ITB this year.

Hoosier has been improving their tire, not sure about Goodyear. I'll be trying out Hoosier's new R4S04 tire next year when I run ITA with my Capri.



[This message has been edited by Joe Craven (edited April 06, 2004).]
 
Any idea on how long their lifespan is?

Hard to beat Hoosier's deal here in the N.E. Go to the site and have them mounted for free. Saves a fair amount in shipping and mounting costs! Plus a less expensive tire to begin with.

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Dave Gran
NER #13 ITA
'87 Honda Prelude
 
Originally posted by gran racing:
Any idea on how long their lifespan is?

Hard to beat Hoosier's deal here in the N.E. Go to the site and have them mounted for free. Saves a fair amount in shipping and mounting costs! Plus a less expensive tire to begin with.
Click to expand...

What do you do about proper heat cycling?



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Jake Gulick
CarriageHouse Motorsports
ITA 57 RX-7
New England Region
[email protected]
 
sales tax is still a lot cheaper than shipping and mounting.

Yes, Hoosier Tire East in Manchester CT.

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Dave Gran
NER #13 ITA
'87 Honda Prelude
 
What is the implication of overall tire diameter (mounted) on a car's performance characteristics. Put another way, two cars equal in all respects, including gearing and final drive, both having 15" wheels but one car's tires have diamters of say 24" vs 23" for the other.

Dave Piasecki
 
Originally posted by dtheracer:
What is the implication of overall tire diameter (mounted) on a car's performance characteristics. Put another way, two cars equal in all respects, including gearing and final drive, both having 15" wheels but one car's tires have diamters of say 24" vs 23" for the other.

Dave Piasecki
Click to expand...

If the diameter of the tire is different, then the gearing is by default different.

24" Dia - slower acceleration but higher top end.
23" Dia - quicker acceleration and a lower top end.

AB

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Andy Bettencourt
06 ITS RX-7
FlatOut Motorsports
New England Region
www.flatout-motorsports.com

[This message has been edited by ITSRX7 (edited April 07, 2004).]
 
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