Bias racing tires -

[cabc26b]

During the same event I noticed a move afoot to allow ( offer ?) Lotus (Loti) the option of moving down a group by restricting the tires they run - So if I give up my Avon ACB10's ( 7x22x13) I can run with the small/medium bore Italian and English and sub 2 liter German stuff.

I don't want to run radials, so bias ply is my pref.

I can run a molded ACB9 and the 6.5x23x13 seems a logical choice but one racer/ authority has waved me off these b/c they are "too tall" .... Has any one run these ? Can anyone else confim the too tall at 23" ??

If I do need to stay with low profile 2 sub 22 inches then my options shrink - to Hoosier and 5x22x13 Avon ACB9's , maybe the Dunlop historic FF or a bias ply slick with a historic pattern cut in them but these get expensive as they typically don't last, but man that first outing is amazing once they come in....


any input would be appreciated

[msd1107]

Just for a bit of perspective:

The stock 155-13 tires have a rated diameter of 22.8 in, 912 rev/mile. 165-13 tires are 23.4 in, 887 rev/mile. 185/60-13 tires are 21.7 in, 956 rev/mile.

The taller the tire, the taller the effective gearing is and you may need a higher numerical differential ratio to compensate. By the same token, a shorter tire gives lower effective gearing and you may run out of revs.

Another post on LotusElan.net

http://www.lotuselan.net/forums/viewtopic.php?t=15596

although labeled simulation software, has various combinations of tire size, differential ratio and rpm tables so you can see their effect on the speed achievable.

You probably should get the tires with the highest grip, since cornering speed is the primary determinate of lap time. Then if the tire diameter is not optimum for top speed on the course, change the differential ratio.

David
1968 36/7988

[cabc26b]

Thanks David,

The pro's and cons of the 23 vs 22 dia were already considered from a gearing perspective and I know people who carry sets of ties as opposed to changing gear ratios dependent on track. There are places where 23" would be nice.

Maybe I was not clear enough on my original posting ( appoligies) I am specifically looking for input on diameter/aspect ratio as it relates to side wall height snd impscy of increasing sidewall height on handleing behavior . particularly as one peerson has mentioned the car gets tippy and I am no joey chitwood .

Second is experience and or insight on the tires I mentioned

[rgh0]

Directionally I believe lower profile is better assuming all other things equal and differences such as ride height are appropriately adjusted.

However differences due to tread compound, wall stiffness and other aspects of manufacture are probably more important than aspect ratio in the range you are talking about so it a matter of try what works best for you.

cheers
Rohan

[bill308]

Rohan,

Your wrote, "Directionally I believe lower profile is better assuming all other things equal and differences such as ride height are appropriately adjusted."

Are you saying that the car will track better, i.e. be less subject to road irregularities with lower profile tires? Put another way, will a low profile tire be less likely to be affected by road features parallel to the direction of travel?

Bill

[steveww]

Give the Michelin XAS FF tyre a try. The FF version is a soft compound, very sticky and it also has very stiff side walls. Although these are treaded road tyres I have used them on many track days and they work very well.

[rgh0]

Bill

By "directionally better" I meant that it would generate higher useable cornering grip on a track for competition purposes.

Sensitivity to road irregularities is however generally worse and the car will move more under braking and cornering when it hits a road bump with lower profile tyres.

Again all this is very dependent on tyre construction, side wall stiffness, tread compound, tread pattern etc but lower profile tyres generally more sensitive than higher profile ones to road surface

cheers
Rohan

[bill308]

Rohan,

Thanks for the clarification. I concur with all your points.

I suspect that a high profile tire will be more forgiving at it's limit, which while not as high as a lower profile tire, may lead to faster lap times for less skilled drivers or on rough surfaces.

Bill

[paros]

I really should look at the site more often but been busy restoring a brute - a TVR 400.

Anyway as to tyres. I like the bias ply best for racing, and have used Yokohama and Avon [ including ACB10 in 6/21X13 ] all on 5.5 Minilites. In slick form the best by far is the Yokohama slick and it just clears the wheel arches of an S4 - 20.2 diameter.
I have also used the radial Yokohama AO48R and 32R in 175/60X13 and these are good, BUT. First they are very heavy with extra inertia and also the side walls are atrocious giving violent breakaway characteristics, and also in long races high wear rates. These are slightly taller than the ACB 10 as a road legal tyre, and are of course radial - with all the comments already recorded in the thread.
In my experience if we take the fastest as being the Yokohama slick and which on a tight 1 min 10 second track, the Avon slick 6/21X13 is half a second to 3/4 second slower, the ACB 10 is a second and half slower than the Yokohama slick, and the radial Yokohama another second slower. The old AO32R was slightly better than the 48R I believe for drivability. However the car is certainly most fun to drive with the bias ply tyre and much easier to race.
In fact for 2008 I am looking at a change in championship which allows me to use any road legal tyres rather than being forced to used Yokohama AO84R tyres. This will be ACB10 or just possibly Kuhmo 175/60X13 which some people in the UK consider excellent - I have no experience of this tyre.
Also with bias ply it is much less sensitive to bump steer and uses less negative. The car can also be tweaked better with toe out to alter characteristics.
If the bias plylooses out anywhere it is under braking, where I believe the AO48 was superior. BUT I used these for the first time this year and had fitted the equivalent of Plus 2 front discs and callipers. This may have been the difference. However I would often lock a cross ply under braking even with standard P14's and very rarely was I locking a wheel with the radial.

I hope this might be of interest

Richard