Tire size

I understand what the first sizing number means, for example 195; okay, but what does the second number mean? For example a 195/60 is a wider tire than a 195/55, right?

I suspect it has something to do with sidewall thickness (ie how fat) vs tire tread width, or am I all wrong here? Does the second number have more to do with fitting the tire in the wheel-well and the desired look than drivability?

I realize this is a rookie question but frankly have never asked for an explanation.

George, anyone with a tire 101 answer?

My understanding is that the second number represents the sidewall height expressed as a percentage of the tire width. So, a 195/60 has a tread width of 195 and a sidewall height of 60% of 195. Therefore, lower profile tires have a smaller second number, but it's always a ratio of the first number.
Gee, and I thought that was going to sound so easy...

tirerack.com, specs chart, click on the heading for gritty details... or, higher the /60, 65, etc. # the taller the tire.

Victor

The second number is the "aspect ratio" and relates to the installed tire sidewall height. Yes, smaller number is thinner tire section, which usually means less overall tire weight and less sidewall flex. For normal street use an aspect ratio of 60 or 65 will probably provide the most comfortable ride (more tire flex on uneven pavement).

George,

I understand. Thanks. My last question is you see some Speedsters with a different second number on front versus rear tires. Is that for fitment reasons in the wheel-well or pertains to tire circumferance, which may slightly effect gearing? If running the same rims front and back is there any reason to have different sized sidewalls?

A rear engine rear drive car tends to oversteer. This is when the rear end wants to pass the front tend in a corner. To remedy this people add wider tires to the back. When going from a 165 front to a 195 rear, it is necessary to lower the ratio to retain the same circumference (gear ratio and speedometer settings). TireRack.com has the specs for overall tire diameter for each tire. This will help in your choice of tires based on width and wheel diameter.

Excellent info, thanks guys.

Well, my IM with four 195/65x15 radials doesn't exhibit oversteer anything like my original swing-arm '56 Speedster, any of the swing-arm or IRS VW's, or early/late Corvairs I've owned; in fact, I haven't noticed any except in extremis and it has been easily controlled with the throttle.

IRS IM's with tube frames have improved weight distribution with the engine/trans mounted about 2 3/4" farther forward than in a VW pan based car. Add to that the 15 gal. gas tank, a full sized spare up front, and the convertible "D" has other additional weight forward (door glass and internal window mechanism, heavier windshield)which can only improve the weight distribution. Mine is probably about 53 rear/47 front. And the car's heavier torsion bars, Bilstein shocks, and front/rear anti-sway bars probably contribute a lot to the control of whatever oversteer there is.

I thought most Speedsters, having much lighter front ends, actually UNDERsteered, not oversteered. Isn't that why reducing the front tire pressure improves handling...increased front-end traction?

Just the opposite

Since I learned to drive in Beetles and love the tail happy experience. I have always prefered the oversteer tendency of cars with light front ends. I also owned three Renault R10s also designed by Porsche (rear engine rear drive).
I do not like the feeling of going into a corner and finding myself going straight as my Toyota Pickup tends to do.
I love the pure fun of taking a vintage beetle in a snow storm sideways around a corner. Needless to say I could never own a front heavy beast like a corvette or camaro. Working with sway bars, tire size and pressure can give you the joy of custom tuning your car to your particular driving style. Enjoy!

I prefer a little oversteer... You can get back on the throttle in the turn and push the car out of a controlled oversteer. In fact, you can be in a little 4 wheel drift under full power at the very end of a turn. I run a little faster with cars that oversteers rather than one that pushes, or understeers. Also, most road courses turn to go around something. So if I must, I prefer to wind up on the inside of a turn... not in whatever I was supposed to go around. You have more room to manuever. Just me, and the truth is I do love all the jazz!

A friend has a 75 911 and the other day was telling me how the car understeered in a corner and scared the hell out of him. I always thought that these tail heavy cars would oversteer and not understeer.

His description was that the entire car slid sideways in the corner like there was no grip in the front or rear. Any thoughts/comments?

Brian Puskas

It's hard not to be impressed with the knowledge - and the civility of those who share it - on this website.
Anyone who also visits other automotive forums knows this kind of online atmosphere is rare.

John, oversteer has to do with weight distribution and to a lesser extent center of gravity, and roll-center; tire pressures normally wouldn't have any discernable effect. One of the problems with swing-axle cars is their marked change in roll-center in certain instances. Under rapidly changing driving conditions in a curve - braking or accellerating - this can transition oversteer into a sudden spin-out without warning.

Wider tires can help compensate for but not entirely cure the problem which is suspension geometry and mass-transfer related.

I would suggest that everyone that owns a replica with the swing-axle rear end install a "camber compensator"; these were original equipment on '67 VW's and some Porsches and are available as an aftermarket item.

(Message Edited 9/5/2002 10:11:56 AM)

Brian, your friends 911 should not have a lot of understeer; he may need to have his suspension and alignment checked. There is a soft cover book available, "Porsche 911 Performance Handbook", that has about all (and I do mean ALL) the info on 911 performance you could want, including handling.

Note that the speedometer is driven from a front wheel, so changes in rear wheel loaded radius will not affect speedo readings.