Tire technology improves odds of sidewall fix

Hey there, time traveller!
This article was published 15/08/2003 (8094 days ago), so information in it may no longer be current.

QUESTION — A flat tire on my car has caused some debate among my co-workers. I drove over a nail and it went through the side wall of the tire about halfway up.

I took it to a local repair shop and they told me I need a new tire. I have only had the tire on the car for a few thousand kilometres and it looks almost like new. One of my co-workers told me he had a tire fixed after a similar incident but others confirm the repair shop’s diagnosis and tell me you can’t fix a radial tire that has been punctured in the sidewall. Should I buy a new tire or can I safely have this one repaired?

ANSWER — It is probable you can have this tire safely repaired. New tire patching technology allows patches to be placed on a radial tire anywhere to within one inch of the bead area. The bead of the tire is the strong edge that touches the wheels. In the past, radial tires could only be patched on the tread area because the sidewalls flexed too much for the patch to adhere.

There are some limits when patching a tire sidewall. The hole must not have cut or damaged the cord material and can be no bigger than the plug available. A cut in a tire sidewall cannot be repaired. Tire manufacturers may void the tire’s warranty after it has had a sidewall patched. They say the speed rating on the tire is no longer valid if a patch is installed outside the tread area.

If you drive at high speeds, then a new tire may be advisable.

Otherwise, there are thousands of tires on our roads that have had patched sidewalls. If in doubt, install a new tire. The tires are your only contact with the road and without that, a car is a dangerous, out-of-control block of metal!

QUESTION — I have a limited slip differential in the rear axle of my 1989 Chevrolet half ton truck and have a question about which gear oil to use. I recently had new rear axle seals installed and they had to open up the rear axle cover. When they put it back together, they installed limited slip gear oil. My owner’s manual says to use SAE 80-W-90 regular gear oil but they tell me I need the other oil because I have a limited slip differential. Which oil should I use?

ANSWER — Drain the limited slip oil out and install regular SAE 80-W-90 gear oil. Your truck has a “locking” rear axle which is built differently than a limited slip differential. A limited slip unit has clutches inside connecting the axles to the differential. A heavy spring pushes the clutches together to prevent slipping; thus the term “limited slip.”

Limited slip gear oil has slip additives in it to allow the clutches to slip a little during cornering without binding or chattering.

The locking differential in your truck uses a small flyweight governor that locks the axles to the differential when one tire starts to spin during low-speed driving. At speeds above 32 km-h the differential will not lock up. Clutches are also used in this differential but are only engaged when one wheel starts to spin. Adding limited slip fluid to this type of unit will prevent the clutches from holding and the locking differential cannot lock.

Most light-duty GM trucks use regular or synthetic gear oil. Some heavy duty trucks use different rear axles and require the limited slip gear oil, as do most GM passenger cars with limited slip differentials. Your owner’s manual has the correct specifications. No damage will have occurred to your differential, but the oil should be changed.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca