Donald McQueen, a retired auctioneer, spends a great deal of time outdoors with his wife and a 2008 Jeep Wrangler. He is not only a regular customer, but a good friend, so it’s always a pleasure to have a cup of coffee with him while his car is getting some forecourt attention.
He can afford to change vehicles quite often, and usually spends a lot of time and money modifying them to fit his purpose, but he’s very careful in his choice of modifications. About six weeks ago, Donald wanted to know whether the ABS and other active safety items would malfunction if he fitted larger and more knobbly tyres. We told him that it was our understanding that these systems would only be disorientated if the front and rear wheel rolling circumferences were different, because then the speed difference between front and rear would be large enough to upset the reference values in the ABS control map.
He then fitted the tyres of his choice, but a few days ago he appeared on the forecourt in his wife’s car, looking crestfallen. He told me the sad tale that his beloved Jeep had broken its front differential and that his warranty claim had been rejected by Chrysler SA because, at the time of the failure, he was taking part in a local hill-climb on a dirt road. In addition, he had switched off the wheelspin control, and fitted non-standard tyres.
He wanted to know from me why Chrysler SA was being so unreasonable. I explained that I could not share his viewpoint. Vehicles are designed for what the engineers consider to be normal stress levels. Racing would introduce abnormal stress levels, thus causing undue wear to a number of drivetrain components, so that even if the differential was replaced under warranty, some other parts might fail at a later date and the company would then have to replace those parts as well.
The oversize tyre question is very interesting. I asked Jake Venter from CAR what effect oversize tyres would have on the differential, and he replied that it was not the size of the tyres but the grippiness of the knobblies that could lead to diff failure. The amount of torque that goes through the diff depends on the traction available to the tyre with the least grip. When the supplied torque exceeds the least grip, that wheel starts to spin. This reduces the torque going through the diff immediately to a very low value, thus alleviating the internal stress. Knobbly tyres tend to grip better on dirt roads than standard tyres, so the engine will be able to feed more torque through the diff and this may exceed the designed limit, thus causing failure.
All of us, including Jake, were surprised that the Jeep’s differential had failed so easily, but we concluded that there may have been other factors involved that we were not aware of.
