Spare the tyre

Compact spare tyres are designed to get you home, not across the country. Like almost all spare tyres, the one on this Ford Focus is recommended for no more than 80 km of driving; anything more and you’re asking for poor handling and a possible blowout. Image credit: Brian Kelly
Date:24 April 2012 Tags:,

Q I had a flat tyre a while ago, so I mounted the space-saver spare. I’ll admit I left it on for way longer than the owner’s manual suggests. Eventually, I replaced it with a full-size tyre, but I’ve always wondered why manufacturers say you shouldn’t use the spare for longer than necessary.

A Since the spare tyre is used so infrequently, motor manufacturers have switched to narrow, compact spares to save space and weight. Of course, a spare tyre is a lifesaver when regular tyres go pop, but leaving the temporary tyre on for longer than the manufacturer recommends invites a host of problems. First, a temporary spare isn’t as durable as a normal tyre. The real strength of a tyre comes from the plies – layers of steel and polyester underneath the rubber – and spares don’t have as many plies as regular tyres. A typical space-saver spare has only one layer of polyester in the sidewall and two belts of steel with a layer of polyester in the tread – about half as many plies as a normal tyre. This greatly limits puncture resistance and cornering ability. In addition, as the name implies, spacesaver tyres are intended to take up less room in car and crossover boots so that those trunks can be deeper. For that reason, these tyres are narrower and have a smaller contact patch. This reduces the amount of traction for the tyre, increasing stopping distances and making handling potentially unpredictable in emergency manoeuvres. It also means ABS and traction control aren’t as effective at keeping you out of danger. And you’re not going to have the same ground clearance. If you’re towing a trailer, you’ll have to leave it behind – spares have much lower load ratings than regular tyres. Longterm use of the spare can cause a serious mechanical issue, too: the smaller-diameter tyre can put a lot of stress on your differential.

The differential has a tricky job. It transmits engine power to the wheels from the transmission, but it also lets the left and right wheels turn at different speeds. This is essential for cornering. In a turn, the path of the inside wheel is shorter than that of the outside wheel, which means they travel at different speeds. When your car is driving in a straight line, the differential isn’t in use and there’s little wear and tear on its gears and bearings. But because the spare is smaller than the opposing wheel on the same axle, it must turn faster to keep up with the speed of the car, making the differential work to account for the variation. It’s as if the car is constantly in a turn. Leave the spare on long enough and the grease lubricating the differential will begin to break down, accelerating wear between the gears and the clutch plates if it’s a limited-slip differential.

For all these reasons, manufacturers suggest keeping speeds below 80 km/h and using the spare tyre only for limited distances if possible. If a compact spare is ever damaged, either the tyre itself or the wheel, the entire spare should be replaced rather than repaired. And don’t forget to check the pressure in your spare every time you check the pressure in your other tyres – it’s important to make sure your safety net is, in fact, safe.

Spare tyre
Dimensions: T125/80/R16
Outside diameter: 605 mm
Max speed: 130 km/h

Full-sized tyre
Dimensions: 215/55R16
Outside diameter: 643 mm
Max speed: 210 km/h

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