CAR CLINIC

How to correctly use your spare tyre

In Summary

• In case of a flat front tyre in a dangerous place, disregard all the above and use the quickest means  possible to fit a donut and move out if danger.

• In fact, iF you think it’s too dangerous to stop, drive on the rim to a safer zone; it's  better to  buy a new rim and tyre than risk your life  trying to save something that has no life 

A spare tyre.
A spare tyre.
Image: BOOSTED AUTO

Three out of every five cars on the road are equipped with a spare tyre. In most cases, the spare wheel is usually a smaller size compared to the rest of the tyres on the car.

The biggest challenge drivers face is failing to understand which is the most suitable place to fit a spare tyre between the front and the rear hub.

 There has been a lot of controversy surrounding the spare tyre fitting issue but at the end of the day, whether you fit the spare tyre in front or rear, the vehicle will still get you to your desired location, the only difference will be the handling of your vehicle and the hazards involved.

   In the light automotive industry, there are different axle formations:

  • Front-wheel drive
  • Rear-wheel drive
  • Four-wheel drive
  • All-wheel drive

All the above driving modes will be affected differently with the positioning of the spare tyre when fitted at the front or rear on each of them.

When a vehicle is on the road, the main purpose of the wheels is to propel the car in the desired direction. Additionally, the vehicles wheel speed sensors give information on how quickly or slowly the tyres are rotating through induction sensors fitted on the hubs of every tyre. The readings are then sent to the vehicles control unit which then helps to determine the amount of braking force to be applied to each wheel the next time you brake to keep the vehicle in a straight forward motion and avoid skidding.

    It is assumed that when the car is moving, the wheels are also supposed to be moving at the same speed but in science that's not the case. Variations in speed are brought about by several factors as listed below: 

 DRIVING WHEELS

If your car is a 2-wheel drive, the driving wheels will always move faster than the rest of the wheels.

ROAD TOPOGRAPHY

The road is never the same all the way. At some point, there will be one wheel with too much weight acting on it while the rest have less weight due to road undulations. This will cause the wheels to move at different speeds

CORNERING (Turning on bent on the road)

When making a corner, the inner wheels usually move a shorter distance which is directly proportional to less speeds.

TYRE SIZE

This is why we are on this topic. One tyre being smaller than the rest means the revolutions made by this tyre are more than the rest and many things are bound to be affected. This includes the braking, traction control and handling of the vehicle in general. Fitting a spare wheel on the car may trigger a traction control light but this should not worry you. It only shows that the system is off until the required tyre is fitted.

 Whenever a car is braking due to inertia, all the braking force goes to the front. If one front tyre is thinner than the other, the contact point on the road will be weaker on the thinner side, forcing the car to swerve towards the thinner side. Even under normal circumstances when the car is driving and brakes are not applied, the vehicle will always tend to move towards the side of the thinner tyre (spare). This is associated with engine weight acting upon it.

 When a spare tyre is fitted to the rear, the setbacks associated with it cannot be compared to the above. Of course, the traction control light may come on, the drag will be there but the pull to one side will not be serious compared to, if it was fitted at the front.

All in all, many manufacturers have recommended fitting the donut tyre at the rear whether it is a front-wheel drive, rear-wheel drive, four-wheel drive or all-wheel drive. However for some all-wheel drive cars (Subarus for example) there is a fuse in the fuse box written AWD that must be unplugged which converts the car to front wheel drive to protect the centre diff/viscous coupling from thinking that the car is stuck thus overworking.

In short if you get a puncture at the front, the rule of thumb dictates that you move the good tyre from the rear hub, fit it at the front then fit the donut at the rear. Whenever these donut spare wheels are fitted, the loading capacity, max speed and max distance are all reduced. However different manufacturers have different recommendations. Refer to  your manufacturer's recommendations.

Nonetheless the above are just guidelines to ensure smooth running of your car in the event of a puncture or tyre burst.  In case of a flat front tyre in a dangerous place, disregard all the above and use the quickest means  possible to fit a donut and move out if danger. In fact, iF you think it’s too dangerous to stop, drive on the rim to a safer zone; it's  better to  buy a new rim and tyre than risk your life  trying to save something that has no life (tyre and rim).

The writer is the owner of Boosted Auto repair shop.