Installing a Tyre

November 29, 2006 at 3:58 am Leave a comment

If you have ever tried to fit a tyre yourself, you will know that it is not easy.

This is because it is not the air pressure that holds a tyre on its rim – nor is it the safety humps on the rim ledge. It’s an interference fit between the beads and the rim that makes it stay there.

The mounted tyre rides over the safety humps, generally with a loud ‘pop’, slides up a tapered ledge, and stops when it hits the rim flange. During this fitting process, the rubber and fabric composite caught between the steel hoop built into the tyre, and the steel or alloy ledge built into the rim, is compressed. It requires localised force to break the seal caused by this compression.

Archimedes said ‘Give me a lever long enough and I will move the world’. Your tyre dealer uses a powered lever instead, called a tyre fitting machine. Air pressure is used to dislodge the beads into the wheel well, and then the steel hooped part of the tyre is levered over the rim flange. Lubricant is used to help the process, and also to stop the rubber bead tearing. If it does tear, the tyre may leak or bubble when refitted.

The choice of lubricant, as simple as soft soap, is basic. Once it has done its job, it should disappear. It’s biodegradable. This is because the part of the bead (the ledge) that sits on the rim is transmitting the power, steering, and braking forces to the road.

The ‘pop’ that you hear is caused by the tyre beads sliding over the humps once there is enough sideways force generated by the inflation pressure, to drive them over. It should happen between 12 and 25 p.s.i. of pressure. Then the beads slam up against the flange, are checked that they are centrally located using guide lines engraved in the sidewall, and pressure is adjusted to road service requirements.

You will also have been charged for a new tubeless valve. This is because the rubber ‘snap in’ valves flex during service. High-speed photos of long valves, or those with extensions to make them accessible when deep hub-caps are used, show them to bend right over and touch the rim at speed. Ultimately they will crack, or de-laminate from the brass core. Some alloy rims may require clamp in tubeless valves, due to service requirements, or the rim thickness at the valve hole. Since these don’t flex, they don’t have to be replaced every time new tyres are fitted, though they cost more.

But wait, there’s more. After fitting, the whole assembly is balanced statically (vertical plane), and dynamically (lateral plane) on a sophisticated machine, which process adds kilometres to the tyres performance, smooths the ride, lessens mechanical wear on your car’s suspension, and reduces the risk of flat spotting.

Finally your friendly tyre dealer will refit the wheel to the car, and replace the rub cap evenly, having kept it clean and unscored by dirt on the workshop floor. Cheap at half the price!

Entry filed under: Installing & Changing Tyres. Tags: , , , .

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