Any - Car tyres - low pressure - kerbed enthusiasm

Probably a ludicrous question but I'm of an age where I'm happy to appear ludicrous in the pursuit of knowledge.

I have a front tyre with a very, very slow leak. It occured to me, as I was inflating it this morning that I don't understand the forces that keep the tyre on the rim. I know that they're glued but presumably that offers very limited adhesion. Air pressure must therefore be a significant factor and consequently low pressure would increase the risk of a tyre slipping and shredding on the rim. Even when tyres are correctly inflated it seems quite incredible that they are capable of bring a ton and a half of car to a standstill within a few seconds without being torn from the wheel.

So, a question for the knowledgeable engineers out there: what are the forces acting on the tyre and what is the relative contribution of the air pressure? Any takers?

Any - Car tyres - low pressure - daveyjp
They aren't glued. What keeps them on are the bead wires in the tyre.

www.popularmechanics.com/cm/popularmechanics/image...g

Air pressure prevents the tyre flattening and running on the bead wires. You are correct that driven for any length of time with little pressure tyres will shred.

Edited by daveyjp on 24/11/2013 at 18:46

Any - Car tyres - low pressure - Andrew-T

Your slow leak may be due to a nail or something similar in the tread, or quite possibly gradual corrosion round the bead (where tyre meets wheel) or round the valve, especially if the wheels are alloys. Check carefully for foreign objects in the tread - if nothing found, get the local tyre centre to reseal the tyre anyway.

But as regards adhesion, the tyre will skid on the road before it skids round the rim. The force of braking concentrates on the front wheels; the sidewalls bulge, forcing the bead harder onto the rim. As Davey said, tyres are not 'glued', but they are fitted with lubricant which makes them easier to force over the rim. Just like a bike tyre (if you've tried that) only bigger.

Edited by Andrew-T on 25/11/2013 at 11:17

Any - Car tyres - low pressure - bathtub tom

Trials cars tend to suffer from the tyre 'creeping' around the rim, but they do go to very low pressures - 5PSI. Some bolt the tyre to the rim!

There's pros and cons for using inner tubes in such cases. If a tube's fitted then the tyre's les likely to roll off the rim under sideways force, however, tyre 'creep' could tear the tube.

I never went below 12PSI. I managed to roll one tubeless tyre off and suffered puntures caused by friction between tube and tyre.

Any - Car tyres - low pressure - gordonbennet

Amazing the punishment tyres will take, but more amazing is how some people take them, and their braking systems, for granted....paying someone to polish the paintwork till it gleams going to the trouble of claying etc, sterilize the interior till you could use it as an operating theatre...yet wouldn't dream of closely inspecting their tyres, and Heaven forbid look at the brakes till they either fail the MOT or start grinding.

Used to be a Berlingo van that turned right at a roundabout around the same time i went to work so saw it regularly, he'd take that 30mph roundabout at approx 50mph, the NSF tyre was probably at a 6" and 45 degree lean inside of the wheel, how it didn't come off the rim is still a mystery.

Modern tubeless wheels have for many years had raised ridges just inside the bead seats helping to fix them in place and make initial inflation easier (by the wheel being effectively 2" narrower to the ridge), you might have heard the tyre pop twice when you've been at the fitters, that was the partly inflated tyre slipping over those ridges when being inflated...part of the reason its such a hard job breaking the bead seal when its time to take the tyres off.

Any - Car tyres - low pressure - coopshere
The worst case scenario on any four wheeled vehicle is to have one tyre with a lower pressure than the one opposite it. It affects the steering and handling of the vehicle and in extreme circumstances makes the vehicle unstable and dangerous. Get it fixed before you live to regret it.