Inspired by a drunken discussion about whether tomato sauce would work as a puncture sealant.

Would it be possible to have a tyre that didn't need any air in it?

Filled with something solid, or at least not runny enough to leak. Or some kind of internal springing.

Given the vast amount of ingenuity devoted to tyre design I suspect not, or somebody would have done it by now. But looking at how low profile some modern tyres are it hardly seems worth putting air in them.

OK let's start with a vehicle with a single axle (I know there aren't any, except perhaps the Segway and a mono-cycle) ... the axle has disc brakes on each side and the vehicle is travelling from right to left when viewed from your position ... by applying the brakes consider what will happen ... the wheels are rotating anti-clockwise so when the brakes are applied, the discs will exert a force on the pads and attempt to pull the callipers (and anything else attached to the callipers) in the same direction i.e. the chassis is rotated anti-clockwise around the axle (the fulcrum) ...

this reaction will occur in exactly the same way no matter where the calliper is on the circumference of the disc

the other force that is acting is the momentum of the whole vehicle and the driver ... now since the centre of mass is usually above the axle, this additional force will act in the same direction as the braking force thus also rotating the chassis anti-clockwise around the axle ... result the chassis and the driver continues to rotate until an equal and opposite force occurs (in this case it would be the driver's head hitting the ground in front of the axle)


deep breath ...........


right, if you're still with me let's consider a vehicle with two axles … the physics will be the same for both axles and the only difference is that there are now there are two fulcra and so it becomes a bit more complicated ... so let's say you can apply the front and rear brakes separately from each other ... applying the front brakes only, the chassis will be rotated around the front axle in an anti-clockwise direction i.e. the chassis in front of the axle is forced down and the chassis behind the front axle is forced up ... and the equal and opposite force maintaining stability is gravity acting on the mass of the vehicle (the centre of gravity being to the right of the front axle and therefore providing a clockwise moment)

... applying the rear brakes only, the chassis will be rotated around the rear axle in an anti-clockwise direction i.e. again the chassis in front of the axle is forced down and the chassis behind the rear axle is forced up ... and the equal and opposite force maintaining stability is the front wheels in contact with the ground

the other force that is acting is the momentum of the whole vehicle and the driver … now since the centre of mass is above the the axle (usually but not always) this additional force will act in the same direction as the braking force thus also rotating the chassis anti-clockwise around the front axle … result the chassis and the driver continues to rotate until an equal and opposite force occurs (in this case it would be gravity as above)

so ......................

if both sets of brakes are applied together all the forces act together and the front of the car is forced down and the rear of the car is forced up

QED (hopefully)

Re: the caliper discussion, remember that the wheel is stopped still at point of contact with the road and is travelling at twice the speed of the forward motion at the top.

Right i'm with you, that's an excellent explanation

Are you insane?
Well the road is not moving is it, so the bit of tread touching at point of contact is not moving either.
Therefore the bit at the top is going forward in relation to the road at twice the speed of the forward motion.

Get a wheel or something that rolls and roll it on the table in front of you to see what I mean.

It's a mind bender.

Are you insane ?

The contact patch must be moving at the same speed as the car, if it's in 100% traction.
The contact patch is part of the tyre.

Do I need to go on?

Can I legally shoot pigeons that annoy me in my garden ?