Might have answered my own question


I assumed that by use of the expression 'going out of shape' Andy was referring to a flat spot on the tyre circumference by being parked up for a long time.



However found this on pure tyres HERE

Quote:

An out of shape tyre is a distortion normally in the treaded area cause by failure of the cords (belt ply), the cords that run under the treaded area is part of what give the tyre its strength and to keep the treaded area flat.
The image below shows the tyre tread on the left is normal and the tyre tread on the right has lost is structural shape (out of shape) Believe it or not this is the same tyre just turned 180 degrees.
Image of out of shape tyre

An out of shape tyre would have a high spot in the tread area and cause vibration through the vehicle, if you feel the vibration more through the steering wheel then its most likely to be one of the front tyres out of shape, if its more of a car shake then it will more likely be one of the rear tyres.
Its not uncommon for a vehicle to develop a pull either left of right if one of the front tyres has gone out of shape.

Safety Notice
If you experience vibration at speeds under 40mph to should immediately have your vehicle checked as this could be a tyre that is out of shape which could fail while Driving causing life threatening danger to you and other motorists.
what causes a tyre to go out of shape?

If an object pierces through the treaded area into the cords but not necessarily completely though to cause air loss this would let water into the corded area rusting them and eventually causing them to break.

Impact damage

Ageing of the tyre

Suspension defects