If a tyre with an extra 3mm of tread on it will make such a difference it will pop diffs, why dont they go 'Bang' every time we go round a roundabout!?!?!?
The action of the diff is to allow a speed differential between wheels, like when one wheel has to go further than the other, either becouse its going round the outside of a bend or over a bump.....
The % difference in radius of a few mm on a 750+mm wheel diamter is blugger all!
Over the life of a car, say 250,000 miles the near-side will statistically have traveled something like 37,000 miles or so MORE than the off-side wheels!
Becouse we drive on the left side of the road, and go round roundabouts clockwise, so we will always travel further round right hand bends than left hand ones!
There IS a grain of truth in the suggestion, you can wreck a transmission on a four wheel drive car by using a 'spare', but it applies to four wheel dricve systems that use viscouse couplings, that CANT tolerate indefinate differentiation, like Audi Quatro's and those Japanese rally replicas.....
I think that there is a warning in the Subaru owners book somewhere, and an over-cautiouse AA/RAC recovery bullatin. (also saying they cant use a lifted tow becouse the centre viscouse will go pop.... as it will on a borg box Rangie, BTW)
But I think that attension was drawn to the issue on an Audi where they at one point fitted a 'space saver' spare that was smaller than the road-wheels, and it had a sticker on it that said something like 'do not drive at more than 15mph or for more than 5miles" on it.
Becouse it was something like 2" smaller than the road wheels that themselves were only about 17" in rolling diameter, so it WAS an appreciable %..... but the warning WASN'T just becouse of risk of transmission damage, but the thing was a 140mph car, and the space saver tyre was not only smaller, but also narrower, and would have upset the suspension and the handling, as well as the torque delivery from the visco units, making the car pretty unstable, and pottentially hazardouse used at much more than cycling speed!
However, it is 'good practice' to 'rotate' tyres, and I have a wonderful bit of insrtuction in a 1960's AA hand-book, that gives instructions and explanation....
.....As near-sire wheels travel further than off-side wheels, and front wheels travel further than rears, and spares dont tend to travel at all...... it says, start at the rear near side wheel, and swap that with the spare, then work your way around the car, so trhe rear near side becomes the front near side, front near side becomes the front off-side, front off side becomes the rear off side and the rear off-side becomes the SPARE! [breath!]......
......and if you do that once a month, when you check your tappets........ you will ensure 'even' tyre wear of your entire set, including the spare, and get the best mileage out of ALL your tyres............
Tappets......... they need adjusting........ do cars still have them?
