What's the difference between a front and a rear?

[Steam]

Simple answer, one goes on the front wheel, and one goes on the back wheel, yeah thanks for that wiseguy.

TO the point:
I don't think you can use a 120/90 -17 front tyre as a rear tyre, even if it fits, but why not?
Is the tread pattern very different? I looked through a Pirelli catalogue and they are mostly pretty much the same pattern, but the front has a smaller width of course.
Is one stiffer than the other?
Do they have a different profile... or curve, or whatever that's called?

[rwh]

I think so. I think they're both part-circles, but the front has more of it, if that makes sense. Something like a 1/4 circle on the front, vs a 1/8 circle on the back. I haven't worked out why, though. Anyone?

Richard

[Ixion]

I have used 'front' tyres on rear without problems. It deends on the tyre of course, some would obviously be wrong.

And I don't think it is something I would recommend for a large machine.

There are differences. Rear is designed to cope with traction (acceleration) loads, front for braking loads.

But on a smaller slower machine , the real world may not care. Offroaders, the differences are even less (not talking competition stuff here). Except of course that the wheel sizes will be quite different front and rear.

Years ago one could get 'Trail Universal' tyres, from memory they were the same front and rear.

I'd not be too freaked by it .

EDIT: Classically , front tyres had a ribbed tread pattern to give directional stability, and rear tyres had a square block pattern. Switching those would be a BadThing (though I recall using a rear tyre on the front wheel of a Bantam for off road use. Worked OK) . But nowdays the tread pattern is often much the same front and rear (I know it's not actually identical).

[riffer]

The front tyre's role is turning and braking.

The rear tyre's job is accelerating, leaning, and braking.

The different forces applied to a tyre - in this case you're talking about mostly cornering and braking on the front, and mostly accelerating on the rear - require different construction for each tyre.

[Sketchy_Racer]

I used a front tyre on the rear of my streetstock (150cc) race bike.

It worked extreemly well. just had to put it on backwards.

I wouldn't say it would be a good idea on a big HP bike though