> Why do dirt bikes use smaller rims at the rear and bigger at the front?

Why do dirt bikes use smaller rims at the rear and bigger at the front?

Posted at: 2015-04-14 
rears are 'wider' but smaller diameter for use of 'fat' tires for more 'contact patch' at rear for power transfer to ground without slipping I soft dirt-- means less rooster tail show off and more 'git going', front tires haven't got the power to ground transfer problem- but are the first to hit the pothole and big rocks- so a larger diameter helps to roll over the bumps and a skinnier tire is lighter, more responsive, easier to dampen for some better quicker handling front braking is about all the power transfer, slipping might be a problem a dirt front tire would be concerned with- and a large contact patch in some situations with big brakes might be counter survival so a skinny front is safer. And a actual dirt bike examples of same tires front and back exists- the Yamaha PW50 and other small dirt bikes of under 100cc with lower power engines under say 15 hp don't worry about rear tire breakaway on acceleration, no need for real fat rear, skinny front tire and having same size 16 to 19 inch tires is simple market economics. The older British and Spanish Trials/trail cycles often had 19 inch front and rear 3.00 to 3.50 tires with small block tread, the desert racers went to 4.00 to 4.50 wide tires front and back- 18 to 19 was old standard and even Harley did that on first Superglide desert racers, sporties XCH.

The bigger front wheel helps get over obstacles better and gives a smoother ride over rough terrain, and the smaller rear wheel helps maximize the acceleration efforts of the bike.

The smaller rear wheel is for putting the power to the ground more easily.

The larger front wheel rolls over ground irregularities easier.

dirtbikes generally have larger rims than streetbikes on both ends .. the front is designed to roll easier over obstacles and be light weight, the rear is smaller than the front to accommodate a larger tire and suspension travel ..

I always supposed that a larger front wheel (with its narrower tyre) was easier to keep in line over rough ground.

It is easier to transfer power to smaller wheel than to a big one .