Why are motor bike tyres different sizes at the front and back?
Educated guess to get things going…
The rear one is where the power is delivered, so it needs to be more skookum to deliver the power. The front needs to be lighter, since that’s where the steering is initiated. (If anyone wants to talk about turns being initiated by leaning, we’ll talk about that later.)
Pretty much it. The front tire occasionally experiences heavy traction demands during braking, but the rear is almost always delivering power to the road. Occasionally it’s a lot of power (as when accelerating from a stop or powering out of turns in the twisties), but even during straight flat cruising it’s constantly exerting a moderate tractive effort against the road to offset aerodynamic drag. If the rear were the same size as the front, it wouldn’t last as long as the front.
IME, the rear tire usually lasts about half as long as the front.
Some bikes are worse that others, some roads are worse than others, and some riders are worse than others (by “worse,” I mean harder on the tires).
My first bike (BMW R1100RT), the front and rear tended to arrive at the wear bars at about the same time; my current bike (BMW R1200RT), the rear wears out faster than the front (and faster than it did on my first bike). There’s certainly a bit more power available on my current bike, and that probably contributes to it. If a bike gets used for a lot of high-powered riding (twisties, drag racing, etc.), then the rear just isn’t going to last as long as it otherwise would. I know other riders with the same bike as me who get much better mileage out of their rear tires, presumably due to a more sedate riding style.
If your rear tire were the same size as your front, it would probably not last as long as it does.
Thank you guys for taking the time and trouble to answer this question. I really appreciate it.
I believe that a fat front tire would be beneficial for traction but it would make the bike harder to turn
The front tire on dirt bikes anyway, is a larger diameter. This does put more rubber on the road compared to a smaller diameter. I guess it also makes it easier to roll over obsticals?
Large front tires on dirt bikes help it over pumps and bridge holes, but also make it tend to sink less in mud and sand. Wider would increase the area and help with flotation also, but would also mean cutting a wider rut which would take more power, and increase steering forces. Many riders claim dramatic improvement between a 19, and a 21" front wheel, a popular upgrade to the BMW R1XXX-GS series of dual-sport bikes. Part of this may be that there is a better selection of aggressive treaded tires available in the 21" size used for motocross.
On very loose surfaces (sand, mud) often the rider does very little steering with the handle bars. Instead the bike is steered by weighting the foot-pegs and with hip motions similar to downhill skiing. Even though it is turning, the front wheel is sort of acting like a ski as well.
The back tire is not nearly as critical as the front. Since it has power, it can partially lift itself up and over big rocks and such, where the front tire has to be pushed forward by the axle, and must “lever” over. The front wheel bogging down in sand or mud usually results in a crash, where this happening with the rear wheel just calls for more power.
A smaller back tire can use smaller, lighter sprockets, requires less tension on the chain, and allows a shorter swing arm for a given suspension travel…all of these save weight, which makes the bike handle better off road.
It is also better for the back tire to be wider as this reduces the depth it digs itself into a hole under power. Essentially, for a given amount of power, you are going to throw a given volume of dirt, and if that comes out of a wider rut, then the rut won’t need to be as deep.
Finally, relative tire size has a substantial influance on which one breaks loose first in a corner. You will often see motocrossers cornering leaning forward, with their crotch on the tank, and inside leg extended as far forward as possible. This is done to weight the front wheel as much as possible, so that it will tend not to break loose (wash out) before the back. A narrower front tire is helpful here.
Large tires wander and don’t track as well as narrower tires.
Up front, you want the best combo going, which gives you the rubber you need to grip the road and brake, but not so much that you sacrifice handling and tracking.
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The tire size thing is almost a religion, I think. Different bikes are set up very differently for different reasons. Sometimes it is just styling. The Harley Fat Boy, for instance, has that huge rear tire just because of appearance.
Small front tires, large front tires…you get all kinds of opinions. Again, sometimes it is styling though without doubt the front wheel setup has a profound impact on performance.
I’m not sure that BMW is a really good example of tire setup to use because BMW bikes are different than any others throughout their suspension setup. I have a K1200LT that is the best handling big bike I’ve ever had, but even though it is a touring bike, its suspension setup is performance all the way. Same with the R1200RT that was mentioned earlier.
Bikes are very complicated dynamically. The choice of tires has a profound impact on the dynamics, but how important the details of the dynamics are just depends on how you ride.
For myself, I switch off between the BMW (which has scrape marks on the pegs where I drag them sometimes) and my Suzuki VStrom 1000. I’m pretty performance-oriented, but any number of my friends are a lot more sedate in how they ride. Our choices in bikes certainly reflect our preferences.
…psst, Johnny, don’t say skookum - no one outside the Way Upper Left Coast knows what that means. 
One other reason for the wider rear tire is because it supports more of the overall (bike + rider) weight than the front tire does.