What's with tire width? Why do some bikes come with skinny little tires while others have bigger buns? What if you want a wider rear tire on your bike, how will it affect the handling?
Most of you are going to think this tire-size question is dumb because the answer seems obvious: Bigger, more powerful sportbikes need bigger tires because bigger tires provide more grip. Bigger tires mean more grip, so bigger must be better, right?
Not necessarily. It’s true, larger tires are needed to support heavier bikes, and wider tires mounted to appropriately sized rims will offer more grip because they have a larger footprint. But, if there’s one thing to remember when it comes to tire performance, it’s that everything is a compromise.
Wider tires, especially really wide setups like you might see on a custom cruiser or a fat-tire Suzuki Hayabusa, aren't good for handling. For starters, the bike won't tip into corners as easily and it will require more bar input to stay on line. Wider tires (and wheels) are also heavier so acceleration and braking performance suffer, and that extra mass makes your suspension's job harder since there's more unsprung weight to control.
Narrower, lighter tires are definitely better for performance and handling, but they can’t handle as much horsepower.
Well, let me rephrase that. A skinny tire could potentially be designed to be super grippy so it could cope with all the horsepower, braking loads, and cornering forces a high-power bike can dish out, but not for very long, because the action of gripping the road tears, melts, and scrubs rubber off the tread.
If you enlarge the contact patch by making the tire wider, you get more grip and that larger contact patch dissipates more heat and provides more material to wear away, so the tire lasts longer. But then that bigger tire weighs more and impacts handling in all those ways I already mentioned. See what I mean about tires being a compromise? It’s always a balance of grip, handling, and wear.
So, smaller, less powerful bikes can get away with smaller, narrower tires, while bigger, more powerful bikes require larger, wider tires so that they offer adequate traction and wear, but not so wide that it ruins the handling.
There’s another factor that comes into play when selecting tire sizes, and that’s style. People like the look of a bigger rear tire, and might even assume it’s going to give them more grip. But putting a bigger tire on a given rim doesn’t guarantee a bigger footprint. Tire profiles are designed with specific rim widths in mind, and in some cases putting a wider tire on narrower rim can pinch the tire so much that the contact patch becomes smaller. Plus, it might rub on your swingarm, throw off your bike’s geometry and speedometer, reduce your gas mileage, and make your bike handle worse. But hey, if you think it looks cool, good for you.
If you want to know what size tire your bike should run, check your owner’s manual. Or look for the spec on the frame or swingarm, or check your wheel to find the rim width, and then match it to the tire manufacturers’ recommendation.
So far we’ve just talked about traction and some basic handling characteristic, but there’s a lot more to tire dynamics. A whole lot more. One other thing I think is worth mentioning, mostly because I think it’s interesting, is how tire size influences how much lean angle can be carried through a corner.
When a bike is following an arc through a corner, the centrifugal force that wants to push it to the outside of the turn is in equilibrium with the lateral acceleration generated by the bike leaning into the turn.
With a wider tire, the contact patch is farther away from the centerline of the bike, so the bike has to lean over farther to achieve that necessary equilibrium. With narrower tires the contact patch is closer to the centerline of the bike, so less lean angle is needed to balance the two opposing forces. That's one of the reasons why wide-tired MotoGP bikes lean over as far as they do, while Moto3 riders, who oftentimes go through tighter corners faster than MotoGP riders, don't lean over nearly as much.
