why different size wheels

[Dream Rider]

this may be the wrong thread but a couple of questions: Why, on the GS bikes, are the wheels different sizes? Handling? What would happen if I put the same size wheels and tires front and rear, on my 2012 F650GS? I want to do a bit of off pavement touring and would like to carry only one size tire for backup. SE washington to Anchorage and back. Seems I read somewhere that a guy changed an 03 1150 GS so that it had the same size wheels and tires. Would you go for the taller front match or the shorter rear match. Wide tires or narrow? any thoughts would be greatly appreciated. thinking about heading out this June. Thanks all.

[holckster]

I find it easier to ride uphill.

The larger the wheel the greater the gyroscopic affect and tracts better thru ruts, potholes, sand and such.

Just my best guess, engineering stuff is figured out by people way smarter than me.
Don't feel the need to second guess smart engineers.

Front tires last twice as long a rears so you only need to carry / buy a rear tire and spare tubes for both.

[sieg]

Just how far out in nowhere do you need to go to have it carry spares? I've been from STL MO. to Fairbanks 2 times and never needed a tire when I couldn't buy one. A credit card is easier to carry and fits any size rim! Really you can't carry enough stuff to cover every thing that can fail, sometimes you just have to wing it. But if you must, it would be easier to carry 2 tires than change the bike to one size front and rear.

[Dream Rider]

I got the part about the front lasting longer, but Iam concerned about slashing a tire and figure an extra tire that fits either is a better option. parts of alaska and canada they use crushed shale for aggregate, Really mean on tires. Took out new snow tires on my 1 ton pickup. Had a local tell me to reduce pressure in the truck tires and there would be less chance of slash or puncture. That worked for the next 1,500 miles. Thanks for your advise

[sailah]

Either way you are going to drastically alter the handing of your bike by changing the rake. Some bikes are more tolerant than others. Putting on a smaller front will make the bike steer much much quicker.

Just buy some rugged tires or buy another. Bring a 21" tube, irons and means to inflate that should get you out of most binds

[tedder]

Originally Posted by Dream Rider

I got the part about the front lasting longer, but Iam concerned about slashing a tire and figure an extra tire that fits either is a better option. parts of alaska and canada they use crushed shale for aggregate, Really mean on tires.

You know you aren't the first person to go to Canada/Alaska on a motorcycle, right?

Look around. There are probably thousands of ADVriders who have done this trip. Some have carried extra tires, most simply ship or order tires, some never bother.

Carry a tube, or two tubes. And duct tape.

[bomber60015]

Interesting in that no one has answered the guy's question, only offered unlooked for advice . . . .

While I am not contemplating changing my bike's wheel sizes, I am also curious about the different sizes.

I understand about the gyro affects, and the fact that a larger wheel will deal with bumps, racks, hole and the like more gracefully . . . . but why isn't the rear wheel the same size?

[Mr. B]

Originally Posted by bomber60015

Interesting in that no one has answered the guy's question, only offered unlooked for advice . . . .

While I am not contemplating changing my bike's wheel sizes, I am also curious about the different sizes.

I understand about the gyro affects, and the fact that a larger wheel will deal with bumps, racks, hole and the like more gracefully . . . . but why isn't the rear wheel the same size?

Ditto that.

[tedder]

Originally Posted by bomber60015

Interesting in that no one has answered the guy's question, only offered unlooked for advice . . . .

There's answering it on face value and reading into the question.

While I am not contemplating changing my bike's wheel sizes, I am also curious about the different sizes.

I understand about the gyro affects, and the fact that a larger wheel will deal with bumps, racks, hole and the like more gracefully . . . . but why isn't the rear wheel the same size?

I think you mean "effects".

A larger front wheel helps the bike roll over bumps better and helps offer slower steering that is more conducive to dirt.

Why not increase the size of the rear wheel? Well, it has less impact on steering. Second, it's hard to put a huge wheel under a motorcycle. Also, generally a wide rear tire is desired, where the front is narrower. Note dirt bikes have 18" and 19" rear wheels, where street bikes have 17". So the size does get increased.

Some older motorcycles use/used matching sizes. For instance, look at the Ural, which uses three identically-sized tires. However, modern motorcycles have abandoned the "same size tire" model. I can think of some examples that are close (TU250), but they are retro-style underperforming bikes.

Certainly same-sized-wheel bikes can be used offroad (Versys, street versions of the F650/F800, Tiger 800, a zillion sport bikes that are ridden by poor riders offroad as well as very capable riders). They have limited tire options available and obviously imperfect geometry for offroad. This is especially true as a bike becomes more sport/street oriented- a twitchy bike is no fun offroad.

The mountain bike market has been going to larger wheel sizes too. In their case, the balanced sizes is easier.

[FixerDave]

Originally Posted by bomber60015

... I understand about the gyro affects, and the fact that a larger wheel will deal with bumps, racks, hole and the like more gracefully . . . . but why isn't the rear wheel the same size?

Wild guess... weight. Back tires are wider, thus more rubber, thus heavier. The front tire has to be larger for all the reasons others have mentioned but this is not important for a back tire. It will follow no matter what, not counting a little drifting (but that's fun). On the other hand, the contact patch on the back is very important in the dirt. You want knobs with teeth to move you forward. You get that by going wide. Thus, wider on the back but smaller to offset the extra weight.

On the street... contact patch on the front becomes very important, and more important than rolling over obstacles. Thus, smaller tire to keep the weight down.

If all you will ever do is gravel, then I don't see too much issue with a smaller front tire. Well, unless you count twitchy with headshake at speed or breaking and probably a very uncomfortable riding posture. Going bigger on the back tire will probably involve lengthening the swingarm and a whole bunch of other stuff to make it fit. Might be useful if you plan on hill-climbing en-route. But, as my father was fond of saying (to me, especially)... if you want something to do that then why don't you just buy something that does that? Converting a trail bike to ride gravel is probably less productive than starting with a street bike and adding winter tires. Then again, you should have seen the crazy things my father came up with... didn't practice what he preached, not at all :) nope.

David...

[motu]

The Norton commando has same size front and rear tyres - they don't make them anymore so it obviously didn't work. I use the same size tyres front and rear on my bike - people tell me it will handle terrible....but those same people have never ridden my bike.

[chollo9]

Birds don't like it.

[JimVonBaden]

Tedder has it right.

That said, there are plenty of bikes that share wheel size front to rear, though width is generally wider in the rear. Lots of reasons for it.

I'm sure you could have a wheel made that matches the rear, or one to match the front, but you would need to go narrower, and match the shorter wheel. Handling will definitely be effected by it, and likely not for the better.

IMHO, carry a spare tube. An 18" mid sized tube will cover both wheels, assuming 17" rear and 19" front. Or carry two tubles. They are not that big or heavy. A swatch of medium thick leather placed between the tire and tube will let you get by in a pinch for several hundred slower miles.

Jim

[P B G]

I prefer to think form follows function.
Anyone got the math on dirt bike tire overall diameters? 17 inch + 110/100/17? what's the overall tire height. Same for the front.

In my form/function debate I would consider that the rear has to convert rotational force from the engine to the ground. Having sidewall is good for this generally. We see this from tractors, to drag racers, to regular road racing cars - Low Profile is not advantageous for getting good hook up/traction under throttle.

It however adds mass, tire tends to have a good bit of weight to it, and rear tires are wider. Again - form/function.

The front being unpowered and doing turning/braking activities has different constraints, surmounting obstructions being a big part of the dirt bike tire game. It also isn't under positive acceleration type rotational forces, but rather primarily braking forces.

[holckster]

I think PBG is on the right track.
Plus the diameter of the rear tire must be considered to obtain optimum power from the engine along with the chain sprocket sizes.
A tall tire takes alot to get it rotating.
Also the increased amount of "travel" that is engineered into modern bike rear suspension prevents too tall of tire or the seat height would be excessive.
The whole package must be integrated into a functional machine so comprimises must be made.