Why do dirt bikes have different sized wheels?


21 replies to this topic
  • Nigel Tufnel

Posted 06 March 2008 - 01:36 PM

#1

I did a search and had no luck. So why do bikes have different sized wheels, in particular why is the the rear wheel smaller? Is there a technical reason or is it legacy left over from old bikes with 2 rear shocks?

I come from a mountain biking background and mountain bikes with smaller rear wheels have been tried but are not prevalent because they have disadvantages. The main one is that they turn/corner like crap because of the different wheel radii when leaning in a corner. Wouldn't dirt bikes corner better if they had the same diameter in front and back?

  • El Marko

Posted 06 March 2008 - 02:40 PM

#2

Many wheel size combos have been tried and in fact in the 50s/60s it was common for both wheels to be the same size.

The front wheel is designed to be able to steer the bike, which a taller, thinner wheel is better at doing; the rear is to deliver traction and power (the reason why it's not the same deal as a MTB; pedal power doesn't come close to matching horsepower).

Various front wheel sizes have been tried repeatedly, including 19, 20 and 23 inches.

Rear wheels have pretty much always been 18 or 19, although there were experiments with 17".

Part of this is that manufacturers also have to deal with limited tire choices if they go to oddball wheel sizes. Those oddball tires are almost always more expensive and harder to find.

  • Ghini

Posted 06 March 2008 - 02:45 PM

#3

it also has to with being on dirt, a mtb is designed to work well on pavement in addition to dirt while a dirt bike is not intedend (due to law) to be ridden on pavement (which is why supermotos have 17s front and rear). Like El Marko said, a taller skinnier tire will track and steer better on unstable ground, while a shorter fatter tire is nessecary for traction. A short fat tire in the front would result in front end tracking where ever the surface takes it, as well as making for some good understeer.

  • busted bones

Posted 06 March 2008 - 02:51 PM

#4

rear tires very in size for what they are being used for. Im not sure how exactly to describe it but the acceleration happens alot faster on an 18 in rear tire which i use for easily wheeling over logs offroad and i use a 19 on track because it gives you a bit more top end in each gear which allows me to strech the gears out on the face of a jump

  • bills442

Posted 06 March 2008 - 03:09 PM

#5

a larger front tire has more "gyroscopic" effect which is generally a good thing ... it also makes the bumps smaller ... as they are smaller with relation to the tire rolling over them. Consider the difference in the way a car with 13inch tires handles a chuckhole vs a truck with 15inch tires.

  • Kyron

Posted 06 March 2008 - 03:11 PM

#6

Im not sure how exactly to describe it but the acceleration happens alot faster on an 18 in rear tire which i use for easily wheeling over logs offroad and i use a 19 on track because it gives you a bit more top end in each gear which allows me to strech the gears out on the face of a jump


Huh? ........ they are both the same height. The 19"s have a lower profile so they dont "fold" over.... and the 18"s with the bigger profile are better off road.......


As to the orig question, I havent a clue why 21" front and 18/19" rear is the normal...... but I have a question for you, if humans evolved how come we have to wipe our buts :thumbsup:

  • jschner

Posted 06 March 2008 - 03:18 PM

#7

Just a guess but I think in MX they use lower profile rear tires for weight reasons and to get more wheel spin so to make up for the tire height they lose from the lower profile they use a 1" taller wheel.

  • Nigel Tufnel

Posted 06 March 2008 - 03:21 PM

#8

Thanks for the comments, but so far I haven't really heard a good answer for the difference. I didn't want reasons why dirt bikes are different than mountain bikes but more of why the rear is smaller on a dirt bike from a turning dynamics standpoint (if any). Street bikes/super moto bikes typically have the same wheel diameters front and rear and they have more hp to get to the ground. I mentioned mountain bikes because they are also two wheeled and turn by leaning and I have felt the difference in turning between a standard mountain bike and one with a smaller rear wheel.

One difference could be a dirt bike will see much more wheelspin but I don't see how a smaller rear wheel would improve the turning dynamics.

  • Thunderbug

Posted 06 March 2008 - 03:21 PM

#9

Dirt bikes have different size wheels to allow us to run different size tires.

Next question....

  • Ghini

Posted 06 March 2008 - 03:23 PM

#10

the rear wheel is also your final drive ratio as some of you guys alluded to. If you stuck a tall rim & tire on there, you would gain top end but loose acceleration.

as for the butt whiping question, we evloved so we whipe our butts, before we evolved we just popped a squat, dropped some heat, stood up and went on our marry way! Now we've evolved to whipe the smelly shit from our asses! ...also our diets had much more fibre in them which makes shit much more firm and we prolly didn't need to whipe...

  • bills442

Posted 06 March 2008 - 03:25 PM

#11

it's not about turning, it's about having a front end that works in *brutal* terrain. Put a 17 on your dirt bike front and then go do some knarly rock infested, ruts and singletrack with whoops, you'll quickly see the advantage of that bigger gyroscopic bump flattening 21 inch tire ....

  • GlobalWarmer

Posted 06 March 2008 - 03:26 PM

#12

My CR has 19/21 combo. Measured from top to bottom with rubber on they are nearly the same height. My 650 has an 18. I haven't measured it but they look like taller tires. Maybe it's a bump absorbtion thing.

  • Nigel Tufnel

Posted 06 March 2008 - 03:32 PM

#13

it's not about turning, it's about having a front end that works in *brutal* terrain. Put a 17 on your dirt bike front and then go do some knarly rock infested, ruts and singletrack with whoops, you'll quickly see the advantage of that bigger gyroscopic bump flattening 21 inch tire ....


I agree a 21 front would be better than the 17 in the scenario you indicated. So then wouldn't a 21 rear wheel bet better than a 18/19.

I am not really looking for what is the best diameter front or rear but why is there a delta?

  • bills442

Posted 06 March 2008 - 03:34 PM

#14

here's a good site refencing a book talking about what I said.

note how it discusses that a smaller wheel has to begin "mount a step in shorter time" to handle a bump, you can see why a larger wheel is advantageous for the front suspension ... it also talks about the stabilizing forces of gyroscopic action arrive sooner with a larger diameter

http://books.google....XFZj6uy4o&hl=en

a 21 isn't needed in the rear so much because you aren't steering with it ... and think how much higher the seat height would have to be to get those inches of travel

  • Ghini

Posted 06 March 2008 - 03:41 PM

#15

Thanks for the comments, but so far I haven't really heard a good answer for the difference. I didn't want reasons why dirt bikes are different than mountain bikes but more of why the rear is smaller on a dirt bike from a turning dynamics standpoint (if any). Street bikes/super moto bikes typically have the same wheel diameters front and rear and they have more hp to get to the ground. I mentioned mountain bikes because they are also two wheeled and turn by leaning and I have felt the difference in turning between a standard mountain bike and one with a smaller rear wheel.

One difference could be a dirt bike will see much more wheelspin but I don't see how a smaller rear wheel would improve the turning dynamics.


turning dynamics take a back seat while other variables are taken into consideration. the front wheel&tire is tall and skinny to track and steer proficiantly on unstable surfaces. while the rear wheel&tire is short and fat to provide traction and a sutable final drive ratio for its intended enviornment. If you were to put a front wheel and tire on the rear, you would get no traction in the dirt and your accel. would be crap. if you were to put a back wheel and tire on the front, your front end wouldn't track well, it would float on dirt when you want it to dig in and bite making it push. Essencially the two different sizes are the happiest medium manufactures could find between turning and traction. And as i said before this is all because it is soley designed to be ridden off road, on a multitude of unstable surfaces. Sport bikes, supermotos are designed to be run soley on pavement which is a very stable surface for traction and for steering. A mountain bike dosn't have to deal with the traction variable that a motorcycle does so the rear tire size dosn't have to compensate for that variable. the dirt brings with it another set of variables that street going bikes don't have to deal with and a mtn bike dosn't have the variable of power. that sir, is the most complete answer to your question you will find.

  • M3A1Scout

Posted 06 March 2008 - 03:46 PM

#16

One of the reasons to a narrower rim in the back may be the torque it has to handle. The bigger the rim, the longer the spokes and the more work the rim has to do to "convert" the power to the tire. By keeping the rim diameter smaller, more of that work is performed by the chain and drive gearset.

  • bills442

Posted 06 March 2008 - 03:50 PM

#17

my 1979 xr250 with a 23!!!! inch front tire, was unstoppable!
not only did the larger front tire add horespower, but I got chicks because of it while my other buddies with their small flaccid 21inch tire were getting no play. [ kidding ]

  • Thunderbug

Posted 06 March 2008 - 03:54 PM

#18

my 1979 xr250 with a 23!!!! inch front tire, was unstoppable!
not only did the larger front tire add horespower, but I got chicks because of it while my other buddies with their small flaccid 21inch tire were getting no play. [ kidding ]


Baby always says..."Bigger is better!" :thumbsup:

  • Nigel Tufnel

Posted 06 March 2008 - 04:09 PM

#19

turning dynamics take a back seat while other variables are taken into consideration. the front wheel&tire is tall and skinny to track and steer proficiantly on unstable surfaces. while the rear wheel&tire is short and fat to provide traction and a sutable final drive ratio for its intended enviornment. If you were to put a front wheel and tire on the rear, you would get no traction in the dirt and your accel. would be crap. if you were to put a back wheel and tire on the front, your front end wouldn't track well, it would float on dirt when you want it to dig in and bite making it push. Essencially the two different sizes are the happiest medium manufactures could find between turning and traction. And as i said before this is all because it is soley designed to be ridden off road, on a multitude of unstable surfaces. Sport bikes, supermotos are designed to be run soley on pavement which is a very stable surface for traction and for steering. A mountain bike dosn't have to deal with the traction variable that a motorcycle does so the rear tire size dosn't have to compensate for that variable. the dirt brings with it another set of variables that street going bikes don't have to deal with and a mtn bike dosn't have the variable of power. that sir, is the most complete answer to your question you will find.


Thanks for the nice summary.

The link posted above looks like it more than covers what I am after. Thanks! :thumbsup:

  • Slackkinhard

Posted 07 March 2008 - 08:35 AM

#20

Can I put a 19" front wheel on my CRF250 and dominate the mini class





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