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18" vs 19" rear, off road and mx bikes

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Posted by: elcamino12sec---------------------

Just wondering why the off road bikes usually use 18" rear tires and the motocross bikes use 19" rear tires. Surely there has to be a resonable explination for this?


Posted by: BRUTE448---------------------

when your off road you want more tire to be able to get traction so u have bigger tire and less rim...19 gets around faster for fast speeds...some how a record player haveing 45's and 33 comes to mind but im not sure to to relate that to this but i mite work...any one pile on what im sayin cause as i read this i think i mite be a bit off!


Posted by: Okiewan---------------------

uhhh.. I'll need to chew on that explanation for a while. There is little is any difference in the overall tire/wheel combinations.

18" are less likely to get pinch flats; more sidewall for off-road (rocks, etc).


Posted by: dirt bike dave---------------------

The 19" has less sidewall flex, which is an advantage when cornering on hardpack.

The 19" tire weighs less. In theory, this could result in better rear suspension performance.

Less tire weight means lower weight for the motorcycle, which looks good on the spec sheet. The magazines have trained many riders to think that if it is not as light as possible, it is not as fast as possible. So a 19" rear is a marketing necessity in MX, whether if offers a performance advantage or not.


Posted by: elcamino12sec---------------------

So at the end of the day there is not much difference for a rider like me that just rides for fun?


Posted by: holeshot---------------------

Quote:
Originally Posted by elcamino12sec
So at the end of the day there is not much difference for a rider like me that just rides for fun?


Yes..and me too.


Posted by: Spokes574---------------------

OK here is the real reason for 18" for off road use.
18" has a larger side wall which is beneficial for off road use.
Larger sidewall helps reduce:
Pinch flats from large rocks, roots, stumps ects.
This is beneficial when you are over 6-18 miles away from your
truck!

Spokes574


Posted by: rmc_olderthandirt---------------------

I'm not buying any of the above explanations.

The sidewall flex will be a function of the height of the sidewall, not the diameter of the rim. When you buy a tire it will have a size specification something like 100/100-18
The "18" indicates rim diameter, in inches. The 100/100 defines the width of the tire, in mm, and the aspect ratio, so the tire would be 100 mm wide and as tall as it is wide (100%). If you bought a 110/90-19 you would have a little bit wider tire but the height of the sidewall would be essentially the same (90% of 110 = 99)

The diameter of the rear tire dramatically effects the final "gear ratio". Smaller engine bikes would run a smaller rear wheel.

The size of the rear wheel also effects the seat height. Having an 18 inch rear wheel will drop the frame a 1/2" plus it will allow the seat to be lowered a bit and still have the same wheel travel. Since the off road bike is intended to be more rider friendly a lower seat height can be an advantage.

Rod



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