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26 Jun 2014
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Rear Wheel 17 or 18"
Hi,
Apart from increasing top speed and reducing acceleration,
Please could someone explain the benefits of having an 18" rear wheel over a 17" with regards to off road capability and RTW availability?
Cheers.
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26 Jun 2014
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RTW
The common belief is that there are many more bikes with 18 inch rear wheel around the world in less populated places .. think 250 trail bikes, 250 farm bikes etc. Therefore you are more likely to be able to conveniently buy a 18 tyre quickly and cheaply. It may not be the 'best' tyre but it will get you to a place where you should be able to get the tyre you want. I've had to inflate the available 18" tyre to 45 psi to get it to stop squirming! But it did get me going, avoiding waiting days for a 'good' tyre to arrive.
The difference in off road capability is much much less than the riders contribution ... the expression 90% rider 10% bike springs to mind. 
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27 Jun 2014
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In terms of travel, as Warin has said it is down to availability when on the road, I have travelled using bikes with both and found 18" easier to find, 17" are around but can be more expensive.
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29 Jun 2014
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Quote:
Originally Posted by Crazy Dave
Please could someone explain the benefits of having an 18" rear wheel over a 17" with regards to off road capability...
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Larger diameter wheels deal with obstacles more easily. Ergo enduro bikes, Dakar bikes etc run 18 inch rear wheels. Motorcross bikes use 19 inch rear wheels.
All run 21 inch front wheels.
Its all related to running the largest possible wheels on a bike designed to be ridden over rough surfaces
Quote:
Originally Posted by Crazy Dave
Apart from increasing top speed and reducing acceleration,
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That depends on overall gearing. Wheel size is just a one component of the gearing equation. The easiest components to change are the sprockets. If you change wheel size by an inch, you could keep the same acceleration and top speed by changing front sprocket 1 tooth, or rear sprocket about 3 teeth. At the end of the day, no wheel size is any more or less restrictive to acceleration or top speed than any other ... because final drive ratios on chain drive bikes are so easy to change.
Last edited by colebatch; 29 Jun 2014 at 20:44.
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1 Jul 2014
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Modern MX bikes use a low profile 19" rear tyre for sidewall stiffness. The overall diameter is nearly the same as a normal 18".
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1 Jul 2014
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Thanks for the replies,
I have just ordered an 18" rear wheel and have been looking on Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator to see how it affects the Speed, RPM and acceleration. It seems like a good site.
I am not too concerned with increasing top speed or acceleration 
I intend to put a more off road tyre on for those occasions were I can use it!
It looks like my stock 120/90/17 has near as damit the same circumference as a 120/80/18.
Add a 46T rear sprocket and RPM at 70Mph only slightly increases.
Hopefully this will make 2nd gear a little more useable.
Right now I am finding 1st revs high too soon, and I'm struggling to go slowly in 2nd because the RPM is too low,
I know 1 tooth will not make much difference but adding 2 means a new longer chain. Also I think a smaller front sprocket will affect it too much and REvs will be too high for any sort of motorway miles.
Waffling on now sorry 
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6 Jul 2014
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18" rear tires are much cheaper than 17" and you have a much bigger range to choose from. (e.g. 140/80-18 are around AU$160-190, whereas 140/80-17 are AU$260+)
On smaller bikes that use 120 or 130 width, the cost-difference is negligible, but for bigger DP bikes upwards of 750cc, the narrower tires overheat and the 140 or 150 width can be very expensive. I think the wider 17" tires are generally made to be tubeless, thus adding to price, but I can be wrong.
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26 Jul 2014
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I got myself a 18" rear wheel!
It came with a part worn 120/80/18 Tyre fitted.
The rim is the same width (2.5) as my 17".
I have read it is possible to fit a 130 or 140 wide tyre!
Is the any benefit of choosing wide/narrow tires?
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6 Aug 2014
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Narrower tires generally provide more grip and less rolling resistance- in the 80's, manufacturers tend to go for wider back tires than they do now on trailbikes (e.g. XR500 came standard with 150 rear compared to 120 currently fitted on CRF450's).
Wider tires (generally) have a higher load carrying capability, and will run cooler and provide longer tire life.
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