MK3 Commando wheel sizes

Evening Ian

The limiting factor in fitting different size tyres and rims is the width of the swing arm and the position of the gearbox. I know that a 4:00 x 18 Avon Roadrider on a WM3 rim will fit and won't affect the overall gearing so your speedo will still read the same. (if you were using 19" Roadriders before) You will also get a deeper tread as the 18" tyre is a dedicated rear tyre, probably another 2k to 3K more miles per tyre. The front can be left at 19"    

Original was 19 front and rear. I did a 18" rear and after one tire I shelved that wheel and laced up a new rim on the spare hub I had acquired and put the 19 back on. I hated the heavy fatso continental?

I've run 19" / 19" WM2 with 4.10 tyres as per original and a WM4 rear with a 110/90-18 using both 19" and 18" fronts....I'm now back on 19" front and rear...My rear rim is a WM3 and I run a 90/90 on the front WM2...With RoadRider tyres, it's my favourite combination so far.

There was a case for trying out the 18" rear I think when only TT100s and Road Runners were available in 19" but happily that no longer applies.

I run with alloy rims and as Dave points out, the narrower 19" tyre is lighter too. It gives a nimbler, more compliant feel in my opinion.

Most modern tyre fitment charts will rarely show a WM2 rim as 'tolerated' let alone 'recommended' for the tyre widths we consider. There is a view that even in the 1970s Norton should have fitted WM3s to the Commando for the 4.10 Avon GPs. WM3 and WM4 might today be the correct options front and back respectively.

I agree with Richard's view that a narrower front tyre will give a more agile bike, but be aware of loads if you carry any luggage or a passenger. You need 110 minimum width on the rear to support the load from passenger and luggage on a MK3. If you want to go this wide in tyre then an 18" diameter keeps the rolling diameter the same. I have just fitted Metzler Lasertecs and the rear 110/90 -18 is on an existing WM3 (WM4 would be better) and it is just within the capacity rating for a fully loaded bike.

It is a wide rear tyre and I have 10-15mm clearance between it and the swinging arm, so whether a 120 would fit is open to question. I would agree David Comeau that I have set it up as a fast tourer and it is not a short track sprinter!

If you're going to change wheel size, you should do both to 18 inch, as advised by a seasoned racer from Thruxton!! Different profiles obviously. 

100/90 x 18 front on 2.50 rim and 120/80 x 18 on 2.75 rim works well on a Commando race bike. But changing the front wheel size means finding a matching mudguard 'cos the original just looks wrong with an 18". Clearance issues at the rear are more to do with the chain and, for road use, chain guard. A 130/80 on a 3.00 rim will go in the swinging arm.

For road I find that BMW R100 sizes work well - 3.25 x 19 front on 2.15 rim and 4.00 x 18 rear on 2.75 rim. Advantage being there is still a wide range of decent tyres available for old boxers.