Rear Rim Size & Tyre Question

My bike came with an 18” rear rim as well with a metzler tire fitted .... can’t remember tire size , it was a tight fit but no issues .... I have since gone to original alloy wheel size front and rear ....

A 110/90-18 tire would have an approximate width of 4.3 inches (110 mm / 25.4 mm/inch) and a sidewall height of 3.9 inches, so it would be both wider and taller than the 3.50-18 tire.

I run a 110/90/18 tire on my WM3 and it fits fine with enough space to go even wider if you don't mind very close tollerances. The original tire that was on that rim when I tested my conversion to the 18" tire was actually 120/90/18 and that tire did just barely fit on my '70 commando swingarm (no gussets, as you discribed)

There's no doubt in my mind that a 4.25"/80/18 would fit with room to spare.

Craig said:

My bike came with an 18” rear rim as well with a metzler tire fitted .... can’t remember tire size , it was a tight fit but no issues .... I have since gone to original alloy wheel size front and rear ....

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This combo would not provide the best handling and feel. On that rim, a 100/90 Avon Roadrider would give the best handling.

Avon 4.00-18. Handles great.

https://www.avontyres.com/en-gb/tyre-details/59/roadrider/?cartype=motorcycle

120/90-18 rear is a great handler and very long wearing.

Most riders could not tell the difference between a 18” and a 19” rear. Until I see lap times of a half second difference I contend rear tire handling is subjective, certainly on the road.

" Am i correct that OE the rear wheel size was 19" "

Yes

Yes... WM2... 1.85

JimC said:

120/90-18 rear is a great handler and very long wearing.

Most riders could not tell the difference between a 18” and a 19” rear. Until I see lap times of a half second difference I contend rear tire handling is subjective, certainly on the road.

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Think that opinion War already been waged on this forum

Craig said:

Think that opinion War already been waged on this forum

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Waged yes, settled no.

As long as a 4.25/85 tyre fitted onto the Akront 18" rim doesn't foul the swing arm i don't give a toss

If the tyre size is smaller in cross-section, it generates heat more easily. A Commando is not a modern superbike, so you usually get more grip, not less. If the diameter is 19 inch where it should be 18, the steering head steepens, so the bike tends to understeer. If the reverse is the situation, the bike will tend to oversteer which can be good. Some guys like to fit fat tyres on the rear of their classic bikes. Apart from stuffing the handling, they often have something on the back, which is not as sticky as it could be. The best handling Manx Nortom probably has 19 inch wheels with skinny tyres. You are only playing with about 50 BHP.

olympus said:

Hi all
Am i correct that OE the rear wheel size was 19"
In the basket case i acquired it came with Akront 2.15 (WM3) 18" rear rim with no tyre fitted. But in the "package" came a Dunlop TT100 Road master K81 4.25/85 x 18 .... so I'm assuming the tyre was got for this rim..
As the tyre has been left outside it has deteriorated to the point it needs to be replaced.
So with the Std swing arm (no front gussets fitted) will it accept the WM3 18" rim & the 4.25/85 tyre and not contact the swing arm???

Thank you

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You don't mention the front wheel and tyre!

You may think the rear has deteriorated due to being outside, but tyres deteriororate over time unless stored very carefully, so if you got a front tyre in the package you probably don't want to use that.

Your best solution will be a pair of tyres that work well together.

Seems like you are happy to run the non standard rear, but you are likely to find that it will ride better with a skinnier tyre than you are planning

90/90 front with a 100/90 rear Avon Roadriders will probably be as good as you will get for balance. If you are worried about gearing with the smaller diameter rim just change your front sprocket.

If it was me I would go with those tyres on a pair of 19" rims.

Not directly relevant but in race compound (AM26) these are indeed the favoured Manx Norton race tyre.

And I plan to use these same tyres on my in build Suzuki T500 race bike which has a non standard 19" rear wheel! Similar power output to a Commando, just peakier delivery!

Your T500 Suzuki does not have the same weight distribution as a Commando. I once won a race with a Cobra. I had to sit right forward over the tank to get it around. Skinny tyres on that might make it too nimble and a bit indefinite in it's handling. Steering depends a lot on rear end squat, with a Cobra - they tend to be light in the front, so the steering head angle increases more easily as you come out of corners . If I was racing one, I'd use 18 inch wheels with slightly fatter tyres to slow the steering up a bit. They you can push it harder.

Thirty years ago I wore out numerous sets K81s canyon racing on a Commando. I tried all the size combinations and settled on a WM3 4.25/18 rear and a WM2 4.10/19 front. A 3.60/19 on the front also worked well. Quick steering but flighty. I should say though that most of the rear K81 rear tires that I wore out were English made 4.10s.

The 4.25s were all USA made.

We preferred the English compounds but with pressures on the low side we could get the 4.25s to work.

All K81s, IMO, work best when they are half worn out