Rim/Tyre size question

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captainkcorb

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So I got my '72 750 working well over the winter and now it's time to replace the rubber for a little safety on the road. When I went out to confirm the sizes I found that my rear is an 18" WM3.

The question is whether this size ever came out of Andover on the back of a 750 of any vintage. Both front and rear rims appear original to the bike, the same proper Dunlop/Made in England stamp, the same age and type of spokes front and rear, etc. The rear hub is a non-cush item indicating it may have come from an earlier year than '72.

In the event that this was a PO mod in the past, is there any handling effect with the 18" rear?

Thanks,

Brock
 
captainkcorb said:
So I got my '72 750 working well over the winter and now it's time to replace the rubber for a little safety on the road. When I went out to confirm the sizes I found that my rear is an 18" WM3.

The question is whether this size ever came out of Andover on the back of a 750 of any vintage.
Click to expand...

That's unlikely, but it is a fairly common modification as the WM3-18 rim allows a wider choice of tyre.
 
If you check out the Avon web site they have a very good info table for road riders. You can get an 18 rear and 19 front with exactly the same overall rolling radius. But the 18 rear fitment is a little wider and deeper.

So, zero geometry issues (same rolling radius), slightly wider contact patch (wider tyre) and slightly added ‘suspension’ effect (deeper tyre).

It’s what I have on mine and I think it’s a great combo.
 
Fast Eddie said:
If you check out the Avon web site they have a very good info table for road riders. You can get an 18 rear and 19 front with exactly the same overall rolling radius. But the 18 rear fitment is a little wider and deeper.

So, zero geometry issues (same rolling radius), slightly wider contact patch (wider tyre) and slightly added ‘suspension’ effect (deeper tyre).

It’s what I have on mine and I think it’s a great combo.
Click to expand...
Do you get any weaving at all at high speed Eddie?
 
No. But I have clip ons and rearsets on it at present which helps keep things tight I think.

I did have a bit of a weave creep in at one time when it was in ‘sit up and beg’ trim, but I’m pretty sure that was due to the isolastics needing nipping up a bit (powder coating had crushed).

Why do you ask Baz?
 
Thanks all for the feedback and information. I've got the tyres ordered (Heidenau) but will check out the Avon site too.

I've had good results with Heidenau in vintage racing. Avon racing tyres have gotten a bit out of sight here in Canada, and they're faster than I am anyway.
 
Fast Eddie said:
No. But I have clip ons and rearsets on it at present which helps keep things tight I think.

I did have a bit of a weave creep in at one time when it was in ‘sit up and beg’ trim, but I’m pretty sure that was due to the isolastics needing nipping up a bit (powder coating had crushed).

Why do you ask Baz?
Click to expand...
When I first built my bike up it was literally from the cheapest sources,I had no spare cash at all
The wheels on my bike are aprillia pegaso or BMW funduro if you like,
The back wheel has a really good cush drive and the hub is very light compared to a commando one I also have a dural sprocket.
Originally it had the 17" rim with a 130 tyre the bike was all over the road weaving at about 90mph I fitted a steering damper it made no difference , shifting body weight made no difference I had clip ons and rear sets
Then I tried an 18" t140 back wheel with I think a 120 tyre it was very slightly better
At the time I had already fitted a yam fz750 forks these have a 40mm offset on the yokes so I fitted a stock commando 750 front end back on it,
This also made no difference
Then I tried a CMA mag on the back that was a 19" and even with a knackered old tt100 it transformed the bike
The weaving was completely utterly gone and handling/stability better
So I had a morrad flangless 19" rim laced to the aprillia hub, I re fitted the fz front end and that's how it's stayed the last 20 years.
I'd also done the mk3 conversion to the swing arm pivot,fitted an extra iso under the gearbox and another iso similar to a norvil in place of the head steady and over and under braced the swing arm all the frame/fork work was already done when first built
The only thing that stopped the weaving was the 19" rear tyre
This is why I'm curious with other people having an 18" and no problems
But like you say if it has the same rolling radius it should be the same
And obviously mine was not!!
 
baz said:
When I first built my bike up it was literally from the cheapest sources,I had no spare cash at all
The wheels on my bike are aprillia pegaso or BMW funduro if you like,
The back wheel has a really good cush drive and the hub is very light compared to a commando one I also have a dural sprocket.
Originally it had the 17" rim with a 130 tyre the bike was all over the road weaving at about 90mph I fitted a steering damper it made no difference , shifting body weight made no difference I had clip ons and rear sets
Then I tried an 18" t140 back wheel with I think a 120 tyre it was very slightly better
At the time I had already fitted a yam fz750 forks these have a 40mm offset on the yokes so I fitted a stock commando 750 front end back on it,
This also made no difference
Then I tried a CMA mag on the back that was a 19" and even with a knackered old tt100 it transformed the bike
The weaving was completely utterly gone and handling/stability better
So I had a morrad flangless 19" rim laced to the aprillia hub, I re fitted the fz front end and that's how it's stayed the last 20 years.
I'd also done the mk3 conversion to the swing arm pivot,fitted an extra iso under the gearbox and another iso similar to a norvil in place of the head steady and over and under braced the swing arm all the frame/fork work was already done when first built
The only thing that stopped the weaving was the 19" rear tyre
This is why I'm curious with other people having an 18" and no problems
But like you say if it has the same rolling radius it should be the same
And obviously mine was not!!
Click to expand...

Interesting stuff Baz, but at least you got there in the end!

So, what tyres do you use, have you stuck with the TT100s?

I know someone who has just swapped his Egli Vincent back to TT100s from Avon Roadriders and he swears it’s much better.

I’m still in the Avon Roadriders camp myself. But then again, I’ve never tried TT100s on the Cdo, so maybe I don’t know what I’m missing?
 
Fast Eddie said:
Interesting stuff Baz, but at least you got there in the end!

So, what tyres do you use, have you stuck with the TT100s?

I know someone who has just swapped his Egli Vincent back to TT100s from Avon Roadriders and he swears it’s much better.

I’m still in the Avon Roadriders camp myself. But then again, I’ve never tried TT100s on the Cdo, so maybe I don’t know what I’m missing?
Click to expand...
I'm also firmly in the roadrider camp I have a 100/90 19 on the back and 90/90 19 on the front ,
Although over the years I have had many different types of tyres fitted including tt100s etc
None seem to make any difference
The only thing that made a difference was rear wheel diameter
 
baz said:
Then I tried a CMA mag on the back that was a 19"
Click to expand...
Baz, what can you tell me about that CMA mag? The only references I have seen about CMA is 2 or 3 about brakes (check my avatar, not my front end but wish it were) Just curious, thanks.
 
CMA made legendary 3 spoke mag wheels.

It was a wheel coming out of the lathe that killed the fella behind CMA.

He started out with the support of Dave Degens, it was Dave’s initial brake design and the promise to buy them from him that got him going.
 
oldbeezer said:
Baz, what can you tell me about that CMA mag? The only references I have seen about CMA is 2 or 3 about brakes (check my avatar, not my front end but wish it were) Just curious, thanks.
Click to expand...
There's not that much to say about them really
The one I had was made in the usual CMA way is ie the Norton hub was machined down and the mag wheel shrunk onto it,this one was a six spoke and the front wheel was a five so they didn't exactly match!!
I was glad the back wheel used the Norton cush drive as I once fitted a Norton atlas back wheel to another commando and it was horrible with no cush drive
 
Rim/Tyre size question
 
oldbeezer said:
Never seen nor heard of a CMA mag till now, does anyone have a picture? or have I hijacked this thread?
Click to expand...
I had a set on a cb750f1 honda and a three spoke set on a cb550 fork
Actually I still have one somewhere that came out of a t150 it looks a bit like a wolfy but it's not the best looking wheel you can still see some of the spoke holes of the comical hub
 
Last edited:
baz said:
When I first built my bike up it was literally from the cheapest sources,I had no spare cash at all
The wheels on my bike are aprillia pegaso or BMW funduro if you like,
The back wheel has a really good cush drive and the hub is very light compared to a commando one I also have a dural sprocket.
Originally it had the 17" rim with a 130 tyre the bike was all over the road weaving at about 90mph I fitted a steering damper it made no difference , shifting body weight made no difference I had clip ons and rear sets
Then I tried an 18" t140 back wheel with I think a 120 tyre it was very slightly better
At the time I had already fitted a yam fz750 forks these have a 40mm offset on the yokes so I fitted a stock commando 750 front end back on it,
This also made no difference
Then I tried a CMA mag on the back that was a 19" and even with a knackered old tt100 it transformed the bike
The weaving was completely utterly gone and handling/stability better
So I had a morrad flangless 19" rim laced to the aprillia hub, I re fitted the fz front end and that's how it's stayed the last 20 years.
I'd also done the mk3 conversion to the swing arm pivot,fitted an extra iso under the gearbox and another iso similar to a norvil in place of the head steady and over and under braced the swing arm all the frame/fork work was already done when first built
The only thing that stopped the weaving was the 19" rear tyre
This is why I'm curious with other people having an 18" and no problems
But like you say if it has the same rolling radius it should be the same
And obviously mine was not!!
Click to expand...


I’ve experienced a weave with a 19” rear Avon. Replaced it with another same Avon, weave gone. Faulty tire? Poor mounting? Later on I switched to a 18” WM4 with a Dunlop 501. Rock solid at any speed.

In all probability, Commandos came with 19” rears for cost and assembly line simplification reasons. IMO, a 18” WM3 or WM4 is a much better choice than a 19” WM2 for the rear.
 
Last edited:
I know of no production motorcycles today that have the same width front and rear rims. Maybe some of the small kidee bikes.
 
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