Official tire sizes 1972 Roadster

According to the factory brochures, 1970 Commandos were supplied with 3.00 x 19 front and 4.10 x 19 rear. The front had a 100% aspect ratio so was a tall tyre and the 4.10 was the first of the lower ratio (90%).

However, the 1971 brochure shows they (and all later models) were supplied with 4.10 x 19 front and rear. 

The Norvil production racer brochure (I think for 1973 as it shows the blue 1972 works racers) indicates a front 3.60 TT100. This was a solo machine of course and the change to 4.10 on the road bikes may have been linked to load capacity. The smaller tyres are close to theoretical limits on a fully loaded Interstate with passenger.

I've tried all sorts and prefer the 90/90 front Road-Rider on my 850 but it hasn't ever been subjected to a technical control.

If you asked Avon today, they would say that the 4.10 is too wide for a WM2 rim, as Norton fitted to both wheels.

Modern cars come with a list of acceptable tyre sizes (winter tyres sometimes differ) but no-one thought of that in 1972 and the UK had no type approval at that time. 

You're probably on a hiding to nothing though, if you're in a country with a Germanic Type Approval system.

If it's any help, I'll happily scan the mid-'72 / 1973 Production Racer leaflet, if you think that you have any chance of arguing the toss.

Modern soft-walled tyres don't like rims that are too narrow, but how does one convince bureaucracy of that ?

There is a passage in the old NOC "Service Notes" which recommends fitting the narrower front tyre.

We are lucky that an “vintage” bike is not tested, of course PC Plod could still pull you over and charge you with vehicle safety issues but bye and large they ignore old bikes. Even when my older bikes needed a test common sense was the bye word. Hurrah for common sense.

good luck, hopefully you can convince the authorities do look at it sympathetically 

Gino

My first 1970 Commando fastback came with a 300 x 19 ribbed front tyre and a 350 x 19 rear. Both Avons.  It handled fine but when I had the front end upgraded to a front disc brake, by Norman White, he fitted a 4.10 x 19 TT100. The rear end got the same change of tyre.  Now surely he would be the chap to ask????

The difference in handling was quite noticeable. The bike felt much firmer and safer in wet corners but did feel that it was always running a little wider than previously.

My 1972 Commando 750 LR was delivered with 4.10 TT100 tyres front and rear. At that time the Club racing at Thruxton was dominated by Commando bikes. Many wearing a 3.60 x 19 at the front end and a 4.10 at the rear. Their riders claiming that this set-up made the steering/handling feel more like a Featherbed chasis.

As stated previously 410x19 were fitted at both ends, but a slightly narrower front tyre makes the bike "point" a lot better, two combinations I have tried are- Dunlop 100/90x19 TT100GP front and 410x19 TT100 on the rear as the front is GP compound it is softer and wears ok on the front but won't last if fitted on the rear, what I am currently running on my Mk3 is 325x19 Avon Road rider Front with a 100/90x19 Road rider on the rear, previously I had 100/90x19's both ends but it was a bit like riding a "Freight Truck", the 325 is the same width as a 90/90 but has a higher profile so fills the guard better.

I've heard some folk-law stating that Norton (and BSA/Triumph) standardised on 4.10 all-round on their bigger motorcycles because they got better deals with Dunlop by buying large quantities of same-size tyres. My first Commando (1970 Roadster) was fitted with smaller tyres (I cannot remember what sizes) but it handled 'quicker' than my later 1974 Mk2A.

Tyre manufacturers used to specify "tubeless tyre, on tube type wheels fit inner tube", or words to that effect. You should be able to find that somewhere and show it to your MOT tester.