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Choosing Tyres for the Norton Commando

posted 25 Dec 2011 05:02 by NOC NSW   [ updated 25 Dec 2011 05:03 ]

Introduction

Selecting tyres for the Commando has always been tricky because of the relatively unusual 19 inch rear wheel and the transition to new and different tire size designation systems right at the time of the Commando's introduction. The trend to much smaller diameter and wider tyres on new motorcycles (which reduces the number of suitable tyres on the market) makes picking new tyres even harder.

The first thing we must do is understand how tyres are sized. Once upon a time, this was very simple. The last part of a tire size, the wheel diameter, given in inches, is still simple, thank goodness. But the rest gets complicated.

At one time, after determining your wheel diameter, one number told you all there was to know: "3.50" meant a tire three and a half inches wide. It was also three and a half inches tall, but we never really worried about that. But in the 1960s, the "low profile" concept began to affect motorcycle tyres. Those 3.50 tyres were still three and a half inches high, but the width was creeping out towards four inches. No one noticed, because the tire manufacturers didn't do anything to clue us in. With the introduction of the Commando, the low profile movement took a giant step forward, and the manufacturers took the opportunity to introduce a new sizing system to draw attention to it.

The standard big bike rear tire at the time was 4.00-18, but road racers still used 3.50-19. These were the same rolling diameter, but the slimmer carcass of the 3.50-19 ran cooler. (The biggest problem with racing tyres is usually operating temperature rather than traction- of course, traction fades as a tire overheats). In the 1960s even a hard ridden street bike could stress a good quality 4.00-18 to the point of disintegration. The lighter weight of the 3.50-19 was also advantageous- heavy wheels and tyres are flywheels and gyroscopes that make a bike hard to accelerate and maneuver.

So Norton decided to equip the new Commando with a version of Avon's 3.50-19 roadracing tire, the "GP". This was a cheap, second rate racing tire- the Dunlop KR series "Triangulars" were essential for serious racing- but the GP was miles ahead of any street legal tyres available then. At the same time or soon after, Avon decided to do something about the fact that a tire designated "3.50" was actually a little over four inches wide. Till then, tire sizes had always advanced in quarter inch steps- 3.00, 3.25, 3.50, etc. The new code used selected intermediate numbers, so the 3.50-19 racing GP became the 4.10H19 original equipment rear tire for the Commando. The 4.10 told you it was equivalent to a 3.50, but "low profile", or wider (4.10") than it was tall.

And what is that "H" replacing "-" or "x" to separate the numbers? It was the first sighting of the now familiar speed ratings, also used on automobile tyres. These were established in kilometers per hour, but those of us who think in miles per hour can regard them like so: S = 110 mph, H = 130 mph, V = 140 mph, Z = 150 mph. H is the basic rating for any sort of sporting pretensions, but if most of your riding at speeds over 110 mph is brief in duration, an S rating will do!

Shortly thereafter, a standard American sizing system starting with "M" for "motorcycle" and then using other letters to specify width, and a European system using millimeters of width, combined with a number representing the aspect ratio (that low profile business again) as a percentage, were introduced. tyres like our Avon GP are about 90% as tall as they are wide, so their aspect ratio is 90.

So a 3.50-19 equals a 4.10H19 or a 100/90H19 or an MM90H19. Today the letter designations are only used for tyres intended for Harley-Davidsons.

Now that we have our language down, so we know what we are talking about, lets look at specifics.

Front Tyres for Early Commandos

On the first incarnation of the Commando, the 1969 and 70 models, the factory equipped front tire was a 3.00-19 Avon Speedmaster Mark II. These were very skinny and very lightweight. The rubber was as sticky as you could get, but the tread was a shallow rib that faded to mere decoration as you approached serious lean angles. Availability in North America may be spotty- Avon apparently does not officially import this size anymore, but you may be able to order them from Britain or find a specialist here who imports them himself.

The Speedmaster Mark II in one size larger, 3.25S19, should be readily available. This will fit and work fine, but some slight bit of steering precision and "flickability" will be lost.

For those looking for suitable replacements in more modern tyres, the 3.00 original width translates to 80/90 in the modern metric parlance, but nobody makes such a skinny tire in 19". The 3.60 Dunlop K81 or Avon Roadrunner Universal, which replaces both 3.00 and 3.25, is an excellent choice, but, like the 3.00S19 Avon Speedmaster, neither is listed by the importer anymore, but the Dunlop at least may be sourced one way or another from Britain. A 90/90 of modern type like the Avon AM20 is a practical choice, but will be somewhat wider and heavier than the original, making the steering a bit on the heavy side.

Rear Tyres for Early Commandos and both Front and Rear for Late Commandos

In 1971 Norton introduced the practice of using identically sized (4.10H19 or 100/90H19) tyres front and rear, with the fork yokes revised to give appropriate steering geometry. Some Commandos left the factory fitted with Avon GPs at both ends, but the luckier ones wore the new Dunlop K81.

The Dunlop K81 incorporated in a street tire some of what Dunlop had learned on the track with the KR series Triangular racing tyres. It was the second tire to adopt the 4.10H19 designation, and was a world better in every way than the Avon GP. It bears two extra names, "Roadmaster", and more meaningfully, "TT100", because they were used on Malcolm Uphill's Triumph T120 Bonneville to attain the first 100mph lap of the Manx TT course by a production bike (1969 Production TT). First made in Britain of course, by the late 70s and early 80s, they were variously manufactured in Ireland, France, and the USA. Since Sumitomo, Dunlop's former Japanese subsidiary, took over Dunlop worldwide, K81s are made in Japan.

Avon answered the K81 with the Roadrunner (now known as "Roadrunner Universal") around 1973. I think it is better in all regards than the Dunlop, but not by colossal margins, and many people will argue the point. Like the K81, the only size now made for 19" wheels is 4.10 or 100/90.

Into the late 80s, manufacturers such as Metzler provided useful options in 19" rear tyres, and Dunlop's K291, K391, and K591 high performance tyres continued to be available in 19" sizes suitable for rear use. Alas, the trend to smaller wheel diameters for all road bikes has caused these to be dropped from the catalogs.

Currently most riders seeking higher performance (or better pose value) are using Avon Super Venoms or the new AM series Roadrunners. Super Venoms are the current development of the Venom introduced for the Hesketh. The Roadrunner AM series are sort of an everyday Super Venom, sharing tread patterns and general construction with the Super Venoms, but are H rated whereas Super Venoms are V rated. The original Roadrunner has been renamed the "Roadrunner Universal". It would have been a lot easier for us if they had used a different name for these completely different tyres! But they didn't, so when you go tire shopping you must keep this clear when shopping for Avons.

The AM series Avons as well as other tyres developed from the 1980s onward, are very different in basic construction from the earlier tyres designed in the 60s or 70s, and this brings up some problems with size comparison. There are always variations between actual size and nominal size and between different tire manufacturers and tire models, but the generational difference we find between tyres designed before 1975 and the more recently developed tyres makes exact comparisons almost impossible. But generally, comparing a tire of the 1980s or 90s and one of the 70s with the same nominal size, we can expect the newer tire to actually be wider and lower in profile than the older one. Due to issues of fender clearance and effect of size and weight on steering characteristics, this affects the front choices most.

For example, even though a 100/90 Avon Roadrunner Universal and an AM20 (either Roadrunner or Super Venom) share a designated size and are the same rolling diameter, the AM20, designed in the 80s, is significantly wider than the Roadrunner Universal. Logic would seem to call for the 100/90 AM series to be better described as 110/80, but not so. Why? I can only speculate that it is to indicate that it will fit the same 1.85" wide (WM2) rim as the Roadrunner Universal.

But this is why one may often want to consider a nominally smaller front tire. Continuing to refer to the Avon AM20, the 100/90 is so wide it j-u-s-t barely fits between the fender braces on a late Commando. I prefer the 90/90 AM20. It fits nicely, being exactly the same width as the 100/90 Roadrunner Universal. But it is lighter, and smaller in diameter, and thus provides really sharp, nimble steering.

Further complicating the issue for the Commando rider wanting a set of AM series Avons is the fact that 19" Super Venoms are available only in 100/90V19 (AM20 front or AM18 front or rear). The AM20 front tread is made as a Roadrunner in 90/90H19 and 100/90H19. The Roadrunner rear tread, AM21, is not available in 19" at all.

All Avons are still made in Britain, a factor which prejudices some of us with British motorcycles in their favor.

Another option, although the idea will upset by-the-book types, is to select a 19" tire designed for front use that looks stout enough to handle the guff, reverse the direction of rotation, and mount it on the rear.

For example, the Michelin Tarmac, sold as a "high performance cruiser tire", is made for front use in 90/90H19 and 100/90H19. The adventurous early Commando rider might try those sizes front and rear respectively, and riders of late Commandos could try 100/90 at both ends. No guarantees, especially of rear mileage.

And at this point, we should remind ourselves that there is no such thing as good mileage on a 19" Commando rear tire!

Then there are Cheng Shins... made in Taiwan, cheap, lousy wear, but recommended by some as giving good value for money. Their C199H is a copy of the Dunlop K81.

Suggested Tyres

To sum up, my suggestions, for 1969-70 Commandos first:

Concours display:
Front: Avon 3.00S19 Speedmaster Mark II
Rear: Avon 3.50H19 GP (Current manufacture in race compound reverts to "plain" inch sizing.)

All around riding:
Front: Dunlop 3.60H19 K81 or Avon 90/90H19 Roadrunner Universal, if you can find either one.
Rear: Dunlop 4.10H19 K81 or Avon 100/90H19 Roadrunner Universal

Very fast riding:
Front: Avon 90/90H19 Roadrunner AM20
Rear: Avon 100/90H19 Super Venom AM18

For 1971-75 Commandos:

Concours display:
Front and Rear: Dunlop 4.10H19 K81 (New old stock if possible- Made in England is best!)

All around riding:
Front and Rear: Avon 100/90H19 Roadrunner Universal

Very fast riding:
Front: Avon 90/90H19 Roadrunner AM20
Rear: Avon 100/90H19 Super Venom AM18

The 18 Inch Rear Wheel

Converting the Commando rear wheel to 18" diameter has been seen as a panacea for the limited choice of 19" tyres. Fifteen years ago, when the 18" rear wheel was still the industry standard, this would indeed expand your choices to include state of the art rubber. Today, as the industry standard moves toward ultra wide (three and a half to six inches or more) rims of 16" (cruisers) or 17" (sport bikes) diameter, the latest high tech radials are not going to be available in any size useful to us. But you will still find a wide range of 4.25/85 or 110/90H18 rear tyres, so many that I won't even attempt to chart them all, except to say that I am partial to the Avon 110/90H18 AM21 Roadrunner.

The conversion can be done without much difficulty. For street use a WM3 rim, 2.15" wide, would normally be used, although racers will want the widest rim allowed for their class of competition. New rims are available from many sources. The premier American supplier of spokes, Buchanan's, will provide a set of stainless spokes and nipples for about $80. A rim and spokes for a Norton Atlas would fit perfectly, of course.

Many bikes have similar spoke patterns, and any 40 spoke wheel with the same spoke flange diameter as a Norton rear hub is a potential source of rim and spokes. I built my first 18" wheel for my Commando with a Borrani aluminum rim intended for a drum brake Moto-Guzzi and an aftermarket spoke set for a Kawasaki Z-1.

But do not think you will make your Commando handle better with an 18" rear wheel. You may think it will provide more traction, but traction is not handling, and a stock Commando has no need for more traction. What a bigger contact patch does provide is a bigger area over which to dissipate traction's enemy, heat (obviously the tire engineers have long since overcome the limitations that caused Norton to select the 3.50-19 rear tire originally), and that is truly advantageous- when you are pumping 100 horsepower through the tire to maintain 130 mph on the banking at Daytona.

Otherwise, the larger tire merely adds weight- increasing the flywheel and gyroscopic effects that make a bike feel comparatively unresponsive. Not that a 110/90H18 rear tire will by itself transform your Commando into a leaden, unturnable cruiser. Rather it will show just a bit more stability and less responsiveness to throttle or steering inputs. But this is not usually the kind of handling we are looking for from a motorcycle like the Commando.

In the case of a Commando built for racing, handling response may have to be sacrificed for sheer grip, and the choice and ready availability of 18" race tyres make an undeniable argument for converting. The poseur who requires that his bike look fast may find a compelling need also!

On the street, the 18" tire can usefully improve load capacity and especially wear. A 100/90 tire is typically rated in the high 400 lbs of load carrying capacity, whereas a 110/90 will be 10 to 15% more, in the mid 500 lb range. And the best 100/90H19 rear tyres last no more than 3,500 miles under most circumstances, while a 110/90H18 can take you right past the 5,000 mile mark. For these reasons, the touring rider, especially if of large size himself, or who commonly rides two up and heavily laden, may well give serious consideration to building an 18" wheel for his Commando.

But most of us, with our mix of occasional commuting to work, Sunday morning sport rides, and hauling a sleeping bag a few hundred miles to a weekend rally, are best off with the standard 19" rear wheel.

(Availability of tyres specified in this article may differ outside the US)

by Ben English