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
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