GROUP 11:- Frame, swinging arm and suspension
FRAME: Now all is to be revealed: the green eyed monster which appeared at the 1967 Show had a
frame with a tubular spine like all Commandos since, but without the smaller horizontal tube below it.
Instead it had a gusset plate wrapped round between the spine and the steering head, and they used to
break at the junction o( the gusset and spine, along with the two front down tubes. This was; caused by
our old favorite, metal fatigue---due to the nodding of the front end of the frame all the time the front brake
isn't on. The process ably assisted by the stiff, short travel forks, and the unbalanced front wheel. The
credit for putting the matter right, putting the tube in, goes to Ken Sprayson of Reynolds Tube. Later
frames don't break (not there, anyway).
Almost every year the frame was changed-it went something like this:1968 First frame, gusseted, centre
stand an frame.
1969 Bracing tube added, Featherbed-type side stand. 1970 Centre stand tube strengthened (it still
twisted).
1971 Centre stand removed from frame--side stand on a peg held by the smallest circlip in the world (it
fell off).
1972 Side stand bolted on.
1973 Steering head angle (rake) change from 27 to 28° (yokes also changed). 1974 Reinforced G6771
rear loop-gusset as continuation of inside rear damper mounting.
1975 Epoxy paint standardized-previously a thin coat of vaguely black varnish covered the rust.
In any case where a new frame is needed 1 would recommend the early 850 type frame (although the
change-over is a bit complicated for a pre-1971 model with Featherbed-type yokes and adjustable
steering bearings). The steering is more positive at high speed with 850 geometry, but to get the greatest
benefit you also need the 850 yokes (but the 750 type will fit and give the desirable increased trail).
It may be appropriate to say a few words about high speed weaves.
1. The worst machines arc 750s from 1971-1973 with top boxes and wide handlebars.
2. Light, short riders on their own are most often affected.
3. Dunlop TT 100 tyres cause more problems, than Avons.
4. I have never had problems with any bike brought back to the works with handling problems, even
hands off at 100 mph, so it is a personal thing, too.
5. The ONLY time I have been worried by the handling of any Commando was a Fastback with a
3.25 x 19 front and 4.00 x 18 rear trials tyre with about 8 psi. At about 90 mph it showed an
unnerving tendency for the rear wheel to run alongside the front one. Worse, it couldn't make up
its mind which side it wanted to be.
All you can do if the bike shows this tendency is to SLOW DOWN, but DONT tighten your grip on the
bars. -There's quite a lot going on with gyroscopic forces centrifugal forces, tyre drag, and the built-in self-
centering provided by the trail-so leave them to get on with it without the complication of extra heave-ho
from the bars.
A few further points on fire high-speed handling or otherwise of file Commando. Initially o disclaimer: 1
have only ridden a Commando for about half a milt, therefore I do not know the problem at first hand hill
the following are facts that 1 have picked up from years of associating with Norton owners:
A run down of the facts affecting Commando wobbles not covered by Mr. Steven's oration. All points on
the Commando have to be set correctly the bike is critical to any maladjustments Tyres intuit be to the
correct pressures, balanced. and in line with each other. Tyres must be of the same breed ; do not mix
Avon and Dunlop. A 3.60 front tyre is recommended for better steering and "the racer's edge". The rear
tyre of 4.10 section should be on a
24
To Next Page
Loading...
