There are Wikipedia articles on Casters and Speed Wobble which are helpful. Wobbles are an inherent feature of casters which affect aircraft landing gear and shopping carts as well as motorbikes. As you can imagine, aircraft designers have directed their attention to the problem. The tendency of casters to wobble cannot be eliminated, but can be reduced by increasing front tire inflation, increasing chassis stiffness, increasing frame mass around the steering head and decreasing sideslip stiffness of the front tire.
How does this apply to our Nortons? Front tire pressure should be kept at the upper range for that tire. Replace the head steady.... it decreases chassis stiffness. Keep the gas tank full to maximise steering head mass. Different tires may have different "sideslip stiffness", but randomly changing tire brands doesn't seem to be practical. If all else fails, install a steering damper.
I have some non-scientific stuff to add. Part of the wobble problem is the antiquated design of the Norton. Ball bearings are not suitable for steering head bearings. Even Triumph and BSA used tapered roller bearings. My "modern" 1982 metric crotch rocket developed a speed wobble due to lose steering head bearings. After adjustment, the sped wobble disappeared. Steering head bearings need to be adjustable, both to reduce speed wobble and to prevent Brinelling of the bearing race. The tapered roller bearings on the crotch rocket were Brinelled by driving the bike with improper clearance and had to be replaced.
Another Norton design defect is the rubber head steady. Both to improve handling and reduce head wobble, it has to go! (see previous thread on home-made puck headsteady).
And we need to fix the built-in offset of the front wheel on disc brake Nortons. The tendency of Nortons to pull to the left is because of this offset. No sensible engineer would design a caster with the contact patch offset from the steering axis. It gives two speeds at which to start a speed wobble instead of one. It also makes it impossible to align the front and back wheels with the steering head... they are not in the same plane.
Take your front fender off and unplug the rubber bung from of the top of the steering stem. Insert a suitable rod down the steering stem to make sure it is aligned with the centre of the tread on the front tire. if not, re-lace the wheel to true the rim.
Once I got rid of the rim offset, my Commando lost the tendency to pull left. With the puck-type head steady, head wobble is minimal. I can sit up and adjust my gloves without a tank-slapper. I can coast hands-off long enough to determine how to fine-tune my wheel alignment without significant wobble.
Manxman
