Vincent, in particular, due to its unusual fork design and mass distribution seems to be somewhat sensitive to extreme tire pressures. Stability is quite directly effected by the size and frictional nature of the front contact patch. This "footprint" area and its dynamic damping effect upon front wheel oscillation has a clear relationship to ones particular tire constructions and design concept. Strongly buttressed by the cord and ply orientation, as well as effected by greater air pressure within, its greater resistance to tread movement lessens the necessary casing and tread movement required for that active footprint.

Contemplate this living patch and its damping effect upon normal excessive movement of the fork assembly, including, of course, the complete wheel. All of that unsprung mass, influenced by caster forces, would dearly love to flail wildly back and forth if it were not for the restraining influences of the steering damper's frictional effectiveness, and that little known and so often ignored front tire contact patch squirming against the road surface. Think about it.

All else being optimum on your machine, consider two different scenarios, in each case the normal damper being ineffective, loose , or forgotten, all together. Case study #1, using an earlier front rib tire: The bike is cruising at a road speed of 60 to 70 m.p.h. and all is as designed and intended by the manufacturer (excluding the damper). One's front wheel is deflected to one side by a sudden or sharp bump to the other side of the tire, or the front wheel is knocked up off the road surface, or perhaps both simultaneously. If only deflected to one side, the tire never leaving the road, a supple contact patch will squirm, locally deforming to the affected tread face, while not actually lifting from the tarmac. Dynamic forces in the rubber bring the wheel to a stop in its swing to one side and then return it towards its original patch. The mass-distribution of the fork assembly influences all of the forgoing actions. The experience outlined above is normally referred to as head-shake.

Case study #2, using a modern sports tire: Head-shake once again occurs, except this time we must remove from the equation the effect offered by the tire's supple dynamic. Now everything changes and disaster looms ahead for rider and machine. Without an effective steering damper and now riding on a stiffly belted carcass, intrinsically more rigid throughout its structure, particularly in the tread area, and pumped up to 38 to 40 p.s.i. within, then we have effectively removed those self centering and directionally controlling influences inherent in the originally fitted softer tire, pumped up to 24/25 p.s.i.

With no force to restrain the wheels swing, the built in caster only promotes the rapid build up of violent oscillations, now increasing to the point where the fork stops take a savage beating on one side and then the other, with one's front wheel turned sideways to the direction of the travel. The forward momentum abruptly alters, and the rider is flung from the gyrating motorcycle which wildly crashes itself. It is rare to recover from such a situation. Few have the strength and the agility, as well as the sufficient presence of mind to ride it out (as an immediate drop in speed will only intensify the wobble because you will be eliminating the gyroscopic effect that might allow the bike to return to an upright position and a straight line up the road). Returning to my point, it is this natural characteristic of rubber to returns to its original shape after being deformed which is utilized to return all disc brake pistons, so releasing pressure from behind the pucks. No return springs or anything else is involved, just a deformed square section of rubber doing its natural thing.

Back to Rip's inquiry; it may also be said that a 19" diameter, as opposed to the original 20" will deflect more easily due to its smaller radius. Later he called me back to report that the bike in question turned out to have greasy steering damper plates and discs! So it was a case of over-greasing the lower head race bearing assembly. Be aware and warned. Too high a pressure in any bike's front wheel, even a modern Jap bike, can cause the same grief, and for similar reasons, most now being devoid of any form of steering damper. In sum, remember to utilize the damping effects provided by properly inflated tires. Lest you fear I somehow forgot, or worse, ignored the stabilizing effect of the spinning wheel in my observations, such is not the case. Being nearly identical throughout, I simply left it out to simplify things.