Hi Tom, the following are not official figures from the time that your Vincent was manufactured as no such numbers were ever required to be produced in the years in which Vincent motorcycles were manufactured. However, here are some values which have been shown to be safe over the last 70+ years on a variety of roads and conditions.
The weight of a Vincent twin motorcycle when new was about 204 kg, dry weight, no passengers, or luggage. This weight is distributed approximately equally between the front and rear axles. So let us say that there was about 102 kg on each axle. Add fuel and oil, both tanks full and this adds about an additional 20 kg which is also roughly distributed equally between the front and rear axles. This gives approximately 112 kg per axle. Now add a rider of 100 kg. Most of his weight is in front of the rear axle and is taken by the front axle so let us say that the front axle now has 192 kg of load. If one adds a pillion passenger of up to 100 kg then that adds almost all that weight to just the rear axle and many bikes carried another 100 kg of luggage on camping trips so the rear axle could withstand a total of over 300 kgs of weight with complete safety and for many years of use. The front axle does not experience much of the weight from the rear of the bike until one uses the brakes hard. Then most of the weight of the bike, riders, luggage, and fuel can be transferred to the front of the bike. It would be unusual to get the rear wheel of such a heavily loaded bike off the ground but certainly over 80% of the total weight would be experienced by the front axle (204+20+200+rider [100]) gives about 400 kg on the front axle. When one considers that many people fitted large sidecars to these bike then the above figures are not the maximum which will have been shown empirically over many decades to be safe.
There is an alternative way to try to estimate the loads. I supply a modified front steering stem complete with springs. These springs were developed, after calculation, by road testing to give comfortable suspension. For twin cylinder Vincents with medium weight riders I provide 36 lbs/inch springs with 3 inches of preload to give a further three inches of movement. Two springs multiplied by 6 x 36 gives 432 lbs = 196 kg. For 100 kg riders I provide 45 lbs/inch springs with 2 inches of preload and three inches of movement to give 450 lbs = 204 kgs. 105 kg riders require an extra 12 mm of compression giving an extra 45 lbs/20 kgs.
Note that the whole of the front wheel, suspension, mudguard and stays, headlamp and stays etc. are all unsprung so some of their weight has to be added to the load on the front axle.
I do not know how best one can influence the administrative mind but if none of the above helps then perhaps you can look up the sheer strength of 16 mm diameter steel bar and point out that these axles have been in use for over seventy years and are not known to break.
Good luck with it.
