H: Hubs, Wheels and Tyres 21" Front Wheel

nobby

nobby

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BigEd said:
I have 100/90 x 19" Front and Rear.
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I am lacing two new wheels: hubs, flanges, spokes and rims. Both 19". Did not decide what tyres yet. Occasionaly I will use the Vincent with a left handed sidecar, also 19" rim.
 
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highbury731

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Tyre size ---- Overall diameter ---- % difference from 20"

21" 3.00 Speedmaster --- 27.2" ---- + 3
20" 3.00 Speedmaster --- 26.2" ---- 0
21" 90/90 Road Rider --- 27.48" ---- +4.9
19" 90/90 Road Rider --- 25.47" ---- -2.8
19" 100/90 Road Rider --- 26.40" ---- +0.8

I see comments that a small percentage increase in rolling diameter is ideal for correct speedo reading. Also, am I correct in thinking that the John Emmanuel fork conversion results in a small rise in front ride height? TT?

Paul
 
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Flo

Flo

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Albervin said:
Peter, physics was never my strong point. I am more into drugs!! BUT. The mass of the cast iron drums plus steel brake plates as opposed to the steel/alloy rims needs to be taken into account. That will take serious computations that a) I can't recall and b) I don't want to. All I can say is the 21" front wheel on my Rapide is a joy and everone who has ridden it are impressed with the light turning/steering on Australian roads.
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The brake plates do not rotate, i.e. they do not contribute to gyroscopic effects re. the rotation axis of the front wheel!
 
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timetraveller

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am I correct in thinking that the John Emmanuel fork conversion results in a small rise in front ride height? TT? I do not know at the level of millimetres. Sorry about that but if it does then it is very small. I have never noticed it and no one else has commented. Instead of the lower link pointing downwards it is important that it is horizontal or a mm or two up at the front with the rider fully seated. However, the rear pivot point has been moved both downwards and forwards so the angle of the lower link is not a good indicator. What is certain is that when one brakes the bike now dives rather than trying to ride up over the front forks. That is the motivation behind the modification. Have you felt that you are higher on your bike than you were?
 
Michael Vane-Hunt

Michael Vane-Hunt

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Avon add ftom 1908
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John Reynolds

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In my article 'An analysis of Vincent Suspension' (MPH November and December 2021) I gave details of the ride heights of the Series A, B, C and D front suspensions, which I defined as the height of the top rear pivot axis above road level when the suspension was at null travel with a load of 232 pounds supported by the front forks. This loading is based upon the weight of a twin, with 40 pounds of petrol and oil on board with a rider weighing 200 pounds and a weight distribution of 42/58 (which was established by direct measurement). Obviously the ride height will change with different loadings. For the C Girdraulic the ride height is 34.7 inches, for the Series D it is 36.0 inches.

Norman kindly supplied me with dimensions of the John Emmanuel fork, which was analysed towards the end of the article. I decided not to quote the ride height for the JE fork because it is dependant upon the poundage of the springs fitted and the amount of packing which is placed in the springboxes. However for the two scenarios which Norman gave me, i.e. 30 ft/lb springs with 3 inches of packing and 45 ft/lb with 2 inches of packing, I calculated the ride heights to be 35.5 inches and 35.2 inches respectively. Obviously, these ride heights can be easily changed by altering the packing.
 
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