Taking a grinder to Britain's motorcycling heritage.
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Wednesday, 10 March 2010

Getting a few figures in place

The above drawing shows the early B31/B33 rigid frame, this frame and the dimensions shown will be used to calculate the lengths and geometry to build the girder fork.
The first dimension of note is the steering neck angle at 27 degrees from the vertical. A neck height can be closely approximated from the figures given, from the front tank mount to the upper front engine mount is 15 3/32", between the mounts is 5 29/32" and from the lower engine mount to the centre line of the lower chain stay is 1 1/2". Don't you just miss the old imperial measuring system? Those added together are 22 1/2" plus another 1/2" for the added thickness of the chain stay gives 23 and scaling the drawing visually there's probably another 2 inches to be added to allow for the rest of the neck height, leaving us with a total of 25". The one measurement that has not been factored in, is ground clearance.
Once again the drawing helps, it shows that the distance between the centre of the rear wheel spindle and the centre line of the lower chain stay is 6" plus the extra 1/2" for the lower half of the tube. So 6 1/2", or approximately 164 mm in new money, a 19" wheel rim with a 3.60 wide tyre measures 324 mm from the spindle centre vertically to the floor. 324 minus 164 is 160, so that will be the ground clearance with the lower rails parallel to the ground. Add this to our 633 mm (25") first figure and we end up with 793 mm. From this needs to be subtracted the wheel/tyre radius of 324 mm, leaving 469 mm vertical height. Armed with this figure and knowing the neck height we slip away to Engineers Edge and let the computer do the figuring, never can remember much Trig from school. The returned figures give a fork length of 536.25 mm and an initial trail of 260mm, these are now working measurements........... to be continued

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