Perhaps we need to be clear when we talk offset. There is the ’fork offset‘ ie the measurement between the steering stem and the fork stanchions. Then there are ‘offset yokes’ ie yokes that do not hold the forks parallel to the steering stem / headstock.
I can’t recall the exact fork offset but IIRC the offset of the top yoke is the same in 750 and 850 yokes, it’s only the bottom at differed on the 850s ‘offset yokes’ as it kicks the fork angle back in.
Earlier posts state the 850 kicked out the rake by one degree, and the yoke offset kicked the fork angle back by one degree (edit: corrected by Ludwig as 1.5 degrees).
Theres lots written about rake and trail, but not a lot about changes to fork angle via offset yokes. But fork angle dictated by yokes clearly makes a big difference (in my tests at least).
Hi all,
I did some careful measurements. My findings and math modelling suggests the following:
Trail ( with std 19" wheels, rolling diameter 647 mm)
750 '71 -'73 : 84.7 mm
850 : 103.2 mm
The difference in trail based on the above is 18.6 mm which is about the same figure FE quoted in posting #1.
The angle in the lower and upper yoke is taken to be 1.0° (I measured 1.04 degrees; NV would have chosen a multiple of 0.5 degrees for manufacturing)
Reference offset at the lower surface of the upper yoke appears to be identical to the 750 yokes (71.4 mm or 2-13/16"), so this is taken to be the datum plane.
Similar to Ludwig, I have opted for a 18" front wheel, which reduces trail a little.
Computed trail for an 850 frame / 850 yokes / 18" tyre is 101.3mm
while computed trail for an 850 frame / 750 paralell yokes / 18" tyre is 89.2mm.
The latter would be the configuration offered by CnW, among others. Contrary to advertising, high-speed stability is reduced vs. the stock configuration, but steering will be quicker.
Playing with the numbers and responding to a question raised earlier in this thread, a frame rake of 30 degrees would have been necessary when using paralell (750 style) yokes in order to achieve
approximately the same trail as for the 850 frame/850 yokes combination.
A MathCad calculation sheet will be shown shortly.
- Knut