850 yoke offset… why…?

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Fast Eddie

Fast Eddie

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I have been using some very nice aftermarket alloy yokes for a while now. They are parallel rather than offset as per the stock 850 yokes.

I recently had the front end out, so compared yokes. I put the bike on the ramp backwards, with the rear wheel in the wheel chock, then tried both sets of yokes with the forks in and using a spirit level, marked the spindle location of the wheel spindle on the ramp. The front wheel spindle is 18mm further backwards with the stock offset yokes, that’s a bigger difference than I’d expected.

Because this difference is via yoke offset and not different frame rake angle, it has a very different effect on trail as would seem intuitive. Curiosity has gotten the better of me and I’m now re-fitting the stock yokes to try out.

All this has gotten me thinking…

Why did the factory do this?

Why didn’t they change the frame rake angle if they weren’t happy with it?

Was changing the yoke offset just a cheaper way of fixing things, cheaper than changing the frame, in other words, is it just a botch job?

Or was changing the yoke offset a very clever way of achieving something with the handling?
 
They raked the frame out from 27 to 28°
Then de raked the yokes by 1°
 
Yes… but why…?

AFAIK offsetting the yokes is not a very common practice.

If they did the frame and yoke angle change at the same time it seems to indicate a deliberate exercise rather than an afterthought / a botch job. If so, what exactly was their intent?
 
Fast Eddie said:
Yes… but why…?

AFAIK offsetting the yokes is not a very common practice.

If they did the frame and yoke angle change at the same time it seems to indicate a deliberate exercise rather than an afterthought / a botch job. If so, what exactly was their intent?
Click to expand...
I believe it was to improve high speed stability without increasing the already long wheelbase
But don't quote me on that !
LAB will know
 
baz said:
I believe it was to improve high speed stability without increasing the already long wheelbase
But don't quote me on that !
LAB will know
Click to expand...
Well, I do believe that’s exactly what it does do.

Still hoping someone clever chimes in to explain the step by step cause and effect (LAB…? Ken…?)
 
The 750's were a bit sensitive and could weave/tank slap at speed under certain conditions, the 850 frame and yoke changes were to reduce the sensitivity. My 850 has never weaved or tank slapped in Roadster or Interstate modes, solo or two up. It is slow to turn but a 90/90 front tyre adds some speed back to the turning.
 
baz said:
I believe it was to improve high speed stability without increasing the already long wheelbase
But don't quote me on that !
LAB will know
Click to expand...
Having never ridden a 750 Commando (or another 850) I can only quote from the NOC Commando service notes, Group 11:

"The steering is more positive at high speed with 850 geometry, but to get the greatest benefit you also need the 850 yokes (but the 750 type will fit and give the desirable increased trail).".
 
I thought moving the front axle back would quicken the steering. Make it feel faster to turn in and lighter than it would be with the parallel yokes. Sort of the opposite of what Norton is claiming. Not that I'm even close to being right about it. You will know soon enough.
 
Schwany said:
I thought moving the front axle back would quicken the steering. Make it feel faster to turn in and lighter than it would be with the parallel yokes. Sort of the opposite of what Norton is claiming. Not that I'm even close to being right about it. You will know soon enough.
Click to expand...

It depends how the axle is moved backwards...

Moving the axle backwards by steepening the rake, via altering the steering head angle, will reduce the trail and do as you say.

Moving the axle backwards by having none parellel offset yokes, and keeping the steering head rake the same, will increase the trail and do as Norton say.
 
So, using 750 trees will lessen the trail, thus quicken the steering on an 850?

Have other manufacturers use off set trees?
 
The factory thinking is pretty obvious. The front forks were inclined to weave and wobble, especially if you took one hand off the bars. Not to mention the occasional random tank slapper. They knew they had to do something.
They thought about putting on a steering damper. But realized if they did this generations of riders would debate if the bike really needed a steering damper, or was it just bling. So they didn't and just screwed around incrementally with the forks and frame...........
 
So far I’d say Baz has the most fitting answer, perhaps raking out the forks enough to achieve what they wanted caused an over long wheel base that caused other handling issues, or just looked wrong, or wouldn’t fit on the firms delivery truck, or sumthin’. Ditto the next option, some rake and a greater yoke offset. So kicking the rake out and the forks back is what they plumped for.
 
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So much of biking at the time was just try it and see. They worked with tight budgets, limited time and tooling and a lot
of 'get it out the door and sell it'.
More investigation of the bikes has been done by people on this list than by those who designed and built the bike!
 
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