Please enlighten me

padlock

padlock

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Hi All, I've heard that some racers in the past changed the standard yolks for sfc yolks on their jotas etc. Can anyone explain please. Thank you very muchly. Mark
 
Ceriani-forked 3Cs had the steering stem between top and bottom yoke at a different angle to the headstock.
This gave ponderous steering which was sharpened up by using parallel yokes from such as the SFC 750.
 
Grant may be correct but from my experience of these things it's not the steering stem which is different, it's the bores for the stanchions in the top and bottom yokes. On the 3C the yokes are not bored parallel to the stem [viewed from the side], they are bored so the fork legs kick back slightly [3 degrees rings a bell?]. The effect is to increase trail presumably in pursuit of stability. On the SFC the yokes are parallel and the reduction in trail sharpens up the steering response
 
Bob,

Actually, it is the steering stem tube that is inserted at an angle in the yokes.  This can be clearly seen on the lower yoke where the steering stem tube passes through it.  The tube exit is not concentric to the round milled surface on the bottom of the yoke, whereas on the yokes that are bored parallel, the tube exits centrally.  I assume this was done in order to gain increased trail without sacrificing steering lock.  The more common method of increasing trail is to use yokes with less off-set.  This unfortunately places the fork stanchions closer to the sides of the fuel tank, thus limiting steering lock, the bevel Ducati SS is a prime example.

Owners of bikes with these forks that choose to fit tapered bearings at both ends of the steering stem unknowingly strain the yokes slightly because these are then placed further apart and the stanchions then need a bit of persuasion to pass through both yokes smoothly.  Not much, but it certainly is there!

Reckon for general road use the stock yokes are quite OK, at least they offer a little more high-speed stability.

piet
 
Ach so, alles klar!
Apologies to Grant because from what you say he was indeed correct. I always assumed the stem must be parallel because I couldn't see how the the taper bearing at the foot of the stem could seat correctly if the stem was out of kilter so presumed the bore in the yokes must be providing the inclination for the stanchions. You learn something new everyday... :D
 
padlock said:
Hi All, I've heard that some racers in the past changed the standard yolks for sfc yolks on their jotas etc. Can anyone explain please. Thank you very muchly. Mark
Click to expand...

You trying to egg us on?  :D
 
sfcpiet said:
Bob,

Owners of bikes with these forks that choose to fit tapered bearings at both ends of the steering stem unknowingly strain the yokes slightly because these are then placed further apart and the stanchions then need a bit of persuasion to pass through both yokes smoothly.  Not much, but it certainly is there!

Reckon for general road use the stock yokes are quite OK, at least they offer a little more high-speed stability.

piet
Click to expand...
So thats why it was much harder to assemble mine after I fitted one of Andys slightly undersized bearing kits in the dim distant resenses of my memory,for years I was thinking I had a slighty bent stem.Another myth done away with Piet,good one.
 
I actually fitted a set of the off-set ceriani yokes to the Motodd. Been on the bike for 25 years and can't fault the handling. They look good to all polished up.

Now that you mention it Piet, the bottom bearing wouldn't be sitting square to the yoke :o Never really noticed the staunchions being tight when sliding them through the clamps either. I'll pay more attension next time I remove them.
Comparing the bottom yokes on the SFC and Motodd you can clearly see the steering stem is off center by roughly 2mm.
 
On POP I had the Ceriani yokes modified to accept a new steering stem parallel with the steering head.
The yokes were filled in with alloy weld and machined back to parallel.
You can see the effect this had on top yoke nut position, way off centre.
 

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In response to Padlock's original comment I'd add that as well as SFC yokes, racers would sometimes fit Ducati 900SS yokes. The reduced offset of the Duke yokes would bring back some of the trail which had been lost when the headset angle had been steepened by letting a piece of extra tubing into the top tube! Those bikes must have had very little steering lock...
 
The Marzo yokes fitted to the later triples are also parallel to the steering stem.
They also have less offset than the Ceriani's, resulting in more trail and thus more stability.

Will the parallel Ceriani's quicken the steering? 
Surely not, as the steering head/stem angle remains unchanged.   
They will however increase the wheelbase.

Tom
 
Tom, It's a valid point you make,  I don't think the difference in wheelbase will make any difference but from in my experience of such things the 'normal' SFC type yokes steer better .[I'm talking for track use here, not normal road useage]. Regardless of the head angle being the same the reduced trail will make it easier to de-stabilise the machine and change direction therefore the effect will be to quicken up the steering. On the road I doubt if the difference would be detectable.
 
I followed the advice of Gijs Van Dijk. He recommended the change.
Good enough for him, good enough for me.
Agreed any change may be marginal, but it was a case of "while I was at it".
 
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