Fork yoke enquiry

I'm in the market for a set of alloy yokes, and sorting through the puzzle of options from featherbed, to early and late 750's:

featherbed/early (pre 71) commando = 7 3/8 centres with 2 1/4 offset, stem attached to bottom yoke
71 onward 750's = 7 3/8 centres with 2 5/8 offset, stem attached to top yoke

both these versions run parallel bores.

Question, has anyone fitted the early commando (2 1/4 offset) to a 71/72 (2 5/8 offset) without handling issues? I'm interested in this option from RGM in particular"



http://www.rgmnorton.co.uk/buy/billet-a ... rettyPhoto

there has been a lot of discussion on this topic here and I think the general consensus was in theory its not a good idea to use the early type yokes
on Commando frames, even though Norton did this, However I fitted a set of my early alloy yokes to my 850 frame (pictured) without issues. I did use
slightly longer rear shocks to counter the so called anticipated handling problems. Would I do it again? yes I intend to. my own personal experience.

As far as I know the 750 frame has 27 degree rake and the 850 frame has a 28 degree rake but the yokes on an 850 are de raked by 1 degree

I'm confused by Commando yokes too (easy to do these days though It seems)...

So, Norton increased the rake on the 850 frame by 1 degree, then decreased the rake on the yokes by 1 degree, thereby ending up at the same 27 degree rake as the 750.

Question 1: is this correct?

Question 2: If the answer to question 1 is yes, then why did they do it?

Fast Eddie said:

Question 1: is this correct?

Click to expand...

Not really.
The steering head angle is the steering head angle (rake), and the fork yokes are a totally separate issue.
Fork yokes CANNOT affect the rake angle. That is entirely contained in the steering column..

The fork tubes can jut forward like the early dommies had,
or even angle backwards a bit like the commando did.
This merely alters the trail. (and wheelbase).

EDIT. I wrote this next bit wrong - thanks Ludwig ! - note the correction.
The 850 was done like that (28 degrees) supposedly to make the steering more stable (no shimmy or headshake), and they INCREASED the trail a bit, by angling the fork tubes back in bit in the yokes,
to give more of that really strong self-steering ahead effect.

All of that rake/trail/wheelbase/rideheight/wheel diam stuff is a juggling act,
and Nortons had tried a few variations over the years/decades.
Referring to the 850 fork yokes as being 'deraked' is technically not correct...

Some other makers have made adjustable steering-head-angle frames
- you wouldn't do that if just fitting different fork yokes could do the same thing...

The reason the 850 yokes were raked was to keep the wheelbase the same. With the greater steering angle & greater yoke offset the wheelbase would have ended up longer than the earlier bikes.

So, Norton anticipated or new that the 850's with the longer wheelbase when fitted with 750 yokes or Featherbed yokes they would handle bad?

Do the 850 yokes run parallel bores? If not, this would explain the instability issues when fitting earlier yokes.

Since I'm attempting to fit the earlier yokes to a 71, the only difference is in the offset. Which I take to mean the forward distance of the stanchion bores from the stem. In my view, this only changes the wheelbase slightly (with everything else being the same), and the rake is unaffected.

"Do the 850 yokes run parallel bores?" apparently not.
" this would explain the instability issues when fitting earlier yokes." correct
"Since I'm attempting to fit the earlier yokes to a 71, the only difference is in the offset. Which I take to mean the forward distance of the stanchion bores from the stem. In my view, this only changes the wheelbase slightly (with everything else being the same), and the rake is unaffected."
No , it changes the Trail.

acadian said:

Do the 850 yokes run parallel bores? If not, this would explain the instability issues when fitting earlier yokes.

Since I'm attempting to fit the earlier yokes to a 71, the only difference is in the offset. Which I take to mean the forward distance of the stanchion bores from the stem. In my view, this only changes the wheelbase slightly (with everything else being the same), and the rake is unaffected.

Click to expand...

No the 850 yokes do not have parellel bores they are de raked by 1 degree, if you alter the offset of yokes that do run parallel you will affect the wheelbase and also the trail

madass140 said:

"Do the 850 yokes run parallel bores?" apparently not.
" this would explain the instability issues when fitting earlier yokes." correct
"Since I'm attempting to fit the earlier yokes to a 71, the only difference is in the offset. Which I take to mean the forward distance of the stanchion bores from the stem. In my view, this only changes the wheelbase slightly (with everything else being the same), and the rake is unaffected."
No , it changes the Trail.

Click to expand...

Got it, so fitting the earlier forks ( 2 1/4 offsett) would lessen the trail, and compromise straightline stability... something I dont want.

I've found scant options otherwise, NYC norton has a beautiful yoke set, but priced a bit out of my range.

Does anyone else have a line on an alloy for yoke set properly dimensioned for later 750's?

Just to muddy the waters further my own 750 commando has Yamaha forks/yokes with only 40mm of offset I am very happy with the way it rides at all speeds with superb handling in my opinion !! Hope this helps but my bike has extra iso's etc,just be careful with what you change

Just for clarity, reduced offset INCREASES the trail, it does not reduce the trail.

Fast Eddie said:

Just for clarity, reduced offset INCREASES the trail, it does not reduce the trail.

Click to expand...

right, so the opposite handling effect is also true, meaning straightline stability will increase at the expense of nimbleness?

acadian said:

Fast Eddie said:

Just for clarity, reduced offset INCREASES the trail, it does not reduce the trail.

Click to expand...

right, so the opposite handling effect is also true, meaning straightline stability will increase at the expense of nimbleness?

Click to expand...

Correct. But by how much, and to what effect, I have no idea!

Here's some reading on the subject:

http://www.motorcycle-superstore.com/41 ... ained.aspx

Take a look at Minnovation racing
Yves

My fastback with an early frame was registered in 1971 though the bike was built in 1970. the frame has the centre stand mounted on a cross bar under the engine and there is no bracing tube under the front iso. The question I have is, is the change to the forward cross brace ( and centre stand on the gearbox cradle) the same time as the change in the rake angle from 27 to 28 ?

no

gripper said:

The question I have is, is the change to the forward cross brace ( and centre stand on the gearbox cradle) the same time as the change in the rake angle from 27 to 28 ?

Click to expand...

The change in frame rake angle (along with 'ANG' yokes) began with the 850 models (so 1973) although it appears the late (230) series 750s also had the 850 frame/ANG yoke geometry.

I have the early yokes with less offset. I also have the commando frame WITH the smaller lower tube supporting the steering head under the main tube. I have the frame mounted centerstand so I assume it's an early frame with the 27 degree rake. I did have handling problems that were related to the sloppy swingarm tube. The Kegler clamp modification cured that. The funny thing about handling is we get accustomed to how a bike is set up, so unless we ride a lot of differently set up bikes, or keep making adjustments and test riding, it might be hard to have a sense of where the balance of straight line stability and quicker, lighter handling is best for each rider.

I've seen this with dirt race cars, where you can't set the car up to be as stable as possible going fast on the straight away, because the car requires a different set up to best run through the corners, so every set up is a compromise.

Having the early yoke set up, the only problem I had with it was that the top yoke casting hit against the interstate tank when I decided to change from the roadster tank. I had to elongate the mounting holes in the brackets to pull the tank backward away fromt the yokes. (which of course crowded the side cover space, so I had to pad the rear of the tank up to clear the side covers)

As far as handling goes. I changed my rear tire to an 18" tire with a 110 width, so I lost only a bit of rear wheel height, which increases the rake by a tiny bit, but since I am a buffalo sized person, I use koni shocks set on the highest preload and dampening so possibly that extra preload makes the shocks longer to cancel out the extra rake induce by the smaller diameter tire....

Got a head ache yet??

Anyway, by changing to less offset you're going to get more trail and more stable straight line performance. How much quickness and lightness of steering you lose is hard to say, as is whether it's enough that you would even notice the difference. Certainly you did the best thing by asking if someone had done this previously and noticed any problems.