Split Topic: Triple Trees

[L.A.B.]

OK, fair enough, but for safety's sake I hope any of our members who may not have been aware there is a potential problem will have taken note.

Quote from the Commando 750/850 Workshop Manual:

"Front Forks/Steering
Section G


"Note : 850 models use a special pair of yokes identified by "ANG" stamped on lower surfaces of both yokes. These yokes must be used for 850's and must not, under any circumstances, be interchanged with earlier types".

("stamped"? Possibly some early ones were stamped I don't know, certainly the later yokes had ANG cast in)

 

[pelican]

Maybe someone with both pairs could measure them out?

Is there really that much of a difference in the 750/850/850mk3 trees? Cnw's trees are made to be universal. Why not different kinds if the difference is so great?

 

[Danno]

I DID measure them and for all practical purposes, they are identical except for the differences in dash lugs, lock holes and brake line drillings.

 

[Hortons Norton]

You put the tubes through the trees and measured the angle? What are you checking? The angle that the tubes run through the trees is what's different. It will only be a couple of degrees.

 

[Ron L]

The angle of the steering stem is one degree different between the 750 and 850. This was because the 850 frame's steering head is kicked out one degree further than the 750. To keep the same wheelbase, Norton decreased the trail by 1 degree. If you mix and match 750 and 850 upper and lower trees you will at the very least have non parallel bores for the fork tubes. If you mix and match 750 trees with 850 frames, or vice versa, you will affect the trail.

 

[Norton-Villier]

My 73 850 born on date 10-73 I bought in 74 has top yoke 064084 the bottom yoke 064083. My 75 850 MKIII electric start born on date 9-75 has top yoke 065721 bottom yoke 064083. My 72 750 combat born on date 2-72 I got from my brother inlaw a rat bike that I will make a real NORTON again had top yoke 061917 bottom yoke 06 with a hole next to it. Hope this helps. Phil

 

[ludwig]

The angle of the steering stem is one degree different between the 750 and 850. This was because the 850 frame's steering head is kicked out one degree further than the 750. To keep the same wheelbase, Norton decreased the trail by 1 degree. If you mix and match 750 and 850 upper and lower trees you will at the very least have non parallel bores for the fork tubes. If you mix and match 750 trees with 850 frames, or vice versa, you will affect the trail.

1. Did anyone ever actually MESURED the the difference between the steering angle of the 750 and 850 frame ??
or do we simply believe the factory data ?
I would do it , but I only have a bare ( English) 850 frame , no 750 frame to compare .
If someone had a bare 750 frame , we could do some precise mesurements .
Knowing their sloppy production toleances , it would probably take several frames to calculate an average .
2. The difference between 750 and 850 tripple trees is easier to be checked :
Anyone with a 750 can check if the fork stanchons are parallel with the steering stem .
On an 850 , they are not : the stanchons are at a 1.5° ( as close as I can mesure ) angle with the steering head of the frame .
This implies that the taper in the top tripple tree must also be cut at a different angle , or the fork tube will not seat properly .
3. Ron L : because of the steeper angle of the fork , relative to the steering neck , trail is INCREASED , not reduced .
( trail is expressed not in deg. , but in cm or inches ) .

 

[Ron L]

Thanks, Ludwig. I stand corrected. You make a good point about the accuracy of the frame manufacture. Plus you would need to factor in the Italian built frame and the English built frame.

Still the factory went to some trouble and expense to change the geometry. I would love to hear the reasoning behind this change.

 

[debby]

I measured the headstock angle on my 850 frame before I built the bike up, and it appeared to match the published 850 specs. Not sure how accurate my measurements were though. I used a plumb line, metric tape measure, and laser pointer, then calculated the angle from my measurements. Finally a real world application for that high school trig! :mrgreen:

I've been meaning to measure the 750, but it's a bit more difficult with the bike assembled. I'm interested in seeing if I can measure any difference, and if my measurements for the 750 match the specs. I'll try to get to it one of these days. I haven't been spending much time in the shop this winter. It's cold out there and I haven't been in the mood.

Deby

 

[ChuckW]

Debby,

Maybe it's time to invest in a good heater for the work space. Best investment I ever made, makes it nice and warm and is so much nicer when working on the bike.

Trig, I don't remenber much from way back when but if I need to I can always ask the wife because she knows everthing.

Chuck

 

[rvich]

This topic is of interest to me as my bike is put together from bits of a '73 850 and a '72 750. Part of the process has been an attempt to document which pieces came from which bike. My attention turned to the front end in the past week as I have been working on the brake assembly. My upper yoke has a number of 061917 and my lower yoke has a number of 061916. That latter number matches the numbers Phil gave above from his '72. It also matches the number given on the Old Britt's parts list for that yoke. (They list 061915 for the upper one).

So it appears that I have 750 yokes on an 850 frame. From the above it sounds as tho mixing the two is the worst case scenario due to misalignment of the tubes. When sighting past the headstock on the bike in relation to the fork rake, mine is wider at the top, thus reducing trail. I am still trying to decide if I should hunt up the 850 yokes. I will turn this back over to the experts.

Russ

 

[bill]

IIRC you can use 850 yokes on a 750 frame but not 750 yokes on an 850 frame as you would reduce the trail to what could be to small of a trail dimension.

 

[rvich]

Don't it figure...something else to add to the parts list...

 

[grandpaul]

The clamps can't be mixed IF YOU ARE WANTING TO RETAIN ALL ORIGINAL FEATURES exactly as they were on the orignal bike.

Other than that, if you don't need the mounting provision for the idiot light panel, and if you don't need the identical steering lock setup (I NEVER use steering locks), then it matters not one whit.

 

[kommando]

Quote from NOC notes on yokes

In any case where a new frame is required I would recommend the early 850 frame . The steering is more positive at high speed with the 850 geometry but to get the greatest benefit you also need the 850 yokes but the 750 will fit and give the desired increased trail.

 

[ludwig]

.. When sighting past the headstock on the bike in relation to the fork rake, mine is wider at the top, thus reducing trail...

Trail is the distance between point A , where the imaginary line trough the axis of the headstock touches the ground , and point B where the wheel touches the ground , a vertical trough the wheel axis :

The drawing is exaggerated to make clear that a steeper fork puts point B further back , reducing wheel base , but increasing trail , not reducing it .
On an 850 , the angle between fork and headstock is about 1.5 deg.
You say that your 750 tripple tree is also wider at the top ( = at an angle ) ...
Can other 750 owners confirm this ?
( that the line trough the headstock axis goes trough the wheel axis is accidental )

 

[rvich]

Yes, mine is wider at the top, just as you have drawn it. So no, I mean yes, it increases the trail! I was thinking the "rake" of the forks determines the trail but it is actually the steering axis? (I hope) And thanx!

 

[ludwig]

Next time you go shopping take a close look at the caster wheels of your shopping cart !
and you are 100 % shure it are 750 tripple trees ?

 

[rvich]

I can verify that they do not have "ANG" anywhere on them and that the numbers are as I posted earlier. So based on information in other posts in this thread it should be assumed (big word) that they are from the 750 donor rather than the 850.

 

[ludwig]

If that so ,(and if other 750 owners confirm this) , then the milion dollar question is :
What the f... IS the difference between 850 and 750 tripple trees ???