Yet again we talk yokes and forks

Hello all,

As you guys know I have been working on a basket case and it seems to be going well.

The latest issue to come up is the triple-tree/frame discussion.

So far I have been able to deduce the following.

1) 750 triple tree in to 750 frame OK
2) 850 triple tree in to 850 frame Ok
3) 750 triple tree in to 850 frame No good
4) 850 triple tree in to 750 frame No good

I am sure there are other combinations.

My situation is that I appear to have an 850 MKIIA frame sporting a pair of 750 triple trees.

Since matching trees to frames is a fairly costly endeavor I'm inclined to say to hell with it and maintain the 750 triple trees in the 850 frame. This apparently can cause handling issues. Can someone please direct me to an article which might describe the ramifications of that decision. I''ve tried to research it and am getting overwhelmed.

The bike I am trying to build is really designed to be a comfortable classic cruiser, able to snort when necessary but not set up to drag race or compete at the Isle of Man.

I'd really like to avoid sinking close to a half grand into this bike over this. I'm open to suggestions.

Thanks in advance

The NOC Commando notes have 750 yokes in 850 frame as being ok as it gives the increased trail, 850 yokes in 750 frame as no no. 850 yokes in 850 frame is best of all.

See page 24

I remember Barry Huron talking about mix and match no-no's at a P.O.D.rally a couple years back. How would you identify the differences between the 750 and 850 Yokes ? Asking because the P.O. of my 750 Crazy Combat mentioned he changed or obtained replacements after a laydown ground away at the lower one.

The 850 yokes have ANG cast into them.
And if you look closely, the fork tubes point slightly 'backwards'.

Uh -oh, Crazy has an MB ANG 064084 cast into one (top) NM 001916( bottom). Will check my other Norton and report back. Could be some changes -corrections coming up.

Now i'm thinking they're mismatched due do a previous laydown and he took what was available at the time ? My other 750 norton has a cast 061917 (The first 1 has a hole drilled through it so am guessing that no.) on top yoke and G R into the bottom one. These are probably O.K. and matching to my knowledge. Can I leave things as be or panic ? B.T.W. I am starting a fork teardown anyways as tubes are worn at bushes.

I would be checking your frame, I have a 72 that came with bent front rim and stanchions, the yokes were fine but the frame top beam was bent and the 2 front down tubes splayed out so it does not take much to bend the frame. Your frame may be bent or replaced with an 850 frame hence the 850 yokes.

In the old NOC 'Service Notes', Tim Stevens stated that in order to get the greatest benefit from the 850 frame, they should be used with 850 yokes but that the 750 type 'will fit and still give the desirable increased trail'.

It is important that the yokes be fitted in pairs (top and bottom).

It would seem logical that the 850 yokes in a 750 frame probably aren't a good idea in terms of high speed stability.

I suggest you should discuss this topic in terms of geometry (rake and trail and bike length, as well as wheel size). My Seeley has 27 deg. rake and 102mm trail, 18 inch wheels and it oversteers under power when laid over. Your road bikes will be considerably different, and it is possible to get the situation where the bike can grab you by the throat. If you buy yourself the weighted magnetic base protractor from the tool shop, you would be able to read off the rake, when you stick it onto the fork leg. The trail is measured by running a rod down the fork leg to the ground, and one vertical to the axle, measuring the difference between the contact points on the ground, then subtracting the forward offset of your fork yokes. The literature on this subject involves a lot of differential equations, however there is information available which gives some idea what works. e.g. The Muzzy ZXR750R Kawasaki used 24.5 deg. rake and 102mm trail. and handled very quickly (see this month's Classic Racer magazine). In general terms increasing the rake and the trail makes the bike more stable, - however too much trail gives problems, and so does too much rake. Both of these measurements are adjusted on production road bikes to cater for reduction in wheel sizes. It seems that going from an 18 inch front wheel to 17 inch requires a half a degree reduction in rake on Japanese two strokes. The most extreme figures that I've heard have been for a GP bike which used 24.5 deg. rake and 82mm trail. I don't know what it was.

Thanks to all of you who answered my post. I think that I will go ahead and re-install the 750 yokes and invest the money I would have had to purchase 850 yokes into something else.

I got the distinct impression that no real harm is done with this configuration, which not ideal, poses no significant safety risk.

I'll start this weekend, and let you know how it goes.

79x100 said:

In the old NOC 'Service Notes', Tim Stevens stated that in order to get the greatest benefit from the 850 frame, they should be used with 850 yokes but that the 750 type 'will fit and still give the desirable increased trail'

It would seem logical that the 850 yokes in a 750 frame probably aren't a good idea in terms of high speed stability.

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This is all wrong ! .
750 yokes are parallel , and 850 yokes are angled to put the front wheel contact point more to the rear ( shortening the wheel base )
So the 850 yokes will increase trail in whatever frame you use them .
All this has been discussed many times ..

Some years ago I had a friends '74 850 stored that we got going so took it out to find I did not like the effort it took to try to "fling" it around twisties or navigate pasture to get to more firm surface. Felt heavy resisting sluggish. Got on my local buddy '71 and instantly felt its easy handiness to fling and spin rear out and had to restrain myself from doing pasture donuts on someones else's ride. On the other hand I have been on my spiffed up sharp steering race shod SV650 riding behind a big boy with camp cargo on his 850 with fairly low aired tires to find I had a damn hard time keeping up with him in the twisties at just his sight seeing rates! One of the main things I love so much about my 750's is how easy low effort they are to steer, even lighter forces than my SV650 girls beginner cycle so If was me I'd try the 750 yokes and creep up on handling extremes to make sure no surprises while enjoying the lighter more handy sense than 850 curiser chopper sense.

Good News. I have located an upper yoke and stem to fix the mismatch issue. Needs repainting but more importantly it is NM cast underneath and not the MB ANG that was fitted. So thanks to checking this thread am now into a front end teardown for proper 750 top and bottom !

Thanks for the post torontonian, i have a 75 mk3 that I love- except the steering seems heavy at slow speeds, seems I have to fight it or muscle it a bit, mine was comprised of parts sought here and there, I need to ck the casting marks and refer back to this thread, But my question is- are there any measurements that can be taken without dissasembling the bike to ck for frame neck trueness ? , I think this bike went down at slow speed with the previous owner... any help?, ps this is my 7th norton Commando, first one to exhibit this issue. -thanks guys.

are there any measurements that can be taken without dissasembling the bike to ck for frame neck trueness ?

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Remove the petrol tank and put a long straight steel rule on the frame backbone, they are all bent to an extent but if you see a gap of more than 1/8" at one end then you need to check further. The 850 does steer heavier than the 750, if your front tyre is the same profile as the rear decrease it one size, I run a 90/90 19 on the front and a 100/90 19 on the rear.

KOMMANDO- thanks for the information, that will be my next move, I will ck that this week-