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MK3 fork yokes on 1971 Roadster

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I have built a MK3 fork yoke on a 1971 frame what alters the angle.

First slow running in test rides up to 65 miles has not shown any strange behaviour

Any experience or comments welcome.

Martin Spain

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Can,t see any negatives but as you have done it you are the expert at the moment. I,d be surprised at any adverse effects on the road.

I,m doing something similar, but Mk3 yokes on an Atlas Slimline to increase trail and hopefully slow down the quick steering a tad. 

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850 yokes increase the trail by setting the forks at a steeper angle than the headstock thereby moving the contact point to the rear. This counters the large offset needed in the yokes to clear an Interstate tank.

I ran 850 yokes with a 750 frame on a race bike for a while. Steering was a bit slow and heavy but you might like the stability on the road. I fixed the heavy handling with smaller section tyres (100/80x18 and 120/80x18) and longer rear shocks.

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Slimline and 68-69-70 (series1 commando ) have a different offset and stem (length) setup from series 2 commando 71-75.

Of course any one with enough time and resources can modify or make custom parts and assemblies.

It is far from a bolt in operation.

Mk3 forks fitted to a slimline Atlas is the easy peasy part compared to other mods done to this bike back in 1978.  Getting it ready for it,s 100k+ mile refurb.

It certainly is a bolt in operation, no custom parts required. Just checked the fit of a new Mk3 yoke and spindle assembly in my 1965 Atlas slimline stem and they go straight in. No mods required using the same taper roller head bearings. The Mk3 bottom nut can be used so the assembly will be identical to my Mk3. The steering head stem on the slimline Atlas and Mk3 frames are the same length at so my setup will be similar to Martin and Stan,s Commandos. 

Stan, that would include the 750 Combat with the Interstate tank? I thought it was only for the 850 models (admittedly no expert on the history of various Commandos)

 

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Testing my memory now - my 1970 750 Roadster had cup and cone head bearings where the stem was part of the lower yoke and the top yoke was fitted above an adjuster ring and retained with a chome nut. Only remember seeing Roadsters and Fastbacks with drum front brakes then. In 1971 the front disc arrived and the yokes were changed to have the stem retained in the top yoke with a free bottom yoke bolted up from below and conventional sealed ball bearings. Although the ball bearings aren't supposed to take an end load they work well, are adjustment free and last longer because the seals keep the water out and grease in.

The new 750 yokes offset the forks  forward by more than the earlier ones to clear the Interstate tank. The 750 yokes ran the forks in line with the steering axis which remained 63 degrees. There were complaints about wobbles and tank slappers. The Interstate was initially 6 gallons but was reduced to 5.25 gallons, by slimming the top section, which helped as did fitting a 3.60 TT100 on the front. But mostly the wobbles would be blamed on Isolastic adjustment, swing arm pivot wear and bad load distribution. I remember putting a heavy pack on the rear rack and not being able to ride off it was so bad; moved the pack to the tank and it was ok (750 Roadster). It was widely joked at the time that running your Commando into a wall would improve the handling.

On the 850s the steering angle was changed to 62 degrees which together with the revised yokes increased the trail and made them much more stable.

I don't know whether all changes : yokes, front brakes, bolt-up/cush rear hub, Interstate happened at the same time or if they overlap. At the time a Commando was just another bike, everyone had one, and I didn't think I'd still be talking about them 50 years on :)

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Check the tube in the cradle that the spindle runs in, the spindle and bushes can be fine but the tube can wallow out on the ends showing up as play. The MK3 had cotter pins to stop this but they may be loose or missing. The fix on pre MK3's is to fit Kegler supports and this will unwallow the tubes and provide support to keep it that way.

 

https://www.inoanorton.com/docs/swingarm.pdf

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The MK3 geometry is no different to the pre MK3 850 geometry. When you follow a 750 on an 850 the difference is notable, the 750 turns in quicker. The change was made to try and remove the wobble when applying power when coming out of a bend.

 


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