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.
Interesting mod, can,t see any negatives but as you have done it you are the expert at the moment! Going the other way and fitting early yokes on a Mk3 may present problems. Keep posted on higher speeds and bumpy roads testing etc but would be surprised at any adverse effects
I may do similar but Mk3 yokes on an Atlas Slimline special to give more clearance to front wheel and possibly slow down the quick steering a tad.
Nice pic of your bike with some well made custom bits
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.
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.
Certainly is a bolt in operation. Just checked the fit of a new Mk3 yoke and spindle assembly in my 1965 Atlas slimline stem and I can confirm 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 160-162mm 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)
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 :)
I,m glad you,re still talking about them 50 years on, some very interesting info there. In the light of your comments I,ve decided to try the later type Commando steering head bearing arrangement but using angular contact bearings of the same size with the O.E. spacer
The original 1978 taper roller bearings in my Atlas steering head are still shiny and covered in (dark colored) grease with no noticeable corrosion or notching on turning. Looks like I filled the entire stem space with lithium grease back then and put rubber seals top and bottom which has kept water out for the 100k+ miles. Will be nice to have a maintenance free, simpler top yoke without the adjuster however and the increased trail you mention looks to be what I,m after
Never had any tankslappers or wobbles to date on my Mk3, either deliberately shutting off in fast sweepers or hands off at low speeds but recently when accelerating hard in a straight line, there is a slight weave feeling that wasn,t present before. Normal cornering at all other speeds not affected nor is the bike worried about luggage on the back, handles the same
Checked the swingarm side play and there is definite flex movement between swingarm and cradle. The spindle and possibly bushes are likely worn with around 65k miles, so a new spindle will be going in first to see if things tighten up. No record in previous owners service notes about having them replaced and his Haynes manual is pristine on that section so maybe the originals. They,re oiled with EP140
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.
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.
I checked the cotters as you suggested and all solid and present, probably spindle and bush wear but will try a new spindle first. Had a friend with a 1980 Kawasaki Z650 around today and checking his solid mounted swingarm found it had more play than mine, but to be fair he cruises rather than uses all the performance