Prep for the Wyoming Rally

Well it has started raining here in Calif. LOL. So I have decided to do a little work to the MK III. As I have taken a few parts off the bike I found that the yokes had different numbers than what I thought were correct. The top yoke has #06-5721 and the bottom yoke has #06-4083??? The Old Britts site has different numbers. They also have ANG marks and GF and what looks like MB on them, The reason I ask is because the bike has a head shake at about 2,500 to about 3,000 rpm, off throttle. Are these correct?? I have new iso's and plan on new head stock bearings but this kinda has me wondering if i also need correct yokes. Thanks in advance for your reply's, Chuck.

I'm certainly not the expert around here. But I recall a thread a few years back in which it was thoroughly discussed that the cast numbers on the yokes is not the same as the part number. It was in essence the casting number. The "ANG" was considered to be the important identifier as to whether or not they were correct for an 850 frame.

Russ

The number on the part is a casting number, and is not the number you'd find in a parts book.
064083 is the casting mark on a Mk I 850, which correlates to P/N 06-4078 in the parts book, so I would say you've got an early 850 bottom yoke. I don't know what the changes are between Mk I and Mk III to signify the change to P/N 06-6228, or if Mk III actually had the same casting number. The lower yoke may have the same critical dimensions for all 850s.

Perhaps somebody with a Mk III can post what the casting numbers are.

IIRC the Mk3 had slightly more rake, to produce slight more trail, to mellow out some of the head shake that previous Nortons shared. IIRC the previous Nortons had steering geometry that mimiced modern sport bikes yielding amazing turn-in and highly responsive results if push steering.

Looking at the pictures you have included makes me think that your front tire may be the source of your head shake. My eyes aren't what they used to be, but your pictures seem to show uneven ware patterns port and starboard of center. What is the manufacutring date on the tire? how many miles does it have?

RS

Hi Chuck,

Usually the headshake is speed-based, not rpm-based. 35 - 40 mph is where they really like to do it, particularly when you take your hands off the bars and coast for a bit. Tire wear and pressure can influence that greatly. Ditto for road conditions.

If you have the correct triples you will note that the steering tube is NOT parallel with the fork tubes. You can easily see it by eye. The 850 frame has one degree more rake than the 750 frame. There is a diagram in the Tech Digest that shows this. Whether or not the factory could actually build frames with that level of accuracy is debatable however! They made the geometry change in the triple clamps to compensate from the headstock angle change. It must not have worked out as they intended. Some people prefer to run 750 triple clamps on an 850 frame however, claiming they get quicker steering.

There were some threads a few years ago where all of this was discussed in detail although I don't know if all the discussion really resolved anything!

Good luck and hope to see you guys in Wyoming.

Debby

If they have ANG cast into them then they have the intended geometry for the MK3 [and other 850] frames. The only other changes that I know of in the 850 triples involve the switch mounting pad and the hole for the brake line. Jim

Thanks everyone for your replys, The bike has had this issue since I purchased it 6 years ago. I had the new rims laced up by Buchannan's back then and put new tires on it, and it still had the shake. Debby your right it's not just an rpm thing now that i think about it, but it's when i'm slowing down at low speed at about that range. I don't feel it much, only when i take my hands off the bars and I don't feel it at any other speed or rpm. In fact it goes away when I get back on the gas. Hmmmm. RS the tires are a bit old but the tread has an even wear, But new tires are in the near future, Thanks Jim for the info. that they are the right ones. I can now look at other things, The iso's did have a little sag. But the head stock bearings feel good, may change them out though. Also the swingarm felt good. One thing I did notice was that it seemed the head steady was out of center???? It has the box type standard for that year. Kinda like the motor was slightly leaning to one side?? I guess I will just start with the new stuff and see how it all lines up. Thanks again for all the help. Thanks Russ and Bill too.

A shake on decel with no hands is pretty common on a Commando. It gets worse if anything is loose- like isos or swingarm bushings. New tires often cure it for a little while but it usually comes back.

My Mk3 did it from the time I built it until I installed the inverted forks and raised the back end with new shocks. Now it doesn't do it at all- anytime. Not sure why but it obviously changed the resonant frequency.

Probably the best answer is to keep your hands on the bars. Jim

Too many things to list that bother hands off coast down but tires top the list by far. There are times its nice to let go to fiddle with jacket or helmet of what not and can be dangerous with immediate tendency to became a tank slapper. its mostly front tire condition compared to rear, that also adds its wondering worn patch effects. The more weight to rear the more it settles head shake hands off. Can't say what made Peel so nice hands off but she spoiled me to use body slams or just shifting weight hands off to steer. Unless body slammed she didn't change direction quick enough for the tighter turns. Hands off fork oscillation is not innate to the isolastic Commando, just more so than solid mounts. Be careful showing off with loaded rear end as Commandos tend to wheelie from lights so easy its tempting not to make a practice of it - if not for the radioactive rear axle adding up the strain loads. My balloon tire bike - about as symmetric and solid as they come also tended to drift L up road crowns hands off and wobble as speed dropped below 40's, until tires renewed then ok, for a time. As tires wear can diddle front a tad lower to help. Do give some thought to bandaids and wraps as once on an adventure ride anything can happen. Just ask Jim on his elite steed.

Nothing like a good sissy bar, lugage rack and tops of hard bags to carry some decent cargo better than all day long with a back pack eh.

comnoz said:

Probably the best answer is to keep your hands on the bars. Jim

Click to expand...

Good advice, I really don't show off much at least on the Commando. What hobot said was true in that at times I would tighten my helmet strap or something like that and it would remind me if i was at that speed. I put up with it because it didn't happen that often and figured that when i had it apart i would look into it, The bike otherwise is pretty much rock soild. You know i really love the bike in all other ways in that it has never stranded me or felt unsafe otherwise. Old Britts did the rebuild of the motor and it just runs great, But the rest of the bike had not been gone through since I owned it. I just want it to be ready for Wyoming so I am starting the project now. I think I"ll have the frame painted and all the other little stuff too, maybe a little polishing and the like. My other bikes don't have any hint of this even the 72 Norton, nothing. So if it is just the nature of the beast it's fine as most of the time my Boss is on the back and I should keep my hands on the bars, lol. :lol:

Mine did that oscillating also but the Dave Taylor headsteady cured it. I went from the stock '72 headsteady (which cracked early on) to the boxed-in 850 type and then the DT. No springs. Worth a try anyway. Took off my steering damper as a result. I fought that problem for 35 years - not to be discouraging or anything... Happy now!

Russ

You have worse problems to deal with. Your shifter is missing!

This is very common subject on Commando lists for decades and almost everyone of them reported way less hands off head shake once new tires fitted, even if the tires look fine but had sat up a good while and rest of the bike still clapped out. Just be careful and plan you hands off fiddles in clear open sections till tire economy gets used up. My Trixie Combat was gone through with adjustable iso's and rubber and swing arm bushes and spindle collars and new fork bearings and spokes all tightened up and trued - but was a head shaking disappointing nuisance - till finally put on good meat tires under 10 yr old and even better after a cheap but new rear fitted last season. The better the forks and linkages the less shake hands off but still present from slight tire shape conflicts with forks slightly oscillating naturally to stay upright, which is more apparent in swing rate about 40 mph down. Most common reason for me to let go is to pull down pants legs to keep the cold breeze out or get an insect out of helmet and/or ears. My jacket has 10 vent zippers and some stick/fold up to zip up when sun goes down behind Mt's into full night time chill. I've got to be real fast and take a few attempts d/t worn tires slapping me around regularly. Play with shifting weight forward and back to see that influence of rider or cargo mass. This head shake form tire conflict occurs in elite racers at limits even with hands on firmly so not unique to Commandos but cycles in general.

This does a pretty good job of explaining low speed wobble. A lot of things can cause it. Commandos seem to have most of them.

http://bmwmotorcycletech.info/instability.htm

Very informative article Bob, thanks for sharing.

Thanks for the link Bob, I feel a bit better about dealing with it now that I hear others have had this issue. And Russ glad to hear the rod link head steady helped as I also have one that was made by one of members here and am going to fit it. So how many people here are attending the rally next year?? I have only been thru Wyoming twice and not on a bike but it sure does look to be a great place to ride. I really get tired of riding in this concrete jungle, early in the morning is ok on the way to work but coming home in traffic is quite the test. I just can't wait to ride in the open, I need to ride an hour just to get to a nice place to ride. Sorry for the rant. Chuck.