Commando tyres

I have a '76 Mk 3 which had a steering head oscillation between 40 and 50 mph after rebuilding it back in the 1980's out in Kenya following hard use by the police from whom I aquired it. Bike had new TT100's 4.10 x 19 back and front and fitting a steering damper cured it. Back in UK in 2010 had a professional total rebuild again with new TT100's and this time steering was perfect without need of damper. Had to be the rebuild set up.

Hello Ken

Well, this is an age-old question, with no perfect answer.  Several will jump on to say that a skinny front and rear is essential, and I don't doubt that that will work.

I have a 1976 MK3 that has gone through many sets of tyres. Yes I used to get 30 mph head shaking on TT100s and Avon GPs. I think that slightly slack isolastiics makes this worse.

Now I have well-set isolastices, and I am running Metzler 3.60-x19 front and 4.10x18 rear with no head shake. I like the big tyres to enable two up running. I did try a damper for few years, yes it does take out the shakes, but seems to slow everything down. I now run higher tyre pressures than I did ten years ago. 

Norm

If you have an MK3 Interstate then with a full tank of fuel the 90/90-19 would be on or over its weight limit with a rider with riding gear - pillon and luggage then it is well over. Of course there will be factor of safety involved. Most owners have no issues with 100/90-19 front and rear. 
I used to use Dunlop TT and the roadrunner, both would cause weave when worn
If you have the narrow handlebars that can be seen on some MK3's then these also don't help with the weave, the Roadrunners have never caused it. 
It is also worth checking the tyre is fitted concentric and equal depth either side on the rim. 

The first sentence of Norman's comment above summarises the situation here to a tee. I have long thought the oscillation may be the result of a flimsy steering head arrangement combined with the weight of the machine being orientated towards the rear, but that is ultimately speculation on my part following significant mileages on Mk1 and Mk3 Commandos both solo and two-up.

I have tried the narrower tyre at the front but found it made no difference and on a heavily laden Mk3 with passenger it is right on the limit in terms of loading capacity, if not overloaded !

My conclusions are that the style of riding, loading/weight distribution and tyre choice/wear and pressure plus isolastics settings all have their part to play, but ultimately it is an inherent design shortcoming which it is not easy to find an effective solution to. Most owners in my experience, after trying to identify the best combination of tyres, weight distribution etc have to adapt their riding style to recognise the potential presence of oscillation and be prepared to counter it accordingly.

Modern tyres usually require higher air pressure. Have you tried 30 to 32psi front 32 to 34psi rear? Because your bike is a Mk3, have you checked the swinging arm pivot shaft cotter pins are tight? Have aligned the wheels by the book with string? using the axel slots can be a big mistake as the accumulation of error in the ISO etc mounts can put the swinging arm a little out of line.

Dear Ken,

You fail to mention whether the machine is in Roadster or Interstate trim.  I have both set-ups and the wobble [which never really goes away] is noticeably more pronounced in Interstate trim.  I would imagine that it is linked to the figure of the rider, now approaching the dimensions of Harold Daniell, being much closer to the steering head on the Roadster version, which, by the way, handles superbly.  Of course, I stick with the 90/90 front and 100/90 rear in 19-inch sizes and don't carry a passenger these days. When I had the Hepco and Bakker pannier set-up and took extra camping gear on the Interstate the handling when slowing down for roundabouts was 'extremely interesting', even with the Ikon suspension set on 'full' and the correct tyre pressures.  I found the best way to cope with the wobble, which surfaces when decelerating between 40mph and 15mph, was to relax the hands and steer with the feet [as usual].

Dear Ken,

Another issue is weight distribution.  Use of the Interstate set-up effectively moves a lot of the weight further back.  The standard rule for carrying extra kit on any motorcycle is that the weight should be, where possible, as low down as is practical and ideally between the axles.  Do you have that abomination which some owners fit to all their bikes, a top-box?  Another suggestion would be to try your Commando with the luggage fitted but with nothing in the panniers.  

Looking forward to a progress report; will you be going to Norton Day?

Regards, Colin.

My experience is all on Interstates. I've found keeping the tank full can help as it creates a fair amount of weight bearing down largely on the front end.

I also had a wobble on my 750 Interstate Ken for a number of years using 100/90 19 Roadriders front and rear. When slowing down from 70 mph to around 30mph the wobble got worse especially if l let go of the bars.
When going to a rally I would put my tank bag full of tools on the tank to compensate for the luggage on the back but it still wobbled a bit.
Last year before riding up to Aberfoyle I fitted 90 90 19 Roadriders Mk2 to the front and 100 90 19 Mk 2 on the rear and had them both balanced. I also replaced the cush drive rubbers in the rear hub. I could not belive the difference riding the bike 250 miles to Scotland. No wobble at speed, no wobble when slowing down and now I can take my hands off the bars and slow down from 70 to 30 without any wobbling. I can't say which of the changes stopped the wobbly but its bloody brilliant riding it now. I have experimented with tyre pressures and now put 34 in the front and 36 in the rear and it just rolls into bends now even though so called 'experts' tell me the pressure is too high. (it's my bike so I ignore these experts). I still put my tool kit on the tank if I've got luggage on the back as my Interstate usually feels quite light on the front. I tried all sorts of things to stop the wobbling before last but none of it made much difference. Now it handles nearly as well as my Atlas did when I had it.

I had really nasty head-shaking on my Mk3 when fitted with Craven panniers in the '90s. Even with only waterproofs in.

I recall noticing that Cravens sold in the 1970s had much sturdier stays to the rear footrests than those sold after the brand was taken over by Watsonian. I've seen them fitted too with a tie-strap behind the rear wheel.

Following a Commando with Cravens soon shows how much they flap about...it felt to me as if the the wobbling started at the rear. I took the panniers off rather than chase the problem so I don't have a definitive answer but any stiffening of the pannier mounts can only be a good thing.

Ken,
Rearward luggage (Cravens and the like) is pretty much unavoidable if you're riding two up, and in my view it's the weight behind the rear axle that creates or at least accentuates the wobble problem? However, if you're solo as you suggest you are, how about soft luggage forward of the rear axle, either sat on the pillion part of the seat or even better, lower down the side of the  bike ?