I have just returned from the Indianos Northern Spain rally and on the return was experiencing serious tank slapping on my 1976 Mk3 Commando when braking for a roundabout etc. The bike has a 5.5 gallon Instate tank, which had at least 3 gallons of fuel in it, and it was reasonably loaded for the trip with full panniers and a full bag on the passenger seat but no passenger. I thought that this must be unequal levels of oil in the front fork legs (changed not that long ago) but each leg contained exactly 150cc of oil, the tyre pressures are also OK. I need to check the wheel alignment but do not think that this is far out, the tyres also have plenty of tread on them and are not damaged, the front brake is an RGM with a Gremica caliper. I have had the bike from new and there there is no frame damage. Any ideas please as the tank slappers have become quite bad and I just cannot think what else would cause this, surely this would not be a problem with the isolastics? Thanks, Ken
I had a situation on my touring bike some time ago. Depending on my seated position I could induce a wobble by moving back on the seat. I adjusted the loading in the panniers and seat pack to bring the heavier items forward. Can you take front and rear wheel weights in the load configuration you had and then without the load? Bathroom scales and equal depth block to level the machine. May give you some indication of what is going on?
Check the head steady bolts to the head have not loosened, did on mine and it felt vague until retightened, only slightly loose so even looser would make things interesting.
The only time that mine has done that under braking was with Craven panniers on. I had the support struts to the footrests but the two sides were not connected below the number plate, as the original Cravens seem to have been. I don't think it's a weight thing but have the impression that the cases were flapping a little at the rear. I got them fairly well forward, but with the Roadster exhausts, they were sitting quite high.
As John says though - head steady security is critical.
With the standard head steady and TT100 tyres my Commando weaved and wobbled everywhere. Now with a Taylor head steady and Avon Road Riders it is as good as any modern bike. I suspect that an Andover head steady will be just as good and probably more durable.
Since this is something new, what have you changed?
The only time I ever managed to make a Commando unrideable was when I loaded a lot of weight on the rack. Moving it onto the tank, although a bit uncomfortable, resolved the problem. Ever since I've been very wary about heavy loads high and out the back.
But if it's something that's happened overnight with the same load setup I'd be looking at : tyre pressures, rear tyre badly worn in centre, rear shocks, wheel bearings, steering bearings, swinging arm bearings, or something broken.
The Commando has always been a bit "light headed" when carrying a weight on the back but this time the "tank slapping" was very bad and had to carry on at a roundabout in Spain when I really should have given way to a car already on it. The panniers were full (maybe I should have distributed the weight in each better but it was not too bad) and I had a full bag on the passenger seat but nothing on the rack, I did move myself as far forward as I could but did I not have a tank bag. I have been to the IOM with the wife and lots of camping gear and luggage in the past and although the steering was quite light I did not really get any of this tank slapping. The tyres are Road Riders which I now fit to all of my bikes. The rear shocks are Hagon and not that old. I will check out the comments made today. Thanks, Ken
Not having a brace behind the the numberplate may allow some flexing / swinging motion to the panniers . I have experienced tank slappers in the past with old worn out bikes, but never with a featherbed even fully loaded . Perhaps you should have taken the SS !!. After your years of sprinting you must be good at controlling a bike with only the rear on the tarmac.
I had a similar Commando with TT100s. The tread was good. I fitted a Hyde Fork Brace, but it was not until I changed the front tyre for another TT100 that the problem went away. I think that the original tyre had gone hard with age and might have had a flat spot. Nothing wrong with TT100s. Try The National Motorcycle Museum and get a free inner tube.
... on my Commando but did note that once the back tyre had flattened out in the centre it had a slight tendency to weave. Not dramatic though.
I would always advise carrying as much stuff as possible in a tank bag; moving the weight distribution to the rear is a bad idea on any normal bike.
However, I note that the problem is said to be tank-slapping while braking.
This sounds like a truly frightening phenomenon but it is hard to see how it can be due to rearward weight bias, since by definition there is weight transfer to the front when it occurs.
Also, since the issue didn't occur on the way out it seems unlikely to be due to weight distribution issues, unless you have stocked up with an improbable quantity of vino.
As others have noted, it must be that something has broken/worn/got out of adjustment.
I have checked the Commando over again this afternoon. The standard head steady bolts are fine, nice and tight and the rubbers are OK, the isolastic clearances are spot on, there is no play in the steering head bearings, the swinging arm is OK the bushes having been being replaced not that long ago, the Avon Road Rider tyres have plenty of tread and are not flat at the base. However, there is "slight" movement in the front wheel bearings when I rock the wheel from top to bottom so I suspect that this "maybe" the problem, it is not that easy to detect though. The bike has done about a genuine 56k miles so I will change the bearings and see what happens. Thanks for all of your advice. Cheers, Ken
" the swinging arm is OK the bushes having been being replaced not that long ago"
Check the spindle cotter pins are tight. I my case I got a slight wobble pulling away.
With regards to Peters reference to the DT head steady. The original link is still fitted and working well, no signs of any play.
Hi David and all,
The swinging arm is OK, the wheel alignment was slightly out but I have now adjusted that. There is no play in the head steady and the rubbers look in good condition.
The front brake has been changed to an RGM disc with a Grimeca caliper and when I took the brake pads out I found that these where "wider" than the disc contact area and therefore the pads where "rebated" over the outside of the disc by 1/8 to 3/16 inch although this was not uniform over the width of the pad so that this maybe have caused the problem which was occurring particularly when braking? I had a new set of pads which I have fitted and although these are still very slightly wider than the disc contact area they are a much better fit. Regards, Ken
Considering all was well before hand, and most mechanical items have been checked. Is the tyre square and concentric on the rim, has it moved, is it damaged.
RGM disc, is it the floating type, if so does it function correctly, is there wear on the fasteners. Might be worth checking the brake closer if it only happens when braking.
The tyre is good and square on the rim and the wheel spins very true wheel checked with a steel rule against the forks, the fastners are very secure as I took the caliper off when I replaced the brake pads. I have a feeling that it maybe those "rebated" brake pads as the tank slapping did mainly occur when braking. I will put it all back together load the bike up again and take it out for a test ride but this will not be for a couple of weeks. Thanks and all the best, Ken
.. that I had the same issue of "rebated" pads with a Lockheed caliper but I just removed the excess material with a hacksaw.
Exactly the same issue was apparent on the genuine PR disc brake that came with the Fastback I owned in the '70s. It can have no relevance at all to your problem. There is absolutely nothing to worry about, apart from relieving the overhanging material from each pad before it can prevent them gripping the disc properly.
Of all the issues mentioned so far, play in the wheel bearings seems the most likely culprit to me.
It seems odd that this device never caught on..
Ken, I would think the instability was caused by all the weight on the back, especially if a particular load can start to 'waggle' at the same frequency as the fork oscillations. I never had full blown 'tank slappers' or anything near that, but the front end would start a weak oscillation when there was a load on the rear carrier and could be triggered at 70mph if I moved my feet to the rear footrests for a leg stretch. Slack isolastics would encourage the effect. For a few years I ran with a hydraulic damper and that that would stop it.
Can you not just go out on the bike now, with no load on the back, and see if the misbehaviour has gone away?
Interesting problem. With all the usual suspects accounted for hopefully the bearings are the most likely culprit as Julian says but I would check the spoke tensions and offset as well. Would be surprised if it,s rear weight distribution under actual braking causing the slapper.
probably never caught on because it,s easier to reach over with the left hand and adjust the knob rather than put all that extra junk on the bars!
The big bendy may be fly by wire sensitive to all sorts of minor issues or combinations thereof.
But I think its a little off that nobody has pointed a finger at the road surface.
A little oil or sand just allowed the Commando to do its thing.
Rider set well back ( Interstate style), well loaded with kit, going downhill, 12kg of fuel sloshing around.
Had exactly the same happen just once, full tank slapper same circumstances had to stand it up or it was terminal.
My view was lack of grip from a 1/2 worn front tyre.
No repeat of this full tank slapper after fitting a new tyre.
Over many years with an Interstate I formed an opinion that they lack grip on the front end and are more easily unsettled than other bikes. Thats in a straight line not cornering.
Front end shake tyre or steering head bearings( full of water) most likely.
Weaving most likely isos or headsteady
Front tyres wear very slowly sort of confiirming my lack of grip comment.
Best tyre on my Commando was the Avon GP front and rear. TT100s were used because they gave better mileage. Then for a long time Avon Am18 front Am20 rear felt good until the front was half worn. Avon Roadriders more recently seem a good choice.
Interestingly Commandos will also wag their tails in mid air, bars static tail oscillating hugely. Not got a clue why but it makes Hondas back off.
Commandos take a little getting used to, allowance has to be make for the bendy bits. Bit like some Meriden Triumphs not shutting off mid corner just keeping some power on seems to help.
Then occasionally they throw in a scary one off.
Finally wonder if "wagging and weaving " should be better documented to help those newer members who experience one or both.
Hi Ken, I have a friend who had a similar problem when loaded up. It turned out the shockers he was gifted were 5 mm longer than standard. Once he put the right length shockers on the issue was solved.
Hi Ken, I agree with Normans comments above.
I recall my first Commando, an 850 Mk 1. I fitted a craven carrier and a small craven top box. I put a heavyish tool kit and some oil in the top box and found when riding solo I had a disconcerting waggle on the bars.
I replaced the items in the top box with some light waterproof gear and the waggle pretty much disappeared. Commandos do not like weight behind the rear axle.
Over the years I have done a lot of distance touring on the Mk3 Interstate I now own. I have a craven rear carrier/comet pannier set up. In order to keep the luggage low and as far forward as possible I have adopted Interpol rear exhaust mounting plates as the swivel allows the silencers a lower run and have reverted to the earlier rear seat without the hinge as this allows the the rear carrier to move forward approx 1.5 inches. Nevertheless, two up and with a full tank of petrol and a tank bag and trying to keep weight towards the front and luggage at the rear to a minimum, I still suffer the occasional waggle on the bars. This is also after having carefully ensured tyres, isolastics, suspension etc. were all correct beforehand. I have not yet tried a steering damper and will do shortly.
As to riding solo and laden at the rear - I try and avoid it. I had to ride the laden machine solo down the concrete ramp coming off the motorail in Düsseldorf about three years ago. At about 15 mph the machine developed a vicious tank slapper that nearly had me off. It was a real ‘change of underwear’ moment.
Not sure whether the comments will have specifically helped you but it does show you are not alone in your experience.
Exactly like Alan,850MK3 and CRAVEN Comet plus luggage carrier, in duo, BUT WITH A STEERING DAMPER and IKON shocks and never had any worries.
A different bike, A BSA A7 but the same problem. Never a handling problems for years until I had a load of heavy tins in the top box and hit a road bump at speed whilst cornering. This produced its first and only tank slapper (despite having a steering damper) putting the bike in a ditch and me in hospital. The top box has since been consigned to a shelf in the shed.