1972 750 Commando weave

Morning Mark,

It's a common problem with Commando's. It is usually caused by inaccurate frame alignment due to production methods.

As long as each isolastic assembly has approximately 0.006" total clearance then they are not the problem. The main problem is that the swingarm spindle is not square in two planes to the steering head axis. This means the rear wheel centre line moves in relation to the frame centre line plane during rear suspension movement.

Also there is no frame centre line reference point to align the rear wheel to. The wheels can be aligned to each other, but this doesn't mean they are aligned to the frame CL. To correct these points requires a complete frame check. I can do this www.parallelengineering.co.uk

A steering damper may mask these problems, but your addressing the symptoms not the cause. How does the bike steer hands off decelerating from, for example 80mph down to 30mph? Should steer straight, over bumps and bad road surface, with no fork wobbles.

Commando's can easily cruise between 80 - 90mph. In my opinion it dosen't make sense to ride around on a bike with handling problems. Obviously very important from a safety point too. I imagine you wont be taking corners confidently at 80mph.

Out on the Mk3 in a bit into Wales, I've invested in some Gerbing heated gloves - toasty!

Dunlop Wobble & Weave.wmv - YouTube

You may find the attached link enlightening (if my dodgy computer skills are up to it, if not, check out 'Dunlop Wobble and Weave' on YouTube). Perhaps the simplest solution is to eat more chips!

Previously howard_thompson wrote:

Dunlop Wobble & Weave.wmv - YouTube

You may find the attached link enlightening (if my dodgy computer skills are up to it, if not, check out 'Dunlop Wobble and Weave' on YouTube). Perhaps the simplest solution is to eat more chips!

i think most of us could manage that and maybe for real handling benifits the odd pie an pint may help the serious sporting rider

As Simon suggests, alignment is probably the ultimate cause. Age / state / type of tyres can be a factor as well. Make sure they're running concentric on the rims.

Is yours a disc-brake model ? Many wheel bulders fail to get the rims for the disc hubs correctly pulled over to the disc side...if centred to the flanges, the rim will be offset to the left and it will be impossible to align the wheels along the centre-line of the machine. You can do a crude check simply by measuring between the fork mudguard mounts and the rim on each side.

Any reasonably well set-up Commando should be rock steady at 80 - 90 mph but the odd flutter on poor surfaces as components wear and clearances increase is probably to be expected.

If yours is stropping on a level, straight, well-surfaced road then it does need looking at. It's well worth getting it right.

Previously howard_thompson wrote:

Dunlop Wobble & Weave.wmv - YouTube

You may find the attached link enlightening (if my dodgy computer skills are up to it, if not, check out 'Dunlop Wobble and Weave' on YouTube). Perhaps the simplest solution is to eat more chips!

I've seen the video - my Mk3 Interstate dosen't do that, even when the petrol tank is on reserve - and I'm 11 stone.

Very interesting viewing. I've only had 2 full blown tankslappers. First when racing at East Fortune - someone had undone my steering damper between races. The public weren't kept out of the pits in these days. Found myself up grass tracking at 90+. Second was on a BSA, heavily laden top box, 75 mph bend. Went through a fence, down a bank into a stream. Ended up slightly shorter.

Worth avoiding!

My own take on the video is that Mr Cholmondeley-Warner's riding assistants must have had cojones of steel.

Whilst wheel and frame alignment are important, If your wheels are parallel and you are travelling in a straight line on a smooth road, as there will be no input to the suspension or steering, the problem lies elsewhere. I would guess that with two people on board the problem goes away. I have just fitted a steering damper to my Commando to improve my straight line stability. My weave has moved up the speed range from 80 to 90 though I haven't ratcheted up the damper yet. Baggy riding jackets can induce a weave transmitted from the rider to the bars. You may consider taking one hand off the bars and see if that works but I cannot be held responsible for any laundry issues occuring.

I don't doubt what Simon says that back in the day Norton manufacturing tolerances were so bad that the swinging arm and forks were all sorts of out of line. The didn't get much else right so why should they get frame alignment right?

But I've rode other bikes that have had hard crashes such that the frame / forks were all over the place, but they didn't display the alarming charcheristics that Commandos do. Nor do other bikes bikes Iv rode with worn steering head, swing arm, tyres and wheel bearings.

My thinking is that you can get the frame/ forks in line as much as possible, put modern tyres on it, get all the frame/wheele bearings as good as possible which will help, but the bike will still wobble and weave because the back wheel is flapping about by 1/4 of an inch side to side on its isolastics.

That wobble/weave is the price we have to pay to be isolated from engine vibration at cruising speed.

There is no such thing as a free lunch

The road would not only have to be smooth, it would also have be dead flat to have zero input to the suspension. The faster you go then undulations on a smooth surface will be of a higher frequency and input to the suspension is greater, resulting in higher amplitude or movement. Go slow and the frequency and input decreases to the extent there is no suspension movement. Try putting maximum preload on your shocks and you should find the weave will begin at a higher speed because the suspension requires a higher input to move it.

Dave, you have mentioned pilot induced oscillations, machine centre of gravity - suggesting more weight on the front would help, but you now contradict this by suggesting a pillion would help matters. Latest theory is baggy clothing - by billowing I assume, thus reducing weight on the front, which also contradicts your earlier C of G theory.

Please let me know where this smooth, flat road is. Riding it would be a novel experience.

Previously peter_stowe wrote:

My thinking is that you can get the frame/ forks in line as much as possible, put modern tyres on it, get all the frame/wheele bearings as good as possible which will help, but the bike will still wobble and weave because the back wheel is flapping about by 1/4 of an inch side to side on its isolastics.

That wobble/weave is the price we have to pay to be isolated from engine vibration at cruising speed.

There is no such thing as a free lunch

Hi Peter,

The F750 TT winning monocoque incorporated isolastics on the front, rear and headsteady engine/cradle mounts. Peter Williams stated this was the best handling bike he ever rode. He also finished 2nd on a Commando in the 1972 production TT at an average speed of 96.53mph. I don't think this bike would have weaved and wobbled it's way around the TT course.

I was fortunate enough to speak to Peter at the Telford Off Road and Race show 2 or 3 years ago, where he told me ALL his race bike frames were checked for alignment at the start of the season and then again at the end, even though they may have not been involved in any accidents.

I'm pretty sure he achieved the 96.53mph average on the production racer without a pillion for ballast or riding one handed.

firstly what tyres do you have? are they 19" front and back? the very worst my commando handled and weaved at 90mph was with an 18" 120 roadrunner on the back and a 19" tt100 110 on the front i have tried many combinations of wheel sizes and tyres but the very best so far has been avon roadriders 19" 90 on the front and 19" 110 on the back rock steady no tramlining and no weaving upto 110mph i also have these fitted on my BMW r100rs with the same good results,and as others have said commando frame alignment can be iffy! all sorts of things can upset them top boxes,fairings one mirror etc or your rear tyre scrubbed square! ,,,,,,,,best of luck with it Nick

I agree with Richard and Nicholas regarding tyres. Size and profile are important as they are on any motorcycle. I've only experienced straight line weaving twice (pre-alignment). First with a new 4.10 19 TT100 rear and part worn Avon Roadrunner 4.10 19 front. The weave reduced as the TT100 rounded off. Second, solo with a top box, increasing rear rebound damping reduced this. Pillion also helped as the air stream wasn't hitting the front of the box.

Now, with 100/90 19 Roadriders front and rear (and after frame alignment) the only problem is a hands off handlebar wobble at about 35mph when the front tyre is approx. half worn. I guesstimated 3,000 miles in a previous post, but this is not correct, more like 6,000 miles. New front and the problem goes away. Rear's can be worn down to the limit with only a slight shimmy when crossing white lines, which have no effect when rear has reasonable tread. I will measure at what tread depth white lines start to have an effect.

The main problem before alignment work was instability (weaving) at speed, through bumpy corners. This dosen't happen anymore. Straight line speeds are no problem, on bumpy or smooth roads. None of the critical frame components were square to each other pre-alignment.

I checked Dave's frame last year as a favour and neither of each pair of the front and rear isolastic mounting bracket holes were square to each other or the steering head axis. He went away and did some welding and filing and I re-checked. It was still out. I explained this was only part of the alignment for a Commando, but he thought it was 'near enough'. The result is the problems he's now reporting on this site.

If a bike is getting out of control in a straight line then you've got no chance through the bends.

Now if you really want to go all the way on alignment, take a look at this.........http://www.vintagenet.us/phantom/wsc.html

From that I was able to suss that my front and rear Isos were out. The rear had 0.25" offset and the front 0.31". Consequently the whole plot was pointing left so I had 0.06" machined off the front Iso and all the ends squared off. An 0.06" spacer was then added to the LH end cap to replace the missing material on the right. Cured the weave even with a 110 x 18 rear and 90 or 100 x 19 front AM20/21/26 or Bridgestone BT45 both ends. I also made the effort to find the actual centre for the rear wheel in the swing arm (aligned with front wheel) and it came out pretty much bang on the cradle offset. That disagrees with some who say the 750 swingarm is offset the other way to compensate the cradle. Well mine was not.

Previously keith_glassborow wrote:

Now if you really want to go all the way on alignment, take a look at this.........http://www.vintagenet.us/phantom/wsc.html

I've mentioned the above article before and it was this which persuaded me to get the required tools, surface table etc so as to carry out the necessary work on my own Commando, which I have done. The handling is transformed. The misalignment of a standard frame is shocking. When you can see the extent of misalignment it's not surprising many Commando's have handling problems. More info at http://www.parallelengineering.co.uk

The frame that we checked and I welded and filed is not the one I'm riding around on today Simon. I have a new frame and it is straight. The bent one had been bent like a parallelogram and my welding and filing, even after a reasonable result when checked on your granite datum, still resulted in wheels well out of alignment. I have given up on that frame for the time being. When time permits I intend heating and bending it straight after repositioning the front and rear iso holes back to their original position.

Previously david_evans wrote:

The frame that we checked and I welded and filed is not the one I'm riding around on today Simon. I have a new frame and it is straight. The bent one had been bent like a parallelogram and my welding and filing, even after a reasonable result when checked on your granite datum, still resulted in wheels well out of alignment. I have given up on that frame for the time being. When time permits I intend heating and bending it straight after repositioning the front and rear iso holes back to their original position.

I stand corrected - forgot about the new frame.

I do remember checking the iso brackets and they were better, but still not correct after your welding and filing. As I explained, that was only the first part of the alignment process. You missed out on swing arm to steering head alignment and establishing the frame centre line. In my opinion the frame, even if bent - which I don't think it was, could still be aligned correctly.

The replacement frame you bought was unused and genuine, correct? Your upgrades include Lansdowne fork dampers, new adjustable iso's, Dave Taylor type rod end head steady and new steering damper. You claim the frame is straight and the wheels are in line. So how come you've still got handling problems? What your doing is trying to address the symptoms and not the cause i.e inaccurate frame alignment. I don't think it's because you're under weight or due to baggy clothing. Good luck with your heating and bending.

Here could be another issue that I have considered. Should your (or my) rear wheel sprocket/brake drum not be running cocentrically to the spindle, or the gearbox sprocket, then the rear chain tension will be changing maybe quite substantially as the sprockets come in and out of phase. this will put a one sided pull on the swinging arm. maybe only enough to manifest itself at higher speeds.

Clutching at straws is the phrase that springs to mind.

Here's another possibility. Have you considered the magnetic pull of the moon. As you apply an opposite force this induces a weave!

Varying chain tension can be checked with the bike on the centre stand, but is usually due to a part worn or worn out chain. Easily proved by fitting a new chain. How many miles have you done since fitting the new o-ring chain and sprockets? If the chain was getting tight enough to bend the swingarm, which is what I think your suggesting, then the chain is going to wear very quickly, if not break. I don't know of any motorcycle where the rear wheel sprocket runs concentric with the gearbox sprocket.

So it's absolutely, definitely nothing to do, what so ever, with the frame alignment then?

My poor writing skills have let me down again. Hypothetically if the drum/rear sprocket was not running true or the gearbox sprocket, then the tension on the chain would be varying. That varying tension may manifest itself as a pulsing force against the rear isos (not bending the swinging arm)

With regards to ..........no I give up you win

Varying chain tension cannot have any effect on the isolastics because the stress is transmitted from the gearbox sprocket, via the chain to the rear wheel sprocket, along the swingarm to the engine/gearbox cradle plates, and then back to the gearbox sprocket via the gear box mounting bolts and gearbox. There's no external force loading the isolastics.

It's the same as trying to push a car along a road by applying force to the dashboard when your sitting inside the car. It's going no where.

If chain tension affected the iso's then every time the bike is accelerated and chain tension is increased the iso's would be loaded. The iso's are only loaded, as in side to side movement, when an external force is applied from the road via the rear wheel/tyre.

Previously mark_chapman2 wrote:

Previously howard_thompson wrote:

Dunlop Wobble & Weave.wmv - YouTube

You may find the attached link enlightening (if my dodgy computer skills are up to it, if not, check out 'Dunlop Wobble and Weave' on YouTube). Perhaps the simplest solution is to eat more chips!

Thanks that was very interesting and makes a lot of sence.

It's interesting the only way they managed to achieve a weave situation with the Laverda was with incorrect tyres and pressures. Whereas all the other bikes tested weaved and wobbled as standard. Therefore you can conclude there is something wrong in design or manufacture with the other bikes.

Just watched another version of the film but with Murray Walker presenting. The emphasis is on keeping below the speed limit (because bikes don't weave at 70mph), keeping your hands on the bars (to prevent fork wobble), checking tyre pressures and only using tyres with lots of tread. It's more a road safety film than a scientific report.

That video was interesting.

Back in the 1976 I had one of those Honda 750s used in the video. One day while I was trying for its top speed, tucked in and lying on the tank every thing was ok. When I shut off the the throttle and sat up with 120mph on the clock it broke into such a violent weave/wobble that I was shaken off it. I got pretty smashed up and the bike was destroyed.

My recently owned 100,000 mile Honda Paneuropean always had a low speed wobble. When the tyres were good it was as stable as a rock up to its top speed,but as the tyres wore down it tended to weave a bit after crossing white lines/over banding.

A Honda Fireflade I used to have was never anything less than rock steady, with new, or worn tyres, with just me on it or fully loaded up for a 2 week camping holiday.

An almost new BMW GS 1200 that I now have is rock solid at all speeds, even when carrying a huge load, until the tyres wear down, then it weaves a bit over lines/banding.

My currently owned girder forked BSA WM20 has slowly been getting into the wobble weave habit ( bearing in mind it's top speed is 50mph ). Since renewing is girder fork bushes and steering head bearings it is now very stable.

The 55,000 mile very standard Commando I've owned for 30 years has always had the wobble weave illustrated on the video. It has good wheel, swing arm and steering head bearings. Always had TT100s on the standard 19" rims. The isolastics are set up on the tight side. It is probably only slightly worse than plenty of Jap bikes that I owned in the 70s/early 80s, and I have spent many an happy hour on it with the clock around the 70/80mph marks without any big problems, knowing that if it does weave it wlll soon get out of it by itself.

So my thoughts are that the isolastics on Commandos probably only make them slightly worse than their 1970s contempories, and they shouldn't be judged against modern bikes, but they would certainly benefit from frame blue printing as suggested by Simon

I guess it is down to "Luck of the draw". If so many frames varied in alignment, at least some must have been right! Whilst searching for an early Dominator, a few years ago, I was offered a newly restored '71 Commando. I never had any handling problems with that bike (lots of other problems though!). Slightly overgeared, it would exceed 100 m.p.h. at 6,000 r.p.m. in third gear, and cruise comfortable at 90 in top.