Commando vibration

Basically you are right the tighter the isolastics the better the handling but more vibration,what head steady does your bike have?,if its the norvil type you will get more vibes but from your description it does sound excessive, regards nick

Hi Tom,

I have an 850 MkIII but it is all pretty much the same. Once mine is much over tickover it is fairly smooth, has more of a shake than vibration. Once under way there is very little vibration at all. This is what I would expect.

Over the years I havetightenedthe isolastics a little too much on occasion and it did bring about vibration.

Adjust according to the book and it should be smooth.

I did fit a Dave Taylor head steady a while ago and this did generate sufficient vibration for me to take it off again. I will have another go one day when I have the time to see if it was just badly set up (by me!).

Regards

Tony

There's something wrong somewhere Tom. Isolastics have to be bolted up pretty well solid before it becomes uncomfortable but it may depend on what you're used to.

Recent manufacture iso rubbers do seem to be harder.

It's worth checking to see that nothing is 'bridging' the isolastics. Did the previous owner do anything silly with crash bars ? Is the fuel tank contacting the head steady ? Footrest against the primary chaincase, that sort of thing.

If the isolatic tubes or caps are not square then false readings will result.

I'd be a bit concerned about vibration which appears to be gear ratio related rather than pure rpm as it may indicate that it comes from the transmission. If it is substantially less in top at the same rpm then it suggests that the cause may lie in the gearbox (in top gear, the input shaft is locked to the output sleeve gear so loadings are diferent).

A rumble from the gearbox doesn't sound good. The achilles heel of this box was the layshaft bearing. This can now be replaced with a superblend roller bearing from Andover Norton.

I have read this thread with interest as I have a 1972, 750 which I've owned for 30 years this year. I rebuilt the engine and gearbox a year ago and while it was apart I decided to have the crank balanced. The bike didn't have a nasty vibration, but would definitely have a good shake at 5000 plus revs. I had it balanced for 4-5000 revs as I like to open it up, especially when keeping up with mates on modern bikes. I'm regretting having this done now as I have a tingly vibration at 2500-3000 revs that makes my hands go numb and pins and needle in the lower region. This might be nice at another time, but not when you can't feel a thing when riding your bike.

I have chatted to the engineering firm that did the work and they are very surprised about this as it was dynamically balanced and they have done many Commandos. I was asked if I had done anything else to the engine and I said yes, I've put an electronic ignition on it. I was told to check my timing and retard it a fraction to see if this makes a difference. It was a bit advanced so I knocked it back a bitand yes it worked. It didn't cure the vibration but it's a bit better. So try retarding your timing a degree and see if it works for you.

I now realise that my Commando was set up really well for most riding conditions, from the factory. My need for speed and search to "improve" the bike has created this problem. The big shake was a lot better than a tingly vibration, but now I have to live with it.

Previously wrote:

Having read a lot about isolastics and vibration I would like to know from some of the more experienced members if my 1972 Roadster's vibration is within "normal" limits.

Hello. Try to determine the size of the gap to start with; use a long bar to move the engine over and insert a feeler blade. Always adjust the isolastics when the machine is on the sidestand.

I find that a good section of road where you can accelerate up to about 70mph is essential as a testing area for isolastics. Start in a layby and ride about half a mile to reach the target speed. Determine where the vibration is worse in top gear. Back to the layby. Adjust the front isolastic with the large bar and socket and reach 6 thou. Tighten up with a torque wrench. Go for the ride. Back to the layby. Open the clearance out to about 8 thou, tightenand try another ride. Aim for top gear again. Back to the layby. Open up to about 10 thou and tighten etc. Another ride. Somewhere here will produce a sweet spot; on my machine [they are all different] the worst patch is about 2,200 to 2,500. From there on it's really smooth.At 5,000 it's unbelievable. If it's about right then when you are sitting on the machine the front mudguard should be moving up and down regularly. When you apply the front brake the up and down movement should stop.

Good Luck!

Colin Cheney, somewhere in England.

Generally violentvibration isfelt in the handlebars and seat from approx 2500 to 3000rpm, where everyting smoothes out. Its worst in 3rd gear under light load, say in town, where vibration is bad enough to disturb the fillings in my teeth!

The bike is fitted with the adjustable isolastics, but before I start playing around with adjustments, I would like to know if I can expect any improvement? I assume that loosening the isolastics will improve vibration, by soaking up more of it, and the opposite will have the opposite effect. At the moment handling is excellent. Straight line stability is great, long fast corners, tight corners and breaking is equally flawless, so I wouldn't like to disturb any of these.

I would be greatful for any opinions?

Seems a bit high. My 750 was balanced at 63% with pistons androds weighed and matched etc. It was quite poor from side to side and a fair bit of material was removed from one side. It didn't make a huge difference to the high frequency vibes from 4200 rpm. Mine is rough at 1800-2400, smooths out and then a tingle/roughness comes through the footrests from 4200 appx. If I depress the gear lever (in top) I can feelvery coarse vibes. At the same time putting a gloved hand on the head/barrels it feels pretty good. Gear shafts are straight, belt drive in place, replaced bearings. I also have a solid head steady of my own make (see pic) and it works well but made no difference to vibes. ISO's are set at 10 thou, any less and HF vibes are much worse. Even went to the trouble of squaring up ISO tubes and getting front and rear cradle offset the same (was 60 thou more on front than rear, pointing the whole assy left). That cured weaving at speed.

http://i424.photobucket.com/albums/pp322/keith1069/NewHeadSteady014.jpg

The bike has had Powermax pistons fitted from before I owned it, The pistons, rings, gudgeon pins, circlips, conrods, shells, conrod nuts and bolts were all weighed and matched with each other. Then weights of this amount put on the bigend journals. Not forgetting to add the weight of the oil that's inside the journals and flywheel. The crank is spun and balanced like a car wheel. I'm sure the company did a good job.

I have a 23 toothed gearbox sprocket fitted, and the bike flies. It could even pull another gear as it revs so well in top. There is no shaking at high revs, just a tingle through the footrests. It still vibrates less than my mates BSA A65 so I should stop moaning about it really. 2500-3000 is about 40-50mph, and you forget how often you ride at these speeds, I can't go flat out everywhere. It's just different than it was and not necessarily better.

The bike has had Powermax pistons fitted from before I owned it, The pistons, rings, gudgeon pins, circlips, conrods, shells, conrod nuts and bolts were all weighed and matched with each other. Then weights of this amount put on the bigend journals. Not forgetting to add the weight of the oil that's inside the journals and flywheel. The crank is spun and balanced like a car wheel. I'm sure the company did a good job.

I have a 23 toothed gearbox sprocket fitted, and the bike flies. It could even pull another gear as it revs so well in top. There is no shaking at high revs, just a tingle through the footrests. It still vibrates less than my mates BSA A65 so I should stop moaning about it really. 2500-3000 is about 40-50mph, and you forget how often you ride at these speeds, I can't go flat out everywhere. It's just different than it was and not necessarily better.

Just add my tupence worth,

I'm on my third Commando, and this one shook something awfull, at low revs

Did Iso's, rebuilt Iso's checked head steady etc etc

So bad I was dreading something was serously wrong, but the engine n gearbox are really quiet.

It's a bit of a rolling renovation and the next thing on my list was to replace the carbs (Knackered Slides)

After fitting the new ones with the same jetting ect I notice remarkably better tickover and bottom end responce but -

The vibrationhad goteven worse about 2000 - 3000 RPM

This got me thinking,

I dropped the needles to centre position (It's MKIIa with plastic airbox,long needles n cutaway jet bodies)

The difference after droping the needles and resetting tick over mixtures is amazing, so smooth

So its allways worth looking at other reasons for symptoms as well

GP

I worked out (lastweek on the road) that if you use the gears to ensure that you keep the rev's at the right place, it isn't really too much of a problem!

Don't labour it, don't over rev it.

However to be fair, I have just adjusted the isolastics on mine, and I did fit a Dave Taylor head steady, which is nothing short of excellent and not too much of a faff to do.

Erm, it also has new brass slide Amals, new Pazon ignition, new stator (uprated), new "power box"; partial rewire (someone had been at the loom with a knife and fork); clutch rebuild (after a "oooo, let's put a 750 clutchwith an extra plain plate,into an 850", bodge...) which I am expecting to have to rebuildagain very soon with at least a proper thickness pressure plate - after proving it just isn't agoodsolution; a starterrebuildwith new brushes, I even re-oiled the oilites!(it actually works a treat when warm and just needs a little helpwith cold gloopy oil); new sprag clutch, crank and starter gear cogs, capacitor (these are service items you know - if you have neverchangedyours do it now. Electrolytic cap's do go dry over time and do not work. You can get a good cheap equivalent from CPC.), and a few other things.

I must have had "saw you coming", on my forehead when I bought it last year!

But some of this work mayexplainits apparent enginely calmness. Or not. I just don't know...but I do know it pulls like a train, steersaccuratelyand tracks like something that tracks very well, and then took some "watch me corner" drugs to make sure it did it better.

Oooops, oh well; back to the ward

Fun times!

Jack