Why do our commandos...

vibrate so much. Why does this particular engine (along with older brit twins) move around so much. Is it due to things not being perfectly balanced?
the reason i ask this is i went and played with a new triumph thruxton 900 and it runs smooth.. and i got to thinking the technology hasnt changed that much, i can only think its the parts that are made better, balanced right.

Any ideas?

Commando engines vibrate because there are 2 pistons moving up and down in unison, the isolastics isolate the driver from the vibes from 3K upwards if the correct rubbers are used and are adjusted correctly. Most modern bikes are either 4 cylinder where 2 pistons cancel out all the primary vibration from the other 2 (but not the secondary) and sometimes have a balance shaft to eliminate the extra vibes. All very boring and lacking in Character

so its the lack of alternating pistons that causes the vibrations?

anyone ever made an offset crankshaft or am i just getting out of my head with stupidity now

Driggs:

Though I can't find it online, I swear the new Thruxton/Bonnie motor has a counter-rotating balance shaft to smooth things out in its 360-degree 865cc twin.

While offering good primary balance, a 180-degree crank sets up a rocking couple on a twin. Picture the engine wanting to swing back and forth across the centerline of the bike - like a pendulum. Not a pretty picture on a large displacement, long-stroke engine.

There are successful offest cranks that do work well for Nortons, including 73-degree, 90-degree and 270-degrees. The crank phasing cancels out much of the bad vibes without creating an undesirable rocking couple. For example, the Yamaha TDM900 uses a 270-degree crank in its parallel twin.

so it would be possible to put said various degree of cranks in the norton and smooth it out a little bit, while maintaining a somewhat degree of balance?


yeah that thruxton i looked at is a pretty nice bike.. not bad for like 8500$ either

Hey Driggs,

Why is YOUR commando vibrating??

Mine vibrates at idle (I call it character) but progressively smooths out as the revs build and is as smooth as silk at 3000 rpm up. Even when its vibrating a low rpm's, it's more of a pleasant throb than an annoying buzz (my old XS400 Yammie was horribly buzzy, I would get off and everything I touched was vibrating!!)

Are your running a Norvil heasdsteady? What do you set your isolastics clearances at? Did you check the clearances all the way around the entire iso, not just in one spot?

You can mess with other crank offsets, but the Creator has given you a set of rubber mounts for a reason. :wink:

Cheers,

Derek

Mine was an academic answer - not a suggestion.

Besides the crank offset, you'd an offset camshaft and ignition trigger - and you'd better hope that each party doing work for you did it spot on or you'll be up the creek with an empty wallet.

Anyhow, listen to Derek and check all the normal stuff, iso's, headsteady, etc. as the vibes aren't unbearable on any of the S'norts I've ridden lately.

derek and dr,

when i posted this i referred to the new thruxton i rode, and wasnt making a reference to my exact bike.. just in general about how our engines are. mine is smooth at about over 3000, and i have no complaints what so ever.. just wondering.

i like to see how companies make things run nicer. for instance how in most cars nowadays, the engines barely vibrate or make noise.. i was just seeing what in particular caused ours to be a little more dare i say violent in running

vibration

Hi Driggs,

There are more articles about engine balance and how to cancell the effects than anyone can read. The new Triumph twins do have a counter balance shaft, but I've ridden both the new Bonneville and the Thruxton and don't believe they are smoother than my 750 Commando. Changing the firing on the Commando crank from 360 degrees to 180degrees will cancel some of the imbalance, but will introduce rocking couple another type of imbalance. All reciprocating engine designs have imbalance and are delt with. Norton's solution was the isolastic mounting. It wasn't perfect, but was innovative for the time and it does work to isolate vibration from the rider. Commandos still have reliability problems due to the vibration shortening the life of carburetors, electrical components...etc. Don't pull your hair out trying to make a Commando into a modern motorcycle. If you want that it's cheaper and less frustrating to go buy the new machine. Forget about the twins and go ride a Triumph Sprint ST....very nice. Keep the Norton if you're nostalgic.

ill,
thanx for the good reply. i dont plan on getting rid of my norton, i love it how it is.. i was just being curious.. it is currently off the road now for the year and im planning on going thru the bike fully. when i bought it two years ago the guy had a long list of parts he replaced, but i dont believe him. i thought the thruxton was a little bit smoother than my bike, but i could have some isolastic issues and what not.

re vibrating commando

My commando has been driving me mad because although the vibration smooths out great above 3500, this is over 70 mph & on country lanes I have been stuck behind someone at below 60 with hands in agony because I am in wrong rev range. Solution, special home made rubber mounted handlebar grips. BAsically a tube clearing bars by 1/16 inch all round, ends are larger diameter to take rubber bushes of soft compound. These bushes are the only point of contact with the handlebars. Tube has lead lining on outside to add extra weight. This dampens vibrations at lower frequencies. I could have used very heavy bar end weights but I thought this may fatigue bars round mountings. I am still experimenting with different rubbers, but as it is the results are a transformation & take the agony out of long journeys. Bars vibrate less than a 4 cyl jap from 2500 rpm

Geoff
What kind of gearing do you have on that thing? Mine gets about almost 60 out of 3,500 rpms, and I think I have a 21 tooth sprocket in there....I know they made 23s...but that isn't for street driving. Thought about changing the sprocket? Or kicking it down a gear? The motor will love you for it.

Aren't those bars a bit like the old BSA? The bars were mounted into the triple tree with rubber grommets. Awful. Sort of like a love affair per internet. Its there, but not quite.

I think I am using 21 tooth rear sprocket, but Hayward belt drive which ups the gearing a bit. I was exaggerating slightly with speeds / rpm, I have been out on it today & 3500 rpm is just over 60mph, so similar to yours

Didn't know the belt drives up the ratio....live and learn. I used to have vibrations too, and the hands that go to sleep on you....but I redid the isolastics and the pain went away. If they are plain too tight, they pass all the bad vibes through to the bars.
Spose you have long bars. I have clipons...so I guess my bars wouldn't wiggle too much. Got to be a fixable reason, for the hand pain though...if the belt drive ups the ratio....maybe for laughs...give a 19 sprocket a try. Mine doesn't shake, enough to get unpleasant, even at low rpms...just not good for the motor over the longrun. You will find a reason...somewhere.

HEw, I avoided lowering gearing when I rebuilt it 12 years ago on the advice of the late Fred Barlow, for reliability reasons. I~solastics are set on the wide side ie around 10 thou. The rubber mountings are purely on the home made grips which are made of steel/lead not on the bars themselves. This does not make a nasty feeling on the steering, the painful rev range where the vibes kick in is now more like 2000 rpm, & the reall rough spot at 2700 - 2900 has disappeared. I think this is because of different amounts of weight on the bar ends altering resonant frequencies in thebars. AS I say, the problem is vastly reduced, & bike is comfortable for 150 miles without numb hands. I still think I can get it better by messing around with the rubbers in the handlebars.

If they work and make the pain go away....got to be some point to them. Still not sure you should need them though....got to be some reason for the vibes that can be fixed...not all machines are as you decribe...at least not mine...anymore. Bet I have my isos set tighter than you do though...my motor doesn't move up and down, even at idle, like it did years ago with the original isos in it. They were so loose..it was amazing to watch the motor wiggle about. Now, my rear fender has conniptions at an idle, but above that, everything smooths out. Come to think of it...most of my mirror wiggle went away when I put in Norvil iso and even more went away when I did a valve job.....motor was running certainly uneven and unbalanced due to a burned valve.
As long as your grips don't make the bars mushy like that BSA setup did...
Just can't figure you would really need them, if everything else was really in order....but then again, I'm not even remotely, correct all the time. Besides...you like them and that's really all that counts...or?

Compared to its predecessor (the Atlas) tha Commando is so vibration-free as to be almost like a rice-burner 4-banger.

We used to reckon that an Atlas headlight bulb lasted 5000 miles, and it didn't matter whether you switched it on or not. The vibration would cause the element to fall off the posts inside the bulb.

My commuter bike when I worked at Marston Road was an old company-owned 650SS, and my initial commute from Kenilworth to Wolverhampton (45 miles or so) was uncomfortable from the vibration point of view. It was a blast otherwise. When I rode the one and only "Villiers Fantabulous" scooter on the same route, I appreciated the performance of the 650SS!

There was a problem on some of the early Commandos where the tube that ran across between the rear set of Isolastics would distort at the ends and screw up both the isolation characteristics and the handling, but I think there was a retrofit kit produced by the factory. That was just about the time I departed N-V for greener pastures across the Atlantic!