How to tell what is worn as vibration increases

Status
Not open for further replies.
Joined
Jan 1, 2017
Messages
349
Country flag
I got the bike used (I'm sure most of us did). So, never having ridden one brand spanking new, how do I tell what is worn as vibration increases. Front shocks I just add or adjust oil viscosity. But rear shocks? And the Isolastic suspension? And the head steady -- was that even standard on a '74 850?
 
Your question is a bit open ended.

Have you had the bike a while and vibration has increased?

If so the bike probably just needs a proper service which you will find detailed in the workshop manual. Any Commando owner shoud have a copy and they are easy to find from most of the suppliers. Your in the States so not my area but I'm sure someone will say.

Yes all Commandos from day one had the head steady. Its a critical part of the whole design of the bike. But there were upgrades plus several aftermarket improved versions. A photo would show what you have.

The obvious places to look for vibration are the isolastic adjustment, chain tension, loose bolts and fasteners, a common one is the tank touching the frame, tappet adjustment, carb sync on twin carb bikes, tyre balance.

I'd give the bike a good service front to back following the manual. That should sort most issues.
 
The reviews in the motorcycle magazines "back in the day" never seemed to acknowledge vibration of any sort. Ok, I recall a review of an BSA A65 Lightning that said all the light bulbs and the clocks stopped working due to vibration around 90 mph. But that was rare.
My point is it depends what you are used to. I get off my Honda VFR and ride my Commando and I am reminded what a difference 30 years of progress makes. If you are not used to riding a Commando, I don't think it is possible to convey what is a normal level of vibration.
 
To make it sound more simple, the isolastic engine suspension was a design to remove engine vibration from the frame, and thus the rider.
"Usually" these bikes have lots of vibration but at about 2800 rpm the islolastic suspension kicks in and if in good shape will smooth out
the vibration almost to feeling like an electric motor. But if your isolastics have never been replaced or adjusted they could very well be
rubbery mush and need replacing after 46 years!! Get a service manual (free digitally in this forum). Yes, other things can affect vibration,
but this would be #1. Find a knowledgeable Norton owner near you for some in-person advice. Most are very friendly and willing to lend
a hand.
 
If you get to feeling it through your feet check for hooking in the counter sprocket. Just saying..... I had to chase a mess of vibes last year and that was where I ended up after going through ios's, primary, front end, swing arm, etc. Everything needed to be done anyway and I feel fortunate to have located the issue. Right now if I go through the motor & trans I've got a new semi-natty Commando. I know I get longwinded........ Yes, do the Vernier ISO's if you replace. They're great.
 
Are you saying you are experiencing a vibration that is troubling you or that you just want to know what items can cause a vibration for future reference? If you are referring to felt engine vibration increasing over time, the Iso suspension/its condition and adjustment would be the primary component involved.

One thing re the ISO suspension...Obviously, the rubber parts will degrade over time and will also "take a set," especially if the bike has been sitting for years. The rubbers should all be replaced. But the rubbers currently available can vary in stiffness from one supplier to another. Therefore, even assuming the same iso clearance, the replacement rubbers my change the isolation characteristics of the original rubbers when the bike was new, thus changing the RPM at which vibration is "dialed out."
 
Mind you, if the petrol tank isn't properly located it can transmit engine vibes straight to your knees and make things seem worse than they actually are....
 
My 74 with original shimmed rear iso's and later vernier front iso's vibrates worst at tickover on the centre stand. On the road it vibrates until 2500 rpm and then like a light switch the vibration disappears all the way to 7000 rpm.

Iso clearances are 8 thou front and 6 thou rear.
 
... And the head steady -- was that even standard on a '74 850?

A Commando has always had a head steady. The one on your 850 is a fairly robust design, but there are better ones out there.

The aforementioned Dave Taylor design is likely the best in my opinion. There are a few options, but gas tank clearance is a consideration. The one sold by CNW looks the best from a tank clearance standpoint but is pricey. Being a cheapass, I have always made my own.

There is also the Norvil type, which I am not a big fan of. Heavy, cumbersome, and transmits vibration to the top of the frame. To be avoided for regular road work in my opinion. FWIW
 
Regarding the MK3 type head steady spring- I became a big believer in the benefits of this after removing the front iso recently.
Several years ago I played with the spring tension and found that too little tension resulted in increased vibration as did too much tension. Increased vibration might be the wrong term as the spring changes brought vibration into the previously smooth as glass zone from 3000 rpm to redline.
After some experimentation the right setting was found and the wide smooth zone returned.
It turns out that the smooth zone is only truly smooth when all the weight of the engine is held up by the spring.
The front iso bolt can be slid in and out easily as the engine is perfectly suspended by the spring.
This also prevents sagging ISOs.
I'm quite sure these ISOs are original but they are not at all sagged, the bike runs smooth so I left them in.
Your head steady, if stock, will be the strong boxed in type, but won't have a spring so you might consider adding the MK3 spring to it.
It's also quite likely that your ISOs are sagged as that seems to occur fairly quickly without a spring type head steady. So my thoughts are change to new MK3 vernier adjustable ISOs (if your ISOs appear bagged and sagged) plus add the MK3 heady steady spring arrangement. On the off chance that your existing ISOs look good, you could just shim them by the book procedure and try just adding the spring. That won't cost much and it might be interesting to see what occurs with the single change.
Set the spring up so that there is zero weight on the iso bolts.
My MK3, once above 3000, is as smooth as any modern I've ridden.

Glen
 
My old mk2a was shockingly smooth compared to other bikes I had at the time ie a65 lightning 860 gt Ducati a t140e cb750
Roll forward 35 years my present 750 commando is not as glass smooth as I remember my mk2a but I have a norvil type headsteady and an extra iso under the gearbox
It's still smoother than my t160 tho
 
I went to the late one piece iso mounts. Must say they raise the speed at which smoothness occurs. It begins at 2800 and
takes full effect at 3k. I turn 70mph @ 3800 and even the mirror is clear. Smoother than any other brit bike of the era for
certain. No idea what a modern bike is like but probably smoother.
 
Replace the ancient iso’s as a matter of maintenance.

I also questioned durometer of parts from various suppliers.
I
got
nowhere.

In the end, bought some vernier adjustable type from a supplier in MA.
They came wrapped in newspaper, no mfg I.D., and the “flat” surfaces where the teflon washers ride, were far from flat, and had a record groove finish. I spent hours reworking them to be usable.
Buy from a known good source.
 
My guess goes first with the isolastic suspension. I have heard there is a version with adjustment possible.
They are all "adjustable". Prior to the MKIII that was done using shims. The MKIII and the aftermarket "vernier" ISO's can be adjusted without using shims. There's another thread about that here now.

I would suggest at least checking the ISO clearance using the methods detailed in the service manual.
 
"I would suggest at least checking the ISO clearance using the methods detailed in the service manual."

Agree. FWIW, my front isos from the original shim-type. Never saw any reason to bother changing them - it's not like it needs to be adjusted very often and it's easy enough though, of course, it requires a selection of shims.
 
  • Like
Reactions: HRD
Status
Not open for further replies.
Back
Top