Rear fender vibration ?

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Jackman

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Why do Commando rear fender and frame rails vibrate some much?

I’ve seen videos were not much vibration and more than a few others that the vibration looks so severe that you would probably think the frame or the fender is cracked or going to crack.

Noticed similar with the cylinder head some move a little and some are whoa that looks wrong
 
Jackman said:
Why do Commando rear fender and frame rails vibrate some much?

I’ve seen videos were not much vibration and more than a few others that the vibration looks so severe that you would probably think the frame or the fender is cracked or going to crack.

Noticed similar with the cylinder head some move a little and some are whoa that looks wrong
Click to expand...
I know I feel somewhat violent shaking in the handle bars at about 2500 RPM. Above or below that, the isolastics seem to be doing a great job. No ghosting in the mirrors and a smooth ride. Perhaps what you are seeing is this RPM range where things tend to shake about.
 
Resonance, at certain vibrations it's made worse, hence vibrating handlebars made less vibrating by adding lead weights which changes the resonance frequency. Its not helped by the rear fender being furthest away from the engine so the amplitude (amount of movement) is at its highest at the lower frequencies. Quickest cure is to ride above 2500 rpm.
 
kommando said:
Resonance, at certain vibrations it's made worse, hence vibrating handlebars made less vibrating by adding lead weights which changes the resonance frequency. Its not helped by the rear fender being furthest away from the engine so the amplitude (amount of movement) is at its highest at the lower frequencies. Quickest cure is to ride above 2500 rpm.
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The only time I encountered the vibration at 2500 RPM was when decelerating in town before making a turn. I soon learned to avoid that RPM range.
 
Jackman said:
Why do Commando rear fender and frame rails vibrate some much?
Click to expand...
A big British twin shakes the ground at idle. They're supposed to. :) Fortunately, we have Isolastic Super Ride that made the Commando better than the others.
 
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Dan1950 said:
The only time I encountered the vibration at 2500 RPM was when decelerating in town before making a turn. I soon learned to avoid that RPM range.
Click to expand...
Here's another data point -

Most of my riding is at about 40 mph, which with a 20T sprocket is right at 2500 RPM. After I installed a cNw head steady, 2500 is now a sweet spot. My mirrors changed from being a blur to being glass smooth. It's truly amazing. This isn't the design purpose of the steady, it's just a wonderful coincidence.
 
I should have mentioned at idle speed I seen the big vibration, when I get home I’ll find and post a video of a shaky Commando.
 
I'm new to these bikes but something seems to be amiss if the fender is shaking at idle.

Here is a clip I took when I went to inspect the '74 MK II OI ended up buying. Notice that at idle there is no shaking but as the engine goes through the 2500 RPM range there is some shake.




Perhaps the isolastics are adjusted poorly in the shaker you saw.
 
Dan1950 said:
I'm new to these bikes but something seems to be amiss if the fender is shaking at idle.

Here is a clip I took when I went to inspect the '74 MK II OI ended up buying. Notice that at idle there is no shaking but as the engine goes through the 2500 RPM range there is some shake.




Perhaps the isolastics are adjusted poorly in the shaker you saw.
Click to expand...

I have welded up my centre stand as it and the cradle were worn, I now have the rear wheel one inch off the ground when its on the centre stand, my rear mudguard and taillight shake around a lot now. Before with the worn centre stand and both wheels touching the floor when on the stand it did not visibly shake but did vibrate.
 
kommando said:
I have welded up my centre stand as it and the cradle were worn, I now have the rear wheel one inch off the ground when its on the centre stand, my rear mudguard and taillight shake around a lot now. Before with the worn centre stand and both wheels touching the floor when on the stand it did not visibly shake but did vibrate.
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My rear wheel is several inches off the ground when on the center stand.

Rear fender vibration ?
 
Jackman said:
I should have mentioned at idle speed I seen the big vibration, when I get home I’ll find and post a video of a shaky Commando.
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It's torque reaction to the inertia of those big flywheels accelerating and decelerating. ( Normal for a Commando )

The previous owner of mine fitted these stainless tubes which stops it to a large extent. With LED bulbs the bouncing is a non issue.

Rear fender vibration ?
 
On my bike I found a lot of the vibration reduced by making sure the rear fender was properly attached to the frame loop.

I used a slightly thicker than stock piece of rubber pad that wrapped fully around the rear loop. Fender and tail piece vibrate a lot less now.
 
Dan1950 said:
I'm new to these bikes but something seems to be amiss if the fender is shaking at idle.

Here is a clip I took when I went to inspect the '74 MK II OI ended up buying. Notice that at idle there is no shaking but as the engine goes through the 2500 RPM range there is some shake.




Perhaps the isolastics are adjusted poorly in the shaker you saw.
Click to expand...

It could be how well the mudguard is mounted. That was the case on a friend's MkIII.
 
cliffa said:
With LED bulbs the bouncing is a non issue.
Click to expand...
They're not immune. I put an LED equivalent of an 1157 bulb on my 850 (from SuperBrightLEDs). My brother was following me and after a ride he told me my tail light was all cattywampus. The tail light section would flicker at idle. OK while under way but at idle it blinked. Replaced the bulb and all is well.
 
The fender design while rigid in the vertical axis, does not resist twisting as well. The side to side also shows itself as up & down with everything that's attached to it.
 
robs ss said:
It could be how well the mudguard is mounted. That was the case on a friend's MkIII.
Click to expand...
Did you dig deeper when you brought it home? So many shaky rear fenders makes me think its just a normal Norton thing but it such a big shake that surely something if not broken already will be broken soon.
 
Jackman said:
Nice short video with a serious rear fender vibration
Click to expand...

Jackman said:
So many shaky rear fenders makes me think its just a normal Norton
Click to expand...

Looks normal enough to me and it's the approximately 18" of unsupported seat tube that's shaking, the fender/mudguard just happens to be attached to it.
One thing about both previous videos is the bikes are on their centre stands attached to the engine cradle so in that situation the frame is likely to be reacting as the shaking of the cradle/engine is restricted by the centre stand being in contact with the ground.

This reaction is often witnessed by owners attempting to strobe the ignition timing as the bike shuffles backwards on the centre stand when the engine is revved as can be seen in the opening few seconds of the video, below.

 
I think it’s fair to say there is a difference between shake and vibration.

Things shake around a lot on twins at idle, especially the extremities at either end. But so long as you don’t spend ages sat idling, it’s unlikely to be an issue.

The rubber mounted engine accentuates that shaky idle, it’s the same with HDs.

IMHO, it’s worse at a slow idle, so excessive shaky idle could be a sign it’s set too low. A slow idle is bad for the engine too (less oil being splashed around) so is best avoided.

Vibration whilst running is more damaging, but of course, a rubber mounted Commando shouldn’t suffer badly here.
 
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