Rear fender length versus vibration

[rick1956]

Hi all,

I know lots of people bob their rear fenders--some barely, and some radically.
Lots of people also talk about breaking license plate and/or license plate mounts from vibration.

My question: Has anyone come up with a "sweet spot" as far as rear fender material removal?

My 81 has the stock rear fender on it, but someone in the past put a small crease in the rear edge, and it has stress cracks already emanating from the edge into the taillight mounting holes. While I could weld it up (an may anyway just for the short term), I don't want to weld it for heat discoloration reasons. I'd rather just bob it slightly, removing the crack-affected zone, and keeping the factory edge trim shape and contour. I just worry I may amplify the vibration issue.

Common sense tells me if there is less fender hanging out in space, there is less to vibrate, but as we all know: Harmonics can be a strange and unexpected thing.

Thoughts?

 

[fredintoon]

Hi all,
I know lots of people bob their rear fenders--some barely, and some radically.
Lots of people also talk about breaking license plate and/or license plate mounts from vibration.
My question: Has anyone come up with a "sweet spot" as far as rear fender material removal?
My 81 has the stock rear fender on it, but someone in the past put a small crease in the rear edge, and it has stress cracks already emanating from the edge into the taillight mounting holes. While I could weld it up (an may anyway just for the short term), I don't want to weld it for heat discoloration reasons. I'd rather just bob it slightly, removing the crack-affected zone, and keeping the factory edge trim shape and contour. I just worry I may amplify the vibration issue.
Common sense tells me if there is less fender hanging out in space, there is less to vibrate, but as we all know: Harmonics can be a strange and unexpected thing.
Thoughts?

Hi rick,
if you crop it short you'll still have the new raw edge to deal with.
My inner cheapskate would weld up the cracks and paint the fender to match the bike.
Or you could check the list's classifieds to find an un-cracked replacement.

 

[59Tebo]

That's for a frame-mounted fender, right? Would it make any difference if the fender were swing arm mounted? The rubber bushings are really all that isolates the fender, so would moving it further from the source of the vibrations make any difference? And what if the fender was even longer? Like the hinged (almost 180*) fenders found on the early HDs? And is this too many questions?

 

[650Skull]

On the 78/79 Specials the Tail-light/Brake-light number plate combo is pretty heavy and being on the rear of the guard doesn't help......on the later specials the Tail-light/Brake-light was behind the seat so it was just the number-plate light and number=plate a lot less weight.........The guards were reinforced around the bolt area but still some cracked.

Some of the Standard guards, (longer than the Special guards), cracked as well but the rear light mounts were spread out over the guard..............may have has something to do with how well the bike was set up to minimize vibration vs cracked guards

 

[NashGTI]

Other way round, the extra mass of the fender hanging out there will dampen the vibration. Shortening the fender will cause it to vibrate more, that's the idea behind mass dampers they put on things like exhaust's and cables and stuff.

 

[fredintoon]

Other way round, the extra mass of the fender hanging out there will dampen the vibration. Shortening the fender will cause it to vibrate more, that's the idea behind mass dampers they put on things like exhaust's and cables and stuff.

Hi Nash,
as I remember my long-ago mechanics class, changing a thing's length changes it's natural frequency.
(The longer the organ pipe, the lower it's note etc.)
So (guessing at the numbers) if a Euro spec. XS650's rear fender shakes worse at 3K rpm, a North American
spec. XS650's will shake worse at 4K rpm and a sawn-off's at 5+K rpm depending on how short it's been trimmed.
Adding weight (like a rear light etc.) to the fender's end will add to the vibration's intensity but the frequency won't change.

 

[Grimly]

If you sit behind some bikes at traffic junctions, you will be surprised how far the tail light assembly vibrates up and down, sometimes at idle, and sometimes just as the engine rises above idle.
I've no doubt there are many owners out there who wonder why their tail light eats bulbs.

 

[X77S]

I plan on mounting my fender to the swing arm, but it wont be the stock one. I've cut the seat frame on mine and welded in a curved seat bar (cant remember whose kit it was).
That being said, I plan on coming up with a rubber mounted system for the swing arm and for the swing arm to rear fender strut. But I'm probably over thinking things.

 

[Moabite]

Not overthinking...positive planning...reinforce with some angle too.

 

[XSLeo]

I can't say a lot about most of what was asked but on the license plate breaking is because the plates re very thin aluminum. I reinforce mine with a backing plate. Aluminum road signs work well but a piece of steel works well too. Bolt the plate to the bracket with the extra plate between the plate and bracket then bolt the bottom of the plate the extra plate.
This extra stiffness prevents the plate from breaking.
Leo