Rear Mudguard Wobble

Just had my bike over at RedLine Norton to have Dave Sundquist assess why my tail light brackets keep breaking in half. He will be welding up one of the 3 brackets I brought him with fractures.

We compared mudgard shake against one of his several exquisite Commandos. His were near solid vs mine which wiggled about 1/2" side to side at the lower lip. Everything is bolted up snug with correct fittings. He didn't have an immediate answer as we didn't take anything apart. I will explore more on my own, but wanted to hear from the collective as to what to check. I'm guessing maybe the rubber pad at the rear hoop bracket is too soft?

Tornado said:

Just had my bike over at RedLine Norton to have Dave Sundquist assess why my tail light brackets keep breaking in half. He will be welding up one of the 3 brackets I brought him with fractures.

We compared mudgard shake against one of his several exquisite Commandos. His were near solid vs mine which wiggled about 1/2" side to side at the lower lip. Everything is bolted up snug with correct fittings. He didn't have an immediate answer as we didn't take anything apart. I will explore more on my own, but wanted to hear from the collective as to what to check. I'm guessing maybe the rubber pad at the rear hoop bracket is too soft?

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Is the mudguard original, a reputable replacement, or what? I've seen really thin ones that don't work well - often the rear end is out of line with the rest and the taillight is not parallel to the ground and you can't get it to be and stay.

Unknown origin. It is stainless while front was chrome. '74 850 mk2.
Will do closer inspection soon.

I would guess you got a thin rear m/g … not sure if there any way to stiffen … best of luck !

Well got it apart now. Lots of wobble movement right where the hoop sits above mudguard with the saddle strap bolts tightened fully. The guard side to side curvature falls away from the hoop immediately each side of the strap......I can pass a sheet of shop towel between hoop and guard right up to either side of the strap....there is nothing to stop the wobble each side of the guard's apex.
When I remove the strap bolts, the guard falls away from hoop a few inches and takes quite a bit of force to make it come back up against the hoop and hold it there when trying to fit strap & bolts back in. Loosening the forward three mounting points and shifting the guard position around does not provide a better fit at rear. It's like the whole guard needs to shift rearward an inch or so. Cannot see any spacers on parts diagram that would need to be at forward mountings to achieve this.

Thoughts? Seems like I need to get creative to make this all work without the wobble.

Are there differences in the 750, 850 MK1,2 or 3 rear mudguards? Perhaps I have the wrong version fitted?

Tornado said:

Thoughts? Seems like I need to get creative to make this all work without the wobble.

Are there differences in the 750, 850 MK1,2 or 3 rear mudguards? Perhaps I have the wrong version fitted?

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My 72 750 Roadster had stainless rear and chrome front fenders .
When I redid the entire bike I replaced the clumsy tail light fixture which was fiberglass with the cast aluminum unit from Old Britts and had the holes fore the original tail light bracket welded up .
It may help you to increase the mass of the end of the fender by fashioning a weight of some sort to fit the underside . It is likely the vibration is causing the cracks you speak of and adding mass to dampen the vibration should help .

I tried adding a stack of much wider, thicker washers underside of guard to the hoop strap saddle bolts. That seemed to reduce the wobble movement beneath the hoop a good bit. Then tried squeezing in a length of hard-ish rubber strip that already had a concave lengthwise moulding to curve around the frame hoop area a little. Cinching the strap bolts reasonably tight and huge improvement in guard movement, basically all wobble gone.

Will give this a go and watch for the usual shaking it's been having for previous while.

My bike has stainless tubes on each side running from one of the grab rail clamps to the number plate support. It was like that when I bought it. I was going to remove them, but I guess they were put there for a reason so I left them. Maybe it had a similar issue?

cliffa said:

My bike has stainless tubes on each side running from one of the grab rail clamps to the number plate support. It was like that when I bought it. I was going to remove them, but I guess they were put there for a reason so I left them. Maybe it had a similar issue?

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That bracing would certainly help reduce shaking.

OK, three rides since this modification to guard mounting bolts and happy to report no big shaking at the tail end of guard nor the tail light/license place bracket. I'm calling this a good'un!
Thanks to Redline Norton Dave Sundquivst for doing the weld repair on one of my 3 broken lamp brackets and for giving me the clue about excessive guard shake on my bike (his bikes were solid in comparison). He also scolded me for having my license plate mounted with its top holes into the top holes on the bracket, which, due to the larger plate dimensions here in Canada, meant the plate lower edge extends below the lower lip of thw guard, hiding it from view. I now have it the proper way according to his aesthetics, with the plate lower holes bolted to the bracket's lower holes, making the top plate edge extend above top of plate bracket, lower guard tip now peeking out from under the plate as the Norton Gods intended


The basic issue was the apex of the guard curvature in contact with a frame hoop tube makes for a very narrow side to side contact patch and thus prone to poor resistance to side to side flex/wobble. The lamp bracket was basically trying to stiffen the lower rear of the guard, which gave rise to stress fractures over a few months of riding. By placing 3/4" diameter, thick washers underside of guard at the frame mounting strap points, the load is better distributed around the holes and the whole guard is now held much more rigidly. Cannot induce wobble with light pushes as before. Blipping throttle shows no wobble at tail light fixture, just a tiny bit of turn signal light shake briefly when going up/down rpm.