I have had an annoying occasional intermittent knocking sound when riding my C15. I have been working my way through possible culprits - and today have found clear indications that the rear of the seat is contacting the welded lugs for the bolts that support the rear mudguard. (I hadn't thought that the noise was coming from behind me, but its the first time I have found something where there are signs of contact.)
Rupert told me a year or so back that there should be no contact between the seatbase and the frame, other than the front and main seat bracket.
There is no wear in the seat bracket - it was NOS unused. And the seat base is in fine fettle (not bent). So I am wondering how to lift the seat to stop this contact. At the moment packing the main bracket off the seat base seems the only solution. The other solution would be to accept the contact and glue in some rubber pads to silence the noise.
Has anyone else had the same issue?
Can you hammer some dimples into the seat base?
I had a C15 and two B40s and would've thought that the knocking would have to be reasonably loud to hear it above the engine.
In the unlikely event it was the seat you would probably be able to replicate it by rocking the seat to see if it see- sawed on the rear mounting or bouncing the bike on the rear shock absorbers.
Is the centre stand knocking on the silencer?
I've had a look at my original C15 seat to frame clearance and there's barely a whisker between the 2.
My initial thought would be to momentarily lift the seat on its frame by slackening the top shock absorber fixings and elevate the transverse U bracket and seat a shade.
See if that stops the noises
looking at my seat base , there is about 12" behind the U bracket, so yours could have been deformed maybe?
I've had another closer look. I have two seat bases both with pristine brackets. In both cases the main bracket is effectively redundant. The bottom of the slot in the main seat bracket does not actually touch the shock absorber bolt. The seat is effectively suspended between the front bracket and and the two lugs on the frame loop. If I lean on the seat in the middle I can make it hit the bolts.
I've had a quick look at sleeving the bolts, perhaps with rubber, but the front to back clearance in the seat bracket slot is quite tight. So I think it is a non-starter. So I think I am back to the original plan of packing under the bracket bolts.
I did really like the suggestion of knocking a dimple in the seat base, and would have given it a try if the 'in-use' seat was still a bare base. But as I have paid RK Leighton a decent wedge to do a beautiful job on the seat, I daren't risk it now.
Can you make another U bracket with slightly longer vertical legs.
They're bolted to the seat base
My initial thought would be to momentarily lift the seat on its frame by slackening the top shock absorber fixings and elevate the transverse U bracket and seat a shade.
See if that stops the noises
Good thinking Rhyatt. In truth I hate relying on bolts for clamping - especially on British bikes with all the vibrations etc. So I have 'assumed' that the seat bracket was supposed to slide over the shock absorber bolt until firmly sat on the bottom of the slot. But maybe the designers intention was that seat was clamped in position at some intermediate position in the slot?
As you said, I can try that. See if it gets rid of the occassional 'doink!. And in the worst event of the clamping eventually working loose, all that will happen is the seat will descend a few fractions of an inch and the 'doink' will reappear.
Quote from: rhyatt on 07 Sep, 2026, 18:10 Can you make another U bracket with slightly longer vertical legs.
They're bolted to the seat base
That's another good thought. But I can do this another way - I could get my welder pal to fill the bottom of the slot by an eighth of an inch or whatever. But I will try the intermediate clamping first.