vibration

[john.nash]

Tonight sees me half way out to the pub (every weds night - the bike club meets) when the vibration on the ariel 350 plunger get's scary excessive.
I mean the front end is pattering, the handlebars jumping around and the seat's going up and down and the peg's are dancing
I am only doing 50mph for heaven's sake and the engine sounds great. there are no other untoward noises.

Pull in the garage - the frame isn't broken the wheels are ok, the head bearings are tight, the obvious front engine bolts are tight and the anstey links seem fine.
Run the engine up on the rear stand, in top gear, and cannot see or hear anything untoward

Crawl home at 30mph and it's mostly good. Up with the front end and the front wheel and forks are fine too. Too dark and late for more.

Non-plussed..
I guess it's a quick tighten of all the engine bolts and making sure the primary chain isn't too tight ..... what else could it be ?.

[PeterW]

Front wheel seriously out of balance?

[john.nash]

It just started ... the bikes been out loads..... I never actually balanced the wheel when I put the tyre on.

[dave.owen]

At less than 80mph I don't think the wheel would need balancing, Are te tyre pressures right
Have you been out in the rain? the brake could be binding on and off

Dave

[nevhunter]

I'd still check the balance of the wheel. Get it down a hill, and go to the speed and a bit above where it occurs and pull the clutch in and idle the motor and see it the shaking changes. Nev

[brenton.roy]

If you have you been riding the bike at this speed for a while (weeks / months), without the shimmy, what's changed, or been altered recently?
I take it no wheelstands, cliff jumps or front brake wheelies?
Any wheel / suspension problems will show up if you jack the frame up under the motor and try "firmly" to move the wheels around.
You should be able to feel brake issues (Dave's thought) then too.
Also, are the wheel bearings adjusted properly?
Are you sure the frame isn't cracked?
broken spokes, loose forks?

[dave.owen]

John,are your wheel nuts secure ,I had a hairline crack in the rearwheel nut on my plunger years ago and that upset the handling.It appeared tight but wasn;t

Dave

[Silas.S]

Front Upper engine bolt maybe broken inside its crankcase tunnel and this is impossible to see externally. The weight of the engine holds the broken bolt ends quite firmly until they suddenly and dramatically fly out as you come in through your front gate. Put a spanner on each end and see if you can rotate the nearside end nut without any movement from the off-side nut. All good? Try the others in turn. Still all goood? Not that then.... Used to be a "Known" failure back a few decades, especially on tuned engines. Oddly enough the apparent imbalance goes away once the bolt is removed. Silas.

[Silas.S]

Was just thumping along the A47 to annoy Norwich when another possibility [well 2x, but one isn't so good] occured to me. No chance that the inner tube you used was injected with 'anti pucture sealant' is there? This is known for its ability to stay fluid for ages and then suddenly take it into its head to coalece into fairly solid gunge without any apparent reason, leaving a great lump of unbalanced matter concentrated at one point of the wheel. The other? Hmmmn. Loathe to even think about a fractured crank-pin. The apparent ease with which a single cylinder JAP, Ariel [or whatever with similar crank-pin design] will continue to run after even a complete shearing of the pin has always astounded me. Sorry for that gloomy idea. Hope it is wrong. Silas.

[john.nash]

thanks for all the suggestions
I have checked everything (including the engine bolts) and the only one thing is that the front rim now appears to have a 2/3mm run out. All the spokes are tight.
I bought myself a spoke key to try and take it out ....

To be fair, I been running it up and down the country lanes and it's not doing it.

HOWEVER, my mate also told me last night that the particular stretch of road I had problems on, he has problems with.
I went along on it, last night on the way back from a different pub (weds night innit), on the ariel outfit (different bike) and it was a little bit rough but nowhere near as bad.

My plan is to try and remove the "run out" and then take it back up to the same stretch of road.