speed wobble on pasenger pegs

Here is a bit of a head-scratcher....

I was riding on the weekend and the bike is solid and tracks true at high and low speed. I have new tires (K100), vernier isolastisc, an 850 headsteady, and swingarm lockrings.
When I put my feet up on the passenger pegs at approx. 90mph I would get a very slight wobble/weave. This would go away as soon as I put my feet on the driver pegs. I've had the bike up to 110mph (speed adjusted for inaccurate gage :wink: ) and no wobbles. It only occurs with feet back?!
Has anyone else experienced this? I will check my iso's but found it odd that rider position would make a differnce. Aerodynamics? Weight distrubution?
Any thoughts?

Cheers!

Wobble

Yes I have, however I found the problem to be based on wind and how I sit on my bike, if I crouch down so the wind does not hit me so much it is not as evident. Wierd, but there are many factors I have been told that causes it on Nortons, Forks, Fork Oil, ISO's and the list goes on. Just think it has to do with the light weight of the Norton's, possibly the frames etc. Much of this is just guess and info passed on by Norton fans on this website. semper fi

Yes it's common. Sliding back on the seat at speed often makes the head wag. You can get a stabilizer or play with lowering your tire pressure.

The dynamic interaction between rider, bike and all sorts of bike-related characteristics is very difficult to define.

When I was at N-V, we got embroiled in a US court case concerning the P-11. Interestingly, nobody at Norton even knew waht a P-11 was. It had been cobbled together by the US importer, putting an Atlas 750 motor into a Matchless 500 Scambler frame.

When testing it, we found a very unsettling, long-wavelength "weave". Starting at about 65 mph, it would start to wander side-to-side - just a foot or so. By the time you got up to about 75 mph, it was weaving about the full width of a motorway lane, on about a half-mile wavelength. I wasn't brave enough to go any faster, but the other test rider reckoned that it smoothed out at about 88 mph and was fine at higher speeds.

We tested the hell out of that bike, but with the lack of decent instrumentation to measure things, we were relying mainly on "observations".

We never did figure out what was causing the instability, but it had resulted in a "tank-slapper" incident at high speed in the Mojave Desert, which was the reason for the lawsuit.

Bottom line - without recording of data and very good analysis of the results, these insidious things are never resolved.

That's pretty wild stuff Frank, you fellas must have pretty big things, about what year was that and how old were you then, if it's not a rude question.
Mike

Thanks for the feedback, very interesting. At least I won't be needlessly tweaking my set-up. I'll just keep my feet in front if I don't like the weave.

fastback said:

Thanks for the feedback, very interesting. At least I won't be needlessly tweaking my set-up. I'll just keep my feet in front if I don't like the weave.

Click to expand...

There you go. Sometimes the simplest solution is the best!

Debby

Mike:

That P-11 testing was about September 1967, when I was 26 or so. The majority of our testing was usually on the Motor Industries Research Association track near Nuneaton, but for the P-11, we were out on the motorways around Wolverhampton, as it seemed a fairly simple test at the time.

We changed front fork innards a couple of times, but without success. My suspicion was that there was some kind of torsional instability in the single down-tube frame, maybe caused by stuffung the 750 Atlas engine into a frame designed for a fairly mildly-tuned 500 single. Also, the effects of rake and trail angles on directional stability were not well understood back then.

I think the lawsuit went away, maybe because they lawyers found that the P-11 wasn't a "real" Norton, and we quit working on the problem.

Fastback,

Have you seen the FASTEST INDIAN yet. There was an example of your phenomina in the movie, maybe fictitious who know?

Something to do with shifting the centre of effort, the further the COE goes rearward the less stable, think of a weather cock!

Cheers Richard