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Cure the wobble...

Bowsh

Well Known and Active Forum User
VOC Member
#1
Just watched this brilliant clip by Murray Walker. Got to say “the rider has balls”. A great tip for those high speed wobbles!

 
Thank You: Bob

davidd

Well Known and Active Forum User
VOC Member
#6
There are lots of different ways to get into a wobble. It does not seem possible to eliminate all of them. The Vincent was subjected to some extra scrutiny because it got a reputation for wobbling. Although you can't eliminate all causes, there is no good reason not to eliminate the conditions that may induce a wobble, such as breaking the link between braking and the front end seizing.

Ultimately, most wobbles seem to have one thing in common: the front getting light. This can happen almost imperceptibly when the front end seizes and the tire starts to "patter" or it can be intentional as in the many wobbles that happen when a wheelie touches down a bit out of alignment with the rear tire. However, you can also just whack one grip of the handlebars and induce a wobble with no weight change at all.

The Murray video is very interesting, but when he said to lean forward to change the weight on the front end it sounded like it made sense if you want to thwart a wobble. I think doing so may change the center of gravity a bit even though much of the weight is still going through the seat. But, I thought Murray was wrong. The rider, when getting into a tuck is changing the center of pressure, not so much the center of gravity. I thought it had more to do with aerodynamics causing a change in pressure than weight shifting causing the change. It seemed to me that when you are upright you are presenting yourself as a giant sail and that sail is in front of the center of gravity. That is a recipe for disaster because a lot of instability is created when the CoP is in front of the CoG in an airplane, car or motorcycle. When the giant sail goes away on the motorcycle by tucking the instability goes away.

I don't know enough to know if this is correct or partially correct, but Bill might be right. You may have to pass on the sweets after all.

David
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#7
Some say the 4 cylinder engine, Across the frame, Was a problem, Like running into the wind holding a bit of cardboard.
But to say nothing will happen if you stay under 70 mph is wrong.
I got thrown over the top at well under 70, And at another time on another bike, Had a bad weave at 50 ish.
On my Comet I had a Huge tank slapper at well under 70, After hitting a pothole while pushing on through a bend, Got away with that one, Don't know how ?, Pure luck.
I am sure a good Hydraulic steering damper, Would have sorted all the problems shown.
Cheers Bill.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#11
In another thread, I said Percy Tate was asked by Yamaha, I think ?, To sort out there front end, He told them to put the brake calipers behind the fork legs, They were in front.
Which is why I thought it might be a bad idea to fit one disc on a Vincent, But I guess what is bad for one ,
Might not be bad for another ?. Cheers Bill.
 

timetraveller

Well Known and Active Forum User
VOC Member
#12
To continue davidd's thoughts on this but to consider sailing boats first. Yachts (sail boats) work by the wind blowing against the side of the boat and the sails being at an angle to that wind. If the sails were exactly fixed along the fore and aft axis of the boat and the wind was exactly from the side then the boat would only try to move sideways. These boats have keels (or fins, dagger boards, centre boards etc.) in order to try to prevent them going sideways. If the centre of pressure on the sails is in front of the centre of lateral resistance caused by the keel then the front of the boat will 'blow off', meaning that the front will move away from the direction of the wind. Similarly if the wind is aft of the centre of lateral resistance the stern will 'blow off' and the boat will tend to head into the wind. It is one of the skills in yacht design to get the centre of pressure on the sails above the centre of lateral resistance due to the keel etc.
On a motorcycle all this pressure is taking place along the line of the bike. Here I am ignoring any side winds. The wind force will occur on both the bike itself and the rider. I imagine that the centre of gravity of a bike is somewhere about the centre of the engine. That is above and behind the centre of the flywheels. The centre of gravity of the rider is probably about his or her navel. The combination of these two centres of gravity (450 lbs for the bike and 200 lbs for the rider) will probably be near to the bottom of the petrol tank. The forces due to wind pressure will probably be at the same height as the combined centre of gravity of rider and bike.
We have probably all had the experience of putting too much weight on the rear carrier frame and found that the steering has got very light with a tendency to wobble. There was a video on this forum some time ago of a young chap trying out bananaman's heavily loaded bike in the USA and scaring himself with the front wheel wobble. So I suspect that the effect described by david is not along the fore and aft line but is a matter of where the heights of the effects are. The human sail moves the centre of pressure upwards rather than fore or aft and my guess is that that has something to do with what is happening in the video. The higher the centre of pressure then the lighter the downward force on the front wheel.
Oh to have access to either a wind tunnel or a computer model of a bike and rider and a computational fluid dynamics (and the skills to use it) program.
 

oexing

Well Known and Active Forum User
VOC Member
#15
Well, hydraulic steering damper onto a Brampton is on my plans. I am thinking of throwing out the friction discs at the bottom and put in an alu plate, same diameter but with an extension and a bolt there to fix one end of the damper. You´d simply wind down the damper knob to lock the plate firmly enough as you did with the friction discs. There is even a slot down there to keep the desired position of the alu plate. Not yet decided if I mount the damper across the frame or along to the head bracket. The sidecar lug looks suitable as well for bracketry.

Vic
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#16
I had a very early and old/ second hand BMW K100, Not the best for steering, But it never did things like that.
It had a big Slab type screen on the bars, I would not go over 100 mph, But was safe up to there, If it was like the film, I would not have had it for 10 years. Cheers Bill.
 

bmetcalf

Well Known and Active Forum User
VOC Member
#17
In a BMW book, LJK Setright maintained that the early /5 BMWs wobbled in the US because of the high handlebars and that in Europe, the standard low bars moved the center of gravity far enough forward to ward off wobbles.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#20
I built a huge rear seat assy', For my Special, I thought it was a copy of Kawa' 24 hour racer, But I was kidding myself !, But over a 100 mph, You could feel it steady the bike, I made it out of fi' glass, But I made it too thick
= Very heavy. Cheers Bill.
 
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