strokerlmt
Moderator
Correct me..I'm no expert.....but don't most wobbles originate from the forks ???
LMT
LMT
Downhill speed wobble
- Thread starter Bonniebird
- Start date
Speed3Chris
I like Dick
A wobble is a harmonic based upon the resonant frequency of the bike...the whole bike...not just the forks. Wind can induce it...but only one element. Higher angular velocity of the wheels at higher speed and wheels and tires that are out of round or out of balance in combination of the wind can cause it.
I have had my bonnie over 100 mph on ocassion with no windscreen and never had a single wobble. The bike has to be right to not wobble at high speed and that means everything I mentioned earlier and also the comment about rear wheel alignment. People tend to think of things in binary terms for convenience. Most issues aren't that easy. Generally a wobble is created due to an interaction of things out of spec. If the forks are too tight or loose for example with a slightly out of balance wheels with borderline tire pressure...can cause it. A motorcycle is one giant spring with resonant frequency...from tires to spokes to fork deflection to the frame flex etc. If the inputs like wind and road surface and rider weight etc match this frequency, it will oscillate. All things have to be looked at if this is as problem...even tire wear affect and specific tire tread pattern relative to road surface can cause it. Its a problem in fact with many motorcycles and just changing a single variable like tires from one brand to the next can either solve it or cause it. Even a death grip on the handlebars can cause it some instances because it changes the overall stiffness of the bike.
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Bullitt
Two Stroke
Great explanation George! :headbang2:
Speed3Chris
I like Dick
Thanks Bullitt.
OT: I received your chainguard. Thanks very much...arrived in nice shape.
Let the surgery begin.
PieMan
Two Stroke
With Bonnies you can have everything right and it'll still wobble, it seems some do and some don't. As has been said, rear wheel alignment is very important. You can't trust the alignment marks, so I made a tool to get the wheel spindle exactly the same distance from the swinging arm spindle on both sides. My alignment marks were a couple of mm out.
I'll have to think about that comment. I have always aligned my rear wheel to my front wheel, never considered the swinging arm spindle other than making sure it wasn't worn and well lubricated. Alignment marks often are incorrect, but when front and rear tyres are sighted correctly (and that's a job for an experienced hand) I expect my bike to track properly even if other welded frame parts are a little out.
BTW next time I align my wheels I'm going to experiment with a recently purchased laser level.
Speed3Chris
I like Dick
Oxymoron. Case in point mine and others don't wobble...ever. You mentioned yours does at times. I do agree its a lottery aka tolerance stack up...but submit to you something is out of spec. If you haven't tried every permutation then you haven't exhausted all root causes.
In my experience root causes for wobble in descending order are:
- Tire tread pattern and condition and even mfr. (BIG)
- Tire balance
- Wheel trueness...radial and lateral runnout
- Fork tightness or looseness...too tight and bike won't track and seek natural castor and too loose will increase displacement and resonance.
Bad fork bearings can also be a culprit...even with correct line to line nut adjustment.
- Wheel bearing, incorrect axial play
Spoke tension
- Rear wheel alignment
- Tire pressure
There are probably a couple of other things I overlooked.
PS: My bike doesn't wobble even with aligning to factory rear marks. Aligning to the swing arm pivots which are clearly frame datums doesn't necessarily yield proper front to rear wheel alignment as Otherwise suggested because of other contributing frame and even swing arm tolerances.
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Speed3Chris
I like Dick
Lasering the chain will only burn microscopic holes in it and weaken it.
Good Luck Mike.
While any of a thousand things can affect harmonics on a bike, I was taking about wheel alignment. You can use a good eye, straight edge or string lines, I will be giving a laser a try just for the sake of it. IMHO chain alignment on a standard bike will look after itself.
For those new to aligning front and rear wheels this simplistic drawing may help.
String line one and two have to be pulled tight and only just kiss points A,E and B,F. When the distance from string line to points C and D are equal and the distance from points G and H are also equal at the same time your tyres are aligned properly (assuming your axles are true to each other horizontally).
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agro
Scooter
If you have checked the setup of the bike and it is all correct and it still wobbles over 85mph, then you'll have to adapt your riding style and position.
I have had this problem for the last three years and after changing to cartridge forks, 17" cast wheels, upmarket rear shocks, steering damper and changing all bearings in the wheels and steering, it is still there above 85mph. (a steering damper won't stop this, it just limits it a little)
The cause of my problem is Hyde Western bars and 28mm risers, but for 80% of my riding they are a great setup and very comfortable. The more upright your riding position and the more you go over 85mph, the more violent the wobble. It seams the wind blast acting on your body lifts the front end and causes the wobble.
The only cure to the problem I have found, is to take the pressure of the wind blast on your torso, in your stomach muscles and put no rearward force on the bars, just hold them firmly. With this riding style I can go up to around 115mph and if I lay on the tank over 120mph with no wobble.
Give it a go next time you're heading to the ton and see if it cures your problem.
Mike, what 17 inch cast wheels have you fitted? I've been pestering Carrozzeria for a set for my Thruxton , but they will only do the the 18 front 17 rear combo.....
re. headshaking with saddlebags.... definitely more pronounced since I fitted the Triumph saddle bag brackets and generic soft bags and removed my low Triumph backrest...will refit the backrest to see if it makes a difference...but have a worn front tire....also to try to slow down oscilations at speed, have fitted an '06 Scrambler swingarm to my '04 Thrux...Scramblers and efi Thruxtons have longer swingarms...longer by 1 chain link....shock mounts still in original position....have Progressive Suspensions progressive springs for a Speedmaster and Ricor Intiminators fitted ...time to fit new longer shocks.....
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agro
Scooter
Your bike looks a treat with those gold Carrozzerias, have been pestering Carrozzeria about a set for my Thruxton, but they won't sell me a set, with a 17 inch front.....
agro
Scooter
Hi Mike, got some pics of your conversion......