Wobble at higher speeds?

[Bonnie850]

I've been finding that my Bonneville starts wobbling slightly at higher speeds on occasional stretches of dual-carriageway and motorways, usually at about 80-85mph. I'm in a more forward-leaning position than on the stock bike with its high handlbars (as I've fitted original M-bars and peg-adjustors, with a small Givi screen), but the extra weight over the front wheel doesn't seem to be making much difference.

The wobble also seems to start earlier when I hit turbulence from vehicles ahead, especially lorries, but isn't so bad when I'm accelerating at higher speeds (rather than slowing down).

What could be causing this, and is there any kind of cure - short of buying a bigger, heavier, more powerful motorway-muncher?

[wiltshire builders]

Hey, they make the new Bonnies just like the originals then!

It could be you. Are you tensing up? Any slight wobble through the bars will go through you and back into the bike and so on. Relax your grip and drop your elbows so that your arms are acting as dampers.

[alanTDM]

Hey, they make the new Bonnies just like the originals then!

Seems so. But some oldies had a damper rod that you screwed in,which went down through the headstock to cure the wobble.
Might be headstock bearings being slightly slack.

[smellygerm]

Could be a number of things maybe the tyres? :-?

[Bonnie850]

Thanks for the feedback so far. I've just ordered a set of steering dampers from Norman Hyde (so its obviously a known problem then!?). As the bike is still almost new, its unlikely to be worn bearings or the suchlike, but he did mention a couple of other things to check: wheels not perfectly inline or properly balanced, tyre sidewalls not evenly fitted?

[NiteW4tcher]

this may sound stupid...............but have u checked your front tyre for damadge? rips, buldges, etc etc??

[pilninggas]

Do you have any luggage/ racks fitted?

sometimes these will just unload the front-end enough or create unexpected aerodynamic forces to allow a wobble or oscillation.

[Bonnie850]

Do you have any luggage/ racks fitted?

sometimes these will just unload the front-end enough or create unexpected aerodynamic forces to allow a wobble or oscillation.

Yes, I do. Of course the problem is worse with loaded panniers attached, and presumably should be worse still with a pillion-passenger - but then surely then some of the extra overall weight will transfer to the front end, loading it sufficiently?

[Mark_Able]

Three causes of wobbles that I've experienced.

1. A screen that is mounted to the handlebars. Any buffeting has an effect on the bars.
2. A top box/panniers, that took weight off the front, and seemed more prone to cross winds.
3. Pulling on the bars at higher speed (without realising) is probably the biggest culprit. Grip the tank with your knees, use your back to lean yourself forward. Try to be as relaxed on the bars as possible.

Of course, it could be none of the above, and just be down to the Trumpet... [smiley=happy.gif]

[wiggy]

Id drop the screen off and try again ;)

[Jon_W]

Stupid one, are the tyre pressures ok???

[Mutt]

High speed wobble

Speed wobbles can occur whenever something starts a vibration that matches a resonant frequency of the wheels. A resonant frequency is one at which your motorcycle will vibrate very easily; a particular motorcycle may have multiple resonant frequencies. The starting point may be a bump in the road, a rough patch in the road, or some combination of these factors. Other potential contributing factors include the small torques resulting from wheel rotation and the tiny lateral oscillations that spinning wheels make if they're not aligned with absolute perfection.
Your motorcycle will go through various "zones" of oscillatory stability and instability as it accelerates up to its highest speed. you might even compare this to musical notes or octaves of relative vibrational resonance.

Steering dampers are made to help with this problem as the main damper ( you ) cannot always handle the correction of the vibrations.
As this can be a fatal but also unavoidable occurrence, the best advice is to slow down. Even though the problem may go away with more speed, the best way to take the bike out of that particular resonance range is to back off.

The tread of your tyres could also be a contributing factor, particularly the front one. different treads have different resonant properties.

Answer

In addition to above, get both wheels statically and dynamically balanced. It reduces wobble, increases road holding and reduces tyre wear and rider fatigue. Check forks and swing arm for correct operation. A lack of fork oil and too much side play in the swing arm are sources of vibration. Check panier box lids are fitting properly and are locked. Lid 'chatter' in the wind can be one of those resonances mentioned above that sets wobble off.
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If your motorcycle is pulling a sidecar or has a handlebar-mounted fairing, this will increase the likelihood of a steering wobble under certain conditions. The wind wash from driving near a semi truck can also cause a steering wobble.
Although many things can contribute to starting a wobble, they generally won't happen if your steering head bearings are properly maintained. Test your steering head bearings by accelerating to 50 mph or so on a straight, smooth road on a calm day with no oncoming traffic. Close the throttle and remove your hands from the handlebars. If your bike's steering seems to pick a direction and stick there, your steering head bearings are too tight. If there is any wobble at all, your steering head bearings are too loose.

Wow I`m knackered now ;D

[pilninggas]

Do you have any luggage/ racks fitted?

sometimes these will just unload the front-end enough or create unexpected aerodynamic forces to allow a wobble or oscillation.

Yes, I do. Of course the problem is worse with loaded panniers attached, and presumably should be worse still with a pillion-passenger - but then surely then some of the extra overall weight will transfer to the front end, loading it sufficiently?
Maybe not, particularly if the centre of gravity of any lugage is above or to the rear of the rear axle and wheel centre-line. Luggage can also create yaw that tends to transfer weight off of the front wheel. My old krauser panniers are mahooosive and create a lot of drag, i can discern the yawing effect at speed [i get a slight wobble].

[Loops]

What sort of wobble is it - a shake or an unsettled 'twitching'?

And if it's only dual carriage ways and motorways....do they have a lot of lorry traffic? And is it usually when you're in the inside lane?

You might be wafting about in the tyre grooves...my car is terrible for that, it wanders all over the place, and I noticed the CBR was a bit twitchy along the M4 at the weekend...

[Bonnie850]

Update: yesterday I ensured my cold tyre pressures were spot on, and then went for ride without panniers. Result was zero wobble on dual carriageway at a range of higher speeds, despite the usual disturbed airflow from trucks and other vehicles. Pleased to be able to rule out the small screen as the cause, or any other problems with wheel alignment or tyre bulging.

I'll have to do another test with empty panniers on, to double-check that the original wobble was due to the heavy weight on the back of the bike, rather than just the panniers oscillating due to airflow.