Speed wobble above 40mph - ideas?

[Scorched yellow]

I have a Suzuki 250 run around that doesn't often go above 40, but when it does - it's very exciting! On a steady throttle it starts to feel uncomfortable but is generally fine - if you lift off the throttle though, you can get the mother of all tank slappers on demand. As a rough estimate, the bars go from +25 Deg to -25 Deg every half a second. Garage has looked at it and is out of ideas - wheel and headstock bearings all good, and the wheel is true. Any ideas where to look next?

This is a long term issue so it's not unique to one set of variables; I've checked tyre pressures (when higher it seems to mitigate the effects a little), and I've taken off the top box and it still does it. The worst circumstances are obviously with a passenger and a full topbox. In which case it will weave uncomfortably above around 40 even at a steady throttle.

My only thought is twisted stanchions. I have tried loosening the axle and pinch pin to no avail, although I note the mudguard is bolted to both stanchions and I can't loosen the chinesium bolts without rounding them. Maybe that is causing some tension in the forks? I might try a bolt extractor kit.

Any ideas very welcome.

[Jazoli]

How old are the tyres on it? Has it had longer shocks fitted to it which have increased the steering head angle and made it unstable?

[Scorched yellow]

Good shout on the tyres, I will check the date stamp but they could well be 5 years old plus given the mileage, I have noticed some minor sidewall cracking. No changes to the forks as far as I am aware but that's a good point too. Thanks

[gareth h]

I’d be going through the whole bike, wheel bearings, wheel alignment, swing arm bushes, steering head bearings etc

[Krikkit]

Jazoli said:

How old are the tyres on it? Has it had longer shocks fitted to it which have increased the steering head angle and made it unstable?

Think this sounds like a good shout, could it also be on the rear end? Has it had shorter/softer springs fitted? Definitely sounds like a geometry issue.

[Tango13]

Krikkit said:

Jazoli said:

How old are the tyres on it? Has it had longer shocks fitted to it which have increased the steering head angle and made it unstable?

Think this sounds like a good shout, could it also be on the rear end? Has it had shorter/softer springs fitted? Definitely sounds like a geometry issue.

Are the tyres the correct size too? Too wide or too tall a profile can make for some 'interesting' handling.

[TooLateForAName]

Does putting your weight forward over the front wheel help?

Couple of old vids on youtube about wobble which point at suspension set up as the big issue.

Search wobble weave

[Scorched yellow]

Thanks all - really good suggestions. Fingers crossed it's an easy win with the tyres.

[black-k1]

Sometimes tyre wesr can cause this, especially if the front tyre has a slightly more blocked tread pattern. I used to have this issue yaers ago on part worn Metzler ME33 tyres a about 40mph. Change the front tyre and see if the problem remains.

[NMNeil]

gareth h said:

I’d be going through the whole bike, wheel bearings, wheel alignment, swing arm bushes, steering head bearings etc

Agreement on the steering head bearings.

[Scorched yellow]

TooLateForAName said:

Does putting your weight forward over the front wheel help?

Couple of old vids on youtube about wobble which point at suspension set up as the big issue.

Search wobble weave

Only barely.

Suspension set up on this bike is rear preload or no rear preload I have it on full rear preload as otherwise it is a kangaroo, but I might see if reducing it makes any difference.

Everything seems to point towards tyres at the mo, so I will try changing the front first.

[Scorched yellow]

NMNeil said:

gareth h said:

I’d be going through the whole bike, wheel bearings, wheel alignment, swing arm bushes, steering head bearings etc

Agreement on the steering head bearings.

As above, garage I trust have already checked these but I will double check myself. Everything on the bike is very budget so bearings were my first thought.

[wsn03]

Make sure your wheels are balance...amazing the effect an out of balance wheel has on a bike

[Scorched yellow]

wsn03 said:

Make sure your wheels are balance...amazing the effect an out of balance wheel has on a bike

I actually removed the weights as I was convinced they'd been put on in the wrong place. No change

[wsn03]

Scorched yellow said:

I actually removed the weights as I was convinced they'd been put on in the wrong place. No change

Id 100% get them balanced. I promise you bike tyres are cery very sensitive to balance, and such a cheap issue to at least cross off the list

[Weso]

It hasn't got a top box full of stuff has it?
Mate had a blackbird that did similar when his top box was full of stuff.
Wes

[black-k1]

Weso said:

It hasn't got a top box full of stuff has it?
Mate had a blackbird that did similar when his top box was full of stuff.
Wes

But we've had the discussions on here many times and it's always claimed that top boxes don't affect handling!

[Chipchap]

Bikes generally dont just wobble. In my opinion fwiw I suspect that it is structural. Please check the following :-

Rear wheel alignment in the swing arm.

Tyre pressures.

Overall wheel alignment.

Check that the suspension moves freely with little or no stiction

Then check around the headstock for cracked paint or evidence of frontal impact.

Same around the swingarm mounts.

Loosen off the engine mounts and see if there is a big gap between any parts of the engine case and the frame, as tightening these to close a gap pre stresses the frame. If there are gaps shim them before retightening.

A

[wsn03]

Scorched yellow said:

I
I've checked tyre pressures (when higher it seems to mitigate the effects a little),.

Is your pressure gauge up to scratch, have you got the right pressure?
I've said wheel balance, I'd also double check pressure. I took a friends bike home to fix, it was like riding a clowns bike. Tyre pressures were wrong. However the fact that you have no balance weight on the front wheel is a big red flag. I'd get both checked first then try it

[Freakuk]

Tyre pressures, wheels out of balance would be my first checks. If it's off the throttle the weight is moving forwards so could it be forks, no leaking seals?