FS650 high speed "waving" issue

Samuel42 · Jul 30, 2023

Hi guys!

I have a 2004 650 FS, and struggle a bit with high speed handling.

From 110kph the front becomes lights and tends to "swerve" left to right, making it a bit scary. It's not a proper wobble, it starts progressively.
I tried sitting closer to the front etc, the only thing to calm it down that helps is to pull hard on the handlebars.
It even does it while cornering above 100kph.

The bike has been completely rebuilt, no free play anywhere, steering bearing is not overtightened, tyres are new, wheels are balanced and straight, the fork has been revised. All suspension settings are as per the manual.
I tried to lower the fork, increase/decrease rear preload without improvement..

Any idea where it could come from? Next step is softer fork springs. I am quite light, 70kg with gear.

What is a good sag starting point for super moto?

Thank you!

HSBteukka · Jul 31, 2023

I had the same issue and i'm 90% sure it is the steering head bearing. Replaced mine and the problem is gone. I'd suggest you tighten it a little bit more, but not that much that the handlebars feels heavier

Samuel42 · Jul 31, 2023

Thanks for the input! The bearing is not new but in perfect shape, I will order a new and try to tighten more in the lean time.

The feeling I had is that the fork is fully extended on straight line. That's also what I can see from outside pictures. I tried removing the few preload spacers but it was only a few millimeters and did not impact.

From the manual I see that the 04 FS650 is running a 4.4N/mm spring with 33mm of preload, while the 05 FS650 uses a 4.8N/mm with 0 preload.

I think that would be good to test that setup. Or at least a 4.6 with 0 preload.

HSBteukka · Aug 3, 2023

I had the exact same issues going 100-170km/h but after new steering bearing i could now go 1 hand 150kmh and keep it steady. even changed new front tire but it must be the bearing. i can throw the bike +130kmh any direction i want and it wont weave

Taffy · Aug 4, 2023

check that the new steering bearings have a thick washer underneath the bottom bearing.

secondly, if it disappears when you lean back it means that the bike likes the extra trail. so either soften the rear spring or push the forks through the top clamp by a further 5mm. the bike wants to lean backwards.

triple clamps should have a code stamped on them, a part number? what is it? ................022 or ...........018 or nothing even (all are possible).

how much do you weigh. 44 springs are VERY WEAK unless you weigh 60kg.

Taffy

HSBteukka · Aug 4, 2023

I don't have thick washer under my bearing and everything is working fine.

Samuel42 · Aug 7, 2023

Taffy said:
check that the new steering bearings have a thick washer underneath the bottom bearing.

secondly, if it disappears when you lean back it means that the bike likes the extra trail. so either soften the rear spring or push the forks through the top clamp by a further 5mm. the bike wants to lean backwards.

triple clamps should have a code stamped on them, a part number? what is it? ................022 or ...........018 or nothing even (all are possible).

how much do you weigh. 44 springs are VERY WEAK unless you weigh 60kg.

Taffy

Actually it does not disappear when I lean but rather when I pull on the handlebars.
I don't have access to the bike at the moment, the only thing I could check last week was the fork springs preload, about 5mm, so it can't be the original 4.4 springs, but I did not have time to measure their stiffness.

Taffy · Aug 8, 2023

the term 'lean back' is the same as 'pull on the handlebars'. sometimes the spring rate is on the end of the springs, on the flat face enscribed gently. otherwise, you can count the length of the springs which will be 495mm or 505mm and measure the thickness of the wire (4.9mm or 5.1mm) and how many coils there are, be accurate. I'll see if i can help but at the moment you are aggresive passive and I'll be phuqed if you're gonna get wrong what I say again and try and help you.

Taffy

Samuel42 · Aug 8, 2023

Sorry for the misunderstanding, when I say pull on the handlebars is that I pull horizontally while leaning forward if that makes any sense.
Measuring the wire diameter would have been nice yes, will do it next week.

Looking at the Husaberg data, they went from:
650FS 2004 4.4@33mm + 84-250@5mm
To
650FS 2005 4.8@0mm + 88-250@11mm

The rear I have is the 84. The front I don't think it's the original, its length is 482mm

Taffy · Aug 8, 2023

482mm can be a very weak, very old 495mm that was probably really 493mm when new but to lose 11mm seems doubtful. they always measured short by 1-2mm.

But i don't think it is, someone has changed the springs over.

if you lean forwards and hold the bars (as opposed to playing an instrument or wiping your eyes) your weight is on the front and off the rear. nothing to do with 'pulling'.

Taffy

Samuel42 · Aug 13, 2023

Correct!
I measured the springs I have, lenght is 485mm when measures properly, wire I 5.1mm diameter so I think they're the original 4.4N/mm.
I replace the steering bearings and will mount a pair of 4.8 with 0 preload and see how it goes.
Any advice on the steering tightening torque, I think it's because of the sealing rubber think I find it difficult to get a feeling of the tightening.
Thanks

Samuel42 · Oct 6, 2024

I solved this issue by replacing the triple clamp with the 2005+ version with reduced offset and slightly more caster.

Taffy · Oct 6, 2024

Taffy said:
secondly, if it disappears when you lean back it means that the bike likes the extra trail.

Taffy

Samuel42 said:
I solved this issue by replacing the triple clamp with the 2005+ version with reduced offset and slightly more caster.

your problem solved was; "reduced offset" which means MORE trail. The castor doesn't (realistically) change. So, like I said.....

Taffy

Samuel42 · Oct 6, 2024

Yes correct, just wanted to confirm the solution, I went from 22 to 16mm offset. Yes you're right, for some reason I was thinking my new clamps had an additional angle but no, so the castor remains the same within 0.5°.

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