Bike oscillating from side to side at speed

samfailed

Recently noticed a strange and scary phenomenon on my mountain bike. As I build up speed I feel like the bike is being pulled and pushed underneath me and this gets more extreme the faster I get and then fades as I slow. It's like the bike is oscillating from side to side, almost like I'm doing a slalom against my will. It's an extremely unnerving feeling and I can't work out what it might be.

I've felt it previously so had the front wheel trued as it had quite a big wobble on it. It's all sorted now but the problem remains, I really don't know where to look next.

Just to make sure I wasn't going mad and imagining it I've had couple of other people ride the bike and they expressed their amazement at how bad it was.

What could be causing this?

reaperactual

With something that wrong check everything for your own safety.

Start at the front and work your way back with a good close look at the frame, check and inspect every nut, bolt, component, inspect and check for play in headset, forks, wheels, hubs, etc.

Will be surprised if it isn't something very obvious and/or visible.

As problem increases with speed and the front wheel is fixed suggests it could be a rear wheel, hub, axle, bearings, issue.

photonic69

Had you managed to ride this bike at this speed without the wobble before? Some bikes just have speed wobbles despite everything checking out ok. My mate had such a bike. We swapped wheels and he still had the problem. Headset was nice and tight/smooth. I think sometimes it's just the geometry of the frame?

me-109

Geometry can play a part, but it wasn't doing it before so unless the front end has sustained an impact to change it (bent steerer or stanchions), then something else has changed. I'd look at the headset first - too loose, too tight or worn. After that, front tyre wear. Then, depending on the frame type, rear suspension main pivot bearings for play/wear.

Edit: Having said that, rear suspension preload/sag could also change your geometry, so also worth checking.

steve_sordy

As it is speed related and gives sideways forces. It sounds very much like out of balance wheels. As the wheels move faster, the out of balance force increases with the square of the speed (ie go twice as quick, the forces are four times worse; 3x faster gives you 9x the out of balance force).

A quick test for wheel imbalance would be to remove the wheel and hold it by the axle or the hubs at least and get someone to spin the wheel for you as fast as they can. You will be able to feel the imbalance quite easily. If it is dead smooth, no matter how fast it spins, the wheel/tyre combo is not the problem.

Is the wheel itself bent? Slowly rotate the wheel and measure the distance between the rim and the fork or the frame. It doesn't have to be a dial gauge or a tape measure. You could use a zip tie that just touches the rim in one place and then see how far the gap changes as the wheel rotates. If its the wheel, then it may be fixable by adjusting the spoke tension. That is something I would leave to the LBS, although those that can do it say it's easy.

In relatively narrow wheels like on bikes, the out of balance forces are mostly static. In other words with the bike off the ground and you give the wheel a spin does it always stop in the same place? If it doesn't then it will probably be quite well statically balanced. (Bent rims are usually well balanced statically). If it does stop in the same place or close enough, then it is out of static balance (rule out brake rub). This is most likely to be the tyre itself, aided and abetted by the tube which may be poorly made with varying thickness around its circumference. If you have tubeless tyres, the sealant may not have been properly distributed, so give the wheel a good spin before the balance test. If you have not used the bike for a while the sealant may have gone off in a big lump. Best way is to remove one side of the tyre from the bead and check.

If your tyres are wider than usual, you may have the situation where the wheel is in static balance but only out of balance when the wheel is rotating, this is dynamic out of balance. In this case, look to see if the tyre is evenly mounted on the rim. Tyres have a line or even a pair of lines moulded into the sidewalls that appear just above the rim. If you look carefully, the lines should be the same distance from the rim all the way around the wheel. It is very easy to mount a tyre such that the line(s) dip under the rim. That tyre is not mounted properly and will be out of balance dynamically. Even if your tyres are narrow it is still worth ensuring that the tyres are properly mounted.

Remove the wheel from the bike. Reduce air pressure substantially until there is just enough to support the tyre. Then grab the tyre and physically pull the tyre until the lines are in the correct place. I find it best to hold the wheel away from me with both hands side by side holding the tyre only and wave the wheel up and down in front of me, letting the wheel weight drag the tyre into place. If the tyre doesn't move, let some more air out. Once the moulded lines are parallel to the rim all the way around on both sides, the tyre will be properly mounted. Re-inflate and test the ride. If you still have out of balance forces, then the tyre itself is out of balance. In that case I suspect that it is a budget tyre (poor quality control at the moulding stage), or maybe one that is too wide for the rim.

When you found out what the problem is and how you fixed it, I'm sure we would all like to know. So please post back.

samfailed

Wheels seem absolutely fine as far a I can tell.

Could it have something to do with the front forks? The lockout has stopped working, not sure if that means anything?

steve_sordy

Are you certain that the front wheel is securely in the dropouts?
Tyre pressures OK?

samfailed

steve_sordy said:

Are you certain that the front wheel is securely in the dropouts?
Tyre pressures OK?

Yes, definitely ok on both fronts!

mully79

You say the lockout no longer works. Does the damping work on the fork ?
Uncontrolled damping can cause all sorts of crazy things

samfailed

mully79 said:

You say the lockout no longer works. Does the damping work on the fork ?
Uncontrolled damping can cause all sorts of crazy things

No damping settings I believe... (I don't know anything about how the forks works except that mine are a coil spring?)

JBA

What bike is it and how old is it?

photonic69

You can get speed wobbles on road bikes that have no suspension, wheels that run true and with smooth bearings. As I said above, some bikes just do this. At what speed on your MTB do you experience this? Is this a new thing that has started to happen? What has changed on the bike?

karljeff72

https://sheldonbrown.com/brandt/shimmy.html

samfailed

I decided to check out the headset as everything else appeared to be working fine and normal. It was super tight and after loosening it off the bike handles FAR better. It's still not 100% and feels strange at speed but nothing like the extreme wobbling I was getting before.

reaperactual

Worth thinking about a new headset or a good service of the current one then correct preload could sort out wobbles fully.

me-109

Exhibit A, my post of 5th Nov.

samfailed

UPDATE: Had a big crash and knocked my bars out of places and the problem is back!!! Any ideas?

karljeff72

samfailed said:

UPDATE: Had a big crash and knocked my bars out of places and the problem is back!!! Any ideas?

New bike

JBA

Get the headset bearings replaced and preload them correctly.

samfailed

UPDATE: Steerer is bent and forks twisted! New headset and forks needed.

loltoride

karljeff72 said:

samfailed said:

UPDATE: Had a big crash and knocked my bars out of places and the problem is back!!! Any ideas?

New bike

2nd that new bike