Luc_D

I am experiencing a bewildering issue with my commuter.

When pedalling, the bike sways left and right, left and right, 3/4 of an Inch approximately.

First I thought It was just an impression, but once identified it is actually very noticeable.

I can’t pinpoint exactly when the issue arised (maybe one month ago)), but I had been riding this frame since one year at last and it never bothered me before.

The front end (1 1/8 — 1 1/2 steerer) was very stable before,

Riding no hand is very difficult (I never had issues before on this particular bike)

I’ve tried to find a specific event triggering this behaviour but nothing seems really relevant. (I’ve swapped wheelset sometime ago though)


EDIT.

it is shimmy and I mostly get rid of it doing the following:

• cleaning and readjusting the headset
• Making sure the wheels are properly aligned and sitting in the dropouts.

For future reference, here it is what I’ve learned so far regarding shimmy :

What is shimmy ?


What can cause shimmy?

Wheels

• Loose spokes
• Loose quick releases
• Play in the bearings
• Tyre/s not seated
• Wheels not seated/ misaligned in dropouts

Weight distribution (rear bias)

• Loaded rear end (panniers, racks)
• Upright riding position
• Short stem
• Sweptback bars

Front end

• Damaged headset bearings
• Headset need overhaul (dirt)
• Headset is too tight
• Misaligned handlebars

Frameset

• Bent Forks
• Asymmetric fork blades
• Cracked fork pivot
• Cracked steerer
• Bent Frame
• Cracked seat/chain stays
• Cracked bottom bracket

rubenextra300

Maybe you have a problem with any front wheel spoke?

Luc_D

Nope, I have even changed the wheel, both are straight with reasonably tight spokes.

In the meanwhile I will check again the headset...

ridelikeaturtle

This sounds like what you'd experience on a motorcycle, called "head shake" or a "tank slapper". There are a lot of causes, from misalignment to poor suspension to bad road conditions... and the bike's geometry counts for a lot. I'd check the straightness of the frame and fork first. It would seem to me that the front wheel and rear wheel are not "on the same path", causing the bike to want to correct constantly, causing the "head shake" problem.

Luc_D

Thanks a lot, misalignment it is what I feared.

Road conditions are good, I run 40mm tyres and the forks are rigid

Frame and forks are aluminum and carbon, and very sturdy. I never crashed them either, so what could have caused them to go out of alignment?!

The bike tracked straight like and arrow and was remarkably stable at the point of being boring until not a long ago...

dsaul

That is a classic sign of a tight headset. When we pedal, the front wheel is constantly turning slightly to the left and right. If the headset is properly adjusted, the wheel corrects itself and you never notice it. If the headset is tight, the wheel stays turned and you have to physically pull it back to keep from turning in that direction. It is a very uncomfortable feeling.

The fact that this happened suddenly points to a rusted lower headset bearing, possibly from riding in the rain or washing the bike.

Luc_D

Will try to readjust it shortly

ridelikeaturtle

It's something I'd check, to eliminate as a possibility. You can do it with some string, then measure the distances from the center to see if either of the chain stays is slightly bent. Here's a youtube video showing how:
With luck, the frame will be OK, and it's the headset (at others have posted), or maybe a wheel bearing, lumpy tyre, wheel out of true, etc.

Koyote

Bent frame would be a longshot. If the swaying is roughly rhythmic with the pedal stroke, my money is on the headset. Remember that you want to tighten the bearing just enough to remove any slop...And not any more than that.

If that doesn't do it, and you can't find another source, pull out the headset (or have a shop do it) and check the bearings.

HillRider

Have you checked for a cracked frame tube, particularly in the rear triangle?

Trakhak

Disassemble the headset and examine the bearing surfaces. If you see tiny dents corresponding to the bearing positions, you should either replace the original caged bearings with free bearings (or vice versa) or replace the headset.

blamester

ljsense

It's speed wobble. One of the more fascinating problems in bicycle physics. It's a whole system at play, and even experts disagree on aspects of it. The same gyroscopic magic that allows a bike to stay upright, stable, lean through corners suddenly turns maniacal through a combination of factors, and changing your headset bearings isn't likely to put this demon back to bed.

Here's one article of many on this fascinating, frustrating and frightening phenomenon:

https://road.cc/content/feature/2198...-speed-wobbles

Trakhak

Probably not in this case. As the OP said: "I can’t pinpoint exactly when the issue [arose] (maybe one month ago), but . . . it never bothered me before."

Frames don't suddenly develop speed wobble. As noted by many experienced folk in this thread, headset damage is much more likely.

Reynolds 531 

Maybe his fitness is improving, and he is riding much faster than he was a month ago?

Andrew R Stewart 

What I sort of "reject" is the assumption of nothing changing in the total system, this includes the rider, from previous riding. I have had speedman's wobble off and on with a number of bikes and have spent a LOT of time, in the saddle and at the computer, trying to make sense out of it. I can say with full confidence that, like ljsense says, it's a system thing. Weather, weight distribution, air pressures, cadence, road surface, are some of the factors. Then there's the bike's steering geometry, the tubing, spoke tensions and general harmonics. The best understanding that I have (and supported by far smarter people then post here, sorry this includes me too) is that the wobbles are a harmonic vibration that enters a reinforcing cycle. Interestingly one common frequency is around that of a shivering body.

I have no idea if the OP's bike, him or the headset went through a sudden change recently to tip the balance and wobble became far more likely. I hope that simple money tossing solves the problem. I look forward to follow up by the OP if he discovers the solution that works for him. Andy

Luc_D

Thanks Andrew. Bewildering it is indeed. I already experienced speed wobble or shimmy and this is slightly different, it increases with speed and cadence, and nearly goes away when coasting.

Luc_D

Not really...this week I am suffering from lower back pain and taking it easy...

Luc_D

Possibily so. the headset is relatively new (less than one year old). it is an integrated 1 1/8 — 1 1/2 headset like this one.

It seems strange to me it broke down already, there is no noticebale roughness.

Also, I've tried to readjust it a couple of time, even letting it slightly loose for a couple of times for a short time but the issue persists.

Luc_D

Will take a look during the weekend...

WizardOfBoz

I'd go back to the suggestion above to check the frame carefully for any cracks or weakness. It's a pretty new frame, so this could still be infant mortality of the frame.
I'd probably remove the fork and check the steerer-tube-to-fork-crown and blade-to-fork-crown joints. See if they wiggle. Listen to hear any crackling or creaking.

Clearly could be a headset, but unless you ride near a body of saltwater or sweat perfusely, or drive the thing through a salt marsh daily, I'd be surprised if the bearings were corroded. But stranger things have happened.

After that, check QR and wheel bearings,

Luc_D

Thanks, I was planning to check frame and fork for cracks during the weekend and bearings too.

base2 

Another simple possibility: Too much weight on the rear, not enough weight on the front. Such an imbalance can cause the front to wander. The corrective action of trail is not able to be put to good use as it could be.

It can be un-nerving to have two totally different bikes depending if the rack/panniers are full or not. The same effect can be caused by rider position. It's the center of gravity being too far aft at fault.

Try moving your seat forward or using your hoods/drops or get a longer stem. Or a combination to move the CG to a more central location on the bike. A loaded front basket would work too if that's how you use your bike anyway and don't want to adjust fit.

With a light front end, you may also be feeling the effect of literally levering the front end around from the torque generated between the seat that is in line with the frame centerline and the contact point on the pedals that are off set from the frame centerline.

ridelikeaturtle

It's easy to overthink these things. Before tearing things apart, check all the "easy" things you can first. To me, checking frame alignment is pretty quick and easy, just to tick that off so you don't have to worry about it. Same with examining for cracks (I do this whenever I clean my bikes).

Another simple thing: are the wheels aligned in the dropouts properly? How easy is it to get the wheel cocked after fixing a flat or swapping a cassette! Or maybe a QR slipped a little. It's the easiest fix in the world, and only takes a few seconds to check - and it's a lot easier than taking the headset apart.

I'm really curious now to see how this turns out

grayEZrider

Also the fact you mentioned the change when coasting tends to indicate problem (alignment or bearing play) at the REAR of the bike. Pedal pressure of any kind will pull the wheel to misalignment with that.