Luc_D

I thought about it, but This is not the case. I've installed a longer stem (130 vs 110mm) and there is no difference vs loaded and unloaded riding. Putting more weight on the front end did not solve the issue...more on the next post.

Luc_D

Hello there,

In the weekend I've dismantled the headset (it was a quick affair being it of the tapered and internal variety) found some crud on the lower bearing, cleaned and reassembled. Took me 20 minutes tops. This specific issue won't arise anytime soon, two months ago I've installed full mudguards.

I've also checked for cracks, there is none.

Now, there are good and bad news.

The bike now WANTS to go straight, but there is still a slight instabiility: it is like the front wheel was not straight, but when spinned on the forks she definetely is well within tolerances.

The front wheel seems to have some play though (going left and straight slightly while I am pedalling), not more than a couple of millimeters per side.

The front wheel also seem somehow "soft" when I pull it sideways, moving by maybe a couple of mm side-to-side (the rear wheel seems to be firmer)

The (slight) play does not seem to be due to loose bearings (I've repacked them two months ago) but to spokes tension...or maybe it is irrelevant.

The only thing I did not check is the rear wheel, but it seemed dead straight in the chain stays. Will check it later along with quick release tension

The problem is clearly manyfold, and the headset is just one part of the puzzle.

Luc_D

I definetely need to check that.

Now that you mention it, I've got a flat rear tyre a month or two ago, so this definetely needs to be checked.
The rear is a disc wheel on vertical dropouts, not so easy to drop in...especially when hastily done on a Parisian sidewalk in chilly winter.

L134 

Luc_D

I am using Vittoria randonneur hyper 700x35 tyres, (actual size is 40mm)

They are foldable 120 TPI tyres.

But maybe I did not remount the rear tyre properly...Even if this seems somehow unlikely.

Trakhak

While you had the headset apart, I hope you inspected the fork crown race and the upper and (especially) lower headset races for indentations. Seems to happen less on modern bikes, but it's still a possibility.

L134 

Luc_D

All suggestions are welcome.

The only thing I have not checked thoroughly yet is the rear wheel, so maybe the tyre didn't seat completely inside the rim, or it is deformed, or the QR is loose, or it is not well centred in the dropouts...

Will check and report back.

madpogue 

2mm might as well be a light-year in this case. You'll never come close to a stable ride with this much play. To determine whether it's the axle/hub bearings or the spokes, put the bike on a stand and induce the movement with your hand. You should be able to tell quickly enough, when you move the rim, whether it moves relative to the hub, or whether the rim and hub move together relative to the axle.

Is the front wheel QR or nutted? Are the fork blades and dropouts aligned? Is the wheel centered (rim walls to relative to locknuts)?

Luc_D

the bearing seats were flawless : this is a recessed headset, so the "races" will correspond to the bearings. They were fine

Luc_D

As I stand I think the spokes are more to blame.

The fork is a carbon fork (it seemed aligned to me)

The front is a disc wheel, with an Allen key QR.

I have another set of wheels, I will give them another go since the headset is now sorted.

I will check and report back on Wednesday.

TiHabanero

Inspect the frame while you are at it. Specifically the BB area.

Luc_D

Bottom bracket it's fine, I've checked it for cracks and there's none.

It's shimmy, speed wobble, whatever you want to call it.

After cleaning and readjusting the headset, the handling changed, going from very, very annoying to.... just annoying.

I've tried to ride no hands and voilà : Front end start to wobble, putting a leg on the top tube stops it.

I already experienced shimmy so I can recognize it for what it is.

IMHO Wheels and/or tyres are to blame, that's the only thing that has changed recently (since when shimmy started)

Apparently unevenly tensioneds spokes, fat or worn tyres can cause shimmy

Rding position/weight distribution* did not really change (and moving more weight forward did not seem to stop the issue).

Will swap wheelsest (one that never gave me issues) in the WE and report back.

speedevil 

Some motorcycles are very sensitive to tire changes, and some front-rear tire combinations have caused a shimmy in the handlebars as soon as they are mounted. Rotational balance doesn't appear to be a factor, but in some extreme cases it progresses to a full-blown tankslapper which is almost impossible to overcome. In some cases, changing the tire pressure has helped to mitigate the behavior.

For reference, I'm talking about a Goldwing here, which is a 900 pound motorcycle with a fairly relaxed headtube angle. it is generally quite stable with predictable handling, but some front-rear tire combinations seem to resonate and cause oscillation.

If I were a betting man, I suspect that swapping for a different wheelset will make a dramatic difference. For curiosity's sake, I would try swapping front, rear, and then both to see if you can isolate the issue to front or rear.

Luc_D

Thanks for chiming in. I will do exactly that.

Beforehand I will pump up the tyres to see if a higher pressue reduces the issue.

blamester

Warped brake disc might do it.

Luc_D

How I currently install my disc calipers :

— First i loosely screw them on their support
— Then I install the wheel without closing the QR
— Secondly I depress the brake lever
— Then I screw the calipers
— and Finally I close the QR

I can Imagine that a bent rotor could place the front wheel not exactly centered in the dropouts, but mine seems dead straight...

Wilfred Laurier

Not on the topic of solving the shimmy, but there is a flaw in your brake setup strategy. You should have your quick release clamped shut when adjusting your brakes. The wheel should be firmly in place with the axle firmly seated against the dropout. Having the wheel detached from the bike when you adjust the brake caliper will have no positive effects.

Luc_D

It could well be the cause of the issue.. Thanks, will check and report back.

speedevil 

I don't recall you mentioning that the shimmy only happened during braking. Whether the disc caliper is properly centered or not should not impact riding while not braking, unless it is so far off that you're getting rub on the disk without applying the brakes. I'm fairly sure you would have noticed that in the first 10 feet of riding or the first couple of revolutions in the workstand, so I doubt that this is the issue.

Having said that, the other responses are correct. The QR should be properly closed before you tighten up the caliper bolts.

Luc_D

I've reinstalled the front wheel and the axle was sligthly off the dropout...Maybe due to improper installation.

Andrew R Stewart 

So is the wheel now centered between the blades, or is the axle fully seated in the depth of the drop outs, or both? We see a LOT of forks that have one blade slightly longer then the other so a wheel install that relies on gravity to settle the position means that the rim is off center within the tops of the fork blades. Carbon forks are not immune to this is the slightest. Andy

Luc_D

This.

Anyways, after swapping the front wheel for a different one, the problem persits, albeit to a different degree.

Will try to so swap the rear wheel during the weekend

Koyote

Huh. So, how do you deal with that problem, Andy?

Andrew R Stewart 

You align the fork best possible. Part of the process is to check splay (hub/axle not being on center WRT the steering axis) and blade lengths being the same (wheel sits fully up in slots and is centered between blades AFTER splay is corrected).

Carbon forks don't lend themselves to the bending (Oh, I mean cold setting) to correct splay but any fork I have seen will allow filing of a drop out slot deeper (or taller to shorten that blade effective length).

It is a false assumption that a molded fork (carbon) is going to be straight and well aligned. We hope it will be. We hope that the cost to create the mold convinces the maker to also take care about the mold's alignment and that the actual production is done in a competent way so that every fork pulled from that mold is also straight. But my experience with production bikes having carbon forks tells me otherwise.

Just because a fork is not straight does not meant that the bike will have shimmy. just because the fork is straight does not mean the bike won't have shimmy. Andy