Fork shimmy when braking
 

Took the freshly built Raleigh out for its first thrashing today. Rides great but I do get a lot of fork shimmy when braking. More noticeable at slower speeds with what looks like an inch of movement at the axle. I don't feel any movement at the headset. I'm worried this is going to lead to something breaking (pardon the pun).

The forks are lightweight 531 but they shouldn't do this, right?

One thing that that might affect this is I had to move the brake caliper forward to clear the headset cup. I'd guess the spacer is just under 1/2". The caliper is tight and seems solid but could this create the issue?

http://i989.photobucket.com/albums/a...ps7tqymjux.jpg

Maybe.

That fork crown is not the most beefy in cross-section between the blades and steerer.
You ARE running a dual pivot caliper.
I would double check the headset and front hub adjustment.
Possibly check the engagement of the allen nut and the brake mount bolt with the extension spacer, different lengths are available, longer is better until it bottoms out of course.
Then I would be tempted to trade out the front caliper for a single pivot unit that clears the headset w/o spacer.
Test ride that set up.

Checklist:
Rims true?
Hub cones adjusted properly?
No bumps on wheel rims themselves?
Both pads striking the rims?

My Professional does the same thing. It's minor but noticeable. I've checked all of the items mentioned by oddjob and have had my LBS check it; everything seems ok but it still shimmys.

I had this happen on a tubular. Not sure but think tire itself was wacky. I ended up lowering pressure and it went away. 5-10 less.

The key may be that <1/2" spacer you put in to clear the headset. Pretty much everything solid can be modeled as a mass-spring-damper system. Harmonic oscillation is what you're getting. Change the mass, the spring, or the damper enough and it will go away. In your case the spring is probably the thing to go after. You need to make your brake system stiffer. If you could get rid of that spacer, I'd bet the shudder would go away.

I'm looking at your picture, and wondering if you could (shudder) file the arm down just enought to clear the headset? You could change either the headset or the brakes to something that wouldn't require the spacer, but I really like the look of that component selection (and I'll be you do to!) Since good looking bikes ride faster, longer, and feel better (a well known, scientifically proven fact), I'd try that.

Here's an article on brake squealing, which is different than what's going on with your brakes, but is the same physical effect.

Completely agree with gugie on this. A file is your friend in this instance (to very carefully lessen the issue), or swap in some other brakes to clear the headset, or consider a different headset. Also re-evaluate if the headset is allowing some of the shudder, if so upgrade.

When I saw the title, I was sure this was going to be a case of cantilevers with a stem-mounted cable stop.
(I had a miserable experience with that).

But I would agree the spacer contributes, however there is no guarantee that is the entire cause.
What you could do is replace the brake pads with a less-grippy pads.
I did this once with a badly-shuddering cantilever brake while waiting for parts for the real solution to arrive.
(I swapped KoolStop salmons for less-grippy Shimano pads).

The St. Sheldon article, written by Jobst Brandt insinuates this fix.

Thank you all for the suggestions.

I'm inclined to first file down the caliper housing so I can fit it without the spacer. Just knowing it is there bothers me anyway :)

If problem persists, I will swap out pads next.

I say a toe in check would be in order. If the pads hit square on the rim or toed out even a little, they can pull forward, release, then pull forward again. This would cause the shudder. With beefier brakes such as newer dual pivots, squealing might be silenced, while vibration or shudder may not be. Of course the spacer could have made things a little worse, but I think the main culprit would be the angle at which the shoes contact the rim. Put a medium size zip tie flat between the brake shoe and rim at the rear, squeeze the brake hard, loosen the shoe, then retighten. It may help, or may not, but it's worth a try.,,,,BD

1-Headset plus slight variation in braking contact.
2-QR, which is doubtful, as it would manifest itself before braking.
3-Hub bearing, which is doubtful, but reacts much as a headset does to variations in braking contact.

If it were totally an alignment/rubbing issue, you'd hear it first. If it's a very slight truing/rubbing issue, you may not, but the headset still "gets it."
If the brakes were perfect, the headset can't transmit a vibration that doesn't exist. No brakes are perfect.

A loose headset will amplify any inconsistencies in braking contact between the rim and pads. Likewise a hub, but at a lower, er, "volume." Far more of us ride around on not-so-perfect hubs and don't know it, than on loose headsets.

My vote is headset. Even a slightly loose headset will transmit variations in load, and again, no rim is perfectly true and no brakes perfectly adjusted. We can live with truing/brake alignment issues, and most of us do, because our headsets aren't loose enough and our rims/brakes not crooked enough to transmit enough to notice.

There's not a better reason to have two headset wrenches in every box. Unless you have a Tange Aero headset, in which case you need one wrench, one strong thin straight-edge, and a supply of profanity.

Another possibility, though more remote, is not your caliper's spacer, but where it meets the front of the fork. Is a concave-like spacer more suited here? I have had your issue a couple of times on the Ironman unicrown fork, and a concave spacer there, flush against the curved front of the fork at the mounting hole, solved the issue. However, at the same time, I made sure the headset was tight and not able to loosen itself, the calipers and pads were aligned well, and I was sober.

If that's a needle bearing headset, which it appears to be by the wide angled cups, then that might cause a vibration also. I had one that would never get tight. It would "feel" tight, but would still thump and thud when I'd hit a bump. I replaced it immediately, and no more issues.,,,,BD

+1. Aluminum bearing cages have always been a sore spot with me. They almost always bend, often arrive bent in the box. Needle and ball bearing headsets, these days, have tinfoil cages. I keep a supply of older headsets on hand and swap out the bearing cages, especially with Ritchey and Tange lower-end models. Some of those old steel headsets use the same size bearings/cages.

I've often wondered if I could make millions producing sealed bearings that go into older headsets in place of the bearing/cages. I continue to contemplate same, but only for a few seconds.

Still, I'd generally spend $40 for a nice used older Stronglight A9 over 2/3 of today's offerings. After that, I can't outperform the headset and it ceases to be an issue.

This is a new to you bike, correct? So you have no knowledge of the history of that fork? Pull it out and inspect it, very carefully.

I had that same shudder appear suddenly with a Columbus SL fork. By 5 miles later the shudder was so bad I stopped using the front brake. Got home, checked the wheel on the truing stand. Perfect. Went back to the bike and pushed the fork blades apart with maybe a pound or two of force. Right blade bent out 8". Base had a full crack 3/4s around at the crown.

Your fork will not have the same issue mine had. Mine was caused by a skipped critical step when it was nickle plated. (High strength steel MUST be hear treated after nickle plating to drive off embedded hydrogen atoms with embrittle the metal. That step was skipped by the plater. Still, I would get to know it intimately, especially around the fork crown since it does have the chatter issue and has a minimalistic crown. This is an area where you NEVER want a failure. Trust me. I lived through one.

Your bike is giving you a clear warning sign. The cause may be as simple as the extended brake location, but it may also be far more serious. I would hate to see that big failure happen as you rule out the possible minor causes.

Ben

Are you 100% sure the headset is adjusted properly? I've had this happen to me with new builds more than once, and it was always the headset.

With that whispy a fork crown, I'd suggest using less grippy pads at front, more grippy at the rear, and just learn to use the rear for most of the heavy braking. It's not ideal, but that's what you probably need to do.

The front brake supplies about 80% of the stopping power. Granted, less grippy pads may improve the situation and apply acceptable braking power, but rear brakes are never going to supply the necessary effect to perform "most of the heavy braking".

That said, the headset adjustment is where I'd start looking.

Make sure the headset shoulders top and bottom where the cups/bearings seat are perfectly parallel?

No way. That's the last thing to do, if you do it at all.

It's like 90% likely to be the headset.

My first instinct would be headset play exacerbated by the front brake extension spacer, combined with rather light fork and crown construction.

I suggest losing the brake spacer first, just to be scientific about it. Some black markie will minimize the file marks. Current set up puts the brake at the end of a lever - bad idea.

Pick the front wheel up off the ground about ten to twelve inches, and drop it. If you hear a thud, then you know the headset is to blame. It's no worse force than hitting a pavement break with you on the bike. How tight was the crown race when installed? If it could be pressed on by hand, or dropped on, that could be another spot to cause vibration, even if the headset is tight.

You can also put your hands in the drops, your body weight on the saddle, and squeeze the front brake. Then push the bike forward and back. If you feel any movement other than fork flex, there is something going on in either the headset or the spacer setup.

Another point? I have run into this a lot lately, when swapping forks. Make absolutely sure the top nut isn't bottoming out on the steerer before it contacts the adjustable race. If this is going on, the adjustable race can back off after a mile or two, and cause this same issue.,,,,BD

I'll bring a little closure to this one. Filed down the brake caliper so I could eliminate the spacer. Looks like it got rid of the the shudder although those blades sure flex when the brakes grab hard. New brakes and new wheels so I'm guessing the extra grab is just flexing those thin-walled blades.

I'm calling this one fixed but will continue to monitor closely. Pics of the surgery:

Disassembled the 5800 brake caliper and filed down the part of the caliper interfering with the headset
http://i989.photobucket.com/albums/a...pspv89slmz.jpg

Quick touch up with a paint pen
http://i989.photobucket.com/albums/a...pshnpamtk5.jpg

Able to mount with no interference!
http://i989.photobucket.com/albums/a...pssfiecowu.jpg

I'd have ditched the headset myself, and got one that fit with the brake. headsets are a lot less expensive. at least the force is going the other way when you brake, hehe.:D,,, Glad it's "fixed".,,,,BD

Easier said than done BD. I was in weight weenie mode with this build so wanted to go with an aluminum headset. Add to that it needed to be threaded, black, a 1" steerer and the options were surprisingly limited. This one weighs 110g and fit the bill.

It is needle bearing in race by the way. Good eye