jamesdak 

So, I wanted to see how the newly built up Circuit did in the mountains so I took it up on of the local climbs. First main descent it tracked just fine at speeds up to 45 mph.

Then on the fast section I saw 55 mph pop up on the Garmin. I was closing in on some cars but really wanted to beat my PR high speed on this road of 55.4 mph so I stayed tucked as long as I could. Wound up having to brake right as I went into a curve. Used the rear only to bleed speed as I didn't want to lose grip on the front at those speeds. But it all went wrong and the worse shimmy I have ever had occurred. My Zurich would start a wobble every now and then but a press of the knee on the top tube would sort it out. Today I tried that and did dampen it a bit. Then I sat up and had to use the brakes again to get off the cars and it got so violent I seriously thought I was going to be thrown. Unfortunately by being stupid and getting is so close to the cars I had to stay on the brakes instead of letting off, relaxing my grip and getting the weight forward. Seemed like forever until I got stopped.

I checked the bike and couldn't find anything wrong. I think it was just my stupidity and the circumstances that caused this but will make another run like that soon to make sure there isn't an issue with the bike. Gonna break out a ruler and string and try to check the frame alignment too since this was the first descent on this newly built up bike.

That was not fun at all today, LOL!

rccardr 

Perhaps 55mph is just too speedy?

Steve Whitlatch 

That is crazy speed! LOL

Wright Bros

Maybe try preloading the headset on your Circuit one flat tight past zero free play.

cb400bill

Many possible causes for head shake. When it happened to me my front QR was just a bit too loose. I've also heard it can happen if the front wheel bearings are too loose.

lostarchitect 

Yeesh. Well, glad you're ok!

jamesdak 

Not normally, lol!

artclone

Glad you're okay. Scary.

Steve Whitlatch 

Oh boy, now I am going to have to check all of my bearings for piece of mind.

Bad Lag

I was also going to suggest you check all the bearings - headset and wheels, especially.


There is also the possibility of resonance - was the road wavy, imparting a time varying impulse into the bike/rider?


Are the wheels balanced?

clubman 

Did your shorts wash clean?

Seriously I prefer 55kph. Mph? Guess I'm getting old

Glad you pulled out of it.

Lazyass

Yeah if you tighten your headset a hair that could solve it. Or get a MTB steering damper haha

Scooper

Jan Heine says using headsets with needle bearings instead of ball bearings cures a lot of front end shimmy issues. I've never experienced shimmy riding my 1987 Paramount with a Stronglight A9 needle bearing headset.

John E

Is your rear wheel perfectly dished, with the rim properly centered?

repechage

I had a bike that provided an experience like you described and I lost faith in it, could not wait to save the funds to replace it.
Interesting that replacement bike 40 years later started the same issue.
Retired the front wheel (38 years old) and have never been able to replicate it since.

It can be quite disconcerting.

Kilroy1988 

Having just a piece of your mind won't help you get your bearings straight.

gearbasher

This wasn't you...was it?

crank_addict

Above as mentioned, hub and headset bearings but for high speed junkies, number one would be proper and even spoke tension. Most will never see those speeds so its rarely critical or shop checked. On the same note, I'd be cautious of the newer wheels with low spoke count in relation to rider weight.

My most fun noodle shimmy crazy descent was on a Fuji s12s touring loaded (and no, not meaning drinking ) Cadillac mtn. at Arcadia - Maine. Usually would keep speeds moderate for any touring rig but that was a moment where it nearly bit me. Plus I never really liked the lack of precise end play in those sealed hub bearings back when.

Rollfast

I'd have to get a Typhoon on top of Mt. Rainier to do that one.

jamesdak 

I'm really thinking it was me more than any equipment issue. But I'll test that out in a day or two to see. While I routinely run this route at those kinds of speeds it was windy today and knocking me around a bit. I very well may have been tensed up without realizing it and when I hit the back brake at speed caused the issue to begin. Then because I was so close to the cars I couldn't just get off the brake and kill the oscillation.

Or maybe it really is an equipment issue but I would be a bit surprised. The wheels have made this same descent at similar speeds several times this summer on the Giordana without issue.

WolfRyder

Glad you are ok dude. I grew up around some sick hills, twists and turns. In Bay Area Berkeley hills and SF Marin a little bit. Used to skateboad and bike um pretty good. You know your going at a pretty good clip when you start passing cars. It is a nice bit of a reward for work of climbing a hill to go down it, but yeah for sure make sure the ride is in order and exercise caution when appropiate. You don't need to hit it hard downhill Criteruim style all the time.

gugie 

There are lots of theories about high speed shimmy. No one seems to be able to predict it, everyone has a theory after the fact. I've personally seen "uncontrollable" shimmy on other's bikes twice. Both times it was downhill, and they locked up the rear brake as the only solution.

Personally, I think it's just gremlins.

TimmyT 

While it's possible that this could work, Heine was talking about lower speed wobbles primarily resulting from fore-aft load relative to trail. Speed wobble is a different problem, and while it could be solved in a similar manner, I am skeptical. The bearings are already pre-loaded at an angle to the direction of travel regardless of whether or not the bearings are cylindrical or spherical.

Sure, it might work, but it would be the first thing I'd try: I'd check preload on the headset and hubs, and inspect the hub bearings.

I would also check to be sure the rear wheel is trued and dished properly with even tension. The rear or main triangles could be out of alignment, too.

jimmuller 

I had a shimmy experience the first time I took my Gran Sport down a steep, slightly bumpy but straight hill, hitting maybe 45. I check the bike over, found nothing amiss. So I took the front wheel by Belmont Wheelworks and asked for an evaluation, in case my judgment was off. The guy said it was fine but that he used to see a lot of customers complaining about shimmy on their Raleighs. Okay...I concluded it was me. I've since ridden similar hills and I make it a point to hold the bar with a light touch. That seems to have helped, as I haven't experienced anything like that again. So it was me.

Wileyone 

Could it have been a Tyre problem?