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Old 10-04-09, 10:02 PM   #1
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Front end shimmy - what other causes?

I recently bought an old Trek 760 racing bike, that has a front wheel shimmy problem. It is significant enough to cause the front forks to oscillate just a bit, and enough so that I don’t dare take my hands off the bars.

The front skewer is tight, the hubs and headset seem to be adjusted OK - no excess slack). The front wheel is straight and true.

The wheels are both 700c X 23, but grossly mismatched, with a V-section clincher rim in the front, and a tubular at the rear. The rear wheel is out of true – but I can’t seem to get it trued. Nevertheless I do not detect any wobble from the REAR when riding.

I am wondering if this will all go away when I replace the wheel set – But meanwhile, is there something else I can check, that I haven’t already checked, that could be causing the front end shimmy?
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Old 10-04-09, 10:35 PM   #2
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http://en.wikipedia.org/wiki/Speed_wobble


basically, you and your bike oscillates and amplifies with the road vibration.

you can't really prevent it, as all inline 2 wheel vehicles will exhibit this behaviour at a certain speed.

how to overcome it: clasp your knees to the top tube, relax your grip, or lift your butt off the saddle.
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Old 10-04-09, 10:54 PM   #3
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Frame geometry is the key here (and a front load), certain bikes will just shimmy no matter what you do. It actually does seem to be some magical combination of frame tubes, length and butting. Some bikes will even shimmy in some sizes, but not others. Then again, almost any bike can be made to shimmy if you put enough weight high enough on the handlebars.

Just live with it, and if you can't, try a slightly wider tire, the slight change in trail might be enough to mollify the effects of shimmy.
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Old 10-04-09, 11:21 PM   #4
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At what speed are you referring to- since you mention removing your hands from the bars.
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Old 10-05-09, 05:54 AM   #5
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At what speed are you referring to- since you mention removing your hands from the bars.
Hi filtersweep -
Most evident around 15 MPH (- and probably faster, but I can't go any faster right now because I'm still recovering from a car-bike crash.)
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Old 10-05-09, 06:27 AM   #6
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Maybe the frame /fork is bent backward from a crash? Check the top tube/down tube for slight bulging of the tubes on the bottom side.
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Old 10-05-09, 06:47 AM   #7
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. . . try a slightly wider tire, the slight change in trail might be enough to mollify the effects of shimmy.
This is good preemptive advice for me fuzz2050, because it happens I still need to buy tires for this thing, and if wider is a way to go for me, I'll do some shopping.Thanks.
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Old 10-05-09, 06:52 AM   #8
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Maybe the frame /fork is bent backward from a crash? Check the top tube/down tube for slight bulging of the tubes on the bottom side.
Hi Mr IGH -
I did look for this and there is nothing that I can see. It looks pretty straight, like the catalog picture. However, true to its "racing" pedigree, it has very little rake/caster to it, which probably does not help matters.
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Old 10-05-09, 07:41 AM   #9
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It won't be the first front end problem caused by the rear end.

A problem at the front end is frequently a result of a problem at the back. Your out of true rear wheel, just may be the culprit.

Can you borrow one for a test ride?
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Old 10-05-09, 07:46 AM   #10
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Have you tried swapping the f/r tires and seeing if the vibration is less or the same or is now on the rear? Shouldn't feel any vibration at 15mph if your wheels are true, the hub bearings are ok and adjusted, and the headset isn't loose. Might have "loaded runout" if its happens at that speed. The tires could have went through some tough/rough conditions and the sidewalls are now weaker in spots. Should have someone ride next to you at that speed and maybe they can see something to help narrow it down.
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Old 10-05-09, 07:55 AM   #11
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As AEO explained, it's a harmonics problem. Lightweight bikes tend to resonate with vibration inputs and shimmy at certain speeds. Putting your knees against the top tube helps, and so does transferring more weight to the front wheel. Sometimes the shimmy resolves itself at higher speeds.

One other thing that often helps is using a slightly wider front tire running at lower pressure
. Most riders today are using overly narrow tires and over-inflating them. This causes their bikes to be more skittish and increases front end vibration, yet has no benefit as tire/road friction is not appreciably lower and in some cases actually higher with narrower tires.
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Old 10-05-09, 08:50 AM   #12
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While the above comments are true, 15 mph is to slow for a bike to experience shimmy. This points to something else being at cause, perhaps the frame is out of alignment ever so slightly, or maybe the rear wheel is doing something funky and resonating.

This problem is fast becoming more complicated, but I'd suggest first, having the rear wheel trued, and second, checking the alignment of the frame.
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Old 10-05-09, 12:41 PM   #13
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I found a new cause the other day - shivering. I couldn't figure out why my bike was shimmying on a pretty long descent, and looked down to see that my whole body was shaking.
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Old 10-05-09, 08:33 PM   #14
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It won't be the first front end problem caused by the rear end. A problem at the front end is frequently a result of a problem at the back. Your out of true rear wheel, just may be the culprit....
So I spent quite some time with my spoke wrench today, and made the rear wheel just a little bit less of a pretzel. It is not possible to true it completely, but THERE WAS A NOTICEABLE IMPROVEMENT, so I believe the rear wheel is the primary contributor.

I have a matched pair of 700c-25 UKAI clincher wheels that I'm going to install, once I get some tires for them that is... (The old tires are shot anyway, and I think a pair of Panaracer Pasela tires would help level this thing out.)

Also- FBinNY's remarks about tires making a ride more skitzy is absolutely right -I have experienced it plenty of times, but this is worse, and probably more the resonance/harmonic issue he mentioned, since the forks actually vibrate from side to side. (Not so much now though, after my 2nd truing attempt.)

Thanks for the advice.
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Old 10-05-09, 10:05 PM   #15
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While the above comments are true, 15 mph is to slow for a bike to experience shimmy. This points to something else being at cause, perhaps the frame is out of alignment ever so slightly, or maybe the rear wheel is doing something funky and resonating.

This problem is fast becoming more complicated, but I'd suggest first, having the rear wheel trued, and second, checking the alignment of the frame.
a bike can speed wobble at any speed.
there are so many variables, that it's possible at any speed. Any speed.

I can't believe the amount of wrong in this thread.

changing parts on the bike will simply change the speed at which the wobble will happen because the harmonics change. It's not a bad idea to change parts, but there's no point changing parts when you don't understand the cause or solution to the problem.

that's like changing the battery on a clock not keeping time, but the real problem is that one of the gears is broken in the clock.

so, solution: shift weight on bike or change speed.
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Old 10-06-09, 10:34 AM   #16
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a bike can speed wobble at any speed.
there are so many variables, that it's possible at any speed. Any speed.

I can't believe the amount of wrong in this thread.

changing parts on the bike will simply change the speed at which the wobble will happen because the harmonics change. It's not a bad idea to change parts, but there's no point changing parts when you don't understand the cause or solution to the problem.

that's like changing the battery on a clock not keeping time, but the real problem is that one of the gears is broken in the clock.

so, solution: shift weight on bike or change speed.
Ever ride a bike with speed shimmy at 15mph? Wanna bet there was something wrong with it. Bike frames just didn't happen, someone took the time to think it out, and tried to eliminate all the problems possible. Frame shimmy being one of them. Shimmy at 15mph would pretty much kill the frame.
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Old 10-06-09, 11:55 AM   #17
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Jan Heine wrote an interesting article on shimmy in a recent issue of Bicycle Quarterly. He thinks imperfections in the facing on head tubes is responsible in many cases, and suggests that headsets with needle bearings (like the Stronglight A9) instead of ball bearings often resolves shimmy problems as the needle bearings align automatically and compensate for the imperfections in facing.

Bicycle Quarterly, Volume 6, Number 3

Curing Shimmy on a Bike

by Jan Heine and Mark Vande Kamp

Some bikes shimmy, others don’t, even if they are made from the same tubes. Some believe that poor frame alignment is the cause for shimmy, but we have ridden modern custom frames from very reputable builders that shimmied, even though they appeared to be aligned very well. Yet many older frames with unknown histories do not shimmy, despite being obviously misaligned.

Looking at the variables involved, Jan noticed that he never had ridden a bike with a Stronglight needle-bearing headset that shimmied, yet similar bikes (from the same maker, with the same tubing and geometry) with ball bearing headsets often shimmied.

Stronglight headsets are different from all others in that the bearings align automatically, and thus compensate for imperfections in the facing of the head tube. On a standard headset, the balls on one side tend to run looser than those on the other, because the top and bottom edges of the head tube never are perfectly parallel. Furthermore, the needle bearings of the Stronglight headset may add a little resistance to the headset, perhaps enough to dampen shimmy, but not enough to change the steering characteristics of the bike.

To see whether a different headset could make a difference in shimmy behavior, we replaced the Chris King headset on Mark’s Ti Cycles, a bike with a very strong shimmy under certain conditions, with a Stronglight needle-bearing headset. Mark rode the bike to the workshop with a load in the handlebar bag, and it was easy to provoke shimmy.

After we replaced the headset, Mark tried to induce shimmy as he had before, but the bike no longer shimmied. After numerous attempts, he finally got the bike to shimmy briefly, but instead of continuing until Mark put his knee on the top tube, the shimmy now attenuated on its own within a few oscillations. We then loaded the handlebar bag with about 4 kg (8.8 lbs.). Still, the bike’s shimmy was gone in most speed ranges. Only when coasting downhill no hands at 40 km/h (25 mph), a violent shimmy occurred that was beyond the capacity of the headset’s dampening. Placing one hand on the handlebars immediately stopped the shimmy. Mark rarely rides no-hands at speeds this high, so the shimmy problem on this bike has been mostly eliminated with a simple headset change.

In conclusion, we still do not know what causes shimmy on a bike. From Mark’s and my experience, it is unrelated to top tube diameter. A handlebar bag does increase the likelihood of shimmy on Mark’s Ti Cycles, but many other bikes with handlebar bags do not shimmy. We found that we could eliminate most of the shimmy on one of the worst bikes we experienced, by replacing the Chris King headset with a needle-bearing Stronglight model. If your bike shimmies, it is worth a try.
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Old 10-06-09, 12:43 PM   #18
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I recently bought an old Trek 760 racing bike, that has a front wheel shimmy problem. It is significant enough to cause the front forks to oscillate just a bit, and enough so that I don’t dare take my hands off the bars.

The front skewer is tight, the hubs and headset seem to be adjusted OK - no excess slack). The front wheel is straight and true.

The wheels are both 700c X 23, but grossly mismatched, with a V-section clincher rim in the front, and a tubular at the rear. The rear wheel is out of true – but I can’t seem to get it trued. Nevertheless I do not detect any wobble from the REAR when riding.

I am wondering if this will all go away when I replace the wheel set – But meanwhile, is there something else I can check, that I haven’t already checked, that could be causing the front end shimmy?
Your wheel and headset bearings should have NO play but also NO drag. "No excess slack" sounds like you might be allowing some slack.

"replace the wheels" sounds like bike shop advice - they don't really know if that will help.

Why can't the rear wheel be trued? It should not cost too much to have a capable shop do it?

Can you have the frame alignment checked?
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Old 10-06-09, 05:22 PM   #19
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http://www.sheldonbrown.com/brandt/shimmy.html
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Old 10-06-09, 05:56 PM   #20
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Ever ride a bike with speed shimmy at 15mph? Wanna bet there was something wrong with it. Bike frames just didn't happen, someone took the time to think it out, and tried to eliminate all the problems possible. Frame shimmy being one of them. Shimmy at 15mph would pretty much kill the frame.
no, I've not encountered a rider/bike combination where I get a speed wobble at 15mph. There are so many variables, that it's not possible to design around a speed wobble on a bike.
there is nothing wrong with the frame. It's just a magical speed at which the resonance amplifies itself.

go read up on speed wobble or motorcycle design.

http://www.pro-am.com.au/speed_wobble_on_a_bicycle.htm
http://www.sheldonbrown.com/brandt/shimmy.html
http://en.wikipedia.org/wiki/Speed_wobble
http://en.wikipedia.org/wiki/Bicycle...cycle_dynamics

see how people designed around it.
http://www.youtube.com/watch?v=RYOg8mLjruE
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Old 10-06-09, 07:17 PM   #21
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It's far from normal for a bike as good as a Trek 760 to shimmy at 15 mph. As others have said, I would start with fixing the wheel situation. If the rear wheel won't come true it's probably due to a bent rim.

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Old 10-06-09, 08:51 PM   #22
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What size is the frame? I skimmed thru the responses and never saw the frame size. A frame with good alignment should have no shimmy. If you have corrected all the obvious causes and still have a shimmy then look at a problem with frame alignment. A common cause of shimmy on older frames can easily be caused when some one has gone from a smaller to larger rear spacing and didn't use a proper frame tool to cold set the frame and mis-aligned the frame. Go to a frame builder and have the alignment checked and the RD hanger at the same time. If I remember the Trek 760 is steel. Check out the tubing for rust. If severe enough it could thin the tubing down in a key place and allow for the shimmy.
I ride a 55cm frame and with all the bikes I have ever owned (since 1955) the frame was never the cause of a shimmy. Loose head set, wheel bearings loose, uneaven loading on a tour but that is it. I always test a frame by slapping the headset at about 30mph with no hands on the bars. If it doesn't dampen immediately then I will s-can the frame. Currently have 4 steel frames and none will wobble or shimmy at any speed. If it is a 60cm or larger then you can see problems. If the frame is the problem then scrap the frame if you can't mitigate the shimmy. Go to an old school LBS or a frame builder in your area.

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Old 10-07-09, 04:51 AM   #23
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Originally Posted by Road Fan View Post
Your wheel and headset bearings should have NO play but also NO drag. "No excess slack" sounds like you might be allowing some slack.

"replace the wheels" sounds like bike shop advice - they don't really know if that will help.

Why can't the rear wheel be trued? It should not cost too much to have a capable shop do it?

Can you have the frame alignment checked?
Hi Road Fan -
If there was any slack, it was not detectable - and given that this bike used needle bearings instead of balls, I would think that would have been obvious.

Replacing the wheels - is going to be necessary in any case - apart from a warped rear rim it is a tubular, and then I have a Capagnolo deep V clincher rim on the front, for which I do not have a mate in my pile of parts.

The rear rim is warped/damaged to the extent that I cannot set it completely straight, and I'm as good at truing wheels as my LBS.

I am taking the bike in for an alignment check today to see if your supposition is correct. I certainly hope the frame is OK!
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Old 10-07-09, 04:56 AM   #24
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Hi Deanster04 -

Arghh ! The thought of scrapping this '760 frame makes me cringe - it's Reynolds 531 and there is no rust. The size is 22" (whatever that is in cm) and the rear hub is a 6 speed, like the original so I can't see a reason why someone would deliberately spread the stays. My LBS has a gage though, and I will have them check it today. The wheel rim however is history.
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Old 10-07-09, 06:12 AM   #25
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Originally Posted by Scooper View Post
Jan Heine wrote an interesting article on shimmy in a recent issue of Bicycle Quarterly. He thinks imperfections in the facing on head tubes is responsible in many cases, and suggests that headsets with needle bearings (like the Stronglight A9) instead of ball bearings often resolves shimmy problems as the needle bearings align automatically and compensate for the imperfections in facing.. . . . [/I]

Hi Scooper -
I'm not presupposing that Jan Heine's theory about needle bearings is incorrect: In fact, it tends to support my HUNCH that the headset is not the principle problem, because the bike already has a Stronglight headset with needle bearings.

I am taking the frame/fork assembly to the LBS today to have them put a gage on it. If it's reasonably straight, I'll put new bearings in the hubs on the replacement wheels before I reassemble the bike.
I am more than curious to know the outcome.
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