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Baffling Front End Shake

Road Cycling ďIt is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.Ē -- Ernest Hemingway

Baffling Front End Shake

Old 02-06-06, 12:30 AM
  #1  
55/Rad
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Near the end of our Sunday ride, I was zipping down a fairly large hill - Taylors Ferry down to Hwy 43 for you Portlanders - and at about 35 mph, my front end started to wobble. Not a little wobble but a somewhat violent shake. Felt like the front wheel had come completely undone - like a spoke had snapped and the wheel was way out of true.

Fortunately, I was able to control it and come to a safe stop. Upon inspection, I could find absolutely nothing wrong. Wheel was fine and true, skewer was tight and the headset/fork combo was rock steady. Road had no undulations I could see that might have caused it.

This was the first time I've experienced this and it has me baffled - any ideas of what might have caused it? Could it be the wheel bearings are going?

Thanks.

55/Rad
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Old 02-06-06, 12:40 AM
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Originally Posted by 55/Rad
Near the end of our Sunday ride, I was zipping down a fairly large hill - Taylors Ferry down to Hwy 43 for you Portlanders - and at about 35 mph, my front end started to wobble. Not a little wobble but a somewhat violent shake. Felt like the front wheel had come completely undone - like a spoke had snapped and the wheel was way out of true.

Fortunately, I was able to control it and come to a safe stop. Upon inspection, I could find absolutely nothing wrong. Wheel was fine and true, skewer was tight and the headset/fork combo was rock steady. Road had no undulations I could see that might have caused it.

This was the first time I'd experienced this and it has me baffled - any ideas of what might have caused this? Could it be the wheel bearings are going? Thanks.

55/Rad

Extort has the some what same problem with his Basso, he doesn't know the reason either, but when we ride together I try to stay ahead of him on the downhills . His is not a violent shake like your is but it's enough to make me want to keep ahead of him on fast downhills.

I was following him on a downhill, the first time we rode together, and noticed his arms shaking, I had never seen anything like this before and hope I never experience it myself.

It has to be a resonance building up somewhere in the front end. I'd have everything up there checked out, that would take ALL the fun out of riding.

Good luck in finding the source.
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Old 02-06-06, 01:19 AM
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Hmmm....a similar thing happened to me on my saturday ride, except not as extreme. The front end didn't feel as stable as it normally does. Turned out the headset was loose, but you said you checked yours. You got me stumped...
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Old 02-06-06, 01:26 AM
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Which front wheel was this?
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Old 02-06-06, 01:33 AM
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Is it what used to be somewhat common on road bikes and was referred to as "shimmy"? I'm wondering if it would go away if you sped up some. You may have been at the speed corresponding to the resonant frequency of the wheel.
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Old 02-06-06, 01:38 AM
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I have a bike that does this too! Funny it happened to you at 35mph; mine starts at 34! Mine also goes into a violent wheel wobble. Very tough to hold on and grip the brakes at the same time isn't it. Scary too, huh.

Of course the 1st time I experienced it I didn't expect it. Then it happened again. Then I put a speedo on my bike and tested it twice. 34mph each time.

Its not the road surface. I took my wheels in to be trued, I tried sitting back further on the seat, I had the frame alignment checked. Nothing was found to be abnormal.

All I can say is, watch your downhill speed from now on (or sell the bike to someone who'll probably never want to go that fast).

Sorry but I don't have a clue as to what causes it. Maybe a loose head tube?

I doubt this has anything to do with it but my handlebar is only 35mm (less control).

If you ever find out what causes it please, please post in CAPITAL LETTERS!

Good luck!
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Old 02-06-06, 01:56 AM
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Originally Posted by 53-11_alltheway
Which front wheel was this?
Perhaps try a different front wheel at the same speed?

You sure that headset/fork is secure? Pick the bike up by the top tube (right near the handlebars) and shake the front up & down, front to back and side to side.
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Old 02-06-06, 03:17 AM
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I think front-end shimmy generally happens when the bike's frame somehow gets into an oscillatiing resonance pattern that shakes the front part of the bike (fork and wheel) back and forth. Like a wave, the shaking starts as a ripple but can build up into a violent shaking.

I have that problem with my steel bike under certain conditions. When I'm descending at 30-some mph (around 35) and there's a strong wind shear which causes the frame to twist a bit, it's very easy to get the shimmy if I just touch the brakes a little. I read an article on the net that says the shimmy is caused basically by a combination of factors -- the frame design, the resonance of the whole bike and the rider's position or grip on the bike and possibly the road condition, too. When I brake, my arms tense up and that puts the bike into a resonating frequency that amplifies itself into a violent shake.

This is the article I found that tries to explain a lot of what causes shimmy:
front end wobble - aka shimmy

What I've found is that to avoid shimmy, I have to keep off the brakes, but that's sometimes difficult if I'm already going too fast and I want to slow down a bit. So I'm always a bit skittish when I'm descending fast in windy conditions -- I tend to just slow way down to insure that I don't start any shimmy.

-Kevin
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Old 02-06-06, 03:47 AM
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I know the shakes you're talking about as I have ridden in front of riders experiencing this......and most commonly with light weight riders. When I rode a shaker's bike down the same hill, I did not experience the shake and I suspected the rider's weight had something to do with this as I weighed at least 30 lbs more than the shaker.....his bike felt solid and stable.
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Old 02-06-06, 06:33 AM
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My first Kestrel 200SCI frame did this on steep long fast decents (35 to 55 MPH). At the time I was running a set of Cosmic wheels front and rear. A buddy on the ride told me to squeeze the top tube lightly with my knees on the way down and use my brakes sparingly. It completely eliminated the shimmy. The next year, I broke the rear Cosmic, and replaced them with some CXP 30's. The shimmy never returned with the new wheelset. I have also noted a slight instability on another bike whenever the headset is even slightly out of adjustment.
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Old 02-06-06, 06:59 AM
  #11  
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Lets talk about wheels and tires. What wheels? How old are the tires? Have these tires taken any big hits? Had any flats recently (on these wheels of course)?

One thing that baffles me is that we don't balance bicycle wheel/tire combos like we do with our cars. I would like to see if your wheels when spun at whatever RPM gets you 35 MPH, hit that balance harmonic and vibrates...as another poster mentioned if you could go faster maybe it goes away...not that I would want to be the one that tries to find out.
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Old 02-06-06, 07:12 AM
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Shimmy is usually caused by not having enough trail. To explain trail for those who donít know: If you draw a line through the center of your head tube and therefore also the steering column, that straight line will reach the ground at a point (Point A.) ahead of the point where the wheel contacts the ground (Point B.)

I always built my bikes with at least 2 Ĺ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it. If you make the head angle steeper it means less trail because you move point A closer to point B. Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.

Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities. As you lean to the left, point A moves to the left and the wheel Pivoting on point B will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail. Fitting a slightly heavier tire to the front wheel may help, and is worth a try.

What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.

Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames also tend to have shallower seat angles to accommodate the riderís longer legs therefore the riders weight is more over the rear wheel.

Any vehicle that has its weight towards the rear is less stable, and a bicycle can be considered such a vehicle. When the rear wheel is being driven it is fine, but when coasting the heavier rear end tries to overtake the lighter front end. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy.

More on the Mechanics of Steering here: http://davesbikeblog.blogspot.com/20...-steering.html

And more Bike Tech stuff here: http://www.prodigalchild.net/Bicycle6.htm
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Old 02-06-06, 07:25 AM
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Originally Posted by Dave Moulton
Shimmy is usually caused by not having enough trail. To explain trail for those who donít know: If you draw a line through the center of your head tube and therefore also the steering column, that straight line will reach the ground at a point (Point A.) ahead of the point where the wheel contacts the ground (Point B.)

I always built my bikes with at least 2 Ĺ inches of trail. Trail is common to all wheeled vehicles, cars and even a shopping cart will have it. If you make the head angle steeper it means less trail because you move point A closer to point B. Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.

Trail keeps the bike going in a straight line, and also assists a two wheeled vehicle in its self steering abilities. As you lean to the left, point A moves to the left and the wheel Pivoting on point B will turn to the left. The gyroscopic action of the spinning wheel also plays a big role in self steering, but this is another subject and I only mention it because the heavier the spinning wheel, the more it keeps straight. Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail. Fitting a slightly heavier tire to the front wheel may help, and is worth a try.

What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.

Large frames are more prone to shimmy for two reasons. Large frames are taller and also should be proportionately longer, but there is a school of thought that believes a race bike should have a short wheelbase, so the builder makes the head angle steeper to shorten the wheelbase, but in doing so lessens the amount of trail. Large frames also tend to have shallower seat angles to accommodate the riderís longer legs therefore the riders weight is more over the rear wheel.

Any vehicle that has its weight towards the rear is less stable, and a bicycle can be considered such a vehicle. When the rear wheel is being driven it is fine, but when coasting the heavier rear end tries to overtake the lighter front end. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy.

More on the Mechanics of Steering here: http://davesbikeblog.blogspot.com/20...-steering.html

And more Bike Tech stuff here: http://www.prodigalchild.net/Bicycle6.htm
Ok that was quite simply the single most informative post I have ever read on BF.

You think this guy knows what he is talking about? (<--- please note the smilely. Yes I know who Dave is)
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Old 02-06-06, 07:35 AM
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Originally Posted by 55/Rad
Near the end of our Sunday ride, I was zipping down a fairly large hill - Taylors Ferry down to Hwy 43 for you Portlanders - and at about 35 mph, my front end started to wobble. Not a little wobble but a somewhat violent shake. Felt like the front wheel had come completely undone - like a spoke had snapped and the wheel was way out of true.

Fortunately, I was able to control it and come to a safe stop. Upon inspection, I could find absolutely nothing wrong. Wheel was fine and true, skewer was tight and the headset/fork combo was rock steady. Road had no undulations I could see that might have caused it.

This was the first time I've experienced this and it has me baffled - any ideas of what might have caused it? Could it be the wheel bearings are going?

Thanks.

55/Rad
My buddy and I have ridden our older bikes downhill at around 42mph with no problems, the new ones we do not know yet as access to that hill has been closed from our usual direction.
I would suggest that you balance your wheels and read Craig Calfee's article on the subject located at
http://www.calfeedesign.com/forksymmetry.htm

Increasing the fork rake may help keep you out of that resonant zone as well as adding weight but who wants to do that? The older Ford Mustangs added a '24lb Hambone' to the pinion nose of the rear axle to 'cure' a vibration problem.
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Old 02-06-06, 07:37 AM
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dunno if it means anything but headshake usually happens on sport bikes (the motored kind) when the front-end gets light... could it be that on those quick down-hills... you're sitting further back on the seat?

I've never experienced headshake on my road bike... but on my CBR I did, but it was just that I was on the throttle much higher in a gear than usual so it was nearing a wheelie (and the redline!)

Rock
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Old 02-06-06, 07:39 AM
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Originally Posted by sestivers
Is it what used to be somewhat common on road bikes and was referred to as "shimmy"? I'm wondering if it would go away if you sped up some. You may have been at the speed corresponding to the resonant frequency of the wheel.
Maybe Rad has to balance his front wheel like you do with car wheels.
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Old 02-06-06, 07:44 AM
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Of course Dave M. is correct. I think many manufacturers routinely use a fork (carbon) with the same offset throughout the frame sizes range. If you look at the frame geometry charts for a lot of manufacturers, they are using a fork with 45 mm of offset. This is fine for a small frame with a 72* head angle. But with a large frame with a 73.5* or steeper head angle, the trail will be insufficient. Because every new bike has brake/shift levers nowadays, the front end can be quicker than it was back in the day of down-tube shifters. Thus, most folks don't notice that the bike is not as stable at speed.
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Old 02-06-06, 08:07 AM
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Originally Posted by Dave Moulton
Road bikes with ultra light wheels and tires are more sensitive to small changes in the amount of trail. Fitting a slightly heavier tire to the front wheel may help, and is worth a try.

What happens in a high speed downhill shimmy the wheel is turned one way or the other by a bump in the road or a gust of wind. (Like when swinging out of a pace line.) The caster action of the trail corrects this, but if there is not enough trail it will over correct and then correct again starting the wheel fluttering back and forth. You can see exactly the same thing on a shopping cart if you run with it across the parking lot the caster wheels will flutter back and forth in the same way.
Awesome Post, it's why I keep reading threads
Umm... Does Rad need a new fork to solve this problem - because the cause may be a geometry issue from what you have stated... ?
What's considered a large frame? I'm pretty sure Rad isn't that tall - rides a 56/57/58..

-simplyred
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Old 02-06-06, 08:24 AM
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Wow, fantastic response - much more than I expected. Thanks everyone.

To answer a few questions....

-Wheels are Bontrager X-Lite Aeros - used but in great shape. Tires are Conti GP 3000 - 6 months old and in great shape.

-Headset was tight - no issue there

-Speeding up was not an option - I was convinced I was going down. Scary? Uhhhh, yea. It went on for awhile and each shake (correction) seemed to get more pronounced. I was actually preparing for the fall in my brain, wishing I had paid closer attention to this thread: http://www.bikeforums.net/showthread...ighlight=crash For some reason, when things like this happen, I seem to have a built in relax mechanism. I don't know why but I've always been happy to have it. Kicked in yesterday at just the right moment.

-Wheel balancing - I may look into this. That's exactly what it felt like - an unbalanced wheel that had hit that speed where all hell breaks loose.

-Body position - I might have been positioned back a tad more than normal - as well as sitting up a tad - I tend to do that on descents with a lot of traffic around.

And finally - Thanks to Dave Moulton for his insighful and knowledgeable response. I will read and re-read this later in the morning - after coffee - to make sure I understand every word.

Question - would a longer rake on the fork possibly correct this issue?

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Old 02-06-06, 08:25 AM
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Originally Posted by Dave Moulton
Any vehicle that has its weight towards the rear is less stable, and a bicycle can be considered such a vehicle. When the rear wheel is being driven it is fine, but when coasting the heavier rear end tries to overtake the lighter front end. So if you are a tall person with a large bike frame, try to keep your weight forward when descending. Also keep your body in a low aerodynamic tuck; if you sit up wind pressure on you chest will push more weight towards the back wheel. Finally if you should get into a high speed shimmy; try not to panic, grip the top tube between your knees, and apply the rear brake first very gently and only apply the front brake after you have come out of the shimmy.
This should be the standard reply to the next 100 high speed shimmy threads we see here on BF.
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Old 02-06-06, 08:26 AM
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Sorry - frame is the Ironhorse Victory (58) with a Reynolds Ouzo Comp fork and a Cane Creek integrated headset.
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Old 02-06-06, 08:36 AM
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Originally Posted by 55/Rad
Question - would a longer rake on the fork possibly correct this issue?
Originally Posted by Dave Moulton
Also if you increase the fork rake you make for less trail; in this case point B moves closer to point A. The worst scenario is a bike with a steep head angle and a long fork rake; trail can be reduced to almost zero.
The way I read Dave's response a longer rake fork would make the issue worse not better as it would make the trail even less than it currently is. Do you know the rake of the fork on the bike? I know the Ouzo Pro comes in the best selection of rakes I have seen...not sure about the Comp.
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Old 02-06-06, 08:41 AM
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Ex-framebuilder? No more Fuso frames? Retired in 1993. I guess I've been out of it for a while. Doh!

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Old 02-06-06, 08:44 AM
  #24  
galen_52657
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Stock fork or replacement upgrade? When I upgraded the fork on my old Bianchi (61) many moons ago, the replacement fork did not have the same offset and it detracted from the bike's handling.

Look at the trail measurements of this Cannondale 6-13. The trail diminishes as the frames get larger

http://www.cannondale.com/bikes/06/geo-613.html
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Old 02-06-06, 08:47 AM
  #25  
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Originally Posted by Grasschopper
The way I read Dave's response a longer rake fork would make the issue worse not better as it would make the trail even less than it currently is. Do you know the rake of the fork on the bike? I know the Ouzo Pro comes in the best selection of rakes I have seen...not sure about the Comp.
You are correct - coffee kicking in now. Don't know the rake for sure but assume it be a 43 - it came stock with the frameset but that doesn't mean they got it right in the first place. It is after all, a fairly generic frame frame from a fairly generic company. One item of trivia - this is the only bike I have with a setback seatpost - meaning my center of balance is a hair further back on this frame than on the others.
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