Oct 2, 2001 4:17 PM
|Sorry to shout, but wanted to get your attention, which is confounded by the lack of email addresses that used to appear with names and profiles.
You commented in a thread a couple weeks ago about induced front end shimmy resulting from too firm a grip on the bars. I've had a shimmy problem for the last couple years and had a bad crash last summer. I thought the problem was my bike, and I've collected just about everything ever written about front end shimmy that results from bent axles, worn hubs, bent steerer tubes, badly adjusted headsets, and the inherent vibration frequency of bicycle tubing. I've got articles by Jobst Brandt and a host of others.
I've changed out my frame and even built a new bike this summer. Guess what? Still got front end shimmy from time to time, even with the new bike. It seems to be random; so according to Brandt, the inherent characteristic of the bike tubes would always shimmy at the same speed. I get it anywhere between 26 and 32 mph. I've got to think maybe it's me. Is this as simple as sometimes I've got a death grip on the handlebar, causing the front end to shimmy, and when I've got a normal grip it doesn't? I tried to test this last week with no real conclusion since no shimmy occurred. Would it make a difference, like with too hard a grip AND a certain type of road surface? If you know more about this topic, I'd appreciate any information. I'm so paranoid of having a high speed shimmy incident that I took little pleasure in fast descents this summer, especially curvy ones.
Rod - email email@example.com
|Road surface is part of it.||nee Spoke Wrench|
Oct 2, 2001 4:29 PM
|I got into some deep, new chip and seal a few years ago which was the only time I ever had a front end shimmy on that bike. I eventually lauanched over the bars and broke my collarbone real bad. Like you, I don't find fast downhills nearly as much fun as I did before the incident.|
|Lemme Respond on Wed. - gotta run!||grzy|
Oct 2, 2001 4:32 PM
|Sorry to put you off, but I gots to go.|
|have you considered trail?||DAC|
Oct 3, 2001 4:45 AM
|Do you know how much trail your bike has? Most manufacurers (such as Cannondale) have at least 57mm trail, some bikes go up over 70mm. the more trail (numerically), the more stable you will be at higher velocities. I don't want to get into a systems dynamics lecture here, but anybody who understands vibrations will confirm this.
I once had a nice old bike, which had originally used 27" tires. When I converted to 700 wheels, I also replaced the fork with a shorter fork, but same offset (rake). Just the difference in the length of the fork reduced my trail enough that it became unstable over about 35 mph. The shorter fork lowered the front, reducing the headtube angle, and giving me less trail. When I calculated the trail, I was left with 52mm, on a 25" frame.
|re: letting go||dzrider|
Oct 3, 2001 5:29 AM
|I had a Vitus 992 that, when riding down hill ond windy days, would sometimes shake like a washing machine on spin cycle. The last two times it started I took my hands off the bars and sat up and it stopped before it got so scary I wouldn't dare let go. Good luck.|
|re: letting go||Birddog|
Oct 3, 2001 5:46 AM
|Letting go would scare the crap out of me because I'm self employed and have no disability policy. I prefer to clasp/lock my knees/quads to the top tube like I'm riding some wild stick pony. It kinda takes me back to my childhood. Fortunately the shimmies haven't hit me in awhile and so far my new bike has zero occurences.|
|What kind & material of frame? nm||George|
Oct 3, 2001 8:47 AM
|What kind & material of frame? nm||Rod|
Oct 3, 2001 10:21 AM
|Both steel. One tigged, 59(c-c)X 57.5, the other lugged, 59(c-c) X 58. Wheels are Open Pro 32s and Velocity Razors 32s. Tires on both are Conti ultra 2000.|
|re: Hey GRZY||grzy|
Oct 3, 2001 9:17 AM
|Well I think one way to look at it is the machine by itself and then the rider/machine together. There are certain things that will make a machine unstable. In fact manauverability and stability are inversely related. One goes up the other goes down. So a sport/racing style frame which is more manauverable than say a touring frame and will have less stability. You can do things to make the bike more/less stable, but it's pretty much limited to the fork, stem and riding position. Then there are the unintentional things which tend to center around the wheels. Out of true and out of ballance wheels tend to show up around 30 mph. A wheel can look perfectly straight and statically ballanced, but all of that changes with load and speed. Due to differences in spoke tension and a tweaked rim you can have some pretty bizzare results. People have also seen that something as simple as a spoke magnet or flexy ti skewer can throw things off. It's probably work going with a set of new wheels just for comparison and leaving the magnet off. Ultimately a frame out of alignment could give you problems. Having lots of patches on your tubes can have an effect as well as poor tires. An overly tight headset is detrimental as is an overly loose one. |
Now, assuming that the bike checks out fine (you said you have a new bike) it's time to look at the rider/machine combo. First, realize that everything has a "natural frequency" a frequency where it want's to oscilate and the oscilations get larger with over time as the driving force is applied. This is all about systems dynamics and vibrations. You as the rider can influence some of this (to a degeree) since you're both a dampening force and/or a driving force. Road surface plays a role since it's part of the driving force. Getting tense on the bike and starting to have a death-grip tends to make things go from bad to worse. Learning to relax a bit and become more confident will certainly help. You may find that changing the position of your hands on the bars can help. Try experimenting when at speed between gripping things tightly and then relaxing. You should see a difference. probably the toughest thing is being able to do ANY of this after expereincing a crash. Even if your crash wasn't speed related you will get tense as the speed cranks up - it took me two seasons to get about 95% of my nerve back after a hellacious high speed wreck at 50 mph. My buds have all had similar experiences. You simply don't want to go down again for anything. I eventually sold the bike (an OCLV) not that there was anythihng wrong with it, but I just didn't ever get all of my confidence back. With other lesser wrecks it's been no big deal to get back on the same bike. Try riding a friend's bike over your route to see what differences/similarities you can notice.
When you find a stretch of road that starts the shimmies going use this as a tool. Go back to the same section routinely and try different things to see what helps and what makes things worse. You'll probably find that there will be some component issues as well as handling issues. Getting comfortable with the very same peice of road will allow you to relax easier and try things. Ultimately, unless you're racing, there's no real reason to push yourself deep into the zone where you're not comfortable. I ride witha mixed group of buds and some of them are total animals on twisty descents while some hold back. Some know exactly wht they're doing and some are blissfully ignorant (at both ends of the spectrum). Just remember the animals tend to crash more often, but they're OK with this. Attitude plays a large role.
Oct 3, 2001 10:46 AM
|Thanks for your repsonse.
Bike frames and forks have been checked and double checked for alignment. Wheels are true. This has now happened on 3 different bikes. The fork and wheels were trashed in the crash. When it continued to occur with the new fork and wheels, the builder changed out the frame. And it shimmied again. Then I built up a new bike this summer, only to experience the front end shimmy on a descent a couple weeks ago. I should add, the shimmy has only occurred three times this year, all on different hills. Two were tense situations, sharing the hill with passing truck traffic. Also, guys (different height, weight, and position on bike) at the shop rode my bike and were unable to induce shimmy at any speed.
My crash a year ago this past summer happened only because my bike shimmied, at 32 mph, downhill, with a curve to the left. I couldn't control the shimmy, so I couldn't steer the bike, therefore I crashed.
Following Brandt's advice, I've learned I can stop a shimmy be unweighting the saddle and placing most of my weight on the cranks so that the head tube cannot oscilate the top tube against the seat tube. (Not a comfy way to spend long descents.) Haven't had enough samples yet, but I'm thinking that when I unweight the saddle I may also change my grip on the bars, which could be a major factor, or THE major factor.
You said again, that a death grip worsens a vibration situation. It seems somewhat likely that I might become more tense (increasing my grip) as the pucker factor increases on a descent. Since the crash, I may tense up at a lower speed, hence experiencing shimmy as low as 26 mph, whereas it previously happened only at 32 mph (which is largely why I thought it was bike tubing related).
I'll continue to test - though only on straight descents. I don't have the nerve to test for shimmy control on curvy downhill runs yet. I don't race, and I don't wish to be uncomfortable on my bikes, but I want to regain the confidence that I can descend at least up to 40 mph. After all the work of hill-climbing, I hate like heck to hold back my descending speed under 30 mph just because the dreaded front end shimmy might happen again.
Oct 3, 2001 1:56 PM
|It really sounds like it's your technique. I have to believe that your bike has been thoroughly checked and the desing is solid - what are you riding? Some will say that certain forks are better than others. i certainly liked going from the original skinny OCL fork to the thicker Air Rail desing, and I liked going from a skinny Time to a Reynolds. Still, there are lots of guys who have no problems going fast on all of this equipement. Some say a Serotta F-1 fork is the ultimate - a buddy has one on his ti Merckx and has hit 62 mph - but he's nuts. |
Tensing up will defintiely do it - and it's sort of a vicious cycle. One trick is to learn to loosen your grip. A way to do this is to be in the drops and push with your palms, but leave your fingers open. Rest them against the brake levers, but don't squeeze. Another thing may be that you're descending and cornering with your pedals at 3 and 9 o'clock instead of 6 and 12. this makes you very unstable. When you place the out side pedal down at 6 o'clock and put a majority of your weight on the outside leg you can "push" with your inside arm and really get into a stable cornering position. Come to think of it I usually do rest lightly on the saddle when descending fast - I'm either trying to get lower with my butt off the back of the saddle or I'm using my legs to absorb much of the shock. However I don't realy consider 30 mph all that fast and don't do anything special here.
Sometimes I'll start to get the shimmies, then realize that I'm all tense for some reason. If I force myslef to relax the shimmy goes away immediately. It can also help, to a degree, to ride with other bikers. You tend to pay more attention to what is going on around you and less time thinking unpleasant crash thoughts. You'll also start to notice their position on the bike - try immitating what they're doing. Also, you'll realize that if they're tires are gripping and their bikes are holding the line then yours should too.
No doubt - you need to get to the point where you can enjoy the 40+ mph descents. Otherwise it's like missing desert.
|shimmies are weird||Dog|
Oct 3, 2001 2:36 PM
|My brother had a motorcycle that got a bad shimmy at, well, let's just say, a really fast speed. Did it every time. We messed with the fork oil, the air pressure, tires, etc., but could not stop it.
I rode the same motorcycle at the same speeds, and no shimmy. Hmm.
I've never experienced a shimmy on a bicycle, either. Maybe it could be the rider.
I read somewhere, but I can't remember, that sitting firm on the saddle of a bicycle can worsen shimmy, whereas lifting off a bit will make it go away (all that frequency stuff). Sounds weird, but might be worth a try.
|here it is||Dog|
Oct 3, 2001 2:47 PM
|Subject: 8h.5 Shimmy or Speed Wobble
From: Jobst Brandt
Shimmy is not related to frame alignment or loose bearings as is often
suggested. Shimmy arises from the dynamics of forward motion and the
elasticity of the frame, fork, and wheels, and the saddle position.
Both perfectly aligned bicycles and ones with wheels out of plane to
one another shimmy nearly equally well. The same is true for bearing
adjustment. In fact shimmy is more likely with properly adjusted
bearings than loose ones. The bearing or alignment concept is usually
offered as a cause of shimmy and each airing perpetuates the idea.
Shimmy, the lateral oscillation at the head tube, depends primarily on
the frame and its geometry. The inflation of the tire and the
gyroscopic effects of the front wheel make it largely speed dependent.
It cannot be fixed by adjustments because it is inherent to the
geometry and elasticity of the components. The longer the frame and
the higher the saddle, the greater the tendency to shimmy, other
things being equal. Weight distribution also has no effect on shimmy
although where that weight contacts the frame does.
In contrast to common knowledge, a well aligned frame shimmies more
easily than a crooked one because it rides straight and without bias.
The bias force of a crooked frame impedes shimmy slightly. Because
many riders never ride no-hands downhill, or at least not in the
critical speed range, they seldom encounter shimmy. When it occurs
with the hands on the bars it is unusual and especially disconcerting.
There is a preferred speed at which shimmy initiates when coasting
no-hands on a smooth road and it should occur every time when in that
critical speed range. Although it usually does not initiate at higher
speed, it can.
Pedaling or rough road interferes with shimmy on a bicycle that isn't
highly susceptible. When coasting, laying one leg against the top
tube is the most common way to inhibit it. Interestingly, compliant
tread of knobby tires give such high lateral damping that most
bicycles equipped with knobbies do not shimmy.
Shimmy is caused by the gyroscopic force of the front wheel that acts
at 90 degrees to the axis of the steering motion. The wheel steers to
the left about a vertical axis when it is leaned to the left about a
horizontal axis. When the wheel leans to the one side, gyroscopic
force steers it toward that side, however, the steering action
immediately reverses the lean of the wheel as the tire contact point
acts on the trail of the fork caster to reverse the steering motion.
The shimmy oscillates at a rate that the rider's mass on the saddle
cannot follow, causing the top and down tubes to act as springs that
store the energy that initiates the return swing. The shimmy will
stop if the rider unloads the saddle, because the mass of the rider is
the anchor about which the oscillation operates. Without this anchor
no energy is stored. The fork and wheels may store some energy,
although it appears the frame acts as the principal spring.
Shimmy can also be initiated with the hands firmly on the bars by
shivering, typically in cold weather. The frequency of human
shivering is about the same as that of a typical bicycle frame.