Speed Wobble

Dear Jim: I bought the bike of my dreams last year, a Seven Alta. It comes in at just over 16 pounds. I love the bike, but I had a scary experience recently while riding in Big Bend National Park in Texas.

I was descending out of Chisos Basin -- a long, winding and very fast descent -- when I picked up a front-end shimmy that got progressively worse. I was convinced I was going down but managed to bring it to a stop. I've been hesitant to let it loose on descents ever since.

The owner of my LBS theorized that a slightly loose front hub was at fault. There was also a fairly significant crosswind hitting the bladed spokes. Do you think there is anything inherently wrong with my bike that would cause this shimmy to happen again? -- Larry M.

Jim Langley Replies: Your e-mail made my day, Larry, because 25 years ago, on my cross-country tour, I rode up to Chisos Basin and down the very hill you're talking about.

What a beautiful place! I almost decided to stay there and work for a few months because it was so relaxing and peaceful, and because the riding was so epic.

I don't think there's anything inherently wrong with your Seven. Speed wobble is one of those cycling phenomena that many roadies experience sooner or later.

I'm not a physicist, but as I understand it, speed wobbles are basically vibrations. Once one begins, it gets worse due to harmonics. You might have seen a video of the bridge in Washington that self-destructed after a strong wind started it swinging faster and faster. That's harmonics at work.

Speed wobble is known to happen to motorcycles, too, so you can find quite a few hits on the subject if you search on www.google.com. Uncle Al has addressed the issue, too, here at RBR. Search for shimmy.

In your case, the loose hub may have contributed. But I think it was probably a gust of wind or rough pavement that set the front end into a slight oscillation that accelerated and scared you to pieces.

When wobble happens, you need to take quick action before you disintegrate like that bridge.

On most bikes, you can stop the vibration immediately by clamping your knees against the top tube, which braces and stiffens the frame.

If the top tube is too low for that (as in a compact frame), you can still press one leg against it. In fact, some riders routinely descend that way to prevent wobble from beginning.

I assume your shop checked for all the usual suspects -- loose headset, loose spokes, bad tire, misaligned wheel, too-high saddle (which affects weight distribution).

If so, I think that future wobbles will be rare. And using the knee-against-the-frame technique should damp it quickly if it happens.

FROM MATT C.: Jim, you're absolutely right that vibration is a downhiller’s enemy. Skis, bikes, airplanes -- wobbles can wreck your whole day. Damping a bike frame with the legs is a good fix.

Larry’s stability woes could be cured by some technical fixes. Without knowing his frame size or his body size, I'd guess that he's taller and bigger than the average 5-foot-6, 130-pound climber. He could try switching to a fork with less rake to increase the trail; increasing his tire width; or adjusting his weight distribution with a longer or shorter stem.

But if he loves to go downhill and the bike still has a vague, floating feeling at speed that compromises his descents, he should re-think the titanium choice. Geometry helps, but ultimately a bigger diameter and thicker gauge (less flexible) top tube would stiffen the bike.

I really wanted to like titanium but couldn't get the ride I wanted for the price I was willing to pay and had a custom carbon frame made for half the cost of a Seven. I'm sure that the folks at Seven would want to help before Larry took a radical step like a different frame.

Jim Replies: Thanks for the input, Matt. I didn't want to suggest that anything was wrong with Larry’s Seven in respect to its construction, but several people have suggested that the frame tubing might be too thin and light for him. That can happen when a person insists on the lightest bike when they shouldn’t.

FROM Uncle Al: Shops can make mistakes, especially when there’s a customer hounding them about their lightweight dream machine.

Some shimmy problems come from how a rider "sits the bike.” A big-butt rider with a small upper body can have poor weight distribution, i.e., too much weight on the rear wheel, which turns the front wheel into a shopping cart wheel.

A short riser stem can make the problem worse. I see a lot of riders making that modification after they've been properly fitted to a bike, once they discover they are neither youthful nor flexible enough to look like the pros do. That's a factor a company like Seven, for whom I have tremendous respect, can't factor in.

FROM GREG K.:  Larry didn't specify much about his weight or the rest of the components on his bike, nor the details of how his bike was built.  The heavier Larry is, or the larger the frame or lighter the tubing, the more likely Larry is going to see this wobble again. Other flexible components in the front end (fork or front wheel) could help induce the wobble.

To isolate the source of the problem, Larry will need to experiment with his bike. Perhaps it was a loose headset, but I expect he will find the frame or other components are too light and flexible for his size/weight. After checking the headset, he should try a different front wheel. If the wobble persists, it's either the fork or frame.

FROM LEON S.:Can't you stop a wobble by either slowing down or speeding up? Isn't it the constant speed that allows the oscillations to amplify?

Jim Replies:  Speeding up is what the motorcycle guys say to do. I've never had luck doing that (which isn't always possible on a bike, of course) or slowing down. I'm not saying that it won’t work on certain bikes, but in my experience a wobble will continue until you interfere with it by bracing the frame.

I'm definitely not an all-knowing expert on this subject. Maybe no one is. I just know that the knee trick is one of those things that seems to work in all situations, so that's the advice I give people.

FROM RICK:  I saw a Bob Roll piece on OLN last year. He was testing a Litespeed and showed that every time he took his hands off the handlebar, the front end wobbled. I have friends that have Litespeeds and some of them say the same thing. I must surmise that, in the case of Litespeeds, it’s bad geometry.

Jim Replies It might be geometry. It could also be that the frame has tubing too light for the person riding it. I have a frame like that. It's okay for me, but if I let my 200-pound friend ride it, watch out! Lots of people buy bikes based on weight, and it can lead to handling problems if the frame's too light for their weight.

By the way, RBR’s Ed Pavelka has a Litespeed Vortex. He weighs 190 pounds and says he has never had speed wobble on the bike.

FROM JOHN B.:I’ve experienced speed wobble problems twice. I’m convinced that crosswinds were the reason, just as they were for the Tacoma Narrows Bridge that you mentioned. Clamping the top tube with my knees didn't seem to work, but zig-zagging across the road seemed to do the trick. Fortunately, a driver saw that I was in trouble and didn't try to pass. I have heard three possible solutions, but I haven't tried them:

1. Change position on the bike (move forward, back, up or down) to change the natural frequency.

2. Pedal to disrupt the natural frequency.

3. Pray.

FROM RON M.:  I have been riding a 1990 Merlin since new and never had a shimmy. Last year, I did some major upgrading, and a part of that was Campy Eurus wheels with bladed spokes. Since then, I have experienced some uneasy tracking in fast descents. The bike tends to drift and shake a little when I get any crosswind. The only thing I can attribute this to is the bladed wheelset. I'm strongly considering going back to a set of 32-spoke Mavics.

Jim Replies: If you wanted to test your theory, you could drive to a hill with both pairs of wheels. Ride down on the Eurus that seem to make the bike shake, then switch to standard wheels to see if there’s an improvement. If so, you know which wheels to be riding.

FROM DAN:  I thought I was the only rider who experienced speed wobble, as it’s happened with every road bike I’ve ever owned (four so far). My most recent episode was just this last Saturday while on a fast (40 mph is fast for me), somewhat curvy descent. I believe I am at least part of the cause as I tend to get nervous sometimes and this causes me to stiffen my upper body, especially my arms.

Another factor may be the low spoke count in my front wheel (Rolf Sestriere). I am planning to get stiffer wheels and try the knees against the top tube. A more relaxed upper body should also help. A friend says I just need to do more high-speed descents until I’m more confident.

Jim Replies: In my experience, only the occasional bike will wobble. So, I wonder if something else is wrong in your situation.

One contributor is a seat that's too high. This makes it hard to put enough weight on the pedals, and that can decrease a bike’s stability. Because all four bikes have wobbled, I wonder if you always set the seat height the same. It could be the common denominator and worth checking.

If it's not the seat, try relaxing as your friend recommends. It's helpful to shrug your shoulders frequently to release the tension that builds up in your arms and neck. I try to do it every 15 minutes. Exhaling through your mouth and breathing in through your nose helps, too. And try standing every now and then.

FROM BILL B.:  Bladed spokes that are slightly twisted will provide a “propeller" effect that can cause a front wheel to wobble at high rpm, like over 20 mph. I would speculate that a wobble not caused by bladed spokes is caused by the wheel being slightly out of true. Also, the destructive force behind harmonics (e.g., the bridge in Washington) is known as an "anharmonic oscillation."

FROM FIL B.: You state that shimmy is caused by harmonics. It's actually resonance -- an increase in vibration amplitude when an input matches a natural resonant frequency of the frame.

FROM DAN N.:I had a wobble problem on a brand new bike. Now when descending, I push down on the handlebar to ensure there's some weight going to the front wheel. I’ve never had the problem again. I think many riders mimic pros who have a relaxed "cool" appearance as they descend the mountains. But we don't really know their techniques.

FROM ROBERT H.:Your response to Larry was correct, Jim. Aside from tightening knees to the top tube, a looser grip on the handlebar works to reduce the wobble. Unfortunately, when a wobble begins at speed, the natural tendency is to grip the bar tighter in order to gain control. That does not help.

FROM GARY B.:  Your column brought back memories of a ride on which I was sure I was going to die. It was 1989 and I was on day one of the Great California Land Rush, a two-day ride from San Francisco to Los Angeles. I was on the final stretch into San Luis Obispo, riding down the Cuesta Grade on Highway 1, and I was humping it -- around 62 mph.

That’s when my front wheel hit a piece of gravel. A harmonic oscillation developed almost instantly. The shimmy was so bad I could not let go of the drops to grab for the brake levers. On top of that, the bike was veering all over both southbound lanes. I had one 18-wheeler on my butt and another ready to pass.

The oscillations were so bad that my taillight’s mounting bracket snapped and sent it flying. There was no shoulder, only a knee-high guardrail with a 100-yard slope of rip-rap below. I was sure that I was going down under one or both of the semis, but I managed to put a death grip on the top tube with my knees. Seconds passed like hours. But somewhere down the slope, I was able to get the bike under control.

I ride a custom-built 68-cm Reynolds 531 frame and always wondered if the frame size had anything to do with it. I rarely will ride above 40 mph now.

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