|   Speed Wobbles | Nat
Jan 16, 2002 11:25 PM | | I was just thinking back to when we rented bikes to roll down Haleakala volcano, a 30 mile downhill trip. "Which bike should I pick?" I wondered. "One set up for downhilling" I decided. All the rentals were rigid Gary Fishers, more or less identical with the exception of a few components here and there. I grabbed the bike that seemed to have the most appropriate parts, meaning a short-reach stem and riser bars. I figured that this setup was similar to what DH bikes use, so it must be good.
A few hundred yards down the volcano and They started in on me: the dreaded Speed Wobbles. Left-right-left went my bars, so strongly that I almost lost it! I was one aloha from leaving my touristy skin, well-oiled with tanning butter, all over the Maui tarmac. I had to keep the bike just under the certain speed where They began, and pray I made it safely to the bottom.
So why did this bike, with short stem and riser bars, get all squirrely on me when the bikes with long stem and flat bars did not? Why do DH bikes use a similar setup and handle speed with aplomb? What forces of nature determine when speed wobbles get summoned to wreak their havoc?
What causes speed wobbles? |
| Nobody knows.... | Retro
Jan 17, 2002 9:14 AM | | This comes up every once in awhile, and everybody has an opinion. Except me, because I've had two bikes that did that, including my all-time-favorite two-wheeled vehicle, and NOTHING fixed it. The thing was still wobbling when I totaled it in a crash (low-speed, unrelated to the wobble). Mine was even checked out by a former USCT mechanic, a friend of a friend, who did everything I'd already tried (headset, new tire, swap wheels with a friend etc.), and HE shrugged and gave it up. It's a complex interrelationship of a lot of stuff I don't understand, is what I finally decided. Even my Atlantis wobbles a little, though not until 50mph. Doesn't bother me, because I only did that once to say I had, and I'm not going to do it again. |
| My stupid skateboard | Nat
Jan 17, 2002 10:00 AM | | It reminded me of my skateboard that I bought when i was 12. Worst thing I ever bought. Get cruising down the bike path and whump-whump-whump get the wobbles so badly I couldn't see. Go careening into the field, running faster than my legs could stride, arms pinwheeling away wildly, land belly-first on goathead bushes. "Heeeeenh-heeeeeenh-heeeeeeenh" for what seemed like minutes with the wind knocked out of me. Brglbrglbrglbrglbuhhhhaahhhhh... |
| My bike when I was 7... | Bonee
Jan 17, 2002 10:17 AM | | had serious speed wobbles, but I only got it fast enough once. And boy was it painful.
I lived on a fairly steep hill by a 7 year old's standards. I took a bike ride to the top with my bros and sisters. They were all older. On the way down they went rather fast, so of course I tried to keep up. Completely unsure how fast I may have been going, in all reality probably very slow (the 7 year old thing).
Everyone was already out of site when the speed wobbles hit. I guess I totally lost control and the handlebars spun around. The bike stopped, I flipped over the bars and did a superman. The first thing on my body to hit the pavement was my chin, and also the only thing to get dinged up.
I must have slide for a while on it - 30 stiches later it stopped bleeding. I was wearing a yellow t-short at the time. A car saw me crash and stopped to help. They took the shirt off me and had me hold it up to my chin. 1/2 mile later we got to my house and it was now a red shirt.
Ahh, memories. I also almost drown as a kid. I often wonder why I love surfing and biking, when they almost did me in as a child. |
| Superman! | Nat
Jan 17, 2002 12:48 PM | | Why is the image of a little kid doing an unintentional superman killing me? Maybe because I remember doing it once or twice.
I hate the image of you in a "t-short" though (is that like a onesie?)... |
| lol | Bonee
Jan 17, 2002 4:17 PM | | yup, shoulda been t-shirt.
Funnier thing is, after 30 stitches, my dad took me to Dairy Queen for a cone. I thought it was totally worth it! |
| I know | club
Jan 17, 2002 11:29 AM | | Things that tend to promote high-speed oscillations include:
a flexy top tube
large frame sizes, especially with a flexy top tube
a flexy fork
a tight headset
a light front wheel
too little trail (too much fork offset and/or too steep a head tube angle
too little weight on the front wheel (riser bar with a short stem would shift rider weight to the rear, there ya go)
a loaded handlebar bag flopping around
When you get a speed wobble, try leaning forward to weight the front wheel, on a road bike it helps to clamp your knees agains the top tube to stop it from flexing and to damp the oscillations. it's possible to accelerate past the speed the wobble begins, but then you have to deal with again when you slow down
Was the bike you rented a large size? Rigid or suspension fork?
Speed wobbles are less common with mountain bikes, as the heavier front wheel tends to stabilize the bike, light-gauge flexy road bikes with real light front wheels, especially large size ones, tend to do it the most.
A good test for a bike's tendency to speed wobble is to ride it no-hands sitting bolt upright in the saddle, so the front end is not weighted much. if it's a wobbler, it'll do it at a lower speed than it would with you holding the handlebar. |
| Yow, bud... | Nat
Jan 17, 2002 12:55 PM | | All makes sense. Do DH bikes, with the rearward center of mass, get wobbly (I've never ridden one)?
The bike I rented was a medium (my size) with rigid fork.
I guess I've gotten the low-speed wobbles before. It was when I went to the supermarket, had forgotten I'd ridden my bike instead of driving my car, and bought six bags worth of groceries. Riding home with 20 pounds of milk, spaghetti, canned corn, RC Cola, hot italian sausage, and candy sure does ruin the handling. |
| I doubt you would feel... | shiggy
Jan 17, 2002 2:35 PM | | ...the wobbles on a DH bike given the terrain they are used on. They are constantly changing direction because of the rough surface so a wobble is hardly noticed. The short stem/riser bar makes riding drops and other technical sections easier |
| re: Speed Wobbles | czardonic
Jan 17, 2002 10:51 AM | | I'm not sure what the cause is, but if you think about it, they are a series of increasingly severe over-corrections.
A short stem is better for making quick adjustments, so they would help you going downhill on a track that required a lot of quick manuvers. On a road, most of your turns are going to be long and smooth. All that extra nimbleness is going to translate to squirrly handling under those conditions. You'd want a less responsive system, so that there is less chance of you over-correcting, and getting into a wobble situation.
I used to get speed-wobbles on my skateboard, and the only cure other than stopping (or falling) was to take a long turn that was steep enough that the trucks couldn't wobble in the other direction. The best way to avoid them was to tighten the trucks. This translates to a less responsive ride. |
| Trucks! | Nat
Jan 17, 2002 12:58 PM | | I haven't heard that term in years! I recall overtightening my trucks. Leaned into a left turn, board went straight, Nat went flat on his Husky-sized Toughskinned ass! |
| re: Speed Wobbles | drsam
Jan 17, 2002 2:21 PM | | I'm by no means a physicist, but I know that when your car tire wobbles in a certain speed range, it usually means you need to get it balanced.
Have you ever had a washing machine start shaking the house down when it goes into the spin cycle? Same thing, out of balance. You open the lid, redistribute the wet clothes more evenly, and kazam! the wobble goes away.
I would imagine that a bike wheel would obey the same laws of physics as my car wheel and my washing machine.
For what it's worth I suffered the wobble thing once on the blue ridge parkway on a schwinn I used to have at about 45mph, with a sheer several hundred foot drop-off a few feet to my right. Scared the geebers out of me!
Cheers,
Sam |
| Just had one today.... | HooKoo
Jan 17, 2002 2:36 PM | | my first one in fact, spooky eh?
I was on my beater ss on the road and cranking hard to overtake my friend on his geared bike (which I did :)). Just as I'd overtaken him I flung my feet off the pedals (for dramatic effect entirely) and when I looked down to see where my pedals were to put my feet back on the front end started to wobble.
Seemed to be caused by a number of things, including a couple mentioned in the post above:-
tight headset - I banged it in with a hammer and a piece of wood (I'm poor)
flexy fork - it's a cheapo RST
Main cause seemed to be lack of concentration though and I'm sure out of true wheels dont help.
Alan |
| It's a major problem with motorcycles ... | Philber
Jan 18, 2002 10:15 AM | | I had a Kawasaki GPZ1100 (a damn fast bike) that would develop a speed wobble at about 120 mph. At that speed, it's a bloody scary thing when when the whole front end of the bike starts quickly pitching side to side. Sure enough, if you slowed down, it would stop. Or, as was posted above, if you speed up it would stop too.
It's caused by harmonic oscillation at a certain frequency which is sympathetic to the speed of the wheel (which is really just a big gyroscope). That's why it only happens at one speed - the speed at which the rotation of the wheel invokes harmonic oscillation in the fork and steering assembly. The amount of flex in the fork will have a direct effect on the particular speed at which the steering assembly will wobble. That's why DH bikes don't develop the wobble - they're way too stiff in the front end for the wobble to occur at any speed that you could actually attain. My GPZ had pressurized air/oil forks (not much different to the SID XC's I'm running) and I could significantly increase the speed at which the wobble would develop by pumping up the shock pressure. But the tradeoff was, of course, decreased control at slower speeds.
The flex generally comes from the forks and the wheel itself. It's possible that it could come from the rest of the steering assembly (headset, stem, bars), but that's very unlikely because those parts generally have an insignificant amount of internal flex compared to the fork and the wheel itself. |
| Damn, that's too scary | Nat
Jan 18, 2002 1:14 PM | | I don't like to even think about speed wobbles at 120. Doing a superman at that speed is not so funny. |
| Scary ... but fun ... | Philber
Jan 18, 2002 3:17 PM | | I have some funny memories of the day I bought that bike. I bought it from a guy on a farm who had lost his license for drunk driving. When I went to test ride it, his dad was sitting at the table outside, whittling wood. I took it for a ride and had the speed wobble, but at that time it happened at about 100 mph. I went back and told him about it. Well the old man's jaw dropped about 4 inches. The son says "Oh yeah, sorry, the shocks musta gone flat over the winter, lemme put some air in em." He pumps 'em up and tells me to try again. I came back and said "Yup, she goes nicely up to 150 mph now, not a hint of a wobble." The old guy went white. He didn't say anything the whole time, but it was pretty obvious that he thought we were both completely insane.
I remember that test ride well, going 150 mph down highway 21 that runs along the east coast of Lake Erie. I would pass cars that were travelling at highway speed, but I'd be going 90 or 100 mph FASTER than them. I remember turning my head slightly and seeing the look of abject terror and amazement on the faces of these people I was passing. God it's great to be contrary. |
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