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Speed wobble when going fast

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Speed wobble when going fast

Old 08-04-17, 01:53 AM
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spectastic
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Speed wobble when going fast

I was trying to take a pic today of the sunset, going down hill at 30 mph or something like that, and with no hands on the bars, the front of my bike started to wobble back and forth pretty good. my body was stabilizing the rear wheel, and i went nowhere, but that wobble was kind of surprising, and I never noticed it before.

I'm a 6' - 6'1" guy on a 56 cm caad9, 15.5 cm head tube, with a slammed -17 deg 140 mm stem. pretty short wheel base.

Does this wobble happen because of the short wheel base?
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Old 08-04-17, 02:45 AM
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get some weight over the front wheel leaning back with no hands at 50kph is asking for trouble.
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Old 08-04-17, 04:48 AM
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Pressing your knee against the top tube often helps stabilize the bike, but the sunset pic is best made from a stop. Having a spectacular pic to show from your hospital bed or during your funeral retrospective is not great compensation.
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Old 08-04-17, 05:55 AM
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140 stem= clown bike. The wobble is designed in for the amusement of spectators. Be sure your AGVA dues are current, for insurance.
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Old 08-04-17, 09:26 AM
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Originally Posted by spectastic View Post
going down hill at 30 mph or something like that, and with no hands on the bars,
Not wise.

High speeds are fun. Riding no-handed is fine. Combining the two is a recipe for medical bills.

Last edited by SquidPuppet; 08-04-17 at 09:34 AM.
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Old 08-04-17, 09:32 AM
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It's your bike's way of telling you to stop being stupid.
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Old 08-04-17, 09:32 AM
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received

now, is anyone going to answer the question?
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Old 08-04-17, 09:32 AM
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Originally Posted by spectastic View Post
received

now, is anyone going to answer the question?
It happened because there was not sufficient weight on the front of the bike for the speed you were traveling.
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Old 08-04-17, 09:56 AM
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Originally Posted by spectastic View Post
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now, is anyone going to answer the question?
It's caused by a combination of factors and not specific to short wheel bases alone, although a short wheel base can exacerbate it.

Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. The restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure. The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component.[2] While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.[5]
Since shimmy frequency is independent of bike speed, gyroscopic effects "are clearly not essential to the phenomenon."[2] The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire.[3][6]
An academic paper that investigated wobble through physical experimentation and computer modeling concludes: "the influence on wobble mode of front tire characteristics, front frame inertia and chassis stiffness were shown. In particular, it shows that [by] increasing front tire inflation, chassis stiffness, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability.
https://en.wikipedia.org/wiki/Speed_wobble

Or read this...

https://www.sheldonbrown.com/brandt/shimmy.html

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Old 08-04-17, 10:01 AM
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speed wobble when going fast
I have had Speed Wobbles going 54 mph. It ended well, Still had to go to the Hospital to remove my saddle from my bum. It was very unnerving to have your front end dancing all over the place at that speed or any speed for the matter of fact.
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Old 08-04-17, 10:04 AM
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...it can be fatal if left uncontrolled.
i.e. hands not on the handlebar.

Dan
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Old 08-04-17, 01:03 PM
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Originally Posted by spectastic View Post
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now, is anyone going to answer the question?
The wobble happened because your hands weren't on the bars. Maybe if you installed low riding panniers on your front wheel it might slow the wobble down and increase wind resistance so you can't descend as quickly.
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Old 08-04-17, 03:42 PM
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I've seen wobble cured by tightening the front Q/R and another time by adjusting the headset. These were folks I was riding with, I've not experienced it first hand. It appears there may be multiple possible causes.
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Old 08-04-17, 04:10 PM
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Originally Posted by shelbyfv View Post
I've seen wobble cured by tightening the front Q/R and another time by adjusting the headset. These were folks I was riding with, I've not experienced it first hand. It appears there may be multiple possible causes.

Shimmy is not related to frame alignment or loose bearings, as is often claimed. Shimmy results from dynamics of front wheel rotation, mass of the handlebars, elasticity of the frame, and where the rider contacts the bicycle. Both perfectly aligned bicycles and ones with wheels out of plane to one another shimmy nearly equally well. It is as likely with properly adjusted bearings as loose ones. The idea that shimmy is caused by loose head bearings or frame misalignment seems to have established currency by repetition, although there is no evidence to link these defects with shimmy.
https://www.sheldonbrown.com/brandt/shimmy.html
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Old 08-04-17, 11:40 PM
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Originally Posted by cny-bikeman View Post
Pressing your knee against the top tube often helps stabilize the bike, but the sunset pic is best made from a stop. Having a spectacular pic to show from your hospital bed or during your funeral retrospective is not great compensation.
Yea, but you're not going to find a better backdrop during the ceremony. It'd have to be closed casket though. You win some you lose some I guess...
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Old 08-14-17, 02:40 AM
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I tried the same descent with my cross/gravel bike, no wobble without hands. wider wheelbase, stiffer frame, wheel is wider, so more lateral stiffness all around, and steering is generally better. I'm guessing it's the wheel/tire that's the difference. the wheels on the caad9 are basic aluminum box rims with 25 mm tires, which isn't really the best handling combination.

also, if you want to be funny or slick, do a little better than making jokes about hospital bills and caskets, or better yet, go read a book. i know it's the internet, but you don't gotta be a prick.
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Old 08-14-17, 04:12 AM
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It's the frame not the wheels
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Old 08-14-17, 06:24 AM
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Is a particular style of frame geometry better for no hands riding?
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Old 08-14-17, 11:56 AM
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I have read from a guy who has extensive experience with this type of stuff say that it could be as simple as tire swap. even a bump in the tire, or slight untrue wheels can cause oscillations.

I think my caad9 is plenty stiff for a road bike. I mean sure the caadx is even stiffer, but I feel like the bigger difference between the bikes are the wheels. the cross wheels i have on the caadx are just solid. they're wider, so tires don't flex as much. the spokes are stiffer too i think. on the other hand, my caad9 has traditional double walled alu box rims. when i corner in a race on them, I can feel the flex. If I were to guess, I could make the wobble go away if I switch to proper carbon clinchers, which i won't do because they're expensive AF.

it could be the frame. they have slightly different geometries.. maybe my friend will let me borrow his wheels for a day to put on the caad9 to find out.

Last edited by spectastic; 08-14-17 at 12:01 PM.
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Old 08-14-17, 04:16 PM
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I had this issue with a front mudflap. I cured it by not taking my hands off the handlebars.

I was discussing it one day with my dad who is an audax rider who said he was having the same issue.
For us we solved it by removing the wide mudflap and replacing it with a slim one - the issue was that the wind was catching the flap on each side and blowing the wheel back the other way, at which point it would rinse and repeat.
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Old 08-14-17, 07:34 PM
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IMO no hands on bars certainly a factor but I wonder if the weight shift alone was a big factor. It would be interesting if you could find a friend with gorilla arms to to sit up like that while keeping hands on the bars. I don't think 30 mph is *that* fast, so I won't chastise you.

I have a bike with steep seat tube and short chain stays that is by far my most "squirrelly" bike - I can't ride it 5 feet without hands but haven't had a shimmy in any high speed descents whereas another of my bikes has had that occasional issue which I counter by a knee on top tube and a lighter touch on the bars.
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Old 08-14-17, 09:07 PM
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I've answered my phone, pee'd, and attempted to work a knot out of my shoulders at that speed. I'm not going to lecture you.

I've only had the speed wobble on two bikes. Both times I had my hands firmly on the bar and both times the only cure was to slow way down.

Once was on a Felt B2 with Hed 3 trispoke front wheel with a side wind. This only happened once, the wind must have been at the perfect angle.

The other was the one that really upset me. On my "dream bike". Brand new Litespeed Vortex with all the fancy shi(f)t. I couldn't correct it at speed and had to inch my way down the descents. I bought it to race the crap out of it. It was utterly useless unless the race had no real descents and ended on top of a hill. I could have raced a $1500 Motobecane faster than that $8000 POS.

In my case, I think it was too many lightweight components, each with different flex properties. All these differences begin fighting with each other at certain speeds. A little heavier bike with good material properties is worth it to me. I'm currently on a Columbus steel frame with Columbus steel fork. It descends like you're on rails, smooth and fast rails.

I sort of think the bike is too small for you? I'm 5'8", 5'9" after a good night sleep, and rode 56cm most of my life due to availability but usually went 55cm. So if you're 6'2 on the same frame as I'd be comfortable (if a little elongated) on, it seems like it has to be too big for you. Do you have access to a friends 58-59cm to preview? Unless your Caad9 is dented, you can almost certainly sell the frame on ebay for no loss on the frame swap.

Last edited by rosefarts; 08-14-17 at 09:17 PM.
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Old 08-14-17, 09:25 PM
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^^ you mean frame might be too small? it's possible. riders my height usually go for 58, not 56. But the bikes built these days all tend to have tall head tubes, which puts me more upright than I'd like. I ride competitively, and my ideal position is to have the back parallel with the ground. To do that, i need to go a size smaller, and compensate the reach with a longer stem. not everyone does this, but if I were to pick between getting lower (while remaining reasonably comfortable), and spending $1000's on aero equipment, I'll choose the free option. besides, I ride a 56 on the caadx as well. as I mentioned, the difference in geometry between the two bikes (one that wobbles and one that doesn't) is pretty small. idk. I'm not too worried about it at this point. i don't normally take my hands off the bars like that anyway.
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Old 08-14-17, 09:30 PM
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stiffer top tube in the next frame .. alignment spot on..
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Old 08-14-17, 09:36 PM
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As long as it only happens with your hands off the bars, I'm with you, I wouldn't worry.

BTW, the steel bike I'm constantly raving about is semicompact, 53 standover, 56 toptube, as I've said so many times and will say again, is the most stable bike I've ridden. So would a 55 seattube and 58 toptube bike be a worthwhile investment? Dunno, and those won't be easy to test drive.
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