scary wobble on 1st decent 66kph



mareng

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Aug 1, 2012
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My first fast decent scared me senseless i was as ridged as a dead body by the time id reached the bottom my bike has been fine for the last 50km and shows no sign of damage etc.

But how is it my bike tried to throw me off my first instinct was to move my bottom further back on my saddle lift my hands onto the hoods and start to ease on the brakes, but this seemed to make it worse really don't fancy coming off at that speed.

Can anyone help me try to understand what it was or what it could be as it is something i want to control!!!!!!!
 

CAMPYBOB

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Sep 12, 2005
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Google "Speed Wobble" and "Death Wobble".

There are a variety of somewhat theoretical sources for the phenomenon. Wind. Harmonic vibrations from the road surface or obstacle run over. Rider-induced shaking. Loose bearings have been blamed (unlikely according to most). Bladed spokes and wind. There are lots of causes suggested.

The suggested reaction from every author on the subject is the same: Grip the bars loosely (I prefer the drops or hooks and most 'experts' think a tight grip exacerbates the wobble), brace one or both knees against the top tube, try to turn the pedals, attempt to take some weight off the seat, brake gently. Work to dampen the vibration.

The 'experts' say frame alignment and wheels out of plane do not affect shimmy...I'm not so sure of that. Forks with increased trail are purported to be less prone to death wobble. I've been told by folks that might know what they are talking about the a fork that has one dropout leading the other can cause shimmy.

Are you tall? Do you ride a large size, thin walled frame? This seems to be an influencing factor, but shimmy also occurs with riders of short frames.

I had a large (60 CM), thin-walled steel tube frame many years ago. The only way I could descend fast was to squeeze the top tube between my knees. I sold that bike and never regretted it. No other frame or bike I've owned has done the death wobble despite the experts saying it can happen to any bike/frame. Weird.
 

alienator

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Jun 10, 2004
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mareng said:
My first fast decent scared me senseless i was as ridged as a dead body by the time id reached the bottom my bike has been fine for the last 50km and shows no sign of damage etc. But how is it my bike tried to throw me off my first instinct was to move my bottom further back on my saddle lift my hands onto the hoods and start to ease on the brakes, but this seemed to make it worse really don't fancy coming off at that speed. Can anyone help me try to understand what it was or what it could be as it is something i want to control!!!!!!!
Well, you made it to the bottom safely, so that's good. Wobbles can definitely be enlivening, at least. Here's the bad news: all rigid bodies have a set of harmonic frequencies (vibration frequency at which the vibrations will on their own increase in amplitude.....er....force or intensity). That goes for bicycles, too. Fortunately, most of those frequencies exits outside the range of conditions in which you'll ever ride the bike. Unfortunately, you discovered one of those frequencies in your range of riding conditions. What those frequencies are depends on the bike (how it's constructed, the material, it's condition, it's symmetry...........), the rider (rider mass, bike/rider mass distribution, whether or not the rider is "loose" on the bike or "tense"), and the road (condition of the road). How do you avoid the wobbles or get them to stop? Generally they can be decreased or stopped by insuring that you don't have death grip on the bike and that you are loose and relaxed. It can also help to squeeze the top tube with your knees. Shifting your weight was a good idea, but it's possible you shifted your weight the wrong direction. It's a bad idea to apply front brakes with a wobble. It's possible to apply rear brakes lightly to slow yourself down, thus moving the bike out of the state causing the wobble. Craig Calfee has proposed that misaligned forks can cause wobble, and I can see how that would be the case. Loosing bearings and things like loose quick releases can theoretically cause a wobble, as can other mechanical issues. Given that, one of the best things you can do to avoid the wobbles is to be sure your bike is in optimum condition: bolts tight; bearings properly loaded; QR's fastened properly; wheels true, and etc. If the wobble is bad enough, beer afterward can help.
 

mareng

New Member
Aug 1, 2012
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hi guys i ride a bianchi nirone 7 c2c alu frame carbon folks 59cm I'm 6'1" tall will try the knee on the tube and try to ease my weight off the seat and some how try to grip the drops in a more relaxed fashion, and see if i can steady the wobble if this fails then i may consider a new set of wheels any recommendations max budget £200.00

thanks for your quick responses!
 

danfoz

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Apr 12, 2011
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There's a particularly long straight decent on a fav riding course that allows one to get and hold speeds of 50mph+ for some time. Little traffic, good shoulder, and the posted I believe is 45, 50 once it feeds into, and becomes the Pallisades Parkway (which is not lawful for bicycles). OP, these speeds may not apply.

I had a blast the first time down, at 50mph the bike tracked true and confident the whole way.

The second time down was a whole nother story. Wobbles and terror at anything which seemed like 40+. I was too terrified to glance at my computer at that point.

What was the difference in the two rides? First time was on my budget alu Cinelli with some cheap Mavic hoops. 2nd time was on my Tarmac SL3 superbike, with the Dura-Ace 7850 deep section carbon/alloy clinchers. It wasn't the headset (which had 2 adjustments) and I'm loosey goosey on the bike. It was either the frame, the wheels, or a combination of both. Since a number of riders had probably exceeded that speed in the recent TDF on their Tarmacs I surmised it wasn't the frame. Although now as I type I reckon it is very likely that those bikes were "team" geo, and not regular geo. That leaves the wheels - my theory, and the fella who owns my LBS figure in this case it may have been the deep section's long inner-tube valves, which at normal speeds, even pro TT speeds, wouldn't hit the threshold required for wheel-balancing, but what is the threshold for bike wheel balancing anyway? Apparently Fulcrum factor the weight of the valve in when milling their alloy rims, maybe they are onto something.

Unfortunately never got to troubleshoot further, sold the bike on ebay when I had to choose cannon fodder from the stable while I was unemployed. It was a case of the reluctant trophy wife vs. the eager butterface mistress. An easy choice, the bike that shimmies at 45mph goes. The bike that handles any input without question stays.

But like mentioned above, many things could be the culprit. And beer...
 

CAMPYBOB

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Sep 12, 2005
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but what is the threshold for bike wheel balancing anyway?

Good question. The out-of-balance mass is not much. Velocity...I've descended at a verified 60+ for minute after minute without a care other than the brakes melting my glue and the 400' drop on one side of the road and rock wall off the other shoulder. Did I ever tell you Arizona is much like Colorado in that they budget $12 of the total state expenditures for guard rail?

It was a case of the reluctant trophy wife vs. the eager butterface mistress.

I nominate this as best analogy of the week!
 

alienator

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Jun 10, 2004
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danfoz said:
There's a particularly long straight decent on a fav riding course that allows one to get and hold speeds of 50mph+ for some time. Little traffic, good shoulder, and the posted I believe is 45, 50 once it feeds into, and becomes the Pallisades Parkway (which is not lawful for bicycles). OP, these speeds may not apply. I had a blast the first time down, at 50mph the bike tracked true and confident the whole way. The second time down was a whole nother story. Wobbles and terror at anything which seemed like 40+. I was too terrified to glance at my computer at that point. What was the difference in the two rides? First time was on my budget alu Cinelli with some cheap Mavic hoops. 2nd time was on my Tarmac SL3 superbike, with the Dura-Ace 7850 deep section carbon/alloy clinchers. It wasn't the headset (which had 2 adjustments) and I'm loosey goosey on the bike. It was either the frame, the wheels, or a combination of both. Since a number of riders had probably exceeded that speed in the recent TDF on their Tarmacs I surmised it wasn't the frame. Although now as I type I reckon it is very likely that those bikes were "team" geo, and not regular geo. That leaves the wheels - my theory, and the fella who owns my LBS figure in this case it may have been the deep section's long inner-tube valves, which at normal speeds, even pro TT speeds, wouldn't hit the threshold required for wheel-balancing, but what is the threshold for bike wheel balancing anyway? Apparently Fulcrum factor the weight of the valve in when milling their alloy rims, maybe they are onto something. Unfortunately never got to troubleshoot further, sold the bike on ebay when I had to choose cannon fodder from the stable while I was unemployed. It was a case of the reluctant trophy wife vs. the eager butterface mistress. An easy choice, the bike that shimmies at 45mph goes. The bike that handles any input without question stays. But like mentioned above, many things could be the culprit. And beer...
Note that two different people likely have two different sets of harmonics in their bike/rider systems, so it's possible for a bunch of people to descend on one frame without issue and for one person to experience wobble. Of course, as I alluded, you have to consider the entire bike/rider system, not just the frame or the wheels. Together it will have a unique set of harmonics dependent on how that rider changes his position on the bike. It's difficult to determine what the threshold for wheel balance must be to initiate wobble on a descent. All that is required is the impulse from the wheel to occur with a magnitude great enough to initiate vibration at a harmonic frequency of sufficient amplitude that the vibration isn't damped so that it is prevented from the expected harmonic increase in vibration amplitude. I got to experience a lovely 45mph wobble coming down Kitt Peak (twisty, big drop off, and often the road was sandwiched between massive rock formations, leaving zero run-off in corners). My belief is that the Lew VT-1 wheels were the cause, as they weren't really close to being balanced. The best way to determine the source of a speed wobble would be to place XYZ accelerometers on all components of the bike; record their output before, during, and after the wobble episode; plot their power spectra to find vibration modes that existed and their amplitudes; then remove the common frequencies from all the plots and then plot those common frequencies on a common plot versus time to see their evolution. That'll let you see which was part was the initiator. Of course all that extra equipment--accelerometers, wires, recording device(s), batteries--would of course alter the dynamics of the bike. Still, it'd be fun, and you'd have the coolest looking bike on the mountain.
 

CAMPYBOB

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Sep 12, 2005
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I got to experience a lovely 45mph wobble coming down Kitt Peak (twisty,[COLOR= rgb(255, 0, 0)] no guard rails, big drop off, and often the road was sandwiched between massive rock formations[/COLOR], leaving zero run-off in corners).

Yup. $12.
Maximum.

My belief is that the Lew VT-1 wheels were the cause, as they weren't really close to being balanced.

How tightly strung were those Lew wheels? I've seen more bikes that had the death wobble with low spoke tension.

What frame? A real lightweight, flexible frame?

I blamed my Peugeot's death shimmy on the 531C Record (thinner than a standard 531 tubeset) tubest and the rolled/welded (seamed) fork blades. I aligned the fork tips on a granite table, but the fork was pretty flimsy.

In the 38 years since I sold that death ride, I've never experienced the shimmy. Typical descending speeds on my weekly training rides range up to 50 MPH with 45-46 MPH hit several times in some rides.
 

swampy1970

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Feb 3, 2008
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Originally Posted by CAMPYBOB .

I got to experience a lovely 45mph wobble coming down Kitt Peak (twisty,[COLOR= rgb(255, 0, 0)] no guard rails, big drop off, and often the road was sandwiched between massive rock formations[/COLOR], leaving zero run-off in corners).

Yup. $12.
Maximum.

My belief is that the Lew VT-1 wheels were the cause, as they weren't really close to being balanced.

How tightly strung were those Lew wheels? I've seen more bikes that had the death wobble with low spoke tension.

What frame? A real lightweight, flexible frame?

I blamed my Peugeot's death shimmy on the 531C Record (thinner than a standard 531 tubeset) tubest and the rolled/welded (seamed) fork blades. I aligned the fork tips on a granite table, but the fork was pretty flimsy.

In the 38 years since I sold that death ride, I've never experienced the shimmy. Typical descending speeds on my weekly training rides range up to 50 MPH with 45-46 MPH hit several times in some rides.
Reynolds never made a 531C Record tubeset. 531C was a general purpose competition double butted tubeset. By definition, the center of the butted tube was thinner than the plain jane 531 non-butted tubeset. 531 SL was the thinwall variant and had the nice red 531SL sticker.

There was a Raleigh 531C Record bike - but this wasnt made by Peugeot. :p
 

alienator

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Jun 10, 2004
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I lied about the guardrails:
1000
1000
 

CAMPYBOB

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Sep 12, 2005
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Wish I had a picture of it. It's been a lot of years ago. Early Py-10, Euro version sold out of Canada. Supposedly ex-factory, but you how sales stories are. The only SL PY-10 I found on the web was 1979 and I thought SL was discontinued by then...could be wrong there. My PY was a 1973 IIRC, despite the fact the PY designation did not officially appear until '74..

All those decals...http://www.classicrendezvous.com/British_isles/Reynolds_gallery.htm Thanks for the point out!

That bike shook like no tomorrow. The shaking was predictable. You could tell when it was getting ready to freak out.
 

CAMPYBOB

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Sep 12, 2005
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I lied about the guardrails:

I didn't!

What, did Arizona double their budget? It's been years since I've been out there and they have probably upgraded things. Do they still allow chains only at certain times and still close the pass when it really snows?

Damn! was it really 9 miles down from the summit to Jerome? I remember the climb taking awhile, but the descent was a blur.






Hitting the rock face might hurt worse than the long drop...or not!
 

swampy1970

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Feb 3, 2008
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Originally Posted by CAMPYBOB .

Wish I had a picture of it. It's been a lot of years ago. Early Py-10, Euro version sold out of Canada. Supposedly ex-factory, but you how sales stories are. The only SL PY-10 I found on the web was 1979 and I thought SL was discontinued by then...could be wrong there. My PY was a 1973 IIRC, despite the fact the PY designation did not officially appear until '74..

All those decals...http://www.classicrendezvous.com/British_isles/Reynolds_gallery.htm Thanks for the point out!

That bike shook like no tomorrow. The shaking was predictable. You could tell when it was getting ready to freak out.
A post about 531...

.... edited at 6:53.

oooooh.

653 being the tubeset that replaced the 'thinner than normal 531C' - 531SL.
 

CAMPYBOB

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Sep 12, 2005
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.... edited at 6:53.

Heheh!

I never owned the later TI tubing alloys and I particulary wanted to try a stainless frameset...and the air hardened 853. I switched to Columbus after the 531 standard tubeset 1974 Schwinn Paramount. The PY was one of only two bikes I've owned that I sold off. I specifically sold it because of the death wobble and unlike the other frameset I sold, I never regretted getting rid of the Peugeot.

OT completely...one of my neighbors has a few Rickman motorcycles that utilized 531 mang-moly tubing.