LesterOfPuppets

Could start riding in leathers I guess. You can't always come back from 'em

LesterOfPuppets

Scariest ones I've ever had were on skateboards. Luckily we didn't have video back then

Duragrouch

Gimme a sec to reach for my big can opener to open this industrial-size can of worms...

Let's start simple. All bikes (except experimental ones probing this subject) have trail/caster; Caster is the perpindicular distance from the steering axis to the center of the tire/road contact point. Trail is the similar measurement from the steering axis/road intersection to the center of the tire/road contact point, parallel with the road. Both pretty much mean the same thing. Henceforth I will mostly use the term trail, except when talking about caster angle on cars, which is analogous to the steering axis (head tube) angle on bikes.

The reason for trail is to have some centering effect on the steering when the bike is moving, i.e., stability, even so much as to be able to ride no-handed. With most bike geometry, having the fork straight and inline with the steering axis, results in too much trail, this can make the steering feel heavy, and also wheel-flop, where, at rest, weight on the bike causes the steering to flop one way or the other, unstable. So, the fork is either curved forward, or kept straight but inclined forward, to reduce the trail to a reasonable amount, which on most bikes is about 40-70mm.

A longer trail provides more steering centering force at speed, generally a good idea, unless excessive. Let me digress for a moment:

The Chevy Monte Carlo, I think second generation so 1973 I believe, GM's goal was for it to behave on the highway, especially high speed interstates, like Mercedes-Benz sedans, designed for autobahn speeds. 'Benzes had a lot of caster angle (for a car) so long trail. GM copied this caster spec for the Monte Carlo. And, the Monte's larger tire diameter meant that for the same caster angle, the trail value was longer, so a longer moment arm from the steering axis, that was the first mistake. The second problem was, the steered mass of the suspension (knuckles, brakes, wheels, tires, steering linkage) of the Monte weighed a ton compared to the carefully designed and svelte 'Benz components. Third, the front-end mass of the whole car, was much heavier than the 'Benz, so once moving, acts as an additional forcing imput. A freeway speeds, you turned the wheel a bit on the 'Benz and it recentered itself positively, with little to no overshoot. The same move on the Monte (as originally designed), hands off the wheel, the heavier steered mass and longer moment arm and heavier car mass, gave the correction more momentum, and it overshot center and went the opposite way, further than original steering input, then went the other way in an even greater overshoot, etc, and you had "divergent oscillation", the car, um, "departing from controlled flight", to borrow an aviation term. At this point the design was fairly sealed, expensive tooling was already complete, so no time to redesign, so GM added a damper, essentially a shock absorber with equal forces in compression and extension, mounted horizontally under the front end and connected to the lateral steering linkage, and this was sufficient to "critically damp" the oscillation to achieve steering centering without overshoot. So regarding that copied caster value, to quote my father, "A little knowledge is a dangerous thing."

This problem became further complicated with the debut of front-wheel-drive cars, so under engine load, the forces on the steering could get reversed, the tire contact patch, behind the steering axis, but being pulled *forward*, unstable, just like backing up a car or bicycle with positive trail. And, they hadn't mastered torque-steer, caused by "spindle offset", the horizontal distance from wheel center at wheel spindle height, to the steering axis. So the same thing happened on cars with much smaller and lighter steered mass and smaller trail. Some added steering dampers, some solved with better suspension design to eliminate spindle offset.

How're we doing? Are you with me so far?

Getting back to bicycles...

So... steering oscillation on a bicycle is a complex matter, involving trail, steered mass and polar moment (not just mass but how far the mass is offset from the steering axis), and, an additional complicator, that the bike and steered wheel can tilt left and right much more than on a car, so additional forces like camber thrust come into play, as well as the tilting inertia of the bike, cargo, and rider, and the rigidity/elasticity of the rider with the bike.

Trail can be a good thing, a stabilizing effect, if not excessive.

Steered mass and polar moment can be a good thing, a stabilizing effect, if not excessive. Notably, small-wheel bikes, less gyroscopic inertia due to smaller wheels so less steering stability due to that (more "agile/twitchy" steering depending on your personal preferance), can benefit from an increase in steered mass and polar moment, to help "calm" the steering, this is known as a "mass damper", which exist in many forms. The location of that mass, forward or behind the steering axis, also matters a lot.

(to be continued, I want to post the above before lost)

It's very late, I'm tired. I will try to continue this tomorrow or the next day.

cyclomath

It used to happen with my (now long gone) touring bicycle when full front panniers were attached AND I held the butterfly handlebars at the sides (parts that run parallel to the wheels and frame). It would take maybe three to four seconds for vibrations to become dangerously strong and very unpleasant, and all it took to stop the vibrations was moving at least one hand to a different spot on the handlebars.
Panniers had to be full and closed tight, because any movement inside them would dampen the oscillations and prevent them from becoming too strong.

Duragrouch

Moving your hands to different position and orientation, changed the rigidity ("spring") and polar moment (mass position), of the steering "system" which includes your arms.

707s had a nasty tendency of dutch roll oscillations if the yaw damper (directional autopilot) was out of calibration. Early pilots struggled to control it for terrifying minutes. Rapid, small but sharp aileron inputs would upset the natural frequency of the oscillations and the plane would straighten out. Like if you have a swinging pendulum on a wire, grabbing the wire midpoint and rapid small shakes in the direction of the oscillations, and it breaks the natural frequency and the oscillations die out.

Damp. Not dampen. One is dissipating energy, one is moisture.

"Chino is a style, khaki is a color." - Jamie Buchman (Helen Hunt), Mad About You, 1990s.

cyclomath

English is not my native language so I definitely don't want and intend to argue, but I had to check this because I have used "dampen" in that context many times over many years.... which doesn't make it any more correct, of course, and could mean that I made the same mistake many times over many years. So I went to the all knowing internet, and it told me this (so it must be true):

to check or diminish the activity or vigor of : deaden
to become deadened or depressed
dampen someone's enthusiasm
https://www.merriam-webster.com/dictionary/dampen
https://dictionary.cambridge.org/us/...english/dampen

unterhausen

I have a bike where that has happened twice. I learned not to eat ice cream at the top of a mountain with that bike.

Iride01 

Just curious LarrySellerz , Does the bike you get this on have a much longer head tube compared to the others? I've had death wobble when going at high speed down hills. The bikes I experienced it on all had very long head tubes. They were also steel bikes from BITD. Haven't had it on the few steel bikes I've had from BITD that had shorter head tubes.

The resonance you talk about does seem to be in the front doesn't it? If you are talking about the resonance you get at higher speeds and higher power into the pedals when in the 12 or 11 sprocket, then I get that too sometimes on most any bike. However that is never perceived by me as dangerous. Just a interesting feeling that lets me know I'm in the "zone".

rosefarts

You need to dampen the resonance. I suggest filling your bike with polyurethane foam.

asgelle

So many words, so much wrong. Read up on Hopf bifurcation. https://silca.cc/blogs/marginalgains...marginal-gains or if you prefer https://en.wikipedia.org/wiki/Hopf_bifurcation

mtnbud

I had it happen to me when I had a loose headset. Tightening the headset fixed it.

merlinextraligh

Jobst Brant pretty much debunked any need to balance bicycle tires. https://yarchive.net/bike/wheel_balancing.html

I think many people intuit a need to balance bicycle tires from the fact that car tires need balanced. However cars with 40 plus times the mass of a bike and rider are subject to much different forces.

I have wheels that obviously were out of balanced and wobbled very noticeably if you turned the bike upside down and spun the tires. However, the effect was imperceptible when riding and didn’t lead to a speed wobble. As Jobst pointed out we don’t ride our bikes turned upside down or in a work stand.

13ollocks

I think what you're describing is chordal action - a vibration or "thrum" you get through the drivetrain when pushing a small (11t or 12t) sprocket - not dangerous

Iride01 

I know that. Did I give you the impression I thought it was dangerous?

But perhaps you are just using my comments to bolster the posting of the link. (which I've seen many times)

merlinextraligh

Few more thoughts on tire balancing: 1) all wheel/ tire combos come from the manufacturer out of balance due to the weight of the valve stem, yet we don’t see all new bikes wobbling around (and good forbid new bikes with wheel reflectors); 2) if you’re riding tubeless you would need to constantly rebalance due to shifting and uneven distribution of sealant sticking inside the tire, and 3) if you’re balancing by adding weight, there is going to be a small efficiency disadvantage.

A quick search reveals no actual data to support a benefit from wheel balancing. With power meters being ubiquitous, any efficiency advantage would be pretty easy to show. A reduction in vibration might be a bit more difficult to measure but could be done with an accelerometer. Yet I see no such data.

So if you want to balance your wheel by placing the speed magnet opposite the valve stem, knock yourself out. But if you’re doing it by adding weight realize there is a price in efficiency albeit small, for no advantage that has been documented or can be perceived while actually riding by the vast majority of riders.

Viich

You joke, but I've definitely stopped a shudder by putting a leg against the top tube. My Immediate Action drill for it. Usually works.

MoAlpha

Yup, a major factor in my hesitancy to do long rides in cold weather. Shivering is in the 8-11 Hz range, the activity is synchronized across muscles, and the bike always seems to find a nice harmonic for its death dance. Just terrifying.

GamblerGORD53

My tour bike sure has a shimmy with the full load on the back, so no riding without a hand on the bar. Doesn't stop me from doing anything else though. Maybe the head angle is too slack, 70/ 70d.
And unloaded doing my downhill speed runs to 45 mph, it has a weird hum from the front somewhere at about 22 to 33 mph, tires likely.
With my old Raleighs I would use my knees against the TT the few times they went fast.

Rick_D

Maybe a variation on what you're experiencing, but I have a bike with a shock-absorbing stem that amplifies a paving irregularity as I descend a nearby bike-pedestrian bridge. First time I experienced this it nearly had me crashing into the bollard at the end of the ramp. Very sketchy.

I feel the undulation on other bikes but they don't amplify it. Learned to use back brake only when slowing my descent and that lets the stem travel but not transmit as much vibration to the entire bike. The whole thing is very weird, and site-specific.

Have experienced oddities on long descents with other bikes and just had to cope in real time, usually getting off the saddle and moving my weight back. Sometimes, rural roads will have periodic irregularities that a road bike will resonate to. Exhausting to ride. IDK how folks race cobbles.

Motorcycles with bad suspensions and whippy frames are a whole other level of "fun." Some were even given a shock to stabilize the steering, which to me was always a sign to "stay away."

79pmooney

Minor point but many rims are made with that counterweight. The inside sleeve that spans the joint. Not on welded rims but there on many "fitted" rims.

Duragrouch

This is known in the auto industry as "ride hop". Some heavy trucks with long frames have enough longitudinal bending flex (with no damping of that) for this to happen on some roads with a regular hop period at certain speeds, like about 50 mph. Some L.A. freeways had/have a nasty ride hop due to slab spacing and some curvature, enough that car makers, when doing final tuning on the suspension dampers, would spend a week in tuning in L.A. on that. I was there on business, they gave me for rental the first model Ford Escape, a combination of the short wheelbase, short overhangs, and suspension springing and damping, and the ride hop was *awful*. End of week I needed to drive to SF and back, I said give me anything else, they had a Taurus, which I happened to know was tuned for ride hop. Rode fine, easy drive.

Duragrouch

My 20" folder on fast descents, I either need to sit, or if standing for more air drag, clamp thighs around seat, to prevent speed wobble.

Duragrouch

Yes, dampen is very often used in those contexts, such as to dampen enthusiasm, analogous to "raining on your parade", drowning a good idea, extinguishing a fire (or idea) with water.

Similarly, dampener is something that can make something wet or quell enthusiasm like noted above, whereas damper is a device to reduce motion or energy, or flow, like a furnace damper.

No worries. I was one of those people who referred to casual cotton twill slacks as "khakis", until I heard the enlightening quote I mentioned. I hope you took my comment in a friendly way. But I can definitely understand issues with language translation; Though I have an ear for accents, I have a terrible mind for vocabulary in a foreign language, that has prevented my achieving any level of fluency in any of them.

And listen, the USA has had leaders in government who pronounced "nuclear" (new-clee-ur) as "nucular". So I'm very impressed with your knowledge and command of english, compared to my zero knowledge of your language (from your profile, Serbian?).

13ollocks

?? If you knew what it was, why didn’t you just say so, instead of the song and dance about that vibration at higher speeds and higher power into the pedals when in the 12 or 11 sprocket, that you get sometimes etc etc? FFS 🙄

downtube42

My 60cm steel Soma Fog Cutter rando build wobbles if ridden no-hands, somewhere around 15 mph. I don't usually ride no-hands but sometimes zipping a jacket or peeling a banana happens. A knee on the TT takes care of it. Different tires maybe changes the speed, but it seems a permanent feature of the bike.

I love how the FC looks and rides aside from the wobble.

This compelled me to buy a new rando bike, so it's not entirely a bad thing