Bike shimmy / high speed wobble.

wind700

Has anyone experienced speed wobble when descending ?
Two weeks ago i crashed at 38 mph on a descent from Coldharbour to Dorking in the Surrey hills. The handle bars started to oscillate violently left and right and I couldn't control the steering. I ended up in the edge of the road in the gravel and dirt at edge of the country lane.
I not sure what caused the wobble but I had my bike serviced the previous day by a very reputable bike shop. I had the wheels that are fairly new trued but I think that maybe one of the wheels my be out of balance. How do you check wheel balance and do bike work shops actually check wheel balance.
I suffered 4 broken ribs a punctured lung and a broken collar bone.

sloppyschleckonds

wind700 wrote:

Two weeks ago i crashed at 38 mph on a descent from Coldharbour to Dorking in the Surrey hills.
I suffered 4 broken ribs a punctured lung and a broken collar bone.

Did you take the Strava KOM?

mercia_man

Unlike the previous poster, I'm sorry to hear about your crash and hope you recover soon. It must have been terrifying. I've had a few high speed wobbles on bicycles and in a previous career in which I tested motorcycles, but thankfully never came off and injured myself.

There's lots of theories about what starts shimmy or wobbles and lots of theories about how to damp down the oscillations. Putting your knee against the top tube is generally the recommended technique combined with very gentle braking. Putting your brakes on hard will make it worse.

On motorcycles, I think it was the combination of me being fairly light in weight and a bike with a not very stiff frame. On bicycles, I have experienced shimmy when I was blasted by the bow-wave of a lorry while descending at 40-plus mph and also at a similar speed when the non-drive side spoke tensions on my rear wheel had gone a bit slack. A wrongly adjusted headset can also be a cause of shimmy. I don't think balancing your wheels will make any difference to the risk of speed wobble.

The geometry of a bike can make it more prone to shimmy. For example, I once met a couple of cyclists touring the Alps with camping gear in panniers on cross bikes. They were suffering frightening shimmy on descents and decided to bypass the Izoard because they were so concerned. Their bikes were designed for racing, not descending with a big load. Steep frame angles and a short wheelbase do make a bike more twitchy and potentially shimmy-prone in certain circumstances. A proper touring bike or a mountain bike has geometry designed for stability rather than quick steering.

marcusjb

They really are about as scary as it gets - I've had two in my life and just about managed to hold on and get the bike back to manageable speeds (and even as you get down to lower speeds, the bike will still be trying to throw you off and all you can think is "where should I aim to land?").

As above, there are so many possible causes and theories on shimmy - wheels, luggage, your weight distribution, bearings, inherent frame design issues etc.

If it is a bike that you've ridden at those speeds before without issue, do think about what has changed. It could be something as simple as a badly seated tyre. You say your wheels were trued the previous day - that is possibly the cause, but hard to tell.

Check everything - make sure everything is tight. Look for cracks and frame issues.

I never really got to the bottom of my issues - but I never truly trusted the particular combination of luggage, wheels and frame again.

Heal well - that sounds a grim old crash.

DeVlaeminck

I used to get it on an old 24inch 531c Mercian. I'm pretty sure being a novice (going back years here) and fairly nervous descender at the time was the main cause but I also think the frame/bike contributed as when I got a Litespeed Ultimate I never really had the same problems.

Best thing is learn to correct it - what seems to work is knee on top tube, unweight saddle and relax - pedal through it if you can or if you are going too fast light braking (pretty much as previous posters said). If you know you can correct it you can at least enjoy descending up to a point but it is hard to get it out of your head. 20 plus years ago I had a vespa T5 that did it on a sweeping bend at about 60mph once - ended up on the other side of the road - and in the end I sold it as that one incident just stuck in my mind.

As I say though I think most of it is down to being tense as a rider - you don't tend to see the pros get it and as I became more experienced I've not really had it on any bike.

bobmcstuff

Sounds like a nightmare - no advice from me unfortunately, hope you heal up OK.

marcusjb wrote:

They really are about as scary as it gets - I've had two in my life and just about managed to hold on and get the bike back to manageable speeds (and even as you get down to lower speeds, the bike will still be trying to throw you off and all you can think is "where should I aim to land?").

As above, there are so many possible causes and theories on shimmy - wheels, luggage, your weight distribution, bearings, inherent frame design issues etc.

If it is a bike that you've ridden at those speeds before without issue, do think about what has changed. It could be something as simple as a badly seated tyre. You say your wheels were trued the previous day - that is possibly the cause, but hard to tell.

Check everything - make sure everything is tight. Look for cracks and frame issues.

I never really got to the bottom of my issues - but I never truly trusted the particular combination of luggage, wheels and frame again.

Hi Marcus - have you ever had issues like this on the Fratello? I only ask because I'm intending to do a bit of light touring on mine this weekend with a tent etc and the posts above have me slightly concerned, and I know you have the same bike

comsense

I've many years of experience with wobbles. I used to be called Shakin 'Stevens out the road.....
Firstly I can tell you for a fact that if you get your backside off the saddle it kills the wobble immediately. This dampens one end (the rear) and places your centre of gravity low to the bottom bracket.

2nd is that assuming a bike is in sound mechanical condition the fault is almost always the rider tensing, sitting up away from the bars ( natural reaction to nervousness) I personally have never witnessed or heard of a bike wobbling while a rider is pedalling and believe this is because when you get tense you tend to stop pedalling.
Wobbles on bikes that are mechanically sound almost always start when the rider gets nervous. A close pass by traffic, unexpected rough roads, or just uncomfortable speed.
The nervous rider then locks his arms. This has the effect of upsetting the front end. If you relax one arm and lock the other you will notice the bike steers towards the side of the locked arm. When you lock both arms the bike front end veers to each side in quick succession and by trying to correct this you cause more veering which quikly becomes a violent wobble. If you ride behind a beginner or nervous rider and they get uncomfortable on a descent you will notice they move backwards on the saddle and lock their arms and sit up straight. All these thing make the front end unstable. Learning to relax, get the c.o.g low, arms bent and relaxed is a big plus, holding the bars in the drops with a very relaxed (but safe) grip is the way to go.

So how do you know for certain that the fault is the rider's or a mechanical? If you can get a "demon descender" to try your bike that is one sure way. If you have to go looking for him and find him in a ditch you might want to get the bike examined by a professional

RutlandGav

Slghtly off-topic, but i've never experienced a shimmy on an adult bike - thank gawd.

As a kid though, i had a pretty big crash on my Raleigh Chopper , going down hill too fast. It just came on suddenly and threw me off the bike.

My next machine was a Raleigh Grifter and that too would shimmy. This one gave more warning so it never gave me the case of road rash that my Chopper did. At 20 mph it would start as a light quivver, which got worse if i let the speed increase. 22mph was as much as i could stand, never went above it. Just assumed all bikes did that like some kind of "sound barrier" for aircraft.

Lo and behold, I bought my first bike as an Adult aged 26, was amazed to discover that i could go as fast as i want, and that the faster i went the more stable it got. Hope kid's bikes these days are better designed....

dj58

RutlandGav wrote:

Slghtly off-topic, but i've never experienced a shimmy on an adult bike - thank gawd.

As a kid though, i had a pretty big crash on my Raleigh Chopper , going down hill too fast. It just came on suddenly and threw me off the bike.

My next machine was a Raleigh Grifter and that too would shimmy. This one gave more warning so it never gave me the case of road rash that my Chopper did. At 20 mph it would start as a light quivver, which got worse if i let the speed increase. 22mph was as much as i could stand, never went above it. Just assumed all bikes did that like some kind of "sound barrier" for aircraft.

Lo and behold, I bought my first bike as an Adult aged 26, was amazed to discover that i could go as fast as i want, and that the faster i went the more stable it got. Hope kid's bikes these days are better designed....

I had exactly the same experience on a Mk1 Raleigh Chopper, the one with the long seat. I had a mate on with me and that did not end well, we got up to 28mph before we where dumped off that bike. My mate came off better than me injury wise. Got rid of the chopper after that incident, replaced it with a Sun 5sp drop handle bar racer, much nicer to ride apart from the 27" x 1 1/4" steel rims and weinmann brake shoes in the wet.

norvernrob

comsense wrote:

I've many years of experience with wobbles. I used to be called Shakin 'Stevens out the road.....
Firstly I can tell you for a fact that if you get your backside off the saddle it kills the wobble immediately. This dampens one end (the rear) and places your centre of gravity low to the bottom bracket.

2nd is that assuming a bike is in sound mechanical condition the fault is almost always the rider tensing, sitting up away from the bars ( natural reaction to nervousness) I personally have never witnessed or heard of a bike wobbling while a rider is pedalling and believe this is because when you get tense you tend to stop pedalling.
Wobbles on bikes that are mechanically sound almost always start when the rider gets nervous. A close pass by traffic, unexpected rough roads, or just uncomfortable speed.
The nervous rider then locks his arms. This has the effect of upsetting the front end. If you relax one arm and lock the other you will notice the bike steers towards the side of the locked arm. When you lock both arms the bike front end veers to each side in quick succession and by trying to correct this you cause more veering which quikly becomes a violent wobble. If you ride behind a beginner or nervous rider and they get uncomfortable on a descent you will notice they move backwards on the saddle and lock their arms and sit up straight. All these thing make the front end unstable. Learning to relax, get the c.o.g low, arms bent and relaxed is a big plus, holding the bars in the drops with a very relaxed (but safe) grip is the way to go.

So how do you know for certain that the fault is the rider's or a mechanical? If you can get a "demon descender" to try your bike that is one sure way. If you have to go looking for him and find him in a ditch you might want to get the bike examined by a professional

This seems to make sense, a mate of mine who is a nervous descender had a huge wobble at around 35mph descending Holme Moss at the weekend. It scared him silly, probably making him even more nervous in future. He tends to sit up, brake into corners not before them, take poor lines etc. he's used to riding a heavy touring bike and seems to try and ride his road bike in a similar way.

I'll descend as fast as I can and haven't experienced a wobble yet, though I'm under no illusions it could happen!

t4tomo

Choppers were inherently unstable at any speed, you either wheelied over the back of the silly small front wheel threw you off going downhill at speed.

I had a Raleigh commando, which was less cool but much better to ride than my cousins chopper.

mercia_man

Confidence on the bike and using specific techniques such as touching the top tube with your knee and getting your weight forward are tried and tested ways of deaIing with instability but I think it's wrong to suggest that nervous descending is the root cause of wobble or shimmy.

Many very experienced riders have suffered from shimmy. It's been the subject of many scientific studies and there are conflicting views. It seems to be set off by some sort of outside influence, such as a bump in the road surface, and then the oscillations get bigger and bigger. It seems to be a combination of circumstances. The position of the rider on the bike, frame geometry, lack of stiffness in top tubes and wheels, and positioning of luggage all appear to be factors than can help trigger those terrifying wobbles.

I know from my own experience touring with heavy panniers front and rear how just a little bit of extra weight can trigger a weaving effect at quite low speeds (the same speed every time), but once you get over the wobble speed, it eases off. And reducing the load by just a few kg eradicates it totally. And lighter riders seem to be affected more than heavier ones.

Shimmy affects motorcycles (it's been a factor in the deaths of highly skilled police motorcyclists, GP bike racers etc), aircraft landing wheels and even supermarket trollies.

robbo2011

DeVlaeminck wrote:

As I say though I think most of it is down to being tense as a rider - you don't tend to see the pros get it and as I became more experienced I've not really had it on any bike.

This.

I had a minor speed wobble just once, when descending off the Furkapass in bitterly cold and windy conditions (in July!) i was so cold that my body was rigid and not relaxed like normal. I managed to keep it under control though. It has never happened any other time and I do rapid descents on virtually every ride. Must have been because I was so tense due to the cold.

HebdenBiker

I sometimes get this around the 40mph mark. Flex in the forks/wheels is almost certainly involved.

I force myself to relax my shoulders, arms and grip. It goes away.

debeli

I feel for the OP; most of us have been in that terrifying moment of absolute certainty that a disorderly dismount at speed is imminent - it is enough to freeze the heart.

I'm glad you came through it. It is rare; very rare. The possible causes are almost without number, but some are more likely than others.

I do not doubt the excellence of the LBS where you had the machine serviced, but I smell a link between the service and the almost immediate tumble. How?

A common cause of speed wobble in bicycles that have been well maintained and have straight frames, forks and wheels is a wheel that is ever so slightly not straight in the drop-outs.

Many people remove and replace wheels either with the machine inverted or in a clamp. Thus, when the wheels are replaced they are lifted up into position. It is then possible to lock the QR with the axle not quite pressing evenly across the drop-outs. You may think this trivial - it is not.

You may think that you'd notice at once or that the brakes would rub... not necessarily so.

When you've had the wheels off, put the weight on the tyres, release the QRs, bear down on the bike and re-tighten them. That way, you know the axle is shoved nicely up into the drop-outs on both sides.

That may not be the cause in your case, but if I were a gambling man I'd put my 15p on that. Maybe all 50p.

mercia_man

There's an informative article about shimmy by Chris Juden, the CTC's former technical officer, who did a detailed study of it over many years. (I can't link to it but it's easily found by googling Chris Juden and shimmy).

He says that "when a bike shimmies, the frame and fork twist so the front wheel goes out of track on alternate sides relative to the rear wheel while the steering turns left and right to accommodate these excursions. Shimmy is a resonant vibration driven by the forward motion of the bike and damped by the inertia and friction of the wobbling wheel - that's tyre friction and aerodynamic drag."

Juden says you should "imagine a weight bouncing from the end of a spring - with someone waggling the top of the spring so the bounces get bigger."

He says every bike has a natural shimmy frequency and speed. The key is torsional stiffness and steering geometry - that is a stiff frame and lots of trail. Such bikes don't shimmy because their natural shimmy speed is faster than the bike would ever go. At most risk of shimmy, Juden says, are lightweight racing, audax and small wheeled bikes. Touring bikes with heavier, stiffer frames, combined with heavier, stiffer wheels to damp resonations, are less likely to shimmy but will do it if they are overladen.

He says shimmy usually starts from something like a gust of wind and can be exacerbated by something as small as a computer spoke magnet. He advises getting out of the saddle, clamping the top tube between your knees and gradually braking. Interesting stuff.

tomisitt

Some interesting stuff in the latest (June) issue of Cycling Active on the subject, too.

g00se

Mercia Man wrote:

At most risk of shimmy, Juden says, are anything with new-fangled materials and measured up in that metric system. It never happened in the good old days. Touring bikes with dynamos, ridiculously sized saddles bags and combined with toe clips and proper leather straps, are less likely to shimmy but will do it if they are overladen with any more self-importance - if that's possible.

Fixed his quote for you...

sloppyschleckonds

g00se wrote:

Mercia Man wrote:

At most risk of shimmy, Juden says, are anything with new-fangled materials and measured up in that metric system. It never happened in the good old days. Touring bikes with dynamos, ridiculously sized saddles bags and combined with toe clips and proper leather straps, are less likely to shimmy but will do it if they are overladen with any more self-importance - if that's possible.

Fixed his quote for you...

mercia_man

g00se wrote:

Mercia Man wrote:

At most risk of shimmy, Juden says, are anything with new-fangled materials and measured up in that metric system. It never happened in the good old days. Touring bikes with dynamos, ridiculously sized saddles bags and combined with toe clips and proper leather straps, are less likely to shimmy but will do it if they are overladen with any more self-importance - if that's possible.

Fixed his quote for you...

marcusjb

Mercia Man wrote:

He says shimmy usually starts from something like a gust of wind

I can go with that - both of mine have been whilst fighting strong crosswinds (and I can also go with lighter riders having a tougher time with shimmy being a sub 60Kg rider). Neither at particularly high speeds and on a bike that has regularly been, before and after, ridden on 80kph descents without issues (BobMcStuff - yes both times it was on my Fratello).

The first one in particular scared the crap out of me as I'd never experienced it before - it was up near Hartside in horrific weather (it wasn't raining at that point, but that was soon to come!).

I hope to never have one again - but having a better understanding of how to get out of them certainly helped me on the second one, I didn't panic and managed to get it sorted reasonably quickly. Still shakes you up though!

mercia_man

marcusjb wrote:

Mercia Man wrote:

He says shimmy usually starts from something like a gust of wind

I can go with that - both of mine have been whilst fighting strong crosswinds (and I can also go with lighter riders having a tougher time with shimmy being a sub 60Kg rider). Neither at particularly high speeds and on a bike that has regularly been, before and after, ridden on 80kph descents without issues (BobMcStuff - yes both times it was on my Fratello).

The first one in particular scared the crap out of me as I'd never experienced it before - it was up near Hartside in horrific weather (it wasn't raining at that point, but that was soon to come!).

I hope to never have one again - but having a better understanding of how to get out of them certainly helped me on the second one, I didn't panic and managed to get it sorted reasonably quickly. Still shakes you up though!

I had always thought lighter riders like me suffered most, but Juden says the opposite. Most at risk, he suggests, are overweight mamils on bikes where ultra light weight has been gained at the expense of torsional stiffness in frame and forks.

I've only had serious shimmy twice, the worst when I was hit by a strong gust of wind from a lorry going the other way as I was descending quickly. I was on a lightweight aluminium-framed Principia RS6 Pro road bike which had previously been totally stable and confidence-inspiring on many mountain descents including the Maratona dles Dolomites.

bobmcstuff

marcusjb wrote:

(BobMcStuff - yes both times it was on my Fratello).

So what combination of luggage was that? I'll be taking mine up the West Coast of Scotland this weekend with panniers and a bar bag, camping so will include a tent on the back... And since it's Scotland it'll definitely be windy!

Maybe I'll take it easy on the descents.

Smokin Joe

The only time I had a bike shimmy was, believe it or not at about 10mph. I'd just pulled away from my house and missed getting my foot clipped in when the front started to shake violently from side to side like it had a hinge on the forks. It was a frightening experience even at that speed but I managed to stop and get my foot down. That was on a Raleigh Dynatec, not the stiffest frame ever built but I owned the bike for nearly ten years and it never did it any other time even on looney speed descents.

marcusjb

bobmcstuff wrote:

marcusjb wrote:

(BobMcStuff - yes both times it was on my Fratello).

So what combination of luggage was that? I'll be taking mine up the West Coast of Scotland this weekend with panniers and a bar bag, camping so will include a tent on the back... And since it's Scotland it'll definitely be windy!

Maybe I'll take it easy on the descents.

Don't worry about it - it's just one of those things.

The bike is supremely stable - it's been at well over 90kph loaded in the mountains. I've done at least 30000km on it and it's done it twice.

I have it's matching Tempo as well - never had an issue (but I guess a fixed is harder to shimmy as you're never able to really let it go on the descents (I'm done by about 55kph on a 67" gear - about 150 cadence is more than enough for my old legs!)).

The problem with shimmy - it could be almost anything on any machine that introduces it. You just have to hope you never experience it and trust that you know what to do if you do.

So rag the *&^% out of the bike!

meanredspider

Only had it once and, had I realised that I was doing close to 60mph at the time, I may not have been quite so calm in my reaction to it. Frustratingly, the GoPro footage I had was lost when, later in the ride the GoPro mount snapped and the camera disintegrated on the road behind me. Picked up all the pieces but forgot the memory card :roll:

Yes, I did get the KoM (though someone has since gone even faster)

To the OP: hope you recover quickly and are soon back on the bike - there but for the grace of god etc etc

bobmcstuff

marcusjb wrote:

I've done at least 30000km on it and it's done it twice.

:shock: That is quite the distance.

I'll not worry then. More worried about the 40mph+ gusts that are forecast for Sunday!

marcusjb

bobmcstuff wrote:

marcusjb wrote:

I've done at least 30000km on it and it's done it twice.

:shock: That is quite the distance.

I'll not worry then. More worried about the 40mph+ gusts that are forecast for Sunday!

It is 4 years old!

I've not ridden the Fratello since about October - been on the Tempo since then. Not in a rush to get back on the Fratello, enjoying the challenge of fixed too much.

imatfaal

Sheldon Brown has an article with Jobst's explanation
http://sheldonbrown.com/brandt/shimmy.html

hazzy87

Sorry to hear you come off.
I do think the the whole bike frame/ wheels not inline and buckled is a load of sh*t. When i was a kid i would ride down any hill on a knackered bike with buckled wheels and a shoddy frame.

I do believe it is all to do with confidence. Two years ago i got very cold on a descent on the Exmoor beast sportive and had a speed wobble at approximately 35mph. It scared the crap out of me and left me dragging the brakes and free wheeling the rest.
over the past six months its been getting worse at slower speeds and i think its down to being relaxed on the bike. Im now at the point where i dont know how to overcome the fear.
All could offer for advice is to keep as warm as possible to prevent shivering which should reduce wobble.
Hope you are on the mend!

bobmcstuff

marcusjb wrote:

bobmcstuff wrote:

marcusjb wrote:

I've done at least 30000km on it and it's done it twice.

:shock: That is quite the distance.

I'll not worry then. More worried about the 40mph+ gusts that are forecast for Sunday!

It is 4 years old!

I've not ridden the Fratello since about October - been on the Tempo since then. Not in a rush to get back on the Fratello, enjoying the challenge of fixed too much.

That's still 7,500km per year on it!

Anyway I didn't really have any issues with shimmy, but I did have issues riding for 25km directly into a 45-50mph headwind on Sunday - there was one point where I was going down a 10% gradient in my lowest gear and I STILL had to get off and push (and could barely keep the bike in my hands). Then the other 105km of Sunday was mainly crosswind which was more like wrestling with a bike for 6 hours. Character building.