I'm scared of my Bianchi!

faffer

I just did a training outing and broke 40mph on my Bianchi for the first time (i'm pretty new to road biking), halfway down the hill the front wheel just started snaking (or at least that's what i'm calling it), basically it was wiggling from side to side uncontrollably, if I braked it made it worse - even if I used the back brakes! I was just getting faster and faster and the wiggling was getting worse and worse and the tarmac was looking more and more like it wanted to become part of my body! I held it through sheer strength/utter luck and forunately arrived at the bottom in one piece. Nothing on the bike was loose (apart from my bowels) and as far as I can see there's nothing 'wrong' with the bike.

It shook me up and now I feel I can't trust the bike, on the ride I did today I got to about 35 and it started doing it again but I managed to catch it in time. Have others had this? Are certain bikes known to be bad for it? Surely manufacturers shouldn't be selling bikes that scare the living cr@p out of people - me in particular!?

feel

Is the bike new ? if so take it back to the shop and explain what has been happening - there are various things they can check.

andrewjoseph

Is the headset loose? I don't mean the actual stem, but the headset bearings. Is there play at the headseat bearings, top or bottom?

S2driveruk

sounds wierd...had my Bianchi up to about 30ish today and rock solid. Something on yours is definately not right, get it professionally checked over if nothing obvious is bent. broken or has movement that it shouldn't in a direction it shouldnt be able to move in

Hope you get it sorted, being on or in anything you don't trust at speed is definately not good!

David

nicklouse

i would have a read of this

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

arankapila

You have suffered from what is known as speed wobble. Without getting too technical, it is where the vibrations from the road match the natural resonant frequency of your bike. When you reach a certain speed the materials in your frame will start to vibrate and flex- hence the side to side movement you described.

On my Trek Madone it appears around 45mph, all bikes are different. There are a few ways of preventing it. Either acclerate through the bad 'spot' or, another method is to clench the top tube between your thighs which effectively acts as a damper on the bike and thus changing its resonant frequency and hopefully preventing the wobble.

Hope that helps.

Lagavulin

Do pros suffer from speed wobbles or just us mere mortals?

andrewjoseph

the above sounds quite sensible, but it would seem then that all frames of the same size and style would suffer the same problems.

arankapila

the pros would also experience the same thing, but you also have to remember that when pedalling it is unlikely that you will suffer it- and the pros are pedalling most of the time!

Yes, bike os the same size and style will suffer it, but it is also dependent on frame material, tyre width, and wheel choice. There are a lot of factors that will affect when it happens and its' severity.

PostieJohn

faffer wrote:

I held it through sheer strength/utter luck and forunately arrived at the bottom in one piece. Nothing on the bike was loose (apart from my bowels) and as far as I can see there's nothing 'wrong' with the bike.

Welcome to the 'shimmy' club, always nice to have a new member.
Bizarrely enough the sheer strength part of your thread, was probably your downfall, making a bad situation even worse.

Gripping the top tube with yout thighs, relax, and let gravity do it's job.
After that, praying is your last option.

Give it a couple of decents on a hill you know well, and you'll be happy as larry, again.

jezcc

This hasn't happened to me on my new Ribble.

I took it up to 50mph last week and didn't get wobbling at any point. I've done a lot of MTBing though and hold it gently (except when sprinting) because it doesn't ever feel like it needs gripping too hard. Maybe I've been lucky, or maybe I need to go faster to get the wobbles.

the_hundredth_idiot

jezcc wrote:

This hasn't happened to me on my new Ribble.

I took it up to 50mph last week and didn't get wobbling at any point. I've done a lot of MTBing though and hold it gently (except when sprinting) because it doesn't ever feel like it needs gripping too hard. Maybe I've been lucky, or maybe I need to go faster to get the wobbles.

Blimey, there are some big numbers being thrown around here! What are you guys doing? Jumping out of planes with your bikes between your legs?

jezcc

what I did was stick it in 50 x 11 at the top of a big straight hill, pedalled as hard as I could for about a minute and then tucked up and rolled downhill as fast as I dared. Wussed out and pulled the brakes on when I got too scared and later when I'd stopped looked at the speedo, it said maximum speed 80.7kph, which is 50.1mph.

robthehungrymonkey

If it is speed wobble, it's when the vibrations match the resonate frequency of the bike. Every part of the bike contributes towards this as does the road. A different road will have a different affect. At least, that's how i understand it.

I've never experienced it myself, but i'd take the bike back to where you got it from ask them to check it, if it's all ok, try it on a different hill.

Juju_uk_68

It's porbably similar to a motorcycle tankslapper -

I found this - from http://everything2.com/e2node/Tankslap


From the world of motorcycling, a tankslapper can be defined as an undesired oscillation of the forks.

When experiencing a 'tankslapper', the handlebars 'wobble' rapidly from side to side, (sometimes so violently that the rider can no loner maintain grip). In extreme cases, (and provided the steering allows), the oscillation is such that the handlebars will hit the sides of the bikes fuel tank, hence the expression 'tankslapper'.
What causes a tankslapper?

The relationship between a motorcycle's tyres and the road is simple physics. The downward force of the tyre on the road surface is matched by an equal and opposite force as the road pushes back. This is Newton's First Law of Motion.

Now consider what happens when a motorcycle's front wheel leaves the ground temporarily, such as when the rider pops a wheelie or accelerates sufficiently such that full contact of the front tyre with the road surface is lost momentarily. If the wheel is still aligned with the direction of travel when it touches back down there's no problem. In fact, the gyroscopic force created by the rotation of the front wheel tends to ensure this. If, however, the alignment is lost, a tankslapper may happen. Alignment with the direction of travel can be lost if the wheel is turned whilst aloft, (don't touch the front brake!), or the direction of travel changes after the wheel lifts, (e.g. when going round a bend).

The important factor here is the castor angle of the motorcycle's steering, (the angle that the motorcycle's forks make with an imaginary line perpendicular to the ground). The castor effect comes into play when a front wheel that isn't aligned with the direction of travel touches back down. The natural tendency in this case is for the wheel to attempt to align itself once more upon regaining contact. Think about how the castor wheels on a shopping cart work, (a shopping cart has a zero degree castor angle, of course).

The rate of castor compensation is inversely proportional to the castor angle of the motorcycle's steering. In other words, bikes with steep steering geometries, (such as sports bikes), are more prone to rapid steering oscillation than, say, cruisers.

In some cases, over-compensation due to the castor effect happens upon 'touch down' and the wheel turns past the centre. The castor effect will rotate the wheel back the other way and handlebar oscillation begins. Normally, at this point, the rider is gripping on for dear life and trying to provide input into the handlebars to correct the oscillation. This is actually the worst thing to do, as the rider can't react quick enough and will tend to put energy into the oscillation. The best thing to do is relax the arms and let the oscillaton reduce naturally.
How to avoid a tankslapper?

Tankslappers are easily avoided - make sure your front wheel never loses full contact with the ground. Modern motorcycles, with their agressive steering geometry, sticky tyres and powerful engines, conspire against this. It's quite easy for a rider to inadvertently lift the front wheel sufficiently to cause it to 'wobble', especially when pulling away out of a curve, when the direction of travel is changing.

Maybe it's time you considered fitting a steering damper?

jezcc

The relationship between a motorcycle's tyres and the road is simple physics. The downward force of the tyre on the road surface is matched by an equal and opposite force as the road pushes back. This is Newton's First Law of Motion.

No it isn't, it's newton's third law.

Newton's first law is "a body will remain at rest or continue to move in a straight line at a constant velocity unless acted on by a resultant external force"

faffer

thanks for the replies, all the comments sound plausible to me although it seems unlikely that it was a one-off combination of road surface/bike oscillation as it happened twice on the same cycle ride on different surfaces, it does seem to be that it happens above a certain speed - unless I was just very unlucky! The tank-slapping description sounds closest to what i experienced, although (as far as I remember) the wheel didn't leave the ground. I think I braked as the speed increased (yeah I know - chicken!) and I wonder whether if the brakes aren't aligned properly and contact the wheel unequally on each rim that could set oscillations off?

Anyway, thanks for the advice.

I will definitely check over the forks to make sure everything's tight. Unfortunately the bike is second-hand so no warranty but I might give it in for a thorough service.

faffer

nicklouse wrote:

i would have a read of this

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

Interesting that the article says it will happen at predictable speeds - that is my experience of it too. I need to get a speedo with an alarm that goes off just befoe i hit 35!!!

Or just get a tourer

Monty Dog

Speed wobble is certainly more prevalent on certain models than with others - original Giant TCRs are notorious. Changing the forks can make a significant difference, but obviously not what you want with a new bike. Avoid a 'death grip' on the bike too.

MoonCircuit

faffer wrote:

I just did a training outing and broke 40mph on my Bianchi for the first time (i'm pretty new to road biking), halfway down the hill the front wheel just started snaking (or at least that's what i'm calling it), basically it was wiggling from side to side uncontrollably, if I braked it made it worse - even if I used the back brakes! I was just getting faster and faster and the wiggling was getting worse and worse and the tarmac was looking more and more like it wanted to become part of my body! I held it through sheer strength/utter luck and forunately arrived at the bottom in one piece. Nothing on the bike was loose (apart from my bowels) and as far as I can see there's nothing 'wrong' with the bike.

It shook me up and now I feel I can't trust the bike, on the ride I did today I got to about 35 and it started doing it again but I managed to catch it in time. Have others had this? Are certain bikes known to be bad for it? Surely manufacturers shouldn't be selling bikes that scare the living cr@p out of people - me in particular!?

Speed wobble. When you go that fast you have to hang on!

coffeecup

I've a couple of Bianchi's and never had this problem. I regularly do 40-43 down a hill on the way to work and wouldn't be getting close to this if the bike was twitching at all

Take it to your local bike shop and get them to have a look

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~