/ High speed wobble!
I have just got back from a ride and had a major wobble going at 47mph and would like to understand why.
The road was straight and steep, I was down on the drops and looking forward, there wasn't much wind. I don't know what order all this happened in but I remember thinking "i'm going pretty quick, better slow down, reaching for the rear break, getting a major wobble left to right a few times, slowing down and the wobble stopping. I don't know if the wobble was before or after I hit the breaks, but it lasted long enough for me to consider and think through the consequences of hitting the road at the speed I was at. I would guess a long stay in hospital, skin grafts, etc?
What caused the wobble?
Gripping too tightly? Adjusting my balance as I went for the break? Applying the break? Poor bike set up? Any other ideas?
I am pretty keen for this not to happen again!
Thanks for any thoughts.
I just installed some spacers on my brake levers to reduce the reach, so I don't have to stretch quite as far to grab a handful.
I got a bit of the same thing last night at 40+mph, but at the time I was moving one hand to the brake hood, thus unbalancing the front end.
Too much weight on the right as I reached for the left break could be the cause.
You may have hit some bit of poor surface just as you hit the brakes which initiated the wobble - I once went over a cats eye as I hit the brakes at similar speed and the back wheel was skimming down the road! You don't say whether the wobble was just front or back or the whole bike.
Why go for the rear brake? The front is more effective - think about it, your weight is being thrown forward as you slow down so more weight is on the front wheel and less on the back.
One way to counteract this to a small degree is to hang off the back of the saddle so shifting your weight backwards.
The device you use to reduce your bicycle's speed is a BRAKE. If you fail to slow down you might have a crash and BREAK something. Spot the difference.
Having got that off my chest...actually, gripping too tightly might have been one factor which made the wobble worse. It's widely believed that holding on too tight can lead to a positive feedback loop, so - although it sounds counter-intuitive - loosening up a bit and letting the bike sort itself out to a degree can help. Watch motorcycle racers: some of them get in to horrific-looking speed wobbles and the best riders actually stand up a little on the pegs to let the bike's own natural directional damping play its part in getting things back under control.
Not sure about the rear brake: it might be throwing weight on to the front wheel (as any braking does) and increasing the forces contributing to the wobble.
It might be worth checking your steering heard bearings for play or sticky spots as I believe these can make a speed wobble more likely.
Keep practising - you'll soon get used to how much brake you need to apply.
I haven't long been on a road bike, and I have to say knowing how much to brake and when i am still finding a little difficult. For some reason I still go for the rear brake even though I know I should use the front a bit more (might be an mtb habit).
I had a nice volvo in front of me today on a long downhill (I swear he was moderating his speed to give me a tow) and I got to 49mph, with no sign of any wobble, but I have got in the habit of sticking my arse in the air, and clamping the top tube with my knees (not hard, but enough to know where it is). I've found this can really help make the bike feel more stable.
Have a look at this: http://www.youtube.com/watch?v=RxXqQqAc2pA
Whenever he's not pedalling he's got his knees against the frame (or he's got one knee out to balance in the corners)
I get wobble if i notice my speed is too high and freak about hitting the deck... I tend to tense up through my arms when reaching for the brakes. If I make a concerted effort to relax, I can bring things back under control.
Anyway, the obvious response to this problem is to change frequency. The quickest way to change frequency is to slow down. So brake.
PS Thinking about it, the faster you go the likelier this is to happen. Because the frequencies will be harmonising on lowest common multiples. And the faster you go the more LCMs you pass through.
I can't understand people who always go for the rear brake, the physics just isn't right. If you look at a car you'll see big disc brakes on the front then often just little crappy drums on the rear.
I thin it's a fear of going flying over the handlebars. Thing is though, in the drops, it's pretty hard to go over the handlebars, even braking hard with just the front brake. What people don't seem to realise is that locking the back wheel at those kind of speeds due to braking is quite likely, and when you're going that fast, it doesn't take long to bun through your tyre!
Speed wobble: Possibly caused by you tensing up at speed? I've had that on the MTB a couple of times and just had to slow down and relax a bit.
Braking: Vastly improve the efficiency of your braking by dropping your heels as you brake and actively pushing with your feet into the pedals - keep your hands 'neutral' (not pushing or pulling too much on the bars) when you do this and it loads your tyres evenly. Well worth practicing somewhere non-serious to get the hang of the effect.
> What people don't seem to realise is that locking the back wheel at those kind of speeds due to braking is quite likely, and when you're going that fast, it doesn't take long to bun through your tyre!
Having said that the rear brake on my mountain bike hasn't been working for months. Probably about time I gave it a bleed.
> The device you use to reduce your bicycle's speed is a BRAKE. If you fail to slow down you might have a crash and BREAK something. Spot the difference.
And while we're on the subject, ablackett might like to note that if the handlebars come LOOSE, he might LOSE control :-)
> I can't understand people who always go for the rear brake, the physics just isn't right. If you look at a car you'll see big disc brakes on the front then often just little crappy drums on the rear.
I agree about it being a mountain biking thing - if you use your front brake when going down a big drop, the bike tends to flip and you end up lying on your front in the mud, with an upsidedown bike on top of you. Not that I would know, of course! *whistles in a casual manner*
So you learn not to use the front brake when going downhill much...
Also, use both brakes, keep your arms relaxed and weight through your feet.
The wobble above is the bike's resonant frequency mixed up with the gyroscopic effect of the wheels, probably coupled with an 'event' to initiate the vibration. Slow down, or get lucky with another 'event' to disturb the oscillation, or apply pressure somewhere to change the resonant frequency (i.e. add a part of your body to the vibrating mass). The key of course is knowing where to apply the pressure and when, so that the wobble doesn't start in the first place. I tend to bring my knees in and grip the saddle hard with my thighs.
My top speed was at 36mph this morning, and I chickened out because of an unfamiliar hill, some bends, some shadows, some strong sunlight and a fear of potholes given the state of the roads hereabouts. And the car behind me.
So, what happens if you hit a 9 inch by 1.5 inch average pothole at that sort of speed and faster on a road bike? Does it skip it, burst a tyre or what? I really don't want to find out myself.
At those speeds there is not a hope in hell of locking the front, unless you've got disc brakes.
Not so sure about that, a set of properly adjusted cable rim brakes is more than enough to lock up a front wheel at high speed. I had a DUKW pull out infront of me and flat spotted my tyres something rotten :-)
> So, what happens if you hit a 9 inch by 1.5 inch average pothole at that sort of speed and faster on a road bike? Does it skip it, burst a tyre or what? I really don't want to find out myself.
Depends how you hit it. If you get a few seconds notice of it you can just unweight the bike slightly and it will skip it, it may well skip it anyway if you're on a straight and the road is otherwise smooth. Also depends where your weight is, if your sat on the saddle and don't successfully skip the hole you'll get a very violent jolt which could throw you off balance and probably blow a tyre. If your weight is on your feet you may get away with it, or could still blow a tyre or buckle a rim - depends a bit on your weight.
If you blow a tyre at speed you could be in trouble, see this example (not actually a blown tyre, but the effect is the same)
just gotta love the cameraman freaking out at 4:15/17...
i think some of the top tube hugging are more him trying to create as small and areodynamic posture as possible, but i could be wrong and it could be for stability.
I thought I could live with it but mine put me in hospital eventually so I decided to solve the problem with the frame which I managed in two stages.
Firstly with the back of a 5lb axe and then a skip.
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