UKC

I am fairly new to road biking, having done a couple of hundred miles over the last 6 months.

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.

Andyu

In reply to ablackett: More weight going to one side as you brake? Are you applying both brakes at once? Try tucking in a little lower and getting your hands further into the curve of the drops?

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.

In reply to bullandbladder: I was scared to stick on the front break at that speed - would it be ok or would I go over the top/loose the back end?

Too much weight on the right as I reached for the left break could be the cause.

Thanks.

In reply to ablackett:

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.

ALC

In reply to ablackett: geometry?
http://forums.roadbikereview.com/scott/cr1-wobbly-geometry-speed-266168.htm...

In reply to ablackett: I can't offer any advice about your wobble, but I can offer this advice:

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.

In reply to ablackett: Yep, use your front brake. The front one accounts for probably 70% of available braking power. Get the rear on first to get things under control, then a split second later feed in the front brake (don't grab, squeeze smoothly). As mentioned, be aware of the road surface - if it's loose/lumpy, don't use too much front. Take extra care in the wet, and try not to brake in corners. Check your speed on the approach, then come off the brakes as you go round (especially in the wet). Get your arse up and back off the seat,and bend your arms and legs a little to allow the bike to move underneath you.

Keep practising - you'll soon get used to how much brake you need to apply.

HTH.

In reply to ablackett: I get it quite regularly at 40 mph plus. Getting off the saddle and low and the gripping the top tube between your knees to dampen the wobble really helps. Having one knee against the top tube when descending seems to avoid it happening so much in the first place.

In reply to ablackett:

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: 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)

In reply to ablackett:

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.

There is a particular kind of wobble you can get at speed that feels to me like a frequency effect. The bike is vibrating in several ways at once all the time, of course, but at a certain speed all the vibrations come into harmony with each other, and this gives you a single very amplified vibration. Hence, a wobble. The fact that this kind of thing is likelier (in my experience anyway) to happen on a smooth road increases my suspicion that it is about harmonised frequencies, because on a bumpy road there's more interference.

Anyway, the obvious response to this problem is to change frequency. The quickest way to change frequency is to slow down. So brake.

In reply to Tim Chappell:

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.

In reply to ablackett:
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.

In reply to ablackett: all covered above I think, relax and lightly grip top tube with knees. I had this once, also at 47mph and I think trumps almost any fear I'be had climbing!

In reply to gethin_allen:

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!

Tim

In reply to ablackett:

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.

In reply to Green Porridge:
Rather lock the back than the front, though.

Having said that the rear brake on my mountain bike hasn't been working for months. Probably about time I gave it a bleed.

In reply to Bob Hughes: At those speeds there is not a hope in hell of locking the front, unless you've got disc brakes. 70% of braking should be done with the front unless on an MTB and you're using the back bake to help you steer/slide.

Ryan

In reply to Martin W:

And while we're on the subject, ablackett might like to note that if the handlebars come LOOSE, he might LOSE control

In reply to gethin_allen:

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...

In reply to ablackett: I've never had this happen, but it's a well known effect know as as 'speed wobble'. General opinion is that the best thing to do is let go of the brake. Despite Tim Chappell's advice, my instinct as a rider (and a physicist if that helps) is that the more you brake the more of your energy will end up in the wobble and it will get worse.

Also, use both brakes, keep your arms relaxed and weight through your feet.

In reply to Oujmik: It's all about technique - when did you last see a pro rider wobbling at 70mph on an alpine descent on a bike that weighs nothing?

In reply to ablackett:

youtube.com/watch?v=xODNzyUbIHo&

This?

You just have to hang on until it stops... happened to me last summer doing 55mph! Scary I tell you!

In reply to ablackett:

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.

In reply to ryan_d:
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

http://en.wikipedia.org/wiki/DUKW

In reply to nniff:

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)

youtube.com/watch?v=Gr89ku-K2WU&

In reply to nniff: learn to bunny-hop?

In reply to ablackett: Whilst in your tuck jam a Knee into the top tube and apply pressure to it.

In reply to balmybaldwin: i've seen that on before, bloomin brilliant

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.

In reply to gingerdave13: oh and also the bit at 3:36 just a bit close to that audi!

In reply to Oujmik: 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)

youtube.com/watch?v=Gr89ku-K2WU&

Armstrong possibly launching the trend for cyclocross bikes there, a quality recovery.

In reply to ablackett: I had a bike years ago that did this. In my case it was at 42mph every time. I asked a lot of people at the time, (club cyclists, mechanics and frame builders) and have read a lot of books/articles over the years since. None provided a definitive answer.
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.

In reply to nniff: I should have said practical answer, nniff's explaination is the definitive answer but I never had any success with gripping, weight shifting, tensing or relaxing.I just had to accelerate or decelerate to avoid the dreaded 42mph.

In reply to ablackett: Hi, I love the speed ! the replies you have had are all great ideas; its just a case of trying them all, you could balance the wheels, I spent 8 years car tyre fitting so enjoy balancing my bike wheels just to be pedantic, if you want the maths, google for centripetal force.