UKC

My bike seems to start wobbling when I hit 40mph down hill. It is a little disconcerting but it isn't a full on speed wobble (from what I've read that is much more severe) but scary nonetheless. I'm a bit of a pussy going down hill (but improving) any tips to deal with the wobble and tips in general?

Three club riders came past me and were sitting back and ducking down as I've seen pro riders doing. I assumed this was just for aerodynamics. I tried doing this and went much quicker, was able to ride out the bumps much better and I gripped the top tube between my knees and it all seemed much more stable (clocked 48.2 mph coming down Waddington Fell - very fast for me!), I assume then that this position is about more than just wind resistance? Seemed to work for me and I was much happier going down hill like that. Cheers

In reply to Escher: That's a fair speed... are you riding a carbon bike? I ride a carbon planet x in summer and a team alu planet x as my winter bike and get less wobble in winter..

In reply to Anonymous: Yes carbon bike. My alu winter hack doesn't seem to cause a problem. Still, won't be riding anything for a bit. Buggered my ITB on my ride today. Not happy. But cheers.

In reply to Escher: Drop Enty a line.. I thought down hill was just down hill until I went out with him.. its like he was riding on tram tracks the way he'd corner... maybe it was just a question of bottle but I was surprised at how much of a gap he could put on me just descending on a road..

In reply to Escher:

Wind resistance.
Lower center of gravity.
Ability to shift center of weight between fore and aft wheel.

You don't necessarily have to 'grip' the top tube between the knees.
often pushing the forward leg's knee against the tube damps any oscillations and allows a better position on the bike.
Also 'gripping' can encourage you to tense up.

Stay relaxed.
Look at where you want to go, rather than where you don't.
Stay relaxed.
Brake only if you really must. Many people will tell you only to brake before the corner, but braking through the line is a useful skill. Learn how the front and rear brake really affect the cornering line differently. Adjust your weight distribution fore and aft depending upon any braking you do.
Stay relaxed.
Don't think of exploding tyres.
Stay relaxed.

Top tip - duck, but don't slow down, when passing underneath camper van wing mirrors as you descend. A couple of litres of euro-lager consumed in the sun helps you to stay relaxed as you do this.

In reply to Escher:

Little things to think about, try lifting your outer hand off the bars slightly.

I sat next to an ex pro rider on a flight a year or two ago, said he was no good at climbing so had to be a good decender. His advice.

Choose your speed, choose your line and commit. I said it was the commiting part I had trouble with

Cheers

Toby

In reply to interdit:

And to add:

- Keep your weight on your feet - try to keep your backside in contact with the saddle, but don't have any weight on it. It helps to keep your centre of gravity low

- Outside pedal down in corners,and try to put more weight on that than your inside pedal

- Super light touch on handlebars - get into the drops (for better braking), and try to hold onto the handlebars with as little force as possible - the bike will tend to keep itself going straight at that speed!

- Remember that on good roads, in dry conditions, your front brake is much more effective than the back, and that it's easy to lock the back up. 1/2 second of a locked wheel at 80kph is over 10m of skidding - it doesn't take long to burn through a tyre like that.

- Relax, look ahead

- In terms of the fear, it's a bit like controlling fear in climbing. Make sure your kit is good, and then forget about it failing. Check your knot before you leave the ground, so when you're getting gripped on lead, you're not thinking about whether you tied it correctly - check your brakes and tyres and QR levers and anything else you want before you leave on your ride - then don't think about them when you're descending.

- Relax

- Boast to your non cycling colleagues

Tim

In reply to Green Porridge:
...
,,,

Agree with every point, and will add...

...enjoy the descent. It's usually been earned.

In reply to Escher: Thanks for the tips all. Didn't know that about the front brake when going fast and the rest makes a lot of sense, especially to enjoy myself whether euro lager is involved or not.

Toby can you expand a bit about taking your outer hand off. Did they explain how that works and why? And what it achieves? Works better for me if I understand the mechanics rather than trusting and giving it a try. Thanks again all, very helpful.

In reply to Green Porridge:

Can someone explain the mechanism of this? Surely braking forces are proportional to the normal force at the tyre to road contact, and unless you're in a superman position more weight will be over the back wheel. So I would have thought the rear wheel would lose adhesion less easily than the front one, and the rear will slow you more quickly.

How can the front be more effective than both close to the point of locking up, but still just rolling?

In reply to Liam M:

Can't explain it in physics terms but when you hit the brakes your momentum wants you to keep going, pitching your weight forward and loading the front wheel. On most performance oriented bikes a fair bit of your weight is over the font wheel anyway.

Look at cars and motorbikes; you'll see they have much bigger front brakes as the front wheels can cope with the braking forces better.

Rob

In reply to Liam M:

It's the same in a car: when you brake, the front dips and the rear rises.

As you brake, the bike and rider rotates as the centre-of-gravity (and momentum) is far above the road (where the decelerating force is being applied). This offset in centre-of-gravity results in a moment about the point at which the braking wheel contacts the road. This means that the rear wheel lifts, making it less effective than the front under braking.

It's more obvious when you consider what happens if you slam either brake on: if you hit the front brake, you stop and fly over the handle bars (very effective); if you hit the back brake, you skid (not very effective).

In reply to Bean Head:

Heh, your reply was a bit more succinct and much less geeky.

In reply to tomski3:

If I'm ever forced to slam on, I normally couple it with rapidly stunting my arse to the back of the saddle ( if I was on a mtb you may be convinced I'm trying to instigated a manual), and releasing either brake as soon as I feel it start to lose grip.

I think I may have to play around with the equations to see what falls out.

In reply to Liam M:

Yeah, of course riding technique and body position come into it; I was just highlighting what would happen 'if'.

Due my complete lack of natural ability on a MTB, I pretty much hang off the back for the duration of any fast descents in the (often) vain hope that I'll get down in one piece.

In reply to tomski3:

Indeed, and your post has give me some useful thoughts. I think I need to convince myself about what gives optimal controlled braking, at least in theory (as opposed to a quick 'eject'! )

In reply to Liam M:

Ok, you're spot on about the static (i.e. not skidding) breaking force being proportional to the normal force. You're also correct that normally, when travelling in a straight line on flat ground at a constant speed, more of your weight is on the rear wheel, so you'd expect there to be more potential for a stopping force there.

However, when braking, you also have a force in a horizontal plane, as well as your weight in the vertical plane. When you resolve these forces, you find that the normal force from the front wheel increases, and the the normal force on the back wheel decreases, and therefore, you have more braking potential from the front.

You can draw a free body force diagram if you want, and it will all work out, or you can think of a couple of extreme examples. If you grabbed on a whole load of front brake, what will happen? Every bike riding kid in the world has done this at some point, and gone flying over the handlebars. Why did this happen? Because they were braking so hard with the front, (and it gripped), and the normal force reduced on the back wheel so much that it became zero and the rear wheel lifted off the ground. For the same reason, you shift your centre of mass backwards when you brake, (hang your arse over the back of the saddle) so that your normal force stays between your wheels, and you don't kip over the handlebars.

The reason I put in the caveat "in good conditions on good roads" is because, even on bad roads in crummy conditions your front wheel will still have better traction BUT the consequences of a front wheel slide are much greater than a rear wheel slide. In good conditions, it's seriously hard (or nigh on impossible) to skid the front wheel when braking in a straight line - you're more likely to go over the bars, and then only if you've got your centre of mass in the wrong place. However, if you're on gravel or it's wet, you could lock the wheel and skid. If your back wheel skids, then it's usually recoverable, but recovering a front wheel slide is much harder, and therefore, if skidding is a risk, then it's better to have the back wheel skid than the front.

Hope that's clearer!

Tim

In reply to Liam M:

Alternatively, put some old tyres on your bike, ham down the road at, say, 30kph, and try to brake as quickly as possible with your back brake only. Repeat with the front brake only (with care). Then live convinced

In reply to Escher: I used to struggle with speed wobble and it is quite terrifying... The change I made that completely sorted it was ensuring my tyres were rock solid: 70-80 psi (on 700/23s) might be all right for most stuff, but stick them up to 100 (obviously dependent on your weight) if you're planning on going fast. Since ensuring tyres were fully pressurised I've never had a problem with it.

In reply to Escher:


You tighten your line in a corner by pushing on the bar with your *inside* hand...sounds absurd but what it does is take the bike out from under you and get you leaning further into the turn. Pushing with the outside hand turns the front wheel more into the bend which paradoxically tends to bring the bike upright and straighten your line.

Best way to experiment with this is slaloming down a quiet back road...don't think too hard about it at first...

In reply to elliptic: Cheers, good motivation to climb some big hills now!

In reply to Escher:

Mr Elliptic is on the money. It's all part of this...

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

Best not to think about it too much whilst you are doing it though.