speed wobbles

[hotmetal]

I'm posting this here cos I'm not sure where the best place is. I'm not really a beginner, having ridden mountainbikes since the end of the 80s, but I suppose I am a beginner on pure road bikes.

Went out for a 40 mile ride on Saturday which involves a very steep hill. Going up was hard work but no problem, however on my way back I was going down it and scared myself half to death!

Basically, I was doing approx 34mph (according to computer), and of course, it's no good being on the hoods at that speed cos you need a handful of lever if you're going to stop. But when I'm on the drops, I find that the bike feels very twitchy in the steering. At 34mph it kind of manifested itself as a high-frequency (and thankfully) low amplitude 'shake' at the bars, which freaked me out, so instead of going hell-for-lycra down the hill I was trying to brake all the way. I realise that my bike is meant for racing (it's an old Giant TCR Composite, more bike than I need if I'm honest ) and therefore will be fairly quick-steering and a bit nervous, but I just wonder if it's something I'm doing wrong? My wheels are true (well, straight at least). Tyres are maybe getting on a bit, I had just over 100psi in them. Could it be that I'm too tense, or my weight is distributed wrongly, or is it just that I'm a wuss and wobbles are part of the deal with fast road bikes? I can live with going slower, but I'd like to know if it's just a question of some technique that I never needed on a MTB. I've done these sort of speeds on the MTB even off-road but the slacker head angle and different riding position seem to take care of it on a MTB. This road race bike certainly is a bit twitchier than what I'm used to - I even find it feels twitchy on the flat when I'm on the drops (I tend to ride on the hoods most of the time and it feels great). Maybe its the way that you end up with too much weight on the bars when holding them on the drops?

 

[Norm]

I think that the road surface might have played a part and the weight is further forward when you are in the drops, further amplified by heading down a hill.

The TCR (slobber) will probably be more responsive (the other side of twitchy) than an MTB, because there is less expectation of kick-back.

 

[Fab Foodie]

Could it be that I'm too tense, or my weight is distributed wrongly, or is it just that I'm a wuss and wobbles are part of the deal with fast road bikes??

Quite possibly...
Assuming that there are no glaring mechanical issues, then it's likely to be 'Shimmy' and it's not uncommon in 'novice' road-bikers descending hills with a certain trepidation! If you search the forum for shimmy you'll come-up with all sorts of reasons, but usually it's due to weight distribution and most importantly having a 'death-grip' on the bars and being scared rigid. The tech stuff is here:
http://www.sheldonbrown.com/brandt/shimmy.html

I have a Giant TCR of the same vintage probably as your early TCR composite. Yes at lowish speeds thay can be twitchy, especially with a short stem, but at high speeds (that they are designed for) they become very stable. I've had mine up to 50mph on twisty roads smooth and rough and it's solid as a rock. But you have to learn to relax, get down in the drops, slide back in the saddle and let the bike run without squeezing the bars to death and you'll probably find she'll run straight and true without a flinch on the bars.

100psi in the tyres is no problem either if that's within the recommended pressure range.

 

[GrasB]

Are your arms bent or straight when on the drops? Many people I see have the bars in the wrong position giving an overly aggressive position on the hoods & end up with straight arms on the drops which makes the bike harder to control.

 

[accountantpete]

My money would be on an untrue wheel vertically or an incorrectly seated tyre(especially where the valve is).

Or possibly the hub has play in it.

 

[zacklaws]

I've just had two very bad shimmy's lately, one just before Xmas down a steep hill into a very strong head wind, it began just over 20mph and even slowing down to 15mph the bike was still shaking itself to bits, 5 times I went down that hill that day and it happened every time, but as its never happened before on that hill, I blamed it on the wind as I have never encountered it that strong before and possibly the shape of the carbon frame of a Trek Madone trying to cut through the head wind.

And about 3 weeks ago, downhill just over 30mph, the bike suddenly wanted to shake itself to bits, even gripping the top tube between the legs never sorted it out. When I stopped at the bottom, I noticed that my front wheel was slightly out of alignment with the bars, probably never set it right when I serviced my headset earlier on that week. After correcting it, its not happened since.

 

[hotmetal]

Cheers for the replies everyone.
Norm certainly has a point, the road surface is pretty rough, which could contribute to instability, though it wasn't surface undulations, it was definitely a wobble, what some of us would probably call a tankslapper if bicycles had tanks...

I don't think it's the wheels or tyres - I did 'check' these for straightness and play as much as you can by the side of the road - the Eurus wheels are in very good condition. I might have a look at the alignment of wheels to bars as Zacklaws mentioned, just because I've never really looked, I assume they're right because they seem straight enough. My bars are not rotated too far forward - if anything I've erred on the other side. My arms would be bent when in the drops, though I'd have to check if this is still the case if I get my weight further back. I reckon the biggest part of the problem is me clinging to the bars for grim death - I'm acutely aware that Lycra doesn't meet the CE requirements for armour or have the abrasion resistance of motorbike gear which is more appropriate for these speeds! I'll look again at bar position and have a play, but in all probability I just need to slide back and loosen up - not having 2" knobbly tyres and hydraulic discs does make a bit of a difference!!

Thanks to Fab Foodie for the anecdotes and links, at least I know the bike should be OK for this once I get the hang of it. I'll have a good look at those links. To answer your (tacit) question, mine is a 2004 model.

Cheers all, safe rides.

 

[zacklaws]

If you do a search online for:- bicycle speed wobble there is plenty of info and you might find the answer your looking for.

Somewhere I have read that all bicycles have the ability to go into a wobble and also that a lot of the reasons why are still unknown.

But there's no mistaking one when it happens as it scares me cackless if I'm going like a bullet downhill and it kicks in like it used to do on my old Raleigh milk race, but that had something to do with the buckle in the back wheel, .

Trouble I find with them is they do not start slowly so you do not get a bit of a warning its going to happen, it justs happens so violently. Perhaps there's scope for manufacturers of cycle computers to build a sensor in which will detect the small occilations building up and warn you with a few loud beeps.

 

[jimboalee]

Older Peugeots were a bugger for it.

They had 74 Deg parallel frames and long stem extensions ( fashion at the time ).

A light grip and pretend you're going through the sound barrier in a Hawker Typhoon in a steep dive.

PS. Does the weight of the front tube valve pull the wheel round and behave like a pendulum when you lift the front of the bike off the ground?
Yes? apply a small weight opposite the valve.

 

[snailracer]

I would blame lateral oscillations of the wheels. Increase spoke tension, get stiffer wheels, or don't grip the handlbars as hard - a loose grip helps dampen oscillations.

 

[Alien8]

Ditto what's been said already but when it starts to get in your head that's when the real trouble begins because it can creep in to all your riding. Try some different hills and cornering etc where you can take it a bit easier and get confident in your own abilities and in your bike. There's no shame in riding in style that you're comfortable with and you'll soon be charging around.

 

[Hont]

Was there any type of wind at all? Only time it's happened to me was coming down a steep hill and got slapped by a 20mph side wind. Pulled the brakes a bit and it got worse before getting better. Agree with everyone else, scares you cackless (comp said I was doing 38 at the time).

 

[jimboalee]

Happens every time when I ride my old Pug down Liverage Hill toward Henley in Arden at ??? mph ( doesn't have a speedo ).
I get 42 - 45 on my Spesh down that hill, into a 40 limit half the way down.

 

[hotmetal]

I would blame lateral oscillations of the wheels. Increase spoke tension, get stiffer wheels, or don't grip the handlbars as hard - a loose grip helps dampen oscillations.

Hmm, I think I'll work on practicing hills and relaxing my grip a bit... Campag Eurus wheels aren't cheap so bettering them will be £££.
By the sound of it, it's 'user error'!

 

[Norm]

Same solution as the Fazer, Hottie, just power through it.