Speed Wobble & My Squeaky Bum

bails1310

Although at mych sower speeds, it felt like this http://www.youtube.com/watch?v=KrNGGA13G9o and I was a few seconds from crossing my hands over the back of my head and assuming the brace postion.

Great ride to Derby although a but windy, first good decent and half way down, circa 36 mph it felt like someone had taken over the bike with the most horrible wobble at the handle bars. Slowed gradullly and passed and put it down to wind.

Next and final one of the day, was nerer 40 mph towards Belper and happend again witout warning, to the extent it pushed me towards the wrong sife of the road. I limped home at a snails pace with adrenalin legs and the 'what if' question running though my head.

Bike was fully checked out last month in readiness for the season so nothing wrong there and having done some reading, appears main cause is incorrect weight distribution when decending which can be cured by holding one or both knees against the top tube to brace the frame.

Many people I am sure already know this but I didnt and only by luck did I not end up on the floor. For sure I will know what to do next time and probably decend every time with both knee firmly agains the bike.

Probably the most frightening expreience I've had for a while, especisally on a bike,

dnwhite88

Squeezing the top tube with your knees can help stop speed wobble but prevention is better than the cure. Try to keep your upper body relaxed, so that your body absorbs the vibrations from the road. Much easier said than done especially when you're going quick, well done for hanging onto it!

peat

Happened to me too. Scary as eff.

The advice i was given, that seems to work, was: Lift bum just off seat to put weight through bottom bracket, then squeeze top tube with knees.

The theory behind it is that it stops the wobble developing by using your body as a damper. As i say, it seems to work for me and i am descending with confidence once again.

IanREmery

I've had that twice in the month or so I've been road riding, the second time at 35mph. Sphincter went 50p-5p-50p-5p.

In addition to knees against top tube and other advice above, I've also been told to sit further back in the saddle, helps even the weight distribution up. Haven't had it since, but then haven't been near 35mph enough times to confirm if it actually works.

Why does it only seem to happen on road bikes? I've done over 40mph coming down Ditchling Beacon on my Cube Nature hybrid, and that was as steady as a rock.

Grill

I rarely have speed wobbles, but had a beauty on Saturday. Of course it was descending at 40mph+ on the aero bars of my TT bike, with a huge crosswind, riding 80mm wheels...

Rarely ever happens on my Foils.

peat

I think it's something to do with vibration resonance within the frame and between components (axles/forks etc). Usually takes a 'perfect storm' of conditions to arise. Due to the lightweight nature of roadbikes, the wall thickness of material is very thin so is prone to flexing. I suspect the hybrid is more heavily built and as such flex is less of an issue.

goonz

Search youtube for the clip of the pro descending at nearly 60mph. Type that in search, should find it.

His position is what you want to be copying.

diy

Its basically the delayed reaction of your body that sets it off. It happens on motobikes at a much high frequency because the weight differences are much greater 200kg bike vs 80kg rider.

On a motorbike the advice is generally to open the throttle to drive the weight off the front. no idea how you could do that on a cycle. But in general a change of speed in the best option to stop the cycle of wobble/react/wobble, plus relaxing the grip.

IanREmery wrote:

Why does it only seem to happen on road bikes? I've done over 40mph coming down Ditchling Beacon on my Cube Nature hybrid, and that was as steady as a rock.

Suspension, wider bars, rougher surface and slacker geometry. It can happen though if you are on smoother surfaces and tucked in holding close to the stem.

eskay

I have been cycling for 25+ years and never experienced speed wobble, I have descended at speeds in excess of 50mph on many occassions and never had any issue.

Early this year however I was going down a hill and suffered frightening speed wobble, my whole bike/body was shaking violently. It nearly threw me off of the bike as cars were passing me by (honestly!). Braking made it worse until I got below a certain speed.

I cycled home very gingerly with adrenalin still coursing through my viens. It then happened again a few weeks later. I read loads of advice and possible causes and it was not until several months later that I discovered the problem.

I had been on my rollers and whilst folding them backup and looking at my front wheel I noticed that the continental GP's were delaminating. It was really hard to spot at first but closer inspection revealed both the front and back were the same and I could feel undulations if I rubbed my hand around the tyre.

I swapped the tyres and no more speed wobble. It has taken me a while to get my confidence back going downhill, something that never used to bother me!

bails1310

IanREmery wrote:

the second time at 35mph. Sphincter went 50p-5p-50p-5p. quote] - audible LOL in the office!!

I'm assuming the wobble only becomes apparent when you are descending but not pedalling?

Some of it may have been my confidence or perhaps a change in technique. Same stretch of road on my old carbon bike which I was very used to not that precious about, actually peaked at a higher speed last year. New bike, first time out, not wanting to be too heroic maybe didn't do as I usually do?

There's a slower decent on the way to work at just over 30mph this morning which I took on my commuter and didn't have an issue. That said I did the things people are saying re weight and knees and all felt good.
Hack is probably 5kg heavier than Sundays and not sure if that has anything to do with it bike but knowing the wobbles do happen and how to deal with it if it happens again, has given me a small degree of confidence back.

dnwhite88

Grill wrote:

I rarely have speed wobbles, but had a beauty on Saturday. Of course it was descending at 40mph+ on the aero bars of my TT bike, with a huge crosswind, riding 80mm wheels...

Rarely ever happens on my Foils.

That would be a suitable time to crap yourself!

Twostage

Thread dredging I know but I had a variant of speed wobble on Sunday, it was actually a low speed wobble. I'd just been descending at about 40mph, reached the bottom and was probably doing about 15-20 when the bike started to wobble. It was like the front and back were connected by an elastic band. Managed to get the bike to a stop by careful rear braking. Stopped and checked the bike and everything seemed sound. Set off again gingerly and everything (apart from my confidence) was fine. Can't figure out why it would be OK with me leaning over the front handlebars at 40 but wobble like hell at 20. Scary even at slow speed.

unixnerd

I had this once at just under 30mph, scary. I put it down to low tyre pressure, check it more often now and it's never come back.

Twostage

I took my tribars off and raised the handlebar the day before. Wonder if the headset is a bit loose ? Don't want to over-tighten it, though, as its a carbon frame.

andrewjoseph

Twostage wrote:

I took my tribars off and raised the handlebar the day before. Wonder if the headset is a bit loose ? Don't want to over-tighten it, though, as its a carbon frame.

You do know how to check headset play, don't you?

Twostage

andrewjoseph wrote:

Twostage wrote:

I took my tribars off and raised the handlebar the day before. Wonder if the headset is a bit loose ? Don't want to over-tighten it, though, as its a carbon frame.

You do know how to check headset play, don't you?

Front brake on and rock it back and forward feeling (and listening) for play at the headset/headtube interface. At the moment there is some barely detectable movement, probably about 1/2 mm of movement if that. Its not over-tight as the steering flops about quite happily.

I think there is about the same amount of movement on my Giant Defy 1 (Alu).

unixnerd

Can't hurt to undo the bolts and gently retorque them in the correct sequence.

bails1310

I justified some new integrated carbon bars on the back of the wobble and asked the guy who fitted them to change the headset just to be sure.

Probably overkill and some will be psychological but have a world more confidence descending now on the bike which caused the wobble in the first place, still doing the knee/weight/relax etc thingies but overall result is sphincter remains far more dilated than before!!

tiglon84

How do you keep your knees against the frame when you're pedalling as hard as you can?

hatch87

tiglon84 wrote:

How do you keep your knees against the frame when you're pedalling as hard as you can?

elastic bands

andrewjoseph

bails1310 wrote:

...overall result is sphincter remains far more dilated than before!!

Dilated means opened wider than normal .

Stedman

Have a look at Chris Juden's article on wheel shimmy. See: http://www.ctc.org.uk/file/member/200911052.pdf

leegreenhough

I had this and shit myself, I've since found peddling out of it helps and taking the weight of the saddle. The main thing though is all in the mind, once it happens get back on and ride hard, don't let the worry settle in your mind, stay confident.

Initialised

IanREmery wrote:

Why does it only seem to happen on road bikes? I've done over 40mph coming down Ditchling Beacon on my Cube Nature hybrid, and that was as steady as a rock.

Bigger tyres at lower pressure absorb more 'road noise' rather than allowing it to transfer to the frame and on an MTB you tend to assume the DH/Attack position on descents. My first time over 50mph was on a Specialized Rock Hopper.

Try dropping the pressure on your front tyre a bit.

Twostage

Happened again this morning in exactly the same circumstances. Fast downhill then level off approaching lights. As I slowed approaching the lights I could feel it start to go. I applied the rear brakes just as it was going into the first oscillation (big word) and it controlled it. Bike (or me) felt a bit weird for a few pedal strokes and then back to rock solid. Following on from the 'perfect storm' suggestion I'm assuming changing something will stop it so I'm going to lower the handlebar by a spacer this lunchtime. Which should also make me 0.0001% faster. I also think the handlebar might not be perfectly aligned although not sure if this would make any difference.

Its a very strange feeling when your bike turns to rubber.

BlueEyedGirl

Stedman wrote:

Have a look at Chris Juden's article on wheel shimmy. See: http://www.ctc.org.uk/file/member/200911052.pdf

This article is well worth reading: you have to scroll past the bit about dogs etc to get to the end of the article. Far better to know all about this before it happens than after!

I browsed this thread because yesterday my new carbon fibre bike did something a little strange at speed. It wasn't a shimmy, which I have never experienced, it just seemed to start drifting on a firm and dry but quite rough tarmac surface. I was on a slow bend, but off that bend was a huge and steep fall into a ravine, so while the drift was only a few inches, it as a bit unsettling. It was like understeer in a car. I changed my steering style into more of a push on the inside bar, the outside pedal and leaned into the bend, rather than trying to turn the bars, and the drifting stopped. I have no idea if what I did stopped it or if it just stopped.

islandman

I had a proper first class frightener last year going about 32mph down a local hill, head bearings were loose. Frame was ancient and it prompted me to buy a Genesis Equilibrium frame, notably rock solid down hill. Shakes happened again down the same hill but not as bad, also happened going fast elsewhere. Started thinking I'm the culprit but couldn't figure it out. Started happening last week but by now I didn't think I was going to die and kept my wits about me, thought " relax hold of the bars a bit," didn't grip top tube but put all weight onto legs, relaxed hands and the shaking stopped. I used to love flying down hill but think the first shaking episode left me so tense I was making it happen.