Bike Shake

Folks
I have been discussing this with friends and we have had a look at my bike and can't see what caused this but it was terrifying.
Over the weekend whilst on a fast descent my front wheel went into a shake best described as a violent vibrate. I tried to control it but the rear wheel then went into a violent shake before I almost lost control of the bike. I didn't touch the brakes and I swear I thought I was a gonner. This lasted about 20 seconds until the speed dissipated a bit. After getting of the bike and sitting on the verge for a while to get my stomach back under control I cycled home with no idea what happened. I have been cycling a while now, have been on this descent numerous times before and nothing like this has ever happened. To be honest it has knocked the wind out of me. I have checked the wheels and all are true. the hubs are spinning clean and the frame looks fine. There was a bit of a cross wind but nothing serious. Any ideas what the hell happened as I think the speed freak is shot
 
I've had a wobble at speed downhill. After a bit of investigation, found the headset was a touch loose, and after adjusting did not get a repeat performance. Had similar on another occasion revealed by emergency braking. Not to say it's the only possible explanation, but I believe poor headset adjustment was the cause in both the cases I had.
 

Mobytek

Well-Known Member
Speed Wobble - phenomonen mostly associated with motorbikes. The physicis is that the rear end is trying to overtake the front end, and as the frame is rigid, pushing from the back, forwards, means it either goes to one side or the other, which then it leans and sends the force to the other side, and you pendulum (v quickly as you now know) from side to side with the handlebars tring to shake you off.

It is that the rear has the speed and is increasing the speed, but the front has the speed but has the air resistatnt slowing it down.

So, pat on the back you are going v effing fast.

Lesson learnt that that was about your top speed ......

Google it on motorbikes, normally on the long straights as they hit top speed, then ease off before they brake for the corner.
 
Thanks to everybody who replied. I was hoping it was something mechanical and not something of chance as I think I'm about done with speed after that escapade. I'll have a look at the headset after work and hopefully that's the fault. They say you learn something everyday and that was the first time on a bike that I thought I was about to knock on the pearly gates. Once again thanks. Can't shake the ''what if '' thoughts though. :thumbsup:
 

Moodyman

Guru
a loose headset is be a common cause, but there are other factors - like saddle being disproportionally high (especially if you're a heavier rider), cross-winds, steel frame (more flex) and even a frame too small.

I used to get it frequently with a steel tourer which was one size too small and had a very high saddle vs the bars. got rid of the bike after the 4th or 5th stomach churning wobble. had many minor ones in between the big ones.
 
I presume the descriptions given by previous posters is what happened i.e. rapid side-to-side wobble/wiggle of the front wheel (followed by the rear from your description)? You stated this happened while descending, was your weight on the saddle or balanced on the pedals? My experience of tank slappers is they are worse/more likely if ther front wheel is lightly loaded, normal when descending, so I'm always careful to lift myself just out of the saddle so my weight is going through my feet to the pedals, keeping my mass more centralised. Not sure how scientific this is but ti works for me.
 

ColinJ

Puzzle game developer
I'm fairly sure that one cause of shimmy is the way the rider rides.

I have mentioned this several times before - I rode the Kirklees sportive/Brian Robinson Challenge 2 years on the trot with the same rider and he got severe and terrifying shimmy both times on the same righthand bend at the bottom of the fast descent off Holme Moss towards Woodhead Pass. He was riding a heavy steel touring bike the first time and a lightweight carbon fibre racing bike the second. The weather conditions were different too.
 

buggi

Bird Saviour
Location
Solihull
I heard that when a speed wobble happens you should clamp your knees around the top tube to give your bike some stability. My mate said it worked with him. I have never reached that speed so can't tell you first hand what to do, only what others have told me.
 
I presume the descriptions given by previous posters is what happened i.e. rapid side-to-side wobble/wiggle of the front wheel (followed by the rear from your description)? You stated this happened while descending, was your weight on the saddle or balanced on the pedals? My experience of tank slappers is they are worse/more likely if ther front wheel is lightly loaded, normal when descending, so I'm always careful to lift myself just out of the saddle so my weight is going through my feet to the pedals, keeping my mass more centralised. Not sure how scientific this is but ti works for me.
Thanks, I was leaning right over with chest weight on the handle bars. Gonna take the headset apart tonight and hopefully that's the problem. If not, my days of fast downhill are over. Will just sit back and admire Peter Sagin. Cheers and Regards
 

andyfraser

Über Member
Location
Bristol
What's the likelihood of crashing from this? What happens if you apply the brakes? I just know if this happened to me I'd either do something stupid or cling on for dear life. It looks utterly terrifying!
 

TheJDog

dingo's kidneys
I had this on my hybrid. Turned out the downtube had snapped almost completely through causing a lack of rigidity in the frame. It would start wobbling as soon as I took my hands off the handlebars. Took me about 12 miles to work out what the problem was.
 
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