Tank slapper.

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Brandane

Legendary Member
Not sure if that is the correct terminology, but in motorcycle terms it is when you take your hands off the bars and the handlebars start to oscillate quite rapidly.
I have experienced them before on a motorbike, but never on a cycle - until today.

I was riding downhill on my Trek 7.3fx about 20mph when it began to rain. I took both hands off the bars to zip up my jacket and it started to do a tank slapper! The bike is very new and not done many miles. Steering head is properly adjusted. Tyres are new Conti gator skins 700 x 28, pressure about 90 psi..

Any ideas as to the cause? It has a rear rack fitted which was carrying one light pannier. I know from experience that sometimes luggage on the back of a motorbike can affect the front end handling, so maybe the same on a cycle?
 
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Profpointy

Legendary Member
I had it when the headset wasn't quite adjusted right - a bit of play.
Adjusted it, and all was well subsequently. Someone disagreed with my diagnosis and said it couldn't cause that, so it may have been co-incidence, but I believe not.
There are no doubt other possible causes
 
TheDoctor

TheDoctor

Noble and true, with a heart of steel
Moderator
Location
The TerrorVortex
It's down to a resonant frequency - my old 531 frame could be provoked into it.
Certainly taking both hands off the bars would lighten up the front end.
I found that pressing a knee (didn't matter which one) onto the side of the top tube would dampen things down.
It also seemed a bit susceptible to tyre pressure. I later fitted different wheels and the problem seemed to go away.
 
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Brandane

Legendary Member
victor said:
@Brandane : it could be due to slightly more weight on the bike's rear, plus the fact that you were distributing your weight further back. There's a discussion of speed wobbles here, if you're interested:
http://cyclingtips.com.au/2011/03/speed-wobble-when-the-bike-shakes-its-head/

As @TheDoctor just mentioned, pressing one or both knees against the top tube tends to stabilise the bike.
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Interesting link. Looks like weight distribution may well be the problem, so I either remove the rack, or keep at least one hand on the bars!
 
Pat "5mph"

Pat "5mph"

A kilogrammicaly challenged woman
Moderator
Location
Glasgow
[QUOTE 3018336, member: 30090"]Its a good skill to have.[/QUOTE]
I can do no hands at 5mph :laugh:
 
Gravity Aided

Gravity Aided

Legendary Member
Location
Land of Lincoln
Can be a lot of things, but Kookas is right, I've had it happen on a fast descent with hands on the bars. Sometimes hub bearings are not quite properly adjusted, sometimes a fork is a very little bit bent, there seem to be a lot of factors. I can do no hands at 20 mph so long as the bike is securely fixed in a turbo trainer.
 
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Brandane

Legendary Member
Pat "5mph" said:
No hands at 20mph? :ohmy:
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fossyant said:
No hands at 20 and on new tyres !!! FFS !!!!

Simples
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fossyant said:
PS I cant do no hands at 1 mph. Tank slappers are a 40 - 60 mph situation where you find your botty meets your mouth.
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I've never had any problems doing it previously, on a variety of bikes. The faster you are going, the easier it is :smile:; I doubt if anyone could do it at 1mph. Gyroscope effect, I think it is called. :thumbsup:
 
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