"Speed Wobble"

[Huggy Bear]

hi all
I raced at the weekend and when I was coming down a long hill (42mph) and the whole bike started wobbling pretty fast - to be blunt I nearly sh*t my tri shorts! I was really hard to get it back under control and started to happen again on a few hills after this so I was losing some time. I am confident on the bike but heading to France next week and would like to sort it out before then
I have heard about it before - and when I "googled" it there's loads of stuff and alot physics (and I didnt try slow twitch!)
It was windy, het jet 60 wheels, tri bike with road bars on. It had never happened before when I had the "cowhorn TT bars" on same bike - same set up otherwise.
Any ideas? (that don't involve physics) could it be the narrower bars?

((on another note as a result of 9 mths of 3km swims (training for 1.9km races) with 2.5km sets with 60% paddlles, PB and band and wetsuit advice. I was 13/90 in swim - not great for you guys but for me to be leading the second group on the swim and be dropping them was exciting for me. No other coaching all from what I read here. Thanks TBB!!!!))

[Matthieu O’Halloran - "M@"]

Hey HB!

I have felt what your talking about in recent few weeks while training in the Swiss mountains.

Basiscaly, when going downhill many ridders tend to tighten up our bodies , straitening the elbows and stop pedaling or simply coast while pedaling softly.

Check out the grand tour cyclist, they are so fluid, in-sync and relaxed on their bikes.

That will come with time and experience...

Also, you become more un-stable while braking, so consistently tapping your brakes, its better to slow down, than sudden and strong jerks!

[Jonathan Caron - "jonnyo"]

mat says the most important with speed wobbles. Been relax....holding the bar loosly. also if the bike start wobbling, squeezing the top tube with your legs will make the bike stop shaking.....


There is a few issue mechanically that can cause wobbles....a tune up and check of your bike at the shop for alignment and everything tight is important.....

[DamienC]

Hey Huggy.

Matt and Jon are correct, being relaxed on the bike is the most important thing, tensing up amplifies everything.  What I'd like to know is where did it feel like the "wobble" came from?  Front, middle or back?

What worries me though is that this "never happened before when I had the "cowhorn TT bars" on same bike".

Check all your nuts and bolts, especially at the front end - headset, stem clamps, skewer, hub cups, etc.

Another possibilty is the change across to drop bars.  I assume that you were in the drops on the descents?  Considering that you were on a tri bike, in the drops, in a crouch with your nose over the stem, it could just be that the front end of your bike was just overloaded.  It might be a simple matter of descending with your butt hanging off the back of the saddle to balance out the loads.

I don't think it is the frame or fork, unless they're starting to fatigue, because like you say, it didn't happen until you changed bars, but it wouldn't hurt to go over your whole bike carefully and check.  What is the frame made of?

To simplify the geek speak, every material has a frequency.  When you ride, vibrations from the road get carried through until they equal the bike's frequency, and then you get a wobble.  And yes, all the bike's parts contribute to the frequency, so changing the bars may have brought your frequency in line with a slower speed.  Try changing the wheels and hitting a fast descent and see if that makes any difference.  Or even dropping your tyre pressures half a bar might be enough.

[Huggy Bear]

Cheers guys
I am pretty relaxed on the bike, the wobble seemed to come from the front.
I'll check the headset, clamps etc.  It's and Isaac carbon frame - not too old
I am glad I asked now as I hadn't thought of it being to do with too much weight forward.

Thats the best explanation of the geekspeak I have seen! (ever thought of being a teacher when you retire?)
I gripped the top tube with my knees through instinct and it did reduce it but not stop it.

Thanks again - I'm keen that it doesnt happen again. I'm not easily panicked but that got the heart rate up!

[Keith Watson]

Huggy, just out of curiosity, where do you have your water bottles mounted? On the front and the rear or on the seat and down tube?

I have had the dreaded speed wobble in the past on a carbon frame (but only AFTER) I moved the water bottles from the down and seat tubes to a fancy rear mounted system. As soon as I put them back on the down and seat tubes, no more wobble.

It seemed that having more weight centered underneath me versus on the front and arse ends gave the bike more stability at high speeds.

Food for thought...

[DamienC]

Hey Huggy.  Nah, no teaching post for me, too impatient! 

However, as it is an Isaac, I'd suggest that you read this, it may solve your problems: http://www.bikeradar.com/news/article/isaac-recalls-all-bikes-built-since-2004-20126

[Huggy Bear]

Hi
Keith - the bottles were underneath, but if my weight is too far forward as Matt said maybe behind could help. I hadnt thought of bottles!
Damien - ooooooh I didnt know that, and I have the old one (on the left in the pic) can you buy those 60mm plugs? I doont think Isaac are in business anymore.

Thanks this is all very useful.

[DamienC]

You should be able to get them through your LBS, or maybe TBB can ship you one?