Unsafe at One Particular Speed

So there I was on Sunday morning, innocently celebrating national Get Outdoors Day by riding down to Aldi for some cycling bargains. The other half had been persuaded that we could just about squeeze into the gap between showers (very satisfyingly, we did) and I was chasing him down the hill just as a car surprised me by turning onto our road (we live on a dead end road that serves just 6 households so this almost never happens). I jammed on the brakes and at that point my bike went from being effectively an extension of my body that seems to think itself around corners rather than needing any actual steering, to behaving like something possessed – shaking from side to side as if it was trying to throw me off its back. For what felt like about 20 minutes, but was only in retrospect probably 20 seconds, I was trying to get the bike back under control, while one part of my brain was planning where best to land when I (inevitably) got flung off. Fortunately, I didn’t have to – I managed to slow down, the bike stopped shaking, and I set off again somewhat shaken myself. (I should also add that thankfully the driver of the car was going very slowly and I was in no danger from them).

It was, as I discovered later, a speed wobble (something I had actually read about a couple of weeks ago for the first time, in this excellent piece by Jasmijn Muller about her recent Land’s End to John O’Groats record attempt). I undoubtedly wasn’t going anything like the speed Jasmijn was going, but it turns out that if you get a wobble that hits the resonant frequency of the bike just right then it can start to oscillate – and that gripping the handlebars tightly (as, indeed, you might do when your bike is apparently trying to unseat you) makes it worse. Instead you need to get your weight out of the saddle (again, easier said than done on a beserking bike) and lean forward with soft hands and eventually the bike will calm down, although whether I actually did either of those things I couldn’t tell you as I was too busy planning where to land.

Interestingly, in most of the descriptions I’ve read of speed wobbles, the rider has, like me and Jasmijn, spent some time contemplating how best to come off – and has in the end managed to bring the bike safely to a halt without needing to put that comfy-looking verge to the test, so – as I said to the other half this morning – perhaps they’re not as dangerous as they feel at the time (his not very reassuring response was ‘or perhaps the people who don’t manage to bring the bike under control don’t survive to write about it on the Internet’) especially as my own bike handling skills aren’t particularly brilliant (did I ever write about the time I managed to fall off my bike while actually stopped at a traffic light in Glasgow?). It has still left me feeling pretty tentative about descending and braking hard – apparently the cure for that is to induce another speed wobble and learn to control it, but yeah, no thanks.

However, today I had to be in town, and the only way was down, so I got back on the bike and everything was fine. I even took it into the bike shop for once over to make sure there wasn’t anything wrong with the wheels or the frame and the owner – bless him, for he has been campaigning to get me to buy a new bike for a couple of years now, and here was his chance – has reassured me that all is well. I also now know how to check a frame for possible weaknesses – as well as learning just how alarmingly a steel frame will bend under load. Of course, this just means that it could happen again, should I manage to find that sweet spot of speed and wobble.

Of course, the alternative could be that my bike genuinely is out to get me, possibly angered by the fact that I’ve taken the Brompton to Aberdeen instead of it, and haven’t bought it any more accessories for ages now. I may just do obeisance to the Angry Bike God and at least clean and oil its chain, by way of a sacrifice…

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7 Responses to Unsafe at One Particular Speed

  1. stcleve says:

    Squeeze the top tube with your knees usually stops them. It’s thinking about doing it while in panick mode which is hardest. Having unusual loading can often trigger them.

  2. Andy in Germany says:

    The Bakfiets does this a lot, but because it’s the weight of a small tank the effect is limited to the front wheel, which wobbles alarmingly at a certain speed. There’s a lot of slack between the front wheel and steering column so I find steering or slowing down stops it. In theory so would accelerating, but this is a Bakfiets after all.
    When loaded, the steel frame flexes when you go over a bump too. It’s quite alarming the first time because you think the frame is going to break, but you get used to it.

  3. disgruntled says:

    @stcleve – it was unusually lightly laden that day, which might have been the problem!
    @Andy – I would have thought the bakfiets would be immune but it shows it can happen to any bike …

  4. Lizzie says:

    This sounds rather scary, to say the least….. I’m not sure if I’m glad to have read about it or not! However I am glad I’m not the only regular cyclist whose bike handling skills aren’t very good! I always feel a bit of a prat because they’re not better. I watched my husband twisting and turning easily around some paths recently and there am I stopping at every turn….. I

  5. Charles says:

    Nice to see that my school boy physics is of some use, I remember resonance, breaking step when marching over bridges etc. Interesting that it has not happened before, that would imply a very specific and rare set of circumstances to set it off. Possible sunshine and lack of rain October went to your bikes head?

  6. disgruntled says:

    @Lizzie – hopefully if it does ever happened, you will be forewarned!
    @Charles – I was trying not to think of the Tacoma Narrows Bridge

  7. […] pressing against the frame, hence the squeaking. This, in retrospect, might go some way to explain my speed wobble the other day, which is also a little […]

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