scary speed wobbles (Boardman 2010)

by DarrenD   April 22, 2014  

Hi all

first post here Wave

I did post on the roadcc FB page and had some replies but I'm keen to know more..

I bought my Boardman road bike in early 2010 (the £999 carbon bike) after not riding since I was a kid - I enjoyed riding it and seemed to go down hills without any issues except getting used to it etc as the more experienced guys (in the tri club I'd recently joined) flew down

I did triathlon in 2010 and entered my first Ironman in 2011 - so I bought some 2nd hand HED wheels (90mm bad boys front and back) - it was around March 2011 I had my first bib short filler of a speed wobble - I never knew what it was at the time but it shook me up BIG time.. descending about 25-30mph with some cross winds and suddenly the bike went nuts and started shaking violently.. need less to say I grabbed the brakes and almost shat myself.. I kept the front HED 90mm for non windy and short rides only and then bought a 60mm HED front for my Ironman in Germany

I asked a few bike experts who tightened the headset and that was that.. about 3-4 months later (with the HED 60mm - again, 2nd hand off ebay) I had the same scary wobble descending a 10% hill (chain hill near Wantage in Oxfordshire) with blustery cross winds - again this had me in a real panic.. I descended very carefully from that point on and never quickly in cross winds - I can descend at speed fine with no cross winds as shown at my ironman in Henley in 2011 and also a cycling trip in Portugal last year..

but then I have had 2 wobbles in the last 8 weeks - using the original Ritchey wheels and my 7-8 year old Mavic Cosmic wheels (52mm).. Chain Hill again for the Ritchey's and a descent of around 5% last week in North Yorkshire..

any ideas? I've tried to look it up.. dodgy forks? wheels? I've never had it happen with my zipp 404 front but maybe that's just a co-incidence with the races/sportives and the weather?

I now always descend with my knees on the top tube and try and relax during the descent - I feel too nervous about riding on the drops in case of the wobble and so I ride on the hoods normally

14 user comments

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Lennard Zinn did some articles on this if you google "lennard zinn speed wobble".

posted by Charles_Hunter [72 posts]
22nd April 2014 - 11:03

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Yeah here's a link to the a Zinn article on high speed shimmy.

http://velonews.competitor.com/2013/11/bikes-and-tech/technical-faq/tech...

In short — it's complicated.

Jim

posted by Cyclosis [33 posts]
22nd April 2014 - 11:09

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I had a scary experience on a long descent in Spain some years back. My bike was heavily laden and I'm pretty certain that was a major factor. But the fact remains that such resonances can and do occur and are often unforeseen. There's no easy answer to them either I'm afraid to say.

High speed 'tankslappers' are well known in motorcycling and I've had one of those too, which spat me off my old Moto Guzzi. For a motorcycle the common cause can be using different types of tyre, while some motorbikes are notorious (the old BMWs with their thin forks and early Honda Pan-Europeans for example, especially the police bikes as they carried a lot of extra kit).

Going by the motorcycle experience, changing tyres might help, but then again it might not. The problems of resonances and harmonics in engineering design are really interesting, but extremely complex and there are many, many factors that could come into play. You could probably load a lot of data into a specific engineering computer model from Autodesk and identify the issue if you have a friendly engineer with an understanding of the subject and a willingness to help.

When I was still an engineer, harmonics and resonant frequencies were issues I used to deal with regularly and I found them really interesting. The maths is complicated though. But I don't do that job any more.

A stiff carbon frame could in theory be more vulnerable to resonances than a steel one. Sorry, but there isn't an easy answer to this problem.

Make sure the head stock bearings aren't loose, maybe try different tyres, check and see if there's any specific speed or road condition during which it occurs and be aware of the risk that it could happen. That's about all I can suggest.

OldRidgeback

posted by OldRidgeback [2132 posts]
22nd April 2014 - 11:43

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I've found it to be more simple, sometimes - you admit to being, like me, a nervy descender. Why does it feel more terrifying accelerating to 30-40mph on a 15% descent than it does going at the same speed on a gentle slope? It's all in the head, unfortunately. What this means is that we will often grip our bars and brakes far more tightly than a confident descender.
Gripping everything more tightly also means our body is more rigid, so the natural dampening that our meat provides to the frame stops working, and the bumps and wobbles the frame transmits to us increase. The more we tense up, the worse it gets, and so we tense up more. The scariest incident I've had in the last 6 months was throwing a massive wobbly going down Titsey Hill on a damp day in February. It's not too steep, maybe 10-15% in places, but the surface sucks and since I'd almost come off on a patch of oil earlier in the ride I was being cautious and braking hard. The wobbles came in, I tensed up and braked harder, and my bike tried to kill me. All going at the high octane speed of about 20mph.

Solution? It's exactly the opposite of what our instincts are telling us. You have to relax. Get comfy in the drops, breathe easy, hold the bars gently. Feather the brakes to lose speed rather than clamping hard. Get your body into the right shape for the bends. Trust your tyres.
I wish it was as easy to put all that into practice as it is to type, but it's possible. Try to concentrate on all that on a dry, sunny day on a wide road without too much traffic. To be honest, I only usually rediscover my descending mojo in mid summer. One day I'll get over it. One day.

posted by bashthebox [610 posts]
22nd April 2014 - 11:51

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I experienced this problem after changing the front fork. I solved it by running a slightly wider tyre at the front than the back. Even a small difference raises the front end relative to the rear, this slackens the head tube angle and increases trail which improves stability at speed.
Also relax the arms, check the front wheel alignment, check for any buckling of the front wheel and any damage or deviation in the fork.

posted by AwkwardEngineer [1 posts]
22nd April 2014 - 12:45

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You may be gripping the bars too tightly. There should be a millimetre or two of play in the bars, between your hands, sometimes more. Also, your arms should be slightly bent and relaxed, *not* with elbows locked out and arms rigid. This allows:

1. The natural self-straightening tendency of the front wheel to do its work.

2. Your body to act as a damper and absorb bumps of the steering (whereas rigid grip and arms increases risk of harmonic oscillation)

3. You to still catch and correct any major deviations.

Also, once you do get in a wobble, braking is possibly the worst thing you can do - hard braking almost certainly so. You transfer load onto the front, potentially increasing the force with which it can oscillate. In the worst case, you might hit the brakes just as the front wheel is significantly deflected and so cause it to tuck and you to crash.

When a wobble happens, you're best off *accelerating* and *relaxing*.

In general, to avoid wobbles, you need to relax on the bars to try avoid harmonic oscillation to begin with, and then to be able to damp out any that do happen.

posted by Paul J [557 posts]
22nd April 2014 - 13:01

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Sounds a lot like my experience (as does Bashthebox's). Basically, and as as Bash... states, it's all in your head. I was getting speed wobble, and so tried to take the descents more slowly and ride on the hoods, but that made it worse. Someone suggested that I was taking too much weight off the front wheel (it's natural to try and sit up and away from it if you're feeling twitchy), and they were right. Relax (I know, easier said than done, right?!), loosen your fingers, get on the drops - this moves the centre of gravity forwards and downwards, and trust your kit.

When I was just getting used to the idea of doing this, someone put a slight detour into what was otherwise a popular route for the club run. Next thing we're on a singletrack road that I wasn't familiar with, featuring pockmarks and bits of gravel. It drops away in front me, speed increases quick, and I tensed up and started to get on the brakes. Straight away I could feel the wobble, at maybe 30mph. Now I'd not long since bought my Trek Madone, which have a reputation for stable handling, plus I'd added GP4000s tyres, so I knew I could trust the kit. I made myself relax, breathed deep, stayed low and on the drops but off the brakes and just let the bike go. Hey presto, I recorded my quickest ever (by quite some margin) max speed of 50mph.

If I could have, say, 6 bikes, would it stop me drooling over others that I don't have?

posted by notfastenough [2940 posts]
22nd April 2014 - 14:00

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I'm off to Andalusia at the weekend (lucky me). If I can't nail some decent descending technique on their lovely smooth roads, I might as well give up. Watch this space.

posted by bashthebox [610 posts]
22nd April 2014 - 14:14

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First off, get the bike properly checked over - wheel trueness, hub condition, headset condition and adjustment, ensure the tyres are seated, even down to seeing if putting the computer magnet on a different spoke alters the balance of the wheel, and make sure the rear wheel is dished correctly and that everything seems in line. Tyre pressure too.

Try and eliminate the bike as a likely cause of the wobble, then you'll know if it's you or not. Indeed if you're confident the bike is spot on, maybe you'll be more relaxed anyway.

If you're descending and feel a bit uneasy, perhaps a gentle shift of your weight back may help.

posted by allez neg [4 posts]
22nd April 2014 - 14:36

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It could easily be your weight distribution too as a lot depends on your build and how you sit on the bike.

As AwkwardEngineer says, changing the forks and some of the other points might help. A lot of resonance issues occur due to the set up of the steering.

You could try either raising or lowering the bars a bit to change the weight distribution on the front wheel, though the rule of thumb would be that more rather than less weight on the front would be better.

With regard to a motorcycle's tankslapper, braking can be the worst thing you can do, especially if the reason for the resonance originates at the front end as this will exacerbate the problem and I expect a similar rule would apply to a bicycle.

Ride safe.

OldRidgeback

posted by OldRidgeback [2132 posts]
22nd April 2014 - 14:43

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My loverly custom Ti bike gave me a speed wobble at 50mph coming down the A470 during the Mille Cymru

After the event I was cleaning the bike and found cracking around the seat tube/top tube weld

It didn't wobble earlier in its career and doesn't wobble now I've had it repaired

I tend to ride a Specialized Roubaix now rather than the Ti bike. The Roubaix descends like it is on rails, it is extremely stable.

I don't go that fast downhill but if I'm getting to any kind of speed I press my legs into the top tube as I understand this dampens the vibrations

vorsprung's picture

posted by vorsprung [285 posts]
22nd April 2014 - 20:43

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Was your headset loose or had play?

If not, and assuming your tyres were Ok and matched, I suspect you just were hanging on too tight and not letting the bike find its own way.

I descend really fast, but I trust the bike to find its own way and do a minimum of braking.

But if you're not comfortable, there is no point trying someone else's method.

People have in the past asked for tips and advice on descending, but as soon as I try and think about what I'm doing, so I can explain I end up all over the place, on the wrong line, having to brake, missing apex's etc. so it's not something you can learn others, you need to 'feel the force'!

I actually find myself more stable at 40 than 30..... Though with less time to weave round potholes.

All Campag

posted by Flying Scot [457 posts]
22nd April 2014 - 20:47

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I put it down to an un-balanced wheel, much like your garage has to balance your car wheels when changing a tyre.

I have seen this exact phenomenon, only once, and it happened to also be on a Boardman road bike. When the (novice) rider had been yelled at to apply the brakes, and then gently calmed down, I noticed that there was a MASSIVE reflector in the wheel's spokes, without a balancing counterpart on the other end of the wheel. Under normal conditions (e.g. low speed, no massive reflector to un-balance the wheel) it doesn't happen.

posted by surfingsimon [3 posts]
23rd April 2014 - 0:46

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bashthebox wrote:
I've found it to be more simple, sometimes - you admit to being, like me, a nervy descender. Why does it feel more terrifying accelerating to 30-40mph on a 15% descent than it does going at the same speed on a gentle slope? It's all in the head, unfortunately.

I don't think its just the head, although thats a big part of it. When you descend a really steep slope, a lot of your weight transfers onto the front half of the bike, massively reducing the effectiveness of the back brake. Without load the tire is also far more likely to lock up. I can't decelerate nearly as quickly on steep slopes as I can on gentle ones.

posted by giobox [238 posts]
23rd April 2014 - 6:04

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