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PostPosted: Mon Feb 21, 2011 11:10 am 
Old School Grand Master
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Location: Completely in the dark, thanks to me good mate Terry....
First time trial outing for the new 631-framed road bike yesterday morning. Acquitted itself well (7th place out of a field of 23) apart from nasty downhill speed wobble at the start of the race (at just below 30mph) which I just about managed to keep in check. Thing is, I've done loads of training miles on it including fast (~35mph) descents and never had this bother before; bike is normally a really stable "armchair" ride. Any suggestions? Culprits at present (or indeed permutations of all 3) are looking like;

1) Less than brilliant road surface
2) Never ridden the bike with tri-bars bolted on the front end before
3) Tyre pressure too high - 130psi in the front (it's a Vredestein Fortezza clincher so still well below the 170 max), 115 in the back (Michelin Krylion Carbon so right on the upper limit for this tyre)

Follow-ups and advice always welcome. Thanks in advance.

David


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PostPosted: Mon Feb 21, 2011 11:21 am 
Pumpy's Bear
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Speed wobble apparently has many causes and can be due to a combination of rider weight, road surface and harmonic resonance of frame so you can be fine in some conditions, not in others. Frankly terrifying when it happens though (and dread to think if you are riding fixed).

Apparently the solution when it happens is to relax rather than hold the bars in a vice like grip (easier said than done) and damp the resonance with your thigh on the top tube.


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PostPosted: Mon Feb 21, 2011 1:48 pm 
Gold Trader
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Mabey more of a modern thing, but I'm sure i've seen rims with an upper pressure limit as well.

Anything happened to the wheels since training rides. Are they still true?

Good result BTW.


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PostPosted: Mon Feb 21, 2011 2:04 pm 
Old School Grand Master
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Location: Completely in the dark, thanks to me good mate Terry....
Ally wrote:
Mabey more of a modern thing, but I'm sure i've seen rims with an upper pressure limit as well.

Anything happened to the wheels since training rides. Are they still true?

Good result BTW.


Nope, no dings or encounters with potholes to report. I was quite pleased with my result. Could probably have got 6th if it wasn't for the wobble but the guys in 5th place and above were getting times far quicker than I could manage in cold weather.

David


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PostPosted: Mon Feb 21, 2011 3:17 pm 
Gold Trader
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Hmm, as ededwards said, hard to pin-point.

Could you try and repeat it to isolate the problem, or to give you confidence there isn't an issue.

Not the thing to do in a race, but mabey worth trying while training.


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PostPosted: Mon Feb 21, 2011 3:31 pm 
Old School Grand Master
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Joined: Wed Jul 25, 2007 2:04 pm
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Location: Completely in the dark, thanks to me good mate Terry....
Ally wrote:
Hmm, as ededwards said, hard to pin-point.

Could you try and repeat it to isolate the problem, or to give you confidence there isn't an issue.

Not the thing to do in a race, but mabey worth trying while training.


I'll probably do that very thing this weekend if I've time as the stretch of road involved is a short ride from my house. Being really scientific about it, I probably ought to do two downhill runs, with and without tri-bars.

I once had a really bad speed wobble episode whilst in the Yorkshire Wolds on a previous bike (due to its back wheel not being dished correctly), which I got out of by using the gravel trap at the bottom of the hill intended for motor vehicles. Came out of that one unscathed but did fly over the bars and land in the gravel on coming to a dead stop!

David


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PostPosted: Tue Feb 22, 2011 2:48 pm 
Retro Guru
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Are all your spokes tight and even?

Are the forks the right size and shape and not bent? I have a bike with a very relaxed steering angle and I expect it might be a bit unstable at high speed.


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PostPosted: Tue Feb 22, 2011 10:04 pm 
Old School Hero

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Jonny69 wrote:
Are all your spokes tight and even?

Are the forks the right size and shape and not bent? I have a bike with a very relaxed steering angle and I expect it might be a bit unstable at high speed.

other way around steep head angles tend to be more be unstable.
David check your headset is done up perfectly and drop the pressures to 100 psi :)


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PostPosted: Wed Feb 23, 2011 10:39 am 
Old School Grand Master
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Location: Completely in the dark, thanks to me good mate Terry....
AndyPA wrote:
Jonny69 wrote:
Are all your spokes tight and even?

Are the forks the right size and shape and not bent? I have a bike with a very relaxed steering angle and I expect it might be a bit unstable at high speed.

other way around steep head angles tend to be more be unstable.
David check your headset is done up perfectly and drop the pressures to 100 psi :)


Cheers Andy - headset seems to be OK with no play (it's a Stronglight threadless job) but will double check it and definitely take your advice re. the tyres. As for comment about head angles, I think mine is 72 degrees so pretty relaxed for an out & out race frame.

David


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