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Something is rubbing or 'resisting' - ideas as to what?

Simon NotleySimon Notley Posts: 1,263
edited May 2009 in Workshop
Ok, this is a weird one. A few times recently when riding TTs on my main road bike I have noticed an occasional increase in resistance (and therefore decrease in speed) which lasts for maybe half a second then goes. The more I think about it, the more I become convinced that it was a figment of imagination or just the wind catching me at a funny angle. However, at the times in happened, I was convinced it was something real so I want to eliminate possible causes.

(The weird thing is that I haven't noticed it when riding normally on the same bike with different wheels. Ss perhaps this should suggest that the wheels are to blame... but I can't find anything wrong with them)

All I ask from you is suggestions as to what might cause such a thing. My ideas so far:

Brakes rubbing (almost certainly not, I loosened my brakes during a TT and it still happened)

Wheel touching chainstay under load (this is possible, it does seem to be tricky to get the rear wheel to align perfectly when fitting it - I think I may have slightly bent a dropout somehow)

Bearings failing in wheels or BB (possible I guess but never heard of this and wheels only have a thousand miles or so on them)

Posts

  • dbbdbb Posts: 323
    given it is a TT, i would go with option a) figment of your imagination.

    but since we want to go through a process of elimination,
    doubt it is the wheel bearings - that would be consistent.
    does it happen when cornering or straight line? (for wheel rubbing on frame) i assume the wheel is tight enough in frame.

    if the rubbing is for about 1/2 a second then this means more than one whole revolution! does this sound right? if so, then i think you can eliminate anything from the bb.

    just some thoughts to keep you going...

    good luck - i hate it when these things mess with your head...
    regards,
    dbb
  • bicebice Posts: 772
    This is probably hopeless: but on my self build I did not tighten the brake levers to the bars enough. As a result I was pushing the hoods forward and tightening rear brake. Then I would pull back on them, and there would be no problem.
  • Captain FagorCaptain Fagor Posts: 1,768
    My guess: inner face of right hand crank scuffing front derailleur when on the big ring.

    or

    If you have a Deda Dog Fang or similar and run a Campag chainset, one of the chainring bolts could be working loose and scuffing the fang.

    Probably horribly wrong on both counts, but worth checking.
  • markos1963markos1963 Posts: 3,724
    For my four pennys worth I would look at the rear tyre. It might not be quite centered on the rim making it slightly oval, as your speed increases the centrifugal force will make a tyre 'grow' and this could be whats rubbing and slowing you. The higher speed of a TT might be the reason for this especially if your frame has tight clearances.
  • on-yer-bikeon-yer-bike Posts: 2,974
    what tyres are you using?
    Pegoretti
    Colnago
    Cervelo
    Campagnolo
  • Simon NotleySimon Notley Posts: 1,263
    An update on this, having had a more thorough look at the bike, I think it is the BB after all. I don't think it's anything 'rubbing' as such, just the bearings failing and the resistance gradually getting more and the smoothness getting less. It's difficult to tell for certain when the BB isn't under load, but just turning it by hand it feels noticeably worse than those on my other bikes. I've ordered a new BB, so we'll see if that solves it.

    Thanks very much for your lateral thinking. Will let you know how it goes!
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