Increasing Bottom Bracket Flex

[Christophe]

Afternoon

I ahve had a Planet X Kaffenback for about 8 years and have noticed recently that there is increased flex in the bottom bracket. The chain has begun to rub on the front mech when on the big ring and making an effort.

I don't think there's anything wrong with the chainset. It's a Campag Veloce with square taper and the bb has been replaced in the last 18 months, The crank arms themselves are good and stiff.

I have made a close inspection of the frame and there are no signs of damage or frame failure.

Is the increased flex a sign of fatigue and the frame is about to give up?

Cheers

Chris

[Biggsy]

Fatigue doesn't increase flex until something's actually cracked or broken.  The old myth that frames go soft with age is just a myth.

The only other causes I can think of:

1.  The crank has gone further up the taper if you've re-tightened the bolt.

2.  Front mech has gone out of adjustment for some other reason, and there isn't really any more flex (unless you're pedalling harder lately).

[mzjo]

While looking for things check the rear wheel (spoke tensions, bearing wear, cassette and freehub play). If any or all of that adds to a bit of maladjustment  on the front mech you could have the cause.

[tom_e]

Is the gear cable about to fail? I usually find it stretches a bit after new installation, then stays stable for most of the life, then suddenly starts changing again when the first strands begin to snap.

Or you have just started going to the gym and gained new strength. Where's the bicep curl smiley?

[Christophe]

Thanks for the replies.

Just to clarify. Front mech is fine. When not pedalling there is 3mm gap on each side between chain and mech. When pedalling, especially under load the chain brushes the mech so there is around 6mm of lateral movement / flex in the BB area. Is this normal?

[tom_e]

That sounds like a lot.  Have you checked the chainring is straight?

Like Biggsy, I can't see that fatigue will lead to increased flex.  Not until the crack becomes big.

[The French Tandem]

The frame has not changed. You are just getting stronger, Christophe!

Fatigue doesn't increase flex until something's actually cracked or broken.  The old myth that frames go soft with age is just a myth.

Agreed!

Back when racers used to ride steel frames, my then LBS used to say that a steel frame normally becomes softer after 2 years of racing.
I suspect this was a very wise marketing strategy   

[Ningishzidda]

The frame is definitely on its way out.

Send it to me and I will dispose of it for you.

[Christophe]

Thanks Folks.

Think I'll make closer inspection of the crank and maybe refit. Then get the magnifying glass out to have a very close inpecton of the BB area. I'll let you know if anything turns up!

[Ningishzidda]

If there’s a crack, you should hear something. Bike frames are good amplifiers, although it might sound like its coming from the rear hub.