So I noticed whilst trackstanding tonight that ther is some serious flex in my bottom bracket, and it isnt just the cranks beacause if I put pressure on one side I can see the other move and can see the chain ring moving, it is a relatively newer shimano UN53. I am a large guy so maybe its just because Im fat, but it just seems odd that there would be any flex in a BB.

sure it's not just play in your chain?

i bet your noticing the frame flex.

or your cranks are terribly loose on the bb.

bb's (even cheap ones) just don't have flex....

Perhaps your cartridge isn't screwed in tight enough? If so, and if you have the tools to extract your cranks and tighten the cartridge, its an easy fix

-Kevin

+ usually that is the frame

Someting is not up to torque spec and is loose. Cranks? bb?

If the cranks are loose enough on the spindle to move around, you are in trouble.

But, it is almost certainly frame flex. I ride several bikes regularly, and it is always surprising to me how much difference there is in the way one flexes at the BB compared to another.

By and large, if the frame is aluminum, there will be minimal flex there. If the frame is made of good steel tubing, there will be little flex there. If the frame is made of hi-ten or low-end Cro-Mo, then there can be a ton of flex there.

jim

My frame is columbus SL and on steep climbs i can watch its bb shell flex back and forth. It's no big deal.

The flex you are seeing at you BB or chainring is the cumulative result of a lot of parts flexing, including your frame, wheels, stem, bars, and BB. The stiffest bike in the world is still going to show some flex down there.

Tube thickness and diameter play a large role in determining stiffness in addition to the material. I have two steel framed bikes, one alu bike and one carbon. The two steels are both the stiffest and the least stiff, with the alu and carbon bikes in between. You can make a stiff or flexy frame out of any of these materials.

Thank for all the help guys, I took the whole thing apart and reassembled it to be sure, everything is tight.

It is definatley not just the cranks because if I am just seated and I put pressure on one side I can see the other move.

I never even considered that it could be the frame flexing, this whole problem is occuring on an aluminum frame I just built, but when I am looking down watching the flex the bb doesnt appear to be moving at all

The frame flexes on all bikes.

A co-worker, a former professional triathlete, holds a bike by the handlebars and saddle and steps on one of the pedals from the side, making the frame flex with much less than his total weight on it.

The first time I saw him do it, it amazed me how much the frame flexed; but he said all frames do it.

I was just suprised, I figured Alu would be stiffer