Frame flex

[sirmy]

This has probably been done to death but how much flex would you normally expect in a steel frame.

My venerable 20 year old bike has what appears to be a couple of inches, certainly more than 1 inch, when the bottom bracket is pushed sideways.

I have considered replacing the frame (most of the components being under 2 years old) with a Surly long haul trucker but have read posting about the amount of flex in that frame.

An opinions/advice/blind prejudice for or against aluminium welcome

[Mick F]

My previous steel frames, including my precious 23 year old Mercian Vincitore, have always flexed when you bush your foot against the bb.

Perhaps an inch or so.

[stewartpratt]

My venerable 20 year old bike has what appears to be a couple of inches, certainly more than 1 inch, when the bottom bracket is pushed sideways.

I have considered replacing the frame (most of the components being under 2 years old) with a Surly long haul trucker but have read posting about the amount of flex in that frame.

And how are you measuring your flex? ie are you eliminating such factors as tyre deformation, lateral wheel flex, fork flex, crank flex, etc etc. I sincerely doubt your BB shell moves two inches with the head tube and rear dropouts held absolutely fast - unless you've had one or two tubes separate from each other altogether.

Having had a Trucker, it's just fine - I certainly didn't notice any excessive/untoward flex at all.

[Mick F]

I'm measuring mine by pressing my foot against the centre of the chainset whilst holding the saddle and the 'bars.

I look down and see about an inch in movement.

The wheels don't flex and neither do the forks or tyres.
(Don't get me on to tyre movement/distortion, because they don't !!!!!! )

It is just the main tubes that flex.

[rogerzilla]

You get some of the flex back through the cranks on the "return stroke", although the rest is lost in tyre scrub. Personally I'm not worried about flex unless it allows the brakes to rub or (in the case of my old Thorn) causes the gears to change themselves.

[stewartpratt]

(Don't get me on to tyre movement/distortion, because they don't !!!!!! )

Don't worry, I've managed to resist getting involved in that thread and have no intention of resurrecting it here

[Mick F]

You get some of the flex back through the cranks

I'm not pressing the cranks.

[rogerzilla]

Ah, but if you were riding, you would be. Frame steel is pretty good as a spring.

[epicurus]

Ah, but if you were riding, you would be. Frame steel is pretty good as a spring.

Yeah. What is wrong with a bit of flex? If you are flexing something that is reasonably efficient as a spring, i.e. the energy put into flexing is largely returned by the unflexing, then outside of Cavendish style sprint situations, I don't see the problem. The idea put forward by the cycling comics that flex is somehow lost energy is completely wrong.

[Mick F]

Ah, Rogerzilla and you are right of course!

My experiment was as described without riding:
I'm measuring mine by pressing my foot against the centre of the chainset whilst holding the saddle and the 'bars. ......I look down and see about an inch in movement.......The wheels don't flex and neither do the forks or tyres......It is just the main tubes that flex.

Great stuff!

[sirmy]

Thanks, that reduces the paranoia a bit. I was a bit worried in case excessive flex could be a sign that the frame could be on it's way out.

Just a note, I haven't measured the amount of flex exactly but placed my foot on the top of the crank arm on the non chainset side and pushed, didn't gauge how much force I used, and judged the amount of flex by eye

[Spinners]

A mate of mine had a Duralinox frame and used to do the bottom bracket flex thing with it. Seriously bendy!!

[Mick F]

Yes, I have a Floosey of a Duralinox. They are described ad "noodle-y"!

A wonderful ride. It makes into a very light bike. When my Mercian was having a re-paint for it's 21st birthday, I transferred all my Chorus Triple stuff onto it, and rode it for a few weeks. Fantastic!

The compete bike weighed 19 pounds or thereabouts. It was comfortable despite having 74degree aggressive geometry. It absorbed the bumps and roughness on the roads, it was responsive and fast, and a joy to ride.