Increasing flex in a steel frame?

[shakedown]

I'm riding a Gazelle Champion Mondial AB from early 90s. Good cosmetic condition, 531, no rust, no dings.
Last few months , I'm noticing how much it flexes when I put weight on the pedals. There's a significant lateral movement of the BB. I'm not any slower because I'm pretty slow anyway but I think about it when pedalling up hills under load.

Just wondering if you chaps had any thoughts?
What is the life span of a steel frame? Could this extra flex be a sign of fatigue? Do I need a stiffer bike?

 

[CTK]

Unlikely to be extra flex more likely that you've just started noticing it!

 

[2manyoranges]

Fatigue​

Fatigue is where a material may fail at well below its UTS because repeated flexions have weakened it. This is why every Aluminium component in an aircraft has a fixed life measured in hours before it is at risk of failure. Steel on the other hand has a Fatigue Limit. Any flexing below that Fatigue Limit will NEVER fail – this is why car springs are made of steel not Aluminium. Every time a steel component is flexed beyond its Fatigue Limit it will weaken and the count-down to failure begin. Better steels have a higher Fatigue Limit (generally around half their UTS) and so again can be thinner and flex more before failure – hence 'spring steel' - but generally speaking a frame in quality steel like 4130 should be designed to be well below its Fatigue Limit in normal use with the gauges used for cycletouring and so last indefinitely.

from: https://www.bretonbikes.com/homepage/cycling-article-blog/163-steel-tubing-for-cycle-tourists


So this is good news for you. Is the flexing only when you are standing out of the saddle and honking? If yes, then I suggest you adopt a habit of upping your cadence and staying seated. I know of someone who borrowed a very lightweight hand-built who was told by the owner 'don't stand up on it on hills....' imagine his upset when the person did exactly that and snapped the frame on a hill in Yorkshire.

You may have simply noticed it and then it's on your mind. I had a Marin ti which you could hold with your hands - one on seat post and one on headtube, and then push the BB maybe two inches to one side! But that felt was just not noticeable when riding...

Note this small fatigue crack in an aluminium Intense frame - happening to a number of Commercial Supremes too....

 

[M_Chavez]

Could it be that you've put on some extra weight recently?

My 531db Bojack flexes like a noodle. When I first started riding it, I had to regularly stop and check that all stays are still attached to the lugs.
I've grown to quite like this wobbly noodly springy feeling eventually.

 

[shakedown]

Fatigue​

Fatigue is where a material may fail at well below its UTS because repeated flexions have weakened it. This is why every Aluminium component in an aircraft has a fixed life measured in hours before it is at risk of failure. Steel on the other hand has a Fatigue Limit. Any flexing below that Fatigue Limit will NEVER fail – this is why car springs are made of steel not Aluminium. Every time a steel component is flexed beyond its Fatigue Limit it will weaken and the count-down to failure begin. Better steels have a higher Fatigue Limit (generally around half their UTS) and so again can be thinner and flex more before failure – hence 'spring steel' - but generally speaking a frame in quality steel like 4130 should be designed to be well below its Fatigue Limit in normal use with the gauges used for cycletouring and so last indefinitely.

from: https://www.bretonbikes.com/homepage/cycling-article-blog/163-steel-tubing-for-cycle-tourists


So this is good news for you. Is the flexing only when you are standing out of the saddle and honking? If yes, then I suggest you adopt a habit of upping your cadence and staying seated. I know of someone who borrowed a very lightweight hand-built who was told by the owner 'don't stand up on it on hills....' imagine his upset when the person did exactly that and snapped the frame on a hill in Yorkshire.

You may have simply noticed it and then it's on your mind. I had a Marin ti which you could hold with your hands - one on seat post and one on headtube, and then push the BB maybe two inches to one side! But that felt was just not noticeable when riding...

Note this small fatigue crack in an aluminium Intense frame - happening to a number of Commercial Supremes too....

View attachment 674151

Came up the hill like Chris Froome this morning no noticeable frame flex in my easiest gear . Good cardio workout too ! Maybe I've lost leg strength?

 

[2manyoranges]

That sounds spot on….

 

[Carge]

I had a Marin ti which you could hold with your hands - one on seat post and one on headtube, and then push the BB maybe two inches to one side!

One hand on the headtube - one on the seatpost.
What did you push the BB with to move it two inches??!?

 

[2manyoranges]

One hand on the headtube - one on the seatpost.
What did you push the BB with to move it two inches??!?

....seriously....my foot....we wanted to see how much movement you could induce with not really much force. It was VERY surprising. indeed about two inches in total, one inch each way....wheels on the ground, hold seat post and head tube, push carefully with one foot. we were careful not to apply too much force.

 

[Carge]

Your foot - surely that's 12 inches

 

[pigman]

When steel bikes we're all the rage, one of the attributes of a good frame was some lateral distortion when you held the bars and saddle and pushed the BB with your foot.

Last year I bought my first titanium frame. That flexes like hell, but I do the longer climbs faster on it than I do on a much more expensive carbon bike. I reckon (no expertise here) that there is a bit of a pendulum effect. So you lose energy as it distorts, but you get some help back as it springs back on each stroke...... I dunno, it's just nicer to ride, the carbon is mothballed (there is no second hand value and I'm not giving it away)