Flex in a steel frame

Just how much is a steel frame meant to flex around the BB area? I know the tubing/lug design has an effect on stiffness but in general what should I expect?

My frame's a cheap SJS one and is 531 main triangle with a tange rear. Now it's pretty light and mostly rides ok but I've noticed it does seem to be a little flexy round the BB.

It's didn't bother me too much during the 18 months it was an SS, but since I fixed it 2 months ago it's been on my mind, particularly when I was applying back pressure to the pedals the other day and the frame seemed to flex quite a lot.

More of a curiosity question than anything really

It's all to do with how the frame is built, not so much to do with frame material. You can get very stiff steel frames, and very forgiving ones. Likewise with aluminium...

BB flex is more 'annoying' when it causes front shifter/chain rub :(

Buy a modern alu track frame. So unpopular with the steel is real crew and yet will drop half a second off your flying 200 :)

If you stand side-on to a steel framed bike and push down on the pedal, the bottom bracket will always flex out, as it will flex when you're pedalling - it's simply part of the nature of steel frames. It's only a problem if you don't like it, or if the frame is fucked (it'd be more than flexing then), in which case it's the manufacturers who are at fault, not the material. Maybe go for a long ride on a alu frame and see what you think? I've never ridden a alu fixed frame - I imagine the braking feels totally different.

No, that's not true. It's to do with how it is built. Not all steel frames will flex if you do that.

No you're right, not all steel frames will - most will to a certain degree. The point is, steel generally flexes, so I wouldn't worry about it too much, unless like I said, you don't like it, or the frame is broke, which will be more apparent than a bit of flexing.

are the cups on the BB plastic or metal??

Plastic cups? Surely the only plastic parts on BBs are the new fangled outboard bearing jobbies that use plastic spacers?

I think they're metal, I'll have to check. It's an old Shimano UN72.

It's not that I don't like it, it's comfortable and forgiving, I was more curious as to whether there was much difference between frames or if quite a lot of flex was simply due to it being steel.

Last time I rode steel it was an old GT mountain bike, which was heavy and stiff. Since I've come back to riding after a hiatus of about a decade, I've ridden an aluminium hybrid and my steel frame.

Seeing the flex made me wonder what sort of variations there were. As I said mine was cheap, only £130 from SJS and a road frame at that. As many ride track frames and frames built by the likes Bob Jackson and Mercian I thought I'd see what others experience is, which seems to be that steel frames have quite a lot of flex around the BB area. I don't think mine's broken.

I wouldn't say its normal to have "quite a lot of flex around the BB shell."

I really don't get any noticeable flex from my track frame, even when sprinting on the 'drome'. Theres definitely a suppleness to the ride which I like esp on outdoor tracks. Had other frames such as a 531 frame where I felt a little more flex. But maybe its because I've always been a skinny git, so never produced enough force to really feel the frames' flex.

I feel though such perceived flex depends a lot on the material and construction techniques used for the frame. Rider size and cranks will also have considerable impact.

I had an old, cheap, steel road bike and it did have 'quite a lot of flex' around the bb area.
No cracks or nuffink but compared to my race bike it was like wet spaghetti!
I is a big, heavy git bag though.

Sorry the "quite a lot of flex" is somewhat vague I know, but wasn't sure how to quantify it. The frame noticeably shift sideways when I press down on the pedal, whilst stopped and with the brakes on to immobilise it. I haven't checked too much when riding as I'd probably end up in the back of a car, but I'll find somewhere quiet and have a look.

What really made me notice was when slowing down. The frame looked like it flexed backwards, towards the rear wheel.

Size wise I'm about 6', medium to large build.

could be the cranks flexing

If you are stationary and push down on the pedals, most frames will move sideways at the BB. Don't know about the flexing backwards thing but it could just be in the description.
Clean and inspect the frame for fine lines, cracks, damage to lugs/joins/welds, ripples in paint/tubing indicating compression. It's probably nothing but it pays to have a look if you're worried.

Thanks for the advice. I'll give it a good clean and have a look.

what if you get flex in the forks when you brake at the front.....is that normal?

Yeah, often. Depends on how hard you brake. Check that your headset and brakes aren't loose though.

cheers for that i'll check that out

You can feel headset play quite easily but standing over your bike, squeezing the front brake tight and then endo-ing the bike and moving it back and forwards (i.e. the rear wheel up and down). You'll feel any play through the bars.

As Hippy said anyway fork flexing is pretty normal. If they didn't flex they'd probably break.

I have a reyonds ZEUS, and even at 150lbs. I flex it too much.

bb could be worn out.

So I've got a wee bit of an issue with flex in the vicinity of the bb area. I thought I'd pop a quick question on here to see if anyone knows anything before I start swapping out parts to figure it out.

To set the scene, this issue is with an 80s carlton 531 frame and what currently happens is the chainring moves from left to right when I apply pressure on the pedals. It needs a fair bit of force to induce the movement, which means to see it I must look down whilst pedalling, so its a bit tricky to pinpoint whats going on.

From what I can tell however, it happens more when I apply force to the drive side, and it may even be the case that it doesn't happen to the left at all and the movement is just it returning to centre. Whats more annoying is the chainring is perfectly straight when going along at a normal pace, but when in higher gears, with the application of extra force, it results in the chain rubbing the front derailleur and I can't move the derailleur out any more as the crank would hit it...

The cranks are a set of old super records and the bb is a second hand campagnolo one i got off of here not too long ago, but the issue was there with the old bb too, so I'm assuming its could be the frame?

If anyone has any ideas they would be much welcomed.

A bit of flex is to be expected, especially with an old steel frame. You need to tune your derailleur a bit so it doesn't rub. Modern groupsets have a trim function to allow closer setup whilst leaving the option to move the derailleur a bit to prevent rubbing.

My plug swings around the bb and the hollowtechs on the cube flex because its so beefed up around the bb, think its just one of those things