fuji_owner

So obviously, by my question, I gave away the fact that I'm not a technical road cyclist.

I read people talking about feeling the bottom bracket flex, feeling the frame flex, so that it's not stiff enough, etc. I understand that stiffness = more power transfer and flexing = more comfort. Am I right?

How do I detect if my road bike flexes, and by how much, and if that's anything I can use as a metric for improving my rides, or when I'm looking for a new bike (I'm looking for a Ti frame bike).

Thanks!

fstshrk

Unless you are Boonen or Cancellera, you should not be able detect bottom bracket flex on a new, modern Ti bike purchased in 2012. There are people that "believe" they can, but I think it is mostly in their head

Right Said Fred

Put it on a trainer. If you can see the front chain rings wobbling back and forth as you pedal, then you will know you are flexing the frame. Very easy to notice on a lugged steel frame. Probably not so much on a modern AL, Ti, or CF bike.

popeye

Crank it up to 1500 watts and you will know.

mustang1

@Right Said Fred: I thought it was quite easy to detect flex on most frames once you hook it up to a trainer.
@OP: To notice flex, buy a steel framed bike. (That's a troll at the steel-is-real brigade, I jest!)

The only flex I notice is in the handlebars, which is an indication to me to change them soon because the aluminium is fatiguing. An Al frame, incidentally, is over-built so you should not notice any performance degradation.

TrojanHorse

I used to get chain rub on the FD when I really got on the pedals on MY Ti bike. It's an oldie though..

iab

Really? Let me guess, you are an armchair material scientist.

For the OP, Sean Kelly rode a Vitus frame, what everyone considers at best is a wet noodle. Somehow, obviously only by an act of a merciful God, he won sprint after sprint. I'm sure God will look down on you just the same. In other words, don't worry about it.

Miyata110

Your question just indicates that you aren't delusional.

As soon as you think you've noticed some "flex" in your frame - find the nearest clinic and check yourself in.

BarracksSi

I noticed flex when the FD rubbed the chain when it was on the big ring, I was standing, and whenever one foot (and not the other) pushed on the downstroke.

But, it went away when I switched from a GXP crank to Shimano HTII. Same hair-close tolerance for the FD in the big ring-small cog combo, too. I stopped blaming the frame (any frame) for BB flex after that.

One of the guys at a shop had a little test for detecting frame flex -- he'd stand facing the side of the bike, put the near crank at 6 o'clock, and push it sideways with his foot. Somehow he thought that my BMC SLX01 had a good amount of flex.

Gluteus

Same here, riding my old bike, going up a steep hill.

oldbobcat

All bikes flex down there. Some flex hardly at all, some snap back nicely in time with your cadence, and some are complete noodles. In my experience the only ones to avoid are the noodles. Fortunately, nobody makes noodles anymore.

BarracksSi

BTW, my personally-owned GXP experience was on two bikes (including a CAAD8, which nobody thinks is flexy), and after switching both to HTII, the "flex" disappeared.

hueyhoolihan

there are a lot of things flexing when i stand and pedal. crank arms are easy to see. just straddle the bike, then with hand on handlebars, step into the left crank and bounce up and down a little with all your weight on the left foot. the crank will flex and so will the BB. and so will the handlebars and stem when you are climbing in or out of the saddle. but it's not so easy to see. things are better, in general, than they used to be, with the oversized tubes, stems, and handlebar clamp diameters, etc.

one one bike, i have found it WAY too easy to throw a chain when i tromp on it as i ascend a short steep hill.

adclark

+1 to putting the bike in a trainer. I can see a huge amount of flex in my steel bike but hardly any in my cf bike when I do that.

Beaker

Often times the movement you see on a trainer in the frame is coming from deflection of the front tire with each pedal stroke

7bmwm3gtr

Be careful testing on trainers, some times it can be the trainer flexing. I made a video about my frame flexing 2 years ago because I thought it was a noodle but I realized a lot of the "flexing" was the trainer itself.

ivan_yulaev

Has anyone shown that flexible frames actually result in significant power loss? I understand the chain rubbing perspective, but assuming that it does not affect the function of the drivetrain, is there any reduction in power delivered to the rear tire? My understanding is that it's a pretty lossless cycle, in that even if you ride a flexible frame, the system remains relatively efficient. Maybe a slightly flexible frame provides a better speed:comfort trade-off anyway; would you sacrifice 1% of your power for a 100% improvement in comfort? I think I would.

hairnet

Forgetting raw performance, a flexy noodle frame feels like pedaling through mud when a lot of force is applied and that just isn't fun to ride even if speed isn't significantly affected.

BarracksSi

That's what was happening with one of my then-GXP-equipped bikes. No matter how closely I ran the FD to the chain, it would throw it, even throwing it off the outside of the big ring. Switched to a different crank (Shimano Hollowtech II, specifically a 4600-series Tiagra), and it never threw a chain after that.

The frame was not flexing, it was the BB/crank (I was told it was the press-fit crank spider that was working loose) that had become wobbly.

Homebrew01

Shhhhhh ..... the marketing guys will come get you if this gets out.

Pedaleur

Though the bike is generally loss-less, the legs aren't, and the bike has to work against the legs to regain the energy used to deflect the bike.

That said, I agree with hairnet. I had an old Vitus, and even if it didn't slow me down (indications suggest it wasn't the bike ;-), I didn't like the feel.

big john

No, and this has been debated for decades. If Sean Kelly winning a sprint on a Vitus doesn't debunk that thought....

I had a 63cm Landshark made from Tange Prestige standard diameter tubing and the thing flexed so much when I would stand the bottom bracket would swing from side to side. People actually though the frame was broken, it was that dramatic. I sold it before it did break.

bikerjp

Or the trainer itself. I have a caad9 which most people say is very stiff but I can see the chainring move back and forth on a trainer. Pretty sure it's not the bike. Check out that video above. On the rear shot you can see the trainer flexing a bunch. That, more than anything, probably accounts for most of the perceived bike flex.

Beaker

That and if you notice the biggest movement is coming from the front end, where the tire can deflect or roll. Without securing the fork, it's pretty hard to pinpoint frame flex. I agree with you - I just think that there are so many sources of flex/movement here that blaming it on the BB doesn't make sense.

Brian Ratliff

If the chainring is moving relative to the front derailleur, it is the frame flexing, not the trainer. But generally, yes, if the chainring is moving in space and the front derailleur is moving with it, it is probably the trainer flexing, not the frame.