Hi guys,

My 2004 Firenze, which is stock save for a carbon post, is flexing quite a bit on my magnetic trainer. When I look down at the bottom bracket I notice considerable sideways motion. I can't be more specific about exactly what part of the frame I think is flexing, just that the bottom bracket sways back and forth when I really try to put power down. Oh, and I know the trainer is stable. Is this something to be concerned about, from either a reliability or a performance standpoint?

I know titanium frames are very reliable compared to aluminum or carbon, but I ride the mag trainer frequently these days and there is a lot of flex in my opinion. Could I be weakening the frame in any way?

Also, I know frames respond to power differently out on the road than they do when locked in a trainer, and I am a very light weight and thus relatively weak cyclist; but I sometimes wonder, how much power is lost in that flex and would I notice a difference by switching to a frame that was stiff?

Thanks for any opinions!

It could be you rocking in the saddle. When you peddle on the road or on rollers the bike can sway back and forth no problem. When your locked into a trainer they bike can't sway so you rock. Try this, place something very close to your pedal and see if you can bump in when pedaling in the trainer. I doubt if you will.

I have noticed the same. With my previous bike, a Giant OCR2 the bottom bracket sway was quite pronounced. I just purchased an '06 Litespeed Firenze and the sway is much less but still there. Yesterday I experimented a bit and it seems that some of the sway comes from the handlebars moving slightly back & forth as I peddle. This is probably what Juanmoretime meant when he said rocking in the saddle. If I steady myself with the top tube instead of the handlebars the sway is reduced, but it is still there.

I would guess that the reason is that on a trainer the bike is fixed in place, so the torque created by turning the cranks twists the frame. On the road the bike would instead rock slightly.

Are there any mechanical engineers our there who can explain this?

Hi guys,

My 2004 Firenze, which is stock save for a carbon post, is flexing quite a bit on my magnetic trainer. When I look down at the bottom bracket I notice considerable sideways motion. I can't be more specific about exactly what part of the frame I think is flexing, just that the bottom bracket sways back and forth when I really try to put power down. Oh, and I know the trainer is stable. Is this something to be concerned about, from either a reliability or a performance standpoint?

I know titanium frames are very reliable compared to aluminum or carbon, but I ride the mag trainer frequently these days and there is a lot of flex in my opinion. Could I be weakening the frame in any way?

Also, I know frames respond to power differently out on the road than they do when locked in a trainer, and I am a very light weight and thus relatively weak cyclist; but I sometimes wonder, how much power is lost in that flex and would I notice a difference by switching to a frame that was stiff?

Thanks for any opinions!

It's the trainer. Even the best trainers sway side to side a lot. - TF

RBR's 11/03/05 Newsletter: Trainer Insanity

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RoadBikeRider [RBRPublishing@RoadBikeRider.com]
Frame Flex on Trainers

As bikes get mounted on indoor trainers at this time of year, riders become concerned about the frame flex they see while pedaling. Experts say there's no reason to worry. In fact, frames flex on the road, too, but because they're free to pivot on the wheels it's not noticeable. Frames are designed to be resilient. They can endure a lot more weight and movement than they'll ever see on a trainer. So don't worry about breaking your frame as you ride inside and instead concern yourself with more crucial issues, like why the hands on the clock have stopped moving.

Thanks guys, good info.

I am really looking forward to getting the bike back outside.