Frame Flex on GCN

[kramnnim]

Originally Posted by Kontact

I didn't say "even pressure". You inserted that.

If you watch this video of what the BB is doing under heavy load, you'll see that the most BB sway is right during the power stroke and is largely gone by the time the pedal is at the bottom. If this was just an issue of putting weight on the pedals, the flex would be at the bottom to match Mark's graphic. Instead, that's when it is going away.

Yes, we agree on when the BB is flexing and when it goes away. But I don't see how the flexing converts to rotational force and propels the bike.

[Kontact]

Originally Posted by kramnnim

Yes, we agree on when the BB is flexing and when it goes away. But I don't see how the flexing converts to rotational force and propels the bike.

The easiest way I've found to thing about it is that the rear center gets shorter when the stays twist along with the BB sway. When the BB straightens out the rear center elongates. Since the chain is already under tension it has to get longer too by yanking on the cassette.

[Kontact]

Originally Posted by kramnnim

It flexes the most at 3 oclock because that's when the leg is applying the most force

Right, but it isn't vectored to bend the crank down and over like it is at 6 o'clock. The BB bends at 3 because of the chain tension. At 6 there is still plenty of down force, yet it is unbending. The thing that is causing the bend is chain, and the chain moves the bike. You can't separate the BB flex from max chain tension.

[kramnnim]

Originally Posted by Kontact

Right, but it isn't vectored to bend the crank down and over like it is at 6 o'clock. The BB bends at 3 because of the chain tension. At 6 there is still plenty of down force, yet it is unbending. The thing that is causing the bend is chain, and the chain moves the bike. You can't separate the BB flex from max chain tension.

This doesn't make sense to me. The BB flexes because of the leverage from the crank arm. If your BB was frozen with the arms at 3 and 6, the BB would flex if you apply pressure on either arm, no chain needed.

[Kontact]

Originally Posted by kramnnim

This doesn't make sense to me. The BB flexes because of the leverage from the crank arm. If your BB was frozen with the arms at 3 and 6, the BB would flex if you apply pressure on either arm, no chain needed.

But they aren't frozen, and if you were coasting there is no way you could make the BB flex from 3 and 9.

[kramnnim]

Originally Posted by Kontact

But they aren't frozen, and if you were coasting there is no way you could make the BB flex from 3 and 9.

You could at 6 while coasting and none of that flex would propel you forward.

[kramnnim]

Originally Posted by Kontact

The easiest way I've found to thing about it is that the rear center gets shorter when the stays twist along with the BB sway. When the BB straightens out the rear center elongates. Since the chain is already under tension it has to get longer too by yanking on the cassette.

This doesn't sound like any frame I have ridden.

[Kontact]

Originally Posted by kramnnim

You could at 6 while coasting and none of that flex would propel you forward.

But we aren't talking about what happens at 6 because, as you noted in the video, the BB doesn't sway at 6.

And I doubt it would sway like it does at 3 if the BB was frozen.

This doesn't sound like any frame I have ridden.

How would you know?

[William]

Came across Dave Kirk's thoughts on frame flex: Three types of flex in the bike...

http://kirkframeworks.com/resources/...al/frame-flex/







William

[William]

Good example of the lateral/torsional flex...

https://www.youtube.com/watch?v=8fbpnn7TUSE





William

[William]

Short but shows bottom bracket flex...

https://www.youtube.com/watch?v=73inseR2zwI





William

[Mark McM]

Originally Posted by Kontact

The problem is, this isn't true. The maximum frame deflection is during the power stroke, somewhere around 3 o'clock. The frame isn't flexing because we are stepping on it, it if flexing because we are trying to move the rear wheel so hard that tension in the chain is enough to pull the BB off center.

Well, that's clearly not true. The frame is flexing precisely because we are stepping on it (or rather, because we are stepping on the pedal). If we step on the pedal when the crank is forward of the BB, then we generate a torque on the crank to drive the front wheel. But if we step on the pedal when the crank is vertical, no drive torque is generated, but we still flex the frame just the same.

[etu]

Originally Posted by william

came across dave kirk's thoughts on frame flex: Three types of flex in the bike...

http://kirkframeworks.com/resources/...al/frame-flex/







william

1+

[Mark McM]

Originally Posted by Kontact

The easiest way I've found to thing about it is that the rear center gets shorter when the stays twist along with the BB sway. When the BB straightens out the rear center elongates. Since the chain is already under tension it has to get longer too by yanking on the cassette.

Maybe you think about this way, but you're thinking is incorrect. You keep claiming that the stays get shorter and longer, but unfortunately there is no evidence that they do to any meaningful extant. So there can be virtually no energy storage due to this type of deflection. The deflection modes of the frame were modelled and analyzed in a web page that was reference on the 3rd page of this discussion (see link below). This analysis shows that the primary energy storage in the frame is torsional deflection around the the longitudinal axis. In other words, the cranks/pedals are going up and down due to a torsional twisting of the frame, causing a rotation of the BB shell as the frame twists.

https://web.archive.org/web/20060214.../Frameflex.htm

[Kontact]

Originally Posted by William

Good example of the lateral/torsional flex...

https://www.youtube.com/watch?v=8fbpnn7TUSE

I honestly can't tell if his frame is flexing or if his wheels and fork are. I can't see the HT and ST going out of plane with each other.