Frame Flex on GCN

[Bonesbrigade]

Originally Posted by earlfoss

All that math won't save you when you're facing a selection in a race, going over the crest of the hill at max effort for 15-30 sec. I can almost guarantee you the dude on a flexy frame is going to be working much harder to keep up with those on stiffer framesets. However, after he gets dropped he'll have ample time to consider the superior comfort and dampening abilities of his rig. He'll feel fresher after the race is over, too.

The ti Hampsten I raced for a season was an awesome bike. I loved it, but when the hammer was down, the little bit of bb flex was a liability. I didn't have ghost shifting or anything that extreme. Going from that to a Giant Propel, the difference was night and day. I'd say that for modern racing, to be competitive you need to have a bike that is at least as stiff as anything the average guy is riding in your event if that makes any sense.

Damon Rinard would disagree with you. For anyone interested - here is an excellent podcast on the subject:

https://cyclingtips.com/2017/06/cycl...ffness-matter/

[skouri1]

its psychological.
which does actually count.
contador refused aero stuff for a long time. Preferred lighter, shallower wheels. Emonda over aero bike etc.

Chances are though--if you felt bad on the hampsten, you would also have felt bad on whatever uber bike you were on. toasted is toasted, and as long as you have the same position and at least 9 gears, you should be competitive.

[earlfoss]

Well, duh, finding a bike with the right stiffness for you is important. I'm not saying that everyone should race on something that's as stiff as a Propel. It's not a binary thing. There's a reason why the aero road setups aren't commonplace in the pro peloton. I bet they beat you to hell. There's a medium to strike for everyone, true, but in competition I think that medium will naturally fall on the stiffer side of things. You're not going to want to race on something that visibly wags the bottom bracket around when the watts are high. Maybe you do want to though and that's ok because who cares anyway? If you enjoy riding it, whatever it is, then do it.

[William]

Originally Posted by earlfoss

Well, duh, finding a bike with the right stiffness for you is important. I'm not saying that everyone should race on something that's as stiff as a Propel. It's not a binary thing. There's a reason why the aero road setups aren't commonplace in the pro peloton. I bet they beat you to hell. There's a medium to strike for everyone, true, but in competition I think that medium will naturally fall on the stiffer side of things. You're not going to want to race on something that visibly wags the bottom bracket around when the watts are high. Maybe you do want to though and that's ok because who cares anyway? If you enjoy riding it, whatever it is, then do it.

Just got to find that sweet spot that feels good for your weight and the right mix of flex and stiffness.




William

[William]

I went back through this thread and took all the physics principles, scientific explanations, and graphs and compressed, collated, and calculated them together and came up with this graph that explains it all...







William

[fa63]

That is awesome I could argue that flex is the lack of stiffness (so one is the inverse of the other), and that rider weight and riding style should be separated, but your diagram captures the essence beautifully

[Fivethumbs]

I watched the video and this is what I saw:

• When the rider stepped on the pedal with the brake applied, the first thing that happened is that the tension was applied to the chain trying to rotate the wheel in the forward direction while the brake prevented the wheel from turning.
  
• Next the chain stretched until it could stretch no further at which time the frame flexed to the side until the pedal contacted the block.
  
• When the brake was released the tension that was applied to the chain now caused the wheel to turn. Then the frame move back to its normal position.

Wouldn't the same thing have happened if the frame did not flex at all and the chain stretched enough to allow the pedal to touch the block? In that case frame flex would be ruled out yet the wheel would still turn.

[Kontact]

Originally Posted by Fivethumbs

I watched the video and this is what I saw:

• When the rider stepped on the pedal with the brake applied, the first thing that happened is that the tension was applied to the chain trying to rotate the wheel in the forward direction while the brake prevented the wheel from turning.
  
• Next the chain stretched until it could stretch no further at which time the frame flexed to the side until the pedal contacted the block.
  
• When the brake was released the tension that was applied to the chain now caused the wheel to turn. Then the frame move back to its normal position.

Wouldn't the same thing have happened if the frame did not flex at all and the chain stretched enough to allow the pedal to touch the block? In that case frame flex would be ruled out yet the wheel would still turn.

Chains don't stretch by very much. Some frames don't flex very much, either. But if you are talking about a frame of somewhat normal flex you get this sort of dramatic result where a notable amount of the pedaling force temporarily gets used to change the shape of the frame rather than going all into the chain.

[Fivethumbs]

My question is: What caused the wheel to turn, the pre-loaded tension on the chain or the frame moving back to normal?

[Kontact]

Originally Posted by Fivethumbs

My question is: What caused the wheel to turn, the pre-loaded tension on the chain or the frame moving back to normal?

Compared to a bike where the BB didn't move, I would say the frame. But someone would object, so consider this:

You take a super stiff bike and the GCN bike and you lock your leg to hold the pedal at 3 o'clock. Your helper grabs the handlebars and pushes you straight back simulating a load on the drivetrain, like during climbing.

On the stiff bike, he won't be able to push the bike back as far before the chain, spokes, frame and everything else wind up enough to stop him.

On the flexy bike, he will be able to push you back further because the BB shell will twist and allow even more chain to be pulled over the cassette. He is pushing with the same force, but getting more flex distance out of the drivetrain as the stiff bike.


When he releases the stiff bike, it will scoot forward a little bit to its starting point.

When he releases the flexy bike it will jump forward more, but also end up where it started.

That extra jump is the flex of the BB that was trapped between your leg and the load on the rear wheel.

[Fivethumbs]

I guess the point I was trying to make was that the pedal started at point A and moved forward to point B. The wheel turned. That would happen whether the frame flexed or not.

To me their experiment showed that if the pedal is moving forward the rear wheel will turn and while frame flex may delay the movement momentarily, it won't stop it. That is so long as the frame returns to its original point.

If people are propositioning that frame flex by itself can propel a bike forward I don't see that. If people are propositioning that frame flex will not rob you of forward motion so long as the pedals are turning, then I can see that.

[Kontact]

Originally Posted by Fivethumbs

I guess the point I was trying to make was that the pedal started at point A and moved forward to point B. The wheel turned. That would happen whether the frame flexed or not.

To me their experiment showed that if the pedal is moving forward the rear wheel will turn and while frame flex may delay the movement momentarily, it won't stop it. That is so long as the frame returns to its original point.

If people are propositioning that frame flex by itself can propel a bike forward I don't see that. If people are propositioning that frame flex will not rob you of forward motion so long as the pedals are turning, then I can see that.

If the frame was stiff enough, the pedal wouldn't have moved to point B.

[Fivethumbs]

Originally Posted by Kontact

If the frame was stiff enough, the pedal wouldn't have moved to point B.

But it did. And the wheel turned.

[Kontact]

Originally Posted by Fivethumbs

But it did. And the wheel turned.

Did you watch a different GCN video with a super stiff frame?

[cachagua]

You lock your leg to hold the pedal at 3 o'clock...

Even if you could hold your leg that still, it wouldn't prove what you're trying to make it prove. The problem here is the same flaw that invalidates both the video and the FEA: they depend on the resistance at the rear wheel decreasing. That doesn't happen when you're riding.

If people are [suggesting] that frame flex will not rob you of forward motion so long as the pedals are turning, then I can see that...

Yes, people are suggesting that, but it's a misconception.

The flex in your frame does not ever make you go forward, period. That's because for this to happen, a lesser force would have to overcome a greater one. It is that simple.

As you're riding along, your legs vary how much force they're exerting between a maximum equal to (overcoming your inertia)+(frame flex), and some amount less than that. Now here comes the crucial point: your inertia never lessens.

The only time when the frame can un-flex is when your legs ease off, and the suggestion that when that happens, the flex energy is going to overcome your inertia -- the greater force, now that your legs are applying a lesser force -- is incorrect.