Frame Flex on GCN

[rain dogs]

Pretty much every component on a bicycle is designed to operate on a very linear plane. The more a frame flexes, every bearing, bushing and connection/contact point is being loaded and operated outside of this ideal plane. That is friction and loss in the same way chainline losses occur. The stiffer the frame the less loss to these tangential loads.

Again, you can argue that this loss is miniscule and insignificant in the real world (I'm guessing you don't invest in oversize pulleys and ceramic bearings) but it is watts. It cannot not be.

The only possible argument for frame flex leading to a more efficient bicycle is rider comfort and fatigue factors on the rider. But 2-5mm wider supple tires and 5-10psi less tire pressure will do magnitudes more to increase comfort than a flexy frame. This conversation is like the the ontological argument for the existence of ____... the sasquatch. I get it.... it's not so much loss to frame flex, but it sure as heck is no gains.

So hey, if you like the flexy frame.... ride the hell out of it!

[Ungaro]

Just from my humble and non-professional experience:
1) Stiffness is a fine line.
2) I've compared two bikes side by side on a computerized diagnostic set up - one was a fillet brazed number made of Columbus Foco tubing (anyone remember that light weight stuff?), the other was a new 2007 Orbea Orca. The Orca was more efficient compared to the Foco tubed bike, but please note that the Foco tubed frame was shot. No really, I had ridden the snot out of that thing for 8 years, and say what you will, a lugged steel frame will hast longer than a non-lugged one.
3) Carbon can be VERY stiff. So stiff, in fact that over a road course, the accumulated micro vibrations can soak into you and wear you slap out.
4) Carbon is it's own worst enemy - What I mean is that it's a great ride for the first few years, but that stiffness helps to also degrade it over time. In the end, it's way less efficient.
5) What happens to plastic when you leave it in the sun? Think what happens to your carbon bike.
6) Last year I reconditioned a 1971 Raleigh Pro for Eroica California. I road that almost exclusively for 2 months. At the end of that time, I got back on the Orbea (now 10 years old) and felt invigorated...like "yeah, this beast is fast!"....until I checked Strava afterward. I was worn out and exhausted and it wasn't even a fast pace. Two days later, I repeated the same course with the 1971 Raleigh. Result was a faster time and I felt refreshingly great at the end of 30 miles. I repeated this little experiment a few times to see if it wasn't a fluke. It wasn't.
7) Sold the Orbea frame and purchased a new lugged steel frame (Bottecchia Leggendaria), and have had incredible rides ever since.

So, for me, yes....a little flex is good. Too much flex is not.
FWIW....just my experience.

[Kontact]

Originally Posted by rain dogs

Pretty much every component on a bicycle is designed to operate on a very linear plane. The more a frame flexes, every bearing, bushing and connection/contact point is being loaded and operated outside of this ideal plane. That is friction and loss in the same way chainline losses occur. The stiffer the frame the less loss to these tangential loads.

Again, you can argue that this loss is miniscule and insignificant in the real world (I'm guessing you don't invest in oversize pulleys and ceramic bearings) but it is watts. It cannot not be.

The only possible argument for frame flex leading to a more efficient bicycle is rider comfort and fatigue factors on the rider. But 2-5mm wider supple tires and 5-10psi less tire pressure will do magnitudes more to increase comfort than a flexy frame. This conversation is like the the ontological argument for the existence of ____... the sasquatch. I get it.... it's not so much loss to frame flex, but it sure as heck is no gains.

So hey, if you like the flexy frame.... ride the hell out of it!

I could see this argument if bikes had shaft drives, but the only component that really has to deal with the flex is the chain, and we are already quite happy operating that outside of the ideal plane by using multispeed gear trains.

I disagree that bigger tires are a better solution than flexible frames. The frame flex appears to be energy we can recover, but sidewall flex of larger rubber tires is heat loss. A balance between the two would be more efficient than a stiff frame and big tire.

[rain dogs]

Originally Posted by Kontact

I could see this argument if bikes had shaft drives, but the only component that really has to deal with the flex is the chain, and we are already quite happy operating that outside of the ideal plane by using multispeed gear trains.

I disagree that bigger tires are a better solution than flexible frames. The frame flex appears to be energy we can recover, but sidewall flex of larger rubber tires is heat loss. A balance between the two would be more efficient than a stiff frame and big tire.

1. Squeaking and creaking is friction, every bearing is dealing with frame flex
2. Bigger tires are faster and more efficient that has been proven time and time again.
3. You cannot recover 100% of anything. It's better to lose as little of it in the first place.

[Mark McM]

Originally Posted by Kontact

Correct. They speed up in the power stroke because less energy is tapped off to make bicycle velocity, and they slow down in the dead zone as that 'stored' crank velocity is used up making power later. You are not disagreeing with me.

Well which is it? Do they speed up in the dead zone to store energy, or do they slow down in the dead zone to store energy? It sounds like you can't make up your mind.

Originally Posted by Kontact

They aren't "supposed to" act like anything but structural members, but we are talking about what they actually do on bicycles, and what they do is twist around their shared centerline. And when they twist, the distance between the BB and the rear hub decreases. When the energy used to twist them is released, the rear center elongates. This elongation could be said to push the bike forward from the rear hub, or it could be said to pull the chain forward, driving the wheel. These aren't actually any different - just "feet push down on the earth, the earth pushes up on your feet" kind of semantics.

You're envisioning things that don't happen. I challenge you to measure any meaningful change in the distance between BB and hub on any rigid (non-articulated) rear triangle. We can easily measure lateral deflection at the BB and we can easily measure rotational deflection at the BB (relative to the rear hub), but there is no meaningful change in actual distance between BB and hub.

Energy stored in a spring is inversely proportional to the spring constant (stiffness). Since the linear stiffness between the BB and hub is so very high, there is virtually no energy stored in this deflection. Instead, there is far more energy stored in lateral and torsional deflection at the BB - but since these deflections are orthogonal to the chain force, you'll have to come up with a mechanism to redirect these forces/energies.

[William]

A simple exercise: Standing my bike up and leaning it like it would be in a spring, I apply pressure to the crank arm (at the bottom of the stroke) and the frame flexes. The quicker and with more pressure I do it and let off, the quicker it snaps back. In a sprint under hard torque where does that rebound go?

Interesting to note how much flex this guy is getting in the handle bars and built up stationary bike frame when he is applying power. He can't rock the bike as much as in the real world but consider if the guy in the GCN video could flex that frame by just standing on the pedal and holding the brake, how much flex will there be in a full on standing sprint when he mashing and torquing the pedals and handle bars back and forth? Quite a bit I would imagine.

https://www.youtube.com/watch?v=kiBw8mSgjRU







William

[William]

If you look at that guys related video with the push down/pull up exercise, there is a lot of torque being generated.





William

[JStonebarger]

Originally Posted by rain dogs

There is no free energy. It defies the laws of physics. 100W in is 100W out.

Certainly, there is a line of thinking that looks very close to: "all flex represents total loss and that no smaller amount is 'coming back' into the frame", which is incorrect, but what's 'coming back' is less than if the frame didn't flex at all in the first place (at least in a lab environment)

The stiffer frame will always lose less energy to flex. Always. But it may be a difference that is immeasureably small and has no real world influence...

Where does that lost energy go? If energy loss is significant at all shouldn't my whippy steel frame heat up as the frame flexes? Funny, I've never noticed that on even the longest climbs.

[Kontact]

Originally Posted by Mark McM

Well which is it? Do they speed up in the dead zone to store energy, or do they slow down in the dead zone to store energy? It sounds like you can't make up your mind.

As I said, the cranks gather velocity in the power zone and that velocity is carried into the dead zone where it is used up. The "storage" is just between power and dead.

I don't know why you don't understand what I mean.

You're envisioning things that don't happen. I challenge you to measure any meaningful change in the distance between BB and hub on any rigid (non-articulated) rear triangle. We can easily measure lateral deflection at the BB and we can easily measure rotational deflection at the BB (relative to the rear hub), but there is no meaningful change in actual distance between BB and hub.

I don't have to measure it - the GCN video quite clearly shows the "expanding" stays pulling on the chain. Instead of letting the wheel go completely, they could have let the brake out slowly and demonstrated exactly how much distance the chain is pulled for the amount of crank deflection.

[Kontact]

Originally Posted by William

A simple exercise: Standing my bike up and leaning it like it would be in a spring, I apply pressure to the crank arm (at the bottom of the stroke) and the frame flexes. The quicker and with more pressure I do it and let off, the quicker it snaps back. In a sprint under hard torque where does that rebound go?

It goes into the drivetrain. None of the flex in bicycle is ever immediately released like a mousetrap because the rider is always pushing on the drivetrain and the tire is always pushing against the road. For the energy to be released in a snap you would have to break you leg or have the rear wheel come off the road.

I was a pole vaulter in high school. Sprint, plant the pole, pole smoothly flexes, you change direction, pole expands to full length and you let it go way up in the air. But one time the pole broke just at the beginning of bending, and a tremendous amount of energy was released all at once into my arms and I just fell forward into the mat. It was awful, but demonstrated how that controlled release of energy is useful and immediate release is not.

[andrewsuzuki]

Originally Posted by JStonebarger

Where does that lost energy go? If energy loss is significant at all shouldn't my whippy steel frame heat up as the frame flexes? Funny, I've never noticed that on even the longest climbs.

Originally Posted by rain dogs

Pretty much every component on a bicycle is designed to operate on a very linear plane. The more a frame flexes, every bearing, bushing and connection/contact point is being loaded and operated outside of this ideal plane. That is friction and loss in the same way chainline losses occur. The stiffer the frame the less loss to these tangential loads.

Originally Posted by William

In a sprint under hard torque where does that rebound go?

I'll post this link once more with the explanation yall:

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

As for heat loss, which yes exists, I've been looking over this paper Elastic Hysteresis of Steel, which happens to use drawn tubing almost identical in stiffness to a 25.4 steel tube with .5mm wall thickness (although admittedly it isn't the exact composition or manufacturing process as bicycle tubing). Given the results on page 535 and some quick "broscience" calculations I did, the heat loss under very hard pedaling (1-2cm lateral flex) is something around the magnitude of 10^-4 watts which doesn't even put it on the same magnitude as oversized pulley wheels. Also, the damping ratio of composite and aluminum is in the same order of magnitude of steel.

Originally Posted by rain dogs

But 2-5mm wider supple tires and 5-10psi less tire pressure will do magnitudes more to increase comfort than a flexy frame

The argument for lateral flex doesn't have much to do with vertical compliance. In fact, I'd almost argue that bikes that are more laterally flexy tend to be less vertically compliant, since flexy bikes tend to correlate with horizontal top tubes, meaning longer seat tubes, meaning less exposed seatpost (which is the only thing that really flexes vertically on the rear end of a bike).

[fa63]

Originally Posted by JStonebarger

Where does that lost energy go? If energy loss is significant at all shouldn't my whippy steel frame heat up as the frame flexes? Funny, I've never noticed that on even the longest climbs.

If there was any heat generated due to frame flex, it would be so small that you would never notice it by touch.

[JStonebarger]

Originally Posted by fa63

If there was any heat generated due to frame flex, it would be so small that you would never notice it by touch.

Sounds insignificant to me. Meanwhile, I suspect the role of frame stiffness has mostly to do with ad copy/sales...

[ultraman6970]

I relate with this post a lot.

Originally Posted by Ungaro

Just from my humble and non-professional experience:
1) Stiffness is a fine line.
2) I've compared two bikes side by side on a computerized diagnostic set up - one was a fillet brazed number made of Columbus Foco tubing (anyone remember that light weight stuff?), the other was a new 2007 Orbea Orca. The Orca was more efficient compared to the Foco tubed bike, but please note that the Foco tubed frame was shot. No really, I had ridden the snot out of that thing for 8 years, and say what you will, a lugged steel frame will hast longer than a non-lugged one.
3) Carbon can be VERY stiff. So stiff, in fact that over a road course, the accumulated micro vibrations can soak into you and wear you slap out.
4) Carbon is it's own worst enemy - What I mean is that it's a great ride for the first few years, but that stiffness helps to also degrade it over time. In the end, it's way less efficient.
5) What happens to plastic when you leave it in the sun? Think what happens to your carbon bike.
6) Last year I reconditioned a 1971 Raleigh Pro for Eroica California. I road that almost exclusively for 2 months. At the end of that time, I got back on the Orbea (now 10 years old) and felt invigorated...like "yeah, this beast is fast!"....until I checked Strava afterward. I was worn out and exhausted and it wasn't even a fast pace. Two days later, I repeated the same course with the 1971 Raleigh. Result was a faster time and I felt refreshingly great at the end of 30 miles. I repeated this little experiment a few times to see if it wasn't a fluke. It wasn't.
7) Sold the Orbea frame and purchased a new lugged steel frame (Bottecchia Leggendaria), and have had incredible rides ever since.

So, for me, yes....a little flex is good. Too much flex is not.
FWIW....just my experience.

[Kontact]

Originally Posted by rain dogs

1. Squeaking and creaking is friction, every bearing is dealing with frame flex
2. Bigger tires are faster and more efficient that has been proven time and time again.
3. You cannot recover 100% of anything. It's better to lose as little of it in the first place.

1. Flexible bikes don't squeak any more than rigid bikes. If you account for the all the PF30 bikes, it would appear that not flexing is more of a problem. Having a BB shell flex with the cranking motion is going to put less side load on the bearings, not more.

2. Bigger tires are faster if we are talking 20s vs 23s or 25s. Above 25s the rolling resistance goes up when you are riding the correct pressure for your weight. The only graphs that show decreasing rolling resistance with size are when all the tires are at the same pressure. No one is riding 28s at 110psi.

3. That's correct, but you can choose where you lose it. Which is faster: A four wheel drive sports car or a mechanically more efficient two wheel drive? If (and this is an if) a flexy bike aligns forces with bearing better and keeps tire contact better, than the flex loss would be worth it for increased bearing and tire efficiency.