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-   Road Cycling (https://www.bikeforums.net/road-cycling/)
-   -   Where are the numbers relating stiffness to speed or power? (https://www.bikeforums.net/road-cycling/902424-where-numbers-relating-stiffness-speed-power.html)

rpenmanparker 07-19-13 06:30 PM
Originally Posted by Pedaleur (Post 15868914)
Yes and no.

Try this: put a coffee can next to your trainer and your pedal just above it. Hold the brakes and press down on the pedal until it rests on the can. You'll note that the pedal can reach the can because the frame deflects. Now let go of the brakes.

What happens?
I'm not so sure your contention that only frame flex allows the pedal to reach the can is correct. More likely play in the chain and derailleurs is also allowing pedal to strike the can. Releasing the brake allows the tension in the drive train to be converted into forward motion of the rear wheel.

Point 07-19-13 06:33 PM
Found the link I was mentioning earlier. http://www.bikethink.com/bicycle-frame-efficiency/

It will take a lot of convincing to make me leave the camp of there being no correlation between flex and efficiency

rpenmanparker 07-19-13 06:36 PM
Originally Posted by OldTryGuy (Post 15868943)
http://www.youtube.com/watch?v=dSXwVC3akVM


Found this video with some numbers comparing some 2013 frames....stiffness starts at 6:15 into video
Yes, thanks. That video has circulated on the 41 previously. Good information, but it doesn't treat the relationship between stiffness and speed at all.

rpenmanparker 07-19-13 06:46 PM
Originally Posted by Pedaleur (Post 15868914)
Yes and no.

Try this: put a coffee can next to your trainer and your pedal just above it. Hold the brakes and press down on the pedal until it rests on the can. You'll note that the pedal can reach the can because the frame deflects. Now let go of the brakes.

What happens?
Yes, but what happens if the pedals are at 12 and 6 instead of 3 and 9? All the pedal travel will be due to frame flex. Will it be converted to forward wheel motion? I don't think so. Your "experiment" is smoke and mirrors.

rpenmanparker 07-19-13 06:51 PM
Originally Posted by Point (Post 15868961)
Found the link I was mentioning earlier. http://www.bikethink.com/bicycle-frame-efficiency/

It will take a lot of convincing to make me leave the camp of there being no correlation between flex and efficiency
Interesting document but totally fallacious. The stool experiment is particularly misleading. Any forward motion that is experienced when the brakes are released would have occurred normally if the brakes had not been actuated in the first place. There is no accounting for the actual losses when the pedals are continued to be depressed up to the point where a vertical rise in the BB no longer acts like a relative depression of the pedals, i.e. on the backside of the circle. No basis for the conclusions at all.

achoo 07-19-13 06:53 PM
Originally Posted by Point (Post 15868961)
Found the link I was mentioning earlier. http://www.bikethink.com/bicycle-frame-efficiency/

It will take a lot of convincing to make me leave the camp of there being no correlation between flex and efficiency
That's a load of crap. He make his conclusion that no energy is lost from frame flex from this childish "experiment":

Try this experiment. It will work best if you mount your bike to a trainer and disengage the resistance roller.
  1. Put the cranks in the horizontal position.
  2. Place a rigid block or stool under the forward pedal so that there is a small gap under the pedal.
  3. While holding the rear brake firmly, stand on the pedal so that it is pushed down to the stool.
  4. Keep holding the pedal down and release the brake.
When you pushed the pedal down, the chain did not move since the brake locked the rear wheel. Since the chain did not move, no work energy was delivered through the chain. The crank moved down with the pedal as the frame was strained. When you released the brake, the frame was able to move the center of the crank back up to relieve the strain energy. But the pedal remained in it’s lower position, so the crank had to rotate around the pedal as the bottom bracket went up. There was a reaction force in the chain and the chain moved as the crank rotated around the stationary pedal. The strain energy of the frame was converted to rotation kinetic energy in the wheel.
That's merely noting that SOME energy is returned from frame flex. The author doesn't even try to measure how much energy is transferred so it is literally impossible for him to come to this conclusion:

For a final part of this analysis, I have used an FEA model to determine the response of a frame to the four loads described earlier. It is somewhat trivial to look at how much energy goes in and out of the frame since we know that it gets released into the drive train.
For making unsupported leaps like that, the author should be competing in the Olympics - the long jump was made for him.

achoo 07-19-13 06:54 PM
Originally Posted by rpenmanparker (Post 15868990)
Interesting document but totally fallacious. The stool experiment is particularly misleading. Any forward motion that is experienced when the brakes are released would have occurred normally if the brakes had not been actuated. There is no accounting for the actual losses when the pedals are continued to be depressed up to the point where a vertical rise in the BB no longer acts like a relative depression of the pedals, i.e. on the backside of the circle. No basis for the conclusions at all.
It's not interesting, it's risible.

Dean V 07-19-13 06:56 PM
Up to page four and if you haven't noticed, still no data. Probably because there isn't any.
If some has been gathered by a manufacturer it must of been unhelpful for marketing.

Point 07-19-13 06:56 PM
Okay achoo - you got something better to disprove it?

rpenmanparker 07-19-13 06:57 PM
Originally Posted by achoo (Post 15868998)
It's not interesting, it's risible.
Hey, I just learned a new word. Sweet. Well played!

Bah Humbug 07-19-13 06:58 PM
Originally Posted by Pedaleur (Post 15868914)
Yes and no.

Try this: put a coffee can next to your trainer and your pedal just above it. Hold the brakes and press down on the pedal until it rests on the can. You'll note that the pedal can reach the can because the frame deflects. Now let go of the brakes.

What happens?
Releasing the brakes ignores the point. My whole point was that it's possible to push the BB shell back and forth laterally without also accelerating.

Releasing the brakes while pushing down on the pedal, at 3 o'clock, moving the chain... that's riding.

Homebrew01 07-19-13 07:00 PM
Originally Posted by rpenmanparker (Post 15868584)
Whether the frame is stiff or flexible, the rider will exert the same amount of force in non-productive directions. A stiff frame will move a little in response to these forces, a flexible frame will move a lot. But the wasted force will be the same, just with two different magnitude results in frame flexing. Once the force is exerted in non-productive directions, it doesn't matter whether it is recovered or not, it never gets channeled in a productive direction. It may be easier to pedal a stiff frame because it is not moving all over the place, but I doubt that a certain amount of pedaling force exerted by a human rider, i.e. not perfectly directed in circular motion, will make any difference on the two types of frames. The fact that the crank arms necessarily push the BB from side to side (as the are not centered on the BB, of course) is paramount. Force pushing the BB from side to side is the same whether the frame is flexy or stiff. It is just the amount of movement (strain) in response to the force (stress) that is different.
That has always been my thought also.

Point 07-19-13 07:06 PM
Just found the supposed scientific test conducted by procycling uk. It proves journalists shouldn't try science. They had 7 racers ride a modern carbon bike and a 1983 steel bike up the same 7.2%, 3km grade. Power output was measured by an SRM power meter, and since those cranks measure strain in the crank spider over time, it's a good measure of what goes into the chain. It can't measure any lost flex into the frame. Summarizing the conclusions, the average power as measured at the crank was an average of 3% lower on the steel bike. Measured time up the grade was an average of 3.4% slower. But if you take into account the extra 2 kg or so of the steel bike, the time differential should have been greater, around 6%. The writer theorized that the lower power was lost in the frame, but if you know how the power meter crank works, that power was never produced by the testers.

Still no scientific proof anywhere that stiffer = more efficient.

Pedaleur 07-19-13 07:09 PM
Originally Posted by rpenmanparker (Post 15868953)
I'm not so sure your contention that only frame flex allows the pedal to reach the can is correct. More likely play in the chain and derailleurs is also allowing pedal to strike the can. Releasing the brake allows the tension in the drive train to be converted into forward motion of the rear wheel.
Some of the motion is due to the frame. A thought experiement where the chain, spindle, etc., are extremely rigid reveals this.

Your 6 and 12 demonstation is exactly the converse: if your leg were a perfect machine, you'd get the energy back. Your leg isn't, so the energy is wasted.

Pedaleur 07-19-13 07:12 PM
Originally Posted by Bah Humbug (Post 15869007)
Releasing the brakes ignores the point. My whole point was that it's possible to push the BB shell back and forth laterally without also accelerating.

Releasing the brakes while pushing down on the pedal, at 3 o'clock, moving the chain... that's riding.
Perhaps I didn't explain it well, but you're flexing the frame, releasing the brake, and letting the frame unwind. Your leg does no work (the pedal doesn't move), but the bike is propelled. The energy in the flexed frame is converted to forward kinetic energy.

Pedaleur 07-19-13 07:13 PM
Originally Posted by Dean V (Post 15869002)
Up to page four and if you haven't noticed, still no data. Probably because there isn't any.
If some has been gathered by a manufacturer it must of been unhelpful for marketing.
That was settled on page 1. Everything after that is nerds doing mental ************.

Pedaleur 07-19-13 07:17 PM
Originally Posted by Bah Humbug (Post 15869007)
My whole point was that it's possible to push the BB shell back and forth laterally without also accelerating.
Again, the energy has to go somewhere. It can do work on the road or work on your leg. One moves you forward, the other is lost (or I supposed stored as potential energy if it raises your leg).

Or it can heat the frame.

Bah Humbug 07-19-13 07:18 PM
Originally Posted by Pedaleur (Post 15869042)
Perhaps I didn't explain it well, but you're flexing the frame, releasing the brake, and letting the frame unwind. Your leg does no work (the pedal doesn't move), but the bike is propelled. The energy in the flexed frame is converted to forward kinetic energy.
The bike unwinds when I stop pushing the BB to the side with my leg and the frame pushes itself back straight and moves my leg slightly outward in the process. I still don't see how this contributes to moving me forward at all.

rpenmanparker 07-19-13 07:21 PM
Originally Posted by Pedaleur (Post 15869042)
Perhaps I didn't explain it well, but you're flexing the frame, releasing the brake, and letting the frame unwind. Your leg does no work (the pedal doesn't move), but the bike is propelled. The energy in the flexed frame is converted to forward kinetic energy.
Okay, but what happens in the real world when you keep the BB flexed down until the crank is just past 6:00. Then rising of the BB doesn't produce forward motion because the upward motion of the BB is countered by upward motion of the crank.

RJM 07-19-13 07:37 PM
Originally Posted by Dean V (Post 15869002)
Up to page four and if you haven't noticed, still no data. Probably because there isn't any.
If some has been gathered by a manufacturer it must of been unhelpful for marketing.
I posted some links to Jan Heine's work into studying this back on page one, there are also Bicycle Quarterly magazine articles devoted to frame stiffness and it's effect on perfromance.

gc3 07-19-13 07:47 PM
Originally Posted by rpenmanparker (Post 15868744)
Hey, welcome to the club. I am the original pedant. Just ask gc3.
Not the original. Just one of the all stars...

Originally Posted by Bah Humbug (Post 15868890)
I say locked on page seven.
my money is on page 9...

Jiggle 07-19-13 08:26 PM
I'd like to see a double power meter test. Take two bikes of different stiffness, eliminate all other variables such as weight, bearing efficiency, or meter calibration, use a power meter at the hub and at the crank, and do some sprints. Compare power at the crank, power at the hub, and watts vs acceleration.

Bah Humbug 07-19-13 08:43 PM
Originally Posted by Jiggle (Post 15869230)
I'd like to see a double power meter test. Take two bikes of different stiffness, eliminate all other variables such as weight, bearing efficiency, or meter calibration, use a power meter at the hub and at the crank, and do some sprints. Compare power at the crank, power at the hub, and watts vs acceleration.
Problem is, with 1.5-2% accuracy on power meters, noise would swamp the differences.

Jiggle 07-19-13 08:46 PM
Well bah, humbug! Then there's no way to ever know for sure.

wphamilton 07-19-13 08:47 PM
Force pushing the BB from side to side is the same whether the frame is flexy or stiff. It is just the amount of movement (strain) in response to the force (stress) that is different.
Originally Posted by Homebrew01 (Post 15869011)
That has always been my thought also.
I don't necessarily agree with this part. A stiffer frame may be more elastic with less bend,wasting less energy in hysteresis. Also pedaling mechanics will be different, so asserting equal stress force in both situations is premature in my opinion.


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