OK, I’m always reading about how a stiff bike is better for power transfer. What I’m curious about is how much actual power is lost on a flexible bike?
How many watts does my noodle-like 979 absorb, as compared to a P3, or a stiff Cannondale?
“Energy can neither be destroyed nor created, but only changed to other forms of energy” -Albert Einstein.
Right, that’s why I said “absorbed” by my 979. Not correct?
OK, how much wattage is prevented from reaching my rear wheel by a flexible frame?
-No idea boss. Good question though.
On my Look KX Light I can hit four hard pedal strokes and there is the distinct sensation that the bike is accelerating. Not so on an old cro-moly bike or some very thin wall aluminum bikes.
OK, next question is, does this wattage “loss” manifest itself at all cadences? In other words, can it be mitigated by spinning a lower gear instead of mashing a big one?
Has anyone seen a wattage loss comparison between frames and wheels?
I can’t give any raw numbers but the power loss comes from the bike “springing” back up. When you press down on the pedal the frame flexes andd energy is used returning the frame back to the original postion. When there is less flex more energy goes towards moving you forward and not up and down.
I would bet how you pedal also comes into play… in relation to how much is “lost” in frame flex
Sean Kelly won many races on the “flexy” vitus frames. Most of them in big gear sprints. I would imagine if the loss was significant he wouldnt have the results he has today. I also remember reading a Davis Phinney quote about sprinting and stiff components. He said when his team picked up Syncros and Specialized as sponsors he was very excited because the new syncros stems would be much stiffer than his old cinelli and the Specialized 3 spoke would be alot stiffer than his old wire spoked wheels.He had always assumed he was losing power in sprints because of flexing components. He said what he found out though was that his bike skipped all over the road when he tried to sprint and he summized that the flexy stem and wheels were better for his style of sprinting. They allowed some the force he was generating to be absorbed rather than translated to the wheels causing them to skip side to side on the road. If you look at old photos from Dupont or other races, he has the Specialized 3 spoke on the front but a regular wire wheel on the rear.
Depends on how flexible the frame is. If you assume that at a higher cadence (and lower force) that the frame doesn’t flex, problem solved. If you assume that the frame still flexes a little bit but at a higher cadence then the power loss may be close to the same overall.
Which frame gets warmer as you ride? The energy has to go somewhere, and it would be heat. I doubt you could measure it.
The energy has to go somewhere, and it would be heat. I doubt you could measure it.
I was thinking that this would be a relatively simple measurement- apply a known force to the pedals, and measure the power at the rear hub. I would have thought this would have been done by one of the bike companies by now, actually- “buy a Cannondale, it transmits 5% more power to the wheels than your flexible flyer!”
What am I missing?
"I was thinking that this would be a relatively simple measurement- apply a known force to the pedals, and measure the power at the rear hub. I would have thought this would have been done by one of the bike companies by now, actually- “buy a Cannondale, it transmits 5% more power to the wheels than your flexible flyer!”
What am I missing? "
Drivetrain losses - chain and bearing friction
Applying a static force neglects dynamic effects
What am I missing? "
Drivetrain losses - chain and bearing friction
Applying a static force neglects dynamic effects
Ack. OK, apply a known power to the pedals. All components except the frames would be identical. Would this work?
The energy has to go somewhere, and it would be heat. I doubt you could measure it.
I was thinking that this would be a relatively simple measurement- apply a known force to the pedals, and measure the power at the rear hub. I would have thought this would have been done by one of the bike companies by now, actually- “buy a Cannondale, it transmits 5% more power to the wheels than your flexible flyer!”
What am I missing?
What you are missing is that it probably is not a measurable difference, but bike manufacturers dont want us to know that, so they can still sell us new stuff. I know a stiff bike “feels” faster, but I doubt it is measurably faster.
Still for most of us, seeing as we arent fast, we my as well feel fast!
"Ack. OK, apply a known power to the pedals. All components except the frames would be identical. Would this work? "
I still don’t think so. People have run SRM’s and Power-Tap’s on the same bike so they could measure power at the pedal and at the hub. This does measure drivetrain losses but frame flex is way down in the noise. Even if you measure the difference between two frames with the same components, the drivetrain losses will still be different because, for example, bearing preload will never be the same, and I suspect the difference in drivetrain losses is still much greater than frame losses.
Heat is one correct answer for energy lost. The only time I really worry about it is when the front derailleur rubs and it can’t be adjusted to stop. That grinding sound is the sound of lost energy…but even it is pretty minor in the overall scheme of things. If you stand up and sway the bike under you on climbs, I would think stiffer would feel better. I’ve had some rear wheels that felt like they would flex under hard pedalling, and that’s one reason I don’t hesitate to run a disc rear, even uphill. I don’t feel the flex in the disc that I do sometimes in a spoked wheel.
WAY back in the 80’s, I had a Cannondale and a Steel SLX bike. The Cannondale was rock-hard stiff, and the SLX was like a noodle. Somehow, I always climbed better on the noodle…don’t know why for sure, I could rub each side of the front derailleur all the way up a hill and still climb better on it…actually won a hillclimb on “the noodle”…my buddies couldn’t believe I didn’t race the Cannondale that day.
Summary…I don’t know.
“Energy can neither be destroyed nor created, but only changed to other forms of energy” -Albert Einstein.
Actually, it was Joule and/or von Mayer that put the first of the ‘Conservation Laws’ in its modern form (they derived it independently, although some credit goes as far back as the Greeks).
Just because a bike flexes does not mean the rider is losing power to an energy suck.
Right, that’s why I said “absorbed” by my 979. Not correct?
OK, how much wattage is prevented from reaching my rear wheel by a flexible frame?
vitus979,
I think that is a very intelligent question. In fact I have often wondered about that myself, but along the lines that any energy that the frame absorbed while “flexing” would be returned when the frame “unflexed”/returned to its normal shape. How does this constitute loss of energy applied to the crank/bottom bracket? Does it mean that your vitus frame will only return that energy back after you crest the climb and start down the descent? I don’t think so.
Maybe it will take a learned and trained professional like Kraig Willet to explain this concept to mortals such as us.
What you are missing is that it probably is not a measurable difference, but bike manufacturers dont want us to know that, so they can still sell us new stuff.
And that’s it right there. Energy would only be lost via heat. The energy lost via a flexing frame is vanishingly, microscopically small. It is so small that it cannot be separated from the background variation in power transmission (or “noise”).
You would lose more time in a bike split by adjusting your huevos once or twice than from a flexing frame.