What makes a bike slow?

Interesting video - shows nicely the stored energy in the frame. It does show that the same effect happens in stiffer frames too, so it doesn't work to differentiate the efficiency of stiff vs flexy frames, and I'd argue that the stiff frame test demonstrates more stored/released energy in that particular setup which exaggerates the forces applied whilst riding.

However, it is thermodynamically impossible for a frame to flex and not waste energy/heat up and I'm not convinced by your explanation of energy going into the rider's body (whether stiff or flexy, the energy is still going to be stored/released in the same manner).

The video only shows that/how the energy is stored. The amount of energy absorbed through a temperature rise by the frame will be so small as to be negligible. With a frame flexing at the rate of 90rpm I'm your pretty sure you're not getting even a temperature rise of 0.01°C.
It has been shown that riders put out more power on flexible frames : https://www.renehersecycles.com/the-biomechanics-of-planing/
(Just in case : please don't come out with any René Herse bashing now, without providing contradictory sources of at least equal quality.)

marmotte27 wrote:

The video only shows that/how the energy is stored. The amount of energy absorbed through a temperature rise by the frame will be so small as to be negligible. With a frame flexing at the rate of 90rpm I'm your pretty sure you're not getting even a temperature rise of 0.01°C. It has been shown that riders put out more power on flexible frames : https://www.renehersecycles.com/the-biomechanics-of-planing/ (Just in case : please don't come out with any René Herse bashing now, without providing contradictory sources of at least equal quality.)

That's much more like it. Finite element analysis and double blind testing, but shame that the issue isn't online: https://www.renehersecycles.com/shop/print/issues/bq-24/

Now that I can see that there is some science/explanation of the benefits of flexy frames, I'm much less sceptical of flexy frame anecdotes.

Assuming only modest differences in weight, I think small differences in geometry, setup, gearing, wheels/tires and fit can subjectively feel significantly faster or slower and have a real impact.  I once received a bike from a very reputable brand on loan while the regular was in the LBS for a couple of days.  I actually looked forward to giving it a spin.  But...it felt like if I was facing a 20 mph headwind the entire ride home (26 km).  Garmin later showed me that there was no headwind to speak of and my speed was indeed about 2 km/h slower while the effort felt a lot harder.  I don't think frame materials by themselves matter a lot if the frame is properly designed and the weight is comparable.  

I've heard the theory about frames being too stiff and not returning the energy of flexing several times but none of the proponents seem to be able to explain how the energy gets transferred back into forward motion, I think it is more likely that too stiff a frame is like a tyre with too much pressure, it does not even out the bumps and therefore slows you down. So yes, frames can be too stiff but not for the reasons they give.

Here they show how the energy gets stored and is returned into the drivetrain: https://youtu.be/BH_AL4rxrp8

I want to see them do it again with the pedal at the bottom of the stroke, because that is where the frame tension is released in normal riding, that experiment only works because the crank arm is nearly horizontal.

Backladder wrote:

I want to see them do it again with the pedal at the bottom of the stroke, because that is where the frame tension is released in normal riding, that experiment only works because the crank arm is nearly horizontal.

I wonder how much of the flexing is the frame/bottom bracket and how much tension is stored in the chain and brakes. With the crank vertical, there should be no tension in the chain and no need to apply brakes, so I agree and think that experiment wouldn't work.

Thanks for the podcast! They mention a table of frame stiffness here by the way ...

Me too. I've noticed everyone on the internet rides faster than me.

perce wrote:

Me too. I've noticed everyone on the internet rides faster than me.

 

I can ride as fast as anyone else on the Internet. It's on the road that I'm slower.

"The problem with this bike is me"

Massive scope for lighter weight / more aero / power gains there, were I more fussed.

Your bathroom scale is also a pretty good indicator of likely performance, of course.