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Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 3:28 pm
by wheelsONfire
Looking at latest is better, talking frames, often tells, more aero than previous and stiffer than previous.
But what deflection in a frameset and possibly wheelset(s) is actually synonymous with power loss?
(not aiming at suspension of MTB's)

Re: Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 4:33 pm
by Jugi
Any lateral deflection which occurs because of pedaling power not delivered on to pavement, means power is lost. Of course the frame or wheel will revert back from deflection and "return" the energy it has accumulated in deflection (GCN's YouTube channel has done a good video on this), but I don't think there are many cases where that energy will actually end up increasing the drivetrain's load in a useful manner, and eventually get delivered on to pavement.

When I had a Specialized Tarmac SL3, with a stiff wheelset the frame felt like a leaf spring while pedaling at high intensity on the top tube. I could actually feel the amount of deflection around the bottom bracket. The bike didn't feel "sluggish" but it had lively springy character. At the time I could make a direct comparison to my Scott Plasma 2 timetrial bike, which was stiffer in all axis and clearly more uncomfortable as a result. Currently I have a Tarmac SL6 which is much stiffer around the BB compared to the SL3 (not much deflection felt no matter how much I hammer it) but it has retained the all day comfort the SL3 had. The difference in deflection wasn't very large, but a laterally stiffer frame is a better frame.

Re: Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 4:33 pm
by Weenie

Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 5:16 pm
by C36
Long story short: out of the saddle it does, sitting... I am not sure (we didn’t measured it the same way we did out of the saddle).

The idea that deformation would give energy back seems to ignore that deformation is storing energy on a plan (“laterally”) different than the one used to pedal (“vertically”). One of the clearest illustrations of the phenomenon was the higher power peaks we measured with stiffer wheels. So stiffer resulted in better efficiency (faster for same power) and easier power generation attacking or sprinting.


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Re: Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 8:57 pm
by wheelsONfire
Laterally stiffer frames and wheels suffer less from power loss? - Yes?
But then again, how about deep slender frames (aero), it seems they would flex more laterally?
I know framesets (as an example) can use shapes and layups that supposedly are stronger talking twisting and/ or lateral flex.
More box shaped. Down tube on both my UP and Vial EVO Race has this design. It ough to be a kill blow talking aero properties!?
Some wheelsets could be argued being overly stiff. But are they faster if they boost such stiffness?
(Corima full carbon, Mavic CU/ CCU and Lightweight)

Re: Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 10:36 pm
by CrankyCarbon
I have a couple older bikes. One is aero frame (aero seatstays, seattube, downtube) made of aluminum and is as stiff as a I beam. I've barely ridden it over the years due to it's stiffness until recently when some wheels made it rideable. It definitely feels like it transmits more power directly to the wheels using the same wheelset as comparison.

My carbon frame I found when setting up a cadence sensor on both bikes that the carbon rear triangle sways several millimeters left to right upon medium to high intensity. The magnet which I initially set up a couple mm spacing to the frame would hit the chainstay on my carbon frame under med/high intensity, not a problem on the alu one which was originally placed in the same location.

But many newer carbon frames stiffen up the chainstays to prevent that horizontal movement. You'll have to do your research.
A couple years ago I was looking at a Fuji Transonic. Nice looking, affordable aero road frame. If it wasn't for the press-in BB (replace with Chris King) I might have gotten one. But that bike, as a complete bike can weigh from 15.21 lbs. / 6.90 kg (SRAM RED .. can only find the frame for $2000 USD) up to 18.32 lbs. / 8.31 kg (Shimano 105 @$1600ish USD). So, it depends upon your pocketbook, and then some of the components are still heavy on that 6.9kg version.

Re: Deflection = power loss (true or false)?

Posted: Sun Aug 25, 2019 11:32 pm
by TobinHatesYou
There are minimal hysteresis/heat losses. However excessive deflection will change/delay the application of power. Let's assume a system with no deflection...the power you put into the cranks gets transferred directly into forward motion. Now let's imagine the cranks and various tube sections are replaced by springs. Now some of your energy is being stored temporarily in the springs and then released on rebound...but on a delay. This delay might be undesirable, especially in accelerations or other technical situations. In a steady state it wouldn't matter as much.

Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 3:03 am
by jfranci3
Some deflection is good as it'll reduce peak torque loads on your joints and muscles, especially as you hit a bump while applying peak forces.

Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 8:42 am
by Marin
Almost no power is lost, it's all being returned to the drivetrain.

Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 10:26 am
by C36
Marin wrote:Almost no power is lost, it's all being returned to the drivetrain.
False... just read what I posted few answers above. First the gain are measurable, then that’s ignoring the biomechanical and the cyclist/bike interaction.


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Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 10:32 am
by C36
TobinHatesYou wrote:There are minimal hysteresis/heat losses. However excessive deflection will change/delay the application of power. Let's assume a system with no deflection...the power you put into the cranks gets transferred directly into forward motion. Now let's imagine the cranks and various tube sections are replaced by springs. Now some of your energy is being stored temporarily in the springs and then released on rebound...but on a delay. This delay might be undesirable, especially in accelerations or other technical situations. In a steady state it wouldn't matter as much.
Indeed. Storing energy « laterally » is storing energy almost perpendicular to the plane you need to spend it to go forward (vertically).
The steady stade doesn’t exist in cycling considering the cyclic nature of the power output (But if meant that sitting on the saddle the effect is smaller, yes, the lateral deformation is significantly reduced).

On a similar post few month ago, I now remember a video showing 2 bikes « loaded » (so with lateral deformation) and then checking how was the energy was released. Quite a misleading representation of the cycling gesture since everything was fixed, static, until they release the energy.


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Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 11:10 am
by Marin
C36 wrote:
Mon Aug 26, 2019 10:26 am
Marin wrote:Almost no power is lost, it's all being returned to the drivetrain.
False... just read what I posted few answers above.

Of course you measure higher power peaks, but you will also measure lower power troughs - overall power is the same.

If crank force can deform the frame laterally, returning lateral deformation will also exert a force on the cranks - unless you add a very clever mechanism to convert this into another form of energy :D

Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 11:59 am
by C36
Marin wrote:
C36 wrote:
Mon Aug 26, 2019 10:26 am
Marin wrote:Almost no power is lost, it's all being returned to the drivetrain.
False... just read what I posted few answers above.

Of course you measure higher power peaks, but you will also measure lower power troughs - overall power is the same.

If crank force can deform the frame laterally, returning lateral deformation will also exert a force on the cranks - unless you add a very clever mechanism to convert this into another form of energy :D
The question is “is the energy stored as lateral flexion of any used to move forward” and the answer is partially no. It’s some sort of equivalent to squeeze and release a spring in a plane that is NOT involved in the pedalling gesture. You put force to load a “spring” (laterally) that releases its energy during the end of the down stroke, once it’s not efficient to make you go forward.

Not sure to understand “higher power peaks, but you will also measure lower power troughs”. If you mean over an entire stroke the final power would be the same? no. Over an entire revolution, 1, 3 or 10 sec powers, the maximum value was consistently higher. You don’t want the high efficiency portion of you downstroke to use force loading a structure laterally.

Finally as I mentioned on the first reply. We did measured higher speeds under same power output (300w in a 8% slope if inrecall correct, all other parameters being equal).




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Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 7:21 pm
by bm0p700f
Tom hates you is closest. Replace tubes by springs. The stiffer tubes/springs still store energy but the less stiff tubes don't nessecilary store more energy but may feel more sluggish doing it due to flex/twist.

Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 11:06 pm
by TobinHatesYou
bm0p700f wrote:
Mon Aug 26, 2019 7:21 pm
Tom hates you is closest. Replace tubes by springs. The stiffer tubes/springs still store energy but the less stiff tubes don't nessecilary store more energy but may feel more sluggish doing it due to flex/twist.

Stiffness doesn’t matter in terms of total energy in the system and you just missed the entire point of this thread. The movement of the spring is the storage mechanism, just like the rotation of a flywheel is also an energy storage system. You theoretically only have hysteresis losses.

Re: Deflection = power loss (true or false)?

Posted: Mon Aug 26, 2019 11:06 pm
by Weenie

Re: Deflection = power loss (true or false)?

Posted: Tue Aug 27, 2019 5:52 am
by JoO