What is the truth behind bike weight? Does it really help THAT much?

TheDarkLord

Amen to that! If you really want to have a debate alienator, put aside your condescending attitude, and put forth in straight terms what analysis you did if any. But if you keep coming up with arguments starting with "lack of data" and then switching to "I did all the analysis", etc. you are just making clear that you are just spewing lies and BS to intimidate the hell out of people. Now that may work with the general public, and maybe you do it on a regular basis in your real life; but it will not work in a forum where there are some people who are scientists. Now, OTOH, I have seen conferences where some bigshots use the same argument that you use here to browbeat people who challenge their theories. I don't understand how a scientist can follow such methods...

artemidorus

John's model used a formula for stored energy that was, indeed, based on the square of the deflection of the frame (0.5kx^2, "^2" means squared). Because deflection of a spring is proportional to applied force, then energy is also proportional to the applied force squared. So you are right on both points, but John has already told us that.

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artemidorus

John's model agrees with you - he assumes for clarity that all the stored spring energy is lost - this is his 0.05% power wastage.

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Crankyfeet

John told us that the loss effect was 0.05% and was linear over a range of wattages. My point is that the stored energy is proportional to the SQUARE of the force applied at the crank (assuming a linear relationship between force and power ie. constant cadence). Refer to post #153 for an explanation. Since k is constant (in the F = kx equation), the pedal force is proportional to the deflection (x), which is squared to determine the energy stored (E = 0.5 kx^2).

Please correct me if/where I am wrong so I can save wasting time on a fallacious point...

Thanks for explaining things BTW.

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TheDarkLord

Here is one more consideration: when people talk of output power as measured by a power meter, is it only the power applied to the cranks, or does it relate somehow to the power output by the rider as a whole? If you look at the final sprint in the race, the riders are standing up, and they are waving their bike one way and the other for balance as they pump the pedals. So, if you talk about the total power output of the rider, some of it is going to be spent in the flexing of the frame during this action as opposed to being applied to the cranks. I doubt that a simple model will take this into account, and this could be important in the pro and Cat 1 type races, where riders win by a fraction of a second. Of course, I am not implying that this is a huge difference that will be felt by a typical rider...

thoughtforfood

Sweetie, I wasn't demeaned. You demeaned someone else. I COULDN'T care less if you demean me, you are a prickly blowhard.

As to the experiment, you have chosen to show no data, why should I? It appears clear to me that you all are discussing the spring effect of the frame solely. I am discussing the known FACT that the alteration of the path of a wheel and tire cause loss of energy. See, we have known that for a long time now. Now, that loss of energy is because of an increase in friction, and it isn't returned like the energy of the frame (acting as a spring). There is a measurable loss of energy, period. Doesn't matter how much.

At one point you say there isn't any loss, then you say that there is, but it is lost in the noise, then you say you don't have data, but everyone else needs some. Then you say you have done the experiment, but YOU have no data.

I realize now what is going on here. You don’t know what you are talking about, and you just like to belittle people to make yourself feel intelligent. I guess if that is how you get your kicks, go right ahead; but please, change your avatar from Mr. Pop to someone else. You don't really carry his gestalt very well.

Oh, and when you DO care less (you know, since you COULD care less) let us know.

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Felt_Rider

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AngryPenguin

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Crankyfeet

I'm also pretty convinced that the losses will be different for the same power at different cadences. The frame will only bend to the extent that the crank (drive train) provides resistance to the pedal thrust (force). At high cadence (lower gear) there is less resistance in the drivetrain, hence less transference to torsional bending in the frame. At higher gears and greater pedal force for the same power input/output (ie. a lower gear), there is more force applied to the crank and BB and hence more torsional and lateral deflection in the frame and therefore more energy absorption.

In fact, if you can push a high enough gear hypothetically, you may be able to put all your weight and thrust into the pedal, and not transmit one watt of power through to the road. You would effectively be bouncing on the pedal, at an incredibly high gear ratio, and your losses due to all elasticities in the system (frame, crank, chainring, chain, rear cassette, wheel, and tire etc.) would be 100% of input force.

There seems to be a myriad of complexities in this model that don't seem to be acccounted for, but again, correct me if I am wrong in this cursory analysis.

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Phill P

I agree... I'm a tad irritable atm though .

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mikesbytes

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sogood

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Crankyfeet

From my calculations, the deflection of the frame is proportional to the force (hence torque) applied to the BB. It is not proportional to power or wattage as power incorporates cadence.

Hence, for the same power value, you can have differing force values on the pedals (with corresponding differing cadences) which produce differing deflections and differing energy absorptions in the frame (proportional to the square of the deflection) and hence you can have DIFFERING EFFICIENCIES AT THE SAME POWER INPUTS.

Also by similar math, you can have the same absolute amount of energy energy lost at two different power inputs.

Also, the system appears to be a highly complex one, that involves multiple springlike deflections from multiple degrees of freedom (eg. torsional deflection of the frame, lateral deflection of the frame, vertical deflection of the frame, deflection of the crank, deflection of the chainring, deflection of the chain, deflection of the rear cassette, deflection of the hub/cassette, lateral deflection of the wheel, longitudinal deflection of the tire, lateral deflection of the tire). The amount of energy that the frame absorbs will be dependant on the interaction of all these points of "give" in the system. The determination of proportional energy absorptions (ie. losses - in the cases where the energy can't be retransferred back) would require, it seems, an iterative process of redistributing residuals and deflections throughout the system. It is enough though to realise that these various degrees of freedom influence/affect each other IMO.

It is also a factor that the crank torque is a varying quantity in the pedal stroke. If a significant portion of the high torque zone of the pedal stroke is taken up hypothetically stretching a very elastic chain, then force won't be absorbed in the frame until the chain deflection (stretch) reaches a point that it counteracts the force applied to the chain by the crank torque. The more deflection, the more that valuable displacement is lost in the high torque zone of the pedal stroke (around 3 o'clock), and hence less available to displace the frame. If springback occurs in the low torque zone of the stroke (in the case of the chain springing back) then it will interrupt the cadence rhythm.

Only when all paths of lesser resistance are loaded up, will the tire push on the road. The more give in the system, the less of the power arc of the stroke goes into propelling the bike. The system's components are interconnected by the fact that only after all lesser resistances are loaded up, will the frame deflect. Hence energy loss in the frame is dependant on energy losses (and elasticities) in the other degrees of freedom.

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JohnO

It's the ghost of Boudreaux... well, he never took it to a personal level, but that guy sure knew his stuff.