Page 16 - What is the truth behind bike weight? Does it really help THAT much?

17-02.-2008, 09:28 PM

#226

TheDarkLord

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

Quote:

Originally Posted by Bigbananabike

There must be SOME difference in speed between a heavier bike and a lighter ('faster wheel') bike.
For training I ride my steel framed single speed or my alloy Felt F80 with R500 wheels. My normal hilly training rides give me an average speed(or lack of speed !

) of 25kph.
Racing - I use my race only bike (Fausto Coppi alloy with Ksyriums and 20mm tyres, not a lot lighter than the Felt). A race example(sure I spent most of the time in the middle of the bunch) of my speed is 36 kph for 48 kms. A race the other week 72kms (a lot of it by myself) at average 31 kph.
There must be a bike component in there somewhere.

Do you train alone? The big difference between riding alone vs riding in a group/race is the drafting that reduces the air drag significantly. About the second race you mentioned above - maybe the extra adrenalin in the race accounted for the difference? And yes, there will be SOME difference between a heavy and light bike. But it will be very small, and probably unnoticeable, especially if your fitness is improving since the latter will swamp the former.

18-02.-2008, 04:49 AM

#227

Bigbananabike

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

Hi.
Yep, you're right - I do train alone(mainly) so its just me plodding into the wind....

I knew that'd be the biggest variable + riding with others inspires one to go faster (even if they're behind).
My fitness would be the same - even in our race season my training pace doesn't get any faster but I get faster racing.
Strange huh?!

Quote:

Originally Posted by TheDarkLord

18-02.-2008, 09:21 AM

#228

artemidorus

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

Quote:

Originally Posted by Bigbananabike

There must be SOME difference in speed between a heavier bike and a lighter ('faster wheel') bike.

Of course there is - on a steep ascent, the difference in speeds is directly proportional to the weight difference of the total system. If I trimmed my 8.7kg bike to the UCI limit (and I have no intention of doing that!) I'd be about 2% faster on a steep climb. On a 20min climb, this would be 24 seconds - a pretty fair margin. Even if you just want to hand your mates a caning, this'll help.
On the flat, there is no difference in steady speed. On a descent, a light bike is a slight disadvantage.

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18-02.-2008, 10:11 PM

#229

Tapeworm

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

Quote:

Originally Posted by artemidorus

Sorry, just curious, why is the light bike a disadvantage on the decent?

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19-02.-2008, 07:21 AM	#230
sogood Registered User Join Date: Aug 2006 Location: Sydney, AU Posts: 2,132	Re: What is the truth behind bike weight? Does it really help THAT much? F=ma. __________________ Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

19-02.-2008, 08:28 AM

#231

TheDarkLord

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

Quote:

Originally Posted by sogood

F=ma.

So? How does that translate to a disadvantage for light-weight bike? Considering just the weight, the accelerating force when descending = m*g*gradient where m is the mass, g is the acceleration due to gravity, and gradient is the gradient (well, actually gradient is the tangent of the slope angle, while the actual quantity that counts is the sine of the angle, but for gradients of the order of 10%, the difference is negligible). So, just by gravity, there should not be any dependence on mass of the bike.

19-02.-2008, 08:29 AM

#232

artemidorus

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

Quote:

Originally Posted by Tapeworm

Sorry, just curious, why is the light bike a disadvantage on the decent?

On a descent, the major retarding force is air drag, which is independent of weight. The major propelling force is gravity, which is proportional to the total system mass. The more the system mass, the higher the propelling force, and drag doesn't change.

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19-02.-2008, 09:07 AM

#233

TheDarkLord

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

Quote:

Originally Posted by artemidorus

Artemidorus, propelling force is larger for a heavier bike, but the acceleration is force divided by mass, and so it is independent of bike mass. If you drop two weights, one light and the other heavy, both will reach the ground at the same time if the air drag is the same on both. So, if the air drag is the same for both the light bike and heavier bike, I don't see how a light bike has a disadvantage.

19-02.-2008, 09:24 AM

#234

artemidorus

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

Quote:

Originally Posted by TheDarkLord

If you drop two weights, one light and the other heavy, both will reach the ground at the same time if the air drag is the same on both.

Wrong. They will hit the ground at the same time if there is no air drag, or if the air drag is the same proportion of their masses. If they have the same absolute drag, then the heavier mass will hit the ground first.

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19-02.-2008, 09:33 AM	#235
sogood Registered User Join Date: Aug 2006 Location: Sydney, AU Posts: 2,132	Re: What is the truth behind bike weight? Does it really help THAT much? Actually, mass independence is only true in a vacuum ie. In a system without resistance. For a bike with a cyclist on top, aero drag and system friction will all retard the rolling bike. __________________ Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

19-02.-2008, 10:00 AM

#236

TheDarkLord

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

Quote:

Originally Posted by artemidorus

Ok, I get it. The drag force is independent of mass, and hence, the drag per unit mass is less for the heavier bike. Is there any estimate on what the actual gain is?

19-02.-2008, 10:21 AM

#237

artemidorus

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

Quote:

Originally Posted by TheDarkLord

Ok, I get it. The drag force is independent of mass, and hence, the drag per unit mass is less for the heavier bike. Is there any estimate on what the actual gain is?

Anecdotally, at 90kg I can roll past most people if none of us are pedalling, often at 1-2km/h more than they are doing. Clearly there are some potential confounders with that example, but the differences are not completely trivial.

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19-02.-2008, 10:35 AM

#238

sogood

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

Quote:

Originally Posted by TheDarkLord

Ok, I get it. The drag force is independent of mass, and hence, the drag per unit mass is less for the heavier bike. Is there any estimate on what the actual gain is?

Not quite. But the common reference of a heavier rider is that their aero drag is not proportional to the increased weight when compared with a lightweight rider. So heavy riders will progressively gain more advantage with increasing weight. But if you are talking about two bikes (same design without the rider but differing weight), then there's no resistive difference b/n the two, but there will be a difference in the propulsive force on a downhill.

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19-02.-2008, 11:00 AM

#239

TheDarkLord

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

Quote:

Originally Posted by sogood

But if you are talking about two bikes (same design without the rider but differing weight), then there's no resistive difference b/n the two, but there will be a difference in the propulsive force on a downhill.

But what counts is force per unit mass, right? So, don't see how that alone will make a difference. Since drag is proportional to the square of velocity, if you write out the equation of motion, it will be

mg sin(theta) - kv^2 = ma,

where theta is the slope angle, and k is a coefficient related to drag, v is the velocity. So, while drag per unit mass is less, the propulsive force per unit mass is the same. So, don't see how increased propulsive force will lead to the difference here.

19-02.-2008, 11:11 AM	#240
sogood Registered User Join Date: Aug 2006 Location: Sydney, AU Posts: 2,132	Re: What is the truth behind bike weight? Does it really help THAT much? I am no physicist and there are lots of far better qualified people here to answer this. My understanding is that given the same bike apart from weight (same resistive forces), the propulsive force on the heavier bike would be greater due to its mass (g is a constant and same for both bikes). So as a result, the net force (propulsive) on the heavier bike is greater than the lighter bike). Am I wrong to explain it this way? __________________ Morphed Bianchi Camaleonte IV 2006, Ridley Damocles 2006, Garmin, Mac

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