Andy_K 

What are you an engineer? Oversimplification is the heart of physics.

mikeybikes

IT Systems Administrator for a small to medium sized law firm. I'm not sure what that says...

Jaywalk3r

Correct. And our air resistance varies with our frontal area, not with our weight. With the same frontal area, the same effort is required to maintain the same speed of a heavy and a light bike.

Clipless or not, a 55-65 pound (including load) long wheelbase commuter with 20-30 pounds in a rear pannier is not easily bunny-hopped, but is very well suited for commuting.

Jaywalk3r

No, because, assuming the same surface area, the heavier bike will have higher surface area, so the air resistance has a smaller effect to overcome, evening things out.

Edit: Already pointed out.

PaulRivers

I would like to comment on your comment commenting about commenting, in order to comment that your train gif was a pretty great contribution to the thread and as well as a fairly accurate description.

mikeybikes

I would just like to point out that F=ma. In other words, a=F/m. Meaning, to achieve the same level of acceleration, more mass requires more force.

mikeybikes

And yet, here I am making FBDs and pointing out basic Newtonian formulas...

Jaywalk3r

Exactly (disregarding air resistance). And to maintain speed, a=0, which implies F=0, so mass doesn't matter.

mikeybikes

Disregarding drag and assuming a level surface.

This equation makes sense if you look at my FBD posted above:
a = g*sin(theta) - Fdrag/m + Fpedaler/m

The hard part is figuring out what Fdrag is. Air resistance, rolling resistance and other drag forces are fairly complex subjects and the mass has an effect on the rolling resistance and other drag forces. (Also, Fpedaler isn't really the amount of force the pedaler is transmitting into the pedals, but that's another complex subject.)

Disregarding resistances, yes, mass doesn't matter when maintaining velocity. Accounting for resistances... heck if I know.

PaulRivers

I think the train really described it as well as anything is going to at this point in the thread. :-)

Jaywalk3r

For maintaining a speed on level ground, weight doesn't matter, only surface area. At the same speed with two bikes identical in every way, except weight, the air resistance on each is identical, and that's the primary resistive force to be overcome.

mikeybikes

So then what's all this hubbub over rolling resistance about?

Andy_K 

True. Aside from losses in the system, I would also note that it is impossible to maintain constant force throughout the pedal stroke so even at apparently constant velocity, you are repeatedly accelerating and decelerating as your input force oscillates above and below the drag force. The difference in max force and min force through the pedal stroke is probably significant, but the time between oscillations is likely small enough that this can be mostly ignored.

The big problem with disregarding the extra effort of accelerating a heavy bike and rider up to speed is that unlike ideal machines used in force calculations, human legs have a nasty habit of experiencing fatigue and so that initial extra effort has a way of staying with you, especially in commuting situations where you stop and start a lot.

mikeybikes

In other words, my FBD is really useless

Jaywalk3r

Because we were trying to isolate the effect width has on rolling resistance.

Wblynch

I learned long ago a slow bike is faster than a fast bike with a flat tire.

mikeybikes

I'm confused. We were talking about a rider's mass and Newtonian physics and stuff. What effect does width have on rolling resistance?

Jaywalk3r

Thread hijack.

There is a negative correlation between tire width and rolling resistance.

Andy_K 

It depends on how slow. I've been passed by a guy with a flat tire in a cyclocross race.

mikeybikes

So... why aren't race bikes built for 4" wide tires?

cobrabyte

Does no one else encounter brick roads or gravel on their way to work?

The fastest tire is the one that's right for the job at hand. In racing, a more narrow tire is suitable, though thay have gone from ~18c t0 ~25c in the past 30 years, so that's something. For commuting, the variables are endless and so the "fastest" tire (if that matters to you when commuting) will depend on the route you take to work.

Andy_K 

They are!

Jaywalk3r

That width is well outside of the range of widths tested, and thus the conclusion cannot be extrapolated to such widths.

mikeybikes

How far can the conclusion be extrapolated, then?

Jaywalk3r

Only within the range of the experiment.