cyccommute 

That's an interesting array of forces that you think are independent of mass. If you bother to go look up the forces, you'll find a mass component in all of their definitions...even electromagnetic forces. Volts, ohms, coulombs, power, etc all have mass as part of their units. Simple logic dictates that if you have "kg" in the units, mass is taken into account somewhere in the derivation of those units. You can't just stick units in willy nilly.

Friction? Really? That one is a gimme. A spring? What pushes or pulls on a spring to give it force to exert?

I'm not conflating anything. You, and others, are forgetting about the effect of the force. Again, the air isn't moving when we ride a bicycle. We are moving the air. If we were standing in a wind, you could make a bit of an argument that the mass of the bicycle and rider aren't dependent on the mass of air moving but since we are providing the motive force, the force that we impart on the air is dependent on the mass of the bicycle and rider.

Finally, you need to study up on density. The density of the air does, indeed, change with temperature. But what is density? It is the mass of air contained in a certain volume. Generally speaking, we don't vary volume when talking about density...it makes the math more difficult. Something has to vary and since you have 2 choices...mass or volume...you are left with mass. When the density of air changes with temperature, it is indeed the mass of air that changes. Higher temperature, less mass. Lower temperature, more mass. Think, for example, nitrogen. At 20C, nitrogen 1.165 g/l. At the boiling point of nitrogen (-195C), the density is 800g/l. A liter is still a liter so the volume didn't change. The mass, on the other hand went up significantly.

cobrabyte

Andy_K 

So do heavy wheels increase or decrease the force of my brakes relatively to how hard I squeeze the lever?

Jaywalk3r

No, we're not. You're forgetting that the air has no knowledge of the weight of the Bike/rider system, and applies the same force to both, and only an equal force, independent of the bike/rider system weight, must be applied in the opposite direction to maintain constant speed.

cyccommute 

Again, we have this issue of some kind of "massless" force. This time you've taken the mass of the bicycle and rider out of the equation. Any force applied by the bicycle/rider to the air is dependent on the mass of the bicycle and rider because the bicycle and rider are providing the motive force. You can't have force without mass. Period. The air you are shoving out of the way has mass so it provides a force on the rider. The bicycle and rider have mass so they are providing force on the air that they are shoving out of the way. Yes, the air being moved is more dependent on the volume of the bicycle and rider but the bicycle and rider's force depend on their mass. The more mass that the bicycle and rider have, the more power...force acting over time and distance...has to be put into the system to move the mass of that bicycle/rider down the road.

If the force that the bicycle/rider express on the air is independent of the mass of the bicycle and rider, then a 40kg rider on an 8 kg bike should have the same speed for the same power output on a flat course as a 90kg rider on a 50kg bike. When has that ever happened in your life? I've never experienced that kind of joy.

cyccommute 

You tell me. Does a more massive object require more or less frictional force to stop? Does it take more force to slide (dry friction) a more massive object than a less massive object? Still haven't seen any "massless" forces yet. But I haven't seen a unicorn, Sasquatch or a demonstration of cold fusion, either. They inhabit the same realm.

wphamilton

You're thinking of the bicycle's motion as a series of accelerations, which is a valid observation since we know that the power stroke is not constant. But that pertains to the simple f=ma of changing the bikes velocity, not to aerodynamic drag.

The mass in the drag equation, (rho) is the mass of the air. The air presents a force on the bicycle, and is not dependent at all on the mass of the bike. Overcoming air resistance at a constant velocity means applying a force against the mass of air. Changing your velocity requires an added force against the mass of you and your bike. Two different things: moving the air, changing velocity of the bike.

Andy_K 

Requiring more force and creating more force are two different things. Sliding a heavy object involves the object's mass because the weight of the object provides the normal force, but in the case of braking the normal force has nothing to do with the mass of the wheel or the brake.

leicanthrope

This is beginning to resemble arguments over religion or politics.

Jaywalk3r

That's because the increased mass of the bike/rider system also causes the air resistance to have a smaller effect on the heavier bike/rider than it has on the lighter bike/rider system. This difference exactly offsets the effect of the increased bike/rider weight. When you do the math, the mass term cancels out on each side of the equation.

Jaywalk3r

That isn't what he's asking.

cobrabyte

At least those discussions offer some insight into the human condition. This pseudo-science nonsense is just amusingly ridiculous.

mikeybikes

At least religion or politics are debatable.

The sad truth, physics isn't debatable. It is what it is. The problem here is we apparently don't understand physics.

Jaywalk3r

Some of us do.

cobrabyte

ftfy

Andy_K 

Ooooh, oooh! Have we gotten to the part of the discussion where we all start trotting out our degrees and work experience?!

wphamilton

Let's skip that part.

I'm confident that everyone can understand this bit of science, if the right concepts are presented in a way that makes sense to us.

Jaywalk3r

It wasn't broken. The physics in this discussion are pretty easy to understand for everyone but cyccommute.

PaulRivers

Clearly the numerous people who don't agree with you don't agree with you.

cobrabyte

I don't agree with any of you. Your opinions read more like dogma than actual real world experience.

Jaywalk3r

You're right. I should have been more inclusive and added your name, too.

kickstart

I'm trying to figure out what it has to do with commuting. Fat tires are far better for 99% of the conditions I ride in when commuting.

Walter S

I agree. Many statements about "physics" are pretty ambiguous. The way to make a point about physics that everybody can fully understand is to support it with the language of physics (math).

Part of the reason we don't see that is that it will be very complex. Show the math for proving or disproving the wisdom of fat tires and include the air displaced, differences in rolling resistance, sidewall flex, and on and on and show how that's applicable to all bikes and riders or only some. Pretty hard to prove mathematically. It's easier to just expouse on your personal and unscientific beliefs. That's fine. But it's not physics.

bikemig 

Wow, these guys are going at it in 2 different threads,


https://www.bikeforums.net/touring/97...ng-gruppo.html

Jaywalk3r

We already isolated the effects of tire width pretty satisfactorily.

I was actually referring to the discussion about how much energy is required to maintain speed of a heavy versus light bike (given identical surface area and shape of bike/rider systems) on flat, level ground. Only one poster seems to think it takes more energy to maintain speed on the heavier bike, and even the calculator to which he linked disagrees with him.

The format of Internet forums makes talking in math pretty inconvenient.