DonChuwish

Hello,

I remember reading an article in a car magazine once that discussed how race car builders are more interested in dynamic weight than static weight. For example, it's more important to lighten up wheels, brake rotors, clutch plates, etc., than it is the frame (gram for gram of course).
Does the same apply at the much slower speeds of a road racing bike? Seems to me you'd be better off spending money on lighter wheels than on a lighter seat/seatpost for example. True?
Obviously some other high end components may be lighter AND offer better functionality than their lower end siblings, but otherwise does the rule apply?

Don

shane45

Yes. The lighter the components (mass), the less energy is required to get those components moving. Therefore, it makes sense to look at components that are not static in nature first.

It takes energy to get your wheels spinning. It takes energy to get your cranks spinning. So not only are you trying to move these components forward as part of the whole bike, but they are also moving independent of the frame as they rotate. So, yes - it is important that these parts be lightweight as they are the ones that move the most.

Of course the overall weight of the bike needs to be considered, as the sum of all the parts needs to be put into motion as well. However, if you get your non-static parts (wheels, crankset) up to speed faster, the natural progression is that anything attached to these parts will accelerate faster as well.

Once you are up to speed, a lighter overall package takes less energy to keep it moving at the same speed.

God bless you Mr. Newton.

LóFarkas

Sure. The most important area is the outer circumference of the wheel(tyre, innertube, rim). These parts are further from the axis of their rotation and so are harder to get moving. Plus you have to get them to spin pretty fast.
False.

That's why time trial bikes are ~4pounds heavier than the ones ridden in a mass start race. When the terrain is flat and you travel at a consatant speed, aerodynamics and drive efficiency (frame stiffness)matter much, much more than weight. Theoretically, weight is irrelevant on flats at a constant speed, and more weight is actually beneficial on downhills.
In practice, of course they try to keep the weight down because there are always minor climbs, acceleration it corners etc.

shane45

No sir, YOU are wrong. Somebody needs to beat you with a Newton stick.

Use cars as an example...torque is what gets a car up to top speed quickly - HORSEPOWER (energy) is what keeps that speed constant. Why do you think race cars are made as light as possible? Lower mass uses less energy (and fuel) to maintain a constant velocity. No different on bikes...

The fact that you indicate that "more weight is actually beneficial on downhills" actually proves my (OK, Newton's) point. More weight means that the bike is not as prone to shifts in direction and absorbs and stabilizes a bumpy downhill ride because MORE MASS IS HARDER TO CHANGE DIRECTION (any direction - forward for example, ie. acceleration, or up and down bumps). Therefore, the heavier an object, the more energy is required to get that object to change direction, or to accelerate.

I suppose you want to tell me that heavier objects fall faster and that's why downhill bikes are heavier, right?

ofofhy

edit - OK, you and Lo Farkas have edited and amended your posts above to make mine nearly void.

Acceleration is not a change in direction. It is a change in velocity. Theoretically an object at a certain velocity will remain at a constant velocity absent any forces acting upon it.

When a bicycle is at a constant speed, its mass is irrelevant in terms of applied force needed to keep it at that constant speed. You need the power to overcome all of the resistive forces acting on said bicycle (friction from air, tire/ground, bearings...), not the inertia of the bicycle itself. When you are accelerating, a heavier bicycle will indeed require more applied force, but it will not require more force to maintain a constant velocity assuming that aerodynamics, bearings, and tires are equal on both bicycles. Lo Farkas is right.

shane45

I did not say "force" - I said ENERGY. What you are saying means that all the NASCAR dudes should be driving around in circles in Hummers, because mass has nothing to do with maintaining a constant velocity? Please. Lower mass = faster acceleration and less fuel (ENERGY) consumed to keep that object moving at said constant velocity.

bostontrevor

Indeed. And heavier objects do fall faster, or rather, denser objects. If I drop a ball of lead and a ball of styrofoam from the top of my house, the lead will hit first. They only fall at the same rate in a vacuum.

As another good example: track bikes are often built relatively heavily given their stripped-down nature. That's because it's more important that they be stiff. In the smooth predictable environment of the velodrome without any freewheeling or braking aside from leg resistance, acceleration and deceleration are generally pretty gradual. There are exceptions to that rule, but not enough to make it worth trading off the overall stiffness and strength of the bike.

bostontrevor

Mass only matters when accelerating or decelerating. Try this thought experiment: take your imaginary bike. Now take it into outer space.

It takes energy to bring that bicycle up to speed but once it's there, it will drift along happily forever until it runs into something, is captured by gravity, or eventually slowed by friction with the not-quite-a-vacuum of outer space.

Energy to maintain velocity here on our planet goes into overcoming friction and has nothing to do with mass.

NASCAR drivers care because they decelerate into corners and accelerate coming out of them. They sometimes brake when coming up on traffic and then have to accelerate around it. They're constantly changing velocity.

shane45

Correct, so now we have come full circle. Weight reduction on rotational (dynamic) components gives greater benefit to the overall package, then simply changing to lightweight static components. Dynamic components benefit because they are in effect, moving twice - once as part of the overall movement as part of the entire bike, and secondly as part of their independant rotational movement.

CTBiker1001

I think you are both right. As an example, take a time trial bike. The forces that are working on that bike the most (in common situations, anyway) are the air, friction of the wheels on the ground, etc. Another component that is working on the bike is the downward pull of gravity. However, in a time trial situation (if we assume a flat course), the other factors play a larger role than gravity. As an example, a time trial bike that weighs 4 pounds more than a road bike isn't greatly effected by gravity. But, if that time trial bike weighed 200 pounds more (say, with a heavier rider), the force of gravity would become more of a factor and would exert a downward pull at all times, thus affecting the amount of force required to move forward.

shane45

Yeah, but unfortunately here on Earth we have this thing called wind resistance and aerodynamic drag, as well as all the frictional aspects of a mechanical device. And you need horsepower (ENERGY) to overcome it. So, you can overcome it on a nice lightweight package, or on a lead ball with pedals - your choice.

CTBiker1001

Well, you are absolutely right - if we biked in outer space. However, we bike along the surface of a large rocky planet that is constantly pulling us downward with the force of gravity. For us, we are always fighting against this force of gravity to move forward. That's one of the reasons why semi trucks must be used to pull heavy loads along the highway and not Volkswagens.

timmhaan

as an interesting side note to this. in the latest issue of scientific american there is an article about how the pioneer 10 and 11 spacecrafts (those sent out in the 70s to photograph saturn and jupiter and set out to space forever) have been steady losing speed at a constant rate - roughly falling about 8,000 miles short of where they should be each year. not-quite-a-vacuum indeed.

bostontrevor

Right, but assuming they both have the same frictional characteristics, they will be equally easy or hard to keep at a constant velocity. Weight and mass are completely irrelevant for maintaining velocity except in the cases where the downforce increases frictional resistance (larger tire contact patch and greater loading on the hub bearings).

CTBiker1001, gravity exists in outer space too. We don't fight against the force of gravity to move forward. We fight against friction. A maglev train in a vacuum on Earth would continue to move forever until it ran into something, even though it's in the presence of a strong gravitational field. So long as that field is perpindicular to the direction of travel, it has no impact except inasmuch as it tends to increase the frictional characteristics of the system.

edit: in fact, a train in general is a great example. A loaded freight train takes forever to get up to speed, but once there it cruises along pretty easily because it has a pretty decent aerodynamic profile and the friction of small steel wheels on steel tracks is pretty low.

Have you ever pulled a trailer? If you have, you've no doubt noticed that once you get the trailer up to speed, even if it's loaded, it doesn't really take anymore effort to maintain speed. It only sucks when climbing a hill (trying to climb out of the Earth's gravity well) or when accelerating.

Phantoj

Rolling resistance at the tire is related to total weight... so if a heavier bike makes you 2% heavier overall... that would take a toll.

ofofhy

But what fraction of the overall resistance to motion is the actual rolling resistance of the tires. I would gamble that the aerodynamic aspect is far more important. Running high pressure in the tires can combat that toll as well by reducing the surface area of the contact patch at the road/tire interface.

Sheldon Brown

Youse guys are misusing the term "velocity." The word you want is "speed."

Velocity is speed in a particular direction. NASCAR dudes driving around in circles are constantly changing velocity even when they're going at a constant speed. You can look it up.

NASCAR dudes don't actually drive around in circles, they drive around in _ovals_. They slow down for the curves and _accelerate_ for the straightaways.

NASCAR drivers don't drive Hummers mainly due to aerodynamics. Bicyclists and racing cars expend most of their energy on overcoming aerodynamic drag, when traveling on the flats. Aerodynamic drag is unaffected by the weight of the vehicle.

A small part of the resistance to a vehicle's motion comes from tire rolling resistance and wheel bearing friction. This bit of resistance is slightly affected by the total vehicle weight, but in practice the magnitude of this is negligible compared to other factors.

The issue of rotating vs static weight only affects _accelleration_ not steady state speed.

Getting back to bicycles, a bike with light wheels will feel livelier and faster, due to slightly better accelleration. Whether it actually _is_ faster is more likely going to depend on psychological effects...sort of the way an old bike with a new paint job is faster than it was when it was ratty looking.

Sheldon "Red Is Fastest" Brown

ofofhy

Of course, shiny new paint contributes less drag than pock marks of rust!

HillRider

Can we get back to the original question?

What the car magazine was refering to was "sprung weight" vs "unsprung weight" and the desirability of minimizing unsprung weight which has a definite efffect on handling and the car's response to imperfections in the road surface.

Unsprung weight includes the wheels, tires, brakes, a portion of the suspension members and the rear axles if the car has a "live axle". The lighter these components the faster the suspension can respond to bumps in the pavement and to transitions during cornering. BTW, the clutch is not part of the unsprung weight. Lighter clutch plates and flywheels allow the engine to rev quicker but have no effect on handling.

On almost all road bikes, the "unsprung weight" is the entire bicycle and most of the rider's weight if the rider is seated but only a portion of the rider's weight if they are standing as the legs act as suspension. Standing to ride over bumps is a good illustration of the effect of reducing unsprung weight.

briancady413

Re: Static vs Dynamic automotive mass - I believe this is in regards to a vehicle with suspension. Reducing unsprung weight makes it easier and quicker for the springs to push the wheels back to the ground after going over a bump. This is good for braking, acceleration and steering beyond its effect on overall vehicle weight.

bostontrevor

Actually I was using the term velocity quite intentionally. It takes energy to change an object's direction. Therefore we could maintain a constant speed but a changing velocity would require more energy than what it would simply require to overcome friction.

edit: and while we're arguing semantics, I'll point out that 'velocity' can be used to refer to the vector or simply as a synonym for 'speed'. In the vernacular it's the latter while in physics it takes on a special use. Of course one can argue that since we're talking about the technicalities of a physical system, we should be careful not confuse our nomenclature. I'm just sayin.

Little Darwin

Looking through this thread, and being a non-physicist, I found myself curious. I did have high school physics, but that was 30 years ago, so things may have changed...

It makes sense that even if able to maintain speed, that turning is actually applying a lateral acceleration... so it would seem that anything that impacts forward acceleration could also be in play when it comes to turning an object. The fact that force is required to turn a bicycle is obvious when you think about leaning into a turn. You are using the position of the bike, and the friction of the tires to overcome the forward inertia and redirect it.

Don Cook

I believe that the focus is "sprung versus unsprung" weight. It has generally been an absolute when dealing with vehicles that have a suspension, that the performance improvements made by reducing unsprung weight provided greater measureable benefits at the race track than the same weight reduction would provide if it were done in "sprung" weight. I can't see how you could transfer this concept to a vehicle that doesn't have any suspension (typical road bike).

LóFarkas

Off: It never ceases to amaze me how ignorant most people are of the most basic laws of physics.

HillRider

Or how often those who claim to understand them misapply them.