simple physics seems to ignore friction and wind resistance. the simple, physical fact is that a cyclist is constantly decelerating due to these facts - unless countering them with an applied force.Kerry Irons said:Simple physics will tell you that rotating weight is only meaningful when you accelerate...This is a simple, physical fact, and not something that is debateable.
I definitely sense something is there on my MTB carrying two large water bottles. I don't throw around my road bike nearly enough to "really" feel anything. My full suspension MTB is 23.3lbs and the road bike is about 16.3lbs.jaekormtb said:Here's a bit of a test for these ideas. Get two big water bottles, fill 'em up, put 'em in your bottle cages and go for a short ride including a hill or two. Now, put the bottles in your rear pockets secure enough so they're not moving around on your back. Do the same ride. Which feels better?Here's my guess. If you've got a really light bike, the bottles on the bike will make it feel heavy and sluggish, making you feel faster with them on your back. The heavier you and the bike are, the less you would notice any of this. I really thinkthat weight questions like this have a lot to do with bike weight relative to rider weight.
Coolhand said:You will notice dropping a pound of bike weight much more readily, because the percentage change is huge in comparison to dropping a pound of body weight. .
Kerry, I always read your posts in the threads we are both in. The simple fact is that riders in many typical group ride, race or hill climbing situations are constantly changing speeds (as the pull out and off, in and out of corners, on different slopes of the climb, whenever they attack, or react to attacks, whenever they sprint for the line or the town line sign, ect.). Only if you are doing lots of long solo rides without climbs or stop signs and long TT's does this become less of an issue and the aero aspects become more important. You were right about one thing- the importance of rotating weight isn't debatable. For most riders, rotating weight is king- those pedals, cranks and wheelsets are what you are going to have to spin, so the lighter they are, the easier it will be.Kerry Irons said:Simple physics will tell you that rotating weight is only meaningful when you accelerate. IOW, a pound of wheel weight is the same as a pound of water bottle weight at constant speed. If your speed is going up and down, then you take double the energy to accelerate the wheels (compared to non-rotating weight) but you get that energy back when you decelerate. Unless you're constantly braking, wheel weight is the same as other bike or rider weight when you average things out. This is a simple, physical fact, and not something that is debateable.
Interesting. The better test would be two indentical bikes. One with a wheelset which is a pound lighter. The other carrying a pound of water (not for drinking) in the bottles. So overall, the bikes and rider have the same weight each time. Assuming (as we must to make the test work) that the rider is able to put out the exact same effort on both climbs of the course, will he have the same time for each bike?jaekormtb said:Here's a bit of a test for these ideas. Get two big water bottles, fill 'em up, put 'em in your bottle cages and go for a short ride including a hill or two. Now, put the bottles in your rear pockets secure enough so they're not moving around on your back. Do the same ride. Which feels better?Here's my guess. If you've got a really light bike, the bottles on the bike will make it feel heavy and sluggish, making you feel faster with them on your back. The heavier you and the bike are, the less you would notice any of this. I really thinkthat weight questions like this have a lot to do with bike weight relative to rider weight.
Some quick things I found already:Coolhand said:If you can find any studies which prove your assertion, please provide me a link and I will read them. I will try and do the same for you. Deal?
Looking for a more accurate model- check this out (not for the faint of heart):Finally, weight is a factor in sprints where inertia (the resistance to setting an object into motion - why it is harder to get up to speed on a bike than to maintain that speed) comes into play. It definitely takes more energy to accelerate a heavier rider/bike combination in a sprint. And extra weight in some bike components (rims for example) may require twice as much energy to accelerate as an equal weight in the frame. This is a result of the fact that with rotational speed you are accelerating these components much more quickly. (Note: this means you should upgrade your tires, rims, crankset, and shoes before you spend your extra $$ to decrease your frame weight an equal amount).
another point made on the list:I am an engineer working at Cane Creek Cycling components and have measured
the moments of inertia of many wheels. Here's some numbers for you:
Cane Creek Aerohead with Ti Spokes (24 front/28 rear):
Front wheel mass: 675 g
Front wheel MOI: 45.5 g*m^2
Front wheel "equivalent mass": 1080 g
Rear wheel mass: 892 g
Rear wheel MOI: 44.3 g*m^2
Rear wheel "equivalent mass": 1287 g
Combined "equivalent mass": 2367 g (5.22 lb)
Cane Creek Carbon:
Front wheel mass: 659 g
Front wheel MOI: 42.2 g*m^2
Front wheel "equivalent mass": 1037 g
Rear wheel mass: 928 g
Rear wheel MOI: 43.6 g*m^2
Rear wheel "equivalent mass": 1317 g
Combined "equivalent mass": 2354 g (5.18 lb)
Rolf Vector Pro:
Front wheel mass: 795 g
Front wheel MOI: 56.2 g*m^2
Front wheel "equivalent mass": 1296 g
Rear wheel mass: 1006 g
Rear wheel MOI: 56.4 g*m^2
Rear wheel "equivalent mass": 1509 g
Combined "equivalent mass": 2804 g (6.18 lb)
Mavic Ksyrium:
Front wheel mass: 740 g
Front wheel MOI: 48.7 g*m^2
Front wheel "equivalent mass": 1174 g
Rear wheel mass: 933 g
Rear wheel MOI: 51.1 g*m^2
Rear wheel "equivalent mass": 1388 g
Combined "equivalent mass": 2562 g (5.65 lb)
All of the above values were from random production wheels.
My term equivalent mass is a combination of the mass and the moment of
inertia. The moment of inertia, I, can be converted to a simple mass, m,
by converting the equation T=I*alpha into F=m*a. Since T=F*r and
alpha=a/r, T=I*alpha is equivalent to F*r=I*(a/r) or F=I/r^2*a. So I/r^2
acts like an additional translated mass and accounts for the rotational
inertia. Using a typical 700c wheel outer radius, 1/r^2 is a constant and
is 8.91 when using the units of grams and meters. [For those not familar
with the formulas; T=torque, I=moment of inertia, alpha=angular
(rotational) acceleration, F=force, m=mass, a=acceleration (translational).
These equations are from Newton's 2nd Law.]
>From the combination, it can be stated, for example, that using a Rolf
Vector Pro wheelset has the effect of adding 0.44 kg (1 pound) additional
mass during acceleration versus the use of a Cane Creek Aerohead wheelset.
You are correct in your assumption that the relatively heavier rim causes
the MOI to be higher than an equivalent mass wheel using more spokes. This
logic is exactly why we place even our spoke nipples at the hub, where they
contribute significantly less to the MOI. So when comparing the masses of
wheels, pay attention not only to the mass, but where it is distributed.
If you add 1g to a rim (at 300 mm radius) it has the same effect on MOI as
adding 225g to a hub (at 20 mm radius)!
Rob Tison
Cane Creek Cycling Components
There are good people arguing your side as well. Take a look, its is a fun (if nerdy) discussion by some of the luminaries of the cycling world.I suspect you haven't raced much if you think acceleration is not a factor
in road racing. I am an active USCF Category 2 racer and have experienced
accelerations as a significant factor in every race except for straight and
flat time trials. As a category 2 racer, I've had the privilege of racing
in many US National Calendar races with the best racers in the US. The
accelerations at this level are fierce and often. If you can't accelerate
hard enough to close the slightest gap to maintain a draft during a race,
you can quickly be off the back. Races are either won by a field sprint,
or a by riders in a breakaway group. Acceleration is a major factor in a
sprint where it is essential to accelerate away from a pursuer trying to
stay in your slipstream. Acceleration is also how a breakaway group
establishes itself with an attack on the field of riders. Most attacks are
unsuccessful requiring repeated attacks before success. These attacks can
occur repeatedly during a road race lasting several hours. In races, there
is nearly always an acceleration out of every turn as well. Lower mass and
MOI mean one can accelerate with less effort, conserving valuable energy.
Or, lower mass and MOI can enable better accelerations with a given
effort. Opponents are "dropped" during quick accelerations to which they
cannot respond. Since there is such a great aerodynamic advantage in
drafting, it is impossible to ride someone off of your wheel by increasing
the pace by a small percent.
In road races differing from the traditional road race accelerations can be
even more important. A good example is racing a criterium (the most common
road races in USA) where a typical race may last 1 hour, and be ridden on a
typical course with 6 right angle turns per kilometer lap. With a
reasonable average speed of 28 mph (45 kph), this results in more than 270
hard accelerations per race. To think acceleration is not a factor here is
unimaginable.
The issue for optimizing the characteristics for a racing wheel
is also subtle. The compromise between aerodynamics (which is
a significant parameter for many events), weight (important in
a mountain TT where speeds are slower) and the wheel's M.O.I.,
is not an easy one to work out in order to optimize a wheel for
a given event.
F=ma, if all forces are balanced, netF=0, a=0.coreyb said:simple physics seems to ignore friction and wind resistance. the simple, physical fact is that a cyclist is constantly decelerating due to these facts - unless countering them with an applied force.
Bicycling Magazine (I realize they get slagged a lot but bear with me) had an interestingBacco said:I was discussing with my LBS owner whether or not losing 1 lb body fat (mine) would provide the same end results in performance for me as buying a bike that weighed 1 lb less? Of course, he believed that having a lighter bike would be more advantageous. What do you think?
I can understand that lighter wheels would make more of a difference due to the rotational dynamics involved, but just considering the weight of the frame, I can't see any difference between body weight and frame weight.
djg said:There is absolutely no way--NO WAY--you can simply substitute a pound of frame for a pound of fat when you are baking. Disaster--don't do it.
Bacco said:I was discussing with my LBS owner whether or not losing 1 lb body fat (mine) would provide the same end results in performance for me as buying a bike that weighed 1 lb less? Of course, he believed that having a lighter bike would be more advantageous. What do you think?
I can understand that lighter wheels would make more of a difference due to the rotational dynamics involved, but just considering the weight of the frame, I can't see any difference between body weight and frame weight.