Bike components or upgrades are sometimes quoted as saving some number of "grams" of drag.
How can a gram, a unit of mass, be used to quantify drag, which is a force?
And for extra credit, why is this a useful way to quantify drag?
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Sign up to join this communityBike components or upgrades are sometimes quoted as saving some number of "grams" of drag.
How can a gram, a unit of mass, be used to quantify drag, which is a force?
And for extra credit, why is this a useful way to quantify drag?
You're exactly right, drag should be measured in units of force, like Newtons. However, although there are small differences in the gravitational constant across the surface of the Earth, a reasonable rule of thumb is to assume it to be constant. Accordingly, drag force in Newtons is proportional to kilograms by a constant term very close to 9.8 m/sec^2, so 1 N of drag force is approximately equivalent to "100 grams of drag."
That said, I try not to use "grams of drag" and instead use Newtons of force. But, as you've undoubtedly seen, many others (including bike manufacturers and wind tunnel operators) do use "grams" so in order to promote effective communication, I'll switch to using a term I know they'll understand.
How can a gram, a unit of mass, be used to quantify drag, which is a force?
It's a misnomer related to the Kilogram-force (kgf) and its submultiple unit the gram-force (gf), 1 kgf being equal to 1000 gf.
A kilogram-force is equal to the magnitude of the force exerted on 1 kg of mass in a gravitational field of 9.80665 m/s² (standard earth gravity), it is by definition equal to exactly 9.80665 N.
This unit is only useful to communicate with people unfamiliar with the Newton unit. The kilogram-force (and its cousin the Pound-force (lbf)) is easier to grasp for the interested layman.
It is however considered an unacceptable unit in the NIST recommendations and is often replaced by the decanewton (dN) in technical documentations, 1 dN being equal to 1.01971621 kgf.
A gram-force is equal by definition to 9.80665 mN (but often rounded to 10 mN for marketing purpose), so when one say "1 gram of drag", it actually means "1 gram-force of drag" which usually mean "10 mN of drag".
Please also note that the drag force is proportional to the square of the speed so any mention of drag force should specify at which speed otherwise it's just marketing claptrap.
How can
a gram
, a unit of mass, be used to quantify drag, which is a force?
Only by using a gram
as a shorthand, or malapropism depending on your perspective, for a gram-force, which is the force exerted on a gram mass by (Earth's) gravity.
why is this a useful way to quantify drag?
In the context of cycling this is simple: it allows the main energy costs that cyclists often seek to reduce to be handled with the same units.
As well as reducing drag, cyclists are also often concerned with reducing mass. The main value of reducing mass is that it reduces the amount of additional power from the rider required to climb uphill (it also reduces the work needed to accelerate, but most riders in most scenarios end up putting more effort into climbs).
This additional power is used to work against the gravitational force pulling the rider and bike down (and consequently back downhill). Reducing mass thus reduces weight. It is really these two forces – drag and weight – that cyclists are usually most interested in reducing.
We could choose to measure them both in a unit of force (Newtons, say) and this would speak more directly to the value of reducing them, but the reality is that it's easier to pop a derailleur on a set of kitchen scales and get a readout in units of grams of mass (though even the scales are really measuring a weight force) than it is to fire up a wind tunnel and get a balance readout in Newtons of drag.
The value of a Newton of weight compared to a Newton of drag isn't a 1:1 correspondence but because the physics that makes the forces important in cycling overlaps a lot, dealing with both in the same units does help with developing an intuitive sense of one when you've already learned a bit about the other. A gram is probably as good a compromise choice for this as any other.
It doesn't make any sense whatsoever to try to substitute a constant amount of weight for a force that is a function of speed.
Obviously the calculation is done by choosing a value for the speed, evaluating the drag force and dividing it by the gravitational acceleration g.
This doesn't provide any insight into the actual benefit of a reduction of drag. The only purpose is to be able to brag about it or sell it to people who don't understand the concept.
To understand how wrong it is, consider that people's average speeds easily vary by a factor of 2, so the actual drag varies by a factor of 4. If you compare climb speeds with flats, speed might even vary by a factor of 4-5 for a drag variation by a factor of 16-25. So your "easily understandable" value is wrong by a whole order of magnitude if you don't know how exactly to use it. Another way to be off by orders of magnitude is because for real weights you lift them up by the vertical climb amount which is rarely more than single digit kilometers, while for pretend weights you "lift" them forward by distances that can easily reach triple digit kilometers.