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I have never ridden a road bicycle, so this question is probably silly (and a duplicate). Still:

Assumptions:

  • Road bike
  • Riding fast, 30km/h nominal
  • Few and gradual corners
  • No abrupt stops (i.e. no traffic lights)

In the face of those assumptions, why are road cyclists concerned about the bicycle mass at all? The large inertia of a heavy bike should be no problem and should even be providing stability and comfort. The only conceivable drawback would be higher friction in the bearings, which should be negligible.

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Hills and acceleration –  Ritch Melton Aug 14 '13 at 8:33
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Try one out and then feel the difference. –  Andrew Welch Aug 14 '13 at 9:12
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Weight does have a lot to do with easier travel, as @tisek explains, but I have always wondered why riders who are riding to "stay in shape" or "in training" worry about weight, which is where I thought this question was going to go. I figure, it is better to ride a heaver bike to train then switch to your ultra-light bike for a race. If you are commuting and using it for transportation, yeah, less weight less effort, less sweat when you get to the office ;-) –  BillyNair Aug 14 '13 at 10:39
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It certainly is possible to obsess about weight, ~20lbs is a good weight. Once you start to get down to UCI weight-limit and lower (16lbs), it would be silly to fret about ounces and titanium bolts unless one's body fat was in the single digits and racing was the focus. –  Angelo Aug 14 '13 at 13:33
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@PeteH, not that there's anything wrong with getting a racy, light bike. There are a LOT worse things one can do in a mid-life crisis! –  Angelo Aug 14 '13 at 16:02
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5 Answers

up vote 4 down vote accepted

There are two main reasons:

  1. Hills: Pushing a light bike up a hill is obviously easier than a heavier bike. Everyone likes to brag about that HC climb he did the other day. Without resorting to lying about actually climbing the hill, the easiest way for a cyclist to make his life easier is to carry less mass up the hill. Nobody will say, "That didn't count because you rode a 3kg bike," and you get no extra points for lugging a 40 kg dutch commuter bike to the summit.
  2. Technical Competence: Bicycle manufacturer have tried over the years to reduce bicycle weight, to the extent that weight (or lightness) has become a proxy measurement of the manufacturer's technical competence. Anyone can build a heavy bike, but it takes real engineering skill to build one that is light.
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+1: Lighter bikes cost a LOT more, so if you are spending that much, you expect the manufacturer to put in better bearings, better cables, better everything. A light bike is a good sign of a high quality bike. No one builds a heavy (road) bike with top quality components. –  mattnz Aug 14 '13 at 22:27
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There are two problems with your assumptions:

  1. You appear to be assuming that the rider is on flat ground but neglected to mention that.
  2. The assumptions that the rider is traveling at high speed with few corners and stops doesn't reflect real world conditions.

When riding hills, weight is a huge concern because of gravity, as others have mentioned. Quite simply, it's easier to move a lighter object uphill than it is to move a heavier object up the same hill.

When accelerating under any conditions, it takes more force to accelerate a heavier object than it would to accelerate a lighter object at the same rate. This is a huge factor because riding in the real world rarely involves cruising down a straightish road with no stops at a relatively high speed. Riders are constantly decelerating because of a hill, stoplight, or corner. Once over the hill, around the corner, or the light turns green, riders once again have to accelerate.

Even if your assumptions were correct, there are other reasons to crave a lighter bike:

A lighter bike feels better underneath you. A lighter bike feels more nimble and maneuverable. Even if all the geometry is the same, it simply feels like you have more control over the bike.

Finally, there is the smugness factor. Light bikes cost a lot of money. Anything that costs a lot of money is a status symbol. And people like owning status symbols.

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It is quite clumsy for me to accept an answer, never having ridden such a bike! However, your answer sounds most logical to me. I am accepting nick g's answer, as yours basically expands on that, and am up-voting yours to put it at the top of the voted answers nd let it stay there! –  Vorac Aug 15 '13 at 8:24
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The heavier the bike, the more weight you have to move with your legs as only motor. Sure you can achieve 60 kph with a 18 kg bike, but as you can achieve it way faster with a 7 kg one, why bother? And every gram weighs 1000x more in tough spots such as climbs etc,... So without being obsessed with it (riding without your valve cap to gain half a gram, with a titan multi-tool and one tire lever only to save the weight of a second one), being weight-aware is the right way to proceed.

Also, the weight of moving parts, and especially spinning parts such as wheels, tires and tubes is multiplied by because of all the inertia, gyroscopic forces and everything: a 10 kg bike with 2 kg worth of wheels, tires and tubes will be faster than a 10 kg bike with 3 kg worth of wheels, tires and tubes. I experienced it myself just by changing tires and switching from 400g per tire tires to 280g per tire tires which in only a 240g improvement (+60g on the tubes); and yet, this change allowed me to use my big chainring on a regular basis whereas I would only use it in long downhills beforehands.

More arguments apply, but yeh, weight is important...

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But F = ma. Once the bike gets moving, you now longer need to keep your motor running in order to maintain speed. Theoretically. Practically, I am sure I am wrong, so where and why? –  Vorac Aug 14 '13 at 11:02
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Well, I suppose that would be true in a vacuum environment (is that the english way of saying that?? not sure but you get my point)... But when you're at 40 kmh on a flat road the friction with air,the friction of mechanical pieces against each other will make you lose speed so you need to keep pedaling and once again, getting from 39 to 40 kph will be easier the lighter your bike is. –  tisek Aug 14 '13 at 12:13
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@tisek Actually, if in the real world, two bikes are going 40kph on the flat and the riders both stop pedalling, the heavier bike will keep going for longer. It has more momentum. A heavier bike resists changes in speed, but this applies to decreases in speed too (and changes of direction). –  JamesBradbury Aug 14 '13 at 15:00
    
@tisek - Weight has nothing to do with air resistance. And weight has precious little to do with friction generally. The main effect on the flat is to very slightly increase "rolling resistance" in the tires, but a few ounces difference will not have a measurable effect, even under laboratory conditions. –  Daniel R Hicks Aug 15 '13 at 1:21
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Roadies are concerned about weight because they believe that lighter bicycles go uphill faster for the same effort. This is obviously true (it's simple physics: the energy required to lift a mass m a fixed distance against gravity is proportional to m), but how much difference does it actually make in practice if you're not racing up mountains?

Let's calculate some back-of-the-envelope numbers: to lift a 1 kg weight up a 1,000 m climb takes 10 kJ; if you climb with a (decent amateur) power output of 200 W then you'll take an extra 50 seconds to do the climb. On less hilly rides, the effect would be even smaller.

Here's a randomized (non-blinded, n=1) trial that looked into this question in a realistic setting:

The author timed his commute from Sheffield to Chesterfield in the UK (about 22 km each way; moderately hilly; ridden at about 30 km/h), randomly choosing each day whether to ride his 9.5 kg carbon frame bicycle or his 13.5 kg steel frame bicycle. The article is open access, so I'll just quote the conclusion:

A lighter bicycle did not lead to a detectable difference in commuting time. Cyclists may find it more cost effective to reduce their own weight rather than to purchase a lighter bicycle.

So given the assumptions of your question (moderate speeds and not too hilly), I think road cyclists do not need to worry very much about the weight of their bike.

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This doesn't really answer the question. Whether or not roadies should care about lighter bikes, there's no doubt that many do. The OP was asking why that is. –  jimirings Aug 14 '13 at 15:56
    
+1 for the great link. Having noticed a similar effect myself, I suspect that in commuting one becomes habituated to route one takes and what speed to do at each point. Traffic certainly has an effect on that. –  andy256 Oct 21 '13 at 22:49
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But F = ma. Once the bike gets moving, you [no] longer need to keep your motor running in order to maintain speed

This is only true if you ignore gravity, aerodynamic drag, rolling resistance, mechanical friction, etc etc..

In reality: when you are travelling on a flat road, you are primarily slowed down by aerodynamic drag ("the wind"). When you are travelling up hill of a certain steepness, gravity becomes the primary source of resistance.

This page has a nice interactive calculator for such things..

Specifically, with the gradient set to 0%, the "force required to overcome gravity" (Fgravity) is of course zero. Weight makes little difference, except to rolling resistance, which is dwarfed by the force required to overcome aerodynamic drag (Fdrag)

Then, increase the gradient to, say, a reasonably steep 10%. Suddenly the Fgravity becomes the significant factor. Changes in weight make substantial difference to the power required for a given speed

For example, if I start with the default numbers on that page: to climb a 5% gradient at 20km/h would require 291watts. If I reduce the bike from 8kg to 6kg, the power becomes 285watts. For comparison, I have ridden an average of 291watts for 16mins, vs 285w for 16min30.

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It should be noted that a 10% grade is damn steep, and a 5% is a grade you definitely notice. And even if adding 5 pounds does raise the power required to climb a 5% grade from 285 to 291 (ie, 6 watts -- as much as a small night light), it makes little sense to obsess over ounces. –  Daniel R Hicks Aug 15 '13 at 11:35
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