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I posted this video on Facebook:

and someone commented that my power readings were a bit "optimistic" I asked why and he basically thought that if I'm getting 600W I should be doing a lot more than 30 MPH. He says he can cruise at 30 MPH at only 250W!

In the end he seemed satisfied that the difference is that he weighs 74KG, I am a bit over 100KG.

On another video someone commented that he would fly on a carbon bike with a nice groupset.

It seems to me that weight of bike / rider and groupset is going to make very little difference cruising on a flat with no gear shifts.

Am I suddenly going to end up cruising at 40MPH at 600W when I loose 10KG?

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Weight affects the rolling resistance of the tires somewhat, especially if they're underinflated. But the wind resistance due to a wider body (and probably less "aero" posture) is much more significant than the actual weight per se. (And I agree that "cruising" at 30mph is doing pretty good already.) –  Daniel R Hicks May 15 at 15:42
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How are your watts being measured? And, the guy who claimed to be able to cruise at 30mph at 250 watts must not be riding a bike like yours -- that would imply very low rolling resistance and a CdA of around .145 m^2, which is lower than most unfaired recumbents and in the range of several faired recumbents. –  R. Chung May 15 at 16:04
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If you're getting 600W you should turn professional –  PeteH May 15 at 20:03
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"A Tour de France rider will average 200-300 watts for a four-hour stage; that’s an intensity most recreational riders can sustain for only an hour or so.".. I think your mate is right –  mattnz May 15 at 20:12
    
Of course, a fancy (and expensive) lightweight bike makes you feel faster. –  Daniel R Hicks May 15 at 22:22

5 Answers 5

up vote 3 down vote accepted

The short answer to your question is, "yes, weight does affect cruising speed on the flat, but not by much."

The longer answer to your question is a special case of one of the answers given to this bicycles.stackexchange question: "How can one estimate drag for a bicycle?" There you can find the equation for power given speed, disaggregated into the components for aerodynamic drag, rolling resistance, acceleration, and climbing (or descending). In your case, you are only interested in the components of drag that are affected by a change in mass.

You have asked about "on the flat" so we can ignore the climbing or descending component.

You have asked about "cruising speed" so we can ignore the acceleration component.

If you look at the drag equation, mass does not enter into the aerodynamic component.

The only component of drag, then, that is affected by mass is the rolling resistance component. Rolling drag force is Crr * m * g where Crr is the coefficient of rolling resistance, m is the total mass, and g is the gravitational "constant", probably about 9.8 m/sec^2 in your location. For "normal" road tires on a standard surface, the Crr will typically be in the range of .004 to .007; heavy tires on rough roads will tend toward the upper part of that range, racing tires on a good surface will tend toward the bottom part of that range. In this case, let's say the Crr of your tires on that road is right in the middle of that range at .0055, and that the all-inclusive weight of you and your bicycle and all of your equipment is a total of 110 kg. Then the rolling drag will be .0055 * 110 * 9.8 = 5.9 Newtons. For each kg that you decrease the total all-inclusive mass, the rolling drag will decrease by .0055 * 1 * 9.8 = .0539 Newtons.

Power is force * speed, so for constant power output the change in speed due to decreased mass will simply be power/change in force.

So, suppose your "cruising" power was 300 watts, your total mass was 110 kg, your CdA was 0.3 m^2 (which is low for a commuter bike setup), and the Crr was .005. We can then calculate your speed as around 10.9 m/s (or 39.2 km/h). If you were to decrease your mass by 10kg, roughly 10% of your total mass, your speed would increase by about 1%, to 11.0 m/s, or 39.6 km/h.

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Translation: good luck detecting the change in speed from losing 10kg. Especially if you're getting fitter while doing that :) –  Nuі May 17 at 9:00

You've got a rack and panniers, so I suppose you're on a commuter bike. It shouldn't surprise you that you cruise at 30mph (which is already quite fast). You would probably get far faster if you improved aerodynamics, e.g. on a road bike with narrow tyres and drops or TT bars. You wouldn't spend so much energy into ripping a gigantic hole into the air.

Losing 10kg of weight will probably not help you much in this regard - at > 30km/h weight doesn't really come into play much. As a Velomobile rider, I can easily cruise 40km/h over a long distance, although the bike weighs about 30kg with tools and food. It's all about aerodynamics.

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This is a road bike with skinny Schwalbe Ultremo tyres but yes I do have a rack with a pannier on the back. –  Roaders May 15 at 18:56
    
@Roaders Okay, then get rid of the rack and fenders if you want to get faster. And duck down. Also: How did you measure your power output? –  arne May 16 at 6:36
    
Thanks for the reply but the question wasn't how do I go faster, the question was does weight make a difference to top speed. Power is measured using a Stages Power meter. Correlation to previous Powertap is good, also comparison to numbers from Wattbike in the gym is pretty good. –  Roaders May 16 at 14:30

From my observation of your vid, the peak power is while you are accelerating, not while cruising. I note that you produce ~700 W at times. I am assuming that you are using power measuring pedals / cranks.

For comparison, when I was a Physics student we did stair runs up 15 floors to measure our power output; I and a couple of other fit people produced over one HP (746 W) and if memory serves me, I produced 1.5 HP. 40 years later, on a recent lazy ride, Strava said I was peaking at 500 W (correction: as I learn to use Strava better, I see that was an average over 300m, the peak was 1200W for 10s over 80m). On flat cruising at 28 to 30 kph, Strava says I produce about 150 W. I weigh 70 kg (same as then).

So I find your results to be reasonable.

Edit: I should add, in answer to your question: no, weight doesn't effect speed on the flat. That is mainly determined for a given power level by your frontal area (but you knew that, it's why we crouch in a headwind). Better bearings can give a small improvement.

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Strava power estimates can be off by quite a bit (either high or low) when compared to actual measurements with an on-bike power meter, especially over short intervals. Averages over an entire ride will typically be closer. This could be a good bicycles.stackexchange question. –  R. Chung May 18 at 17:04
    
@R.Chung yes, it can only calculate from my weight (mass), change in position and time. It doesn't know the mass of my bike, what I'm carrying, head winds, ... . Before your comment I was guessing Strava would be within about 10% of directly measured power levels; close enough for the point I was making. A Google found this blog article, which finds the errors to be in the range 7.9%-15.5%, with errors at low power levels being much higher. –  andy256 May 19 at 1:00

There are various cycling power calculators out there that can help you with your questions, but the short and simple answer is that the main factor affecting bike top speed on flat ground is aerodynamic drag.

http://bikecalculator.com/

Drag varies as the square of velocity, so 40 mph will take nearly double the power of 30 mph.

Where weight makes a huge difference is in your acceleration rate. Acceleration rate is the key difference maker in bike racing. Losing 10kg will dramatically affect the time it takes you to go from stopped to 30 mph, it won't do much about your actual top speed.

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Thanks for that, I'll have a look at the calculator –  Roaders May 15 at 18:56

The more you weigh, the more power required to get to, and maintain a certain speed.. but at the same time, generally, the more power you can produce. On the flat you're mostly working against air resistance, a larger framed rider (less aerodynamic) will need more power to overcome the resistance than a smaller rider. Generally aerodynamics of rider & weight of rider will have a strong correlation.

30mph is not at all slow, if you were able to maintain that for 20 km you'd actually post reasonably competitive times in a pro TT!

A better groupset might save weight (although that's more important with climbing & accelerating), it might also give you a better selection of gears & better power transfer. A carbon bike might be more aerodynamic.

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The more you weigh, the more power required to "get to" a certain speed. But once you are travelling at a constant speed, the only thing you have to provide power for is wind resistance and friction (rolling resistance of wheels, bearings and other things). A heavier rider, assuming the same aerodynamic profile and friction forces will actually not require more energy to maintian a constant speed on a flat surface. –  Kibbee May 15 at 15:08
    
Although at the same time the friction will be greater, and probably surface area, so there will be more forces to overcome. Assuming the same aerodynamic profile is absurd. Have you ever tried to shelter behind one of those featherweight climbers? Compared to a big sprinter? –  7thGalaxy May 16 at 14:40

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