Does Bike or Rider Weight / Groupset affect cruising speed on the flat

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.