# What impact does weight have on flats

So, if I apply what I have learned during physics class, then the theory would be something like, it doesn’t matter for steady speeds and the acceleration/ braking will take a bit longer. And if we think about it a bit further, the even when we are riding along at a constant speed, we are continuously accelerating as we move through the power phase and dead phase of the pedal stroke. However the added weight here should be a wash as more weight takes more power to accelerate, but you also need to accelerate less because less speed is lost during the dead phase.

So far so good. But then I came across this video: How Much Difference Does Weight Make For A Pro Cyclist? - YouTube

In short, they do 2 runs at the same speed of 41.8 km/h over a 3.5 km flattish stretch of road. The only difference being the second run performed with a 20 kg vest. Now the surprising bit is the average power on these runs: 317 W for the first and 369 W for the second one!

Using a calculator I get about 330W for holding 41.8 W on a flat. So the first run looks consistent and differences could easily be due to air temperature or slightly more aero position as he was holding his arms flat on the bars rather than in the drops. But the 50 W increase in power on the second seems a lot. Could it be from the weight moving around side to side or added wind resistance?

I skimmed the video, but I’m not sure we can read too much into it TBH. You look at the weather conditions - it’s clearly pretty windy. He’s also got some kind of vehicle doing the filming so will influence the power. Also, he keeps stating the road is rolling and he can feel the weight up the rises. Too many variables at play here.

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You’d need way more than two runs for reliable statistics.

Apart from the points mad above about the wind and the rolling road, I’d also guess that his body position changes with the vest on - that’s a lot of weight for the core to hold in an aero position. Plus the extra bulk from the vest will affect his drag.

On flats, weight impacts your acceleration.

The key takeaway in the GCN video is the momentum a lighter rider conserves (power saved) on a ROLLING course. NOT a flat course.

In racing, this is why there is lots of strategy regarding positioning entering a short punchy hill, because heavier riders can effectively block and make lighter riders lose momentum, while still racing “positively”. This is also really easy to test and replicate for yourself if you have a rolling course handy.

Given the nuances of rolling courses, it is important to not conflate the GCN takeaways with a truly flat course. Or get too hung up in the minutiae of aero/Rho/vest, event tho these variables are valid concerns. Momentum in regards to weight is the true takeaway in that vids.

For flats, all things being equal EXPECT weight, the amount of separation is based on W/kg.

It’s F = ma, meaning you’ll accelerate and thus travel slightly more distance if you weigh less than someone else with the exact same aero, power, frictional losses, environmental conditions, line choice, etc.

It matters if you find yourself doing everything right, but often get pipped at the line by a centimeter.

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This. I’m heavy (95kg at the moment) and find that on flat course races I can definitely keep up with (in some cases out perform) lighter riders who I know having rode with them in the road, are much better up any sort of hill than I am (significantly better w/kg). But even if I am ideally positioned (3rd wheel) running into a sprint finish I just don’t have the w/kg to quickly ‘kick’ a killer sprint finish to out sprint the lighter guys, even though the post race data shows I put out a lot more watts than they did.

Fat guy problems #thestruggleisreal

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That could explain the difference, if it is rolling enough, you would expect that with more weight it takes more power to average the same speed.

You have to overcome gravity on the flats in the same way you do so on climbs. You just keep more momentum on flats than climb so it is relatively easier. More body weight is more resistance to overcome in all scenarios. So that’s why the rider in the video had to put out much more watts to maintain the same speed with the 20kg vest.

So when your extra weight doesn’t allow your body to produce more power (ie it is not muscle mass), then it’s detrimental. Heavier riders have a much easier time keeping up on flats than climbs, but it’s still more labor intensive for them in all scenarios.

Sorry, no. If you’re on perfectly level ground, it’ll take more power to accelerate, but if you’re doing a constant pace, the only difference between you and much lighter rider is that the you probably also have a larger frontal cross-section, and therefore more wind resistance. The physics on this is quite straightforward.

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The thing to remember is that, even on the flat, you’re always subject to decelerative forces such as aerodynamic drag and rolling resistance. As a result, you’re effectively always accelerating in order to maintain a steady speed. The amount of force you need to generate to achieve that acceleration is proportional to mass.

I think.

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So in the case of the video, the difference comes from:
Drag from vest
Different body position because of vest
Fighting gravity up the rolling hills

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Right, but more mass also means less deceleration. In the end the power that goes in should only depend on the resistive forces, air drag and rolling resistance.

There really are no upsides to being a heavier cyclist. Worse heat dissipation, less aero, heavier for climbs, more mass to accelerate, more fuel required to maintain the same speed as a bunch of lighter riders.
Cycling is an efficiency based sport (bar the shortest of track events) and so even at the highest level (say Matt Beers or Brennan Wurtz at 80kg/500w ftp) they are coping rather than thriving.

But in a physics sense we have done more work If I take my 95kg plus bike and equipments and move it through space and time alongside a 60kg rider and we arrive at the finish line together, I’ll have done way more kcal of ‘work’. But until someone invents weight handicapped bike racing, there’s not much joy being outside of the bell curve for your chosen event.

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True, but it’s not gravity you’re fighting there. It’s the wind resistance. On flat ground and at a steady pace, the vast majority of your power is overcoming wind resistance.

No. You are not accelerating. In fact, the opposite is true. A heavier rider will maintain speed better once power is cut than a lighter rider given the same Cda.
Frontal area is usually larger for a heavier rider, so in that sense speed can be inversely proportional to weight.
Heavier riders need to air their tires up a bit more (and they usually do) to maintain equivalent rolling resistance with a lighter rider.

In a pack, skilled heavier riders can very successfully navigate rolling courses by minimizing braking.

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You are constantly decelerating on a bike on the flat due to gravity… wind is another factor, but setting wind to zero, the statement is still true

So the question then is do those resistive forces scale with mass? In the extreme, the contact patches btwn tyres and road would. It’d be interesting to see a) by how much and b) how significant that might be.

Only because gravity creates the tire deformation and rolling resistance, which heavier riders equalize with tire pressure.

Given identical riding positions and W/kg, a heavier rider will always be at least a little faster than the lighter rider.

Yeah but that’s not what I’m saying. The point is that gravity is why you slow down on your bike. I’m not saying gravity’s relative impact on a large to smaller rider. There is no ‘constant’ speed in cycling on flat without putting out power. You must be accelerating to maintain a constant speed. The first law of physics (object in motion stay in motion) is true in the absence of gravity. But here on planet earth, we are dealing with it constantly.

Gravity is not why you slow down on flat ground. It’s frictional (in the hubs and air resistance), and to a lesser extent rolling resistance (which is indirectly influenced by gravity since we use deformable tires).

With properly inflated tires, rolling resistance is constant between lighter and heavier riders, so gravity has no impact.

A rider’s mass does mitigate slowing down when the power is cut, but this doesn’t have to do with gravity.

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