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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Actually what you mean to say is that you must apply a force to remain at a constant speed. If we were able to apply a constant force equal to the resistive forces, there would be no acceleration while going at a constant speed.
The accelerations that take place in practice while going at a “constant” speed are micro accelerations as we slow down just a little during the dead phase of the pedal stroke and than accelerate back up again as we push down on the pedals.
So with more weight the acceleration is smaller but during the dead phase the deceleration is also smaller. So in the end the same power would be needed.
