Tadej Pogacar breaks 3 Strava records during his reconnaissance for Paris-Roubaix 2025 including fastest time on Mons-en-Pévèle

Going over bumps is equivalent to going over many small climbs. Lower weight is always a good thing (ceteris paribus) except when going downhill, and even then it's not that advantageous if the downhill is at all technical.

That’s not quite true, even steep climbs and sudden gradient changes can be considered as remaining tangential to the wheel whereas the occasional dip between cobbles a wheel drops into cannot, as such riding over cobbles can become like a permanent surmounting of small obstacles that hit back into your wheel slowing you drastically if you don’t have the right rhythm to “fly” on the mounds, here again the greater the momentum the less these obstacles reduce your speed. But, it is but one factor, you cannot decide on this alone. Like with natural surface ripple formation on desert tracks used by vehicles, there is an optimal speed range to efficiently pass over such surfaces, too fast or slow and your car gets shaken to pieces from the “harmonics” in the vibrations caused. Pretty sure the same applies to riding on cobbles.

I disagree. If you have lower weight, you will bounce more at the same speed, meaning that you will spend more time in the air and shorter time on the ground, as a consequence, when you are on the ground you have to produce more force for the same speed

So, you're saying that doing a bunch of small hops on a bike is easier if you're heavier? Why do people buy lighter bikes, then? Why do they generally feel better and faster?
An appeal to "rhythm" is not helpful either, as the right "rhythm" might be on the lighter or heavier side of things, and as it varies, it's likely a wash either way. Then we are left with the physics truth that it takes less power to lift lighter things than heavier things.

You might bounce more for the same power, but not the same speed. Sending 20 kgs over a jump at 20 kph will fly just as far as 10 kgs at the same speed. Again, Galileo demonstrated this.

You are completely wrong

Ok guys, both of you, this is far too complex to debate as a whole as there are dozens of variables and none of the conditions have been rigidly defined, what’s more, everyone is looking at things from a different angle and for a different purpose so let’s go back to the real basics first to see if we can agree on some starting principles.

If you throw a golf ball or ping pong ball (assuming same size here) at the same speed in the same direction, which is going to land further? Ditto if you launch them with the same power?

If two identical riders of 70 and 80kg (lets assume the extra 10kg is just bones and drinks and lunch) on identical bikes ride side by side at speed X with zero wind (or alternatively a really strong head wind) and both stop pedalling at the same time, which is going to roll further or come to a stop sooner?

The wind force does inevitably come into play but I want to keep an open mind as to the effect it has in situ in practice because it obviously isn’t the only criteria and with acem82’s example it’s highly likely that wind is more minor but that wouldn’t be true for every real-life situation.

Regarding cobbles, that’s an extremely complex motion to analyse, you can call it bouncing or hopping or climbing or what you like but in reality it’s indescribable and only high precision analysis can tell you exactly how much of each effect is contributing for each rider at each speed and every tire pressure/behaviour, you will NEVER manage to say that one criteria alone governs the whole efficiency equation.

If you throw 2 identical golf balls at the same speed, but one is twice the weight, it will go a bit farther (wind resistance is the same, so the amount of energy of the heavier ball is higher). If you throw 2 identical golf balls at the same power, but one is twice the weight, it will go less far (the amount of energy is the same, so the lighter one is going faster).
Remember that in a flat TT, once up to speed, the amount of weight is basically negligible (except a tiny amount of increase in rolling resistance). It's all power vs CdA.
Probably the best way to think about this is with a truck (with a covered bed) over a flat but very bumpy road, and ideal tire pressure for the weight. Now, load the truck up to 3X the mass, change the tire pressure to the new ideal pressure, and do the run again. What will take the most amount of power? It's quite obvious that it's the run with the more mass. (Many of us have done this test in real world conditions without realizing it.)
Why? Because there's deformation loss to the tire, which is somewhat worse when there's more mass, but there's also the loss from having to lift that much more weight over and over again.

All this seems right but the problem is that two riders of differing weight going at the same speed will have different power and they will also have different power per weight yet are both (in theory, i.e. when trying their hardest) at their max,which means the lighter one is proportionally losing more to the wind (working harder to overcome), the heavier one proportionally losing more to rolling resistance and bounce. The extremely difficult calculations to know which of the 2-3 causes the bigger effect under what exact conditions is what makes it impossible to say who will benefit most in many situations when they aren’t different enough to make it obvious, i.e. a puny climber of 50kg vs a sprinter of 80kg on either the flat or a mountain.

I agree about the sprint or drilling it on the flat. Lighter rider versus heavier rider. Inertia is the natural tendency of objects in motion to stay in motion (e.g the heavier riders momentum) and objects at rest to stay at rest, unless a force causes the velocity to change.

You have to keep all else held equal or you're not isolating the change. A lighter rider is faster at everything, save downhills, all else held equal, as I keep showing.
As for wind, just as in flat TTs weight basically doesn't matter, except that lighter equals slightly less rolling resistance. In the same manner, for a 30 kph headwind, weight doesn't matter, as the relevant equation is power vs CdA. The reason why more weight is helpful downhill is the same reason it's hurtful uphill, it "gives back" the energy (not speed) you gained by going up the hill.
Lastly, in a sprint, weight is again counterproductive, assuming one has to accelerate up to speed. Accelerating a mass takes energy, and a heavier one takes more energy than a lighter one. Now, at most sprint speeds, almost all energy goes into wind resistance, but the acceleration of mass still does play a part.

Not sure exactly what you are saying, but if you're implying that being heavier is helpful if you're going against the wind or sprinting, then no, it isn't. Ceteris paribus (all else held equal) being a lighter cyclist is always better, save downhill. The reason why heavier cyclists do relatively better than lighter ones on flatter courses is because they will have to have a higher power than the lighter cyclist in order to have any chance at all against them, and flatter courses mean that the lighter cyclist has nowhere to display the better power to weight ratio, meaning that significant ratio is power to CdA, which is usually an advantage to the heavier cyclist.

I am saying I have an open mind at the moment for your cobbles theory but cannot agree with your wind statement (and by extrapolation sprinting since that implies speeds where wind is by far the limiting factor).
Had you argued all things not being equal I could have agreed with you but if we thus assume 2 objets of the same size and speed but with differing weights, it is completely obvious that the lighter one will suffer faster in the wind. Extrapolate to the extreme, drop stuff off cliffs or just remember your momentum equations to verify. Now, obviously in real life we have to assume riders of different weights won’t have the same wind hitting surface but you cannot generalise at all then as you can have very unaerodynamic light riders and very aerodynamic heavy ones because it isn’t weight that defines CdA, Teslas have an incredible CdA and are ridiculously heavy, and someone like Harry Levreyssen is a Tesla in cycling.

No, weight doesn't affect the ability to go into the wind at all. It's all CdA vs power at that point.
For example, it's "obvious" that a heavier object falls faster than a lighter one. But, absent wind resistance, it's not true (Galileo demonstrated this). An appeal to "obviousness" doesn't matter when the physics is clear.
And yes, you have to be able to hold everything else constant, or you can't determine anything! So, imagine a 300 watt rider at 80 kgs with a CdA of 2.0, then imagine that same rider at 70 kgs with a CdA of 2.0. (Assume ideal tire pressure for both, same bike.) Which one can ride over a bumpy road fastest? The lighter one, as he doesn't have to lift as much.
That's also why when they test bikes on 5 star cobbled sectors, the MTB goes faster than even a cobble-build road bike. Even though rolling resistance is higher on a flat road, and the CdA is junk, it smooths out the cobbles, allowing the person to not lift as much as they otherwise would.

So why did Tadej say, in an interview, when he won Flanders and was asked if he would try Roubaix next, that he would need to gain more body weight before he thinks of Roubaix? Tadej weighs 25 lbs. lighter than MVDP, last I checked.

...Cause cycling at this level is always about the compromise of weight vs power. Pogacar could be stronger at a higher power, but his power to weight ratio would suffer.
All else held equal is very important to understand they physics here, but that's *never* how cycling actually works.

Being a heavier rider means you have more momentum entering a cobble sector, as your wheel hits every cobble you are going through them at a higher momentum than a heavier rider, so the lighter rider needs to pedal faster to overcome the cobble, furthermore as specified above you have to also consider the fact that the rolling resistance will affect riders of different weights, tire pressure, techniques.. There is also the fact that a heavier rider has more absolute power.
TT riders tend to be good when they are of a higher weight. Explain then why are Ganna, Van aert, Kung, Tarling, etc, favorites on flat ITT? Pogacar has lost several flat TTs to heavier riders..

Alsene, I tend to agree but it’s incredibly complicated to calculate who has what advantage in which particular situation as the variables are largely interrelated. As you note, the rolling resistance alone is affected by numerous criteria, all things equal it is easy to change one variable to determine whether the heavier or lighter rider has an advantage, however, when EVERY rider is free to play with ALL variables (tire pressure, bike stiffness, wheel seperation and dozens more), many of the advantages/disadvantages melt into a smaller overall a/d much harder to determine when there is not an enormous weight difference (we have riders from mid 50 - mid 90kg but most are much closer together). In the meantime each team has optimised the set-up for each rider so differences are nowhere near what they used to be 30-40 years ago but obviously there is and always will be some but without highly advanced scientific research (which no-one is likely to provide for free and investing teams will keep for themselves) I feel there is now little point in people like us trying to make any factor or point the determining one, none will ever outweigh the gains from drafting and strategy :-)

Most of us really overlook the reality of what happens physically when a bike tire goes over cobbles, we are too focused on experience, feeling and intuition to actually look at what should be a microscopic slow-motion analysis. I think it would help many to better imagine what is happening by comparing the situation to differing boat types advancing on wavy waters. You start with typical boats and play around with wave height and amplitude and then you progress onto more advanced designs, motorboats, catamarans, hovercraft, jetskis, gliders on different types of waves. Nce you can eliminate most of the wave and just skim the tops, the wave topography reduces to something minimal, this is the grail of bike design for cobbles, tough because no cobbled sector has exactly the same pattern. Does anyone know if wheel size is UCI regulated or is there a margin?

Rolling resistance goes up with weight. Also, I did specify "all else held equal".

Yes, you did say that but what is the point of stating the obvious when obviously a lighter rider won’t be able to avoid reality which is that above a certain speed rolling resistance becomes insignificant compared to wind resistance, so it pragmatically all things being kept equal is irrelevant for them in their task. Momentum is what you’re left with after overcoming resistances, and that’s what going to help you keep up your speed except on a longer hill where weight will slow you faster.

(I was really replying to the other person...)
In Paris Roubaix, the bits with the cobbles are slower than the paved sections, so wind resistance is less than it otherwise would be on a "flat" road, though as I keep pointing out, these are equivalent to a bunch of little hills.
While momentum will help one maintain velocity given resistances, you aren't taking into account that one had to get that extra mass up to speed in the first place, which costs more power. So, you do get back most of the extra power you've put in, but that's not helpful. The less rolling resistance (and climbing over the bumps) of less mass will always be more helpful than having more mass, all else held equal.
Actually, this could be easily demonstrated with a cyclist on cobbles, 2 runs, start at the same velocity, do the same power, have a backpack full of foam on run 1, then full of rocks on run 2 (ensuring the PSI was perfected for both) and see which run went faster. And when we think about it, we know which one will be faster intuitively.

Sorry then

By the way, try to specify details a little deeper next time, I know lengthy text is unpopular but often it’s very useful, like to distinguish between a cobble having the effect of many little hills or a cobbled section :-)