They won’t, because the heavier ball’s extra mass will make it overcome the aero drag forces and let it accelerate further/faster. Assuming they’re not moving in a vacuum.
In that case, the same size and the same material will eradicate the different rolling resistance from the equation.
Which is my point neither initial acceleration nor terminal velocity due to aërodynamic drag are influenced by mass, only the coefficient of drag and frontal area
You are confidently incorrect here.
Over any interval, the energy difference is mg(h1-h2).
That energy doesn’t all go into increased speed. Energy is dispersed in rolling resistance & air resistance. Once you leave classical mechanics into the real world of friction, turbulence and hysteresis, you can’t cancel out the mass in the energy difference because not all the energy terms are linear with mass.
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There is no mass in the formula for aerodynamic drag nor rolling resistance
- That would prove my point. You can’t cancel the mass out of the energy due to height change if it’s not there in the energy loss terms.
- Also, that’s not the case. Rolling Resistance with a rubber tire absolutely depends on mass, at least in the presence of gravity, and Zwift models that. Aerodynamic Drag in Zwift absolutely depends on mass as that is used to calculate a rider’s CdA. But it’s not linear so still can’t be cancelled.
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That’s why bobsled’s don’t have weight limits right? You must be talking about Fig Newton….Why the heavier rider is fasterOn a descent, gravity provides a significant “assist” by pulling you downhill. This gravitational power is proportional to mass and grade: heavier riders get more free power from gravity.
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Approximate gravitational assist on a 5% grade at typical descent speeds (~40-60 km/h or 25-35 mph): roughly 1.3-2.0 W per kg of total system mass (rider + bike).
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Assuming similar ~10 kg bikes:
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Heavier rider (146 kg total): ~190-290 W from gravity.
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Lighter rider (78 kg total): ~100-160 W from gravity.
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Effective total power (pedaling + gravity):
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Heavier: 300 W pedaling + 190-290 W gravity = 490-590 W.
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Lighter: 200 W pedaling + 100-160 W gravity = 300-360 W.
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The heavier rider has far more effective power driving them forward.Other factors
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Air resistance (drag) — The dominant force at higher speeds; scales with speed squared (and cubed for power). Heavier riders often have slightly larger frontal area, increasing drag, but the extra gravitational pull usually outweighs this on moderate grades like 5%.
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Rolling resistance — Slightly higher for heavier riders (proportional to mass), but minor compared to gravity and drag on descent.
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When actively pedaling (as here), the heavier rider’s higher output (300 W vs. 200 W) amplifies their advantage further, as they add more on top of greater gravity assist.
If both coasted (0 W pedaling), the heavier rider would still be faster due to higher terminal speed (where gravity balances drag/rolling resistance more favorably for mass).This aligns with real-world cycling physics: heavier riders descend faster on straight, moderate grades, which is why weight penalties hurt more uphill than they help downhill. On very steep or technical descents with corners, skill and braking matter more.
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your logic would be right if Zwift was even close on heavier rider mechanics. Lighter riders get benefit in incline, decline, and only slightly at a disadvantage on flats, but when there is a Pack doing 10 kph faster than they should IRL, the lighter riders can stay in it easier.
You’re confusing yourself by not using the same terms.
Aerodynamic Drag is a force, so you have to use the FORCE due to gravity, not the acceleration due to gravity. That force is completely linear with mass because yes, acceleration due to gravity is a constant. A special one we call “g.”
The force from drag (dependent on CdA, not mass, as you say) acts against the gravity force. Which is m*g
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I read threads like this and my brain hurts.
I can barely do simple arithmetic but some of you guys are at Stephen Hawking levels of intellect.
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I could have been clearer and maybe for the same tyre pressure heavier riders have slightly more rolling resistance though not necessarily a higher surface area but heavier riders should definitely not be faster downhill as suggested by OP!
Vibe Zwifting here. Nothing to add, just the feels.
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They definitely should be, for reasons already explained in this thread.
Plus, they are in real life. You’ve very clearly never freewheeled downhill IRL with another rider who has a significantly different weight to you. I have, and the difference is extremely clear.
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Who’s he? 
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Dude, yes they should. I explained where you went wrong in your physics already.
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Done me there. Chapeau. 
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