I’m just having a debate with someone…if a small person were to race someone larger, of the same power to weight ratio, would there be a difference in speed?
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On a flat road yes. On a steep incline yes.
Heavier rider with bigger FTP would likely be faster on the flat (no gravity to overcome)
Lighter rider with lower FTP would be quicker up a hill (gravity)
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Is it true? Isn’t that true that power is power. So if both rider has 4w/kg, they should be riding side by side, right?
No not necessarily. Wind resistance and gravity mean that it isn’t always equal.
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I disagree. On a flat terrain w/kg is not important at all. The main focus should be absolute power. W/KG kicks in when climbing comes into picture. If a small guy has FTP of 240 at 60kg and 80kg rider with 320FTP, they can go side by side during the climb. At the end it is 4w/kg for both.
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So you’ve just disagreed with me in your first post when you said that it shouldn’t matter (overall absolute FTP) and then disagreed with me now again saying it does matter?
no 
I said on a flat w/kg does not matter, absolute power is the key. W/Kg is important when the riders are climbing but if both rider has same w/kg ratio, they can climb side by side. You said that lighter rider can go up faster that what I disagree.
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Ah ok.
So we agree that on the flat absolute FTP means that two riders with the same w/kg may not ride at the same speed because the bigger absolute FTP rider could be faster.
But we disagree when it comes to climbing. I still believe that a lighter rider with a lower absolute power may be faster up a steep climb than a heavier rider with the same w/kg
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There are so many variables to take into account. Generally, on the flat, absolute watts will be
king. So two rider at 4w/kg, but one putting out 250W, the other 300W, the rider at 300W will be faster. However if they both ride at 250W, they will be a similar speed.
Uphill, if they both ride at 4w/kg, then in theory they should get up the hill at similar speeds. But if they ride at the same watts, the lighter rider will be quicker.
None of this takes into account real world situations like size and shape of the rider, wind speed etc.
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W/CdA will play a big role, especially on the flat… So the original question should probably include the premise “CdA’s being equal.”
You have to be going quite slow for CdA not to make a big difference.
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Often the smaller lighter rider will have quite a lower CdA wouldn’t they?
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If the two riders have the same position and shape, then yes, but many things play a role in this.
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Not always. In TT, some of the big guys can get very aero whilst some of the climbers don’t.
Just to exemplify how much influence CdA has: A friend and I, have done several practice time trials, both totally flat and slightly rolling. Our weight (including bike) has been the same (within 1 kg). We did them on road bikes, with clip on TT bars. On all occasions, I have been slightly faster than him. Here’s the kicker: he puts out 70 (seventy!) watts more than me!
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Obviously, steeper the incline, more the weight matters. For example, at 1%, more powerful and heavier rider may still be faster, while at 3% they may go equal, and where steeper, lighter rider will have an advantage.
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Best Bike Split is great for this kind of analysis. Create a profile & race then you can analyse estimated time using sliders for CdA/Power/Weight/Crr.
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W/CDA wins on the flats
W/KG wins on hills 5% or more
Somewhere in between it depends
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On same W/kg at different weights there is also the kJ factor to consider. The heavier rider is going to have to expend more kJ to ride at the same W/kg. At the amateur level this can be a major consideration on a one day event as you are often kJ limited. At a pro level this can start to become a consideration during the longer stage races like the tour.
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They talked about this on a TR podcast in the past. Even at the same W/Kg the lighter rider will go uphill faster at a certain grade. I think they said something like 6-8% or higher.
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Yes, which is why W/kg is a good proxy because it’s easy to weigh yourself. Not so easy to measure your CdA.
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