Shouldn't Watts/Kg INCLUDE the weight of your bike?

The Nine Hammers Tavern (General Chat)
#1

If you want to truly do an “apples to apples” comparison for different riders on different bikes, shouldn’t the weight of the bike be considered? I know it’s a moot point if the bike is on an indoor trainer, but if it’s riding outdoors the OVERALL Watts/Kg would make a big difference.

I don’t know what got me thinking about this, but by my calculations two riders of the same weight and 20 min FTP would have a disparity of about 2% for every 5 lbs. (3 kg) of bike weight.

So if my FTP is 230 and I’m riding with a friend who weighs the same and has the same FTP but whose bike is 5lbs lighter, I would need to generate 234.6 Watts to match her speed at 230 Watts.

Of course, if our bikes weigh the same and our FTP is the same, I could lose some weight from my body (instead of my bike) to match her total Watts/Kg. Losing 6Kg would improve my Watts/Kg by 6%. And it’s a heck of a lot less expensive to eat less than to buy titanium and carbon fiber…

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#2

The watts/kg is used as a metric to determine a rider’s ability not what his/her bike is capable of.

#3

What if you have more than 1 bike? For instance, my TT bike is lighter than my road, which is heavier than my steel fixed wheel commuter, whilst I am as fat/fit.

Edited to change heavier to lighter.

#4

The wattage you can produce is independent of the bike and the bike is irrelevant. Even if you need to produce more power because you have a heavier bike, you are still only capable of putting out as much as your body is able. Your bike only helps you out by being lighter or more aerodynamic or more efficient - which makes you and the bike more efficient so your watts can translate into more speed. But regardless of your bike you are still only able to produce the same amount of power regardless of the bike you’re riding.

#5

I think it depends on what you’re trying to do. If you’re trying to work out who’d be faster on a particular climb, total weight is what’s important. However, if you’re comparing two riders to see who’s fitter, W/kg using only rider weight is closer to what you’re trying to measure.

#6

In Sufferlandria, everyone rides same bike.

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#7

If you go down this rabbit hole you would also need CdA to make it “fair” and the significance of that and mass would vary as a function of velocity and gradient.

#8

I think that Strava must takes bike weight into account when it calculates estimated power for those of us who don’t have power meters. I’ve seen my estimated power for uphill segments change significantly after updating which bike I rode after it has done the calculations for my default bike which is a FS MTB 3kg heavier than my Hardtail.

#9

Bike weight (and clothing, bottles, etc) only matters if you are trying to simulate times for 2 riders over a specific course on their respective bikes. If they are both on similar bikes e.g. carbon road bikes then the difference in weight is likely to be pretty minimal. 2kg pretty much covers the entire spectrum from sub 7kg climbing racer to 9kg sportive bike. To put in perspective that’s about the same weight variation an average person will see during the day with food, hydration level etc.

When measuring fitness, as we usually are, the bike is totally irrelevant.

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#10

From a big picture, when looking at Watts/kg it provides the most value for giving relative climbing capacity. For riding on the flats, it’s watts/CdA (aerodynamic drag) that matters the most. Watts/rider weight is the simplest way to keep things relative from person to person (just like VO2 max can be expressed as Liters/minute or milliliters/kg/min). That being said, actual climbing speed is related to not just rider watts/kg, but rider + bike + clothing/etc. system mass (kg) versus the watts that the rider puts out. Even with that, two individuals could have the exact same system mass (kg) and put out the same watts, but still go different average speed/time on the climb due to pacing strategy (see: From the Coaches: Pacing for Performance) as well as variations in the wheel mass in one equal system mass versus a different wheel mass in another. Another practical reason in cycling that we tend to look at rider watts/kg is that there is a legal lower limit of 6.8kg for the bike mass that is required for all UCI events. Of course at 6.8kg bike is a great percentage of the rider + bike system mass for a rider that weights 50kg than it is for a rider who weighs 75kg, but the bike mass still has an absolute limit. Hope that helps!

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#11

I got the new BikePacking model

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#12

:crazy_face:

#13

True enough…hadn’t considered aerodynamics…

#14

Thanks Sir Neal! It makes sense to me that comparing the riders watts/kg is a more practical metric for most of us, and that the “marginal gains” that can be made from making ourselves (or our bikes) lighter and/or more aerodynamic will be just that–marginal.

I think my statement about it still being a lot less expensive to reduce my “total package” kg by dropping a few Kg of body weight vs. a few Kg of weight from the bike is still valid (though admittedly less fun).

Was not familiar with the pacing strategy, so thanks for sharing that!

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