Speed Tests: Steady w/kg Across Various Body Weights

In an earlier post, we dug into how rider weight affects speed on Zwift, specifically calculating how much each kilogram of weight slows (or speeds) a rider on Zwift.

Many readers asked us for similar data based on watts per kilo. That is, if we hold w/kg steady, how much does each kilogram of body weight (and the corresponding additional wattage required to maintain the same w/kg) slow or speed the rider on Zwift?

First we summarize the data, then we discuss conclusions. Enjoy!

First, a Disclaimer

Zwift is a rich simulation of outdoor riding in which many variables are taken into account to provide a realistic experience. What we’re trying to do here is show simplified test results to illustrate basic cycling physics which apply in Zwift and outside.

There are many, many variables which these tests do not include: varying rider heights, drafting, bike weight or aerodynamics, rider posture, etc. Our goal here is to focus on w/kg. But in practice, there is a lot more going on!

All of our tests were done using a single rider in isolation – so no draft effect. This rider was 183cm (6′) tall and used the Zwift Aero bike with 32mm Zwift wheels.

The Data

Climb Test

Climbing tests were completed up Alpe du Zwift, an 8.5% average gradient which is 12.2km long.

Power to weight ratio (w/kg) is a very good predictor of speed on steeper climbs. For example, we see here that two riders of wildly varying weights (50k to 100kg) are separated by only .6kph when they both hold 2w/kg.

• When holding 2w/kg: every kilogram of weight + 2 watts makes you ~0.012kph faster
• When holding 3w/kg, every kilogram of weight + 3 watts makes you ~0.016kph faster
• When holding 4w/kg, every kilogram of weight + 4 watts makes you ~0.022kph faster
• At 75kg, each additional w/kg increases your speed by ~3.55kph

Why is there a speed difference at all, and why is that difference greater at 4w/kg than 2w/kg? Read “Conclusions” below for further discussion.

Flat Test

Flat tests were completed over two laps of Zwift’s Tempus Fugit route, a 0% average gradient which is 34.6km long.

Speed varies significantly on flat roads between riders of different weights who are holding the same w/kg. For example, the speed difference between 50kg and 100kg riders both holding 2w/kg is 3.8kph. And like the climb test showed, this speed difference is even greater at higher w/kg: the difference between 50kg and 100kg riders at 4w/kg is 5.5kph.

Notice also how the speed gap between 2w/kg and 3w/kg is wider than the gap between 3w/kg and 4w/kg? Again–it’s just physics! As you speed up, each additional kilometer per hour requires more added power than the one before.

• When holding 2w/kg: every kilogram of weight + 2 watts makes you ~0.076kph faster
• When holding 3w/kg, every kilogram of weight + 3 watts makes you ~0.084kph faster
• When holding 4w/kg, every kilogram of weight + 4 watts makes you ~0.11kph faster
• At 75kg, each additional w/kg increases your speed by ~4.4kph

Descent Test

Descent tests were completed down Alpe du Zwift, an -8.5% average gradient which is 12.2km long.

Here we see the largest speed difference between rider weights, and this makes sense because gravity is now working in favor of the heavier rider.

• When holding 2w/kg: every kilogram of weight + 2 watts makes you 0.206kph faster
• When holding 3w/kg, every kilogram of weight + 3 watts makes you 0.21kph faster
• When holding 4w/kg, every kilogram of weight + 4 watts makes you 0.214kph faster
• At 75kg, each additional w/kg increases your speed by ~1.95kph

Conclusions

Our Zwift test results don’t tell us anything an experienced cyclist hasn’t already learned outdoors. Still, let’s discuss some of the findings and how they’re applicable to riding and racing on Zwift.

Don’t Attack on Descents

Look at the speed difference between each w/kg band on the climbs or flats versus the descent. Notice how the speed difference is much greater on climbs and flats. This (along with safety concerns outdoors) is why you don’t see riders putting in big digs to attack on descents: the speed you gain just isn’t worth it!

Example: on the Alpe descent, if a 75kg rider bumped up their wattage from 2w/kg to 4w/kg, their speed would increase by 3.9kph, or 5.7%. If they had put in the same effort on flat ground their speed would increase by 8.8kph, or 28.2%. More than double the difference! But get this: put in that same effort while climbing the Alpe and the rider’s speed increases by 7.7kph, or 92.2%. This is why racers attack on climbs–because you can put massive time gaps into the competition.

Heavy Wins

As the road gets steeper, riders at the same w/kg will see their speeds coming together. On a steeper climb like the Alpe, speeds stay quite close together, varying at the most in our tests by just 1.1kph (between 50kg and 100kg riders both at 4w/kg).

But why is it that when two riders are holding the same w/kg, the heavier rider will always be faster? There are multiple reasons, actually–but one big reason is what we’ll call “true w/kg”–that is, including the weight of the bike in our w/kg number.

Suppose we have two riders, 100kg and 50kg, both riding at 3w/kg. But let’s say they’re on 9kg bikes. If you add that bike weight to the rider’s weight and calculate the true w/kg, you get this:

• 100kg rider + 9kg bike @ 300 watts = 2.75w/kg
• 50kg rider + 9kg bike @150 watts = 2.54w/kg

So even though both riders are holding 3w/kg, the heavier rider is holding a higher true w/kg.

Another reason heavier riders go faster is that, unless you’re climbing straight up (which is impossible), your effort isn’t only lifting you up the hill–it’s also driving you forward by overcoming the forces of air and rolling resistance. Heavier riders are putting out more pure watts than lighter riders, meaning (in simple terms) there are more watts available to overcome air and rolling resistance after the lifting is done.

But Light Really Wins

But let’s not misinterpret the data. While the heavier rider always wins at the same w/kg, what we see in reality is that it’s really tough for a heavy rider to hold high w/kg. Making 400 watts of power is much more work for a 100kg rider than making 200 watts is for a 50kg rider.

So while our theoretical 100kg rider would be faster up the Alpe at 4w/kg than the 50kg rider at 4w/kg, there are very few 100kg riders who can hold 400 watts all the way up the Alpe! By comparison, there are many lightweight riders who can hold 4w/kg all the way to the top.

At the upper echelons of our sport, there are riders who can hold 6w/kg+ for over an hour. But those riders never weigh 100kg! The biggest are perhaps 80kg (Cancellara), but most are closer to 60-70kg (Froome, Dumoulin).

But Heavy Wins (Until the Climbing Starts)

On the other hand, if we look at Zwift’s standard w/kg-based race categories in light of the data above, it’s not hard to conclude that lighter riders face a significant disadvantage in flat races. Yes, you won’t find many heavy riders able to hold 5w/kg to win A races, but you will find plenty of heavier riders who can hold 3.1w/kg, meaning they could race as a C and really put the hurt on lighter riders. (In a flat race, a 100kg rider at 3w/kg will travel over 3kph faster than a 60kg rider holding 3w/kg.)

This is why many flat C races are won by racers weighing 90-100kg. Check out the category C results of this recent race, for example.

So what’s a lightweight D, C, or B racer to do? Skip the flat races. Enter events with significant climbs, where speed differences between weights are much smaller! If you’re able to hang with the heavier riders on the flats, then punch it up the climbs at a higher w/kg than the heavy riders can sustain, you can create the winning selection.

Longer-term, we’d like to see results-based categorization take the place of w/kg-based categories. But until that happens, lighter riders will need to race smart on the flats, attack on the climbs, and perhaps lobby for more climbing races.

Larger Variance at Higher Power

As watts per kilo increase, the speed difference between riders of varying weights also increases. So we see up the Alpe the 100kg rider is .6kph faster than the 50kg rider at 2w/kg, yet at 4w/kg the 100kg rider is 1.1kph faster. Almost twice the speed difference. Why is this?

Again, our “true w/kg” idea explains most of this difference:

• At 2w/kg, the difference in true w/kg between a 100kg and 50kg rider is 0.14w/kg
  • 100kg rider + 9kg bike @ 200 watts = 1.83w/kg
  • 50kg rider + 9kg bike @ 100 watts = 1.69w/kg
• At 4w/kg, the difference in true w/kg between a 100kg and 50kg rider is 0.28w/kg
  • 100kg rider + 9kg bike @ 400 watts = 3.67w/kg
  • 50kg rider + 9kg bike @ 200 watts = 3.39w/kg

Your Thoughts

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