UPDATED: W/KG FAIL 2: THE LIGHT RIDER'S CURSE

The heavy rider's disadvantage

You couldn't exactly call it unfair perhaps, but heavy riders suffer a disadvantage in Zwift races. The lighter riders have an easier time uphill and it is so hard to match the Watts needed to keep level with the lighter rider's W/kg there. 

The above is a very common complaint on various Zwift forums. But is it true?

I like to question those self-evident truths we all take for granted. If they are indeed truths then there is no harm in validating them. Sometimes, however, they turn out not to be true after all, once you actually take a serious look at them. So what about rider weight and race results in Zwift? Let's take one of those serious looks for a change instead of just passing on what some other guy said on the internet.

The below is partly based on a discussion I was involved in on the official Zwift forum recently centered around what I like to call the Light Rider's Curse.

The light rider's advantage

Without any prior knowledge about the impact of weight in Zwift racing we could assume three things:

1. There could be advantages to being light

2. There could also be disadvantages to being light

3. If there are both advantages and disadvantages to being light, maybe depending on scenarios, then you could compare those and come to some kind of conclusion regarding the net effect of being light - is it more good than bad to be light, or is it the other way around?

So let's start by looking at possible advantages to being light, since people say there are such advantages. There are no obvious advantages on the flat, and everyone seems to agree (we will get into details on this further on). What heavier riders say is instead that they have a hard time against light riders in climbs. 

On the flat speed is mainly maintained by momentum, so pure Watts is king and heavier riders can usually (not necessarily) push higher Watts than a lighter rider with a smaller frame and less muscle volume. But in a climb W/kg is king. Body weight comes into play, and maybe it is easier for a light rider to attain a better ratio between Watts and body weight than it is for a heavier rider, especially a heavier rider with a few surplus kilos.

The above is reasoning taken from riding outdoors and in a different setting than Zwift racing with its unique and uniquely stupid rules. But it is actually a flawed argument and you need to understand why. First some physics. 

Question: A rider at 90 kg is time trialing against a rider at 70 kg up the Alpe du Zwift climb. Both are keeping the exact same lines and both are able to keep dead steady, ER-like Watts. Both are doing exactly 3.2 W/kg. Who will win?

Answer: The lighter rider will win. By a few seconds. But it has nothing to do with Watts or weight. The lighter rider has an ever so slight advantage in drag, having a smaller frontal area. 

It's similar to choosing between bikes in your garage before a Zwift climb. One frame will be ever so slightly faster than the other. However, if this is a TT on the Road to Sky course, then this advantage, a mere seconds, is more than offset by the heavier rider's advantage on the flattish approach to the mountain. So if the race route is Road to Sky, or even Ven-Top with its very short approach, then the heavier rider will win!

And if it wasn't for the small difference in drag between the two riders, then if both started at the same time at the foot of the climb, both riders would arrive at the finish exactly simultaneously. Bar the drag issue, 3.2 W/kg is 3.2 W/kg. It doesn't matter what you weigh. You will travel up the mountain at exactly the same speed. That's what the measure W/kg implies, it's its purpose, to equalize riders to make a comparison possible.

Now put this in relation to the W/kg cat system, with the performance ceilings in cat B-D. To be competitive in any cat B-D, you typically need to be able to put out W/kg at or very close to the performance ceiling, be it 2.5 W/kg, 3.2 W/kg or 4.0 W/kg. So to win a race on any course in, say, cat C, you need to be able to hold 3.2 W/kg, or someone else could come and do the 3.2 W/kg and beat you - there's plenty of such riders. To win a race up AdZ you thus need to be able to hold 3.2 W/kg. Assume you are contender, someone who could actually win in cat C. Then you will be able to race Road to Sky at 3.2 W/kg. If you are one of those riders who could, then as we just concluded your weight doesn't matter at all. In fact, you might even be advantaged being heavy since you will be naturally faster in the approach and might thus save some energy for the climb. Thus...

GET THIS:

There is no advantage to being light in regular Zwift racing!

So if there are no advantages to being light, could there still be disadvantages?

The light rider's disadvantages

The Light Rider's Curse is a nasty tendency in Zwift racing. Many light riders have first-hand experience of improving fitness to the point where they reach the top of their current race category. Or rather what should have been the top of the race category. Only it isn't. 

You would think that being able to average e.g. 2.5 W/kg in cat D would make you competitive there. But that is not necessarily the case. First, you have to beat the cruisers. But even if we take the cruisers out of the picture it still seems awfully hard for a light rider to get anywhere near a podium in the average Zwift race.

And then fitness improves further. Shouldn't that help getting to a podium then? No, it's just that final little push that tips you over to the bottom of cat C. You got upgraded before you even saw a podium. 

Why is this? Is this real or just some bad excuse from a light failed racer? It all seems so counter-intuitive. As a light rider you should have an advantage against the heavies in the hills, and being able to do 2.5 W/kg you should have no problem getting to the podium, right? So why don't you win?

It's because of this:

Someone doing 300W on the flat is going faster than someone doing 275W.

Yeah, of course he is! So what?

Well, what if it’s a cat C race and the guy doing 300W weighs 94 kg? That’s 3.19 W/kg, within ZP's cat limits. And what if the guy doing 275W weighs 77 kg? That’s 3.57 W/kg, way over limit. See the problem?

The heavy guy wins the race and the light guy isn’t anywhere near a podium but is still a disgusting sandbagger who deserves a DQ. But this never happens in real-world cycling, only in Zwift. And it’s because of the W/kg cat system that no other sport uses. 

Specifically, it's because of the W/kg ceiling of the lower cats in combination with ZP disqualifying racers afterwards that they themselves let into the race. You can't have a performance ceiling in sports. And you should never have to disqualify a contestant for being "too good" in sports.

Most races consist of mainly flattish stretches and then some shorter climbs. At the W/kg ceiling of a cat a light rider can in theory never match the speed of a heavier rider without going over limits and getting a DQ or even an upgrade. It's simple maths.

If it's simple maths in theory, then it should show in data too. So does it? Let's find out!

Weight study #1

I grabbed some fresh data from ZwiftPower, a sample of 50 consecutive cat C races of all sorts (distances, elevation etc). I only skipped races where i) weight data was missing, ii) there were fewer than 6 cat C finishers according to ZP, or iii) the race type didn’t lend itself to this test (like e.g. Hare & Hounds, age category or TTT races).

Then I compared the average weight of the 3 riders on the podium to the average weight of the other riders in the race (hence why I wanted at least 6 finishers).

Results:

The podiums in the races had an average weight of 81.3 kg.
The remaining riders in the races had an average weight of 77.5 kg.

This nearly 4 kg difference between the average podium winner and the average loser turns out to be highly statistically significant, even at the 1% level (p = 0.00118). For those who aren’t into statistics, this means that it is extremely unlikely that this difference wouldn’t appear again and again if we picked some other random set of 50 races from the ZP history. And thus we have to conclude that we can't refute that there is indeed a difference in average weight between winners and losers. Winners are somewhat heavier on average. It is not bad to be heavy in Zwift racers, quite the opposite. It is bad to be light in Zwift races. The results prove it.

The W/kg cat system screws light riders. I will give a more detailed example than the the simple one above of why. Let's work through this.

Assume the following:

-You are racing in the front group in cat C (for some reason there are no sandbaggers this time…)
-The group keeps a steady pace and you are at least 20 min from finish
-You weigh 75 kg
-You are on the wheel of a bigger guy @ 85 kg
-You are both in draft
-The big guy is able to hold a 20 min average of 286W, i.e. 3.2 W/kg according to ZP (286 x 0.95 = 272. 272/85 = 3.2)

The only way you can stay on his wheel is by matching his 286W. This would put you at (286 x 0.95)/75 = 3.6 W/kg. Keep at it for 20 min (if you can) and ZP will give you a DQ. People might even call you a sandbagger! You simply can’t win this race as a light rider and get away with it on ZP!

Guys weighing 75 kg with a 1 hr FTP of 272W according to ZP will already have been upgraded to cat B. They will have seen very few podiums back in cat C if they were up against heavier riders. Which they were. Hence data supports our simple maths theory and the existence of a Light Rider's Curse.

The objection

But wait a minute! "Assume you are both in draft..." Draft in Zwift doesn't give as much help as outdoors but it certainly is a factor. What if these heavier winners are just better at drafting? It seems unlikely. Why wouldn't drafting skills be evenly spread out over riders of all weights and sizes? But it's a good idea to eliminate draft when you are doing a study like this. So how could we eliminate it? By studying only individual time trials instead. On a TT bike you can't draft.

Weight study #2

So instead I scraped 40 consecutive iTT races in cat C from ZP. What were the average weights for the podium vs the rest of the field? Was there a difference? And was it statistically significant (i.e. not random)?

Results for iTT’s in cat C

Podium avg weight: 83.9 kg
Losers avg weight: 78.1 kg
Difference: 5.8 kg
Statistical significance: p=0.00004 (probability of a random sample/event resulting in such a difference)

Conclusion: The difference is not random. In fact, a pharma company doing a study on a new promising medication would do wheelies and open up the champagne if getting results of this magnitude. So heavier riders do have an advantage in cat C, even in iTT’s where there is no draft.

“Ok, but maybe this is exclusive to cat C. I don’t care about the fat noobs in cat C anyway. I race in B.”

So let’s look at cat B too.

Results for iTT’s in cat B

Podium avg weight: 77.7 kg
Losers avg weight: 73.0 kg
Difference: 4.7 kg
Statistical significance: p=0.00007

Conclusion: The difference is not random. We can see that people weigh less in cat B, just as I predicted in this blog post, but there is still a clear advantage for the relatively heavier rider, even without draft.

“Uh-oh… and you mean the reason for this is that both cat C and cat B have a performance ceiling (3.2 W/kg and 4.0 W/kg) that will weed out lighter riders trying to match the speed of heavier riders?”

Exactly!

“A-ha! Gotcha! But cat A doesn’t have a performance ceiling! So if their iTT winners are heavier than the losers too, then your argument implodes!”

Yes, that's right. It would. We’d have to come up with some other explanation for the differences. Not that I can think of any. But let’s worry about that later. First let’s look at cat A the same way. If we see the same difference, then I’m in trouble. However, if we don’t see the same difference… then the W/kg cat system is in trouble. If I lose, I’ll go jump off a bridge. If the W/kg cat system loses then… it can go jump off a bridge.

Results for iTT’s in cat A

Podium avg weight: 68.8 kg
Losers avg weight: 69.9 kg
Difference: -1.1 kg
Statistical significance: p=0.18

Conclusion: There is a small difference, but it is pointing in the other direction (better to be light) and it is quite possibly just random. We would get a difference like this almost every 1 in 5 samples from the ZP database. So we conclude that there is no difference in weights between podiums and losers in cat A iTT’s. There is no disadvantage to being light in cat A, where there is no W/kg ceiling stopping you.

Q.E.D. 

GET THIS:

There is no advantage to being light in regular Zwift racing, but there are clear disadvantages. Hence the net effect is negative. Or to spell it out: It always sucks to be light in Zwift.

The Light Rider’s Curse is a reality. Now where’s that bridge? 

And don't you ever come to the forums and complain about being heavy again. You are wrong!

I have tried. I have really tried. But I can’t think of anything positive to say about the W/kg cat system whatsoever. It just sucks any way, any angle you look at it.

So what is your takeaway from this post? That you should go buy some cake, french fries and some jars of peanut butter and start gaining weight? No, it's not weight per se that gives an advantage but the part of it that is muscle volume. And it's muscle volume in absolute terms, not relative terms where you factor in body fat, that matters. If you are obese but still on top of your category in terms of W/kg, then you more than likely have higher muscle volume than a rider 30 kg lighter than you. And that still gives you an advantage, even if that lighter rider can also produce the same W/kg. Of course you only stand to gain from losing excess body fat, but that's a different story.