Cheat School: 6. Optimizing the First 20 Min

In the previous lesson we learned how to manage the trailing 20 min W/kg average so that the highest such trailing average in a race does not reach the performance ceiling in your cruising cat.

We made the distinction between 20 min blocks on the one hand, which any race can be divided into, and on the other hand 20 min periods, or trailing 20 min averages, which is what ZP measures and which can intersect two 20 min blocks.

We also said that in most races the highest 20 min period in your race, which ZP will judge you by, is likely going to coincide with the first 20 min block since it includes the start, where everyone typically pushes hard for early positioning, thus driving up the average.

But is it optimal to push hard the first 20 min? What does in-game physics have to say? Let’s find out!

Zwift physics is modeled on real life cycling physics. I wouldn’t say this has been tested thouroughly yet, but all the little studies so far, and some of them you can find on Zwift Insider if you dig around, indicate that, although with some convenient simplifications, Zwift closely follows the generally accepted physics model of cycling, parts of which was published in a report by Martin et al (1998) but which is fundamentally plain old Newtonian physics. And this matters because it helps us make a crucial (cruisial) prediction regarding the first 20 min of the standard race.

I’ll throw some physics in your face:

These hieroglyphics are not nearly as difficult as they may seem at first, so bear with me for a minute. The above formula can be used to calculate the power in Watts needed to pedal your virtual Zwift bike forward on a flat surface, a simplified case but the Tick Tock course does spring to mind. Let’s pick the formula apart.

P is power in Watts. Fv or F * v is force, measured in Newton (like on your expensive Park Tools torque wrench), times your speed. They are the same. It’s how you can define Watt.

Then comes an expression in two parts. First is the Cr * m * g * v. It’s the contribution of rolling resistance. Friction between wheels and tarmac and friction within the drive chain means you don’t get fully rewarded with speed when you pedal. Some is lost due to rolling resistance and you need to counter it. Little is known about how rolling resistance is modeled in Zwift and it really doesn’t matter for our purposes. Likely it’s just a fixed number regardless of rider and chosen bike. For simplicity, let’s assume it is zero, no rolling resistance to worry about.

Then comes the second part of the formula on the right hand side, and this is where it gets interesting. The second part says something about air resistance, or drag. There is no wind in Zwift but it seems drag is accurately modeled.

The second part of the expression is ½ * Cd * rho * A * v^3. (It’s not a ‘p’, it’s the Greek letter rho, but that’s unimportant.) What does it all mean?

Cd is the drag coefficient. Every 3D geometrical shape, or its front side rather, has its unique Cd, we could say. A low number is preferable because that means we will need to push a lower P, Watt, to keep the bike moving. A ball-shaped object has a lower Cd than a cubic object, e.g. a cardboard box. A pointy bullet or a rocket has an even lower Cd than a ball. And an aero bike frame has a lower Cd than a MTB. What about the rider? Even though it is a simplification, Zwift most likely sets a fixed number for any rider regardless of their real-life bodily shape.

The funny looking ‘p’, rho, is air density. The thickness of the air. We don’t normally think about air and the resistance it actually gives unless we take a walk on a windy day. Or when we get on the bike outdoor and try to be fast. In complete vacuum, like in outer space, rho would be zero, because there is no air at all there. And when you multiply something by zero, it becomes zero. So if rolling resistance is zero, which we assumed for simplicity, and if there is no air, then no power is needed to keep a certain speed on your space bike. You just need to worry about the initial acceleration up to the speed you want to keep, but that’s a different formula. This is the reason why it is possible to go back to Earth from the Moon without a crap ton of fuel or why a satellite can go around the Earth at very high speeds without using any engines – in vacuum you just keep going. But down here on Earth we have to keep applying Watts to keep the speed. And the closer to sea level we are, the higher the air resistance because the air density is slightly higher due to gravity, whereas the density is slightly lower at high altitude. So what about Zwift? The Zwift physics model is probably simplified, meaning air density is just a fixed number regardless of whether you ride the Ocean tunnel or sprint to the finish on Ven Top.

Next comes the constant A. That is you and your bike’s frontal area measured in square meters. A big frontal area gives high air resistance. This is how a sailing boat can go forward without an engine. A huge frontal (or aft) area in the form of the sails, the bigger the better, sort of. But for a bike rider you want it as small as possible. Aero frame. Aggressive riding posture. Etc. Like with Cd, the drag coefficient, a smaller frontal area means you can push lower Watts to keep a certain speed. Frontal area has been shown to vary among bikes and riders in Zwift. A short rider is more aerodynamic because he has a smaller frontal area. Possibly, although not quite confirmed, a tall light rider has a smaller frontal area than a tall heavy rider in Zwift. Outdoors this is so. But regardless, like with the drag coefficient and air density, this is a constant that you cannot change (unless you lower your height in the Zwift settings). These factors are what they are for you in any race.

So what the hell does all this have to do with the first 20 min when cruising a race? Well, there is one more factor to consider, the cruisial one. Next and finally comes v^3, velocity (or speed in meters per second) raised to the third. A cubic speed. And this has huge implications for Zwift racing in general and for the first 20 min when cruising.

Let’s visualize with the help of some fictitious and simplified (an thus unrealistic but illustrative) examples. Let’s assume rolling resistance is zero. Let’s assume Cd, rho and A are all 1. So then the Watt needed to keep a certain speed, v, is:

P = 0 + ½ * 1 * 1 * 1 * v^3

which is the same as

P = ½ * v^3

So under these artificial circumstances, the Watts needed to keep 25 kph would be:

P = ½ * …

Wait a minute. v is measured in m/s in Newtonian physics, not kph. We need to convert 25 kph to m/s. 25 kph = 6.9 m/s.

P = ½ * 6.9^3 = 164W

These are unrealistically high Watts. That’s just because we set the other factors to 1. They are constant and don’t change and they are lower than 1 in reality. But it keeps the example simple, so let’s stick with those numbers.

What about keeping 30 kph instead? 30 kph = 8.3 m/s.

P = ½ * 8.3^3 = 286W

But that’s no way to start a race. If we were to race a TT without drafting in the lower categories we might want to go at 35 kph. 35 kph = 9.7 m/s.

P = ½ * 9.7^3 = 456W

In a normal race we would be drafting in a group as cruisers. So we would probably be looking at 40 kph on the flat mid-race rather than 35 kph. 40 kph = 11.1 m/s.

P = ½ * 11.1^3 = 683W

But we were supposed to talk about the first 20 min. At the start everybody will be pushing hard. We would want to go at 45 kph at least to keep up with the others. 45 kph = 12.5 m/s.

P = ½ * 12.5^3 = 977W

Nah, let’s stomp it at start and sail with the sandbaggers for a bit and then compensate the high initial Watts by going under cat limit for the latter part of the first 20 min. Let’s do 50 kph at start! 50 kph = 13.9 m/s.

P = ½ * 13.9^3 = 1342W

Hmm… there is something going on with the Watts needed. And it has to do with raising speed to the third in the formula. Let’s visualize it in a graph.

We can see that the line is not straight. The relationship between speed and the Watts needed to keep it is not linear. It’s cubic. This means that for every additional kph we want to add to your speed, the Watts needed to drive it grows exponentially. It is not hard to keep 20 kph even without drafting. To increase speed by 10 kph to a total of 30 kph is harder but still not hard. But to increase with yet another 10 kph up to 40 kph is notably harder even though the speed increase (10 kph) is the same in both cases. And going 50 kph without drafting starts to get really hard. For some people it will mean actually sprinting, something you can’t keep up for long.

So what about the first 20 min then? Let’s simplify again. Assume that the cat limit translates into the W/kg needed to go 40 kph. And assume that you have the choice between going a steady 40 kph throughout the first 20 min and going 50 kph for the first 10 min and then 30 kph for the last 10 minutes to compensate for the hard start. (50 + 30 ) / 2 = 40, so the average speed would be the same in both cases.

In the first case, a straight 40 kph for 20 min, would mean 683W with our simplified and exaggerated numbers. And let’s say 690W is the cat limit. You have nearly optimized the first 20 min then, all else equal.

But what about the hard-start option then? For the first 10 min you go at 50 kph, meaning 1342W, and then 30 kph meaning 286W for another 10 min.

(1342 + 286) / 2 = 814

So by going hard rather than keeping an even pace in the first 20 min, your Watts needed jump from 683W to a whopping 814W! And if the cat limit for you with your weight was 690W, then you would go over limit by a mile and get a WKG on ZP.

In a real Zwift race the drag coefficient and the other constants will be lower, so the actual Watts needed will be lower of course. But the cubic relationship between Watts and speed remains the same. It is much more efficient to keep an even speed just below limit in the first 20 min block of a race, or actually at any point in a flat race.


Can you? There could also be tactical considerations to take into account, things that might force you to go hard at start. Let’s assume, again simplified, that at start you have the choice between two groups. The front group is full of sandbaggers and will keep a speed above your limit, so you can’t stay with it for long. But later in the first 20 min the group will shatter into two and you could go with the slower subgroup and keep within limits. Or you could go with the second group which ends up traveling at a decent margin below cat limit. Although going with group two will safeguard your average, you will be going too slow later in the race and will probably miss the podium. Trying to bridge to the slower part of the front group from group two would mean pushing very hard Watts solo without draft later and might not be possible.

So there are reasons, fuzzy uncertain factors to consider, when choosing speed in the first 20 min block. But you should be aware of the… let’s give it a cool name, the Cubism of Speed, when you cruise. The best thing to do is rarely to go with the worst bunch of the sandbaggers at the start, even if you intend on dropping later in the first 20 min block. It is a better choice to look for a somewhat slower group already from the start, a group that you will have to drop into later anyway as you drop from the sandbaggers. And when you have dropped into that somewhat slower group, your average will be higher than theirs because of your early sins at the start. Very important point! So it would have been better to go with the slower group already from the start. Going too hard at start is a complete waste of useful W/kg wiggle room as a cruiser. It is what is going to prevent you from having to drop to save your average if there is an early climb, or what is going to prevent you from bridging to a somewhat faster group if it turns out your group slows down too much.

But what about drafting? Drafting means you get less air resistance, right? So how does that affect the whole argument? If you can secure good drafting in the first 20 min, wouldn’t it change things? No. Well, a little bit. But in the two group example we assumed draft in both groups. And the way draft is calculated in Zwift, it seems it gives at best something like a 30% reduction in drag. But even so the cubic relationship between Watts and speed remains, although with this reduction. If the choice is between going with a fast group in draft and going solo behind them at start, then provided the fast group doesn’t send you over limit, in which case you would have to drop anyway, then going with the group is the better option because of the 30% reduction. But in most cases when cruising you should avoid taking wind like the plague anyway, regardless of which group you choose to go with. Cruisers suck wheels and only take wind when attacking.

So how do you choose the optimally paced group at start then? Well, that is more an art form than science, at least then and there from the saddle with numbers and riders flashing on the screen all over. It will take a lot of practice. Practice with caution. Err on the side of prudence, push it slightly more in the next race instead and you will have a long and fruitful career as a cruiser. Good luck!


Race Report: CeramicSpeed Series Qualifier Race – How to Cruise Out and Back Again

This was one race I just had to cruise! Sponsored event featuring a high-profile brand. Is there ever a better opportunity to display the filth Zwift swept under the carpet? Now, nobody is interested in anything but cat A here. They are the ones to be crowned ambassadors, i.e. a few of them will get to be a virtual ad sign for a bit. Almost like going pro, no? Well…

For best effect, although from a low category, you would of course want to win an event like this as a cruiser, and then brag openly about how you cheated yourself to a win… without actually breaking any rules. If it wasn’t for the usual cat A focus, I would be furious as a sponsor with a brand name to protect if I was fully aware of what a piece of unfair crap Zwift racing really is. And as an undercover cruiser I would just love to help create a little awareness.

The only problem was this was going to be a very difficult race, for a number of reasons.

First, the start list was huge. Three minutes before start I counted 98 ZP registered participants and a few more probably dropped in while I warmed up. We can safely assume there were many unregistered as well even though the rules said you needed to be registered in order for your result to be taken into account, although in the end only 86 finished in cat D. It was a long and tough race that would still attract a lot of people who don’t race regularly and who then got more than their fill during the race and bailed out. Anyway, it is more difficult to cruise a big field since the probability that there is someone in the race who can beat you increases with every additional participant.

Second, how can I be so sure there were lots of unregistered without actually looking? (I never bothered.) Well, it’s a fair assumption that people just wanted in, not caring too much about the rules, since there were plenty of sandbaggers too. I counted 33 cat C’s or worse 3 min before the race. They would all get UPG’s of course, but that didn’t stop them from joining. There were probably quite a few unregistered sandbaggers as well. And cruising a race with a lot of sandbaggers in it is difficult for two reasons. They may confuse you and have you believe they are threats to your placing, and if you don’t watch it you might be tempted to follow which could bust your 20 min average. Also, perhaps even worse since you can’t control the damage yourself, sandbaggers set up the heavies for a win. As a light cruiser I can’t follow a sandbagger for long or I will go over limit. But a heavy might be able to suck the slower sandbaggers’ wheels and still stay within limit e.g. by getting early separation in the first minutes that may be hard to bridge, if you are even aware of their presence somewhere in the far distance in front of you.

Third, an additional problem with huge start lists is that they are near impossible to keep track of during the race. Who is in your group at the moment? Are they on the ZP registered list? Are they strong, will they last? Are they legit, sandbagging or cruising? With so many participants you would need your own personal directeur sportif to shout in your ear what to do and how to respond to what is going on around you. Do the dishes and call in the favor from next-of-kin is my advice. It would actually be really useful when cruising. (I do the dishes and more but I still don’t get my directeur sportif – life isn’t fair!)

Fourth, I noticed many threats in the start list. There was an unusually large number of “nearly C”, loads of heavies and quite possibly cruisers too (I didn’t have time to go through the race histories of all the stronger participants of course).

Too many factors to control. I would have to focus on my average and just play the race by ear. The result? It was OK, I think. I got a 5th on ZP and missed the podium (the bronze to be specific) in a sprint, and if you have read earlier posts you may recall that I am not a strong sprinter.

Actually, it was a mistake to allow the short sprint to happen at all. I was planning on doing the usual thing and go on a push or long distance sprint about a km from finish, but being low cat C trash and after quite a few races recently I must admit I was a bit tired and decided to gamble instead. In hindsight it might not have mattered to the result, looking at the race histories of the small group I was in, but at least there would have been no real downside to trying the long sprint as the worst that could have happened was 6th. Note to self: Don’t let it go to a sprint ever again if you can avoid it…

So what about the rest of top 5? The race was won by a superheavy who finished minutes ahead of us. Since all my 20 min blocks (3 ½) were near limit I know for certain that it would have been absolutely impossible to stay with a guy like that without getting a WKG, not that I ever even knew of his presence ahead in the race. He was pushing almost 300W!

Silver was a heavy who worked hard and who I think pushed solo for the last stretch, which kills speed quite a bit. If a long sprint would have been successful (I doubt it) I just might have connected with him, but then he would have destroyed me in the sprint as I would have been toast and wouldn’t be able to match his Watts even if rested.

As for the little group I was in, which included 3rd, we were all cruisers except for 6th who worked hard and had a legit history. He had a clear weight advantage on me but most of the others beat him on the scales, so all in all he was double disadvantaged. If I could pick a deserving winner in this race, it would be him for sure.

How to cruise Out and Back Again then? I think the easiest cruises would come in cat C. The reason is that the Volcano KOM would start just outside the second 20 min block for them. So they would have somewhat better opportunities to go rather hard in the climb in a sort of late post-20 min push, and then recover the average in the descent. But that is only the forward 20 min average covered, and there are good chances of getting your 20 min average starting at the foot of the climb recovered with the help of the descent as parts of it can be supertucked. If the tempo was only slightly below limit in the approach to the Volcano KOM, however, then there would obviously be the risk of raising the trailing 20 min average above limit.

Note: In this race I actually had to drop in the early climb to get my average down before 00:40:00. Then I went on a hard P20P push. It was probably what caused a best 20 min average on ZP slightly above limit and gave me an upgrade post-race (the result still stands though). Doing so helped me reconnect with the riders I later finished with though, so the result would probably have been worse if I hadn’t pushed. But be wary of your trailing average in that climb. It could easily rob you of your cheater’s license.

The Zwift KOM reversed, which is also included in this course, happens at a relatively safe distance in time from the Volcano KOM. So just be mindful of your average as usual there, no special treatment needed. Apart from those two climbs the rest of this longish race is pancake flat favoring the heavier rider. And for the heavy rider able to ride at limit for the duration of the race (essentially cruising, since this is a 60+ min race in cats C and D) not even the climbs will set him back.

As a final note, am I sorry about the upgrade? Not at all! I was about to write something about it a few days ago that it was probably about to happen soon and that I wouldn’t go out of my way to avoid it. Racing is my preferred WO and cruising is really bad for your fitness if you, like me, have the physique where high intensity gives better results than quantity, the long and slow. As if I would have the time anyway… So back to bottom cat C again where I belong right now. I will likely keep posting even so with course advice for the cruiser, more dissections of the Zwift racing system, etc. Oh, and a new data analysis festival – new and improved, smoking hot and shocking – that I have planned for when I get the time. But first that new lesson in Cheat School that I promised the other day.

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