Drafting in Zwift: is it easier to draft behind a larger rider?

It’s a question that has popped up in Zwift discussions many times over the years: do larger riders put out a bigger draft on Zwift? Conversely, is it harder to draft behind smaller riders?

It’s a logical question since anyone who has ridden road bikes outside knows there’s a big difference between drafting behind a larger rider or a small one.

It’s also a strategic question, because the answer could affect how you plan your next team time trial on Zwift.

Zwift calculates a CdA for each rider based on their height and weight – a key part of accurately simulating in-game speed. Years ago, I had a conversation with someone within Zwift (I forget who) who told me that the size/strength of the draft does indeed change based on the rider’s height and weight.

So I had always assumed that was the case. But I had also assumed that the difference was so small that I wouldn’t be able to measure it with the rudimentary methods I had at my disposal.

Then recently the question popped up again – twice in the same day, in fact – and I started thinking once about how I could test it. That’s when I realized Sauce for Zwift could help me make it happen.

Test Methodology

To test if the size of the front rider affects the draft received by the second rider, I devised the following simple test:

1. Create a Meetup on Tempus Fugit, the flattest route in Zwift, and invite my two bots to the party. (A Meetup was used instead of a Club Event because this allowed me to dynamically change the height and weight of the front rider while remaining in the event.) No other riders were visible on course, and both bots were on the same basic bike: Zwift Carbon with 32mm Carbon wheels.
2. Set the front rider to a very small size (152 cm, or 5′, and just 50kg in weight). Set that rider’s power so they are cruising as close to 40 kph on the flats as possible.
3. Set my drafting bot to our standard dimensions (183 cm, or 6′, and 75kg in weight) then tweak this bot’s power until it is sitting on the first rider’s wheel at the lowest wattage possible.

I stayed in the Meetup (it was a 100-lap Meetup, after all) for quite a while, modifying the front rider’s dimensions and then tweaking both bots’ power until they were drafting nicely at 40 kph. Sometimes I changed that rider’s height, sometimes just the weight, sometimes both. I tested plenty of permutations until I was satisfied with the result.

With draft strength potentially differing by only a small amount, accuracy was crucial if my results were going to be useful at all. Zwift’s HUD shows no decimal points for speed, so I employed Sauce for Zwift in order to get at least one decimal place, which helped immensely!

I also used Sauce to show each rider’s current stats, and I kept the draft gauge up just out of curiosity.

Here’s a quick video summarizing what I saw:

Test Results

Conclusions

The conclusion is clear: the strength of the draft does not change based on the size of the front rider.

I reached out to internal contacts at Zwift to confirm this conclusion, because I was surprised with the result after what I’d been told in the past. The contacts confirmed that this is in fact the case: height and weight are used to compute individual rider CdA, but that CdA does not affect the size/strength of the rider’s draft “shadow” in any way.

Not yet, at least. It sounds like Zwift may be looking into making some changes in this area. But of course, there’s nothing set in stone.

Zwift could simply change the overall draft strength based on rider dimensions – but that isn’t truly how it works outdoors, is it? Outdoors, what changes is the size of the draft. A taller rider casts a taller draft shadow. A wider rider casts a larger one. Could Zwift get so detailed that they’re actually changing the size of the draft shadow based on the rider’s size?

We’ll have to wait and see. I can only promise to report on any changes when they roll out!

Your Thoughts

Does this conclusion surprise you, or confirm what you already thought was true? Would you like Zwift to change the draft so it more accurately emulates the outdoor experience, or do you prefer how it works today? Share your thoughts below!