Saving Watts In the Draft: Climbs vs Descents vs Flats

Saving Watts In the Draft: Climbs vs Descents vs Flats

Cyclists know that staying in the draft saves watts. It’s been tested outdoors, and it’s been tested on Zwift. This knowledge is important to understand and put into practice because bike racing is all about conserving energy so you have it when you need it. Drafting is the #1 way a cyclist conserves energy while remaining in contention for the win.

But how many watts do you save in the draft – and does that change based on whether you’re on a climb, a descent, or a flat road? We decided to to run a few tests in the Zwift Insider Lab to get some hard numbers.

About the Test

For our test we had two riders: Rider 1 pulling on the front, and Rider 2 drafting behind Rider 1. Both riders were 75kg in weight, 183cm tall, and riding Zwift Aero bike frames with Zwift 50mm wheelsets.

We took these riders over the Watopia Hilly KOM segments (forward and reverse) with Rider 1 pulling at 330 watts for one test, then 270 watts for the other. We also took them on Ocean Boulevard for a flat test at the same wattage. Rider 1’s job was to hold the wheel using as few watts as possible.

These tests were made with other Zwifters on the road, and required constant adjustment of Rider 2’s wattage to properly hold the wheel. This means the test results aren’t perfect. But they’re still certainly useful!

Our test bots, hard at work

Drafting Up Climbs

On the Hilly KOM Forward climb:

  • drafting a rider holding 330 watts requires 320 watts – a 3.1% savings
  • drafting a rider holding 270 watts requires 264 watts – a 2.3% savings

On the Hilly KOM Reverse climb:

  • drafting a rider holding 330 watts requires 292 watts – an 11.6% savings
  • drafting a rider holding 270 watts requires 244 watts – a 9.7% savings

The steeper the climb, the slower the speed. The slower the speed, the less effect drafting has since drafting is all about reducing wind resistance and wind resistance isn’t an issue at slow speeds. So we see on these climbs there is certainly some savings in the draft, but it’s not nearly as significant as we’ll see on flat roads or descents.

The savings is greater on the reverse climb because that climb isn’t as steep as the forward climb. Therefore, speeds are faster on the reverse KOM, and drafting has a greater effect.

Similarly, the savings are greater when drafting the 330-watt rider compared to the 270-watt rider, because speeds are faster behind the 330-watt rider, so drafting has a greater effect.

Drafting On Descents

On the Hilly KOM Forward descent:

  • drafting a rider holding 330 watts requires 207 watts – a 37.3% savings
  • drafting a rider holding 270 watts requires 145 watts – a 46.3% savings

On the Hilly KOM Reverse descent:

  • drafting a rider holding 330 watts requires 227 watts – a 31.3% savings
  • drafting a rider holding 270 watts requires 222 watts – a 17.8% savings

Descents are much faster than climbs, and the main thing slowing riders down on a descent is air resistance. Drafting greatly reduces air resistance, so riders in a draft on a descent can sit in with very low watts even when the rider ahead is hammering.

The steeper the descent, the greater the savings from the draft. So we see the steeper forward descent gives us a greater savings than the reverse descent.

And just like on the climbs, the savings will be greater when drafting the 330-watt rider compared to the 270-watt rider, because speeds are faster behind the 330-watt rider, so drafting has a greater effect. We don’t see that exactly in the results above, because holding the wheel over changing gradients was difficult in our tests!

Drafting On Flat Roads

On Ocean Boulevard:

  • drafting a rider holding 330 watts requires 241 watts – a 27% savings
  • drafting a rider holding 270 watts requires 208 watts – a 23% savings

We would expect the power savings on flats to fall somewhere in between the savings we saw on the climbs and the descents, and that is indeed the case (apart from the hilly KOM reverse data at 270 watts which is probably just a testing error – like we said, these tests weren’t perfect!)

Here we see once again that riders achieve greater savings when drafting a more powerful rider because speeds are higher, so drafting has a greater effect.

Takeaways

There are several useful takeaways from these test results:

  1. Descents provide a wonderful opportunity to recover, if you play it smart. Stay out of the wind, surf the wheels, and apply just the power needed to stay in the pack. Of course, there’s always the supertuck as well, but that is best used on longer, steeper descents.
  2. Drafting is a significant factor on shallow climbs like the hilly KOM reverse. So play it smart and stay on a wheel unless you’re attacking.
  3. The faster you’re traveling, the bigger that “wattage window” is where you’ll stay with the rider you’re following. That’s why our descent test results were a bit messy. (The “wattage window” is the minimum and maximum wattage you can hold and without being dropped or dropping the rider you’re trying to follow.) This means you can potentially waste a lot of watts by hammering too hard–so play with it, ease up, and see how little you can work to hold the wheel on descents and even flatter roads. With practice, you’ll get a feel for it.
  4. If you think this savings is impressive, consider this: the draft effect is even stronger in larger groups on Zwift! This is difficult to test (requiring many computers, Zwift accounts, etc) but we know there is a stronger draft behind 4 riders than behind 1.

Your Thoughts

Do these results make sense to you? Got questions, or additional takeaways from the data? Share below!

About The Author

Eric Schlange

Eric runs Zwift Insider in his spare time when he isn't on the bike or managing various business interests. He lives in Northern California with his beautiful wife, two kids and dog. Follow on Strava

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Chris
Chris
1 year ago

The data are helpful, and my anecdotal experience matches the results you folks observed. Since I’m a heavier rider, however (94kg), my typical race strategy is to reserve recovery drafting to the flats. Going uphill, I’m constantly trying to just minimize time losses. I can beat other riders up very short climbs, and hang with good climbers up short climbs. But even if I hammer all the way up the Zwift forward KOM, I’m going to lose some seconds to a good climber. So in a race, I’m always hammering the downhill sections, in order to maximize the benefit my… Read more »

Joe
Joe
1 year ago
Reply to  Chris

dude you need to drill it and get a gap at the beginning of a descent with a couple of other big guys, once you establish a gap it’s harder for the lighter guys to catch up. Shhhhh, it’s a “big guy secret”….

Tyler James
Active Member
Tyler James (@mrtyside)
1 year ago
Reply to  Joe

As one of the little guys, I’ve definitely seen people take advantage of this. I have to make sure I don’t leave it all on the climb so I’m ready to grab a wheel when the big guys come screaming down after they crest.

ChrisB200SX
ChrisB200SX
1 year ago

I believe the draft effect is less at the back of a group as opposed to in the middle of a group. Although, this could just be sticky draft playing tricks on my mind when the other riders ahead but toward the back of the pack are swapping positions, making it difficult to overtake.

Eric F
Eric F
1 year ago
Reply to  Eric Schlange

I think you’re probably in a better position than anyone, Eric, to recruit a bunch of people for a stop and go group ride where you measure the draft effect at various positions in the pack – everyone in a line vs one big bunch, etc… Would love to see those results!

Joe
Joe
1 year ago
Reply to  ChrisB200SX

I agree, it feels like that to me too, it feels easier up front than in the back of a group. We need some data on this…. 🙂

Peter
Peter
1 year ago

#doubledraft?

Gary Crays
Gary Crays
1 year ago

I’ve always wondered if the size of the rider in front influences the amount of draft the drafting rider gets? My guess is no but it would be more realistic if it did.

Tyler James
Active Member
Tyler James (@mrtyside)
1 year ago
Reply to  Gary Crays

I would love to see this tested too, although it is obviously hard to tell the size of the riders in front of you, but I know how big I am! Eric this would be one to do pretty easily by replicating this test but with a couple different rider weights/heights up front yes?

James
James (@jeastwood)
1 year ago

Any idea if the watt saving differs depending on where you are in a group? i.e. 3rd back versus right at the back?

MATTHEW YANKOW
MATTHEW YANKOW
1 year ago

As a guy who has swept group rides a couple of times, I have a bit of knowledge on the subject of the draft when it comes to group rides. I usually sweep for B and C group rides (power between 2.5w/kg and 3.5w/kg). Riders usually save about 1 to 0.5w/kg when drafting behind just me. When there is a team of riders working to get the dropped rider back, that rider sitting in the draft will save 1.5 to 1 w/kgs. These are not scientific numbers so they may not be exactly right but near enough to be useable.

Danny
Danny
1 year ago

What about size of rider you are drafting behind IE their height any evidence this effects it or not?

Cameron Allan
Cameron Allan (@cam_allan)
1 year ago

Hi Eric, a couple of questions/thoughts… 1. If the forward climb is steeper than the reverse climb, doesn’t that mean the reverse descent is steeper than the forward descent? But I would agree the savings would be bigger on a steeper descent. So, is the descent section labeled the wrong way around maybe? Or that part of the test just didn’t work out as expected? 2. Generally speaking, I agree the higher the speed, the higher the savings. But because weight helps on a descent I think it complicates things a bit. The front rider needs to put in some… Read more »

Cameron Allan
Cameron Allan (@cam_allan)
1 year ago
Reply to  Eric Schlange

ah, right, so Hilly KOM Forward descent means doing the Hilly KOM Forward climb in reverse. Got it! haha sorry.

Yes, I see the issue with the auto-braking. Not suggesting you should bother, but I’d be interested to see a whole bunch of data points at different power levels when descending.

Alan S
Alan S
1 year ago

How does trainer difficulty affect your descent speed? If a light rider had 100% and a heavy one had 10% would it be easier for the light rider to draft?

Alan S
Alan S
1 year ago
Reply to  Eric Schlange

So going over the top of a climb without more pedaling you free wheel just as fast regardless of of your trainer difficulty?

John C
John C
1 year ago

Hi Eric Not sure who to contact for this issue….love Zwift….noticed over the last year that when I race and cross the finish line, the times crossing the finish line and those that appear in the zwift app and zwift power results are always off by 15-20 seconds. I raced today in CP and crossed the line in 26:50. However zwiftpower has me at 27:07 even though I was way past the finish line. When I use the slider on the app it has me at 26;50 as well. Strava has me at 26:50 for the 2 laps as well.… Read more »

Chuck
Chuck
1 year ago

Here is a thought on how to test the effect of a large group draft. Golden Cheetah has a Aerolab calculator which allows a ride to be analyzed for Crr and CdA. A fit file from one of the leaders of a group ride on Tempus Fugit (like PACK or ZZRC) could be analyzed using this tool. The group leader is always in the bunch draft, you know the Crr so the CdA can be calculated on the flat course. A hilly course would also work but there is the chance a leader is not in the bunch draft on… Read more »

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