Zwift roads come in various surface types including pavement, dirt, cobbles, and more. Just like outdoors, the wheels (tires) you use on your virtual Zwift rig affect how your virtual bike performs in game. So road bikes roll fast on pavement, but don’t do as well in dirt. Mountain bikes are slow on pavement, but better than road bikes in the dirt.
We already have a post that covers the details of rolling resistance (Crr) on Zwift – see “Rolling Resistance on Zwift: Crr and Watt Savings of Various Wheels“. Today, I want to chart these performance differences in new ways so you can better understand how your bike choice affects your in-game performance and make smart choices heading into races/rides.
In the charts below, I simplified things for clarity:
- Makuri Islands’ sand rolls the same as pavement, so it was combined into the “pavement” surface type.
- Bricks, Cobbles, and Wood all roll nearly the same as pavement for all wheel types, so they’ve been removed as redundant.
- Grass is a surface type only available on Repack Ridge, where only MTB can ride, so it was removed.
Numbers are for a name-brand gravel wheelset instead of the stock Zwift gravel wheelset, again for simplicity.
Power Cost by Wheel Type and Speed
Note: numbers above assume a 75kg rider weight.
As speed increases, the power needed to overcome rolling resistance increases proportionally. The additional work of riding a gravel rig on pavement at a social pace may be hardly noticeable, but trying to keep up with roadies at race pace may quickly sap your legs.
- At 50kph on pavement, a gravel bike requires 41W more than a road bike to overcome rolling resistance. But at 30kph, the difference is only 24W.
- Descending the Jungle’s dirt at 50kph? A road bike will require 91W more than a gravel bike. But as you climb at closer to 30kph, the road bike only requires 55W more.
Power Cost by Wheel Type and Rider Weight
Note: numbers above assume riders traveling at 40kph.
As rider weight decreases, the power needed to overcome rolling resistance decreases proportionally. That’s why this chart looks similar to the one above – both illustrate proportional increases/decreases.
In a race scenario, what’s interesting here is the power costs for riders on different types of bikes, at different weights.
- An 85kg rider on a road bike trying to keep up with a 65kg rider on a gravel bike up the Sgurr’s gravel climb at 40kph will need to put out 100W more just to overcome rolling resistance.
- But once those two riders begin descending the paved side of the Sgurr, the lighter rider on the gravel bike only has to put out an extra 20W to keep up at 40kph.
Road Bikes Are Costly
And I’m not talking about purchase price (although they’re stupidly expensive these days). Check out the road tire performance on gravel and dirt. Rolling resistance (and thus power cost) is so high on these surfaces for road tires that you should really consider your options when choosing a bike on a course that includes significant stretches of dirt or gravel.
MTB Are Rather Useless
MTB tires perform worse on every surface type except dirt, and even there, the power savings is very small compared to gravel tires. Factor in a MTB’s higher weight and poor aero performance and there’s really no place (certainly no full routes) within Zwift where a MTB is the best choice.
If you’ve ever wondered why Zwift road races stretch out and selections happen in dirt sections like the Dirty Sorpressa or Ocean Boulevard’s Marina, the first chart spells it out. If your pack is traveling at 50kph on pavement and they hit the dirt, you suddenly have to hold 214W more if you want to keep the speed at 50kph!
Dirt in a road race is a lot like hitting a hill. Expect power numbers to jump, heavier riders to pay a higher cost, and for things to be generally more suffery when they’re more dusty.
What About Drafting?
People often ask, “What about when you’re in the draft?” on bike performance posts like this. The first thing to understand is that your setup’s rolling resistance, and therefore the wattage you have to put out to overcome it, isn’t affected by whether you’re drafting or not.
Example (using round numbers to keep it simple):
In a race, you might be traveling along in the peloton at 50kph, putting out 300W while the riders on the front of the pack are doing 400W. If you’re a 75kg rider on a road bike, racing on pavement, you have to put out 41W to overcome rolling resistance.
Now you roll to the front, where you have to put out 400W to go the same speed (50kph). Rolling resistance hasn’t changed, because you’re going the same speed. So it’s still requiring 41W to overcome rolling resistance, even though you’ve gone from fully drafting to no draft at all.
The second part of the answer, though, is that drafting is a huge factor to consider when you’re deciding which type of bike to ride for a “mixed surface” route, and how exactly you will attack the route.
Example: Jungle Attack
You’re entering the Jungle loop in a pack of roadies. You’re considering whether you should swap to a gravel bike for the loop. You’ll lose the pack’s draft when you stop to swap, but then as they’re descending the dirt at 40kph (having to put out 204W to overcome rolling resistance), you only have to put out 131W to match their speed. So you’ll be able to catch the pack.
You can’t just sit in the pack, though – you need to break away from the pack so that once the pavement arrives, you can swap back to the road bike and still be ahead of or in touch with the peloton. Happily, you have a 55W advantage when traveling at 30kph in dirt, so you should be able to push and get away.
Example: City and the Sgurr
A gravel bike delivers the best overall time on City and the Sgurr, at least in solo rider tests. But you must know that the short paved lead-in will require extra work! You’ll have roadies pushing the peloton at speeds near 50kph. Roadies sitting in the draft have to do 41W to overcome rolling resistance, while you on your gravel bike have to do 82W.
This is a manageable power difference, though, for a short time. What becomes unmanageable is you matching the peloton’s speed if you’re not drafting (perhaps you’re chasing off the back, or attacking on the front). Lose the draft and you’ll have to put out ~100W extra to match the pack’s pace, in addition to the added rolling resistance you’re forced to overcome. Even if you can hold this power and keep the group within reach, you won’t have any legs left to attack the climbs. Staying the draft is crucial for this lead-in.
Questions or Comments?
“there’s really no place (certainly no full routes) within Zwift where a MTB is the best choice.” What about the Jungle Circuit?
Close, but no cigar. If you’re using name-brand gravel wheels, your gravel bike is faster than the MTB on the Jungle Circuit. By just a few seconds on the circuit, but much faster on the paved lead-in!
I’m a bit confused by this. So if I’m racing a course like Dust in the Wind and wanted to consider a bike swap for the Jungle portion, would I want to swap to a gravel bike or a mountain bike for the circuit?
Gravel. MTB are heavier and less aero, although they have slightly lower roller resistance in dirt. Overall on the Jungle gravel bikes perform just a bit better.
Great to know! I think MTB is listed under the zwiftinsider-page on that route as the fastest bike. It seems like there is a place for the gravel bike after all! Perhaps a rejoinder to this article would be one pointing out which routes you would recommend gravel bikes for. Thanks!
Snow? Where are the snow roads or is snow swept pavement at the top of things like the radio tower climb and alpe de zwift considered snow?
As far as we know, the only ice/snow section is the Radio Tower climb.
Does this mean I should be switching to gravel down radio tower to hit that pesky Uber fast badge?
Definitely not, Jeremy! Road tires are fastest on snow/ice. Gravel and MTB are slower. (See charts above).
Wait, wait, wait, wait, wait, wait. So you’re telling me that not only is the Radio Tower climb a nasty brutal steep climb at the end of a nasty long steep climb but that it’s on a slower surface too? No wonder I hate that road more than any other road on Zwift.
The Radio Tower climb is intentionality brutal and is Zwift’s source for the raw material in DROPS. I have insider information that DROPS are made from the TEARS of riders collected when struggling up the Radio Tower Climb.
The tears are collected, refined in a proprietary and highly guarded process, and then distributed as DROPS. Who knew.
Massive kudos to you Eric for putting this together.
Great post. Thanks 👍
For a long time, I’ve been comparing gravel/dirt sections to hills where big guys like me (100+ kg) have to put out more work than lighter guys (it comes up most frequently when planning TTT strategies), and I’m always met with (dirt/gravel hurts us all equally). Nice to have some data that backs it up.
I had read your suggestion to use a gravel bike during the sgrrr course in the tiny race series the first weekend. Almost nobody did. At my 85kg / 300w pace up the climbs, It felt like dropping those 15-20 kgs I need to lose (or lie about) to be competitive. I’d never experienced that on a climb – usually I entertain myself looking at the different jerseys and skinny legs as people fly past.
I experimented on this course using the tron the first week and gravel bike the second. I found that with the tron I rode at 289w for 3:35 on the first climb, whilst on the gravel bike I used slightly less power 280w but was 7 seconds faster at 3:28. Overall I was 17 seconds faster on the tron, but there were bigger fields the first week, so might have benefited more from drafting. I rode the same course in Race Scotland yesterday and using the tron managed 3:15 but was up at 330w. I am one of those skinny… Read more »
Rolling resistance is almost entirely hysterisis losses. Given the same tire pressure, a heavier rider will have more tire deformation and more rolling resistance. Heavier riders, however, are smart enough to air up their tires with more pressure than lighter riders, resulting in the same tire deformation and the same rolling resistance. This is why heavier riders have an advantage in events such as gravel races when rolling resistance is high. Based on your charts above, Zwift has got it wrong here.
Zwift has long favoured w/kg vs pure watts. Look at all the high ranked junior racers in Zwift Power. IRL they wouldn’t be able to stay mid pack let alone jump free and win some hard sprints. Yet some dominate with relative ease. I came across one with a ZP rank of mid 50s from memory. I feel sorry for them when they eventually try some local crits (when old enough obviously) and get hard dropped pretty quickly.
Excellent points. Zwift does indeed have it wrong. https://www.renehersecycles.com/why-wider-tires-are-not-slower/
Amazing work Eric. I love these in depth articles for stat geeks like me to obsess over. As a heavyweight at 95kg I now have some data as to why I get smashed going up dirt climbs! Thanks!
On the ramp coming out of the tunnels, I can hang with the group at ~102 kg (usually). But going right onto the dirt toward the Italian villas or the desert (depending on which way I’m going out of the tunnels), is always where I struggle to hang on. The fact that the dirt toward the desert is inclined just makes it worse. People had almost been able to convince me that it was all in my head that I was taking more of a penalty than the lighter folks through here. Part of me is glad to know it’s… Read more »
Finally, something written down about the difference weight has on rolling resistance. Being a heavy rider, anything of road hurts and know I know I’m not just imagining it.
That’s some nice data, thanks! Though as a mathematics teacher I can’t help but notice that the lines connecting the data points hold no value, as there is no data between e.g. snow and gravel…While they do look fancy, it’s just the program fitting a graph there 😉
Great post! Challenging your “draft doesn’t matter for crr” statement. I do accept your example where you ride at same speed in front of group Vs drafting in same group. However, if you ride in a pack, you tend to have higher average speeds for a segment/course compared to riding solo. For me personally it can easily be at least 5 km/h difference. In this post you also proved speed to impact the power cost of crr. So this is your correlation to speed.. in races you will generally (have to) ride faster, so the wheel choice will matter more.… Read more »
If i understand your comment about riding in a pack correctly, I think you are experiencing zwift’s total riding resistance which is Cda + Crr, where Cda is a measure of “air drag” or “aero drag”, and Crr is a measure of rolling resistance. So I believe Eric is correct in that statement about “draft doesn’t matter for Crr” because rolling resistance has nothing to do with air/aero drag. On the other hand I believe that your experience is also correct because of how Cda plays out in a group vs alone or at front of a group because cda… Read more »
The rolling resistance force comes about because of contact between the road and your tire, and because of deformation of your tire (and to a lesser extent, the road surface). The aerodynamic drag force comes about because of contact between yourself and your bike, and the air you are moving through. Drafting affects the aerodynamic resistance force. It has no effect on the rolling resistance force. The rolling resistance FORCE is not affected by speed. It is constant. The POWER CONSUMED by the rolling resistance force IS affected by speed. The faster you go, the more POWER you have to… Read more »
Dirt is the jungle and the bits around Watopia and Makuri. Gravel is just Scotland?
I am wondering the same: How to distinguish dirt from gravel?
I think gravel is more grey in color, dirt more brown.
Great article and new information, thanks Eric!!! I really need to try the Gravel bike in the Jungle. And in Scotland when I ride it for the first time
Is it fair to assume that rock (Repack Ridge, some of the corners of Sgurr Summit, other places) is the same Crr as road and Makuri’s packed sand? The texture looks the same as the packed sand other than the color.
I was sort of surprised to see that cobbles are listed as having similar Crr to pavement as there was a race series on Champs de Elysee where the steamroller was an option and we were encouraged to use it on the cobbled portion to lower our Crr down to road.
I guess it’s only on road tires (which I was using for the Paris races) that those other surfaces make a difference after looking at the table again. If I could delete that part of my comment I would.
Does rain and other weather (hot pavement in the desert) have any impact on power costs? I was in London yesterday and it was raining a lot. Felt like I had to do more effort, but then it could be just me.
It does not. The difference is all in your head… sort of like trying to do an FTP test on a long Zwift descent!
Descents during workouts are the worst. I have to stop watching the screen.
One thing that would REALLY help me is for the Drop Shop to be organised by bike type rather than Brand. As it is you have to chose the brand then look at each entry to find out if it’s road or gravel. Even then it’s not obvious if you’re not familiar with what the different frames/wheels are called.
Is this information available anywhere?
Sure, see https://zwiftinsider.com/frames/