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?