Depending on the type of trainer you ride – dumb vs smart, wheel off vs wheel on, as well as which brand of any trainer type – you will find that either your wattage is higher or lower indoors vs outdoors (there isn’t even a “rule” one way or the other). And in the case of smart trainers, you will probably even find a discrepancy between how it calculates wattage output vs your power meter. With myriad variables, it can not only be confusing, but downright frustrating.
The punchline here is that your FTP (Functional Threshold Power) and associated power training zones are almost assuredly different when riding indoors. For example, my Stages power meter records about 20-25w lower than my Wahoo KICKR in Erg mode. And, when I’m riding in a virtual world such as Zwift, the discrepancy oscillates between 10-30w difference. And, there is even a difference between Zwift, Kinomap and FulGaz, at least in my experience. Couple this with the fact that the resistance of riding indoors also seems greater indoors. It’s as if the inherent forward momentum created when riding outside is nearly (or completely) non-existent indoors.
Here's a few pointers for you to help with all this.
Rear Wheel On Trainers
This is the most common type of trainer. It is also the most challenging to dial in compared to outdoor riding. Riding on the road, you apply force to the pedals generally from about noon to 5 o’clock. The bottom and back of the pedal stroke, the legs don’t create any meaningful force because of how the body is oriented to the cranks. So, there’s a little built in rest for the legs with every pedal stroke. Not so, indoors.
On a rear wheel on trainer, the only momentum of the rear wheel is that which is created while you are pedaling. When you stop pedaling, the rear wheel comes to a stop almost immediately. Because this type of resistance is applied to the entire pedal stroke, your legs are tasked all the more with every pedal stroke. Thus, your cardiovascular system is tasked more greatly when riding indoors than outside. Your HR climbs more quickly and your legs crater more quickly – during intervals and during complete workouts. This greatly impacts your ability not only to produce watts but also ride for a long time. 90-120min indoors can be both a soul- and leg-crushing experience.
The ”good news” is that the more you ride indoors, the more your body will adapt and a large discrepancy in wattage will reduce over time, even if it never completely levels out.
Wheel off Direct Drive
Wheel off trainers like the Wahoo KICKR overcome the dynamic resistance issues of the wheel on trainers. By taking the rear wheel off, and increasing both the size and weight of the flywheel, wheel on (or direct drive) trainers better simulate the type of momentum you gain when riding outside. And, wheel on trainers also create a smoother, more realistic “road feel”.
Now, how the direct drive trainer interprets force and measures watts is still not perfect. Like my Stages vs KICKR example above, where my Stages actually reports lower output, some athletes I train talk about the discrepancy going in the other direction, where the KICKR records lower wattage than their own power meters. It would appear that how and where the power meter measures your wattage – crank arm, crank, pedals, rear wheel – will impact the indoor vs outdoor relativity as well.
Various spin bikes will calculate power. Stages has come out with a spin bike that uses the same technology on the crank arm of the spin bike just like on a road bike. Arguably, this could produce a very similar power output when comparing a Stages spin bike to riding a Stages-equipped road bike outside. However, spin bikes increase resistance by applying a brake to the flywheel. This creates the similar challenge as with wheel on trainers in that resistance is applied during the entire pedal stroke rather than just when you’re able to apply force between noon and 5 o’clock. Yes, the spin bike’s larger fly wheel does create more momentum than the small flywheel on the back of your trainer, but this only mitigates the issue of constant force to a degree, not fully.
You hear about how important it is to calibrate your indoor trainer at the beginning of every single ride. In my experience, this is a bit of an exaggeration. Flywheels heat up and, thus, calibrating 20 minutes into a ride can alter the calculation of watts and could result in more accurate output. However, I have found that whether or not I calibrate my KICKR after completing a full warm-up, the discrepancy is still really big and really isn’t that different in any case. So, whether or not I calibrate my KICKR, the output will never come into line with my Stages power meter, and calibrating it creates a negligible difference compared to non-calibration. So, I don’t bother with calibration one ride to the next.
Now, if I decide to test and re-test my FTP indoors, I would calibrate the unit after warming up and do so before every test in order to ensure as consistent of results as possible one test to the next.
Other Variables to Consider
1. A Fixed Position
Being fixed into one position without the ability to move the bicycle or move your body as much as you might outside causes you to use more isolated muscles and create fatigue all the more quickly. When riding outdoors, we are constantly shifting our body on the bicycle which allows us to recruit some of our upper body to help the core and legs to produce power. This also gives some muscles a rest, helping them to recover and produce more power. Losing the ability to effectively wobble the bike back and forth reduces the ability to produce peak power. When athletes I coach are sprinting indoors, I tell them to ignore wattages as they just aren’t representative of what can be done outdoors. If the effort is there, call it good.
When riding at L3 (tempo) to L5 (VO2max), more strain is applied to the working muscles, especially some that are rarely leveraged outdoors, such as those of the inner thigh (adductors). It is not uncommon for the adductors, quads or hamstrings – or all of them – to crater and even cramp up toward the end of a challenging workout.
Temperature plays a significant role in the ability to produce wattage when riding indoors, so staying cool is critical. Heat is a byproduct of effort, and without the cooling wind we experience when riding outside, overheating becomes a real challenge. The hotter you get, the more you sweat. The more you sweat, the thicker your blood gets and the harder your heart has to work to pump that thickening blood to the working muscles. If you’ve heard the term “cardiac drift”, this is what is occurring. A rising core temperature coupled with exacerbated dehydrating from excessive sweating will do two things: 1) cause your HR to continue to rise; and 2) cause your wattage to drop.
Get a powerful fan blowing on you to provide some cooling. You can also set your trainer up in the garage where the temperature tends to be lower than in the house, or set it up by a window that you can open to allow cooler air to flow over you to complement the blowing fan.
3. Get Mental
The mind is very powerful, so don’t underestimate its ability to impact your workout – positively or negatively. Riding in a cellar staring at a concrete wall is not nearly as stimulating as riding outside with nice scenery. Nor will it ever be. Watching movies or race videos helps keep you stimulated and pass the time. Virtual training programs like the ones I mentioned earlier are also quite a bit of fun. And what I like about them is that they dynamically alter the resistance of the ride based on the terrain you’re covering, forcing you to constantly shift and alter your cadence and riding style to match the virtual topography. This increases the “outdoor feel” of the ride in a way that you simply cannot replicate on a “dumb” trainer.
Adjust Your FTP?
So, should you adjust your FTP when riding indoors? I don’t think there is a clear cut answer, quite frankly. I think if you say “yes”, then you open yourself up to the slippery slope of interpretation, which gets messy quickly. My suggestion is to understand the correlation between riding outdoors and riding indoors, and then keep both numbers in mind as you complete both your outdoor and indoor workouts. Remember, our goal is to shoot for certain training zones – in other words, there are no absolutes. So, if I have a 20-25w discrepancy between my Stages power meter and my KICKR, then I can allow a delta of 15-30w and know with confidence that my indoor workout is very close to what I would accomplish doing a similar workout outside.
And don’t forget, HR is the ultimate governor. You need to know what your HR zone is for a particular type of workout and that will help you more accurately define your indoor power zones compared to your outdoor zones.
When you look at all these variables, you realize that even if all of them have small impacts on wattage (up or down) that adding just a few of them together magnify the discrepancy you’re experiencing. You can either be aware of the discrepancy and account for it in your head by ensuring your HR is where you need it to be, or you can deliberately create and recalculate indoor power training zones. There’s no right or wrong answer. It all depends on how much hand-wringing this all causes you. I will admit, when I was younger, I would have wrung my hands raw over this sort of thing. Today, not so much. I know what a particular type of workout is meant to feel like and if I complete an indoor ride feeling the way I should, then job well done.
In conclusion, there is a much greater chance that outdoor vs indoor watts will be different than similar. If you train exclusively outdoors (for example, if you live in a perpetually warm part of the world) or always indoors (for example, if you live in the heart of a busy city and, therefore, riding outside is neither safe nor practical), this isn’t a challenge for you. However, most athletes are forced indoors during the winter months, some exclusively so. We put a lot of time and energy into our training so the last thing we want is to emerge from our winter pain caves only to realize we are not as fit or prepared as we thought because we were tracking to the wrong numbers for several months.
If you are able to create a mix of outdoor and indoor riding through the winter months, then cut yourself some slack. Be aware of the differences in power outputs and account for them in your various rides. If it is uber important for your indoor zones to be spot on accurate, then you need to complete a separate indoor-specific FTP test. It is really up to your and your tolerance for “fudge factor”.