By Rick Schultz
For those that have been cycling both indoors and outside (In Real Life – IRL), these two related questions come up all of the time.
- “Why is my trainer harder to ride (i.e., harder to pedal) than my road bike IRL?”
- “Why is my FTP lower inside than IRL?”
Basically, you can have two different FTPs. Usually riding outside, your FTP will appear higher and your road bike will seem easier to ride. This is why I recommend either two bike computers or one computer with two profiles (indoor / outdoor). The reason your FTP is lower on a trainer is due to all the additional resistance, i.e., moving parts that you are turning before the power is reported to the computer.
Riding and measuring power outside on the road, as you push on the pedals you are physically bending a strain gauge which directly measures the force applied and is reported at this point. It’s kind of like measuring the horsepower of a car. They can measure crank-based horsepower (like riding outdoors) or rear wheel horsepower (like on a trainer). There is a lot of parasitic and mechanical loss through the drivetrain. Motortrend magazine estimates 14-20 percent horsepower loss for a car (Drivetrain Power Loss – The Brutal Truth (motortrend.com)). I have spoken to Hunter Allen on this topic as well as researched several articles from Peaks Coaching Group, Training Peaks, Wahoo Fitness and the estimates are 5 – 30w difference between indoor and outdoor riding. Some just take a 5 percent difference.
So the bad news is that you should probably do two FTP tests (I know, twice the fun) to determine your indoor vs outdoor numbers and then set your computer accordingly.
Reference: https://www.roadbikerider.com/indoor-vs-outdoor-ftp-numbers/
Coach Rick Schultz is an avid cyclist who trains, races and coaches in Southern California. Rick is an engineer by trade, and in addition to being a coach, he’s a bike fitter and prolific product reviewer. He’s the author of Stretching & Core Strengthening for the Cyclist in the RBR eBookstore. Check his product reviews website, www.biketestreviews.com, and his coaching site, www.bikefitnesscoaching.com. Click to read Rick’s full bio.
True; UNLESS you are pairing the same or equivalent power meter used outdoors with an indoor training app rather than relying on a trainer based power meter inside.
I use a Wahoo Kickr indoors and measure my performance stats of my indoor rides and log them in Strava. When I ride outdoors, I use a combination Garmin 1030 and Stages dual power meters. I also use the Garmin and Stages power meters indoors, but independent of the Kickr,. If I put both ride profiles into Strava and compare them, I find that over a long period of time, 15 minutes or more, I get the same results within about 5%.. I calibrate both systems every 3 to 4 days. I have not been able to compare data successfully on a micro level of say 3 to 10 seconds, only longer term.. Still, I perceive an indoor ride to be much more stressful than I remember an outdoor ride with the same values of stress. I believe there is a psychological element of being distracted by outside stimuli when riding outdoors that relieves the perceived stress. As an example, I am riding the same bike with the same shoes, pedals, clothing, minus gloves, indoors and outdoors. I can ride 2 to 3 hours outdoors, with no break other than stopping for traffic control and maintain 70% of my average FTP over that time. After one hour indoors on the same profile, using a planned exercise that produces the same level of stress, I am toast. It helps me if I have something like a TV program to distract me, but that does not make a dramatic impact on the perception. I have also noted that in general, I can stress myself more outdoors when riding with someone else than I can solo. This applies if I am riding with a weaker rider and setting the pace, or if I am riding with a stronger rider, and they are setting the pace. This is based on measured power, heartrate and time, and should not be impacted by drafting.
This is a good point about drivetrain power losses creating the difference between power measured at the pedals/crank arm vs. cassette. Of course there are physiological/psychological/positional differences between riding mostly seated indoors vs more freedom of movement outdoors. Bound to create differences. Finally, one has to consider that single sided crank arm power meters measure only left leg power and double it to get total power but for many/most(?) riders the left leg is weaker. We are not symmetrical beings.
Concur with RickO above. I generally use my Assioma pedals as my power sensor/source both indoors and outdoors. Thus, my FTP should be roughly equivalent. I have not confirmed this through testing! :-). As an aside, my Favero pedals are failing on me, so my current set up indoors (no outdoor riding) is with my Kickr configured as both the controllable and the power source. Thus, I am apparently having to input more power (at the pedals) to hit my %FTP-based targets (as measured at the Kickr) during my training workouts.
Thanks for the article, Rick.
Is it possible that bike fit is part of the issue? I have an admittedly bad habit of constantly fooling with my fit. Several times, a fit that felt perfect on the indoor trainer felt way off (i.e., much slower) when riding on the road, especially uphill. (Still haven’t figured out in exactly what way the fit may need to differ.) Perhaps there needs to be an “indoor fit” and an “outdoor fit?”