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.
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.