Sometimes I want to ride on my eMTB with people who are on normal/manual/clockwork/acoustic/old-school bikes and want to experience the same effort that they have while still having the ADA OPDMD ability of the eMTB to bring me out of the woods quickly if needed, so I set out to see if there was a small amount of assist that would make up for any motor drag and added weight without giving extra boost. I have seen people suggest 10%, 15%, or even 5% to make up for the weight and motor drag of the eMTB. My theory was that about 10% assist was needed to match the effort, so I devised an experiment to find out. I will define success as coming within 1 mph of the same average speed for the segment with the same amount of effort, as defined by average heart rate.
My manual bike is a 2013 Giant Trance X 0 29er with a Stages power meter and is approximately 31lbs. Mods include Easton Arc 30 rims with tubeless Nobby Nic Addix rear and Maxxis DHF MaxxTerra EXO+ front – both 2.6 inches. My eMTB is a 2020 Specialized Turbo Levo and is about 50lbs. Mods include 35mm internal rims, Nobby Nic Addix rear and Magic Mary – both 2.6, DeSlackinators for the brakes, and a 38T Chainring . Tire pressure was about 15psi front and 18psi rear for both, which is appropriate given the 2.6″ 29er tires, and my weight of about 155lbs with full gear, including backpack.
I rode the same Strava segment, named “VC-Turkey Hill Loop,” with each – matching my heart rate the best that I could. This segment is a mix of single-track through forest, some paved bike paths, some gravel roads, and some fire roads.
First, the manual bike. My heart rate was 151 bpm average, and my Stages power meter reported 172 watts average. I completed the 7-mile loop in 36:13, for an average speed of 11.6mph.
For the eMTB, my heart-rate was an effectively identical 152 bpm average using the same Polar chest strap, and the Turbo Levo’s pedal force sensor reported 156 watts average. I completed the same 7-mile loop in 33:39, for an average speed of 12.5 mph. Assist was on 10%, with the “peak power” setting at 49%. If the heart-rates were the same, why was the power input to the pedals different? Based on my 1000+ km of riding the Turbo Levo and seeing this issue time and time again, I believe that the Turbo Levo power meter under-estimates power compared to my Stages meter and appears to do so by approximately 9.5%. If there is a way to calibrate it, I would like to know how, but it is to be expected that no two power meters will give the same results, unless carefully calibrated to match beforehand. I will make use the heart rate to say that the efforts were equal, since heart rate data from a chest strap is very reliable.
As we can see, 12.5mph of the eMTB on 10% assist with peak power on 49%, is faster than 11.6 mph of the manual trail bike, so setting a Turbo Levo to 10% assist is a modest-amount easier than riding a manual trail bike for this course. Perhaps a value of 8% would be a more exact match to the manual bike, but then again, this Giant is not as fast as a cross-country bike with faster rolling tires, like Rocket Ron, Racing Ralph, or Racing Ray – so 10% is on the high side, but probably fair enough compared to a more optimal manual bike with faster tires.