Perspectives on Personal Power
Helping you figure out how much electric assist boost you need
Humans are powerful! However when it comes to navigating the wild world of electric assist options, to choose the right e-assist for your needs, it is helpful to know how powerful a human can be and how much additional power you really need your e-assist kit to add to your bike.
This article below was written by designer Alan Scholz to help you understand the amount of power a human can produce and then compare it to the amount of power most electric assist kits can provide. Enjoy!
-Hanna Scholz (Bike Friday President)
How Many Horses Do You Really Need?!?
By Alan Scholz
In the lab, experiments have shown an average “in-shape” cyclist can produce about 3 watts/kg-1.4 watts/lb for more than an hour (e.g., around 200 watts for a 70 kg/154lb rider), with top amateurs producing 5 watts/kg (350 watts 154lb person) and elite athletes achieving 6 watts/kg for similar lengths of time.(6 watts per kg – think world hour record of 400 watt output for 60 minutes and distance of about 30 miles/48 kilo) Elite track sprint cyclists are able to attain an instantaneous max output of around 2,000 watts (& can sometimes break chains doing it!), Elite road cyclists may produce 1,400 to 1,700 watts as an instantaneous maximum in their burst/sprint to the finish line at the end of a five-hour-long road race. (now you know how they do those 40 mph/65kph sprints at the end of those Tour de France stages!!)
Now look at this in terms of horsepower. Mechanical horsepower of 550 foot-lbs per second is equivalent to ~745/750 watts. A healthy human can produce & sustain ~ 0.1 hp/75 watts indefinitely; trained athletes can manage up to about 1800 watts briefly & 0.3 hp/220 watts for a period of several hours. Regular folks bicycling are riding at between 25 watts & 150 watts, if they are not a skilled cyclist. If we have some cycling skill we are not only able to put out more wattage within our comfort zone (for an hour) but also going faster per each watt we put out by riding a more efficient bike, doing it more ergonomically (because it fits us correctly), we are trained, fit, efficient in the proper movements, and our bodies are trained to be better heat exchangers (dumping the excess heat is also easier outside the “lab”).
Example Situation: A Moderate Cycling Commute Distance
Assume a cyclist riding on flat ground, no wind, & motivated. We can expect about 200 watts of power output in an upright position (mountain bike) that should result in an average speed of about 16 mph, a road bike in its better aero position = 20mph, both have street tires on pavement. Let’s assume a distance of 8 miles one way. That will take 30 minutes to get to work, and the same to get back, a road bike rider in better more efficient aerodynamic position takes 24min. The total energy used of 200 watts X 1 hour = 0.2 kilowatt hours. So the cost would be about 2 to 4 cents if it was electricity. (the road rider 200 watts x .8hr = .16 kilowatt hours.)
A very comfortable commute (or slow touring pace) of 75 watts will produce a speed of about 10 to 13 mph. It takes more time but no sweat. These slower speeds makes the commute about 48 minutes to 37 min.
For watts vs. speed reference: 100watts = 12/15mph, 125watts = 13/17mph, 150watts = 15/18mph. (Thank you to Grin Technologies for their great power/load simulator!)
Typical speed in American cities has been shown to be about 30 mph. Portland Oregon averages 23mph. During rush hour & in contested areas it runs more like 15mph. Sometimes this includes a lot of expressway parking & engine running at stop. In 2016 it was reported that the average speed in the U.K.’s busiest cities had dropped below 10 mph! Clearly travel by bike in cities is very competitive time wise.
Bicycles are very efficient if designed for actual riding and not as motor bikes or for extra large loads. Total weights of riders & loads over 100kg/ 220lbs take considerably more energy if there are hills and repeated start ups and stops are needed. And we don’t all have the same capacity legs, skill, or motivation all the time.
What the paragraphs above show is that anyone with an addition of 100 to 200 watts of assistance can ride like the breeze with predictable trip outcomes and feel the thrill of a gifted athletic while doing so. If you can do an easy 100 watts and 12 mph, an additional 100 watts will put you in the Sweet Spot that I have found to be 17-18mph. Fast enough to make real fast predictable trips and still stay safe.
This fun 3 min video gives you another way to see the power a human can produce compared to the power it takes to run a house hold.
By Hanna Scholz
How many horse power/watts a human can produce:
1 Horse Power = 750 watts
- A healthy person can produce around . 1 hp / 75 watts which gives them a cruising speed of 10 to 13 mph
- A fit cyclist can produce over .25 hp / 200 watts and cruise at close to 20 mph or more
- A top amateur athlete cyclist can product up to .5 hp / 350 watts for more than 1 hour and ride at 25-28 mph
- A top elite athlete cyclist can produce about 400 watts for 60 minutes for a 30mph (World Hour Record) pace
How many watts / horse power to add to your bike to boost your human legs:
When an average human adds between 100 to 200 watts (1/10 – 1/4 hp) to their output with electric assist they become just as capable as a fit athlete cyclist!
If you can already comfortably cruise at 12mph on flat ground then if you add an e-assist boost of 100 watts then you can cruise at 17-18mph with the same effort. This is the ‘sweet spot’ speed that makes for predictable and safe trips.
If you have some hills or plan to carry some heavy loads then you may want to add 200 watts of electric assist to smooth out the tough parts of your trip.