# How can we calculate the efficiency of the Siva Cycle Atom generator?

The Atom is a bike electricity generator made by Siva cycle and launched via kickstarter.

I was curious about how this generator compares to others (eg. hub dynamo/generators) in the market in terms of efficiency.

Quoting from the siva cycle FAQ:

How much resistance does the Atom add to riding?

The resistance is very small, and is proportional to how large your charging load is. At full generating capacity, it is akin to riding with your tires at 70 psi instead of 90 psi, or pedaling up a 0.3% grade. In our use around town, it is very difficult to tell that it’s on the bike.

I think the full load on the generator implies drawing 1A of current through the USB port at 5V so thats 5W of output assuming perfect voltage conversion.

Is it possible to calculate the efficiency of this generator using the above information? How much power would 0.3% grade take away compared to 0% grade?

Thanks!

• I'm from Siva Cycle. Thanks for asking these questions, it's exciting to hear anybody really cares. The efficiency of the system was designed to make sure the rider never experiences over a 10% boost in effort...in other words, our algorithms are maximizing power at various speeds based on the rider's perceived effort. To throw out another comparison, the added effort of riding with a Siva Atom is the equivalent of riding up 23ft for every mile ridden on the flat.
– user16970
Jan 11 '15 at 16:33

Taking 100kg of bike plus rider up a 0.3% grade generates a retarding force of 100kg*g*0.003=3N. At 24 km/hr=20/3 m/s that is 20 watts. If that is generating 1A at 5V, the efficiency is 25%.

Yes, I picked numbers that were easy to calculate with. But I believe this is all marketingspeak and you shouldn't trust the numbers to an order of magnitude.

Note this claims the efficiency doubles if you slow to 12 kph. I suspect the efficiency is roughly constant and either the force decreases or the current available increases as you speed up

• I agree this is all marketing speak. I doubt they have any maths backing up their statement. Jan 9 '15 at 4:23
• If they had the math to back it up, they would have just said that it is x% efficient and that it adds a resistance of for y watts over a conventional hub. They're own wording doesn't make sense. I don't think most riders could tell the different between 0.3% grade and 0% grade, but most could probably tell if their tires were 20 psi under where they should be. Jan 9 '15 at 15:14
• I calculated 20 watts drag as well. My Schmidt dynamo hub has a quoted efficiency of about 65% at 15 kph. At 30 kph the drag is about 6 watts. 6 watts of drag is barely noticeable, but 20 watts of drag will start to feel like riding on a false flat. Jan 9 '15 at 16:29
• @Kibbee: I can't compute the effect of tire pressure, as it depends on friction in the tire. A slope is basic physics, so that is what I used. Jan 9 '15 at 17:43
• Crr scales exactly like slope, so an increase in slope of 0.3% is exactly like saying the Crr increases by .003. Regular road tires (not knobbies) typically have a Crr in the range of perhaps .004 to .008 on smooth roads, so using the Siva would feel like switching from low rolling resistance racing tires to Continental Gatorskins (or from Gatorskins to heavy-duty belted hybrid tires). Jan 9 '15 at 22:08

it's exciting to hear anybody really cares

Yeah, because some of us actually use bike chargers as more than toys. If you are just going to ride a few miles cross town and would like your phone to be more full when you're done than when you start, a plug in battery charger, which I always have in my backpack anyway, is a cheaper and better solution. People steal anything they can pry off of a bike in Seattle, so it's safer in my backpack anyway.

But as a toy, this is still great. I like toys, so fine. But that's what it is.

A useful generator is for when you are going to be out in the dark and you need power for lights, guidance, or phone for safety reasons. In that case, of course one cares about efficiency, and 25% is too low to be useful.

Or, another way, 20 watts of drag is a big deal for a strong rider on a long ride or a typical rider on their commute home. I could figure out what kind of headwind that would be equivalent to for a typical commuter, but I'm pretty sure it's "too windy to ride today" for the typical commuter.

• Shimano, Schmidt and others already make hub dynamos with efficiency somewhere above 50%. USB charger adapters are available for those. I don't understand why anyone whould bother about Siva.
– ojs
Nov 14 '15 at 15:58