Calculations: Speed loss from battery vs dynamo, dynamo vs. no dynamo

1. Speed loss from dynamo vs. no dynamo

Power = k1 * v + k2 * v3 (when there's no wind)

where v = the speed, k1 = rolling resistance constant, k2 = wind resistance constant. k1 and k2 depend on tyres, bicycle shape, position of the cyclist, etc.

Assuming a rolling resistance of 30 Watt at 30 km/h, we can easily calculate the required power to pedal at another speed, or calculate the new speed when given a certain power. I will apply this to show how much influence the efficiency of a dynamo has, on cycling speed. Here I assume the use of 3 Watt lighting, so no use of e.g. 3 high power LEDs in series powered by the dynamo (which is possible)...

1. Take someone who rides at a brisk pace (as I usually do): Suppose I ride at 30 km/h, position a bit bent forward. This takes about 200 Watt (Note: This is on a standard bike with ca. 37mm tyres, fenders, racks etc. Not a road bike and especially not a time trial bike! Only then is it possible to go 30 km/h with about 120W. That is a figure I read in some test about bicycle lighting but which is irrelevant in case of lighting!).

2. Now we take someone who rides slowly: Suppose I ride at 20 km/h without a dynamo (+standard 3 W lamp), then the required power is approximately: 20 W + (20/30)^3 * 170 = 70 Watt. N.B. This is the power for the same somewhat bent-forward position, but people who cycle slowly at about 20 km/h usually sit up fairly straight. I estimate that a total power of around 100 W is a more accurate amount in that case, which means the influence on speed by the hub dynamo is a little less than calculated here.

As you can see, the faster you ride, the less the influence of the dynamo. The question is: Is a high efficiency dynamo of interest more to those who ride slowly or those who ride quickly? (answer: Those who ride quickly, as those who ride slowly don't ride slowly because it's hard to cycle faster but because they just ride at a slow pace; The fast riders want every bit of speed, but in my view it's not worth a lot of money as the speed gain is minimal at speeds of say 30 km/h).

2. Speed loss from battery vs dynamo

To come

What must be weighed here is the drag from the additional weight of the batteries (if the batteries + lamp are heavier than the dynamo + dynamo lamp), which gives additional rolling resistance, vs. the power needed to run the dynamo lamps from the dynamo.

To email me go to the email page