I’ve already written a little about the weight implications of choosing a dynamo hub (in short: there needn’t really be a significant weight penalty).
However, there is still another objection to dynamo use – the friction which must constantly be overcome as you ride along. I hope to demonstrate that dynamo hub drag is not much of an issue for the majority of riders, and more of you should be giving serious thought to investing in one (especially as you can now enjoy a discount from our favourite mail order stores).
Power loss versus a regular hub
Let’s start by putting some hard numbers on friction losses in ordinary hubs, before moving onto dynamo hub losses.
Manufacturers of ceramic bearings have helpfully quantified this (because you can’t sell a fancy bearing without demonstrating how ‘bad’ ordinary ones are) and it seems that losses in an ordinary hub are around 1W at 25mph [1].
So at a more reasonable 12.5mph, a ‘typical’ front hub will lose roughly 0.5W.
With the lights switched off, dynamo hub friction means a loss of 1-2W at the same speed, for a penalty of 0.5-1.5W over the ordinary hub. With the lights on, 7W is a good estimate. “Mini” dynamos like the SON 20R / SONDelux or Shutter Precision SV8 save around 1W on “full fat” ones like Shimano’s Deore, 3n72, or 3n80 series [2] [5].
Calculating a per-day power loss
The ratio of darkness:sunlight during the summer months most favoured by long audax rides is not far off 1:5 (civil twilight in mid-June gives 19.5h of light and 4.5h of ‘dark’).
This gives an overall hub dynamo drag of under 2.5W (averaged across all the hours of a day), so let’s use that as the basis of comparison.
What’s the impact of 2.5W?
A rider on a long distance ride might be producing as little as 100-150W, in which case the drag of a dynamo is ~1.5-2% of the rider’s output. Let’s take that at face value and imagine the impact of a 2% speed penalty:
1% of an hour is 36 seconds, so double that means you will fall back 1 minute 12 seconds per hour compared with your battery powered self: twelve minutes over a ten hour event.
A robust 200W effort reduces the impact to 0.75% (27 seconds per hour) or 4.5 minutes over a ten hour event – should you be one of the few who can put in that kind of effort for a whole event!
Another way this is popularly expressed is in terms of synthetic gradient, with the hub dynamo friction made equivalent to an extra X feet per mile of ascent. It turns out this is not such a difficult calculation, and comes to around 28cm per mile for a dynamo during daylight and 2m per mile for a dynamo at night. Scroll to the end if you want to see the maths!
Comparing with other sources of friction
So far you are perhaps thinking “OK, so 5-10 minutes over a ten hour event isn’t such a big deal, but it’s still something I’d rather avoid”. Fair enough.
This makes it a good point to contrast hub dynamo drag with some of the other things that can hold you back. For instance, it was recently reported that some types of chain lube can add as much as 10W over the factory lube [3] while, at only 12.5mph, the difference between a Continental GP3000 racing tyre and Vittoria Open Corsa racing tyre is already 20W [4].
Remember in the graph and example above we’re just comparing two racing tyres – the effect of changing from something like a Gatorskin or many other types of puncture-protected tyre will be significantly larger. Finally, I’ve already written about the tremendous efficiency advantage of the recumbent riding position: my own drop-bar racer is left for dead at roughly a 100W deficit to a midracer. 100W is a lot more than 2W yet many riders seem just about as loath to try a dynamo as they might be to try the dark side…
Conclusion
When the difference between ‘good’ and ‘bad’ chain lube and two superficially similar tyres is around 30x more than the difference between an ordinary hub and a dynamo (daylight hours) and 4x more than the difference between an ordinary hub and a dynamo (at night), you’d better start asking hard questions about your chain cleaning products/routine and whether or not you should be splashing out on new tyres!
Most people, of course, can’t tell the difference between different brands of chain lube and perhaps not even between comparable models of tyre. Certainly it’s a popular comment by dynamo users that they can’t tell whether their lights are on or off (in daylight!)
Even sample variation between production runs of any particular tyre probably exceeds the drag of a hub dynamo with the lights on… but how many of us spend as much effort making sure to buy just the right examples of a particular model of tyre as we do avoiding dynamos?
For no-holds-barred maximum efficiency, obviously you want to bring your power with you in the form of batteries; but for 99.9% of us, there’s little real reason to avoid all the goodness of a fully self-powered bike…
After holding out for years, I bought one nervously (and now have three…)
As I mentioned up-page, even Wiggle are in on the action: get yours here.
Disagree with the maths or had an experience (either way!) that you’d like to share? Please drop me a comment below!
The maths…
We can take advantage of straightforward formula here, with the following parameters:
- It takes 288 seconds to ride a mile at 12.5mph
- The power lost to the dynamo (lights off) is just 1W
- The weight of rider plus bike (all in) is 100kg
Then it follows simply that the equivalent height gain per mile is 28.8cm, so:
Power (W) * Time (s) / Force (N) = height (m)
(1 * 288) / 1000 = 0.288m
Run the same sum for a dynamo which is on (at 7W) and you get
Power (W) * Time (s) / Force (N) = height (m)
(7 * 288) / 1000 = 2.01m
Note these are roughly the 1 foot / 6 feet measure you will see referenced in popular articles! It’s easy to subsidise different values; for instance swapping chain lube on your bike (10W, if you’re unlucky):
Power (W) * Time (s) / Force (N) = height (m)
(10 * 288) / 1000 = 2.9m
