Hub Dynamo Friction

What exactly is the cost in friction / drag of running a hub dynamo system? Perhaps not as much as you expect…

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.

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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.

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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].
Screen shot 2013-03-05 at 21.32.26

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!

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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

Long distance bike lights

Almost anything will do if all you need is to get ten minutes down the road to work! As your rides get longer the question of illumination gets a whole lot more interesting…

Almost anything will do to light up your bike at night if all you need to do is get ten minutes down the road to work! As your rides get longer, however, the question of illumination gets a whole lot more interesting…

This article was prompted when long-term reader Nick F asked me about lighting for this summer’s 1400km London-Edinburgh-London audax; pretty much the ultimate lighting challenge and one with a large number of competing considerations.

Lighting options

We can break down the general approach to one of three:

Hub dynamo

The ‘fit-and-forget’ option; I’ve had a dynamo hub and lights in daily use since Autumn 2011 (without any maintenance at all). It’s been so successful that my wife and I now have three hub dynamos between us.

Initially, it’s undoubtedly more expensive to buy and build a dynamo wheel and the counterpart light. There’s no question it’s cheaper in the long run (I’m looking at nearly £100 to replace the Li-Poly rechargeable batteries for our MTB lights, where a dynamo has no real recurring costs) – but in fairness, if you’re speccing a lighting system for one event like LEL you might not get enough use out of the dynamo system to benefit from such long-term savings.

randokit

With the lights off, the extra drag of a modern dynamo hub is said to be equivalent to climbing an extra gradient of one foot per mile (19cm per km). Over the course of 1400km that’s 265m extra to be climbed, but we should strive to see this in a proper context: using a tyre of moderate rolling resistance compared with a top quality one can add many times this much extra effort, while wearing a jersey or jacket that’s a little too big and flappy? All bets are off!

When the lights go on, the friction from the hub will rise to the equivalent of around 5-6 feet per mile (a little over 1m per km) or around 1/10th of a percent. This is still far too small to be detected while riding and still significantly smaller than the differences between tyres – for comparison, if you change from an Open Corsa with latex inner tube to a Gatorskin the extra friction will be 2.5x higher than the hub dynamo on full blast.

There are a couple of other things to consider with hub dynamo systems, such as making sure the wire can’t get damaged (not rocket science) but also noting that you can’t easily perform maintenance on your own bike using a dynamo light. Get a flat tyre in the middle of nowhere and you’ll wish for at least a keychain AAA micro torch to make a repair…

Of course, it also offers you the ability to charge accessories via USB (whether that’s a phone or a GPS unit) rather than carrying a dedicated USB battery pack. More drag again, but keeping things relatively simple and removing that worry over remaining charge.

When it comes to the actual lights I have meaningful experience of only three: the Philips Saferide and the B&M IQ Cyo and Lyt. The latter would struggle at speed on open roads, in my opinion, but both the Cyo and Saferide put out lots of light.

The Saferide, with two LEDs, is considerably brighter than the Cyo but on the flipside it seems to stop the wheel more quickly when spun by hand; I still have to do a detailed report on this light but wouldn’t be surprised to learn it’s drawing more power from the hub (in which case, you can choose how much friction and how much light you want buying one over the other).

On balance if I had to choose just one for a brevet bike, I’d probably settle on the Saferide, although it’s hella ugly.

Rechargeable batteries

At the other end of the scale you have rechargeable battery lights, which range from things like my Ay-Ups (with an external Li-Pol battery pack) to Fenix-style torches mounted with velcro or rubber bands.

ayup

The obvious disadvantage is that you have to match your battery life to the amount of light you need, either by carrying sufficient spares to last the whole ride, finding time to recharge your batteries, or running your lights on a dim setting (or a combination of all three).

On an event like LEL, it’s unwise to rely on charge points anywhere on the route (unless you provision them yourself!). Having said that, the efficiency of LEDs is now such that many riders do manage to get through even the longest events like PBP and LEL without too much difficulty using battery lights.

If you’re lucky enough to spend time in a group, for instance, all but the first pair of riders can take advantage by running almost in the dark (this also makes life easier for the front-runners, as they won’t have to deal with riding into their own shadows if their lights are outclassed by those behind). That might not always be appropriate, but at least it is an option.

Battery lights inevitably have symmetrical beams which waste much of the light output compared with a dynamo light’s shaped beam, but don’t worry about this too much (except insofar as it means you have to carry that much more spare battery!)

AA (lithium primary)

For dedicated eventing there is another option, in the form of AA lithium primary (non-rechargeable) batteries. These batteries aren’t cheap or environmentally friendly, but they do have the benefit of being extremely light and long lasting.

With the aid of a battery holder pack from Maplin you can make up a 4, 5, or 6 cell (6-10V) DIY battery pack and wire on a connector as appropriate.

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I did Paris-Brest-Paris with my Lumotec IQ Cyo wired into a 5AA battery pack (yes, it works off DC too!), and one of my Ay-Ups wired in parallel on a separate switch (providing a ‘high beam’ mode).

I managed to ride almost right through three nights (and half of a fourth) on just ten AA cells, a total weight of 150g. See the separate article for more detail on my Paris-Brest-Paris kitlist.

If you were to start this approach from scratch there’s also the option of the B&M IQ Ixon, which runs off 4AA batteries and is still perfectly respectable (if a little long in the teeth), rather than repurposing a dynamo light – although it must be said that the venerable IQ Cyo may actually be found cheaper, at street prices.

If you really want to blend options you can charge the Ixon using a hub dynamo while you ride (not with primary cells!), although this begs the question of why you wouldn’t just have a cheap and straightforward dynamo light in the first place 🙂

One thing that has to be said about the AA approach is that you will need to wire up at least part of it yourself. You may consider this a bonus (you’ve carefully put something together and know exactly how it works) or a downside (your cack-handed soldering gives way at 3am in the middle of moorland half way through the ride).

The advantage over stock LED lights really comes down to the low power consumption and high bang-for-the-buck you get with a dynamo lamp like the Cyo, so you can get away with much less in the way of spare batteries.

On PBP I also had lithium AA (4x) in a USB battery pack to keep my Garmin Edge topped up, and that did give some leeway in terms of common battery spares early in the ride. However, because I didn’t want to carry enough batteries to power the Edge for all 90 hours, I turned it off at controls and thus lost some definitive data – not a problem if I’d had a dynamo to charge it constantly.

Summary

Each type of lighting has its pros and cons and it’s not possible to say with any authority that you should choose one over the other.

For starters, if cost is a major issue and you’re preparing for an event as a one-off, you will probably want to find a way to use your existing lights, appropriate or no (carrying spare batteries for a light which can use AA or li-ion 18650 cells).

Alternatively, you might justify the purchase of a hub dynamo on the basis that it will see use on a commuter too (the wonder of always-on lights that need no thought – not leaving work to find you’re out of charge – can hardly be overstated).

That being so, I can recommend Shimano’s newer dynamo hubs as a good cost-effective option (or for a little more, the Shutter Precision which is as efficient as it gets).

For my part, I’ve just built a set of brevet wheels around the Shutter Precision SP8. I don’t struggle for time on events, so for me the drag (which you can’t feel) is an irrelevance; I’d rather know that I’m always going to have light, as well as enjoying the ability to charge my phone (which I hope to do via the new B&M Luxos light).

As an unexpected bonus, the hub dynamo system actually saves weight over my original Ay-Up battery setup.

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I still have my Maplin battery pack for Lithium AA’s should I need to obsess over ultimate efficiency, in combination with my reliable old Cyo and Ay-Up LEDs. I’m not sure I will bother though, even for the next PBP.

 

Bombproof new audax wheelset

I’ve just finished putting the last touches on my new audax wheelset, which will be seeing some heavy action in 2013…

SON Archetype w/ Shutter Precision dynamo… very nice!

I’ve just finished putting the last touches on my new audax wheelset, which I hope will be seeing some heavy action in 2013.

Based around the SON Archetype rim, Shutter Precision SP8 dynamo and a Novatec lightweight rear hub, laced 32h 3-cross with Sapim Lasers, the total weight for the finished wheels is just 1930g. Not bad when you consider it’s saving me around 300g of LiPo batteries on the longer rides… effectively a 1630g wheelset!

I should probably also offset the substantial cost of new batteries I won’t be buying against the cost of the parts (totalling £300.77). I sourced the hubs separately, but got rims and spokes from DCR Wheels in the UK, a pleasure to deal with 🙂

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Continue reading “Bombproof new audax wheelset”

Dynamo Hub Weight

Just how much of a weight penalty do you pay with a hub dynamo? Not (necessarily) much at all…

Hard currency illustrates illumination information

I’m building a new dynamo wheel and thought it would be interesting to compare the weight with the Li-Po battery LEDs I used previously.

Based on the Shutter Precision PV8 hub (not the light weight SV8 version), I expected the dynamo setup to be quite a bit heavier, but in fact the weight difference was just 25g.

On the lights themselves, the IQ Cyo is about 30g heavier than the Ay-Ups, but the RSP Astrum is 30g heavier than the Toplight Line Plus rear light, making that a wash.

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Let’s throw in 10g for the tail-light wire and you get 35g – roughly the weight of five 2p coins. If you wouldn’t throw away that much change after a commercial control or cafe stop, you probably shouldn’t try to justify a battery setup on weight grounds alone 😛

(Yes, it’s true that there are many other reasons why you might prefer batteries – I used them exclusively myself, until recently).

Of course, the battery pictured is only just enough battery for one summer night. In reality I took two on all long brevets, which tips the scales in the dynamo’s favour by a cool 100g…

dynamo_weight1 (1)

This doesn’t touch on the two real issues that split dynamo / battery use: drag (which is now the subject of a dedicated article on hub dynamo friction) and efficacy, but I wanted to take a few minutes to illustrate that what many percieve to be a significant downside of dynamo use just isn’t…

Even Wiggle are getting in on the hub dynamo action: get yours here.

Lumotec IQ Cyo dynamo headlight

Dynamo or battery driven, its winning combination of (relative) cheapness, light weight, reliability, and lumens on the road make this a real star.

~£70 shaped beam photon cannon

Dynamo driven dazzler defends against dim drivers

For me, the Lumotec IQ Cyo is the bike light that revolutionized night time riding.

Dynamo or battery driven, its winning combination of (relative) cheapness, light weight, reliability, and lumens on the road make this a real star.

45mph downhill on the ‘wrong’ side of the road, at 4am with 900km in your legs and 3 hours sleep in the last two days? You got it.

Working heavy traffic, blasting windscreens to help keep the rush-hour cretins at bay? No problem.

Navigating a treacherous canal towpath in the pitch black without dazzling oncoming riders? Priceless.

Continue reading “Lumotec IQ Cyo dynamo headlight”