Dynamo Lights

Dropped you a PM

Anyone know anything about rear dynamo lights?

SJS do a Supernova E3 Pro 2 for £65, but it doesn't have a connector for a rear light. £100 less than the version that does have rear light connector.

Is there a rear light that I can purchase seperately that runs straight from the hub? And I presume it will be OK to wire in a front + rear + D1 all from the same hub (although not all will be running at the same time...).

I use the Supernova Triple front and have it wired to their rear light option:
http://www.supernova-lights.com/en/supernova-e3-tail-light-2

Are you sure it won't work with E3 Pro 2?

"In use with an E3 Pro 2 front light and an SP hub dynamo, and it's a fit-and-forget joy"
http://road.cc/content/review/127121-supernova-e3-tail-light-2-dynamo-rear-light

"There's a single button to turn the light on and off, and a wire for the dynamo with an output loop to link the E3 with a tail light."
http://road.cc/content/review/70888-supernova-e3-pro-2-dynamo-front-light

@hippy Yeah it will work, but only if you get the E3 Pro 2 with the rear light cable.

See here, SJS sell both - but £100 difference between the two (click on the options):

https://www.sjscycles.co.uk/lighting/black-supernova-e3-pro-2-dynamo-front-light/

Fancy saving a bit of money if i'm able to find a rear light that i can wire directly into the hub...

Ah, ok. Didn't know there were two versions. That's annoying.

I'd probably just use a battery blinky.

Rear lights don't have the internal gubbins to regulate the power output from the dynamo so need to be connected in series from the front light. You cannot hook them straight up to the hub as this will damage the light. If you get the cheaper one then you will need to use a battery powered light out back.

Yeah so am I - but my google fu is not strong today.

Cheers.

Ah, thanks - that's what I feared.

Hmm. Decisions.

On my touring bike my current approach is bright dynamo light up front and a battery powered rack mount cateye on the rear. This way I didn't need to faf with running rear cabling and I find the battery powered rear has enough life for what I need.

That's my learned thing for the day. I can go to bed now :)

See also: http://blog.dutchbikebits.com/2012/01/selecting-and-installing-dynamo.html

Q: Why can't you use just a dynamo rear light without a front light ?
A: The simple answer is that a rear dynamo light will always be damaged if used without a front light. The explanation of why is quite long and technical, but you'll find it below:

It is perfectly safe to use any of our dynamo front lights with our dynamos without also connecting a rear light. This will not damage the headlight. However, you must never use a dynamo to power a rear light without a headlight also being connected as this is quite likely to cause damage due to driving the rear light with too high a voltage. The reason why is as follows:

Sources of electrical power can approximate either a constant voltage or a constant current.

Constant voltage: A battery is an example of a constant voltage source, and it will attempt to supply the same voltage whatever load is attached. If you short-circuit a battery, then the battery will attempt to discharge at an infinite current and this is why lithium batteries, which are capable of very high currents, can cause fires.

Constant current: A bicycle dynamo is an example of a constant current source. Bicycle dynamos are rated at 6 V 3 W but what this really means is that they are current sources which attempt to deliver half an amp across whatever load is connected. The expected load of a rear and front light together is the equivalent of 12 ohms. A dynamo tries to supply sufficient voltage always to deliver the same current. With a short circuit, the dynamo will still deliver about 0.5 A at zero volts, but with an open circuit the dynamo will produce so high a voltage as it can in an attempt to deliver 0.5 amps.

You don't need to know any maths to safely connect dynamo lights. Simply pick a front and (optional) rear light, and wire them in parallel to a dynamo. However, if you want to know why you should never use a rear light on its own, read on:

The 12 ohm equivalent resistance for front and rear lights together comes from the front and rear lights being connected in parallel. The front light has an equivalent resistance of 15 ohms while the rear light is the equivalent of 60 ohms. To combine the two we use 1 / (1 / 15 + 1 / 60) = 12. Now that we know the equivalent resistance of each light we can calculate the current and power for each if used on their own.

For a front light alone, V=IR, V=0.5 * 15 = 7.5 V. P = IV, P = 0.5 * 7.5 = 3.75 W. This is a little over the rated value of 2.4 W, sufficient to damage older front lights with bulbs unless the bulb is changed to 3 W. However, modern LED based front lights can cope.

For a rear light, V=IR, V=0.5 * 60 = 30 V. P = IV, P = 0.5 * 30 = 15 W. This is a long way over the rated values of 0.6 W and 6 V and this is why dynamo rear lights do not last long if connected on their own to a dynamo. The front light is required to keep the voltage down to the correct level for the rear light.

I like the rear dynamo light on mine because it's fit and forget. During TCR I did also mount a seat-stay battery blinky for backup/safety.

Interesting, that is suggesting wiring in parallel (both lights connected to the hub) rather than in series (hub to front then front to rear). Would suggest any old AC rear could be used. Sounds wrong to me but they have lots of maths so must be right.

Yeah that's what's appealing, it's for my TCR too.

I think I'm going to go with the cheaper option for now, have a couple of rechargeable/battery lights on the rear for the next few months and then see how it's been working in the summer and I might upgrade then.

Cheers for the info by the way - didn't completely understand it other than I can't do it.

^ I reckon for the sake of killing a cheap light (rear dyno lights really are very cheap - unless you buy supernova etc), you should just connect a rear light up and see what happens... I'd err on the side of doing it in series. The internet (ie not here) may have your answer.

@fussballclub did your Luxos stop doing that thing where the top diodes just stay on forever? When mine did that I sent it back. Reckon it was still on when they unpacked it, as they replaced it without quibble.

So the reason I came here is this: has anyone worked out how to attach a front light to a quill clamp bolt? Would I just get away with an extra long bolt and a nut to space the light bracket?

That is funny yes. New one after just after just under two years.

Not necessary a lights question, but anyone have experience with the Kemo M172 dynamo powered usb charger?

Any tips for replacing a son dynamo hub bearings?

Mine feels really notchy but iirc they do feel like that as standard.

I'd check for sure if it wasn't the magnets that felt notchy and then bring it to someone who's certified to service it. Because magnets

Go Igaro for less shit to go wrong.

In other inspiring news. Killed my E3 Pure. Not fucking pleased at all. It still relays power to the rear, but nothing up front. My worries over moisture build up seem to have come true.

Mind if I ask some basic questions?

I have an uber-practical commuter bike which needs a new front hub. So I'm thinking of getting a dynamo and lights to get rid of the need to charge my USB lights.

If I get a cheap Shimano hub am I going to be disappointed? Also, what's a good option for city riding for lights? Lots seem like they'd have no side visibility, which is no good. Also, how do you route a cable to the rear? That seems like it'd always be a bodge.

If I get a cheap Shimano hub am I going to be disappointed?

Maybe, but it depends if you care how ugly your front hub is. I have the 'high end' shimano one, the 3N80 or something, which to me didn't look as boxy as the lower end ones and is apparently more efficient, and is almost certainly lighter. For me the extra 15 or 20£ seemed worth it for something that will last a long time, and I think if you search this thread (and the hub thread?) you will find most people are happy with their Shimano dynamos, no matter where they are in the range hierarchy.

I can only comment on the lights I have - the B&M Luxus and toplight taillight. But routing the rear light was pretty simple. Mine is just taped under the top tube, and mounted on the rear rack. My bike is black though, so the cable is discreet.

B+m lumotec with a toplight tail light to an alfine dynamo light is my commuter, never faltered over my 3 or so years of owning it, been left out in all weathers and never had problems with side visibility.

I have high end shimano hubs (3d80 and t780) on two bikes. The hubs work well and give plenty of juice, no issues with them at all. I have B&M lights front and back, they've all been good. You can run cables neatly if you want, mine runs in the bed of my GB mudguard to a light mounted on the mudguard. I've heard some people suggest did e-tube covers for routing dynamo wires.

I really rate those lights for sideways visibility. As with all b&m front lights they come in different brightness levels. Brightest one is good. Not descending mountains in the pitch dark good, but good. And, unlike some of the more expensive b&ms it has plug connectors for the rear light not a cable that exits the light directly.

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@sparky tl:dr Shimano are more than adequate for your 'practical' bike.

I think have been told (BC iirc) that the advantage of getting the spendy Shimanos is that the bearings are accessible by independent mechanics, so good for a long life with servicing, and a bit better for the planet.