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It is a thing that I noticed even on cars it is missing: when rear lights fail, the driver does not realize it.

In bicycles it is much more worse it happens, as the cyclist's own life is more exposed.

I would like to know if someone knows about the existence of REAR lights for bicycles that give some feedback about their own failure, without the needing of the cyclist to lift up from the bicycle.

My thanks in advance.

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  • Reflectors work well and are quite fail safe.
    – ojs
    Commented Oct 1, 2016 at 11:43
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    Or simply have two lights -- one "main" one and one small blinkie. (And, any more, light failure should be quite rare.)
    – Daniel R Hicks
    Commented Oct 1, 2016 at 12:08
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    Look down briefly and if you don't see a red glow, your tail light is off?
    – Batman
    Commented Oct 1, 2016 at 13:44
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    @Batman's suggestion doesn't always work: on a dry road with streetlights and cars you're unlikely to see a reflection from a well aligned light, and if the light is well aligned and efficiently designed the setting be much light reaching you directly
    – Chris H
    Commented Oct 1, 2016 at 14:22
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    Multiple redundant lights is all you need. In the depths of winter I will have 5 rear lights (helmet, backpack, two on the seat post and one on the seatstay. None are particularly expensive. I could loose four and still be lit.
    – Criggie ♦
    Commented Oct 2, 2016 at 5:25

6 Answers 6

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The problem is that you have to start considering the failure modes of the failure detection system. You're quite likely to end up with a less robust system or a lot of false alarms if you have a system designed to warn you of failure.

You also have to consider what faults would be detected:

  • a dead battery? You'd need another power source to feed your warning
  • a briefly open contact causing the light to switch off (I've had a proper brand light do this every pothole on a road bike)? Can this be distinguished from a deliberate switching off?
  • accidental switching (e.g. bumped by a pannier)? How is that different from switching it off?
  • unlikely failure modes like LED burnout? These shouldn't happen in a well-designed system, and I wouldn't trust failure detection in any other system.
  • the light falling off? Quite a likely failure mode but an extra fixing would be more reliable and less effort - and your failure warning service could fall off too.
  • Damage (which could be things like a failed connection or loss of focussing optics) due to vibration/fatigue/impact. My helmet-mounted rear light has lost an LED to this, and another flickers, so it happens. With series-connected LEDs (common on e-bike rear lights) or a failure in a common electrical path, you could easily lose the whole output.

So instead we consider a system designed to confirm that the light is on. If the confirmation fails you stop and check. This could be a design or mounting that deliberately directs a little of the light to where you can see it. Simply mounting your rear light on your seat tube and having a silver-coloured pannier rack is an easy solution to this. A light with some front-facing white/yellow lights that shine between your legs would cost battery power but might increase visibility.

There are big advantages to multiple independent lights:

  • you're more visible from more angles (and further away if you add lights up high)
  • approaching vehicles can get an idea of how fast they're gaining on you (this really needs multiple constant lights to work well)
  • they indicate your size better - important if you're wider than a driver might expect (trailer for example)
  • and of course the big one here: if one dies you've still got some minimum level of lighting at the back.

I take this approach with two steady rear lights and a flashing one on my helmet (also with yellow LEDs on the sides). Sometimes I have a third rear light as I have one on the top of my daughter's seat and one on my spare pannier.

Another consideration is what you'd do if you detected a failure and it wasn't easy to fix at the side of the road, an hour from home, in complete darkness. With a backup light or two, just ride on. Of course this does mean you should check your lights at home - this is just a matter of looking at them as you switch them off and not doing it by feel.

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  • Well, there is many products that make your bike frame looks like a moving Christmas lighting display. ;-)
    – mootmoot
    Commented Oct 13, 2016 at 14:16
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    I have once try the cheapo, cable tied an IKEA drawer LED light bar(2 AAA NiMH) on the down tube and make visual inspection from 100 meters, well, it works, except the battery doesn't last more than 4 hours.
    – mootmoot
    Commented Oct 13, 2016 at 15:26
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    One more failure mode -- having the lens cover fall off the light when it has the focusing lenses. The light is still "on", but is less effective. Happened once to a riding partner - I told him that his battery was nearly dead because I could barely see his light, turned out that the lens cover had fallen off, and the tiny unfocused surface mount LED's were very hard to see without the focusing lens.
    – Johnny
    Commented Oct 13, 2016 at 17:40
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    Don't forget the failure mode of failing to notice that the failure indicator is active :)
    – Móż
    Commented Oct 13, 2016 at 23:14
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    @Johnny I've added that in with the electrical failures due to damage etc as the cause and effect are both quite similar even if the mechanism is different.
    – Chris H
    Commented Oct 14, 2016 at 8:15
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Most of the Busch & Müller IQ Dynamo lights seem to have this feature. Their lights have a well designed beam pattern, si it's hard to tell whether they're on or off during the daytime. So they have a pair of indicator LEDs on the back of the light, one for the front light, and one for the rear light. On my light they're orange and green, so it's easy to tell the difference.

B&M have changed their product range since I bought my light, and they all look different now (this is a common "problem" when products last a long time). What I have is (I think) an IQ Fly from about 2010, and it looks more like the current "Avy" light than any of the others. If you look closely there's a transluscent bulge on the back of the light, where the LEDs are:

enter image description here enter image description here

They don't mention the feature in the manual for that light, but my light definitely has it. And some of their new lights have a lit up ring around the button on the back, so it's likely to be present.

The other solution is more common: have multiple rear lights. Many people use two (I use a battery powered flashing light as well as the constant, dyno light)

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    This seems to concentrate on the front light -- how does it apply to rear lights?
    – Chris H
    Commented Oct 10, 2016 at 10:04
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    Mine has two indicator lights on the front light, for both. Generally dynamo lights run power from the dynamo to the front light, and from there to the rear light, so having both indicators on the front light works. It also makes sense, as the rear light is harder to see while riding :)
    – Nuі
    Commented Oct 12, 2016 at 19:39
  • Fair enough - for those of us who've never/rarely used dynamo lights that makes a lot more sense than your original answer. That would seem to only indicate whether the rear light is turned on, not whether it's working (a wired light would at least still be attached to the bike if the mounting bolts failed, but a failure of the light -- see edit to my answer wouldn't be detected).
    – Chris H
    Commented Oct 13, 2016 at 8:17
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Use also helmet with rear light. Assuming that you charge and maintain both helmet and "official" rear light, at least one of them likely should stay working, and they may supplement each other.

You can also simply use two rear lights.

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There is an expensive but modern approach to this problem:

Smart lights that pair with GPS head units - connected devices usually report the battery status to the computer and you get a warning when any kind of paired device (such as a sensor or light) is low on battery - which is probably the most common failure mode.

Any other kind of failure such as losing the light or a severe hardware defect will also give some kind of feedback on the computer (visually and audibly) because the ANT+ connection is lost and most head units will give some kind of notification about such events.

Bicycles SE is not for product recommendation, but I will give the Garmin Varia series as an example. Of course, ~500$ worth of hardware are not a practical solution on a reasonably priced commuter bike but I wanted to give the other end of the spectrum as we already had cheap but redundant combinations of LED lights or just looking down (on my road bike, I get nice reflections on the rear seatstays so that I can see it from my regular riding position, though...).

The Bontrager Flare RT seems to be a cheaper light with BT/ANT+ connection if you don't need the Varia's radar features.

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Sensor just too complicated and not cost effective. Rear view mirror is sufficient to let you notice the failure at night, human eye is sufficient to detect the red light patch on the road at night. If you can't see the red patch, it is either fail or the light angle are wrong : you are not suppose to beam the light to the driver level.

As a backup ,
- attach a battery operated LED light on the seat tube, facing left or right.

There is one hacks beside the mirror : lay a cheapo optical fibre cable at one point pointing right on the rear LED light, another point tie on your stem. So you will see the red light in front of your stem end when your turn on the LED. If anyone insist some home product DIY fiber optics, checkout this youtube video use a transluscent silicon cable as fiber optics cable

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  • How is a light facing to the side a backup for a rear light? Why would you want red lights facing the side, anyway?
    – David Richerby
    Commented Oct 13, 2016 at 10:29
  • @David Richerby : You don't want your backup fail without knowing it. Rear LED light attach to the seat tube facing rear is difficult to spot with rear mirror when it failed.
    – mootmoot
    Commented Oct 13, 2016 at 12:08
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    OK, great, I can see if my sideways-pointing light has failed. But I don't care if it's failed or not. It's pointing sideways so it's a really bad rear light.
    – David Richerby
    Commented Oct 13, 2016 at 14:03
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    (@DavidRicherby) a small seat-stay/pannier upright mounted light like a knog frog (one on each side if you like) shining mainly to the rear but showing a little light to the side is quite an effective backup to a bigger rear light. It's also checkable from a glance behind (between your legs).
    – Chris H
    Commented Oct 13, 2016 at 14:43
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    @ChrisH Yes, I much prefer your idea of a backward-facing light that's slightly visible to the side, compared to the answer's idea of a sideways-facing light that's slightly visible to the back.
    – David Richerby
    Commented Oct 13, 2016 at 15:05
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The previous suggestions have all been good but I'd like to mention that I've had a decent amount of use out of one of these http://www.wiggle.co.uk/lezyne-macro-drive-duo/

You can strap it to the top of your helmet and the light points where you are looking which is good for forward vision but it has a red light on the back which flashes. It's foolproof to know whether it's on or not.

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  • Could you clarify? It's foolproof because you can easily see that the front light's on and they're always either both on or both off? (LED failure is extremely rare, so that would work.)
    – David Richerby
    Commented Oct 14, 2016 at 8:34
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    I'm not a fan of helmet-mounted lights. The front light dazzles people when you look at them, they cease to be visible when you turn your head to the side, they display the lights the wrong way round when you turn your head to look behind you, and they're much higher than regular lights, which makes distance judgement very difficult.
    – David Richerby
    Commented Oct 14, 2016 at 8:36
  • Yes it's simply a case of seeing and using the front light would cause one to assume the rear light is on and working. I've never had an led burn out but when I'm using lights it's generally pretty cool outside which helps. I agree that the light on a helmet could dazzle which is why I keep mine angled reasonably downwards.
    – Chris
    Commented Oct 15, 2016 at 22:40

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