When riding in the dark, rear lights serve a very different purpose than the front lights. They are not for lighting your way, but rather make you detectable from behind. They're also not reserved for the darkness, but are handy in doing that same job of making others aware of your whereabouts in broad daylight and especially in grim weather conditions.
In our last Collective Favourites roundup, we looked at the front lights, so now it's time to take a step back and look at our – and your – favourite rear lights. With the growing number of rear radars, which incorporate lights, this category blurs a bit and so we'll cover favourites of both styles.
Suvi Loponen
Brightness - Lumens and candela
Most bike lights, whether they are front, rear, or side lights, are marketed by their lumen number, which indicates the maximum brightness they can produce. Most rear lights are much dimmer in brightness than front lights, as their primary purpose is usually to make you seen, rather than make you see the path behind you. As such, lumen numbers from 10 up to 100 are common, though some extra-bright 400-lumen lights exist, as well. For comparison to how bright each is, car tail lights are commonly 100 lumens.
But much like with front lights, the lumens don't tell the whole story; they ignore how the light is distributed (which is determined by reflector geometry), and they don't give you any idea of the angle of highest visibility. If you thought you knew all the useful light measurements, I bet there is one that you've not paid much attention to. What seems purely as yet another confusing measurement system, candela (abbreviated cd) is actually a measurement that is used in connection to rear lights.
When we spoke about front lights, we looked at lumens and lux, which measure how much light falls on a surface at a given distance (illuminance), but because the rear light seldom is pointed out to the ground as the front light might be, candela measures the intensity of light in a specific direction (luminous intensity). So, candela is about how bright a light source is in a specific direction, and due to the history of the unit, a common candle emits light with roughly one cd of luminous intensity.
The truth is, though, that many brands don't list candela (cd) in their light spec sheets – even though, for example, the UK road regulations state that if you have a flashing rear light, it must emit at least four candela. One candela is equivalent to 12.57 lumens, which means that a four-candela light is roughly equivalent to 48-50 lumens. However, again, the legislations on this vary geographically, and the UK rules apply only to flashing lights, which means that if you run a steady light, it isn't bound by the four-candela rule. So really, you don't have to worry about regulatory concerns too much.
More useful to consider is the use case of your rear light. If you want to use it as a daytime light, something 100 lumens or more is recommended for good visibility, because in bright conditions a light has to be more powerful to stand out, whereas for simple commuting in dim or dark conditions, a lower lumen count is sufficient.
Flashing or no?
You should not always have your rear light in flashing mode. In daylight, a flashing rear light helps you stand out at a distance. But at night, depth perception improves with a steady element, so a sensible approach is steady or steady pulse after dark, and a dedicated "day flash" in daylight. Ultra-rapid strobes are never a great idea and are, in fact, banned in some countries (among other issues, they can trigger seizure episodes in people with photosensitive epilepsy).
It's also worth thinking about the people you ride with. In a group after dark, etiquette is to use a steady (or peloton) mode so you don’t blind anyone behind you.
Side visibility
Similar to front lights, rear lights differ quite a bit in their designs, and as such, also in how visible they are from the side or at an angle.
For best visibility when turning or crossing traffic, light reflected off axis is important. Designs that wrap LEDs around the body (even D-shaped lights), or feature side windows, are more effective at junctions than beam that only directs light straight back.
Mounting system and placement
You've chosen your lumens and your light style, and then it's time to mount it, and you find it doesn't fit anywhere on your bike.
The placement of your rear light matters in how well it's seen, but also how much road spray it gets – and should be something you consider when picking a light. Some lights have saddle-mount options, but most are primarily designed to be mounted on the seatpost or seat tube.
James Huang
However, if you don't have much seatpost available, and run a saddle bag and bigger tyres or a rear rack, the light might get blocked. Certain brands also offer mounts for seatstays to overcome this issue. Bear in mind that the height of a light has some impact on its visibility; lights mounted low are more easily obscured by other traffic.
You can also run your light on the back of the helmet, on your backpack, on the saddle bag, or attached to the rear rack. The complicated part of this is finding a mount that fits the desired location. Round seatposts are easy; most lights come with an attachment system that allows for straightforward mounting there, but if you run a D-shaped or aggressively tear-shaped aero post, you might look into a second, alternative mount option.
Not all lights have saddle rail mount options either, and some are not going to work attached to your helmet. Anything you attach to your helmet should be lightweight, too, so you don't want to put your rear radar there, even if there is a mount for that.
Battery life and durability
Micro-USB isn't quite gone from bike lights yet. Most new lights will feature the easier USB-C interface though.
If your light doesn't have enough battery to last through your rides, then it's pretty much useless, so worth looking at what the claimed battery life numbers are. Most lights go from a couple of hours to 24 hours of runtime, but these depend on what mode you use.
Modern lights utilise fast charging, which allows you to get the battery back full in an hour or two – older lights take a bit longer and are sometimes still charged via a mini-USB port instead of a USB-C, which means one more cable to keep track of.
The longevity of your light is also dependent on your riding conditions. If you ride your bike without mudguards, the rear light is bound to get a heap of water and dirt thrown at it on each ride, and such, it needs to be made durable and water-resistant.
Most rear lights on the market are rated with IPX6, which should mean "protection against powerful water jets from any direction," whereas one step up, the IPX7 gives protection against temporary immersion in water up to 1 meter for 30 minutes. The higher the rating, the better.
A note on IP (Ingress Protection) ratings: they are technically a two-digit system, with the first number relating to solid-particle (dust) ingress protection and the second to water; an "X" means a product wasn't tested for a particular mode of ingress. Many bike brands ignore the dust ingress rating and simply list lights for waterproofness. Given that many rear lights are exposed to significant amounts of grit, the IP waterproofness rating only tells you so much about their protection.
Smart features
For rear lights, the smart features now included in some modern lights are usually a brake light and automatic light detection. The latter means that the light might have a built-in ambient-light sensor to boost output in bright conditions, while a brake light function relies on accelerometers that add a temporary “brake” pulse. Some lights also have head-unit integration for battery and mode control, and charge-through so you can top up on the move.
Radars: the all-in-one solution
Radar systems (e.g., the Garmin Varia) add more awareness by pairing a rear light with radar for detecting vehicles as they approach and giving you an alert through your head unit, a phone app, or even the front light. Some units also triple up as POV cams, and are able to record your ride. However, the more features you have, the heavier and more battery-hungry the light is going to be.
It’s clear from the favourites in this article that many of us now choose to ride with rear radars – these are rear lights that can detect approaching vehicles. They combine two functions and, for road riding, add a layer of awareness by alerting you when cars are closing in from behind.
Suvi Loponen
But as with nearly anything, the more tech you cram into something, the more chances there are for faults. There are now a lot of different rear radar systems available, but very few of them work totally reliably.
It's also worth highlighting that a rear radar light is never so reliable that you should rely on it when, for example, changing lanes or turning across the road. Always do a check over your shoulder in these situations.
A note on regulations
Germany's StVZO standard only allows steady rear lights, unless they have a brake light feature, and also dictates that your light must not be brighter than 25 candles and it must be mounted at least 25 cm above ground. , and defines a few other qualities. The approved units carry a K-mark. The UK requires a red rear light after dark and allows flashing within set rates; many other regions permit flashing but specify minimum visibility distances.
Escape staff and member favourites
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