Bike lights, and why the lumen number is not what the law asks for

A bike light is one of the few things on this site that the law has an opinion about. That makes it a useful shelf to read, because the numbers the titles shout are not the numbers the regulations ask for, and the gap between the two is where the money goes.

What the law actually wants

Between sunset and sunrise, a bicycle on a UK road needs a white light at the front, a red light at the back, a red rear reflector and amber pedal reflectors. A light is allowed to flash, and if it does the regulations say how fast: between 60 and 240 flashes a minute, and at least four candela bright. The regulations also name a British Standard, BS 6102/3, that a lamp is meant to meet.

Now read the twenty titles on this page against that list. Eight front lights, seven rear lights, four sets: the ends are covered, and two of the seven rear lamps say red in the title while the rest leave the colour to the photograph. Not one title states a flash rate. Not one mentions a standard. Not one gives a brightness in candela, which is the unit the regulations use, rather than lumens, which is the unit the sellers use.

That is not unusual and it is not a reason to avoid this shelf. It is a reason to know that the number on the box was chosen to sell the light, and the number that matters was left off.

Lumens, and the one that gives the game away

Four titles state a lumen figure: 1,000, 1,200, a 5,000 with the word max in front of it, and 99,000.

A lumen is the total light a lamp puts out in every direction. For scale, a car’s dipped headlight is around 1,500 lumens. A good bicycle headlight for an unlit lane is somewhere between 400 and 800, and for a lit street where the job is being seen rather than seeing, 100 is enough. A phone torch is about 50.

So 99,000 lumens is a stadium floodlight, and it is on a title beside a 5,500mAh cell that could not run one for a minute. The number is not a measurement. It is a search term. Once you know that, the 5,000 “max” from a handful of LEDs on a single 18650 cell reads the same way, and the two honest-looking figures, 1,000 and 1,200, are the ones to take seriously.

What none of the four says is the shape of the beam. A round torch beam pointed down a road puts half its light into the eyes of whoever is coming the other way. A proper bicycle headlight has a cut-off, flat at the top like a car’s dipped beam, and no title here claims one. If you ride unlit roads, that is the feature to search for by name, and it is not on this shelf.

USB, twenty times, and never which one

Every one of these twenty titles says USB. None says USB-C or micro-USB.

That matters more than it sounds. The two sockets take different cables, and a light that arrives with the older one is a light you charge with a cable you have to keep specially, in a house where every other cable is USB-C now. If you own one charger for everything, ask the seller which socket the light has before you order. It is a one-line question and the answer decides whether the light gets charged.

Two titles give a cell size, 1,100mAh on a rear light and the 5,500 on the floodlight. None gives a runtime, which is the number that decides whether the light is still on when you get home. A rear light sips power and runs for hours on a small cell; a front light on high does not, and a title that will not say how long is a title you should assume means “less than your ride”.

One title says the light doubles as a power bank. A bicycle headlight that can charge your phone is a headlight that will be flat when you need it.

Waterproof, and IPX6

Ten titles say waterproof. Two say IPX6.

An IP rating is a test. IPX6 means the light survived a jet of water from a nozzle; IPX4 means it survived being splashed. Waterproof, on its own, means the seller typed it. A bicycle light lives on the outside of a bicycle in a country where it rains, and the difference between a rating and a word is the difference between a light that works in February and one that fogs up in November.

Buy the number.

Brake sensing, which is real

Three of the rear lights say brake sensing or smart brake. Inside is an accelerometer, the same part a phone uses to know which way up it is, and when the bike slows the light goes brighter or solid for a moment.

It works, it costs almost nothing to add, and it is the one thing on this shelf that a plain lamp cannot do. A driver behind you reads a brightening red light the way they read a car’s brake lights, without thinking about it. If you ride in traffic, it is worth having, and two of the three that offer it carry a maker’s name.

Modes, and the button in the dark

One rear light here lists twelve modes, one set lists six, and the rest say nothing. Every mode is a press of one button, and to get from the mode you are in to the mode you want you press through all the others, in the dark, in gloves, at a junction.

Two modes are enough: steady and flashing. Six is a menu. Twelve is a puzzle, and the light will spend its life on whichever mode it was in when you gave up.

If you use a flashing mode, remember the regulations: between one and four flashes a second. No title states its rate, so the honest position is that you do not know, and a steady light is the one that certainly complies.

The helmet light

Two titles are for a helmet. A rear light on a helmet is the highest red light on the road, and it is the one a driver sees over the roof of the car in front. It is also the one that points wherever you look, which is worth knowing when you turn your head to check over your shoulder and your rear light is suddenly aimed at the pavement.

Run one on the bike and one on the helmet if you ride in traffic at night. That is what the sets on this page are for, and it is why the helmet light here that also brake-senses is the pick.

The mount, which is the thing that breaks

No title here says what the light clamps to or how. A rubber strap that stretches over a handlebar fits everything and slips on everything; a bracket fits one bar diameter and stays put. The one exception is the light sold to hang under a Garmin-pattern computer mount, which is the cleanest answer on the page if you already run one: the bar stays clear and the light sits low and central where a beam belongs. The CYCLAMI mount already on this shelf is that mount, and it has sat here unpicked since the catalogue was built because nothing to hang from it had been imported.

What I would actually do

For most riders, the set with a stated 1,200 lumens up front and a rear lamp in the same box. It is the only title here that gives a believable brightness and covers both ends of the legal minimum at once. Ask which USB socket it takes before you order.

For the back of the bike, the ROCKBROS. A maker’s name, brake sensing, and a rating rather than a word for the rain.

For a helmet, the SHANREN RAZ Pro. Same brake sensing, same maker’s name, sold for the highest point on the rider.

And if you already run a Garmin-pattern mount, the IPX6 front light that hangs beneath it. It is the neatest installation on the page and it states its rain rating, which is more than most manage.

Whichever you choose, take the lumen figure as a mood rather than a measurement, and check which cable it wants. The law asks for white at the front, red at the back and a flash you can count. Nothing on this page states the last of those, so buy the steady mode and use it.