Brake Box FAQ

21 Mar 2020

What is this?

120dB bicycle horn.


I recently saw a car travelling a considerable distance with its left wheels bumping repeatedly along the raised barrier that was meant to protect the bike lane - before abruptly hooking straight across it.

Do you think it will actually make a difference?

If you already have a helmet, cat-like reflexes, healthy suspicion, and hydraulic disc brakes, this is the fifth item of safety equipment. It's a few extra percentage points.

Isn't it better to brake instead?

Obviously you'll be braking-and-evading at the same time. The box is triggered by a microswitch that's mounted next to the brake lever, so you can use your thumb to trigger the horn as you brake.

What about a bicycle bell?

Inaudible inside a vehicle.

What about yelling?

Tried it. Not loud enough (approx 80-90 dB). Also, drivers instinctively brake for horns, but don't listen out for voices.

What about hearing damage?

Having a two-tonne car smash your skull can severely damage your hearing.

(But seriously, it's effectively mounted right next to your face, so don't use this often, or for long).

When can I use this?

When there's 1) an imminent collision 2) on the road and 3) you have right of way.

What about for when I've evaded the collision but I'm really angry about what they just did?

No. It's not your job to teach people, and I don't want people getting too used to this, or I'll have to build a louder one.

Also, the more people (justifiably or not) hate cyclists, the harder this gets for all of us.

What about on shared cycle/pedestrian paths?

Oh god no. I don't even like using a bike bell on those.

(Also, it probably wouldn't work very well. The idea here is to be parsed as "imminent collision with speeding SUV" rather than "oh hello there's a bicycle" - but if you break people's mental models, they react slower, not faster).

What are the logistics?

Handlebar-mounted bag. Approximately 1kg, 20cm x 10cm x 10cm. Attaches with velcro, so you can take it with you to avoid theft. Plugs into the (glued-on) microswitch with a DC jack.

2200maH LiPo battery provides way more sounding time than you'll ever need (many minutes).


Denali Soundbomb 12V aftermarket motorcycle horn, 12V 30A relay (30A!), Turnigy Lithium-Polymer battery, microswitch, fuses, thick (30A!) wires, BTwin bike bag. Solder. Glue.

Why's it called the Brake Box?

If you can convince the driver to also apply their brakes, you've effectively doubled your braking force.

What next?

The bag blocks my normal bike light mount. For now I've added a mount to the top, but it would be better to integrate a bike light using the same battery. Perhaps 5W "normal" bulb and a separate 100W bulb (hey, we have the amperage available, so why not) that lights up only when the horn is sounding.

Perhaps an intermittent buzzer, like on a reversing forklift, with a toggle switch for those situations where it's not an emergency yet, but you have a bad feeling about this..

Perhaps a glued-on rear light, with cables running along the bike to serve it from the same battery.

Maybe an integrated dashcam?

Also there's room in the bag for a first-aid kit.