Introduction: Bike Party Sound System - Easy Rear Rack Style

By MonkeyLectric
About: Dan Goldwater is a co-founder of Instructables. Currently he operates MonkeyLectric where he develops revolutionary bike lighting products.

Spring is coming, and there's nothing more fun than a group ride through town with 10, 100 or 1,000 of your closest friends.  I've been on a lot of group rides and if there is 1 thing that really ads to the event its COSTUMES.  and if there is a 2nd thing, its MUSIC.  having a good sound system really ads to any bike event - both for riders and spectators.  I have several friends that consistently bring the music to local events, but its hard going.  Most of them have a trailer with a huge DJ speaker, weighty amp, maybe an inverter, and a car battery to power it.  Here in San Francisco that means a squat team has to help push them up the hills to keep up with the ride!  Today's project uses high efficiency components to make a much more convenient 25 pound (10kg) setup that mounts on any rear rack and gives the 150 pound (60kg) sound trailers a run for their money.

This setup is easy to build and if you buy all new parts will cost about $150.  The main cost is the speaker and you may be able to re-use one you have around.

If you need a more complete reference on off-grid party sound systems, check out my other article.

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This article is sponsored by Momentum magazine and MonkeyLectric.  The article appears in Momentum Issue 45.  Here on Instructables i've posted an expanded build section that has tons more inspirational photos annotated with building tips.




Step 1: What You Need

The basic stuff:
  • rear bike rack
  • mp3 player with belt clip/holder
  • small wood screws
  • mount from an old bike light
  • mp3 player cable and speaker cable
  • tupperware box
  • 4 bolts around 3" x 5/16"  (80 x 8mm) with 8 washers and 4 nuts
  • about 2' (70cm) of wood around 1" x 5/8" section (25 x 15mm).
  • if you want 2 speakers instead of 1: a piece of plywood to mount them on
The key components:
  • Amplifier
  • Speaker (1 or 2)
The key to this project is efficiency.  Home and car sound gear is not designed to be efficient or light - it just isn't a concern in those applications.  Low efficiency means you need lots more power to make the same amount of sound, and lots of power means a big heavy amp and a big heavy car battery.  An efficient speaker can produce 5x the sound volume from the same amount of power.  An efficient amplifier is 2x better than a standard one, plus its far smaller and lighter.

Efficient speakers:

PA and DJ speakers are the best place to look for efficiency.  Auto and home speakers are usually poor.  Large speakers are nearly always more efficient than small ones - this is one of our real tradeoffs because large speakers also weigh more.  

The smallest speaker I recommend is the Gemini RS-308, you can get that at Amazon and other places for $70.  It fits into a milk crate and weighs 15 lbs.

For the current project I used a somewhat larger Behringer S1020, I got it from parts-express.com for $100, it weighs about 20 pounds.   American Musical has a lot of speaker choices too.

Go MONO!  Stereo doesn't make a lot of sense when you are riding around on a bike.  Using one larger speaker you get will more sound and less weight than two small ones, and its less work to mount it.

How big a speaker can you fit on a bike rack?  The speaker in my project photos has a 10" woofer, most models with a 10" woofer are about this size.  The Gemini 8" speaker is quite a bit smaller, it fits into a milk crate that you might already have on your rack.

Hacker Tip:  Occasionally I've seen hackers try to use a bare speaker cone with no box to save weight.  Do not do this!  The box of a speaker is critical to its ability to create sounds. When you get rid of the box you lose most of your sound output.

Efficient amplifiers:

For amplifiers there is really only one choice: a Tripath-based amp, sometimes called a T-amp, Class-T amp or Digital amp.  These ultra-efficient wonders are inexpensive but obscure.  T-amps are available in a couple of different sizes.  parts-express.com sells the 10 watt DTA-1 for $45 and it includes a built-in battery holder.  It runs at full power for 4-6 hours on 8 rechargeable AA's - wow!

On Ebay.com there are several vendors with higher power T-amps.  For a 20 watt amp I recommend the TA2020 amp from ebay seller indeed-hi-fi-lab.  It's only $20 and is very good quality with durable construction.  indeed-hi-fi-lab seems to have 2 variations called the "Tripath TA2020 Mini Cute Class T Amp Amplifier Ipod MP3" and the "NEW Class T Amp TA2020 Amplifier Tripath Chip TA 2020".  Rest assured the one I bought is the "mini cute" type, but the other one looks equally well made in their photos.

The larger T-amps don't include a battery holder, you will need to supply an appropriate 12V battery.  12V "SLA" type batteries are very inexpensive, and also easy to find at recycle yards.  For a new one try batterymart.com.  A 7.0 Ah capacity should last you all day and costs $20.  They also have a nice charger for $15.  Amazon has these batteries too.

Much more details about speakers and amps I have tested if you need it.


Step 2: Align the Speaker and the Rack

before doing any drilling and cutting:

match up your bike rack to the bottom of the speaker and mark where the 4 bolt holes will go.  check that your mounting blocks won't hit the struts of the rack.

** if you want to use 2 speakers **
you won't be able to bolt them straight to the rack like this.  but don't worry!  cut a rectangular piece of plywood and bolt both your speakers to that.  then attach the plywood to your rack using the method shown.


Step 3: Mounting Bolts

we need to get 4 bolts into the bottom of the speaker to hold it on the rack.

unscrew the grill & woofer from the speaker to get access to the inside.  most speakers the grill is just screwed on, a few it is press-fit and you just need to pull hard.

drill the holes and put the bolts in so they stick out the bottom of the speaker.  Put some glue into the holes to keep the bolts in place and waterproof it.  make sure to use big washers on the bolts so they don't tear through the speaker box.  these mounting bolts will take a lot of force when you are riding on a bumpy road.

with a wood speaker box the bolting is very easy.  if you have a plastic speaker, some parts of the box are stronger than others - avoid bolting a thin spot.  most likely you want to drill through any existing mounting feet because those are already designed to take the weight of the speaker.

Step 4: Mounting Blocks

cut 2 lengths of wood about 9" (230mm) long.  mark and drill 2 holes in each to match the bolts in the speaker.  these wood blocks clamp the speaker firmly onto the rack.  if you have any trouble with vibration you can wrap your rack with an innertube.

If your speaker case or wood blocks are fairly thick, it will probably be easiest to drill through both of them together.  that way the bolt holes are sure to line up correctly.


Step 5: Waterproofing the Speaker

The fabric box of most PA speakers is waterproof.  If you are using a home speaker then spray paint it.  The woofer cone on many speakers is made from thick paper, you can waterproof it by applying a thin coating of water-based urethane wood finish such as Varthane, available at home improvement stores.

Step 6: Waterproof Amp Box

waterproof amp holder:  screw the tupperware bottom onto the top of the speaker.  drill 2 or 3 holes in one side of it for wires.  cut a piece of foam to fit in it and put in the amplifier.

Step 7: MP3 Handlebar Mount

The easiest thing will be to just throw your mp3 player into the plastic box with the amp.  If you want it on your handlebar though, it isn't too hard:

Separate the mounting from your old bike light.  glue or screw your standard mp3 player belt clip onto the bike light mount.  Now you have an mp3 player bike mount!  put it on your handlebar or top tube.  route the stereo cable from the holder to the amp

- connect one channel of the amp to the speaker, add batteries and prepare to rock out!