Step 1: The Bike in Question.
The back rack.
Step 2: The Parts.
Altec Lansing BXR1120 2.5 music and gaming speaker. It was originally powered by a 9 volt ac wall wart. I first tried powering it with a 5 volt dc USB Battery pack but the sound was noticeably distorted over the wall wart especially as the volume was raised. I then went to RadioShack and bought a 8 cell aa battery holder for $3. This gave me a little over 10 volts dc with rechargeable cells at 1.2 volts each. This seemed to match the sound of the original setup.
I also wanted to have bluetooth wireless capability I found a cheap unit on Amazon that looked like what I was after. The Enerpak Tube 5000 mAh external battery pack was from living social but these things are everywhere now days. I actually have a smaller battery pack that I was planning on using if it last long enough for a bike ride.
I had found a good deal on some saddle bags over Christmas not realizing they would stick up above the rack surface. This seemed to work to my advantage though as these speakers seemed to nestle in between them.
I decided to try and mount the speakers to some foam board I had left from another project. I cut out a piece to fit just inside of the area around the top of the rack. I then had to notch out for the saddle bag clips it felt a little flimsy with one layer so I cut another piece and glued them together with some Liquid Nails I had on hand. That seemed to really stiffen it up. The speakers have a plastic bottom so I tried using some drywall type screws through the foam board bottom into the speakers. One nice thing about these speakers is that the grills will come off along with the bottom cover so I could see where the ends of the screws were going to be. I had to be careful with the speaker that has the power wire as it has a circuit board close to the bottom. The other speaker just had a wire in the back. I drilled pilot holes for 3 screws and screwed them in until they were flush with the foam board. The bike rack had 2 holes about 1 inch from the front edge that I used to help hold it in position with 2 screws through the foam board. Next I had to mount the battery pack somewhere. I had a shampoo bottle that was just empty so I washed it out and cut the bottom off and much to my surprise it was almost a perfect fit. I drilled 2 holes in it so to avoid the battery pack metal rivets from coming into contact with them in case it might possibly cause a short circuit some where. I used 2 screws with nylon lock nuts to hold it in place.
So far I have only had the chance to try it in the living room due to the weather. Hopefully it will perform on the trail. I will try and leave an update when that happens. I hope this gives you some ideas and the motivation to do your project.
P.S. I have a new found appreciation for those who post projects here. I think I spent more time creating this post than doing the actual project.
Update: I have used it probably 10 times now and every thing still works. It does take a beating some times though. I had trouble with the battery pack falling out at first also the battery holder would some times allow the batteries to come partially out loosing contact thereby stopping the power to the speakers.
I added a heavy rubber band around the batteries to hold them in the battery holder tighter and it also kept it from slipping in the shampoo bottle that holds all the batteries.
I also hook a womens hair type band from the bottom and across the top of the battery pack to help hold it in because a big bump can still pop it out.
It also seems to get peoples attention and some give positive comments and so far no one has complained although I would imagine that could depend on what you are playing.