Recently I have started to use 4 cameras on my bicycle. I am using a variation of my rig described in http://forum.mapillary.com/t/my-4-camera-rig/289
I had tried mounting all cameras on the handle bar, but had big issues keeping the images clear of obstacles. So I tried mounting 3 on the back and 1 in the usual place on the handle bar.
The rig is mounted using a Manfrotto Super Clamp. It is fairly cheap and very heavy duty. You might see two twigs. They are there to keep the cameras level. I have since moved to a piece of metal. I still have the issue that the rig turns around the spigot that goes into the super clamp. Currently I use one of the rubber bands I usually use to secure luggage to the bike. I might get another super clamp so it is perfectly secure.
I have had to tighten all screws related to the luggage carrier, on which it is mounted. It vibrated too much before.
An issue that does not go away so easily sounds quite simple: Remember to turn on all cameras all the time. I always look an extra time, but some times I miss one. When they are located in the back it takes a lot longer time to discover.
All my scripts and tool are put in GitHub - simonmikkelsen/mapillary_scripts: Shell scripts that helps with a multi action cam workflow for mapillary.com . They still contains some undocumented hacks for my cameras but should be fine besides that.
I used the hand held 4 camera rig so I got 8 images with 45 degrees between them (I walked the same way 2 times). For bicycle it was much easier getting both a front and back shot. An example of the result could be Mapillary (the far majority of images in the town of Tranbjerg are shot in this way. I think Mapillarys navigation handles this better than the 8 image version, but there is still a lot of work until it is perfect. I trust the Mapillary team to perfecting it while I try to push the limits of their system