Velo Lumino’s connector of choice has been the 2mm, gold-plated banana plug. Over ten years of experimenting with various connectors from classic spade-type terminals to MOLEX-type terminals, I settled on the 2mm banana plug as my all-time favorite connector for several reasons:
- They’re slender and unobtrusive looking anywhere on the bike
- The design of the “banana” shaped spring on the male end maximizes the contact area with the interior wall of the female barrel end for very reliable electrical continuity. The contact area is much greater than the contact area between a pair of spade terminals.
- The tension of the spring is “just right”: there’s enough friction that the plugs never come loose on their own, yet they’re never difficult to disconnect and there’s little risk of ripping the wire out if the connector is pulled by the wire. There have been many a cyclist who’ve forgotten to remove the spade connectors on a SON hub before pulling the wheel out of the dropouts, only to wind up with torn wires!
- Unlike spade terminals, they can withstand hundreds of cycles of connection/disconnection without diminished tension or compromised electrical contact (see this video my videographer shot demonstrating this).
- The gold plating means they’re corrosion-proof.
The only downside to the banana plugs is that they cannot be crimped, they must be soldered. Some people don’t own a soldering iron, and even for those who do, soldering these little connectors can still be tricky. My 13 year old videographer shot a video of me demonstrating how to solder a pair of banana connectors and insulate them with shrink tubing. It’s fairly long at almost 12 minutes, and since I’m the type who can ramble on, I had to refrain from a few details in order to keep it under and hour! I will go over some points in finer detail here, using still shots from the video.
Before you get started, you will need a soldering iron. I use a Weller 40-watt iron that comes supplied with a stand and a sponge. You need a damp sponge to wipe the molten solder off the tip frequently, because the longer the molten solder stays on the tip, the more it oxidizes, and oxidized solder won’t wick onto its substrate. For a soldering tip, I prefer a flat screwdriver shape, such as the one that comes with the Weller. You will also need either a heat gun or a high-wattage hair dryer to apply the shrink tubing. You will also need a soldering stand (or jeweler’s stand) to hold the parts to be soldered in place. You can find these all over the internet:
First, strip just a couple of millimeters of insulation from the wire. Hopefully you’re using Velo Lumino’s premium wire with the super thin cross-linked polymer jacketing. It’s the highest quality wire I’ve found, and it snakes very well through tight passages such as in the fender’s rolled edge and through frame tubes. Twist the strands several times to create a tight bundle and prevent stray wires from splaying outward during soldering.
Clip the wire into the alligator clip and wick some solder into the exposed wire. This is called “tinning” the wire. Wires with pre-tinned ends fuse more quickly with the pre-tinned banana plugs when re-melted. You only want to wick a tiny amount, don’t go overboard. I’ll explain why later. The best way to do this is to touch the iron tip to the wire to pre-heat it (just a second or two), and while doing so, touch the solder to it. It should melt and wick almost instantly.
As soon as it’s molten and wicked, remove the iron.
Next, tin the banana plug. I’ll start with the female end. When you look closely at the female banana plug, you’ll notice that one end has a stop about 2mm into the barrel. That’s the “cup” end and it’s the end that accepts the solder. You want to fill the cup with solder about 1/2 to all the way full. This is a little trickier than tinning bare wire. If you use the same approach as tinning a bare wire, you will end up with an air bubble trapped in the cup and a little ball of solder on top, or you’ll end up with solder wicking down the outside of the barrel.
Instead, insert the cold solder into the cup FIRST, THEN apply heat to the outside of the cup until the solder starts to melt and fill the cup:
I love this shot because it illustrates a few things. First, note the flat end of the iron tip contacting the side of the barrel. This maximizes heat transfer, which would be less efficient if not impossible with a pencil-tip. Second, note the barrel isn’t placed all the way into the alligator clip. You want at least half of it sticking out the end to minimize heat transfer to the clip. If the clip acts as a heat sink (this is a phenomenon well known by frame builders), you might not conduct enough heat to melt the solder. Lastly, you can see the solder already beginning to melt and fill the cup. Now’s a good time to pull out the cold solder and remove the heat. You only need the cup partially filled, but all the way is okay, too.
Next, get the tinned wire ready, and reapply heat to the solder cup.
As soon as it’s molten again, press the wire into the cup. As soon as it’s in, remove the heat and hold the wire steady until the solder solidifies. This should take just a few seconds. (Note the photo below is not of the same female plug shown above, but the male plug, but the process is identical):
The soldering is done. Now apply the shrink tubing. The 6″ long shrink tubing sold by Velo Lumino is the optimal diameter for the 2mm plugs and is enough for four pair of banana plugs. Cut a 2cm piece for the female end, and a 1cm piece for the male end. This leaves enough shrink tubing to extend beyond the solder cup and a few additional millimeters along the wire. Getting back to my comment earlier about not applying too much solder when tinning the wire: soldered wire is brittle and will snap if bent back and forth. The farther the solder wicks into the wire (a function of time and amount of solder applied), the longer a section of wire will be brittle. This is why you only apply a tiny amount when tinning, and it’s also why you remove the heat as soon as the tinned wire is inserted into the cup of molten solder. Having the shrunken tubing continue over the wire past the solder junction is essential for providing rigidity to the soldered area to prevent bending.
For the female plug, hold the 2cm long tubing in place with one hand such that it overhangs just slightly (0.5mm, max) off the end:
While holding it steady, shoot it with the heat gun (or hair dryer), aiming for the end opposite your fingers first (for obvious reason). As soon as the far end shrinks, you can let go and move your fingers further along the wire away from the connector so as not to get burned. (If you don’t hold it in place initially, and just hit it with heat, the blast of air from the heat gun will blow the tubing away from where it should be). Hit the rest of the tubing with the heat gun, rotating the wire to distribute the heat evenly so as to avoid hot spots that might melt it (although, my 1500W heat gun has never melted the shrink tubing). Think roasting marshmallows over a fire.
For the male end, position the 1cm piece of tubing just aft of the barrel spring:
Hit it the same way with the heat gun, being careful not to burn your fingers!
All done! (Apologies for the blurry video still).
You can see plenty more examples of soldered and insulated banana plugs in the Velo Lumino gallery.