Adding dual disc brakes to the front of an SOHC is a great way to improve performance and safety on the bike. Some SOHC's such as the 550 and 750 feature forks which are both drilled for brakes, even though only one brake is fitted. Adding a second brake to the empty side is an easy and confidence inspiring exercise.

Parts Needed:

You will need to obtain an entirely new caliper, caliper arm, rotor, and brake hose from the same bike as you are installing it on. The parts and spacings differ from the 550 to the 750, so stick with duplicating what you've got. This may also work on the 450/500 twin, though I forgot what the forks looked like...You will also need six bolts that are about 1/2" longer than the stock bolts that are holding the single rotor on the wheel hub. As well, you may need other small parts outlined here, depending on how sophisticated a job you want to do.

Fitting the Second Rotor to the Wheel:

The most difficult part of the process comes only if you want to retain the use of a speedometer. If so, the second rotor must be slightly machined to be able to incorporate the original speedometer drive unit. The speedometer drive unit consists of the Speedometer Gearbox, on which the speedo cable is mounted, and the Gearbox Retainer, which mounts to the wheel hub, and the Gearbox Retainer Cover, which is the chrome steel ring that holds the retainer onto the hub. The inside of the rotor must be machined so that the Gearbox Retainer will be flushly mounted in it. The rotor will take the place of the Gearbox Retainer Cover, and this part will not be used in the additional brake. So, once all is said and done, the Gearbox Retainer will be put inside the new rotor you machined, and the rotor then bolted to the wheel, and the Gearbox Retainer Cover put into your junk bin, or Harley-Davidson.  The Speedometer Gearbox will then be slid onto the axle shaft and fitted to the wheel assembly, as usual.

The machining required can be seen in the picture. The Gearbox Retainer is slightly larger than the center of the rotor.  So, material needs to be removed from the inside (hub side) of the rotor such that the Gearbox Retainer will fit into it flushly.  This will be a cut that is a circular shoulder, with the two corresponding tabs on opposite sides cut as well.  

I used an "End Mill" bit on my drill press. The End Mill bit looks like a barrel, with a flat bottom, with teeth all over it, including the bottom. I think the diameter was about 1/4 to 3/8 inch. The hardware store also had a bit that had a curved bottom, but this won't suit the purpose. I cut small depths with each pass of the tool, and ended up cutting as much as 1/8-3/16 inch in order to get the Gearbox Retainer to fit exactly flush with the inside surface of the rotor. Probably don't want it loose, either. I just put the rotor on the press table, and brought the tool down so it cut into the backside of the rotor about 1mm of depth, then worked the rotor around to create the right shape.

Final Hookups:

You will need to attach the second hose to the brake system. There are a few ways to do this. You can go the cheap route, and put a brake hose and banjo bolt where the brake indicator switch is mounted on the brake line junction box at the bottom triple clamp. Then, you lose your front braking signal. Or, you can get an extra long banjo bolt, and double two lines together using three brass washers. Or, you can get a different junction box, from a dual disc brake bike, such as the 77-78 Goldwing, or the 77-78 CB750F.

Master Cylinder:

There is some debate around the proper master cylinder to use. Arguably, adding a second caliper to the system would necessitate a Master cylinder with a larger capacity. The more capacity the master has, the less the lever will move until it canÕt be squeezed anymore. The stock Honda master has tremendous capacity, which is why a properly bled system leads to a rock hard lever feel, with almost no travel on the lever. A master that is working well, recently rebuilt, should push enough fluid to safely push both brakes.

A dual disc brake master is also an option. I found a master from one of the above mentioned bikes, they look identical, though have more capacity. I have the rock hard feel back, which I donÕt particularly like. In addition, the dual disc master from these bikes has a built in brake light switch, so the original brake line junction box fits the bill perfectly.

A racer may choose to use the smaller master cylinder, as these give more leverage, therefore more braking with less squeeze. This may be at the expense of more maintenance and safety risk though. Brake fluid can become contaminated or heated so that its properties change. If you are running a master that has just barely enough capacity at standard conditions, then you may be in trouble on a hot day, or if your fluid gets contaminated, or the seals inside the master start to deteriorateÉ.your lever will go straight to the bar without the solid stopping you had come to depend on.

Special Notes:

In many cases, the second caliper doesn't mount on quite straight. It may be skewed in relation to the rotor running through it. This is a problem because if the pads strike the rotor at an extreme angle when the brakes are applied, they will twist and jam inside the caliper, and cause the brakes to bind. If this happens, then bleed off some of the fluid in that caliper, or push the caliper against the rotor when the bleed valve is open, this will loosen the brake quite a bit. Watchout! Brake fluid may squirt out. This stuff eats paint!!

I have a couple ways to deal with this problem. What I do is shape the pads on that caliper so that they strike the rotor squarely before I take it for the test ride. I have done this with a sheet of coarse sand paper on a flat cement floor, and more recently on an upright bench belt sander. I install each pad into its caliper half, hold one up to the arm squarely, and swing the arm so that the pad touches the caliper. You have to make sure that you are actually holding the caliper up to the rotor the same way it would be when it was bolted tight. In this position, I note the high and low sides, then go to the sander, and try again. I do this for each caliper. It saves the hassle of screwing and unscrewing the caliper halves so much.

You can also shim any of the three mounting points on the caliper arm bracket, and try to get a better alignment this way. Have not tried it myself, and have seen that different bikes will yield different amounts of misalignment, sometimes none at all.

Good Luck!