This post is intended as a nothing-goes-unexplained, no-steps-skipped approach to converting a HONDA CB550 or CB500 from a single-disc to a dual-disc front brake set-up in the hope that it would help HONDA aficionados and cafe racer enthusiasts.
Much has been written about such dual-disc conversions and the available opinions range from “it’s fairly simple” to “it most definitely wasn’t” with some detailed descriptions in between such as this excellent post provided by the builder of the Huelligan’s Projects.
But let’s get to work and let’s do this in two steps: 1) What do we need to know? and 2) What do we need to do?
WHAT DO WE NEED TO KNOW?
HONDA made at least two different front wheel hubs for the CB550 bike: HONDA part numbers 44601-300-030 and 44601-300-040.
One hub design featured a flange face on the right hand side (opposite the side having the original brake rotor) that had “cut-offs” for the speedometer drive bracket (the term for this part according to HONDA is “gearbox retainer”). I do not know which HONDA part number is assigned to this particular hub design since the HONDA microfiche does not clearly show the differences in design between the two hubs. But it lookes like this:
Why was this needed? Skip this, if that is of no interest to you, but for the curious, here is the answer. The gearbox retainer needs to be “affixed” to the hub. The two “notches” in the cylinder part of the gearbox retainer in the image above fit into corresponding “teeth” in the speedometer drive (the original HONDA term is “speedometer gearbox assembly”). When the wheel rotates, the hub rotates with it. As the hub rotates, the gearbox retainer rotates with it because it is “affixed” to the hub. This causes the “teeth” inside the speedometer gearbox assembly to rotate and this, in turn, rotates a “worm screw” inside the speedometer gearbox assembly, which rotates the speedometer cable, which then causes the needle in the speedometer to go up or down showing the speed at which the bike is moving.
Alright, what does that have to do with anything and how is it related to a dual disc conversion?
Before answering this question, let’s call one of the two hub designs “oval” and the other “round”.
Now, understanding the design of the front hub is important. For the obvious reason that the additional (second) rotor will have to mount to it. And when that additional rotor is actually mounted, re-using the original gearbox retainer without modifications will become impossible. A good understanding of how things work allows for a good planning and, hopefully, eliminates frustrating mistakes.
Because things could go wrong. In one particular case where the builder Ben Hiles proceeded courageously without a thorough initial understanding of the hub design and fork sliders design, he reported:
BIG mistake… what happened was, when I squeezed the front brake the caliper wasn’t parallel to the disc, and jammed the whole show up!!!, I mean major drama…
The difference in flange design (oval and round) described above is not the only difference between the two hub designs. A somewhat more important difference is this: the “oval” hub is symmetrical with regard to the LEFT and RIGHT rotor mounting surfaces, the “round” one is not! This is absolutely crucial. To appreciate this, take a look at the image below showing a side view of the “oval” hub:
If you measured the distance A between the disc-mounting flange on the left and the center of the left ridge, and you then measured the same distance B on the right, you will find that A = B. Since the rim and tire are symmetrical to the ridges holding the spokes, then the two rotors will be symmetrical to the rim, spokes and tire.
and you will find that C is bigger than D as shown above. What this obviously means is that when the two rotors are mounted to the hub, they will be asymmetrical to the rim, spokes and tire. The right rotor will sit closer to the spokes and will be further away from the fork slider when compared to the left rotor. By precisely 3 mm. This would lead to the suspicion that the additional right side caliper, needed for the conversion job, will not align with the right side rotor. But that assumes that the right and left fork sliders are themselves fully symmetrical and have the exact same mounting points for the calipers.
Well, let’s take a close look. Here is what we see:
Amazingly, the right side fork slider has a caliper bracket mounting point/post, which is offset by exactly 3 mm when compared to the left. What this means is that the caliper will sit 3 mm closer to the rim, tire and spokes and will, therefore, line up with the rotor perfectly. So far so good.
There is a gap between the slider and the top caliper bracket! It is exactly 3 mm. That’s an easy one to fix. If you are re-using the original front fender, its mounting bracket will fill some of that 3 mm gap, the rest can be taken up by standard 6 mm washers (one at each mounting point). If you are not using a front fender at all, then just put two 3 mm thick washers to fill the gap when installing the calipers.
Okay. It took some time, but now we have a clear understanding of the hub design and fork sliders design, and, more importantly, we know that converting to a dual disc set-up is relatively simple. Which brings us to:
WHAT DO WE NEED TO DO?
Purchase all parts needed: a second caliper (with mounting bracket, adjuster screw and spring), new brake pads, new caliper piston seals, a second rotor, six bolts 10 mm longer than the originals (the original bolts are M8 x 102 mm long but replacing them with M8 bolts 110 mm long will do the job just fine), six M8 flange nuts or lock nuts, new oil seals and new ball bearings for the hub (now is the time to replace them). And some other parts if needed as described below.
Make the necessary decisions reflecting your personal preferences:
To drill or not to drill? There are many pro and con arguments about drilling rotors. No need to dwell on them here. I personally think that drilling the rotors reduces their weight substantially and makes them look cool. If you decide that this is the option you would like to pursue, I highly recommend Thomas Neubauer of ANUBIS CYCLE. Tom did the two rotors shown in the headline image for this post. The work was superb, prices were very reasonable and the turn around was super fast. Make sure you send both rotors at the same time. Tom will need to match them for synching.
Symmetrical or asymmetrical rotors?
Symmetrical. This is the only option for the “oval” hub design with cut-offs for the gearbox retainer. For the “round” hub design, we already know that if the right side rotor is mounted to the hub, it will sit 3 mm closer to the rim, spokes and tire. So, to make it “symmetrical”, we need to offset the right side rotor by 3 mm. This would require a 3 mm thick spacer between the hub and the rotor. It is easy to fabricate such a spacer and here is a very crude drawing of it:
Just about any machine shop could do this for you. Using such a spacer will, of course, require cutting 3 mm off the caliper mounting post on the right fork slider. This, depending on skill level and available tools, may not be as easy as it sounds. And, keep in mind that cutting 3 mm off the mount post will eliminate 3 mm of the available thread.
Asymmetrical. No need for the special spacer shown above. If this solution is implemented, the caliper will be closer to the spokes, but there is still sufficient clearance between it and the spokes. Just make sure that the two bolts holding the two halves of the caliper together do not extend past the caliper surface facing the spokes. Grind them to a perfect fit, if necessary. The fact that the two rotors are not symmetrical to the center line of the wheel does not affect wheel balance.
To retain the speedometer or to eliminate it? Getting rid of the speedometer is very tempting. First, there are excellent speedometer apps you can get for your smart phone for free. They are very accurate, easy to use and have a number of features the original HONDA speedometer does not such as maximum speed attained, average speed, audible alert if the bike’s speed exceeds a preset speed, etc. There are also hundreds of different handlebar mounts for smart phones, which make it very easy to mount your smart phone anywhere on the bike that you see fit. Second, both the speedometer cable and the gearbox retainer can be discarded. The purists may decide that such an approach is nothing short of anathema. No objection.
Here are the solutions for both options:
Retain the speedometer. In both cases of hub design, the gearbox retainer will need to be modified. Obviously, once the right side rotor is mounted to the hub’s flange, there will be no way to install the original gearbox retainer as intended regardless of which of the two different hub designs shown above you actually have. The modification requires cutting/grinding off the “skirt” and leaving only the flat surface so the gearbox retainer can be inserted in the rotor’s central bore and be secured to the hub with screws. (If your bike has the “oval” hub, it will be possible to reduce the diameter of the gearbox retainer so it fits in the rotor’s central bore and the trim the gearbox retainer’s tangs to a good fit.) Alternatively, a custom gearbox retainer could be machined. Use flat-head screws to avoid any interference between the gearbox retainer and the speedometer gearbox assembly itself. But if/when doing so, make absolutely sure the gearbox retainer is mounted absolutely perfectly centered to the hub. If it is not, the speedometer gearbox assembly will be offset in relation to the opening for the front axle and this will, obviously, create issues. In addition, please note that the original HONDA set-up (in the “round” hub design) uses an O-ring between the gearbox retainer and the hub to prevent any dirt or water from getting into the internals of the hub. Modifying the gearbox retainer as crudely as described above in order to make it fit easily is going to make it impossible to use an O-ring to seal the hub!
Eliminate the speedometer. This requires some work, but the result is a much cleaner set-up and look. Obviously, the gearbox retainer and the speedometer gearbox assembly will have to be replaced with something. That “something” is actually 4 things: a custom spacer, a bushing, an O-ring and an oil seal.
The custom spacer can be made using the drawing below. THIS PARTICULAR SPACER CAN ONLY BE USED WITH THE “ROUND” HUB DESIGN! Although the principle is the same, a spacer with different dimensions will be needed for the “oval” hub design.
The oil seal is the original HONDA oil seal that sits on the left side of the hub inside the bearing retainer. The HONDA part number is 91252-300-003. That oil seal has an I.D of 22 mm and an O.D. of 36 mm.
Here is what the spacer looks like (with the seal installed):
The O-ring needed is 50 mm I.D. (any I.D. between 50 and 54 mm will do) and 2.5 mm cross section (CS or thickness).
The bushing needed is made from stainless steel and is 22 mm O.D., 15.06 mm I.D. and a length of 28.4 mm.
Here is how these 4 components work together. The O-ring is mounted to the “back” of the spacer on the 2 mm tall flange with a 55.5 mm diameter. The oil seal is then installed into the “front” end of the spacer. The spacer is then pressed into the center bore of the right side rotor. The original cover for the gearbox retainer is then placed on top of the spacer and the two rotors are bolted to the hub. As the bolts are tightened, the cover will press against the spacer and will push it against the hub. This will compress the O-ring both against the rotor and against the hub surface, thus sealing everything perfectly. Here is what it looks like:
The friction created by this compression is going to secure the spacer and it will not move or rotate. The bushing is then slipped into the oil seal. In essence, for the purposes of installing the front axle and the wheel, you have the same set-up on the right as you have on the left.
Here is a picture showing all the parts needed for a dual disc conversion, which eliminates the speedometer gearbox assembly (and consequently the speedometer cable and the speedometer itself):
- BUSHING O.D = 22 mm, I.D. = 15.06 mm, LENGTH = 28.4 mm
- HONDA OIL SEAL part number 91252-300-003
- CUSTOM SPACER (see drawing above)
- GEARBOX RETAINER COVER
- M8 FLANGE NUTS
- M8 BOLTS, 110 mm LONG
- ORIGINAL LEFT SIDE BUSHING (it inserts into the oil seal in the bearing retainer)
- O-ring I.D. 50 mm, CS 2.5 mm
- NEW BALL BEARINGS
- BEARING RETAINER with oil seal installed
- FRONT AXLE
- FRONT AXLE NUT
- HUB SPACER
Once the job is done, your dial disc set-up will look like this:
A dual disc conversion is, of course, not complete without all the necessary brake lines, master cylinder and other components. This will be covered in the next post.
In the meantime, let me know if I’ve missed anything and correct me if, perhaps, I may have misspoken.QUESTIONS?