DUAL DISC CONVERSION PART 2: CALIPERS, BRAKE LINES & MASTER CYLINDER

DUAL DISC CONVERSION PART 2: CALIPERS, BRAKE LINES & MASTER CYLINDER

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  • On 27th July 2017

This is Part 2 of the Dual Disc Conversion series and it covers calipers, brake lines and master cylinder.  Installing the second disc was covered in DUAL DISC CONVERSION PART1.  This will be, again, a no-steps-skipped, down-to-the-last-detail approach (in full appreciation that it could go a little long and be a little boring, but the idea is to get the job done right).

WHAT DO WE NEED TO KNOW?

Not much this time.  Just that having two brake discs (rotors) means having two calipers and so, consequently, the master cylinder will need to be able to, theoretically, supply twice the amount of brake fluid for about the same travel (squeeze) of the brake lever.  Which essentially means that a bigger master cylinder is needed.  Why?

The master cylinder is a tube with a spring-loaded piston inside it.  When you squeeze the brake lever, the piston pushes brake fluid into the brake lines and from there into the caliper(s).  The brake fluid then moves the piston inside the caliper and in turn the piston pushes the brake pad toward the brake disc (rotor).  The friction between the brake pad and rotor stops the bike. To appreciate the importance of the correct sizing of the master cylinder, imagine the extreme:  The master cylinder has a very small diameter and the piston inside it can only travel a very short distance.  Now, you squeeze the brake lever, the piston inside the master cylinder travels a very short distance and pushes a very small volume of brake fluid into the caliper, which causes the piston inside the caliper to travel a very short distance toward the brake pad.  The brake pad moves toward the brake disc (rotor), but not enough to touch it.  The motorcycle will not stop.

An excellent description of the importance of matching master cylinder diameter to the number of caliper pistons and their diameter can be found here.

Many builders have reported that using a master cylinder from a HONDA Gold Wing works very well.

WHAT DOW WE NEED TO BUY?

second caliper and all of its associated components including the two O-rings.

New caliper pistons.  After all, it makes little sense to convert to a dual disc with the idea to improve the braking power of your HONDA CB550, if the calipers are not performing optimally.  The calipers must be rebuilt. New caliper pistons are available from a number of suppliers.  I bought mine from Motorcycle Solutions LLC.

A master cylinder.  See WHAT DO WE NEED TO KNOW above.

Brake lines and a 3-way connector.  Now is the time to decide if you prefer to have the calipers mounted forward of the forks (as originally by HONDA) or behind the forks as many builders have done. Here is what that looks like on this beautiful build by Dustin Kott (http://www.kottmotorcycles.com/). Dustin’s work is exquisite and very inspirational.

Dustin Kott’s “Exec 550”

The choice of caliper location will determine the length of the brake lines and the location and mounting point of the 3-way connector. Many different 3-way connectors are available.  Some have mounting holes, others do not.  Here is a possible option from GOODRIDGE:

Banjo bolts come in a huge variety.  Two things to keep in mind when selecting yours: thread size and pitch need to be 10 mm x 1.25 and the length has to be such that the bolt sits not too low and not too high once screwed into the caliper as to allow the banjo brake adapter that the bolt goes through to be mounted and seal properly.  See image below, which shows the correct length banjo bolt fully screwed into the caliper:

The banjo bolts normally come with two copper washers.  The natural inclination is to assume they’ll fit.  They probably won’t! Check to make sure.  Here is a picture of a copper washer that is too large to fit in the caliper’s seat:

And here s a picture of the right size aluminum washer:

WHAT DO WE NEED TO DO?

The rest of the job is pretty simple and easy to work out.  The one thing to keep in mind is the “spring load” on the right caliper.  What is that?  Most modern bikes have “floating” discs (rotors) and hard-mounted calipers.  “Floating” discs means that the discs can “float” side to side in response to the brake pads’s pressure/force. The HONDA CB550 has the exact opposite: hard mounted discs and “floating” calipers.  This floating is regulated by a spring.  The spring needs a certain amount of tension in order to allow the caliper to “float” properly in response the brake pads’ pressure/force. Here is what the original spring set-up looks like:

Note that there is a bolt that goes through the spring, which screws into a mount on the fork’s slider and comes through at a sufficient length to allow a lock washer and a nut to be used to secure it.  The challenge is that on the right side the distance between the caliper’s bracket and the mount is much greater so when the spring is inserted into the bolt and the bolt is mounted to the fork’s slider, there is almost no tension to the spring and the bolt does not come through the mount.  The solution is to put the locking nut and a few washers as shown below:

This will allow for sufficient tension on the spring and at the same time for locking everything in position once the brakes are properly set and aligned.

The rest is easy.  Bleed the brake lines and adjust the gap between the brake pads and the discs as described in the HONDA SERVICE MANUAL. Spin the front wheel by hand and apply the brakes.  Repeat several times to make sure everything works as it should. Adjust, if necessary.

QUESTIONS?