Honda’s Dual Combined Braking System

In my post about braking, I mentioned that my current motorcycle features a “linked braking system”.  Honda refer to the system fitted on my motorcycle as “Dual combined braking system”.  It’s their contribution to “rider safety”.

Hydraulics work on the principle that liquid cannot be compressed.  When a brake lever is “squeezed” this moves a piston in the master cylinder.  Moving the piston, displaces brake fluid.  We know that this brake fluid cannot be compressed, so the end result is that it must be displaced elsewhere in the system.  At the other end of the brake line, is the brake caliper.  It contains one or more cylinders with pistons in them as well.  The brake fluid therefore pushes the piston(s) in the caliper – which in turn pushes the brake pads into contact with the disc.  Typically the pistons in the calipers are much larger than the piston in the master cylinder.  This acts as a “gearing” allowing a greater pressure to be applied to the brake pad than the rider exerts at the lever.  But, because the surface area of the piston is greater, the piston won’t move as far.  

On the Honda, there are three brake calipers, each with three pistons. Two of these calipers grab large discs on the front wheel, whilst the third grabs a smaller disk on the rear wheel.  Larger discs mean better leverage and thus can be described as having a larger “braking force”.  (Remember that up to 90% of efficient braking force can be applied to the front wheel)

If the bike were fitted with a conventional braking system, the right-hand lever would activate all six pistons of the calipers used on the front wheel, whilst the right-foot pedal would activate the three pistons of the rear caliper.

The right-hand lever on the VFR activates five of the six pistons on the front calipers of the VFR.  That is, all three pistons on the front right caliper and the outer two pistons of the front left caliper.

Everybody knows brake fluid is blue!

The right-foot pedal activates the outer two pistons of the rear caliper via a “Proportional Control Valve” (PCV), and the middle piston of the front left caliper.  As near as I can tell (based on Internet research) the PCV acts as a “pressure reducer”.  Judging by the workshop manual, the PCV is strictly a mechanical device with no requirement for electrical input.

Or maybe it's green?

Those who have been paying close attention may now be asking: “How does the middle piston of the rear caliper get activated?”.  Well, that is answered by how the front calipers are mounted to the bike.  The right-front caliper is attached in a fairly standard way to the right fork leg.  On the left hand side, the caliper is not.  Instead, it is mounted at the bottom by a pivoting joint.  At the top is what is referred to as a “Secondary master cylinder”.  The piston for this master cylinder is attached to the fork leg.

It used to always be that clean!

Once the left front caliper grabs the front disc, the anti-clockwise motion of the disc (presuming the bike is moving forward) pivots the left caliper, forcing the piston further into the secondary master cylinder.  This secondary master cylinder is attached to a brake hose, through another PCV and then onto the middle piston of the rear caliper.

So, to summarise how the lever and pedal work in isolation:
When the rider applies the “front” brake lever:

  • Five of the six front pistons are applied.
  • The left front caliper exerts force on the secondary master cylinder.
  • This in turn applies the middle piston of the rear caliper.

tada!

When the rider applies the “rear” brake pedal:

  • Two of the three pistons in the rear caliper are applied.
  • The middle piston of the front left caliper is applied.
  • The left front caliper exerts force on the secondary master cylinder.
  • This in turn applies the middle piston of the rear caliper.

Pity the mechanics who bleed 19 hoses

The next obvious question to ask is: Does it improve braking performance?  The only answer I can give you is that I expect it does.  – I should qualify that by stating I expect that it reduces stopping distances for most riders.   Honda fit DCBS to several of their models.  Over time, the system has evolved.  Differing pistons have been utilised in previous versions – as have “delay valves” which staggered the intervals at which pistons were applied.  I suspect (but do not know) that maybe even the PCV has been developed over the years in an attempt to improve the system.  I know that some readers may want a better answer than “I expect it works”, but that’s a story for another time.

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