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#989933 - 06/19/17 06:54 PM Linked brakes - do they compensate for pressure I apply on rear brake?
poodad Offline
Newer Member

Registered: 08/12/16
Posts: 24
Loc: Wandering and wondering
My understanding is that camheads have linked brakes so that when you apply front brake, rear brake is applied at 30% of front brake pressure.

What I'm wondering is that if I apply rear brake as well as front, is the rear braking I'm applying add to the system's 30%, or does the system back off its application of rear pressure as I apply pressure to maintain the 30% ratio?

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#989939 - 06/19/17 07:55 PM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: poodad]
dirtrider Offline
The Oracle
Member

Registered: 06/05/10
Posts: 12868
Loc: Ohio
Originally Posted By: poodad
My understanding is that camheads have linked brakes so that when you apply front brake, rear brake is applied at 30% of front brake pressure.

What I'm wondering is that if I apply rear brake as well as front, is the rear braking I'm applying add to the system's 30%, or does the system back off its application of rear pressure as I apply pressure to maintain the 30% ratio?



Afternoon poodad

It is a somewhat complicated brake system.

First I don't know where you got that 30% from but there is no fixed front/rear apply percentage. The ABS system is an adaptive system so it looks at front & rear apply pressures vs tire slippage then adapts the braking bias to keep the braking even, even if there is a rear seat passenger or a load change on the motorcycle.
(This is IF the braking is basically done using ONLY the front hand lever).


When the front brake lever is pulled that runs the rear servo pump to apply the rear brake (there is no direct hydraulic link between the front hand lever & rear brake).

Now the rear brake is a strange affair (or at least the hydraulic plumbing is)-- The rear brake is basically an either/or brake apply system.

If you ONLY use the rear brake pedal (alone) then you get a direct hydraulic link to the rear brake (no front brake & rear servo pump)

There is a pressure bias valve between the front & rear systems to keep the rear pedal isolated from the front calipers & to keep the front brake from blowing brake fluid out of the rear system with front brake apply.

In a simple explanation on how it works-- If you use the front lever alone then you get front braking in proportion the what lever pressure that you apply, the ABS computer then determines how much rear brake to add based on wheel slippage, pressure applied, spin down rate, etc.

As mentioned above when using rear brake pedal you only get rear brake but no rear servo assist.

The part that a lot of riders don't understand is the either/or for rear brake apply.

If you stop the bike from speed using the front lever then the servo applies (lets say 30 psi to the rear caliper), now you step on the rear brake pedal lightly (lets say 20 psi) then NOTHING is added to the rear brake apply as the either/or rear brake valve remains biased to the higher 30 psi front pressure.

Now without changing the front apply pressure, & assuming no tire slip, you step on the rear brake pedal harder (lets say to 40 psi) that rear pedal 40 psi is higher than the front's 30 psi to the rear so the either/or valve closes off the front servo input to the rear & opens the rear input (so now the front braking remains the same but you have increased rear braking pressure).

So now you find that you need to stop quicker so you squeeze the front lever harder & increase rear servo pressure to 50 psi. The either/or valve now closes off the rear pedal input as the front pressure to the rear brake is now higher than the rear pedal pressure so now it is back to allowing the front to control the rear brake. (note: that either/or valve is my name for it, that is not an official valve name)

If you want to do a little test just put the bike on the center stand, now have someone spin the rear wheel (or do it yourself using your foot)-- (you will find NO rear braking from front brake lever)

Now do the very same test with the ign key turned on, the rear wheel should lock up just as you start to spin it (that is the rear brake servo kicking in & sending pressure to the rear brake)

When in ABS lock-up mode things change as the front is accumulated & the rear make-up fluid is servo only.

(the above a is a pretty simple explanation of complicated braking system)
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#989958 - 06/19/17 11:23 PM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: poodad]
sardineone Offline
Member

Registered: 10/01/05
Posts: 973
Loc: Indiana, USA
JMHO, I know my hexhead is different somewhat (100% servo) from the camhead, but it does have linked brakes and you might be having the same experience I had early on with my BMW. Even though the anti-lock system will do just that, the brake feel from my rear brake pedal is no where near as linear as the front hand lever. My riding experience is when the front brake lever is used and the foot brake was added, I felt a bit of an unsettled feel. I felt a conflict between the two inputs not blending harmoniously into the system. What I've found that works very well is to trust the linked braking, only using the front brake lever except for when I only want the rear applied. I found the brake system on my BMW is the best in my 48+ years of riding. As Dirtrider has alluded, the system has to be more complex than a constant 70/30 ratio from the great results I've had for over 45,000 miles.
_________________________
You are what you have ridden!
-------------------------------------------------
George Martin
R1200ST(ESA equipped)

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#989967 - 06/20/17 12:17 AM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: dirtrider]
6speedTi Offline
Member

Registered: 05/26/12
Posts: 186
Originally Posted By: dirtrider
Originally Posted By: poodad
My understanding is that camheads have linked brakes so that when you apply front brake, rear brake is applied at 30% of front brake pressure.

What I'm wondering is that if I apply rear brake as well as front, is the rear braking I'm applying add to the system's 30%, or does the system back off its application of rear pressure as I apply pressure to maintain the 30% ratio?



Afternoon poodad

It is a somewhat complicated brake system.

First I don't know where you got that 30% from but there is no fixed front/rear apply percentage. The ABS system is an adaptive system so it looks at front & rear apply pressures vs tire slippage then adapts the braking bias to keep the braking even, even if there is a rear seat passenger or a load change on the motorcycle.
(This is IF the braking is basically done using ONLY the front hand lever).


When the front brake lever is pulled that runs the rear servo pump to apply the rear brake (there is no direct hydraulic link between the front hand lever & rear brake).

Now the rear brake is a strange affair (or at least the hydraulic plumbing is)-- The rear brake is basically an either/or brake apply system.

If you ONLY use the rear brake pedal (alone) then you get a direct hydraulic link to the rear brake (no front brake & rear servo pump)

There is a pressure bias valve between the front & rear systems to keep the rear pedal isolated from the front calipers & to keep the front brake from blowing brake fluid out of the rear system with front brake apply.

In a simple explanation on how it works-- If you use the front lever alone then you get front braking in proportion the what lever pressure that you apply, the ABS computer then determines how much rear brake to add based on wheel slippage, pressure applied, spin down rate, etc.

As mentioned above when using rear brake pedal you only get rear brake but no rear servo assist.

The part that a lot of riders don't understand is the either/or for rear brake apply.

If you stop the bike from speed using the front lever then the servo applies (lets say 30 psi to the rear caliper), now you step on the rear brake pedal lightly (lets say 20 psi) then NOTHING is added to the rear brake apply as the either/or rear brake valve remains biased to the higher 30 psi front pressure.

Now without changing the front apply pressure, & assuming no tire slip, you step on the rear brake pedal harder (lets say to 40 psi) that rear pedal 40 psi is higher than the front's 30 psi to the rear so the either/or valve closes off the front servo input to the rear & opens the rear input (so now the front braking remains the same but you have increased rear braking pressure).

So now you find that you need to stop quicker so you squeeze the front lever harder & increase rear servo pressure to 50 psi. The either/or valve now closes off the rear pedal input as the front pressure to the rear brake is now higher than the rear pedal pressure so now it is back to allowing the front to control the rear brake. (note: that either/or valve is my name for it, that is not an official valve name)

If you want to do a little test just put the bike on the center stand, now have someone spin the rear wheel (or do it yourself using your foot)-- (you will find NO rear braking from front brake lever)

Now do the very same test with the ign key turned on, the rear wheel should lock up just as you start to spin it (that is the rear brake servo kicking in & sending pressure to the rear brake)

When in ABS lock-up mode things change as the front is accumulated & the rear make-up fluid is servo only.

(the above a is a pretty simple explanation of complicated braking system)





Uhh, can you repeat that again, I missed the first part of the explanation. :-)

Hate to ask for the detailed longer version.
Thanks for the breakdown. All in all the brakes on the camhead work really good. I use the front brake lever only about 90% of the time for that reason.


Edited by 6speedTi (06/20/17 12:18 AM)

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#989973 - 06/20/17 01:20 AM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: 6speedTi]
lkraus Online
Member

Registered: 05/28/11
Posts: 1153
Loc: Central Ohio
I think the gist of it is:

Applying the front brake also applies some pressure to the rear, the amount of rear braking varying according to what is suitable based on various conditions. Pressing the brake pedal will have no additional effect until you exceed the pressure already applied by the front brake "linkage".

If that basic concept makes sense, go back and read dirtrider's post to appreciate some of the finer points.


I think this why there are complaints about the rear brake "feel". It changes depending on how much the front brake is used. Sometimes the pedal does a lot, sometimes it does nothing.


Edited by lkraus (06/20/17 01:26 AM)
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2006 R1200RT

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#990010 - 06/20/17 02:05 PM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: poodad]
poodad Offline
Newer Member

Registered: 08/12/16
Posts: 24
Loc: Wandering and wondering
Thanks for the detailed explanation DirtRider!

I probably should have mentioned the reason for my question. Prior to the RT, I always used the front and rear brakes together as is standard practice on non-linked brake bikes. I want to maintain that habit for times when I'm riding someone else's bike without linked brakes.

But I was wondering if by applying rear brake pressure on my RT, I was in effect reducing the front brake effectiveness by reducing front wheel traction due to lessened forward weight shift.

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#990023 - 06/20/17 03:40 PM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: poodad]
dirtrider Offline
The Oracle
Member

Registered: 06/05/10
Posts: 12868
Loc: Ohio
Originally Posted By: poodad
Thanks for the detailed explanation DirtRider!

I probably should have mentioned the reason for my question. Prior to the RT, I always used the front and rear brakes together as is standard practice on non-linked brake bikes. I want to maintain that habit for times when I'm riding someone else's bike without linked brakes.

But I was wondering if by applying rear brake pressure on my RT, I was in effect reducing the front brake effectiveness by reducing front wheel traction due to lessened forward weight shift.


Afternoon poodad

You can keep using the rear brake in conjunction with the front brake but just use very light rear pedal apply pressure. That keeps your braking habits sharp but shouldn't effect the computer balanced adaptive braking.

Just resist the urge to use a lot of rear brake in an emergency type stop as that not only upsets the braking balance but can induce early rear wheel ABS action.
_________________________
D.R. ___
Sent from my rotory dial wall phone!

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#990331 - 06/23/17 12:19 PM Re: Linked brakes - do they compensate for pressure I apply on rear brake? [Re: dirtrider]
Deek Offline
Member

Registered: 07/05/08
Posts: 1106
Loc: Near Memphis, TN
Originally Posted By: dirtrider
Originally Posted By: poodad
Thanks for the detailed explanation DirtRider!

I probably should have mentioned the reason for my question. Prior to the RT, I always used the front and rear brakes together as is standard practice on non-linked brake bikes. I want to maintain that habit for times when I'm riding someone else's bike without linked brakes.

But I was wondering if by applying rear brake pressure on my RT, I was in effect reducing the front brake effectiveness by reducing front wheel traction due to lessened forward weight shift.


Afternoon poodad

You can keep using the rear brake in conjunction with the front brake but just use very light rear pedal apply pressure. That keeps your braking habits sharp but shouldn't effect the computer balanced adaptive braking.

Just resist the urge to use a lot of rear brake in an emergency type stop as that not only upsets the braking balance but can induce early rear wheel ABS action.


This is my understanding of how the "linked" system works also, and I think it's a good design. I use the rear brake often when I am doing things like we did in the MSF Rider Courses where we had so much fun(!) doing those figure eights. Slightly pressing on the rear brake only while using the clutch <no throttle> is the only way I could ever get through that dreaded event!
_________________________
.
Deek aka adVentureMan
2013 BMW R1200RT
Gone:
2012 Triumph Tiger 800
2009 BMW R 1200 RT
2010 Triumph Bonneville
2006 BMW R 1200 RT
2007 Royal Star Venture
2007 V-Strom DL1000
2006 VTX 1300R

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