Brake Master Question

Suspension, including wheel, tire and brake.
User avatar
JordanTr
Posts: 900
Joined: 20 Feb 2013 22:27
Location: Langley, BC, Canada

Re: Brake Master Question

Post by JordanTr » 28 Apr 2019 12:30

As has been alluded to, a larger master leads to a higher pedal effort to achieve a given brake pressure proportional to the ratio of the square of the bore diameters. A larger master results in a shorter pedal travel and thus an inversely proportional higher pedal effort.

Without actually being able to measure the stock parts, I assumed the numbers below for some calculations.
Pedal pivot to clevis pin: 2.375 inches
Pedal pivot to pad center (parallel to above): 12 inches

These yield a pedal ratio of 5.05:1 which seems like a reasonable starting point for a manual brake system. Seems perhaps congruent with this thread.

If a longer pedal travel can be tolerated, geometry changes in the brake pedal can alter the effort/travel to return them closer to the stock feel. With the dimensions above, just under 2 inches added to the pedal (like Keith's) will return the feel of a 7/8" master to 13/16" factory feel.

Alternatively, the distance from the pedal pivot to clevis pin can be shortened. Or a combination of both could be used.
In some cases (final row of chart below), the pedal system could be modified to achieve a lighter pedal feel than stock at the expense of more travel. I will look at my setup to see if can afford an increase in travel to reduce the pedal feel.

Obvious disclaimer that if you're messing with brake stuff such as this, you are taking tangible risks.

The table shows the foot force required to achieve 1200psi brake system pressure just as a point of reference which Mark Williams says is a good target pressure for emergency stops.

Image
Image

Interestingly enough, a combination of longer pedal length with short clevis-pivot length can be as effective as a 5.5" section added to the pedal (shown in row 7 above) but the combination of the 2 works within the constraints of the dash/foot room etc.

I will have to look into my brakes since I stand to reduce the effort by ~30% (98/143) since I'm already used to the feel with the 7/8" master. This would come with the required 45% increase in travel from current so it may not be achievable but I am certain I can work within my travel constraints to reduce my current pedal feel somewhat.

Hopefully someone finds this interesting. :roll:
Jordan | '72 2 door KA project | '94 240sx RB26DETT | '97 Silvia RB25DET | '90 Audi 90 Quattro 20V (DD)

User avatar
James
Posts: 2070
Joined: 26 Nov 2007 19:58
Location: Laguna Beach, Ca

Re: Brake Master Question

Post by James » 28 Apr 2019 13:21

That is interesting. Would the geometry change of the modified clevis position run the risk of added wear etc to the master?
Finished is better than perfect......

User avatar
JordanTr
Posts: 900
Joined: 20 Feb 2013 22:27
Location: Langley, BC, Canada

Re: Brake Master Question

Post by JordanTr » 28 Apr 2019 14:31

I think it would mean a short life for the seals in the master. Maybe an offset clevis could be built so it attaches higher on the pedal but still pushes straight into the master?

I am envisioning reworking the bracket so that the pedal pivot changes and the clevis position remains unchanged.
Jordan | '72 2 door KA project | '94 240sx RB26DETT | '97 Silvia RB25DET | '90 Audi 90 Quattro 20V (DD)

User avatar
James
Posts: 2070
Joined: 26 Nov 2007 19:58
Location: Laguna Beach, Ca

Re: Brake Master Question

Post by James » 28 Apr 2019 15:16

I could see that - but I would imagine the offset clevis would still have some axial pressure on the master - I may have to draw that out and see how it could work.
Finished is better than perfect......

User avatar
James
Posts: 2070
Joined: 26 Nov 2007 19:58
Location: Laguna Beach, Ca

Re: Brake Master Question

Post by James » 28 Apr 2019 15:17

I like the idea of nothing changing but the clevis position and the master increasing in size, and it still works the same. Intriguing.
Finished is better than perfect......

User avatar
JordanTr
Posts: 900
Joined: 20 Feb 2013 22:27
Location: Langley, BC, Canada

Re: Brake Master Question

Post by JordanTr » 28 Apr 2019 15:32

If you do measure it up, I can re-run the spreadsheet with actual numbers and see where things stand.
Jordan | '72 2 door KA project | '94 240sx RB26DETT | '97 Silvia RB25DET | '90 Audi 90 Quattro 20V (DD)

User avatar
JordanTr
Posts: 900
Joined: 20 Feb 2013 22:27
Location: Langley, BC, Canada

Re: Brake Master Question

Post by JordanTr » 28 Apr 2019 17:15

Here's a thread to take you down the internet rabbit hole. https://www.jalopyjournal.com/forum/thr ... d.1082627/

Offset brake push rods are a thing.

Image
Jordan | '72 2 door KA project | '94 240sx RB26DETT | '97 Silvia RB25DET | '90 Audi 90 Quattro 20V (DD)

User avatar
James
Posts: 2070
Joined: 26 Nov 2007 19:58
Location: Laguna Beach, Ca

Re: Brake Master Question

Post by James » 28 Apr 2019 20:22

MASTER CYL..pdf
(16.39 KiB) Downloaded 23 times
This was my thinking - I'll see if I can do some measuring tomorrow.
Finished is better than perfect......

User avatar
bertvorgon
Supporter
Posts: 10750
Joined: 04 Aug 2003 20:45
Location: White Rock, B.C. Canada

Re: Brake Master Question

Post by bertvorgon » 30 Apr 2019 13:14

I was over at my garage today and thought...just because...I would measure my pedal travel for my setup ( 2" extension in the pedal)
With just a gnats eyebrow for travel BEFORE the brake arm actually pushes the rod ( I get 99% of all my cars pedal travel eliminated on the threaded adjustments), I had a TOTAL of 1" of travel till the firewall was starting to move, so not very much.

My pad knock back is minimal and all my calipers are totally square to the rotor.

In the 1"of travel, I have a very lineal feel of grip as the pressure/force goes up.
"Racing makes heroin addiction look like a vague wish for something salty" - Peter Egan

Keith Law
1973 2Door Slalom/hill climb/road race / canyon carver /Giant Killer 510
1968 Vintage 3HP Mini Bike
1971 Vintage 13' BOLER trailer

Post Reply