Disk brake issue on T6 conversion

For those who obsess about exactly how their 356 left the factory!
Message
Author
User avatar
Adam Wright
356 Fan
Posts: 8378
Joined: Wed Mar 12, 2008 1:00 am
Tag: KTF

Re: Disk brake issue on T6 conversion

#16 Post by Adam Wright » Tue Aug 21, 2012 9:00 pm

The lines are like artiries before a heart attack sometimes, almost completely blocked. Check to see if this is the case, it makes it very hard to push the petal when they are like this. But they look great from the outside!
I always need stories for my Barn Find column in the magazine, if you have a good story to share, please let me know.

Kevin Wills
356 Fan
Posts: 347
Joined: Wed Jan 27, 2010 9:04 am
Location: Northern Virginia

Re: Disk brake issue on T6 conversion

#17 Post by Kevin Wills » Tue Aug 21, 2012 10:48 pm

Mechanical advantage. It seems much more noticeable on a track bike. People often run into this changing m/c's on motorcycles. Everything needs to be thought of as a system, not a component.

User avatar
Larry Coreth
356 Fan
Posts: 1901
Joined: Sun Apr 20, 2008 12:48 pm
Location: NE N.CAROLINA

Re: Disk brake issue on T6 conversion

#18 Post by Larry Coreth » Wed Aug 22, 2012 12:42 am

Vic et al,
Hydraulic advantage is not that complicated, Think in terms of pressure as the common term.
So a 19mm M/C piston has 0.4395 in2 now we apply 50lbs. of force to the pedal and we get 50/ 0.4395=113.77 psi in the line. This same pressure is exerted on the 48mm caliper piston. Since the caliper piston has a greater square area (2.805 in2) than the M/C piston we will get more force, i.e. 113.77psi*2.805 in2=319.11 lbs. If you notice the ratio of the forces, pedal/ caliper piston is the same as the square area ratio. i.e. .04395/2.805=6.382 ! Keeping this ratio in mind, then a smaller M/C gives more advantage, a lager dia, less, while at the caliper end it is the opposite.
This is why the rear caliper pistons are smaller than the fronts because of the weight transfer to the front during braking which if they were the same diameters the rears would lock up too soon.
BTW, this also why the rear brakes of drum brake cars are single leading shoes while the fronts are double leading shoes.
Larry Coreth
Roanoake Rapids, NC

John Montgomery
356 Fan
Posts: 24
Joined: Fri Apr 06, 2012 9:48 pm
Tag: Taxidodger
Location: Jersey City

Re: Disk brake issue on T6 conversion

#19 Post by John Montgomery » Thu Aug 23, 2012 2:37 pm

Thanks all - lots to keep me busy here. BTW pads are new Mintex.
John

User avatar
Neil M. Fennessey
356 Fan
Posts: 257
Joined: Wed Mar 26, 2008 9:20 am

Re: Disk brake issue on T6 conversion

#20 Post by Neil M. Fennessey » Fri Aug 24, 2012 9:35 am

Ernesto Cabrera wrote:Smaller pistons have less mechanical advantage at the master cylinder so, there is less travel at the pedal but more pressure is required: P1/V1=P2/V2 if temperature is a constant.
Ernie
Pascal's Law might be a better choice for an incompressible fluid than the Ideal Gas Law, which is OK for an incompressible fluid that's far from the triple point. Unfortunately, your algebra is also incorrect.

PV=nRT => P1V1=nRT and P2V2=nRT then

For the same gas at the same temperature:

P1/V2 = P2/V1

Let F1= the pedal force required to generate a pressure P to stop a car. Let D1 = 19 mm, the diameter of a MC piston of diam 19 mm. Let F2 be the pedal force necessary to generate P in a car with MC piston diam D2 = 21 mm.

F1=PxA1 and F2=PxA2

Rearranging terms:

P = F1/A1 = F2/A2

Rearranging terms to solve for F2,, the pedal force required with the 21 mm MC.

F2 = F1(A2/A1)

Since we don't know F1, we can solve for the ratio of F2/F1 and express as a % difference in force.

F2/F1 = A2/A1

A1 = pi (19 mm)^2/4 and A2 = pi(21 mm)^2/4 then

F2/F1 = (21 mm/19mm)^2 x 100

= 122%

Then to stop in the same distance under identical conditions, it will take about 100 lbs of pedal force on the 21 mm MC versus 80 lbs of pedal force on the 19 mm MC.

Vic, you don't need to dust off that old textbook.

The Motivated Student may use these principles and relationships to better understand Larry's Lecture.
Last edited by Neil M. Fennessey on Fri Aug 24, 2012 1:59 pm, edited 2 times in total.
Back to the Ivory Tower I go!
 - Neil
'67 912/356D (Ol' Blue)
'82 HP 34C

User avatar
Ernesto Cabrera
356 Fan
Posts: 390
Joined: Wed Jun 18, 2008 9:32 am
Location: Atlanta Metro
Contact:

Re: Disk brake issue on T6 conversion

#21 Post by Ernesto Cabrera » Fri Aug 24, 2012 10:45 am

Kudos to professor Neil. I stand corrected on the equation (Its been more than 45 yrs since I used these formulas). The point is the mechanical advantage increases as the fluid is pumped from a smaller chamber to a larger one and vise versa as Ashley and Larry illustrated.

Best

Ernie
 

Conrad Carter
356 Fan
Posts: 327
Joined: Tue Aug 05, 2014 9:54 am
Tag: Let's be careful out there!
Location: Durham, NC

Re: Disk brake issue on T6 conversion

#22 Post by Conrad Carter » Fri Mar 15, 2019 3:28 pm

Vic,
I keep my slide rule on my desk at all times. Always check calculator answer with slide rule!
Conrad

Martin Benade
356 Fan
Posts: 3730
Joined: Wed Nov 23, 2011 10:52 am
Location: Cleveland, Ohio

Re: Disk brake issue on T6 conversion

#23 Post by Martin Benade » Fri Mar 15, 2019 7:59 pm

In case I got lost in the various maths, the larger caliper piston gives more leverage, correct?

User avatar
John Brooks
356 Fan
Posts: 764
Joined: Tue Jul 08, 2008 11:50 am
Location: Whidbey Island WA.
Contact:

Re: Disk brake issue on T6 conversion

#24 Post by John Brooks » Fri Mar 15, 2019 8:49 pm

I agree with Jacques, replace the flex lines. Use TWO wrenches on the hard to flex interface and buy some flare nut wrenches.

viewtopic.php?f=1&t=42405&hilit=brake+lines
John Brooks

62 Roadster
66 912
getting pushed around in porsches since 1965

Conrad Carter
356 Fan
Posts: 327
Joined: Tue Aug 05, 2014 9:54 am
Tag: Let's be careful out there!
Location: Durham, NC

Re: Disk brake issue on T6 conversion

#25 Post by Conrad Carter » Fri Mar 15, 2019 9:31 pm

I agree also. Even the stainless steel braided hoses do not live forever. Ask me how I know!!
Conrad

Mike Murray
356 Fan
Posts: 35
Joined: Sat Sep 27, 2014 3:09 pm

Re: Disk brake issue on T6 conversion

#26 Post by Mike Murray » Mon Mar 18, 2019 12:15 am

Force equals pressure per area.

500PSI behind a smaller piston creates a greater force.

Edwin Ek
356 Fan
Posts: 1860
Joined: Sat Jun 21, 2008 11:16 am
Location: Santa Monica, CA

Re: Disk brake issue on T6 conversion

#27 Post by Edwin Ek » Fri May 17, 2019 12:53 am

Pressure is defined as force divided by area. So force equals pressure times area. In symbols:

P = F/A

So F = P x A

Mike, I think you posted F = P/A
#6386

Post Reply