MadaboutKitcars

Jason

Is your PM working?

Mike. Some thoughs about your forum post #195

1) While researching a brake upgrade for my classic (front drums to discs) I have found that the braking ratio for road cars is usually set at 65% Front / 35% Rear.

2) Unless the front / rear ratio was way out at original build then the improved servo should not worsen the tendency for front /rears to lock first, only get it to happen with less pedal effort / sooner?

3) When Jason did his road tests did he notice any tendency for fronts or rears to lock first?

Re your post #197: I will be using my car (Hunter) through the winter but due to other planned jobs it will likely be end Jan before I get to test the servo on the road. Peter.

As for questions on locking, I don't know which wheels locked first... All I know is I started to slide.

That said, I know that at IVA, on the rollers the fronts locked before the rears, so she passed. The master cylinder hasn't changed, and neither has any of my brake lines or routing - so the bias will be exactly the same. Only the level of assistance has increased.

If people want me to do further testing, I may need to ask for contributions to a new set of boots when I flat spot them.

Guys

I don't know if any of you looked at the specification figures for the boosters, or are even interested in the numbers, but I have made some calculations based on the data given by MBM, which show a very interesting result: see if you agree with the following;

My brake details are:
Pedal centre to pivot centre 32cm - excuse mixed units - it does not matter
Pedal connection to servo to pivot - 8cm
Therefore my pedal ratio is only 4:1
My master cylinder bore = 22mm = 7/8" = 0.6 sq inch
Now assume 23” of vacuum and 100lbs of applied foot pressure for comparative purposes.

Based on my brake's details:
[edit: it will not hold spaces to make figure line up below]

Booster size “Manual Pressure” “Free Pressure” Total Brake Pressure
7” Metro 666 psi *1 587 psi *2 1253 psi
7” Dual 666 psi 815 psi *3 1481 psi *4

*1 = (100lbs x 4:1) / 0.6sq.inch = 666psi
*2 = Metro booster constant 352lbs / 0.6 = 587psi
*3 = Dual booster constant 490lbs / 0.6 = 815psi
*4 - The dual offers an increase of 18% of the total over the Metro. This does not sound too impressive.

I then considered if I reduce the distance of myservo connection to pivot by 1.6cm to 6.4cm

The new figures are:
New pedal ratio is 32/6.4 = 5:1
Same master cylinder bore = 22mm = 7/8" = 0.6 sq inch
Same 23” of vacuum and 100lbs of applied foot pressure.

Proposed brake's details:
Booster size “Manual Pressure” “Free Pressure” Total Brake Pressure
7” Metro 833 psi *1 587 psi *2 1420 psi
7” Dual 833 psi 815 psi *3 1648 psi *4

*1 = (100 x 5:1) / 0.6 = 833psi
*2 = Metro booster constant 352lbs / 0.6 = 587psi
*3 = Dual booster constant 490lbs / 0.6 = 815psi
*4 - The dual offers an increase of 16% of the total over the Metro, appears not so good

However, if we turn the calculation around to how much foot pressure is required to achieve the same total as with the original Metro, we see a significantly different result:

Original foot pressure to achieve 1253psi was = 100lbs
New foot pressure to achieve 1253psi = 52.4lbs *5

*5 = 52.4lbs x 5:1= 262lbs / 0.6sq.in = 438psi + 815psi = 1253psi
ie almost only HALF the effort

This has got to be worth a try!!

What master cylinder bores do you have?
What brake pedal ratio do you have?

Does anyone have a view on the calculations?

Not looked inside my master cylinder to check bore but raised number 23 on casting maybe 23mm? From Google I found the following DLJ website link
http://www.dlj-autoparts.com/eng/product.php?mode=list&cid=485
which lists several master cylinders including one of 23mm bore which is for a Mondeo! Peter.

Mike. From memory I don't think pedal travel was noticably short but the pedal effort required to stop was very noticable! I am thinking that the larger master cylinder diameter was fitted at POB because I have discs all round which theoretically may need more fluid displacement with eight large pistons? Can't do any practical checks at present as car on stands for smaller 24mm dia front ARB fitting (which has been on my 'to do' list for ages). Will do some measurements with and without engine vacuum when I can and let you know. Hopefully with a large master cyl bore I should get much improved braking with less tendency to lock the wheels. Peter.

Pedal centre to pivot centre 32cm - excuse mixed units - it does not matter

Pedal connection to servo to pivot - 8cm

Therefore my pedal ratio is only 4:1 

My master cylinder bore = 22mm = 7/8" = 0.6 sq inch



Now assume: 

23” of vacuum

100lbs of applied foot pressure for comparative purposes.



Based on my brake's details:



Booster size  “Manual Pressure”  “Free Pressure”  Total Brake Pressure 

7” Metro       666 psi  *1	  587 psi *2	  1253 psi

7” Dual        666 psi 	          815 psi *3	  1481 psi *4



*1 = (100lbs x 4:1) / 0.6sq.inch = 666psi

*2 = Metro booster constant 352lbs / 0.6 = 587psi

*3 = Dual   booster constant 490lbs / 0.6 = 815psi

*4 - The dual offers an increase of 18% of the total over the Metro. 

Mike - Where did you get the 'constant' figures in the calculation?

I've tried to do the sums my self, but as a bear of very little brain, what little there is is now smoking.

Taking all the figures as stated, I got as far as:

666 psi Manual Pressure.
Metro Boost Ratio is 1:1.9, so total brake pressure is 666 * 1.9 = 1265 psi
Free Presure is then 1265 - 666 = 599 psi.

Then I get confused. I don't know the boost ratio of the dual 7"...

Thank heavens for all you mathematitions, I will leave that to the intelligencia.

All I know is from what has been written so far is that the brake pedal should require a little less pressure to stop the car. Not quite up to my every day car standard but never the less better than it was.

Thank you one and all.

Dohhhhh, my brain hurts !!!!!!!!!!!!!!!

Peter

I suspect your master cylinder is a part of your brake issue. Peterux has said his MC is 19mm. If you calculate the surface area of the two you will see his has 30% less area than yours, therefore the same pedal pressures in your two cars will produce 30% different psi at the brakes, assuming the pedal ratios are the same. The servo will be diluted in your car too.

I am not too happy either, as my Cabrio has a 22mm MC, which means 25% more effort than Peterux pedal pressure. This may well explain why Peterux says he could lock his brakes up.
I believe Jason has a Ford type MC, so it would be interesting to know what size MC he has, as he says he could not lock his brakes up, prior to fitting the Dual servo.

try dropping your own numbers in to the calculations I have made and see how the different sized MC makes a big difference.

Of course the flip side of a smaller MC is that the pedal will travel further to get the same brake pad movement. So it will be a play off between pedal effort and pedal travel.

Do you currently get very limited brake pedal travel?

Mike

Good news.

The new servos have arrived, and certainly look good finished in Powder Coat Paint black. They are almost identical to the unit we tested, so I will go ahead with the main order on Monday.


Regards to all

Mike

Hi Mike. If there is a chance I would certainly like to change to a powder coated servo provided it doesn't compromise the order quantities. Peter.