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The line pressure out of the single bore size mc will remain constant at all four wheels. If however that pressure passes through a prop valve of sorts the resultant outlet pressure will then be less. Adding a proportioning valve will only reduce or limit the rear line pressure more, never increase it. Yes rears are usually the ones needing the adjustment.
Have no intention of adding a rear proportioning valve. Friend of mine with a nearly identical car has a front valve
And it's an odd thing that you'll find the harder you brake the less rear brake you get- thus the value if the prop valve.
I would say that is weight transfer
Light braking allowing for ample feed to both ends, hard braking reducing the rear pressure to prevent lock up.
Front pads have lost 1/8" relative to the back with not all that many miles.
When it comes to the calipers; The smaller pistons of a rear caliper will create less, the larger area front more, clamping pressure relative to that line pressure. Knowing the area of each end will give a rough bias % of the system. In the case of the D8s you have 5.56" on each front and only 3.0" on each rear.
Got about the same #'s over the phone
You can see that's roughly a 65/35% bias. Adding a rear prop valve to this can reduce the rear input pressure more but not increase it.
That would surely violate Newton's laws in a passive system
Not sure that helps a lot but more food for thought.
I'll make the measurements because I am now interested in the results. Any discrepancies with what I got over the phone from Wilwood tech, I will relay back to you privately and you can have that conversation..
A proportioning valve installed up front? That's a terribly wrong thing to do in reality...yikes!~
I'm not clear on your question about installing the rear however....I did word it poorly in saying input pressure...actually it would be output of the valve. I was thinking input to the rear caliper...
A prop valve running 200psi would perhaps 200 out, however as the input pressure goes up the outlet pressure goes down proportionally. Ie. when you brake hard and put 500psi into it the outlet pressure may be only 350psi. All that is somewhat relative to whats called the 'knee point' or the cross over at which you choose to have the reduction begin.
Here a random sample.
The purpose it to allow higher rear pressure at lower levels as demands on the rear can be used. But under hard braking that demand has to be less for that weight transfer you mention.
Two Longacre hydraulic brake pressure gauges arrived yesterday from Sparks, NV. Easiest way to collect the data was just to take some iPhone video with two helpers while I pressed on the brakes. Result is that front and rear went to 650 and 640 psi respectively. Close enough to identical. Also, no discernible difference in time constant.
Since these are manual brakes and this seems to be on the low end of pressures I think it unwise to dial down the front pressures. If the car were moving, the additional force from my mass decelerating would presumably take pressures somewhat higher.
So next I decided to approximate the maximum weight transfer. Wheelbase is 98", so call that 100" for convenience. Guessing at the center of mass at about 15", that means that 15% of the weight transfers at 1G. My car is around 3000 #'s, so 450#'s gets added to the front and 450 #'s subtracted from the rear. The ratio is (1500+450) / (1500-450) = 1950 / 1050 = 1.857.
The brake piston area ratio front-to-rear is 5.56 / 3.0 or ta-da 1.853.
My system would appear to be perfectly balanced. I have no explanation for why subjectively to myself and observers it feels like the rears are not doing their share, but the maths seem to indicate they must be.
I also have to conclude that the rear cylinder bleed is close enough. That's great because I have no idea how to get anymore air out of them.
The last thing I can think of doing is to get out on the street and see if I can get all four to lock up, which sensibly they should given the numbers involved.
Because you do not have a twin mc set up populate the bore for each to what you have. Then adjust your "pivot offset" to zero as you have no balance bar.
Now..for more rear brake needs and better rotation look for a higher Cf rear pad. One that bumps from say .4 to .5 will make a huge change. Pick one that operates at lower temps however than a hard core track pad.
For giggles and grins take that same into to the full calculator and it will give your the necessary force needed for say .6g decel.
I'll play with the calculator but my main objective is to be at the friction limit.
Originally Posted by Todd TCE
Now..for more rear brake needs and better rotation look for a higher Cf rear pad. One that bumps from say .4 to .5 will make a huge change. Pick one that operates at lower temps however than a hard core track pad.
Hawk autocross pads all around. I don't know that there are other choices for the rears.
I'll play with the calculator but my main objective is to be at the friction limit.
Hawk autocross pads all around. I don't know that there are other choices for the rears.
Same pads all around however will keep that bias the same. Mix it up; put some grippier ones out back. Try talking to the folks at Porterfield Enterprises and see what they can plate up for you. Keep in mind you cna also 'de tune' the front and achieve the same thing.
Just an quick note. I have installed two set’s of Wilwood replacement calibers. No need for shimming and bleeding was easy with rear calibers bolted on. First bled with vacuum and after that the traditional way.
Happy with the weight savings and leak free pistons.
Just an quick note. I have installed two set’s of Wilwood replacement calibers. No need for shimming and bleeding was easy with rear calibers bolted on. First bled with vacuum and after that the traditional way.
Happy with the weight savings and leak free pistons.
06-23-2015, 03:47 PM
