• From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Topic: Help.. too much rear brake! What direction do we go?

I thought it would be a great idea to take the HUGE Wilwood brakes off of our old Caprice and put them on the little RX7 we run now.  The problem is that we now have too much rear brake, and I'm out of adjustment and am at the point of needing to change hard parts and am conflicted on what to do next.  On the caprice we ran the factory master, which physically won't mate up to anything on the mazda.   On the Caprice we had no issues with the setup, but he car was 1200 lbs heavier and had a much longer wheelbase. 

Current setup is:
6 piston front calipers (1.75, 1.375,1.375 pistons)
4 piston rear calipers (1.375, 1.375 pistons)
1.125" straight bore master
Factory booster
wilwood distribution block & prop valve
13" rotors all around
Same compound pads all around (hawk DTC60)
same diameter tires all around.

Even with the prop valve turned all the way out (max rear restriction) the car will wheelhop near the end of the braking zone in the rear under heavy braking in the dry, and is un-driveable in the wet- the rear just locks first and comes around. 

My initial thought was to go to 1.25" piston rears to reduce clamping force and also go to a DTC50 in the rear.   When I called Wilwood to confirm, the tech (who seemed to be unsure of himself) said I should actually go BIGGER on rear pistons so that they take more volume to move.    My issue is that while in theory this sounds good, the next size up piston for our caliper is 1.5" so it would result in a significant increase in clamping pressure, and in my mind, make the problem worse. 

So here are the ideas we're bouncing around in addition to going with the less aggressive rear pad:
1- go to smaller piston rear calipers
2- go to larger piston rear calipers
3- add a restriction to the rear brake line before the prop valve to reduce the volume of fluid
4- try to find a stepped bore master

or any other ideas ?

Team Orca
  • From: Elverson, PA
  • Registered: 2017-08-15
  • Posts: 1,308

Re: Help.. too much rear brake! What direction do we go?

BigPappy wrote:

[snip] When I called Wilwood to confirm, the tech (who seemed to be unsure of himself) said I should actually go BIGGER on rear pistons so that they take more volume to move.    My issue is that while in theory this sounds good, the next size up piston for our caliper is 1.5" so it would result in a significant increase in clamping pressure, and in my mind, make the problem worse. 
[snip]

Interesting issue.

I believe the Wilwood tech suggested this so that the front brakes lock up first. If the rear brakes lock up after that, presumably the driver would be getting off the brakes already, or at least be able to control the car.

Just to make it easy for me, here's my hypothetical:

Let's assume the volume of the fluid is initially the critical part of the system - and _not_ either the pad contact area or clamping pressure.

Using your piston diameter data, and desiring a 1.0" piston travel, here's a volume comparison:

F: 1.75+1.375+1.375 = 4.50 x 3.1416 = 14.1372 cu.in. fluid
R:           1.375+1.375 = 2.75 x 3.1416 =   5.6947 cu.in. fluid

Looks like you need about 2 5 times rear line fluid volume to make the front calipers move the same distance - when, presumably, contact area and clamping force would come into play.

Please - somebody- school me, also.

  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

ye, tech is very wrong lol. Your bias is 55.7% front and 44.3% rear. In my experience front bias of 75% is pretty aggressive but works. 80% is nice and mild predictable brakes.
If you do DTC80 front and DTC50 rear, that will bring bias down to 64.5%F / 35.5%R.
If you keep DTC60 all around and use dual 1" diameter piston caliper, that be 70.4%F/29.6R. Basically if you change caliper, need something in 1.25 in^2 piston area.
I am curious if you put 2 bias adjusters in line, does it work? got to think about it, but I think it just might.
If you find a good stepped bore master cylinder, let me know; been looking for them.

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This car....Is said to have a will of it's Own. Twisting its own body in rage...It accelerates on.
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  • From: Camarillo, CA
  • Registered: 2021-03-26
  • Posts: 431

Re: Help.. too much rear brake! What direction do we go?

I would start with running a less aggressive rear pad. It's not uncommon for people to mix DTC30/60, R10/R12, etc to adjust their bias without spending extra $. Typically people will adjust bias rear-ward and run factory brakes/prop valve, but nothing wrong with going the other way around.

Also make sure that your suspension and tires are sorted. Our first test and tune day we thought we had brake issues, rear kept locking up with the lightest of brake pedal touches, but turned out we forgot to bolt the sway bar in completely which was giving us dynamic sway bar engagement, and the tires were heat cycled to rocks. We tightened the sway bar, got new tires, stepped down to "sport" pads to reduce the bite and everything was perfect. As time went on, and we got sick of replacing $30 sport pads every 6 hours, we tried going back to an endurance pad and everything feels fine. Long story short, unless the BBK was the only variable you changed, don't rule out suspension or tire gremlins leading you down the wrong path.

Full Ass Racing (Captain)
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  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

Lemon_Newton-Metre wrote:
BigPappy wrote:

[snip] When I called Wilwood to confirm, the tech (who seemed to be unsure of himself) said I should actually go BIGGER on rear pistons so that they take more volume to move.    My issue is that while in theory this sounds good, the next size up piston for our caliper is 1.5" so it would result in a significant increase in clamping pressure, and in my mind, make the problem worse. 
[snip]

Interesting issue.

I believe the Wilwood tech suggested this so that the front brakes lock up first. If the rear brakes lock up after that, presumably the driver would be getting off the brakes already, or at least be able to control the car.

Just to make it easy for me, here's my hypothetical:

Let's assume the volume of the fluid is initially the critical part of the system - and _not_ either the pad contact area or clamping pressure.

Using your piston diameter data, and desiring a 1.0" piston travel, here's a volume comparison:

F: 1.75+1.375+1.375 = 4.50 x 3.1416 = 14.1372 cu.in. fluid
R:           1.375+1.375 = 2.75 x 3.1416 =   5.6947 cu.in. fluid

Looks like you need about 2 5 times rear line fluid volume to make the front calipers move the same distance - when, presumably, contact area and clamping force would come into play.

Please - somebody- school me, also.

the clamping force is area of piston * line pressure * pad mu. the volume only plays into how far the pedal needs to travel before pads touch the rotors.

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This car....Is said to have a will of it's Own. Twisting its own body in rage...It accelerates on.
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  • From: Elverson, PA
  • Registered: 2017-08-15
  • Posts: 1,308

Re: Help.. too much rear brake! What direction do we go?

kakarot1232001 - Thank you!

[*** Sorry - my disclaimer of not knowing anything about brakes didn't make it into my post (it _was_ in an earlier draft - I swear) ***]

And, thanks for being kind.

  • From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Re: Help.. too much rear brake! What direction do we go?

thanks for replies so far...   I think we're good on chassis- when we ran the stock mazda stuff there were no issues except longevity.   Tires were all new, susp had been nut and bolted ahead of time.   I definitely will change pads, but I think the pad alone won't get me there.

Team Orca
  • From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Re: Help.. too much rear brake! What direction do we go?

Interesting issue.

I believe the Wilwood tech suggested this so that the front brakes lock up first. If the rear brakes lock up after that, presumably the driver would be getting off the brakes already, or at least be able to control the car.

Just to make it easy for me, here's my hypothetical:

Let's assume the volume of the fluid is initially the critical part of the system - and _not_ either the pad contact area or clamping pressure.

Using your piston diameter data, and desiring a 1.0" piston travel, here's a volume comparison:

F: 1.75+1.375+1.375 = 4.50 x 3.1416 = 14.1372 cu.in. fluid
R:           1.375+1.375 = 2.75 x 3.1416 =   5.6947 cu.in. fluid

Looks like you need about 2 5 times rear line fluid volume to make the front calipers move the same distance - when, presumably, contact area and clamping force would come into play.

Please - somebody- school me, also.

I agree-  I think for a street car this makes sense.   But we were seeing lockup of the rear near the middle / end of braking zones so I'm thinking clamping force is more critical than timing.   Just driving in the pits at 25 or so, they would all lock at the same time, but we're not unloading the rear as much at that low speed. 

Based on a my calculations / pedal ratio etc, currently our front clamp force is 5500 lb and takes .32 cu/in to move all 12 the front pistons .015",   rear clamping force is 3071 lb and takes .176 cu/in to move the rear pistons .015".   

if we went to 1.25" pistons it would drop the force to 2538 lb but then only take .144 cu/in to move them.   

So smaller pistons would reduce the clamping force by about 16%, but yes, they would engage even sooner... but with the reduced force would they "lock up" sooner?  My gut says no, but the math on this is beyond me.

Team Orca
  • Registered: 2020-02-06
  • Posts: 19

Re: Help.. too much rear brake! What direction do we go?

Maybe the system doesn't like the brake booster.

  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

Lemon_Newton-Metre wrote:

kakarot1232001 - Thank you!

[*** Sorry - my disclaimer of not knowing anything about brakes didn't make it into my post (it _was_ in an earlier draft - I swear) ***]

And, thanks for being kind.

smile

https://www.facebook.com/greatglobsofoil/
This car....Is said to have a will of it's Own. Twisting its own body in rage...It accelerates on.
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  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

BigPappy wrote:

Interesting issue.

I believe the Wilwood tech suggested this so that the front brakes lock up first. If the rear brakes lock up after that, presumably the driver would be getting off the brakes already, or at least be able to control the car.

Just to make it easy for me, here's my hypothetical:

Let's assume the volume of the fluid is initially the critical part of the system - and _not_ either the pad contact area or clamping pressure.

Using your piston diameter data, and desiring a 1.0" piston travel, here's a volume comparison:

F: 1.75+1.375+1.375 = 4.50 x 3.1416 = 14.1372 cu.in. fluid
R:           1.375+1.375 = 2.75 x 3.1416 =   5.6947 cu.in. fluid

Looks like you need about 2 5 times rear line fluid volume to make the front calipers move the same distance - when, presumably, contact area and clamping force would come into play.

Please - somebody- school me, also.

I agree-  I think for a street car this makes sense.   But we were seeing lockup of the rear near the middle / end of braking zones so I'm thinking clamping force is more critical than timing.   Just driving in the pits at 25 or so, they would all lock at the same time, but we're not unloading the rear as much at that low speed. 

Based on a my calculations / pedal ratio etc, currently our front clamp force is 5500 lb and takes .32 cu/in to move all 12 the front pistons .015",   rear clamping force is 3071 lb and takes .176 cu/in to move the rear pistons .015".   

if we went to 1.25" pistons it would drop the force to 2538 lb but then only take .144 cu/in to move them.   

So smaller pistons would reduce the clamping force by about 16%, but yes, they would engage even sooner... but with the reduced force would they "lock up" sooner?  My gut says no, but the math on this is beyond me.

So the earlier part of clamping dont matter one bit. there is almost no brake force generated until all 4 calipers bring the pads in; in fully functioning system. This is due the master cylinder balancer. basically when you press the pedal, the master cylinder chamber 1 and 2 moving together, and the pressure they generate would be the same in a straight bore. aka the chamber 1 and chamber 2 oppose each other, so pressure must be equal or else. when one of the circuits fail, than the chamber would have to collapse than all force is on the other chamber. that is why when system leaks, you have to push the pedal 1/2 way before other chamber kicks in.

is the 1.25" single piston?
If you really want to get math heavy, need to get the height of your center of gravity. than its basically straight easy math to get the right brakes setup.

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  • Registered: 2020-02-06
  • Posts: 19

Re: Help.. too much rear brake! What direction do we go?

Is the stock, fixed proportioning valve still connected?

  • Registered: 2015-10-19
  • Posts: 20

Re: Help.. too much rear brake! What direction do we go?

"Using your piston diameter data, and desiring a 1.0" piston travel, here's a volume comparison:

F: 1.75+1.375+1.375 = 4.50 x 3.1416 = 14.1372 cu.in. fluid
R:           1.375+1.375 = 2.75 x 3.1416 =   5.6947 cu.in. fluid"

Bad math here. Area is Pi*R^2 or 1/4*Pi*D^2.

F: 3.14/4*1.75^2 + 3.14/4*1.375^2 + 3.14/4*1.375^2 = 5.37
R: 3.14/4*1.375^2 + 3.14/4*1.375^2 = 2.97

You're currently at 64% front, 36% back.

The other wild card here is you brake rotor diameter. If you rears are bigger than your fronts it could cause an additional bias.

  • From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Re: Help.. too much rear brake! What direction do we go?

thanks kakarot1232001  makes sense to me.... so basically it sounds like Wilwood folks gave me bad advise to go bigger.   The 1.25" calipers would still be 4 piston- just like the 1.375" rears that are on the car now.

couple answers to other questions and notes:
1- the stock distribution block is gone- we're running a Wilwood adjustable combo valve
2- the rotors are same dia front and rear as are tires


The reason my volume calculations are so low is that when trying to figure all this out I wanted to have an idea on where we'd be on pedal travel, so I picked .015" of piston stroke as a safe # which seemed to work out well.   

I took apart our old master from the caprice- it's a straight bore 1.25".  I went smaller with the mazda to get some more pedal stroke due to pedal ratio being different.

Team Orca
  • From: Elverson, PA
  • Registered: 2017-08-15
  • Posts: 1,308

Re: Help.. too much rear brake! What direction do we go?

weakhobo - good catch - thanks!

  • Registered: 2015-10-19
  • Posts: 20

Re: Help.. too much rear brake! What direction do we go?

I would start by downsizing your rear calipers. If it was my car I would  want to be able to leave the adjustable proportioning valve open in the dry and start turning it on in the rain as needed.

  • From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Re: Help.. too much rear brake! What direction do we go?

Well, 4 days later Wilwood answered my tech email:

"You have two options to reduce the amount of rear brake bias in the car. You can go to a pad compound with less friction, calipers with smaller pistons, or possibly a  combination of the two. Both options will reduce the rear brake bias. "

Finally I have confirmation of my bias!!!  This completely contradicts TWO tech people I spoke with at their place.

Team Orca
  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

weakhobo wrote:

I would start by downsizing your rear calipers. If it was my car I would  want to be able to leave the adjustable proportioning valve open in the dry and start turning it on in the rain as needed.

When it rains, got to get more rear bias. Cause there is less weight shift forward; so rear start to work more proportionally. Still got to be able to modulate the brakes properly.

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Re: Help.. too much rear brake! What direction do we go?

BigPappy wrote:

Well, 4 days later Wilwood answered my tech email:

"You have two options to reduce the amount of rear brake bias in the car. You can go to a pad compound with less friction, calipers with smaller pistons, or possibly a  combination of the two. Both options will reduce the rear brake bias. "

Finally I have confirmation of my bias!!!  This completely contradicts TWO tech people I spoke with at their place.

Having done PR/Mailbag stuff for an aftermarket company before, I can tell you that it's frequently an enormous PITA to get in-depth tech questions answered by someone who knows what they're talking about. It's ungood that they first gave you an answer without saying something like, "Hey, that's a good technical question so I'm going to take the time to get you the right answer."

But YMMV.

Eric Rood
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eric@24hoursoflemons.com

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  • Registered: 2013-11-15
  • Posts: 699

Re: Help.. too much rear brake! What direction do we go?

Are you absolutely sure they are bled fully?  I had a heck of a time with rear lockup on my car until I got a Motive power bleeder and finally got all the air out of the system.

Team whatever_racecar #745 Volvo wagon
  • From: Buffalo Grove, IL
  • Registered: 2013-03-14
  • Posts: 55

Re: Help.. too much rear brake! What direction do we go?

So final update… the smaller piston rear calipers seem to have done the trick

Team Orca
  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

BigPappy wrote:

So final update… the smaller piston rear calipers seem to have done the trick

nice!

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  • Registered: 2013-12-09
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Re: Help.. too much rear brake! What direction do we go?

just go buy an adjustable proportioning valve from Summit.

"get up and get your grandma outta here"
  • From: Queens, NY
  • Registered: 2014-05-17
  • Posts: 635

Re: Help.. too much rear brake! What direction do we go?

Mr.Yuck wrote:

just go buy an adjustable proportioning valve from Summit.

he already has that one.

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  • Registered: 2014-11-11
  • Posts: 152

Re: Help.. too much rear brake! What direction do we go?

This whole thread is biased! Fake news!