My $600 Big Brake Kit

[mike the snake]

Couldn't a proportioning valve be utilized to balance the bias out?

[eikond]

Originally Posted by Dave-ROR

What's the swept area like for the 300z spec pads on those rotors? They were for 280mm rotors IIRC originally so I'm curious how well the pad matches the larger disc radius.

The pads sit on the extreme outside of the these 317mm discs. When I was test fitting them I was actually a bit worried they might "overhang".. where the top of the pads isn't actually touching anything.. That is not the case. The top of the pads is flush or just slightly under the edge of the disc face. I credit KNS Brakes for a good design here.

I should take a side-by-side pic of the pads so we can see size differences. I think the Z32 pads are slightly taller, but not as wide.. maybe a tad smaller in overall surface area. They seem to sweep the top 2/3rd of the LegGT rotor.

I read online that the further away the center of the swept area is, the better the braking power. Anyone else know more about this?

[eikond]

Originally Posted by mike the snake

Couldn't a proportioning valve be utilized to balance the bias out?

I believe pad choice can also impact brake bias as well.


Guys.. I hope I don't come across like I'm trying to "defend my choice" and save face or something like that. I'll share my thoughts on the performance of the brakes through my real world usage (when I get a chance). But I'll also submit to the knowledge of the forum members.. If *we* can do the math and figure out that brake bias is severely impacted or some other input comes forth that makes these turn out to be a bad modification.. then I'll happily admit that the case. So far though.. I like how they feel.

[mike the snake]

I appreciate your posts and this thread! Thanks!

I love to explore alternatives to high-dollar parts, and I'm just trying to gain as much info to determine if this is a workable alternative.

Part of me wants big, cool looking calipers too! I know a big improvement can be made with simply changing fluid, pads, and brake lines.

If I can find salvage yard parts at pennys on the $$ and it works, I'm all for it.

[Dave-ROR]

Originally Posted by eikond

Dave: Where did you read that WRX 4-pots suck? Have you tried them? I've read one review on this forum where the guy said he loved them. Where did you hear that they move the bias toward the rear?

Piston area is smaller, which means less brake torque at a given pressure (which with no other changes is constant for our purposes). That means that given the same rear setup in both cases, bias will shift towards the back. I meant suck for this (and the WRX) application, not that they suck for any other reason (ie, high flex, crap seals, poor construction, et cetera).

Originally Posted by eikond

Here's my math:
The stock BRZ calipers have 48mm pistons
if my high school geometry is correct, that would make for a radius of 24mm. Pi x R ^2 = 1810 x 2 pistons = 3620

The Z32 calipers have 40.45mm pistons
same math gets me 1282 x 4 pistons = 5127

It actually doesn't work like that for piston area on a slider. You measure only one side of both calipers. If you want to measure both, you'd just double the OEM ones (so 7,240 based on your math for the WRX calipers).

Originally Posted by eikond

So, in theory the Z32 caliper should inpute more clamping force right? So I'm not sure why that would result in moving the brake bias toward the rear?

Again, i'm not a physics expert..

Based on the correction above, the piston area is actually smaller than stock, so it'll shift towards the rear if all else is equal. Until now all the comments I said about shifting towards the rear was directed at the WRX calipers as I didn't bother to lookup the Z32 ones. Given the better leverage though it could be a wash (leverage = further from the hub, no different then using a longer lever to apply torque to a bolt vs a shorter lever)

Originally Posted by eikond

One thing I noticed is that the Z32 pads are slightly smaller surface area than the BRZ stock pads. I don't know how much smaller though.
Does a larger pad surface area increase stopping ability? It seems logical that it would.. but I'm not sure about that.

Swept area does have an effect combined with pressure and piston area as well as leverage. However, brake force by itself isn't the issue (OEM calipers have PLENTY - they can lock up R compounds so an increase is force is simply not needed). A potential change in brake bias can lengthen braking distances due to easier lockup (more ABS intervention). This assumes Subaru did the job correctly and optimized the bias as well as they could, which isn't a sure bet given the parts bin nature but clearly they tried (hence the LGT rears with the WRX fronts vs all LGT or all WRX, etc). Prop valve/abs pump/etc play a roll in that tuning as well so we should assume that it's at least pretty good from the factory.

[Dave-ROR]

Originally Posted by eikond

The pads sit on the extreme outside of the these 317mm discs. When I was test fitting them I was actually a bit worried they might "overhang".. where the top of the pads isn't actually touching anything.. That is not the case. The top of the pads is flush or just slightly under the edge of the disc face. I credit KNS Brakes for a good design here.

I should take a side-by-side pic of the pads so we can see size differences. I think the Z32 pads are slightly taller, but not as wide.. maybe a tad smaller in overall surface area. They seem to sweep the top 2/3rd of the LegGT rotor.

I read online that the further away the center of the swept area is, the better the braking power. Anyone else know more about this?

Put it in the other reply, but just think of a breaker bar vs a normal ratchet. Same story.

[Dave-ROR]

Originally Posted by eikond

I believe pad choice can also impact brake bias as well.


Guys.. I hope I don't come across like I'm trying to "defend my choice" and save face or something like that. I'll share my thoughts on the performance of the brakes through my real world usage (when I get a chance). But I'll also submit to the knowledge of the forum members.. If *we* can do the math and figure out that brake bias is severely impacted or some other input comes forth that makes these turn out to be a bad modification.. then I'll happily admit that the case. So far though.. I like how they feel.

You aren't coming across like that. I'm not attacking your choice either. Just having a conversation on a mod.


Yes, pad choice can impact bias (I fine tune the race cars brakes with pad choice often).


Sucks that it's not using all of the friction surface.


If anyone has a correct year (10+?) LGT front caliper you should install that on one side with matching pad material and do some testing, easy way to see which is more effective in terms of force lol

[eikond]

Originally Posted by Dave-ROR

It actually doesn't work like that for piston area on a slider. You measure only one side of both calipers. If you want to measure both, you'd just double the OEM ones (so 7,240 based on your math for the WRX calipers).

I'm not sure I agree with this part..

And again, I'm no physics expert

Here's what I think I know.. Brakes work by having the master cylinder push brake fluid through the lines into the caliper to fill in behind the pistons. That hydrolic pressure pushes the pistons outward and that in turn pushes the pads into the rotor face. The friction created by the pad pushing against the rotor allows you to slow down.

Knowing this.. I don't think you should "double" the calculation for the OEM sliding calipers. They only fill with fluid on one side. The hydro pressure from the master cylinder is only filling in behind two pistons. That force is distributed to the other side as well.. but it's not double force.. it's the same amount of hydrolic fluid.

If anything, I'm worried they have too much front bias.

I'm planning to do the 2 pistons rears as well.. just need to come up with the extra $400 to finish it off. Doing so should help even out the brake bias (whether the Z calipers have more or less force.. it should help equal it out going to fixed on all 4 corners).

Any thoughts?

[Dave-ROR]

Originally Posted by eikond

I'm not sure I agree with this part..

And again, I'm no physics expert

Here's what I think I know.. Brakes work by having the master cylinder push brake fluid through the lines into the caliper to fill in behind the pistons. That hydrolic pressure pushes the pistons outward and that in turn pushes the pads into the rotor face. The friction created by the pad pushing against the rotor allows you to slow down.

Knowing this.. I don't think you should "double" the calculation for the OEM sliding calipers. They only fill with fluid on one side. The hydro pressure from the master cylinder is only filling in behind two pistons. That force is distributed to the other side as well.. but it's not double force.. it's the same amount of hydrolic fluid.

If anything, I'm worried they have too much front bias.

I'm planning to do the 2 pistons rears as well.. just need to come up with the extra $400 to finish it off. Doing so should help even out the brake bias (whether the Z calipers have more or less force.. it should help equal it out going to fixed on all 4 corners).

Any thoughts?

Equal and opposite reaction......


Do the rears fit? I can't recall 300z rear brake specs, haven't installed them for years it seems like. By fit I just mean wide enough to fit new pads and the rotor since you can make any caliper "fit"

[mike the snake]

Found these. The guy is selling brand new (front only) calipers, rotors, lines and pads for $750 (pics are of used ones).

More than I want to spend, but maybe someone might want to jump on these.

https://sfbay.craigslist.org/nby/pts/4547823750.html

[Dave-ROR]

Originally Posted by Dave-ROR

Equal and opposite reaction......


Do the rears fit? I can't recall 300z rear brake specs, haven't installed them for years it seems like. By fit I just mean wide enough to fit new pads and the rotor since you can make any caliper "fit"

Just checked, width is identical so rock on Diameter is close too so pad should fit nicely assuming a good bracket. Didn't check piston sizes or do any math

[eikond]

Originally Posted by mike the snake

Found these. The guy is selling brand new (front only) calipers, rotors, lines and pads for $750 (pics are of used ones).

More than I want to spend, but maybe someone might want to jump on these.

https://sfbay.craigslist.org/nby/pts/4547823750.html

Those are not oem z brakes. Wilwood calipers, etc.

Ebay usually has a dozen listings for full sets of 4 calipers for 300 to 350.
Salvage yards are normally under 75 per front caliper. Rock auto or other online stores have them too. Hell.. i found mine on amazon of all places.

[Ryephile]

Great find @eikond!

I have some questions though, since my Google-fu is terrible right now:
*Aren't the stock front calipers 2-piston sliders with 42.8mm pistons?
*Your Z32 TT's are fixed calipers with 4x 40.4mm pistons?

If that's all true, then the piston areas are 2,877mm^2 and 5,128mm^2 respectively. That would be a massive front-bias movement. Sorry @Dave-ROR, your equal and opposite reaction idea isn't a perpetual machine that creates magi-double piston area in the formula. There's a fixed amount of piston area, and thus force, for any particular caliper, regardless of it's piston amount and orientation.

Now, what's happened here is since there's so much more front piston area, the master cylinder requires much more pedal travel to get the same line pressure. Chances are the car now has a longer and softer pedal. The pad mu is totally unknown and of course impacts brake torque significantly.

Using Legacy777's spreadsheet, the rotor diameter change is resulting in about a 5% front bias change.

That would all add up to a huge front bias increase. I don't feel like calculating it right now, plus without knowing the pad mu's it's fruitless.

If I were tackling this from scratch I'd want to find a front caliper with smaller pistons, but for $600, an inch more rotor diameter and 6mm more thickness, on top of EBD that everyone has seemed to forget about, this is probably a decent DIY solution even for light track use.

Bravo!

[eikond]

Originally Posted by Ryephile

Great find @eikond!

I have some questions though, since my Google-fu is terrible right now:
*Aren't the stock front calipers 2-piston sliders with 42.8mm pistons?
*Your Z32 TT's are fixed calipers with 4x 40.4mm pistons?

If that's all true, then the piston areas are 2,877mm^2 and 5,128mm^2 respectively. That would be a massive front-bias movement. Sorry @Dave-ROR, your equal and opposite reaction idea isn't a perpetual machine that creates magi-double piston area in the formula. There's a fixed amount of piston area, and thus force, for any particular caliper, regardless of it's piston amount and orientation.

Now, what's happened here is since there's so much more front piston area, the master cylinder requires much more pedal travel to get the same line pressure. Chances are the car now has a longer and softer pedal. The pad mu is totally unknown and of course impacts brake torque significantly.

Using Legacy777's spreadsheet, the rotor diameter change is resulting in about a 5% front bias change.

That would all add up to a huge front bias increase. I don't feel like calculating it right now, plus without knowing the pad mu's it's fruitless.

If I were tackling this from scratch I'd want to find a front caliper with smaller pistons, but for $600, an inch more rotor diameter and 6mm more thickness, on top of EBD that everyone has seemed to forget about, this is probably a decent DIY solution even for light track use.

Bravo!

Thanks @Ryephile I appreciate you sharing some of your engineering genius with us.

I thought for sure I saw that the oem caliper had 48 mm pistons. I'm sure it still results in more front bias.. but I don't think as bad as you calculated at 42.8mm.

I need to find rear piston sizes and see how those change and how that affects overall bias

thanks again for your input

oh.. and you are correct.. the pedal does feel a bit longer. I was going to bleed them once more just in case I had a little air. But that makes sense.