Brake calculations OEM vs different rears

[frankc5r]

As others have noticed, the way FFR has you do the brakes with the prop valve on the front so you can reduce their performance to match the small rears that now have to deal with
60% weight is not the best way. Wayne P has gone to 12.88 Wilwood rears with 4 piston for racing and Bob & Cincy has gone to oem fronts on all 4 corners which works well. Main reason
is oem rears have about 1.7 vs 4.7 sq inches of piston area.

I did some calculations of various setups and what I ended up with which is the Wilwood 12.19 rear setup but with different calipers with twice the area. ( 4 piston)


This is stock oem setup. Note 54 pound pedal force gives about 1/2 g stock


This is with 4 piston rears with 1.35 rears. Now same pedal force gives 0.8 g. To get same you need MUCH more pedal force with stock


This is bias graph. Neg is front bias. It is so front biased its off the graph for stock brakes


This is with Wilwood 4 piston rears and prop valve slightly. Don't want rear bias at high g stops.


This is the weight transfer graph. Still have more rear weight and need for rear brake at a 0.9 g stop

[frankc5r]

Here are the Wilwood rear brakes, 12.19 rear rotors, drilled and slotted with E coating and 4 piston calipers with dual 1.35 pistons

[Frank818]

So what you're saying is to get the Wilwood standard WRX rear setup for the disc but use the Wilwood std WRX front calipers?

[frankc5r]

So what you're saying is to get the Wilwood standard WRX rear setup for the disc but use the Wilwood std WRX front calipers?

No, what I said was some, like Bob & Cincy, used WRX fronts in the rear and it worked well but I used stock WRX in fronts and got the Wilwood rear kit with 12.19 rotors but did not want the
stock caliper in that kit (120-9706) but asked to substitute a 120-9703 that has twice the piston area. the rear caliper in kit( 9706) is almost same as stock and thus does not work very good
except the bigger rotor helps. Some have used a bigger rotor only (H6) with stock wrx rear caliper . That helps some but not the best.
You need almost same piston area in rear as front for best braking.

[Frank818]

Ok this is getting interesting.

Wildwood sorry Wilwood kit consists of

• Dynapro 4 Bolt-On Front Hat & 12" Disc Brake Kit (Part #140-9193) for Both Front Wheels
• Dynapro 4 Bolt-On Rear Hat & 12.19" Disc Brake Kit (Part #140-7006) for Both Rear Wheels

These are the rear ones that come with the Rear Kit: http://www.wilwood.com/BrakeKits/Bra...no=140-7006-DR (120-9706)
Piston area of 1.58.
These are the front ones that come with the Front Kit: http://www.wilwood.com/BrakeKits/Bra...no=140-9193-DR (120-7378)
Piston area of 4.12.
The ones you suggest in the rear are: http://www.wilwood.com/Calipers/Cali...temno=120-9703 (120-9703)
Piston area of 3.00.

Are those 9703 a direct replacement of the 9706, no disc swap, no bracket modification (those that came with kit 140-7006), nothing at all?
I feel my rear brakes don't brake much, I can't get them to lock.

[frankc5r]

You do not really need the front Wilwood unless you are tracking the car. That is why I kept the stock fronts.
I never installed the prop valve on fronts cause I knew it was wrong unless you WANTED to keep low performing rears.

Yes 9703 is a direct replacement for 9706. Bolts right in. You are seeing it in the pictures of 2nd post.

I just did a quick calc with brake program and you need 85 pounds of pressure to get .8 g stop with stock setup.
You and many others have the no lock problem with rears.

[Frank818]

Yeah I knew that big front and tiny rear calipers setup was wrong on such a car. It still performs better than a real WRX of course, but it's not optimal.

150 a piece on those 9703, there goes another part on my list for this winter.

Tnx Frank (not sarcastically, I meant it for real )

I guess you need different pads to go with those calipers?
And with those, no need for a proportioning valve at ALL or you can install it on the rear like it's normally done?

[Hindsight]

Great info thank you for posting this! I didn't like how small the rear calipers were on the Wilwood kit and didn't know they had a caliper with more piston area that would bolt right in and still work with the rotors.

One question though: I think Wilwood makes two different rear kits: a 12.19" and a 12.88" - I think the 12.88" works with 06/07 spindles and the 12.19" works with 02-05 spindles. I take it you have 02-05 spindles?

[frankc5r]

Yes, my donor is 02 WRX. Wayne P uses Wilwood 6 pot
fronts and 12.88 kit in rear.
The caliper change is a no cost option if you order the FFR
designated kit for rear WRX..Why FFR chose such a small
piston area is strange.

If you or anyone wants me to run their car
data thru brake calc program, I would be
happy to do. Look at the 2nd calc sheet to see car
data I used.

[DanielsDM]

Nice job on the calculations Frank.
I'm using a completely different setup, but went with 4 pot 1.75" piston dia. calipers in both the front and the rear. Tilton pedal set with dual masters and a balance bar.

[Hindsight]

Thanks Frank - do you have any idea if the calipers you upgraded to will also work with the 12.88 kit?

[frankc5r]

Nice job on the calculations Frank.
I'm using a completely different setup, but went with 4 pot 1.75" piston dia. calipers in both the front and the rear. Tilton pedal set with dual masters and a balance bar.

Thanks. That should be a great setup.

[frankc5r]

I would think so because I was able to upgrade within same class of caliper..Expect Wayne Presley would
know for sure cause he has 12.88 rotors and 4 pots
on his 818R. Appreciate all the comments.

[Bob_n_Cincy]

Great info frankc5r.
Now I have math to support what I did.
Bob

[Frank818]

I guess you need different pads to go with those calipers?

Not at all, uses the same 7812 Type as the smaller 120-9706 calipers.

And with those, no need for a proportioning valve at ALL or you can install it on the rear like it's normally done?

That, I have no idea, maybe someone else knows?

[frankc5r]

Great info frankc5r.
Now I have math to support what I did.
Bob

Actually your post that a mild adjustment of prop to
rear brakes was a trigger to post the results I got because
the brake calc program predictred exactly rhe results you got. Some data input was a guess since I did no have your
corner weights etc but close enough.

[Bob_n_Cincy]

Actually your post that a mild adjustment of prop to
rear brakes was a trigger to post the results I got because
the brake calc program predictred exactly rhe results you got. Some data input was a guess since I did no have your
corner weights etc but close enough.

With 275lbs driver 1000 front 1355 rear full of all fluids
scale.jpg
Just a note. My car has a 50lbs front an 50 lbs rear bumpers added.

Bob

[frankc5r]

Send me your tire sizes so prog can calc brake
radius and I will recalc and send u results for
your car. Stock pads??
Frank

[Bob_n_Cincy]

Send me your tire sizes so prog can calc brake
radius and I will recalc and send u results for
your car. Stock pads??
Frank

Frank
I'm running Toyo R888 235/40-17 front and 255/40-17 Rears.
CENTRIC StopTech Sport Brake Friction Pads 30909290 pads
https://www.rockauto.com/en/moreinfo.php?pk=2022418
Thanks
Bob

[frankc5r]

This is front vs rear bias without prop valve.


This is with 10% prop


These are your para I used


This is weight transfer graph.

Frank


Here are the calc for your parameters. Not much change. Goes to front bias at 0.8 g -pretty ideal. Assumption was 0.9 prop valve or 10% drop rear line pressure.

[Bob_n_Cincy]

Here are the calc for your parameters. Not much change. Goes to front bias at 0.8 g -pretty ideal. Assumption was 0.9 prop valve or 10% drop rear line pressure.

Thanks Frank,
This is great data. I only understand about 90%. I will figure it out and ask questions later.
I'm and electrical engineer, but Physics was on of my favorite subjects.
Bob

[Frank818]

This is the weight transfer graph. Still have more rear weight and need for rear brake at a 0.9 g stop

Frank, I don't understand all you said on this thread. I'm trying to figure out if those twice-area-1.35 rears would lock the rear wheels, assuming no prop valve installed anywhere.

Do you have an answer to that in your calculations?

[frankc5r]

Look at the calculations sheets fro stock vs 2 x 1.35. The rear retarding force in rear on stock is about 1/3 of front and with 2 x 1.35 its close to same as fronts. So with 3x the force, the answer
is pretty clear-yes , you should be able lock. In fact, it you don't have a prop valve, under hard braking you may be tail happy. That is why you have one on rear.

[Frank818]

Ok that would mean the prop valve needs to move from front to rear, or keep front but add a rear.

How can I re-use your calculations to factor in the different weight my car has?
Officially for inspection I got F902/R1408 but that will slightly be balanced back to front once I lower the front by 1".

[RetroRacing]

Prop valves work fine for street, or if you only hit the brakes once per braking zone, but if you pump the brakes, you can defeat the valve by building pressure. Personally, if your on track, I would use a Wilwood type set up, with a balance bar and tune your pad choice front and rear. We use DTC 70 up front, DTC 60 rear with a pretty square set on the bar and same size masters, as the piston size on our calipers are the same front and rear.

[Bob_n_Cincy]

Frank, I don't understand all you said on this thread. I'm trying to figure out if those twice-area-1.35 rears would lock the rear wheels, assuming no prop valve installed anywhere.
Do you have an answer to that in your calculations?

Frank 818,
when we had no PV in the rears. We would come into the finish line of some autox between 60 to 80 mph. Then we would do a panic stop to see how fast we could stop. I suspect we were in the 1.2 to 1.4 G braking range. The rears would chirp or lock and get a little squirrelly. That is why we added the pv to the rears.

Listen to this braking at the 1:10 mark. this was no PV.

[frankc5r]

Ok that would mean the prop valve needs to move from front to rear, or keep front but add a rear.

How can I re-use your calculations to factor in the different weight my car has?
Officially for inspection I got F902/R1408 but that will slightly be balanced back to front once I lower the front by 1".

Be happy to run your numbers but probably wont be much
change. I need tire sizes, master cyl size, if hole in brake pedal is where ffr said to put,and what rear brakes u intend
to run. You cant just buy WW calipers by themselves. Need to make or buy adapters and rotors or do what Bob did.

[phil1734]

The caliper change is a no cost option if you order the FFR designated kit for rear WRX..Why FFR chose such a small piston area is strange.

The factory five kits are simply the off the shelf offerings from Wilwood for the WRX. I highly doubt they ran any sort of engineering calculations on the system or the changes they made.

You cant just buy WW calipers by themselves. Need to make or buy adapters and rotors or do what Bob did.

The kit from Wilwood has everything one would need to do this swap. http://www.wilwood.com/BrakeKits/Bra...&axle=Rear+Kit You just need to contact Wilwood and have them swap the calipers.

[frankc5r]

The quote about making adapters was for a forum member
who thought the calipers were a bolt on.

[Frank818]

Be happy to run your numbers but probably wont be much
change. I need tire sizes, master cyl size, if hole in brake pedal is where ffr said to put,and what rear brakes u intend
to run. You cant just buy WW calipers by themselves. Need to make or buy adapters and rotors or do what Bob did.

I think you're right, yours are 20kg less than mines so I won't bother seeing the difference, may not make a diff. Thanks!

[2bking]

Does the program predict wheel lockup or does it assume all the breaking force results in stopping force? Seems like some of the g values are too high with the heavy bias on the front wheel.

[frankc5r]

No, it is a simple force calculator. Need alot more parameters to do what u ask.
Graph does not say u get that number of g only that
is the value compared to other axis.
Even at .9 g stop, still more weight
on rear on 818 unlike front bias on wrx.

[Sgt.Gator]

While reading thru a NASIOC thread on Subaru brake interchangeability I found this spreadsheet. It may be very useful to the engineers in here.
I scanned the XLS file, it's free of infections.

NASIOC Thread: https://forums.nasioc.com/forums/sho....php?t=1529797

The Spreadsheet: https://redirect.viglink.com/?format...Fbrakemath.xls