More cost effective Brake upgrades ??

[Roy_Josef]

I seem to remember someone saying that when you put Box 'S'/996 front brakes on you can take your 'old' front calipers and rotors and put them on the rear. (Front left to right rear, Front right to left rear) Any truth to this?

[shlim8]

Hmmm, thanks for all the inputs.

I've currently got pretty new Michelin PS3 upfront and PS2 behind (both less than 8k km).... Made the mistake of changing the front to PS3 then realized that they don't make PS3 of the right size for the rears. The fronts are not N specs. But I presume this shouldn't be the cause of my problem ?

Oh, my front calipers were rebuilt afew months ago (one side sticking).

Does the ABS system on the Box come on to prevent ALL wheel locks ?? I did lock my front wheels once or twice and the ABS didn't come on.. No ABS warning light on at the moment. I've never felt the ABS coming on before at all (I presume there would be juddering feeling from the brake pedal ?).

Maybe I need to have that looked at first. Maybe a faulty ABS sensor ??

[insite]

Originally Posted by Roy_Josef

I seem to remember someone saying that when you put Box 'S'/996 front brakes on you can take your 'old' front calipers and rotors and put them on the rear. (Front left to right rear, Front right to left rear) Any truth to this?

you CAN do this, but DON'T! the brake system is designed to have smaller caliper pistons in back than up front. the pistons on the boxster front are the same size as the boxster s front. this means that by putting the front calipers on the back, you change the brake bias.....a lot.

since the car dives when you brake, roughly 70% of the car's weight is over the front tires during a heavy stop. this means that about 70% of the car's braking power is designed into the front brake system. if the rear brakes are too big, the rears will lock up well before the fronts (particularly if braking in corners). the potential for spins is very real. just ask pedro.

[ekam]

Well first of all that's called trail braking. As a novice you're not supposed to have your foot on the brake when you enter the corner anyway. It's an advanced driving technique and definitely not for someone who's not experienced.

That being said, I'm going to do the swap, but don't to what I do. It's bad for you.

[insite]

it's not called trail braking on the street. how about a panic stop in wet weather on a slight bend? the bias is too heavily rear for street OR track. if you do this, it's not a matter of if, but WHEN you will spin (street or track). a brake system is supposed to be balanced. moving the fronts to the back is a dumb idea.

Originally Posted by ekam

Well first of all that's called trail braking. As a novice you're not supposed to have your foot on the brake when you enter the corner anyway. It's an advanced driving technique and definitely not for someone who's not experienced.

That being said, I'm going to do the swap, but don't to what I do. It's bad for you.

[Topless]

Shlim8,

Sounds like you indeed have brake issues. Take your car out to a barren road and do a few full simulated panic stops from 50-0. Your car should go into ABS judder every time with no wheel lock up. If it does not you need some brake work. My car is really set up for track rubber and I get used to threshold braking and very high braking forces. When driving on the street with stock 205mm PS2 tires I get into ABS a lot.

[thstone]

I had a brake situation similar to the OP when I first bought my used Boxster - I'd slam on the brakes at about 50mph and it seemed like the car just didn't want to stop and the ABS wouldn't kick in. I also tried pad break-in runs with no improvement.

I also replaced the tires with Michelin PS2's. Whether PS2 or PS3, the tires should now be fine and giving you all of the grip needed to stop very well.

I decided to start at the obvious and work my way up so the first thing do to was to replace the rotors and pads (I went with Cquence drilled rotors and EBC Redstuff pads but any good rotor and pad would be fine for the street). I also installed the GT-3 brake ducts at the same time (cheap and easy). Didn't touch the fluid or ABS system.

Now I can stop on a dime and as another writer commented, I have to be careful about out-braking the car behind me on the street and worry about getting rear-ended. Also, now if I get on the brakes hard, I can get the ABS to engage and feel the judder.

If this hadn't addressed the issue, then a full brake fluid flush would have been next.

If pads, rotors, and fluid hadn't worked, I'd have taken the car in to a mechanic or dealer for evaluation and see what they recommended.

[shlim8]

thstone, before u did all the mods on your brakes.... did your brakes lock but the abs didn't kick in ? Or the car just refused to stop but brakes didn't lock ? In my case, I think the brakes are working well but the abs has yet to cut in. It does take pretty hard braking to lock the brakes.

[Roy_Josef]

How about a brake bias valve off another car? We used to do this (a lot!) right or wrong back in the day for both the 914 and the 944.

Originally Posted by insite

you CAN do this, but DON'T! the brake system is designed to have smaller caliper pistons in back than up front. the pistons on the boxster front are the same size as the boxster s front. this means that by putting the front calipers on the back, you change the brake bias.....a lot.

since the car dives when you brake, roughly 70% of the car's weight is over the front tires during a heavy stop. this means that about 70% of the car's braking power is designed into the front brake system. if the rear brakes are too big, the rears will lock up well before the fronts (particularly if braking in corners). the potential for spins is very real. just ask pedro.

[insite]

Originally Posted by Roy_Josef

How about a brake bias valve off another car? We used to do this (a lot!) right or wrong back in the day for both the 914 and the 944.

this is actually okay. rather than use one from another car, you can replace the proportioning valve with an adjustable one. they allow 'full force' to the fronts & let you limit pressure to the rear until appropriate bias is achieved.

just be sure you know what you're doing first. rear-bias settings that seem fine in a straight line on dry pavement may be to aggressive for wet weather & curves.

[Roy_Josef]

Thanks, good to know! I have all the bits necessary to do this and was thinking of doing it when I have everything apart to install my coil overs when warmer weather finally arrives. Any suggestions on a 'perfect fit' bias valve? I remember when we did 'Medium Red' brakes from the 944 Turbo 'S' on the regular 944 we would use the 928 S4 bias valve and when we did did the BMW 320i front brakes on the 914 we would completely take out the factory bias valve and install a VW 'Tee' bias valve fitting instead. Does nothing with a Porsche part number on it work?

Originally Posted by insite

this is actually okay. rather than use one from another car, you can replace the proportioning valve with an adjustable one. they allow 'full force' to the fronts & let you limit pressure to the rear until appropriate bias is achieved.

just be sure you know what you're doing first. rear-bias settings that seem fine in a straight line on dry pavement may be to aggressive for wet weather & curves.

[insite]

roy -

you'd have to scale the pressure back proportionally to the area of the total piston size for the calipers.


BEGIN ENGINEER SPEAK:

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the fronts, if you put them on the back, will now have 40mm leading pistons and 36mm trailing pistons. the OEM rears have 30mm leading pistons & 28mm trailing. the difference in disk diameter is only 6mm and negligeble.

so, the new rears would have total area of 9100 square mm (remember, two of each piston per caliper) and the old rears would be 5300 square mm.

now let's consider the OLD system. the fronts were 9100, the rears were 5300 and the OEM proportioning valve has a .46 reduction factor. if we multiply the 5300 by .54, we get 2860 sq mm equivalent area from the rears.

back of the envelope to check our work: roughly 75% of the braking is done by the front. what is 9100/(9100+2860)? it's about 76%, so we are spot on.

now let's calculate the reduction factor required to give us the same proportion as OEM, front to rear.

our NEW boxster S calipers have, believe it or not, the SAME piston sizes as the base boxster. the disks are 20mm bigger, though, so first we need to calculate the EQUIVALENT area at the OLD disk diameter. let's assume that the center of the brake pad works on roughly the outer 40mm of the disk. this means that the OLD lever arm is 109 mm and the NEW lever arm is 119mm. the increase in braking torque is then 10/109 = 9% with equivalent piston sizes & system pressure.

so now we increase our 9100 by 9% to get the EQUIVALENT piston area in terms of the old system: 9920.

so what proportioning valve reduction will get us from 9920. to keep the same bias as before, we need our rears to be reduced to (9920 / .74)*.26 = 3485.

we need to reduce our new 9100 sq mm effective rear piston size to 3485. this is a reduction of 5615, or .62.

BTW, proportioning valves keep the F/R bias at 1.0 (NO reduction to rear force) until some pre-set pressure value. porsche's factory valve begins biasing at 25bar / 360PSI.
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END ENGINEER SPEAK


you want: proportioning valve w/ a reduction factor of .62 and a divergence pressure of 25bar.

or an adjustable valve like this one:

http://www.amazon.com/Wilwood-260-8419-Adjustable-Proportioning-Valve/dp/tech-data/B002G37IEA

take care.

[R-J]

Awesome explanation! I had to read it a couple of times but it makes sense. Do you teach this stuff, because you do it well...

I was hoping for a Porsche bolt in solution to get away from re-plumbing/bending the brake lines to use this aftermarket solution or is it less complicated than it seems?