boxsey911

I'd say that Bill has already answered that above by indicating that the standard p/v (55 bar) gives the correct bias to the rear. Those of us with C2s that upgraded from 2 pot to the same 4 pots that you have, will have changed our 45 bar to your 55 bar. If we had left it at 45 bar the bigger rear brakes would be under utilized. If we bypass it, there would be too much rear bias.

KaiB

There is indeed a "safety factor" built in, but the individual driver must determine how much rearward bias he needs.

I, for one, prefer far more rear bias than is typically recommended in threads like these. This is driver dependent, of course.

My car is touch and go and will swap ends quickly if attention isn't paid to driver input, but at the edge, I really like being able to rotate the car at entry and catching it with the throttle.

For guys seriously tracking their cars, (and for those with no intention of racing in a PCA stock class), I would highly recommend tossing the prop valve, running as much rear bias as possible, and then taming it to taste with a simple bar type Tilton bias adjustment valve. I do this and change according to track condtions.

Cheeksyboy

Thanks Kai, maybe an adjustable bias valve is the way I'll go in the future, just doing this as a winter project to my Westy!

Bill Verburg

The factory is always pretty conservative for legal reasons.

It is considered safer to lock the fronts before the rear and abs also muddied the waters

here's what you have now @70bar line pressure, this is a hard but not extreme stop
brake torque front/rear in N w/o p/v then w/ p/v
964 1974/1217 ratio 1.622 1974/657 ratio 3.003
964RS 2381/1390 ratio 1.713 2381/751 ratio 3.171

the Cup cars had the same brakes as the RS but w/o a p/v so they always have the 1.713

you can use lot more rear brake than you get even w/ no p/v

alexjc4

As Bill says ^+1

But you know I can't help myself so I've been spreadsheeting again; a picture's worth a thousand words:

x axis is line pressure in bar, y axis is clamping force in newtons. the full scale of the chart represents roughly the range you'd see in a non downforce car I think.

Assumes:
Front 36/40mm
Rear 2 pot 44mm
Rear 4 pot 30/34mm or 28/30mm
And rate after knee point is 0.5

As ever happy to be corrected...

alexjc4

Thinking about it, perhaps the pv is there to cope with driver behaviour, that is; on the road we mostly brake lightly <50bar and the rears can do their fair share, but when we brake heavier its often an emergency and in an emergency and we may be turning or on a low friction surface at the time in which case you'd rather lock the fronts and go straight on than spin. On a race track we mostly brake hard in a straight line and we want to extract every ounce of performance taking the rears up to their limit. Or we might brake more subtly in turns (less likely to be an emergency stop) but in a controlled manner so you are ready to manage the rotation and if the surface does let you down you just end up in the gravel hopefully

Just a theory, not sure that's what really is happening...

KaiB

Remember, at a certain level, on the track we use the brakes as an important tool to set the balance of the car at and into turn-in. As we improve, we brake less, and often braking becomes less of a straight line adventure.

I just looked at my graphs. My bias valve (the adj Tilton) is currently set at a shade over 60bar - keep in mind that I have 322mm fronts, and have moved the big former front calipers to the rear of the car...this is one hell of a lot of rear brake.

Balance and smoothness is everything in this game.

alexjc4

Kai, I don't suppose your data shows the maximum line pressure you get up to does it?

KaiB

Nope. Bill has torque value and pressures for almost all possible brake combos.

On a smooth, flat track I'll run the valve wide open...given my f/r brake combo, this is as much as these cars can take and demands absolute attention and no mistakes.

ras62

The formula for brake torque on a disk brake system is
Torque = (Brake Pressure) X (Piston Area) X (Effective Rotor Radius) X (Pad Coefficient of friction) X 2

Given this calculation it would mean that the twin 44mm piston caliper has a greater piston area than the later four piston 28/30 caliper and therefore generates more braking torque!
The four pot 34/30 caliper has a 6% greater piston area and has a small increase in torque as indicated on Alex graph. So by changing to the 4 pot calipers some are actually decreasing braking power in many cases.
I guess the biggest benefit of the 4 pot caliper is the larger pad dissipates the heat better so is less prone to fade.

boxsey911

Yep, that is exactly why I upgraded. I found the original 2 pots damn good from a stopping perspective but the fluid soon boiled..and the pedal suddenly gone! Also, there is very little choice of pads for those two pots.

alexjc4

Give me a min I'll add the regular 28/30 caliper on the chart

alexjc4

So here's the brake torques front and rear with and without pv for a few different combinations:

(which unlike the clamp force plots above this takes into account the changes in the rotor diameter, I think the maths is for each wheel but the NM figures may be out by a factor of 2 - but the ratios front and rear will be as per the charts)

Assumes friction coef of 0.5 (track pad)

Kai is running the combo in the top left - with the bias valve wide open the rears follow the green line, you can see why that might be hairy, but also fast. With 55bar pv though there's a reasonable margin for error.

Most 964s with "big reds" leave the stock 4 pot rears, which is bottom left, the rears are really just along for the ride, especially with the proportioning valve in place - all very safe.

Middle left is the next biggest you can go front and rear after Kai's setup without going custom adapters two piece rotors etc. Still quite a step down from Kai's. Maybe you could go with high mu rear pads to balance that up, and just watch the rear temps.

As ras62 points out the 2 pots offer more performance than the 4 pot rears, though unfortunately for only a short period of time before they overheat.

PS all made possible by Bill's excellent data http://members.rennlist.com/1976c38/brakes.htm, and inspired by this ready reckoner for Subaru's from user Legacy777 one nasioc.com http://www.main.experiencetherave.co.../brakemath.xls (forum thread http://forums.nasioc.com/forums/showthread.php?t=720871)

ras62

Nice graphs there Sir! I wonder what sort of brake pressure is actually generated for example on road tyres on a dry road when the ABS is triggered. Anyone have some ballpark figures?

alexjc4

That's a good question.

There is an EC regulation that says you must be able to generate .5g decel with <700 Newtons pedal force. You could work back from that if you know the master cylinder size.

The chain goes:
Pedal force > Brake line pressure > Clamping force > Braking torque > Wheel torque > Deceleration

If you can pin down a real world figure for any of them you can work back and figure the rest if you know the dimensions of the components.

I googled a bit when making my spreadsheet and 50kN clamping force seemed to be mentioned in as a necessary level for electronically actuated brakes to be viable which I used to validate the first charts.

Looking at this today, I think my y axis are out of whack on the second set of charts, I think I've got mm and m mixed up somewhere so the brake torques are 10 or 100 times to high, though the relative slopes fr vs rear are correct - so they still allow comparisons. I'll see if I can fix that. EDIT OK i've updated the charts should be closer now (excel workings are attached.)