What is the bigger factor, pad surface area or over piston size?

Are 2 or 4 larger pistons better than 6 or 8 smaller ones?

 

I think I may have been blowing smoke in yer *****!!! After looking at it closer, I don't think pad area comes into the equation as I originally......so yes, I agree with Todd that the line pressures for the AP system will be higher, 40% higher given the same cF(don't know what the stock cF would be).

Sorry about the multiple and confusing posts, I haven't worked with the brake math for a while now!

Pros of fewer pistons:
-Cheaper
-Less seals to damage upon installtion(Important when you do a few million/year)
- Caliper winds up to be physically smaller.

Pros of more pistons:
- Able to apply an even pressure to a larger, more banana shaped disc. Look at the AP website and check out some of the big racing calipers. Due to wheel size restrictions in racing, engineers need to package a lot of pad area on a smaller rotor. Therefore you wind up with long curved brake pads that need a bunch of small pistons to push on. Also small pistons can mean shorter(in height) pads, so the rotor can have a big outer radius and also retain a big inner radius.

Randy

 

All this makes for good conversation eh?

The size of the pad in general has no impact on the brake torque. The larger the pad, the longer it lasts. BUT you can correctly state that the mean radius of the pad comes into play: thus a tall skinny pad is more effective than a short fat one.

I did not crunch the numbers as far as you have here, but it's pretty much what I thought. The less piston area up front is not a minor change as I and others often do, but rather a very big change. You can often reduce front area for three reasonsl; pedal feel, larger rotors and higher Cf pads. This when done yields the same or close original bias in the car.

If you go too small as here the rear pressure is going to be way up on it. In this case a Prop valve is in order at the least.

I'm going to crawl under my truck and see what's up down there. There is really little market for a rear kit in my mind, but maybe a modest caliper change to keep things in check appearance wise. You say the rear piston is 2"? That's about 3+sq" so a small four pot of about 4 x 1.38 sounds about right.

Also the radial mount is nice but I'm certain the bracket for doing so on the rear will be a bit harder this way too. And still there's that nose to hat problem....hmmm, that'll effect my ideas too. Damn!

 

Todd how des the ABS play into all this?

 

Not RPV but Prop valve to the above.

ABS is uneffected by any of these so long as the sensors and such are still in place. They simply measure rotation. Or lack of actually.

 

I guess what I meant was does the ABS not have a built in proportioning valve? If an aftermarket valve in introduced how will that affect the ABS if at all?

I am too far into this to turn back so I need to resolve any issues now.

 

I don't believe the ABS system has a proportioning function in it. It is out of the loop until it senses wheel-lock up(the reason why bleeding our brakes without cycling the ABS isn't bleeding them completely).

The stock rear piston diam is 2", rotor diamter is 13.2" and the mean effective radius should be pretty large(thin rubbing surface). The rotor thickness is also pretty small.

I remember from an old post that someone had recorded some high rear rotor temperatures, higher than we'd expect. Any chance of getting a vented rotor back there easily?

I can't find the piston diamter of the C4 Corvette rear caliper, but if it close to what we need, it's what I will probably try to use. It has an integral e-brake and fits an .810" rotor (12.3" diam??).

Ayrton, what rear rotor size are you using with the CP5200?

Randy

 

Okay. Now it's time for me to throw some more stuff against the wall.

I have always thought that my truck has incredibly bad pedal feel. The pedal has been spongy from day 1 (I much prefer the immediacy and firmness of Porsche brakes).

When at a stop with the engine running, it takes little pressure to bottom out the master cylinder. Granted, if you applied this much pressure while running, the ABS would kick in. But it still seems to me that the pedal should require much more effort. All of the free play bugs me also. I would like to be able to just touch the brakes to begin slowing. I rented a Focus one day. The first time I put on the brakes, I almost went through the windshield. Just the way I like it.

And another thing that bothers me is the little love taps that the ABS gives me even on low-pressure stops, or when turning the front wheels while braking.

In general, I think that the L brakes are crap from a "feel" standpoint. Yeah, they stop the truck, but little more good can be said about them. The Brembos definitely gave a firmer pedal, but I attributed it to a good bleeding. Form reading the above, the smaller piston area may be responsible. If so, I would welcome some really tiny pistons.

 

Same as OEM (330mm) but it is 1.25 thick...

 

Tim,
I'm so glad to hear someone else is frustrated with their brakes like I am. I have taken my truck to the dealer several times complaining about lack of feel and the ABS kicking in for no reason. Ford just looks at me like I'm stupid and says "oh that is normal". BS...my 99 L never acted like this and neither has any other car I have owned, Ford or other.

Does anyone else have this problem? How about pulling up to an intersection after coming off a freeway exit ramp and just have the pedal go to the floor and then the ABS kicks in. I have warned my dealer that when I finally have a wreck, and I believe I will, that I blaming them and will sue.

is there anything that can be done for this?

 

My brakes on my 03 suck also! My 99 had great feel. I have the same problem as Tim described when braking while turning. The problem that drives me crazy is completely different. When I make a long slow gradual stop, right before a get completely stopped, the pedal will all of a sudden get hard! At that point I can push hard as hell and the truck does not stop any faster! This has only happened six or seven times in 8000 miles. This could be trouble if it happens going into a turn at high speed at the track. It has never happened when braking hard......only when coming to a stop very slowly. My pedal feel did not change at all when I changed to the TCE big brake kit.
I also have another problem. Just like when your rotors need to be turned because of uneven pad deposits, I get a mild shake in the stearing wheel when I brake. Once again this does not happen all the time. It does not happen during hard braking or when braking easy from 60 mph or so. It happens (when it happens) when I brake easy from 85 or above.


Jerry

 

Somewhere I thought I posted a further reply....maybe not.


The issues here vary a bit.


Rear changes: The PBR may work but I am considering a caliper only change to the existing rotor. And 13 x .81 or .75 is as big as it'll come.

The issue with Jerry is one I spoke to him on and I feel it's a problem with the booster or MC. The piston area on the SLIIIs is 5.1sq" thus representing a reduction of about 20% over stock. I don't like to do much more of a reduction than this due to the other problems associated with it as we discussed above. This WILL produce a much shorter, firmer pedal. Why Jerry has the pedal fall low and go down is not related to the caliper in use but something else happening here. I'm guessing that vacuum pressure is being lost somewhere or a bleed back of hydrolic in the MC itself bypassing the piston perhaps?

When you push harder and harder on a pedal and the vehicle does not slow that is a pretty sure sign of a too low a Cf of pad in use. This is usually evident on the track where a person uses street pads which 'give up' after a few laps and you get 'pad fade' not fluid fade. Fluid fade comes next as the system works overtime to get the vehicle stopped and temps go up and up, then the caliper gets hotter and hotter until it boils fluid.

If anyone has a better suggestion for Jerry's problems please let us know. I'll keep it on file should it come up to me again. His is the only one to do this.

 

I don't have it nearby, but I remember the rear rotor is super thin, like .5" or under. Do you think it's worth it to put more heat into such a thin rotor? Sure would be nice if Ford had a vented version. I can't think of anything off-hand that would have it and would have the same bolt pattern.

Ayrton, that is a monster rear rotor, certainly won't have too many temperature problems back there! But I agree the rear bias will be too high with that caliper. The reason I asked is that a smaller rotor diameter would lower the rear bias. For cost and complexity, I'll probably use a bolt-on style GM rotor(12" diam) with the PBR caliper(if its piston diam is close to what I need).

The brake pedal feel on my '01 has always been pretty good, IMO. Obviously nothing nearly as good as my friends 944 turbo or even girlfriends Miata(4 floating calipers and rubbers lines can be designed to work perfectly well, BMW M5 for another example), but definitely what I expected considering the weight and other compromises(pads chosen for low noise, etc.). My pedal has never gone to floor though, that is just freaky!!

To me, Jerry's problem sounds like a booster. Losing vaccuum after a long stop? Intermittant vacuum leak? Especially noticable after the engine has been at low RPM for a while(i.e coasting to a stop). How about a vaccuum guage?

 

I had a long chat today with Todd and in order to try and get back some of the correct brake bias I will be running a 12.6x1.25” rear rotor with the 14” I have up front and will fine tune it with some pads.

 

I want to make sure everyone knows this has nothing to do with TCE. My truck did this before I ever installed the big brake kit. I don't think it is the booster or I think it would idle bad with my foot hard on the brakes. I think the M/C may be bad. I just drove in from the airport this afternoon. After driving 20 miles and making many stops the brakes worked perfect until the last stop I made....it did it again.


Jerry