collected the last bits for a 911 rear e-brake conversion today
so I figured I'd compare the rear 911 calipers (they're from a 86 911 3.2)
to my GT-style 914 rear calipers (widened version which fit vented rear rotors) which I already had...

well, so much for the "upgrade"
they both use the exact same size pads


I guess the pistons on the 911 caliper is a little bigger (haven't taken out the pads to compare yet)
but so far, it looks like a total waste of time & money


Unless someone will tell me that the slight piston size change will make a world of difference in the braking capacities
I don't think I'll bother converting the rear trailing arms and fabbing the cables for the 911 e-brake setup

yup ...with Erics rear spaced calipers..it doesn't make much sense or cents

Why would you "upgrade from rear 914-6 widened rear calipers (with integral e-brake) to go with 911 rear calipers with 911 e-brakes? Or did you space out a set of 914-4 front calipers for use in the back?

I thought it was common knowledge that the pad size for 914/4 front and 911 rear are the same size, but different thickness (914/4 front pads are thicker).

I do believe that the piston size in the 911 rear calipers is larger.

Sorry you didn't know this prior to your "upgrade".

yeah... me too
usually I do my homework better than this

quicker pad changes

You do realize, I hope, that pad size has almost exactly nothing to do with the "stopping power" of the brake?

The caliper piston diameter (or rather, the total piston cross-section area) has more influence. A larger piston will, all other things equal, exert more force on the brake pad. If the front and rear pistons are sized correctly relative to each other, the rear brakes are doing "enough" of the work of stopping the car. If the piston sizes are mis-matched, then you have the rears not doing enough work.

The key is, as in so many areas, balance. You want a good balance of the front to rear brake effectiveness, so the rear brakes do their fair share of work but don't lock up.

Also remember--the area goes up with the square of the diameter. So a 40mm piston will give you about 10% more braking force than a 38mm one--and that's a difference that is hard to see with the naked eye...

That all said, it may indeed be better to stick with the widened -4 calipers. Remember that a lot of the 911 rear brake adaptations were done before we had people machining new spacers for the 914 calipers...

--DD

Stock Rear Pistons - 33mm
914-6 Rear Pistons = 38mm
911 Rear Pistons = 38mm
Later (post 84) 911 Rear Pistons = 42mm

Jeroen, if you are going from a stock 914 to a 911 and you have the 911 handbrake assembly I would say this is still a worthy upgrade, given the fact that you would have A or S-Calipers up front.

Dave / Eric,

Thanks for setting things straight...

I just measured the pistons
The widened 914 rear calipers are modified /4's (33mm pistons)
The 911 rear calipers apear not to from a 3.2 but earlier (38mm pistons)
So I'll go ahead and convert to the 911 e-brake

Dunno what's going on, but this past week or so it seems my head is stuck up my ass

Wait a minute, you say the pads for a 914/4 rear caliper are the same size as fits the 911 rear caliper? That can't be so. The 914/4 front caliper, the 914/6 rear caliper and the 911 rear caliper all take the same pad, and it is larger than the 914/4 rear caliper pads.

I said that the stock 914/4 front caliper used the same size pad as the rear of a 911. The difference in pads being that the 914/4 front pads are thicker than the 911 counterpart (911 rear pads fit the 914 front calipers).

I had assumed that widened 914/4 front calipers were mounted on the rear of this car. We have had a similar setup on our track car, but are now going with 911SC rear calipers and rotors.

Hell I'll take the 911 e brake stuff off your hands

Dave,

I beg to differ. Although you do have a point with piston size, the swept area of the braking material has a great deal to do with stopping power.

The braking material combined with surface area determine how much energy (heat) can be generated by the brake system. This in turn needs to be dissipated by cooling capacity of the rotor, caliper and brake material. The more effective the cooling capacity, the more effective heat can be generated (stopping power).

If a brake system has the capability of overheating and can't effectively cool itself, a larger piston (more pressure on the brake pad), will only make matters worse (quicker). On the other hand, a properly designed brake system has the components matched as far as available pressure (piston size) and cooling capacity (rotor venting, etc).

Swept area, does indeed play a part in brake systems. Otherwise, it would follow that brake pads should be the same size as the piston in the caliper (sic).

just my humble opinion.

I don't think that has to do with stopping power per se, but more to do with heat management. As in, if the pad is too small it will go past its operating temperature too quickly, and you'll get "pad fade". Either that, or it will wear excessively quickly... That indeed does affect your stopping distance, but IMHO it is an indirect relationship and is not what I am talking about.

If you'll go back to HS physics textbooks, you'll see that the force caused by friction does not actually have a term for the area of the things that are in contact. Now I realize that the real world does not quite work the way highschool physics texts say they do (or how could we corner at more than 1 G?) but I believe that the pad area does not have that much to do with the actual stopping power.

I haven't looked into a good thorough discussion of this in a while, but the last time I did that seemed to be the consensus of the people who were making the best case. I wish I remembered more of the arguments, but those were the ones that held up...

--DD

Early -4 fronts are built off the same basic housing (the fluid tap is machined on the side vs. the ends). I suppose to widen one you would use 911 rear spacers...? However, I believe their pistons are actually too big for this application. Yes, there are people who have done this but I think they are in danger of swaping ends unless they have their p-valves set up properly. Going with the Earlier 911 or SC caliper is a much wiser move for a more balanced system.

My $0.02

I think you are on to something. (THE REAL WORLD)

Force caused by friction?

I feel that you mean in this instance the force that CAUSES the friction.

When you are going back to those basics, then you would be referring to using the coefficient of friction between two surfaces (unitless).

_____________________________
Laws of Friction for Dry Surfaces:

1. Friction between two given bodies is directly proportional to the pressure; the coefficient of friction is constant for all pressures.

2. The coefficient and amount of friction for given pressures are independant of the area of contact.

3. The coefficient of friction is independant of the relative velocity, although static friction is greater than kinetic friction.


The preceeding laws are only approximately true. The coefficient of friction is slightly greater for small pressures upon large areas than for great pressures upon small areas. The coefficient of friction decreases as the speed increases.


Eshbach's Handbook of Engineering Fundamentals
_______________________________

They don't necessarily reveal in HS Physics that some of the laws they teach are not 100 percent accurate, but more rather an approximation.

The reason friction brakes work is that they convert kinetic energy (motion) into heat energy. The more surface area the brakes (pads, rotors, calipers) have, the better they are at dissipating the heat (real basic heat transfer).

just my 0.02 and I'm done on this topic.

Dissipating. That's what happens after the heat is generated by friction--after you stop. It affects subsequent stops (particularly when the heat builds faster than it can be dissipated and the pads get out of their working temperature range), but there should be little or no effect until they get out of that range.

These are, however, small quibbles.

--DD

I think you are correct. From what I can tell the early 914/4 front caliper (to end of 72 MY) is virtually identical to the Carrera rear caliper with the exception of the brake line location. I believe the castings are the same, just finished differently. I believe the outer caliper halves of these two (with 42mm pistons) are the same casting as the outers halves of the 911 rear caliper and 914/6 rear caliper (both 38mm pistons), with just bore changes and the 914/6 also has the adjuster. Thus the spacers (10mm & 14mm) can be used with 914/4 fronts and 914/6 rears. Sort of a whole mix and match system.

The early 914/4 front pads and the 911 rear pads should be the same pad. The late 914/4 front pad is the same size, just thicker. The 914/6 rear pad is the same size as the early 914/4 front pads and the 911 rear pads, but the ear holes are two sizes for the regular size pin and the large size pin.