Date:         Tue, 15 Jul 2003 09:10:43 -0700
Reply-To:     mark drillock <drillock@EARTHLINK.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         mark drillock <drillock@EARTHLINK.NET>
Subject:      better brakes on 86 up
Content-Type: text/plain; charset=us-ascii

For North American models, VW changed the front brakes in 1986 models
and all Vanagons from that point on used that same type including
Syncros. I have done a lot of driving for years with both types and find
the newer brakes to be noticeably better.

To figure out what was different, I have measured the pads, caliper
piston diameters, and the rotor diameters. Then I pondered the
differences and made a few rough calculations.

The newer piston has a bore with 55% more surface area. For a given
brake pedal pressure the newer type has 55% more force pushing the pads
against the rotor. More stopping power with the same pedal pressure if
all else is equal.

In addition the newer pad area is 10% larger and is shaped to put much
more of the pad surface area farther out on the rotor. While the newer
rotor diameter is smaller the newer pad size and more importantly the
new pad shape makes up for this. I measured and calculated the mean
effective radius of the old rotor/pad combo. This radius is the point at
which there is the same amount of pad surface area on either side of the
circle. Because the newer tapered pad is over 4.5" wide at the top
versus only 3" wide for the older pad, the newer pad puts more contact
area at the outer edge of the rotor making it more effective. The newer
pad has just as much surface area above the same mean effective radius.

Another thing I thought about was heat dissipation. The newer caliper
exposes much of the metal pad backing plate on the fixed side to cooling
air flow. Little heat from this pad could be transferred to the brake
fluid as there is none on that side. The other side with the piston has
cooling fins cast into the caliper around the piston area and where
airflow would be generated by the inside of the wheel turning very near
it. The earlier caliper surrounds both pads and looks to have less
exposure to air flow. This would seem to cause the pads to run hotter
and also would seem to allow more heat to be transferred to the brake
fluid from both of the pistons. The fluid should boil sooner with the
earlier type.

Also, the newer caliper is 2 pieces, connected only by the 2 pivots of
the floating mechanism. The pads ride on both parts with metal to metal
contact between the pad and the steel bracket bolted to the steering
knuckle. Thus the pads can transfer some heat directly to this massive
metal area and not to the piston area. On the older caliper, the metal
back of the pads touch only the piston and adjacent area. All pad heat
transferred through this direct contact could heat up the pistons and
brake fluid. I can see now why many modern vehicles have switched to
floating calipers as the heat transfer advantage seems obvious.

My suspicion is that the disc rotor diameter was reduced at least partly
to make room for the cooling fins that are on the top of the newer
caliper. Without fitting larger wheels (15"), these fins would have hit
the rim so the brake rotor was made a little smaller so the caliper
could be moved in and VW could keep using 14" wheels. Lucky us.

Mark