Yet another take on cables for 911 parking brake conversion Options

[Larry.Hubby]

I posted a picture of my approach to this some time back on another thread about the cables problem, and have since gotten some interest from others in more details about it. I’m therefore posting the details of what I did here in case there are more folks out there who might find them useful.

Since the mechanical adaptation of the 911 parking brake assembly to the 914 trailing arms has been well-covered in other threads, I’m going to cover only issues concerning the cables and the anchoring of the parking brake shoes to the trailing arm – things about which there still seems to be a variety of approaches.

Just so that we’re all on the same page, a little background first. The diagram below is the one used in the 911 service manual to explain the operation of the parking brake. It shows an edge-on view of the brake assembly without the brake rotor and wheel, both of which would be in a vertical plane at right angles to the plane of the diagram.



When the cable is pulled within its sheath, the two “bow ties” are squeezed together, the cable pulling on the outer one and the cable sheath pushing on the inner one. Because of the angle at which their arms sit, the bow ties generate tremendous leverage, and force the two parking brake shoes apart and into contact with the drum formed by the inside of the “hat” portion of the brake rotor. Friction forces then tend to lock the two together.

Two things are worth noting: first, the cable sheath end must be free to move along the length direction of the cable in order for this to work as described. If it isn’t, i.e. if it’s anchored rigidly to the trailing arm somewhere, the force needed to oppose that of the cable (which would otherwise tend to pull this end of the parking brake assembly out of the rotor) must be supplied by the stamped metal backing plate through the shoes and the single outer bow tie, while the inner bow tie does nothing. While this may be OK under most circumstances, the greatly increased friction between the shoes and the backing plate, and the possibility of slowly progressing deformation of the backing plate due to repeated application of the brake, particularly if the brake is ever used as true emergency brake or for parking on steep hills rather than just to keep the car from rolling on level ground, are significant concerns IMHO. Actually, even if the cable sheath is free, there is some force that must be supplied by the backing plate, but it’s just the much smaller force necessary to flex and move the cable and sheath by half of the cable movement.

The second thing is that locking the shoes to the rotor doesn’t stop the car from rolling. That must be done by something which prevents the shoes from rotating with respect to the trailing arm. In the original 911 system, this is done by a feature cast into the trailing arm itself that provides a very strong mechanical stop for the bottom ends of the shoes. These two photos, also from the 911 service manual, show this feature very clearly.



Since the 914 trailing arms lack this feature, something similar must be added. As always, different folks have used different approaches to perform this function. The old Velios trailing arms that were available when you could catch George, and when George felt like it, replaced the entire cast portion of the trailing arms with a custom casting that accepted the 911 parking brakes and included an identical feature. This was a rather high-buck approach, although it also upgraded the wheel bearings to the full 930 dual bearing configuration. Others have welded metal stops of various kinds to the backing plate, or to the trailing arms. Of these, the latter is by far the best approach IMHO, because the former causes the torque from the wheel trying to roll to be transmitted through the relatively flimsy backing plate.

My particular solution to this problem was to design and machine steel “anchor blocks”, and TIG-weld them onto the bottom of the trailing arms at the “6 o’clock” position. This is the drawing for those parts:



…and this is what they look like installed:



If you go this route, I recommend clamping them onto the arms with the parking brake assemblies in place to get the alignment just right, tack welding, then removing the brakes and finishing the welds. The heat of the welding will nearly always cause enough distortion to bend the long side of the “L” out of the plane where you want it, but this can be easily “adjusted” with a hammer once a solid weld has been formed.

My approach to the cable problem was to use the stock 914 cables on the inboard end, the 911 cables on the outboard end, and splice the two together externally, shortening the 911 cables as necessary to match up with the ends of the stock 914 cables. Single, custom-made cables would of course be cleaner, but this was appealing to me for several reasons:

1. It kept everything as stock as possible. All critical hardware is Porsche spec.
2. It kept the non-standard things I did out in a readily-accessible location, in case anything fails.
3. It allowed me to use the 911 cables that I obtained at relatively little additional cost along with the parking brake assemblies.
4. Only three fairly simple special parts per side are required (well, OK, four, if you count the bracket on the trailing arm), and I had, or had access to, all the required tools.

This is an overall view of the result: