Unusual Brake Set-Up

[Tom Oxner]

I have a pair of struts like this in some parts I bought. This appears to be a way to use a slightly larger diameter rotor on a 911 strut with an early S caliper. Has anyone seen this done before? Thanks.

Tom

[Walt Fricke]

Haven't.
That blueish bit is an adapter plate? Machined to interlock? Holes on the caliper threaded for a larger diameter bolt?

I don't see why it wouldn't work, though the shear forces on the bolts/mating faces are going to be larger due to the larger diameter rotor. That ought to be something which can be calculated. I don't think aluminum threads can take quite the torque of steel ones, but that, too, could be calculated. But driving ought not to add greater tensile loads.

You would have to consider the change the larger diameter rotor creates in the front/rear brake bias: more front bias.

The pad sweeps the entire surface of the stock rotor. Unless your new rotors have the same wear surface width, you might get differential wear, but that ought not to matter. You won't get the heat transfer benefit of having a wider wear area - aka more pad area. But for stopping power, pad area is not part of the equation. Hydraulic and geometric lever advantage leading to pressure squeezing the pads together, together with friction coefficients of rotor and pad, are what determines stopping power.

Stock rotors would work, with stock calipers, on 14" rims (not many of those, but was part of a comfort package). 16s or larger allow physically larger calipers and greater diameter rotors.

But you don't hear about this much beyond fitting other calipers, as from a Boxster. I think the adapters for that are made from aluminum, which might say something about the strengths involved.

With the A pad on the S caliper, you should be able to fit - if the in/out spacing works - a slightly wider rotor, too - maybe 3.2 width?

[Tom Oxner]

Hello, Walt. Yes, you are correct, the blueish bit is an adapter plate. It would seem that the adapter could also be used to adapt the larger calipers to the struts with 3" spacing using a larger diameter rotor..
This set up was in some stuff that was from the estate of the late Grady Clay. He did lots of really cool stuff with the early cars. I am sure this was part of a plan to have better braking, I am just trying to figure out what the plan was. Thanks for your insight.

Tom

[Walt Fricke]

Grady had quite a stash. His intuition about strength was as good as anyone's.

Nowadays it doesn't make sense to adapt the 3" struts for the 3.5" calipers (which all the Ss are)? Easy to get the better strut. Which looks to be what you have - just ditch the adapter?