RE: Big Red Stuff

["Mark Nelson" <tahoe@msn.com>]

QSHIPQ wrote:
> A 12.5 rotor will just fit in most 16in wheels, but watch the
> offset and the
> manufacture, it's still tight in there.

Correct...you have to use a wheel spacer with some 16" wheels...5 mm or
there abouts should be adequate.

> My experience with the
> Big Reds is that the problem isn't usually rotor clearance, but caliper
> clearance, which
> has more to do with the manufacture of the wheel and it's offset.

Right again...the ends of the caliper stick out a good .5" or so, hurting
clearance.

> Specifically, it's the lip and the spoke design of the wheel
> itself.  Many 17
> in wheels are a "stretched" 16 (or even 15), which means there is
> a step on
> the inside of the wheel that can whack the caliper.

Exactly...fortunately for some wheels, like a couple sets of Borbets I have,
the inside of the wheel doesn't have the "step"

> For instance, the Groupe A (ex FIA - '95 S2) car that ran last
> week at Lake Superior PR will take a SQ specified 15in steel wheel with
the
> sport quattro brakes.

That setup uses an 11.5" rotor and the compact AP 4-pot caliper.

> But, when we changed to ABT 16in wheels for the tarmac stage, the
> caliper was so close to the wheel that after the first tarmac stage, we
> changed the RF because of caliper rotor grinding.  When we
> changed it, the new
> spare wheel still got ground down as the car pulled from service.
>  So, don't
> automatically assume that a wheel tire combo will clear a
> specific brake.

Some rims will need a wheel spacer to get the proper caliper clearance
because of a flat backing.  Some smaller 15" wheels, like the Fuchs, have a
curvature in the spokes so you can clear the calipers on the sides.

> Regarding the Big Reds specifically (and this will apply to the
> 928 as well).
> The Big Reds are designed for a 13-13.5in rotor.  Going to a 12.5 in rotor
> means you have to regrind or buy special pads, because the pads
> won't fully
> cover the swept area of the rotor.

Here's the deal that I found out from our friend at the Porsche dealer:

The 993 TT has a .7" larger diameter rotor than the 928 GTS.  The 928 GTS is
supposed to have a 12.5" diameter rotor (320 mm) which would put the 993 TT
caliper at 13.2", so says the dealer.  I've heard that the 993 TT uses a
12.68" rotor (322 mm) (PN 993 351 045 / 046 10), but haven't seen the
proof...today we're going to have a 993 TT caliper and rotor and a 928 GTS
Caliper and rotor and see what the differences are.  The pad height on the
928 GTS caliper is 2.96" at it's tallest point, with some curvature in the
center.  The 928 GTS caliper is supposed to have the same diameter berillium
pistons and use the same pads as the 993 TT, but is machined to take the
smaller rotor.  I will know for sure today...BTW, the 928 doesn't use the
"big reds" just as an FYI...the internals are all the same, but the
machining on the inside is different.

> Bottom line:  "Bolt on" is relative to the wheel and offset of
> the wheel you
> plan on using.  My suggestion is that if you are looking to do a brake
> conversion, hold off on buying the wheels untill you know they
> will work with
> what you are doing.  Me, I don't automatically "guarantee" all
> 17in wheels to
> fit Big Reds.  But I can guarantee it to be the minimum
> requirement as far as
> wheel diameter.

One also needs to check the hat offset.  I got the shallowest hat that
Wilwood makes (~1.4") and the bolts that hold the rotor to the hat, rub on
the brake bracket ears and ball joint.  Mov'it has some shallower hats, so
I'm going to see about getting one of those.  I've also got the PN for the
Sport Q hats, which are pretty shallow.  That will take care of the
clearance on the back side of the rotor, but in my particular case with the
15" Fuchs, I'll still need to run a 30 mm spacer to clear a 12" rotor.  I'm
hoping to learn some more today after looking at what the dealer has.  Worst
case for me is that I will have to buy the Sport Q brake setup for the car.

cya!
-mark nelson
mailto:tahoe@msn.com
http://www.quattrosport.com