What wheels will work with my 13" SN95 Cobra
conversion?
I get asked that a lot. Especially since Street or Track started
producing a bracket conversion kit to allow 1965-73 Mustang owners to use SN95
13" Cobra calipers/rotors on their cars (link).
The first thing most people are really surprised about is that their beloved
vintage style wheels, Torq Thrust II's etc won't work without some
tweaking. In this 'how to' I'll explain why they won't and what can be
done about it.
The first thing to note is that most late model vehicles use a rotor with a
very shallow hat offset. Hat offset is the distance between the rotors
friction surface (where the pads bite into) and the wheel mounting pad (plane at
which the wheel bolts to). On a late model car, this is very shallow,
pushing the caliper outboard toward the backside of the wheel spokes.
The SN95 Cobra rotor measures only 0.71". Compared with the
1.19" hat offset with the 13" rotor used in Street or Track's
13" Front Disk Brake Kit for DRUM spindles or for
DISK spindles and our 13"
Rotor & Bracket Kit.
SN95 Cobra rotor measures only 0.71" hat offset
13" rotor supplied in Street or
Track's kits measures 1.19" hat offset
Lets now measure a SN95 Cobra caliper with a fresh set of
pads. What we need to determine is the maximum distance the caliper will
extend from the rotors friction surface. This measurement is taken from
the interior friction surface of the pad to the furthest point of the caliper
body. This measures 1.45".
Now we know the rotors hat offset and the dimension the caliper
will extend from it we can determine the caliper will sit 0.74" above the
wheel mounting pad using a Cobra rotor (1.45" minus 0.71") and 0.26"
(1.45" minus 1.19") using the Street or Track
13" rotor.
Now, lets talk about wheels.
The most common wheel I hear people using and are wanting to
convert to the 13" Cobra conversion is the American Racing Torq Thrust
II. At a minimum you'll need to use a 17" wheel, this is the most
popular size so the following dimensions are based off of that size.
Backspacing choice does not change the following dimensions. The following
picture is the backside of a 17x8 Torq Thrust II:
Backside view of a 17x8 Torq Thrust II
And now for some dimensions. By laying a straight edge
across the wheel mounting pad and measuring down we can get a pretty good idea
of the profile of the backside of the spoke. Starting from where the spoke
intersects the wheel mounting pad there is 0.8" of clearance, ranging up to
1.09" at the center point and dropping to 0.42" where the spoke
intersects the rim. If we use the dimension of 0.74" that the caliper
will extend above the rotor's wheel mounting pad we can obviously see that a 17x8
Torq Thrust II wheel will have negative 0.32" minimum clearance from
backside of spoke to front side of caliper when using the Cobra rotor. Allowing for 1/8" to
1/4" of
clearance you would have to use at least a 1/2" wheel spacer. If you
do that, you will need to recalculate your wheel backspacing because the spacer
will push the wheels out the thickness of the spacer. Street or Track can
supply wheel spacers and custom backspacing Torq Thrust II's.
If you use the above Torq Thrust II wheel with the Street or
Track 13" rotor, you'll have 0.16" of minimum clearance without using
any spacers!
But wait! There are 2 other solutions
other than using wheel spacers!
The first one we've already discussed and that is the Street or
Track 13"
Rotor & Bracket Kit. Secondly, if you really want to use a vintage
looking wheel with a Cobra rotor, we usually stock and have measured a American Racing Torq Thrust M Shelby
wheel. These are available in a 17x8 with 4.5" of backspacing and
will clear a 13" SN95 Cobra rotor/caliper without spacers. These
wheels are available in black # 105MS7865B
or anthracite # 105MS7865A
They even include a cool Shelby center cap!
Just because we are geeks, we also measured the backside spoke
profile to prove these will clear. Here is a shot of the backside of the
TTM Black:
American Racing Torq Thrust M wheel mounting pad
By laying a straight edge across the wheel mounting pad and
measuring down we can get a pretty good idea of the profile of the backside of
the spoke. Starting from where the spoke intersects the wheel mounting pad
there is 2.03" of clearance, ranging up to 2.49" at the center point
and dropping to 2.11" where the spoke intersects the rim. If we use
the dimension of 0.74" that the caliper will extend above the Cobra rotor's
wheel mounting pad we can obviously see that a 17x8 Torq Thrust M wheel will have
1.29" minimum clearance from backside of spoke to front side of caliper
(2.03" minus .74").
So, in plain English what are my options I
hear you ask?
-
If you don't want to use wheel spacers and want to
use a wheel like a TTII, you'll need to use our kits that feature a higher
hat offset rotor:
-
If you want to use OE Cobra rotors use our
13" Bracket kit for DRUM spindles or
DISK spindles. For wheels you'll either need to use something like
a 17x8 Torq Thrust M wheel #'s 105MS7865B
or 105MS7865A
that don't require a wheel spacer, or a 17x8 TTII wheel with a spacer or, 1994 - 2004 17"x8" (30mm offset) GT, Cobra,
Bullitt, Mach I OE or Aftermarket wheels with a spacer. Or, any other wheel you can
measure that will fit!
We hope this helps explain all of the different 13" brake
conversions we offer. If you have further questions, please
let us know!
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