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PrOpeller
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Discussion Starter · #1 ·
Oh great... here comes ANOTHER wheel question! :rolleyes:

I searched the forums but could not find ananswer to this one...
What's the physical difference between the '75 Manta rims that allow them to be used with the larger brakes?

I have '75 brakes on my GT , but would like to get some lightweight 13" racing rims which can still fit. I'm guessing the Manta wheels have less backspacing... so what measurement actually differentiates them from a stock GT wheel?

TIA,
Walter

(Yeah Sox!)
 

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Walter,

That's a neat question! I just happen to have a pair of '73 Manta rims and a pair of '75 Ascona rims in my garage. They came care of Kat, who's SportWagon I bought. It had two '73 Manta rims on the back and two '75 Ascona rims on the front. Kat later delivered the other two '75 rims on her way to Oregon, warning me that I would need to re-mount the good tires from the '73 rims to the '75 rims, so that I could rotate the tires and still clear the "Big Brake" package. I don't happen to have the car here (it is sitting in John Warga's driveway waiting for me to get my crap in a row and get the engine re-built), but I believe Kat when she says that the '73 rims will not clear the '75's front brakes. They look very similar, with the '75 rims having "double" holes between the spokes for a total of eight, while the '73 rims have single holes for a total of four. Here is what the wheels look like (the '73 rim is on the left, the '75 is on the right):
 

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The most significant difference is the wheel size. They are both marked "ET 37" meaning they have a 37 mm (1.46 inches) positive offset, but the '73 rim is a "5Jx13" while the '75 rim is a "5 1/2 Jx13". Both rims have back spaces that measure approximately 4 inches, but the calculated back space of the '73 rim is 3.96 inches while the '73 rim is 4.21 inches. As a comparison with the stock GT rims which are 5Jx13 with an ET25 offset, they have a 3 1/2 inch backspace. So the GT rim has LESS backspace than a Manta or Ascona rim, and the '75 "Big Brake" rim has the MOST backspace.

Here's a view from the back:
 

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After measuring a bunch of different ways, I really don't see much difference, except the wheel "centre" might be a bit taller as it attaches to the outer rim on the '73 wheel. I even used my profile tool and the shape of both rims are very similar, albeit the '75 rim has that extra 1/4 inch of back space. So perhaps I will have to try the '73 rims on the Ascona this weekend and see where the interference occurs. Here is the profile tool with the '75 rim profile sitting in the '73 rim:
 

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Super Moderator
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FWIW, I've measured the GT wheels with approx 4" backspace, the Manta wheels with 4.25", and the 1975 wheels with 4.75".
With 1/4" spacers, the 'non-'75' wheels will clear the calipers of a '75.

Bob
 

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Are the 13 inch rims that clear the big brakes a hard to find part??? I think I have a couple that I will not be needing.

Garry
 

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The 13" wheels that clear the big calipers are only on the 75 model cars. I have a full set here, but, they are already spoken for. They're not hard to find.
 

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Opel fan
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rim sizes

I know I read a post where someone had a comparison of the differences in the rims for use with the big brakes, I can't find it using the search. This post was not that long ago, anybody remember who posted it, might be easier to find with the posters name.
 
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