Cobalt 4 Bolt Rear Disk Brake Solution

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Apr 15, 2020 | 09:33 AM

Boyes50

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From: Caledon, ON

Cobalt 4 Bolt Rear Disk Brake Solution

Hi Everyone,

I have been working on finding a rear brake disk which would fit with a 4x100 Hub, and be close to the same size as a 2.4L Cobalt SS Rear Rotor. This way I could use the OEM rear calipers from my 2.4L SS, an SS Rear Axle with Caliper Provisions, and 4x100 LS Rear Wheel Hub Assemblies.

I've found a 1984 BMW 318i, front brake rotor is almost identical thickness. It is .4" smaller in diameter, which should be fine and still allow plenty of brake pad engagement surface. It is 4x100 and 66mm centre bore, so it should fit the hub correctly. The rotor surface is roughly 9.9mm too far away from the hub, since the rotor height is 9.9mm shorter than the OEM Cobalt SS rotor. I would need to determine a way to shim the caliper out from the car roughly 9mm. Otherwise, I cannot see any possible issue with using these brake rotors. They are solid, non-vented. An example is Raybestos part number 9829R.

Can anyone see any issue with this approach to rear, 4x100 rear disk brake setup. I need 4x100 to install the correct required Aero Racing Wheels in 15x7 that my track requires. Beyond that, I will be using a complete LS 4x100 Front Brake Assembly to clear the 15" steel racing wheels.

Apr 16, 2020 | 08:03 AM

jdbaugh1

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From: Kansas

Why not just have the 2.4 rotor re-drilled to 4X100?

Apr 16, 2020 | 08:35 AM

ECaulk

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From: Houston, Texas

A rotor diameter difference of 0.4" is significant, that's 0.2" of your pad that isn't gripping. Also shimming a rotor hat out 9mm (~0.35") is not a good idea. Jd's suggestion is a better one just to re-drill the JL9 rotors (JL9 brake code from SS/SC (05-07) SS/NA (06-07) or Sport (08-10)) and run the rear twisting beam you're planning on.

Are you running out of braking ability at the track with the drums? I know the rear does a minimum amount of braking so I don't think there is a huge advantage to upgrading to a rear rotor.

Apr 16, 2020 | 08:41 AM

jdbaugh1

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From: Kansas

If you have a rear drum I definitely wouldn't swap to discs without setting up a proportioning valve unless you want to lock up the rears waaaay before the the fronts have reached the grip threshold. I assume you are doing dirt track? You may get more bang for your buck by getting better pads or otherwise upgrading the fronts since like Ecaulk said the fronts really do most of the work. When you are braking hard the rear end gets very light.