Tuesday, February 17, 2015

298mm Twin Piston Brakes - A Cheap Alternative for the E21

Much earlier in this blog, on a post that would appear to have one of the highest view counts (continually rolling in) on this site, I mentioned a potential cheap alternative for big brakes on an E21. 

That alternative, was the 298mm diameter, 28mm ventilated rotors from a Holden Commodore VT, in combination with the factory caliper from the same.

I've since contacted the original poster referenced in that blog post, Duane Vanarey.

Duane is responsible for the rather tasteful E21 above which is (or was? - it's currently undergoing some changes, I believe) powered by a Turbocharged 3.8L V6 from a Holden Commodore.  

Duane advised that the bolt pattern for the caliper bracket is the same, so I purchased a pair of rotors and a pair of calipers from a local bloke who parts out a few of this model Commodore (for the princely sum of $40).

Above is the standard E21 single piston caliper and (tiny) brake pads for demonstration.

Even ignoring the fact the rotors are bigger and the caliper is twin piston, the surface area of the brake pad looks to be more than triple that of the standard E21 part.

In this image, the two brake rotors are sat back to back. You can see the significant size difference.

An interim inspection looks like even the line lengths and connection setup will interchange.

Here's the VT Caliper bolted to the E21 upright. I've butchered the heat shield as I initially misunderstood what had to be done to make this all fit - if you're going to do this yourself, you will definitely need to trim both the top and bottom of the heat shield (or fold it out of the way) to clear the bigger caliper, but you may be able to get away with doing just that.

Here you can see there is quite a bit of room for the brake rotor to come forward from the hub before it's centered in the caliper bracket. Initially I thought I'd have to shim the caliper bracket back but it soon became clear that the Commodore Rotor was going to hit the upright and that instead it was the rotor that needs shimming.

This allowed me to fit the wheel and verify clearance (yes, it clears my 15" wheels!) 

This is the hub adaptor/shim I need to have made (left) and on the right is a rough idea for a way to hold the rotors together so I can drill the holes on the drill press. I've had a change of mind to construction method for that so please disregard it.

Key specs (measurements) are (bear in mind not done with digital verniers so may be "close" rather than spot on. If I machine this myself I will be checking against the actual rotors; if I have it machined I will take them to the machinists:

Thickness of Rotor at Hub Face (between Hub and Wheel) - E21 8mm | Commodore 10mm | Effect to Offset +2
Backspace from wheel mounting face to front face of braking surface - E21 54mm | Commodore 55mm
Rotor Thickness - E21 24mm | Commodore 28mm
Hub Diameter (in rotor) - E21 63mm | Commodore 71.5mm

To get the overall height of the rotor from the hub (for upright clearance) add the Backspace and Rotor Thickness and remove the Thickness at Hub Face: E21 54+24-8=70mm | Commodore 55+28-10=73mm

This means the required shim between the Hub Face and the Commodore Brake Rotor to allow clearance to suspension componentry per factory geometry is 3mm

The Hub on an E21 is 122mm in Diameter. I have chosen to use this as the dimension for the shim instead of the inner diameter of the brake rotor as otherwise it may cause issues due expansion etc, and also to keep unsprung weight down.

The overall effect on offset is worth noting - because of the thicker area at the hub face (2mm) and the required shim (3mm) the offset will increase by a total of 5mm. I will have to check the thread depth of the wheel bolts - they may need replacing with bolts that are 5mm longer to make up for this. 

This may also mean that wheels that don't usually fit the front of an E21 without a small shim may now fit (doubtful) or that clearance issues with wide wheels/incorrect tyre sizes may become amplified. This is of potential concern on this vehicle as both left and right sides scrub at extremes of steering.

The Holden Commodore VT is a vehicle produced for and sold almost exclusively in Australia.  The following links are to eBay Australia where you can buy second hand, new or reconditioned calipers. Many senders will ship internationally. Buy VT Calipers | Buy VT Rotors - These vehicles were one of the most sold vehicles in the country and therefore parts are currently plentiful and hence cheap.

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