Rear Drum to Disc Conversion

This page describes how to convert VW Drum brakes to Disc brakes cheaply and effectively. The conversion is well worth the effort and brake feel is much better. Disc brakes clean themselves great in mud and are not as affected by water as drum brakes. Those of you who have driven through a puddle with drum brakes know what I'm talking about.
Type of Vehicle: Any Bug Rear
Vehicle Component: Rear Brakes
Cost: $70-$140
Time to complete: Completion time really depends on how long it takes you to find the parts. Take your time when doing this job. Quality is important.
Tools Required: Socket for Rear Axle Nut
Welder
Lathe to cut drum
Parts Required: Porsche 914 Rotors
Super Beetle, Celica Supra, 280Z or other small car calipers
Submitted by: Todd Kirkwood
Finished Rear Disc Brake Setup

Finished Disc Conversion.

This conversion is serious business as it affects your stopping ability. If you do not have fabrication experience, mechanical experience, and a large amount of common sense I would suggest you get a fabrication shop to do the conversion for you or buy one of the premade kits.
Parts


On a Bug IRS setup, a lot of people use Porsche 914 1970-1975 rear rotors. Front rotors work also and are slightly thicker but I believe they may have a smaller offset. I put a worn-out 4 lug bug drum on a brake lathe and machined it down to barely fit inside the porsche rotor. Other people have used Super Beetle rotors.

I have not used them, but some rear 914, Nissan 200SX, Maximma, and 280Z calipers have emergency brakes on the caliper. Other small car calipers such as a Super Beetle will also work. Find Calipers that look like they would be easy to mount, easy to get parts for, and use about the same width rotor. And try to pick one that has floating pads. The Pad/piston assembly should move side-to-side on the caliper. The entire caliper should not float at the mounting points. This is just because it's easier to mount properly and it will tend to follow a bent rotor/axle a little better.

If you're the technical type, there's a book called Brake Handbook by Fred Puhn. The book is out of print, but the Local Library may have it. It's a good book to learn about hydraulic theory, piston area, etc...

Technical: In general the larger the piston area, the more brake force is applied and the more pedal travel you will have. It seems most small car calipers have a piston area that is about the right size.



While you have things apart you may want to put studs in the Hub. I had the holes drilled and pressed generic studs in place. Just watch out because some VW drums (such as the one pictured) have a taper where the stud would seat that causes the stud to not sit properly. Either machine the surface flat or find another old drum with a flat surface.

I used 14mm studs from an autoparts store. These studs require an oddball size hole (39/64), so I would suggest against using them unless you want to get the holes machined. Better to find an SAE size stud and make sure it uses a "common" size bit. If you do use the 14mm studs, harbor freight sells the bit.

Next time, I will try threading bolts in from the back and tack welding them so they don't back out or possibly just using locktite. This would be much quicker, easier, and (I believe) just as strong.


Cut brake drum


The caliper mount is very important. Make the mount strong enough to pick the car up, they get a lot of torque applied to them. Also make sure you mount the caliper perfectly straight in relation to the rotor or you will need extra pedal travel to straighten the pads (you will notice this most as a steering brake that doesn't work). I had someone hold the brake down while I tacked the mount in place. This aligns the caliper and mount and holds it in place for welding. Before final welding, double check the mount alignment and make sure the pads are centered in their travel. Then re-check it. Welding may cause the mount to creep out of alignment, so weld short beads on each side and continuously re-check the alignment.





When I built the reinforced trailing arms, I didn't anticipate disc brakes and that the rotors and calipers may interfere with the arms. It was 11:00 am and I was driving to the mountains at 5:00 so the vertical tube was clearanced with the help of a torch and a 3 pound sledge. If I remember correctly, Front Bug rotors have less offset than Porsche rotors so that may be an option if you have clearance problems.




Finished setup.
Relatively cheap and easy to do. The steering brakes work great and I'm not adjusting brakes or pulling the drums every weekend to clean out dirt.

Depending on your vehicle, tires, weight distribution, etc, you may have too much or too little brake force on the rear wheels. An adjustable brake proportioning valve can be installed to lower brake pressure. In addition if you need to lower brake pressure to the front wheels and you use VW drums, you can replace the wheel cylinder with a bug rear cylinder which is a couple millimeters smaller. The piston size is marked on the cylinder. (Smaller wheel cylinder = decreased brake force.)

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This Tech Tip courtesy of : Todd Kirkwood


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