From the Boutique:

Drum to Disc Brake Conversion

By Doug M - Updated 02/02/2001

I have had a lot of questions about my disc brake conversion, I hope this will clear at least some of it up. Note that my bike is unique in that it has a '71 HD frame with a '81 HD engine. So I already had "provisions" for the brake pedal/master cylinder. I also did not have to worry about having the shifter pedal on the wrong (right) side of the bike.

The only place I was able to find the swingarm was Bill's Customs Cycles in PA. When I bought mine, I paid $100 for it. I told him what I was doing (and offered him my old swingarm). When the next catalog was released, the price jumped to $200 and actually has a note that says "a must for disk brake conversion". Bummer...

But as far as the conversion itself...

 The swingarm is a bolt on. Just use the 79-81 XL swingarm. You can use your old pivot hardware or upgrade to the newer style, they will both work fine. After that, basically change to a 79-81 rear end.

I used a GMA caliper for 79-81 XLs, and cast wheel from a 96 XL. I used a sprocket for 79-81 also, but it is dished and I had to space it out. A flat sprocket would do much better (at least with my wheel combo). I also used the axle from the 79-81, but had to be creative with the spacers. (Again, I am sure this was due to the new wheel.)

After this, you will find that the shocks will not work. Since the lower mount is at the rear of the swingarm, and you cannot move it due to the calpier (the caliper will not allow the room for the lower shock mount to be easily moved), you must move the shock.

So I bought some Progressive 413's (PN 413-4039 15" Black) and moved my top mount from the original place (see image) to the other bolt for the strut/frame mount. (Using a grade 8 bolt of course.) If you are doing the switch just like I did, you can now (I believe) get different spring rates. If you intend to do some two up riding, you will likely want the next stiffer spring rate. Mine are fine for just me (150lbs) with the preload cranked up. Also, you will need to relieve the fender strut or the shock will hit. I just used an angle grinder to 'dish' out the strut enough to keep the shock from hitting. Be sure to check its full range of motion. Of course this will ruin any chrome. Which was OK with me, I wanted to paint them black anyway. (see update below).

Finally, you need to find a place to mount the master cylinder. My engine is an 81 in a 71 frame, so I already had the correct sprocket cover. I don't have any idea how you could do this on a bike with left side brake pedal, but I am sure it could be done. If your bike is new enough, I believe the 80-81 sprocket cover is a bolt on and it will give you the correct pedal setup. Then just plumb the brake line. I used a Russel braided line all the way from the master cylinder to the caliper. I also used Russel's banjo bolt on the mc so I could easily attach the brake light switch. UPDATE: I recently discovered that my fender and struts were not the factory width. When I used factory width struts, they cleared the upper part of the shock without the need to grind them away. Also, I am currently moving my shocks back to their factory (upper) mounts. To do this I just used a 3/8" threaded coupler cut to 1 1/8" in length. I welded this to the swingarm 6" forward of the rear and 1/8" out from the inside of the swingarm. I will take some pictures soon to update this.

If you do this conversion, let me know how it went and what year, parts, etc you did it on. That way I can keep this page updated for others who want to try it.
--Doug

From: Zippy68n1@aol.com

I too, changed the swing arm using the 79 arm and stock brake bracket (79 lower and flatter than 80 or 81) removed shock brackets exept inside portion on right side to mount chain guard. I drilled and welded lower shock mounts into swing arm in the same position as old arm. My frame is a 64 and motor is a 76 used forward controls for 76 for shift and 79 for brake with master located on forward control. All items purchased from BILL"S as well.

From: Hank patyerger@home.com

I have 72 that I put a disks on (f&R) from a 79 about 20 years ago and I now have pictures if you want them. Uses stock shock and stock 79 parts. see picture. I don't know how to get them out for any one else to see so if ya want to show a early setup let me know.

Hank

From: Walter americancycle@netzero.net

In 1990 my eng came apart on my 77 sportster, As I was about 150 miles from the house I left my bike at a shop that was near by to have the eng rebuilt, A week later the shop owner told me that it would cost about $1700.00 to fix it, That's when I came across a new 1983 eng for $2000.00 with no miles on it, I but the new motor on that same day with a new set of forward controls so i could use my rear drum brake, Over the next year the forward controls broke six times and I replaced four sets of shoes, Thats when I knew I had to go disc brakes, But how could I do it no one could tell me and no one made a disc setup for a 77 Sportster, The frist year for rear disc brakes on a sportster was 79 ok Pm makes a nice kit for a 79 so i got one had the tab welded to the swing arm, got a 79 rear wheel and a flat sprocket and a axle for a79 the axle didn't fit though the adjusters and was to long so I drilled out two sets of adjusters cut off the studs from one set so I could use them as spacers put it all toghter with a washer on the end of the axle that was to long , It looks and works great.

Walt from AMERICAN CYCLE

From: Red76Harley@aol.com

Hi there Doug, as you prolly guessed, we have a sporty too. Ours is a 76 XLH. We did the conversion before we found the official Sportster page :(. How we did it though was as follows. The frame is a 76 the engine is a 74 and we used the stock swing arm...... with the stock swing arm we used PM 4pistion caliper with the '81 style mounting and had to cut a U shape between the upper and lower stops ( the ones that touch the swing arm on the top and bottom) used a 81 rear wheel with the dual bolt pattern and a flat big twin 48 tooth rear sprocket and yes we had fun with the spacers to.. also ( here is the trick) we ground down the what looks like a casting mold mark on the top of the left side of the swing arm. this allows the brake to work when you have to adjust your chain in or out... if you do not do this you will not be able to move the wheel in towards the motor. The advantage was being able to use the original shock location. Roy & Peg's home page our home page shows a decent look at the brake side.... hope this helps some.... thanks

Roy and Peg

From: Steve Hillion kbd@gci.net

instead of changing swingarm you can hang the calaper on a bracket that mounts to the axle ( look at a flattrack bike ) . the calaper anchor mounts to the frame, where it should be( not on the swingarm). an airhart master cyl. will mount on the top of the swingarm right side behind the shock, bolting through the old brake anchor hole(two adapters and two flat plates,adapters in hole plates on each side cyl. between plates) a sprocket adapter, and disk adapter will allow you to use the stock rear wheel. if I could draw in this program a sketch would make this clear. the stock brake pedal is used.I have had this set up on my bike (68 XLCH) for over 20 years and it works great.also dual brembo disks on the front, my sporty will stop.

Steve Hillion