I arced my 850 Eldorado 4LS Ferodo brake shoes on a vertical mill, and posted the technique on the LoopFrames forum. Here is the link to my forum post.
https://motoguzzi.groups.io/g/LoopFrames/message/81029?p=%2C%2C%2C20%2C0%2C0%2C0%3A%3Acreated%2C0%2Carc+brake+shoes%2C20%2C2%2C0%2C47934544If you cannot access that forum site, here is the text from the post.
"I want to share a technique I used to arc brakes shoes for my 1974 Eldorado with 4LS front brake. I had arced the old shoes using adhesive backed sandpaper on the inside of the drums and sanding the shoes until they lost all of the pencil line I had scribbled across the lining surface. The front brake was better, but the rear brake just didn’t perform like I though it should – I was not able to lock up the rear wheel at slow speed with the full amount of pedal pressure I was able to apply. The linings measured about 4 mm thick after arcing, so they were well above the 2.5 mm minimum thickness specified in the manual. Compared to my recently acquired 1976 T3, I was longing for better brake performance on my Eldorado.
I decided to replace the Eldorado brake shoes with new Ferodo shoes. The lining thickness on the Ferodo shoes were such that they would not even go into the drums when installed on the backing plates, so the sandpaper technique was not viable. Lots of online sources describe arcing brake shoes on a lathe. I have an old 10” Logan lathe, but the front backing plates are too large to swing in my lathe. I also have an old Taiwanese round column milling machine, so I decided to try and use the mill to arc the brake shoes. I locked the axle vertically in the vise with a vee block, and held the backing plate against the axle shoulder with a combination of a large washer, an old steering head bearing set, a collet that fit axle to center the bearing, a spacer, and the washer and axle nut. I tightened the nut enough to hold the backing plate firm against the axle shoulder, yet allow the backing plate to be turned by hand.
An inexpensive (<$25) Westward drum brake resetting gauge I got from Amazon (
https://www.amazon.com/Westward-1EKA5-Brake-Resetting-Gauge/dp/B001GIUCSK) was used to measure the inside diameter of the drum and serve as a gauge to check the progress of the arcing. I used a Gator brand 1-1/2” diameter by 1-1/2” long sanding drum I got from Lowe’s to sand the brake linings. The linked photo shows the setup on one of the front brakes.
In the photo you can see the dial indicator I used to find the high spot for starting the process and to measure the out of roundness as I went. I sanded the linings in about 0.002” increments until a 0.020” feeling gauge would slip between the lining and the resetting gauge. I used 0.020” based on the recommendation on the Vintage Brake tips webpage (
http://www.vintagebrake.com/tips.htm).
I just went for the first ride after installing the brakes, and am pleased to report the rear brake now easily locks up the rear wheel, and the front brakes are much better. It will be interesting to see how they perform as the linings bed into the drums. I’m hoping for the type of performance from the 4LS front brake that has been described in a number of online posts.
I suspect this technique could be done on a drill press with a good cross slide vise."
Good luck getting your new shoes to fit. The performance on my Eldorado was much improved with the new shoes.
Dennis
