Lightening a /2 flywheel

The stock BMW flywheel is about 13 lbs of tool strength precision steel, drilled and balanced. It has the rear main seal surface on its flange.  Timing marks are stamped into its circumference. When lightening a flywheel, the goal is to reduce rotating inertia while retaining the timing marks and balance.

I start off by taking some measurements on the backside. This gives me a diameter where I can begin to cut into the depth of the flywheel. I want to be in control of how thick the finished flywheel will be.  To maintain strength and the rigidity to withstand the clutch spring without flexing, I want the flywheel to be no less than 1/4" at any given diameter.

 I am beginning to thin the flywheel.

Steel is removed from the face of the flywheel.  The flywheel remains in balance during the entire process and, unlike milling holes in the flywheel with a turn table on a mill, when lightening on the lathe, the flywheel does not need to be rebalanced after lightening. 

Using a caliper, the flywheel is shaped to follow the contour of the opposing side.

The left side of the flywheel is cut, leaving a thin ring about 1/4" wide which contains the timing marks. 

This ring can be undercut to minimize the amount of steel out near the "heavy" edge.  

The ring, about 1/4" wide and supported by enough metal to be strong and light, bears the original and unchanged factory timing marks. 

The final product weighs approx. 8.25 lbs, a weight savings of about 5 lbs.  Versus cutting holes in the flywheel on a mill, most of this weight loss on the lathe-cut flywheel is at its circumference, where weight matters most when the flywheel is spinning.

An 8 lb flywheel is not very aggressive. It is just enough of a weight savings to notice an improvement in engine response and faster shifting speeds but, because the flywheel is not excessively lightened, starting,  idle quality and drivability are not adversely affected. 

The most beneficial effect of lightening the flywheel is to allow the motor to spin down faster.  This is really helpful when upshifting. It's been my opinion that one of the things that slows /2's down is slow up shifts.  One must wait for the motor to "spin down" in order to let the clutch out into the next higher gear. For example, you're in 2nd and wound way out past the "II"  redline mark on the speedo. You begin to the pull the clutch in and apply upward pressure with your left toe. Click, there's third. The motor is still winding down. Taking its good old time. Meanwhile I'm ready to release the clutch. Even if I let the clutch out smoothly, the bike will lurch forward and jerk the whole driveline. So I'm left to wait until the motor spins down low enough where I can release the clutch and get back on the gas.  Anywhoosit.  Scottie's Workshop offers this service on /2 flywheels. The cost for the service is $150.  

Getting better brake performance

Some people say BMW /2 drum brakes are terrible.  Some people say they are great.

They are both right.

BMW drums brakes can perform great, but oftentimes, they don't.

This article is about how to get the best performance from your BMW /2 drum brakes.  Although I'm talking about /2 drum brakes in this article, specifically the front brake, theoretically this article can apply to all drum brakes, even those on cars.

You may be replacing your shoes because they are worn, cracked or soaked in oil.

Even if your original shoes are in super condition, you still may want to replace them.  New friction materials have been invented (and are constantly being developed) that perform better than the stock friction materials available from BMW.  There are lots of different kinds of friction material available. Considerations when choosing a friction material are:  braking power when cold, maximum braking power, longevity, how much heat they generate, and how much heat they can withstand without getting losing their braking power, and how aggressive the friction material is on the machined surface of the brake drum.

I have been testing various compounds of friction material and I have come to appreciate a lining sold by Vintage Brake called 2520V (the VB3000 is also quite good).

The braking performance of the new linings is almost double  that of the stock BMW material.  Here is what Vintage Brake describes this friction material:

High friction compound with consistent feel and performance at all duty levels and rotational speeds, for off road and street use. Good first stop capability and with excellent water dispersal. Commonly used as a road race rear with front compounds that require heating."

In any case, I have found this to be an excellent street compound for both front and rear wheels on BMW /2 motorcycles.  I have been told by Mike Morse at Vintage Brake that this compound does wear the brake drum more aggressively than the stock fiction material, but not too bad.  Since most /2 motorcycles are not driven on a regular basis, one should expect a very long service life out of the drum.

Left to Right:  BMW factory material,  material sold by Benchmark Works, and Vintage Brake's VB3000. (Note: Benchmark Works and Vintage Brake products come uncut and undrilled, more on how to do that, below. Also note that  greasy hands and surfaces should be kept away from brake linings. The staining you see on the left most linings is not good for the linings.  These linings have been bumping around on my parts shelf for a while and with the newer and better friction materials out now, will likely never get used. 

For a drum brake to work properly, several things need to happen at once.  It is essential that all of these mechanisms function properly.

• The rider's fingers needs to squeeze the lever 
• The lever needs to pull the cable. 
• The cable needs to pull the brake arm(s). 
• The arms need to actuate the brake cams.
• The cams need to move the brake shoe closer to the brake drum.
• The friction material needs to come into contact with the brake drum.

In some future articles, I'll discuss how to improve the lever action, cable performance and so on. 

However, let's begin by discussing the guts of your the brake and how to improve its performance. 

Achieving Maximum Contact Patch:  Truing the Brake Drum 

It goes without saying that, to get the best braking power, as much of the friction material should be contacting the brake drum surface as possible. If the brake shoe is irregularly shaped (in other words, if it has "high" spots and "low" spots), if the brake drum is not perfectly round, barrel worn or bell mouthed, if there are glazed hard spots, or the surface is rusted, cracked or worn thin....then the brake friction material will not contact the drum braking surface as much as it could ... if both the material and the brake drum surface were true, "square" and circular. 

TIP:  The machined hub braking surface must be square, smooth (and of course, perfectly round) 

Examine the braking surface for grooves and, using an inside micrometer (or a pair of inside calipers and a vernier caliper), measure brake drum braking surface.  

Measure the inside of the brake drum at several points along its circumference. Always measure across the axle hole.   To test for barrel or bell mouth, keep both tips of the caliper the same distance from the edge of the shoe.



If the drum is wider at the bottom or middle, it is barrel curved.  If the drum is wider nearer the opening, it is bell mouthed. 

If you find that the surface of the drum is scored, grooved, barrel curved or bell mouthed -- or if the drum appears to be "out of round" or oval-shaped more than 0.004" then you should have the drum surface corrected to square and round.   This is done by mounting the wheel in a lathe and cutting the brake drum machined surface, such as our old Van Norman that we have modified to accept BMW Motorcycle wheels. 

Most motorcycle shops do not have a brake drum lathe. Head down to your  local independent brake shop mechanic and see if they can help you. Or send your wheels and brakes to us at Scottie's Workshop and we will set up your brakes for you. 

If your brake drum is not square and round, it will be very difficult to get good brake performance.  I always start by measuring brake drums and correcting them on our lathe if they are not perfect. 

Achieving Maximum Contact Patch: Bedding the Friction Material

What's the difference between front and rear brakes on a /2 ?   It's true that both the front and rear brakes on a /2 are identically sized.  Both use the same size drums.  Both use two shoes with the same amount of friction material surface area on each shoe.  However, the front brake of a /2 is much more powerful than the rear brake. Why is this?  

Answer: the /2 uses a twin leading shoe design while the rear is a single leading shoe (with a single trailing shoe). 

In a twin leading shoe brake, both shoes are driven by cams at the leading edge of each brake shoe.  In other words, the cam is pushing the leading edge of the shoe into the brake drum surface.  The other side of each brake shoe merely pivots on a hinge. As each brake shoe is actuated by its cam, the friction between the shoe material and the rotation of the brake drum causes the shoe to press itself harder into the shoe, thus increasing the effectiveness of the brake with less brake lever effort.






The rear brake is a single leading shoe (or "Simplex") design.


Note that this phenomenon only occurs when the wheel is rotating in the direction of travel.  If the wheel is turning "backwards", the front dual leading shoe brake has very little braking power, since both cams would be pushing on the "trailing" end of the shoe.

This is why, when you are trying to hold yourself from rolling backwards on a steep uphill,  the front brake of a /2 seems to barely work!  But try the rear brake!  It will work MUCH better than the fronts to hold the bike from rolling backwards. The reason for this is because the rear brake is a SINGLE CAM brake with two shoes: one shoe is leading, the other is trailing. A rear brake has equal braking action whether it used to stop forward or reverse motion.

The reason I bring all this up is to impress upon you how the brake shoes move.  Note that they do not move laterally into the brake drum surface. The leading and trailing ends of the shoe do not move equal distances. 

Unlike a disc brake where the entire shoe is moved into contact with the spinning rotor equally across the entire surface of the brake pad, in a drum brake, one side of the shoe is moved by the cam and the other side of the shoe simply pivots on a hinge.

Understanding how brake shoes pivot on their hinges is essential when bedding in brakes. 

A newly installed brake friction material will not contact the brake drum machined surface until it is "bedded down".  Bedding down means that the "high" points of the friction material have worn off and the "low" points can contact the brake drum surface, too. How much friction material should contact the brake surface?  As much as possible!

You have probably heard that you should "bed" in your new brakes using some special technique handed down by wise men over the ages.  Throw out all that hog wash. 

I've waited patiently for countless new brake linings to bed down on many, many vehicles.  While you are waiting, you can look forward to poor braking performance and the hazard of "glazing:" the brakes. Glazing occurs when the brakes get hot enough to transfer some of the brake shoe material to the brake drum surface. The resulting "haze" lowers braking performance dramatically and more importantly, it is very difficult to remove.  Sometimes glazed brakes need to be dismantled and hit with a wire brush, and in very severe cases, the drums need to be recut on a lathe to remove the glaze.

My goal when installing new brakes is to have them work at peak efficiency when the customer drives the bike out of my shop.  In order to do this, I always...

Tip: Minimize the Bedding-in Process by Cutting Friction Material On Lathe

Here is the basic idea: using the same technique you did when measuring the brake drum for defects, measure the inside diameter. If the brake drum is in good shape, the diameter should be the same at all points along the circumference.  Always measure across the center and always check diameter at the bottom, middle and top of the brake surface (check for barrel curving and bell mouthing). 

With the shoes mounted on the brake backing plate (or rear differential, if working with a rear brake), measure the outside diameter of the friction material surface.  In order to perform this measurement,  you want to do it with the brake shoes in a position where they are just beginning  to be moved towards the brake shoe when your fingers squeeze the brake lever. To  do this,  shim the lever cams about 0.020" to simulate that the brake is being actuated by the cam.   

With the brake shoes mounted on the backing plate as shown, mount the entire assembly in a lathe.  With the cams shimmed about 0.020", cut the friction material to have the same outside diameter as the brake drum inside diameter. 

On front brakes, be sure to remove the shims before assembling the brakes on the motorcycle. 

To cut the rear brakes, it is very difficult to spin the entire differential.  To facilitate cutting rear brakes, I made a plate which has the same measurements and geometry as the differential inside cover (rear brake  backing plate).  One of the posts is stationary and the other has a sleeve. I can change the thickness of the sleeve to "shim" the shoes outward.  I mount the rear shoes to this plate, clamp the shoes in place and spin the entire fixture in the lathe. 

With the cams shimmed to simulate the brake lever being pulled ever so slightly, the outside diameter of the brake shoe material should be cut to the exact inside diameter of the brake drum.

Once BOTH the brake drum and shoes are perfectly square and circular, they will have the largest possible  contact surface -- and will not have any high or low spots.   This means that the brakes will work with nearly 100% effectiveness the moment they are assembled.  I say nearly because the lathe's cutting tool can't make a super smooth surface on the friction material.  Because the friction material is relatively soft, it comes out just a little bit "fuzzy".  After machining in the lathe,  take some sandpaper in your hand and smooth out some of this fuzziness.

Even though the brakes will continue to improve as the friction material beds down even smoother, because the two surfaces are so closely matched, this final bed down process does not take very long.  No bedding in process is required.

The brakes will be impressive on the very first drive! 

Installing an LED tail lamp

A couple of years ago my wife was following me home from downtown San Francisco. She was in her car and I was on my /2.  She noted that, even wearing my bright silver Arai, I was nearly impossible to see at night. The /2 is a very "skinny" bike and it is very low.  Adding insult to injury is the stock /2 tail lamp, which is a candle in the wind. Since then, two manufacturers have started making LED tail lamps: Jim Franzen at CULayer.com and bulbsthatlastforever.com by Bruce Branstad.

On the Vintage BMW Forum, Jon Miller pointed out that the Branstad product has an edge in that the LEDs are dual brightness; when the tail lamp is on it is very bright. When the brake is applied, the same set of LEDs grow even brighter.  When the tail lamp is on (as it usually is for most motorcyclists who run the lights on all the time), this makes it very apparent that one is applying the brakes.

The Franzen product, on the other hand, illuminates a series of LEDs for the tail lamp.  When the brakes are applied, another set of LEDs illuminate with the same intensity as the first set of lamps.  At a distance it is difficult to see any difference in brightness.

Branstad's products are sold by Scottie's Workshop in either 6v or 12v versions.

63 24 8 654 131.1--$150.95--full LED brake & tail light 6v, replaces bulb holder and reflector R26-R27 R50-R69S
63 24 8 654 131.2--$150.95--full LED brake & tail light 12v, replaces bulb holder and reflector R26-R27 R50-R69S

Whichever system you choose, installing an LED tail lamp on your /2 is a great idea.  They draw less power and never burn out.

In this example, I'll be installing a Branstad LED tail lamp on a 6V  R60/2.

Update: I spoke to Jim Franzen at CULayer recently.  Unfortunately he has discontinued production of the CULayer /2 tail lamp.   

Update 4/1/17: I heard from Bruce Branstad recently and unfortunately, he also will be discontinuing production of his lamps.  Scottie's has a limited supply of Branstad lamps in stock. Hurry and get one before they disappear forever!

Unscrew the retaining ring screw and remove the tail lamp ring and reflector.

Trim any loose, excess or mangled wires...

...so you end up with a neat bunch like this. Tin the ends neatly. 

Gray is Ground -

Black is Tail lamp +

Red is Brake lamp +

Using a screw driver, press the long end of the springs and release them. Cover with a towel so they don't fly across the garage.

Remove the reflector assembly from the chrome ring and peel away the gray gasket.

Say GOODBYE old fashioned bulbs!!

Find the locating tab.  This is the top of the lens. 

Find the license plate LEDs.  They point towards the BOTTOM of the lens.

Align the LED unit and the lens, as shown.

Reapply the gray gasket to hold the LED and lens together.

Find the locating dimple in the chrome ring.

Align the locating tab in the lens with the locating dimple in the chrome ring. If you are right handed, hold the unit in your left hand as shown as you reapply the clips. 

Once the clips are attached you can connect the wires, install the chrome ring and LED assembly and screw in the chrome ring screw.

Test the tail lamp and brake lamp to ensure everything is working! 

Enjoy your new level of safety and reliability! 

Replacing brake shoe linings on a BMW /2 Motorcycle

It's not often you replace your brake linings.  They really last a long time. However you should inspect them every time you have the wheel off to make sure that they are clean and dry, and not too worn.

These are getting a little thin.  See how close they are to the rivet?    

The rivets are made from copper which is a soft metal which will not damage the brake drum. 

New Brake Linings. These are new old stock OEM from West Germany when Germany had an East and West, however you can also buy raw lining material and cut your own. The benefit of cutting your own material is that the performance is GREATLY improved due to the new technology employed in the friction surfaces.

Several different kinds of brake linings for /2. My current favorite is the VB3000 lining from Vintage Brake in Sonora CA.



To remove the old shoes, use a SHARP 1/8" drill bit to drill out the head of the rivet. You only want to remove the head. Do no drill too far. Don't damage the shoes, or enlarge the hole in the shoes.  You can also use a lever to pry off the old brake shoe, breaking it apart. Use side cutters to remove the loose rivets.

Clean shoe and line up the new lining.

Grab yo self a box o rivets.

The rivets are inserted with the heads on the lining side.  From underneath, a center punch held in a vice supports the rivet head.

Squeeze the pad to the shoe when peening the rivet. Make sure it is tight!

You can use a pop rivet tool to start the rivet, however I recommend hand-peening to get maximum tightness. I made a small peening tool to form the rivet head, however hand-peening with a ball-peen hammer is perfectly acceptable.

Stock linings last a long time!  I think that date is 12/66 !!

Looking so fine!

When reattaching shoes to the front brake, don't forget to attach the small lever return spring.

And the other side.

The brakes are ready to install.

And I'm ready to call it a day!