Saturday, July 13, 2013

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! 

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