Tuesday, March 5, 2019

Tim's front brake link-rod adjustment procedure

The Twin Leading Shoe Brake Setting and Adjustment Procedure below is the work of the Royal Enfield enthusiast who signs himself "Tim N.Z." on Royal Enfield message boards across the Internet.

Physics: For the Mechanical Twin Leading Shoe Drum Brake to work with 100 percent  efficiency, the brake levers and their link rod MUST be set as a True Parallelogram!

The greater the divergence from this, the more ineffective the brake will become! The greater the levers' deviation from parallel, and the larger divergence in angularity, the worse the brake becomes!

What is wrong with the method of 2LS brake shoe adjustment described on many web sites?

Most everything.

For the best of misguided reasons (ease of adjustment) owners are advised to drill out one trunnion, to allow independent adjustment of the brake arms.

DON'T DO IT! It is a misinformed method of brake adjustment that has been on the Net in a multitude of places for many years, and was written by some well intended person who had a complete lack of understanding of the functionality of mechanical drum brakes.

In the real world it means that you will be forever readjusting the brake, as one shoe will wear quicker, and one will glaze up sooner than the other, and you will never have 100 percent braking efficiency. With the ongoing "misadjusting" of the link-rod to compensate for the uneven brake shoe wear, lost efficiency is a never ending and ever worsening down-hill spiral.

The degree of accuracy required to accurately set a twin leading shoe brake is just as exacting as that required for setting ignition timing!

To which end, in the case of racing applications, we are talking about 0.002 inches or less in accuracy of measurement!  (Though for typical "road" use we don't need to be quite so particular.)

Once correctly set, the levers should NEVER need to be disturbed again, and braking efficiency will remain optimal.

Before commencing to correctly adjust and set the brake levers, the link rod and the trunnions that the rod screws into must be inspected:  IF either of the trunnions has been drilled-out you will need to buy a new link rod, trunnions, and nuts!

IMPORTANT:
To try to reset brake shoes that have long been misadjusted can prove to be a exercise in frustration due to the unevenly worn or glazed shoes. Often times it is advised to start afresh with a new pair of front brake shoes.

OR: Have the existing shoes fettled by a specialist with the linings radius skimmed back to a true pair.

Even after the brake levers have been correctly set, it can take a few weeks for used shoes to become fully and correctly bedded-in. If the brake remains ineffective after a program of rebedding by means of a series of controlled stops from increasing speed, it is advised to either find a brake shop that can retrue the linings back to a matched pair (see your local brake shop) or buy a new pair of shoes.

With new or reworked shoes fitted, the linings will need to be "broken-in" with a series of controlled "normal" stops with a minimum of a five-minute gap between each application to allow for cooling:

30 mph to a halt five times.
50 mph to 30 mph 10 times.
50 mph to a halt 10 times.

Followed by the same schedule but with max applied lever pressure (emergency stop application) .

Mechanical twin leading shoe brake functions on a very simple principal: Leverage.

The moment the levers no longer function as a true parallelogram, i.e. one lever is set at a greater or lesser angle than the other; you will have more or less pressure (leverage) being applied to one brake shoe, and more or less lift applied to the other.

For a twin leading shoe mechanical drum brake to function with its full potential, BOTH levers MUST be set so that they operate in perfect harmony; balanced leverage. Otherwise over a very short period of time you end up with only one brake shoe with full applied pressure in contact with the drum; thus one shoe is more inclined to glaze, or wears faster than the other.

The situation we are heading to will result in both brake plate levers set "square" to each other.

Both brake shoes lifting of their respective cam face in unison. With the angle of both levers set to be nearing 90 degrees in relation to the foot of the brake shoe.

AND that the angle of the brake cable pulling on the primary-lever (longer of the two levers) is also approaching 90 degrees to its arm.

The optimum point for the exertion of maximum leverage is when ALL of the levers and cams are moving up towards 90 degrees; at which stage you will have attained optimal leverage and thus full braking potential at every juncture in the system. Over-shoot that point and sweep past 90 degrees and braking efficiency drops! So you need to make allowance for future wear.

First step: lightly clean the drum surface with a pot scrubby or fine emery paper. You want a new, clean and fresh surface for the linings to bed onto again

With the brake plate assembly removed from the front wheel, the initial need is to commence by adjusting the external link rod that connects the two brake arms, so that the distance between centers of the trunnions is a nominal 144mm and that BOTH levers are set parallel to each other.

Occasionally, allowance must also be taken into account for "slop" between the trunnions and the brake arms, which will sometimes see one cam or the other commencing to operate before the other even though the theoretically correct distance between the centers has been set correctly.

Front brake plate exterior with brake arms set parallel to one another.
Adjust the external link rod that connects the two brake arms so that
the distance between centers of the trunnions is a nominal 144mm
and BOTH levers are parallel to one another.
Then turn the brake plate over and loosely sit the brake shoes back onto the plate. Closely inspect how the brake shoes sit against the actuating cams. Hopefully BOTH shoes will be sitting square and flat on BOTH of the brake cams. (They frequently do not).

Note: Permanently mark each shoe so that whenever they are removed, they are refitted to the same position on the brake plate! DO NOT swap brake shoes about!         

Interior of front brake plate.
...flip the brake plate over...
A tip: With the two levers set as a true parallelogram and centers at a nominal 144 mm (individually check your brake cams to confirm the center distance due to minor manufacturing variations) flip the brake plate over and sit the edge of a 12-inch  steel rule across and between both cam faces. The rule should sit flat and square to both cams at the same time (one edge of the rule on one cam face, the other edge of the rule on the other cam face). If the rule does not sit square on BOTH cam faces at the same time then the splines are not set true to each other; or one cam or lever is advanced/retarded from the other.

Ruler laid across interior of brake plate.
The ruler should sit flat and square to both cams at the same time.
Due to manufacturing tolerances, the precise location of the splines machined onto the external ends of the two brake cams, and the corresponding splines on the two brake actuating arms, may vary slightly. If need be the secondary (shorter lever) brake arm cam be turned over to help optimize spline alignment. Or, rotate either cam 180 degrees as there may be a variation in the face of the cams or the spline teeth on the cam.

Consideration must also be taken into account of manufacturing tolerances between the trunnions and their respective brake arms. Trunnion diameters, and eyes in the brake rod ends vary enough that it contributes additional ‘lost movement’ in the linkages, affecting the "true" length of the top side (the link rod) of the parallelogram. In this situation, set the link rod ever so slightly "long" to take into account the lost-motion "slop" in the system (We are talking less than 0.5mm).

With the correctly assembled and adjusted brake plate refitted into the drum, lightly apply the brake (centralizing the brake plate to the drum and axle), put a drop or two of Loctite on the Brake plate retaining nut threads (taper faced nut) and HAND "snug" tighten the nut. Do NOT over tighten this nut!

When refitting the now fully assembled front wheel into the front forks, ensure that the brake plate retaining nut is positioned HARD up against the side of the fork leg and axle cap. If need be a thin copper washer may be required to shim up any clearance. Having the axle nut flush hard up against the fork leg will prevent it from ever working loose in service.

If ever the brake plate rattles (loose axle nut) you are loosing leverage effect, thus further reducing efficiency.

Final adjustment to ensure maximum brake efficiency is to adjust out 99 percent of free play in the front brake cable, so that the brake shoes are just clear of the drum: Spinning of the wheel should be free and with out any scraping noise resulting from lining and drum making contact.

Excessive free play in the front brake cable seriously detracts from operational efficiency of the brake.

As a check on optimal brake shoe contact, strike a series of diagonal chalk lines (five or six) across the face of both brake shoes prior to refitting the wheel to the bike. Follow the braking bedding in instructions as initially advised, then remove the wheel, and brake plate and inspect the brake linings for signs of the chalk lines. If all has gone according to plan there will be very little residual chalk marks on either shoe.

Minimum acceptable contact area is 80 percent (only 20 percent or less of the chalk lines remain)  for a bedded in brake. But don't get too worried at this time, as it can take several months for linings to fully bed down. Check again after six months.

If you want a SERIOUSLY (racing) good front brake,  there is more that one can check and fettle and will require the use of a mandrel for the brake plate axle hole, plus two dial indicators, to manually "true" the cams with careful hand filing so that they both have exactly the same lift at the linings center point, through no less than 10 degrees of cam rotation.

There is benefit to be had in checking that BOTH of the brake cam pivot holes in the brake plate are on the same Pitch Circle Diameter. Not all brake plates are machined true and square to the axle in this regard. To check PCD,  mount a dummy axle into the brake plate and measure directly to both cam pivots.  If there is in excess of 0.5mm in their PCD it is advised to correct, as shoe-lift will be different. In which event either a new brake plate will be required, or specialized repair.

B0th shoe pivot posts and brake-cam holes should be equidistant from the axle, and that all three reference points are in the same plane.  If the shoes' fixed pivot posts, or the brake-cam holes in the brake plate are off center, then the central axle hole will need realigning (bushing) to match the fixed pivots center, after which both the holes for the cam pivots are checked, and cam lift confirmed.

Don't assume that any new brake plate is correctly machined!

All in all, fettling can sometimes take a couple of hours work, and the wheel can be in-and-out several times. The end result is a brake that is as equally effective as Royal Enfield's single piston disc-brake, albeit with marginally heavier lever pressure.

The all-up cost to have a "perfect" 2LS drum brake? Usually less than the price of a disc brake conversion in both money and time.

6 comments:

  1. Spelled brake wrong in heading. lol

    ReplyDelete
  2. Roger, THANK YOU for pointing that out. My mistake, not Tim's. If you hadn't commented I might never have noticed it and got it fixed.

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  3. I was shown how to do this by The Chief Engineer at Madras back in the early 1990's. Mr. Murali was his name, nice chap and rode a lovely 350 Bullet. He was one of the only Executives who rode a bike to work! The others drove cars. The Chalk method works well and the instructions are also in the Enfield Bullet Manual...which Mr. Murali wrote.

    Cheers

    Lloyd Gibbs
    Rider of "Old Clunky".

    ReplyDelete
  4. Quite an article Dave, you'll have to start on your bike today. lol

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  5. This is a excellent article. I was always of the "drill out the rod" school of thought and can see I was way off base. A side note - RE could not understand why we were worried about a basically ineffective front brake. At that time using the front brake was considered suicidal by Indian riders. No one used it. Marty Scott also worked with them to try and retool some parts like brake shoe and operating cams which were out of spec. I have thought about arching the shoes myself - do shops still do this? It was common in my day asbestos and all. The shoes RE used were as hard as concrete and didn't help the equation. I was told by many old timers and RE racers that the front brake could be made to work well - just never believed it. This should open up a new world for drum brake owners

    ReplyDelete
  6. Excellent write up by Tim, a very knowledgeable bloke On Enfields.

    ReplyDelete

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