Tuning For Speed: the Vincent Black Shadow
Part 4 - Anchors
by Sam Q. Fleming
My misspent youth involved a fair amount of riding around the country with a tent strapped to the back of a 1970 BMW /5 while wearing inadequate weather protection. That /5, currently hanging from a hook in my basement awaiting time and attention to bring it back to running order, has a twin leading shoe front brake and a single leading shoe rear drum brake. With an incredible amount of care and attention they could be made to work acceptably, for a 1970 motorcycle, when all brakes absolutely sucked.
In the mid-1980s a neighbor recklessly invited me to ride his 1950s BSA Goldstar. Imbued with the overconfidence of youth, I set off. I arrived at a green light only to find that an elderly woman had decided to roll through her red light in an approximately 5000’ long El Dorado. I got on the binders only to find that the low speed initial bite almost immediately faded away to nothing as I helplessly drifted through the crosswalk and narrowly missed her tailfin.
Drum brakes indeed.
My first ride on this particular Black Shadow eventually ended with a complete failure of the front
brake system. This was caused by the tip of the brake cable socially distancing itself from the cable.
There is a whole art to properly soldering a control cable which involves spreading the wires after
feeding them through the hole just so, then using your solder pot to attach the properly cleaned
and fluxed pieces together. It is definitely a skill that either involves guys with a lot of ear hair, or
people who aspire to having hairy ears. This cable seemed to have failed because the wires were
not spread and cleaned before soldering. We subbed this out and just bought a new cable.
(Photo by AOD Ministry of Information)
A drum brake has a round drum attached to the inside of the wheel. The drum is, say, seven inches in diameter and one inch in width. There are then shoes which are pushed up against the inside of the drum. That is where the trouble starts. For a given brake shoe friction compound, the brake force is initially contingent on the surface area of contact between the shoe and the drum, as well as force with which the shoe is being pressed into the drum. A single leading shoe drum brake has an eccentric (a little cam) that rotates when the lever is pulled, pushing the shoe outwards into the drum. However, the pivot that the other side of the shoe is rotating around isn’t moving, which means, depending on how cleverly the shoe is tapered, only a portion of the brake shoe is ever in contact with the brake drum. Double leading brake drums have two cams so each shoe has a leading edge pressed outwards and more friction material can make contact with the drum. Before decent hydraulic disk set ups obsoleted all this rubbish, there were double-drummed, four leading shoe drum brakes! Remember, all those eccentrics have to be very carefully adjusted to move exactly at the same rates! And all those cables, splitters and eccentrics all had friction.
The Vincent Black Shadow deployed all the tricks the Phils could devise to maximize brake performance.
The front end has two single leading shoe brakes. The big shiny T-Bar is the axle. It is designed so you
can remove the front wheel without tools by stepping on the T-Bar with your boot. The brake cable is
grabbing the eccentric lever which twists a cam pushing the shoes out into the drum. (Photo by
AOD Ministry of Information)
And that is just to get the shoes to generate heat. Once they are converting the kinetic energy of the moving bike into heat, the real problems start. The friction material can start gassing, which causes the shoe to partially float off the drum. Or, in the case of my BSA stop, the inside of the friction surface of the drum (which has more mass) expands at a slower rate that the outside of the drum (where it’s thinner) and the drum basically becomes more of a cone than a cylinder. As it does this the shoe loses contact with the drum and you become much more likely to become one with a Cadillac trunk.
This is the front brake balance bar. As the primary cable is currently missing, being broken,
there isn’t a cable going through the hole on the right hand side of the bar. However, once it
has that cable back in there and the rider is trying to crush the level to the bar, the balance bar
will get pulled down to the right as the eccentric lever on the drum is pulled up. That, in turn, pulls
up on the beam’s left hand side cable to actuate the second drum. (Photo by AOD Ministry
Here is the balance beam with the primary brake cable installed. Using the adjuster on the left
and right side of the beam one can try to get the shoes as close to the drums as possible without
them actually dragging. However, given the tenuous frailty of the physical world, all these parts
flex to some degree. The eccentric in the brake plates flex, the levers attached to them flex, the
cable stretches a tad, the balance beam flexes and, finally, the brake lever and lever mount flex.
All of this means that the braking experience for the rider is like squeezing a squash ball.
(Photo by AOD Ministry of Information)
In 1952, the roads were terrible and the tires even worse, so, even though the brakes were not very good, there wasn’t a whole lot of traction available downstream in the system. That said, our intrepid Phils (Irving & Vincent) did the best they could for the Black Shadow with the technology available to them. Mainly this involved cleverly fitting as many drums to the bike as they could (four!) and installing robust drums.
This is the right side front drum. After that balance bar gets pulled down to actuate the primary drum,
it lifts up on the cable to actuate the secondary drum. One of the big upgrades for the Black Shadow
compared to the weak sauce base model, Vincent Rapide, is the ribbed drums. Those ribs provide
additional surface area for the drums to shed heat but, also, they provide concentric physical
support to keep the drum from distorting from thermal expansion during long braking periods.
The big drive is for the speedometer which allows you to quantify, specifically, that you are currently
going faster than these brakes will allow you to stop. (Photo by AOD Ministry of Information)
With modern sporting motorcycles we’ve engineered the bikes so the front tire is primarily
used for braking and turning and the rear tire is primarily used for acceleration. In 1952,
the idea of unweighting the rear wheel with the front brakes was ludicrous. Phil Irving wanted
to take advantage of that hefty proportion of weight still firmly sitting on that rear tire, in all
circumstances, so he doubled the friction area of the rear brake by fitting dual drums to the rear
wheel. The rear drums are ribbed like the fronts to shed heat and help preserve concentricity
under load. The rear axle can also be removed by stepping on the T-bar. (Photo by AOD
Ministry of Information)
There are dual leading drum kits and even disk conversion kits available for the Vincents but, short term, I set about trying to optimize what I had by getting it all working as well as possible. After doing almost everything we could with the stock set up, I determined that if I was going to actually log serious miles on the bike I would have to upgrade to something more substantial, but in the short term I’d have to use rider discretion to never override the brakes. Rider discretion has not really been my strong suit.
The rear brake pedal is on the left of the motorcycle. The whole footpeg assembly is held
on with a tapered cone fitting pressed together by the single nut by the cable holder. That
means, if the brake lever or peg isn’t where you want it, loosen the nut, rotate the whole
assembly, tighten the nut again. Adjustable rear sets in 1952! It is actually a little crazier
than that because you can actually flip the rear brake pedal to the back of the mount as there
is a secondary boss on the back to hold the brake cable. If you do that you can remove the
front pegs entirely and ride with only the passenger pegs and still reach the foot controls
because you can flip the shifter as well. The big circle with bolts on the right of the photo
is the 6 volt horn. The aluminum piece at 10:00 is the world’s most labor intensively
produced dip stick. (Photo by AOD Ministry of Information)
The Black Shadow has a rod that goes from one side of the bike to the other through the
swing arm pivot. Actuating the rear brake pedal rotates that rod forward which pulls on
solid rods, on either side of the wheel, to pull on the single leading shoe eccentrics. In this manner,
both of the drums’ stopping power can be accessed. (Photo by AOD Ministry of Information)
There is a lot going on here. First, that big aluminum knob at 2:00 is a combined seat support
and auxiliary friction damper (compression and rebound!) for the rear suspension. The long
horizontal shiny rod is the secondary rear drum brake actuator rod with a brass wing nut to be able
to synchronize the drum action between the left and right drums. The piece with the electrical
connectors and the spring is the brake light switch. There is no switch for the front brake
requiring modern riders to constantly remember to always use the rear brake as well. The exquisite
aluminum piece under the switch spring with the knurling is the chain adjuster. Although replacing
the front sprocket on the Black Shadow is daunting, adjusting the chain can be done without any
tools! You can loosen the axle with your boot and adjust the chain using the finger-tip adjusters.
(Photo by AOD Ministry of Information)
After the last test ride of the season we rigged a ramp up the stairs into the house and moved the bike into its winter quarters in the dining room. Dr. Dwyer is fettling his Vincents and, coronavirus and weather permitting, we hope to soon be able to let the hills of Maryland reverberate with the roar of these dinosaurs.
The AOD Black Shadow, with restored carburetors, optimized brakes, 6v LED lighting, and modern
ignition magneto, along with a drained tank, sitting in winter storage in the AOD dining room astride its
mandatory drip pan awaiting warmer weather and dry roads. (Photo by AOD Ministry of Information)