YESTERDAY WE ENJOYED SEVERAL TIDBITS from Pur Sang, a publication of the American Bugatti Club. Today in Part 2, we focus on fascinating applications of the differential: one in Bugatti cable braking, the other in a Chinese Imperial Cart of two millennia past.
Ettore’s Cable Brake Equalizers. Bugatti famously said he made his machines to go, not to stop. And, indeed, though he assayed hydraulic brakes on the Type 28 prototype and some Type 30s, Bugatti Revue observed, “the system didn’t work reliably, Ettore did away with it and invented the best ever all-mechanical brakes.” These continued in use into the late 1930s.
A feature of these cable-actuated brakes was discussed in this particular Pur Sang in “Further Thoughts on the Differential” by a member with pseudonym Knuckle.
The brake compensator assembly from a GP Bugatti.
“Take a look,” Knuckle wrote, “at the brake compensator assembly. It is formed by the [spider] gear mounted on the forward side of the brake pedal forging and the two side gear sections which rise from the brake cross-shaft.”
(Not shown are levers actuating cables, chains, and cams linked at the other end to the brake shoes. As you can imagine, one challenge with mechanical brakes is obtaining equal braking at the four wheels. And, note, the physics of hydraulic pressure solves this inherently, albeit with the tradeoff of leaks.)
A close look at the right front wheel shows the cable, pulley, and lever actuating its brake. From the Pur Sang centerspread description: “… the driving is like the driving of Jehu the son of Nimshi; for he driveth furiously.” 2 Kings IX, 20. F’auto Delta. (Not your typical magazine caption, eh?)
Back to Bugatti’s brake-pedal differential: Knuckle continued, “If you press on the brake pedal, you will see that as the pedal moves the spider gear forward it turns a little one way, and possibly a little the other, as it takes up the play on each side of the brake system’s cables, chains, levers, and cams and presses evenly forward on both gears.”
Knuckle noted, “If you were to press harder on the pedal you would see that the force would be taken equally by each of the side gears, the spider turns no more as it is splitting the effort to the bevels.”
All very clever and logical, given the distrust of hydraulic systems.
The Emperor Points. Discussing differentials in general, Knuckle said, “Whoever might take the credit for modern use, the differential was well known to the clever artificers of the last emperors of the Han Dynasty of China as early as the second century A.D. These amusingly inventive men created all sorts of entertainment to please the court. One of the most delightful was a curious wagon in the form of a chariot with an imposing figure of a man aboard.”
Image from Simple Working Models of Historic Machines by Aubrey F. Burstall, MIT Press, 1969, via Pur Sang.
“The man’s arm was outstretched,” Knuckle wrote, “and his finger always pointed south no matter how the wagon was pulled and turned about. This effect was achieved with differential gearing as shown in the drawing wherein gears #5 and #8 are the bevels and #6 and #7 are the spider gears.”
Which reminds me that my own source for this marvelous cart was A History of the Machine, by Sigvard Strandh, A&W Publishers in association with AB Nordbok, Gothenburg, 1979. See “Understanding the Differential” here at SimanaitisSays.
Exhibit in the Science Museum in London, England. This conjectural model chariot incorporates a differential gear. Image by Andy Dingley from Wikipedia.
What fun perusing old magazines and following up on their topics. ds
© Dennis Simanaitis, SimanaitisSays.com, 2024