(first posted 12/27/2011; updated 10/26/2022) The development of brakes pretty consistently lagged the development of engines and speed, which made most really old cars very hairy to drive. One of the breakthroughs was the mechanical brake servo, given that the modern vacuum servo was still some time in the offing, as well as hydraulic brakes themselves. How to stop the giant luxury cars of the teens? In 1919, the brilliant Marc Birkigt invented the solution for the superb Hispano Suiza H6, and Rolls Royce quickly licensed the technology. And kept using it well into the 1960s.
This diagram shows the first version of the earlier mechanical system, purely mechanical, where the foot pedal operated both the front and rear brakes, both assisted by the servo. The hand lever operating on the rear brakes is an emergency system. The picture after the jump shows the later version where the servo works on a hydraulic master cylinder. Here’s how the servo worked:
(click to enlarge to very large original size)
The servo is very similar to a clutch, and driven off the side of the transmission. Pressing on the brake pedal brings the other face of the servo disc in contact with the driven face, essentially like slipping a clutch. That generates a variable degree of force, which here is transmitted to the master cylinder. In mechanical brake cars, there were linkages running front and back to the brakes from the servo (the Hispano Suiza was one of the earlier adopters of four wheel brakes).
This diagram shows the later version, whereby the servo worked on two hydraulic master cylinders. Either way, the mechanical servo system worked quite well, and was reliable. Which probably explains why conservative RR kept it for the new Silver Cloud in 1955.
Update: This system was replaced by a fully hydraulic servo unit in 1966 for the Silver Ghost in 1966, but it was kept on the Phantom VI limousine all the way to 1978.
Was it any easier to adjust than bleeding brakes now?
All-mechanical brakes required regular adjustments, and probably fairly often. But then pretty much everything about old cars required more frequent attention.
Amen. I’m doing this from memory, but I believe that the mechanical rod brakes on my 29 Model A required adjustment either every 500 or every 1000 miles. In addition, there were close to 30 grease fittings on the car that were to be lubed every 500 miles.
My great grandfather was a service manager for a Buick dealership in New London, Connecticut in the 20’s. Reportedly to check the brake adjustments they’d go out to the back lot, jump on the brakes and measure skid marks to see if they were even.
They had pretty much regular adjusters at the wheels. Servo would not retract very much, as long as the car was going in the same direction.
The travel of the brake pedal had more to do with the amount of boost that the servo would provide; that didn’t wear too much.
Very clever and complicated and expensive no wonder it was on high end cars, now back to the repairs on my 52 year old Lockheed hydraulic system.
A Hillman, perchance?
50 years (roughly) using the same technology? Sounds about par for the course. It’s probably just me but it seems like braking and tires always took a back seat to to power and style.
I do have to admire our forefathers though. I couldn’t imagine cable operated brakes and I know for a fact I don’t like 4 wheel non-assisted drums at all!
I was cruising with non assist 4wheel drums on Xmas day admittedly with a leaking rear axle seal and leaking front wheel cylinder but the stopping sensation is nearly nonexistant but untill I get the car registered it has to stay stock after that a disc front end is going in other wise its another $500 for a chassis modification cert and I simply dont have the money.
What an ingenious solution. I had no idea about these. How elegant! I misread the title at first, and thought that Rolls was still using mechanical brakes into the 60s. Oops. “The safety of steel from pedal to wheel” may have been good enough for Henry Ford in the 1930s, but I think that his cars were about the last to move to hydraulics. That mechanical power servo would have been somewhat helpful on my Model A, although not as helpful as siginificantly more lining area.
Yes, that title needed a bit of adjusting too. Done.
Yes, juice binders (hydraulic brakes) didn’t appear on Fords until 1940.
yeah, except it was actuating drum brakes, which are garbage.
I don’t know what you lot are moaning about.
The very best car I ever drove, 1903 Mercedes only had rear wheel brakes and the linings were cast iron.
If you had a real emergency you just put the car sideways.
Front wheel brakes are for sissies.
Ford’s mechanical brakes were also know as “press and pray brakes”……!
More about these- that picture doesn’t look correct. I recall that there were 2 master cylinders; 1 to stop when going forward & another to stop when backing.
After an amount of wear, braking action was fine until backing; the drum would travel around for a while until putting on the brakes for backing. The same thing happened when going forward & trying the first stop. You may remember the “Grey Poupon” commercials- 2 RRs going back & forth & not lining up.
There was also some extra emergency mechanical system.
An interesting addition- the system was pretty much anti-lock. When the rear wheels stopped turning, the drum would quit putting on the brakes, pretty slick.
It doesn’t quite work for me either. The two rods coming off that servo would want to move in opposite directions, eh? But it’s the only one I could find.
Looks like those may be cables, not rods. Cables would work, would they not?
Here’s a bit more detail about the brake servo-
http://www.classicrollssa.za.net/BrakeServo.html
Your picture does show the worm gear to drive the servo; it turned a lot slower than the driveshaft.
Bleeding the brakes involved being under the car with a big lever & manually operating the levers.
I just found a better diagram showing the purely mechanical system, and I’ve updated the post.
Ill dtick to one english car with brake problems that are far simpler thanx
Very simple in fact I took a drive and bought 2 r/h wheel cylinder NOS off the shelf it helps to know where the shelf is though The vendor will allow me back to photograph his52 Morris Minor convertablehe bought used 3 years old owned 56 years.
In the first picture, it is clearly visible that the pedal operates both front and rear brakes. The lever is for the handbrake and operates the rear brakes only. “Pedal”, by the way, comes from the latin word “pedes”, which means “foot”, so there can not be a “hand pedal”.
The purely mechanical layout was used from 1924 until 1939, when car production in England was stopped because of the war.
In 1946, Rolls-Royce and Bentley cars used an arrangement similar to the one in picture two – there was one hydraulic master cylinder for the front wheels, operated by the servo only, and a mechanical brake for the rear wheels. The latter was operated by the servo, and directly by the pedal as a kind of emergency brake.
From 1956 on the where two master cylinders, as in the second picture. Both where driven by the servo only, the larger, upper cylinder supplied the front and the rear wheels, the smaller, lower cylinder the second pair of wheel cylinders at the front. In addition, there where servo-driven mechanical brakes at the rear axle, as well as the emergency arrangement by direct pedal connection.
As opposed to the original layout by Hispano-Suiza, which worked only in the forward direction, the RR servo worked in both directions (forward and reverse) in the same manner: in the second picture, there are two rods going from the servo rearwards to the master cylinders, crossing each other on the way. The rod going from the top of the servo to the bottom of the vertical lever works in forward direction, the one going from the lower part of the servo to the top (hidden by a bracket in the picture) operates the brakes in reverse.
Because the servo has to turn a few degrees to work on the brakes, there will always be a slight delay. This delay from applying the pedal to operating the brakes by the servo will result in a distance of about 15cm that the car drives with emergency brake only – if it is more, the system needs to be serviced.
The lining in the servo drum is similar to a clutch lining; it has very little wear.
Thank you for the clarifications and additional information. I will correct the text. I hardly pretend to be an expert on this system.
Birkigt was a prolific engineer; he also designed a SOHC V8 engine used in SPAD & SE-5 fighters & a 20mm cannon used by the Western allies during WW2. The HS.404 was heavy but had excellent ballistics.
I was recently at the Rolls-Royce Museum in Dornbirn, Austria with a friend. It’s a truly magnificent collection with an on-site restoration shop.
When touring the shop, there was a disembodied chassis there and I noticed and we (both automotive chassis engineers) then marveled at the complexity of the brake linkages and wondered if they could ever be kept properly adjusted to work well.
I guess that they were in use for so long by R-R is a testament to the value proposition offered by this system (as well as to the “conservatism” of R-R and its customers.)
I have been lucky enough to have briefly driven two 1929 vehicles with mechanical brakes and one was a Rolls Royce (LHD Springfield USA assembled one). The other was a much lighter Chevy pickup.
The unloaded Chevy had sufficient braking power (for the time). No matter how we adjusted them, the result was a constant fight to stop in a straight line. The RR, while not acceptable by modern standards, seemed to self correct jerks and lurches and stayed pointed straight ahead. The owner of the car called it the first anti-lock brake system. Now I understand how it works.
I discovered recently that Rolls-Royce continued to use this system on the Phantom VI limousine until 1978 — 54 years in all. (Regular Rolls-Royce and Bentley cars dropped it when the Silver Shadow and T Series arrived in late 1965.)
I read that too after I wrote this, but forgot to update it. The Phantom VI was a rolling time capsule from the 1940s, or 1930s. I’ve just updated the post to reflect that.
Minor correction: The Phantom used it through 1978, not 1976. The first Phantom VI without it was a special car for QEII, delivered in March 1978, and the change was made in production later that year. Adding to the sense of antiquity, even subsequent cars with hydraulic brakes still used drums, allegedly because they were quieter.
Ok. Yes, squeaky discs would not be a good thing on a parade through London. 🙂