Summary

From 1953 to 1978, British automaker Rolls-Royce manufactured its own versions of the General Motors Hydra-Matic transmission for use in Rolls-Royce and Bentley luxury cars. One version was very similar to the Hydra-Matic used in American cars and trucks, but Rolls-Royce later created an improved aluminum-case version with modifications to provide smoother shifting. This version was offered in some versions of the Rolls-Royce Silver Shadow I and Bentley T Series from 1966 to 1969. The older version remained in use in the big Phantom V and Phantom VI limousines until 1978.

SIDEBAR: Understanding the Rolls-Royce Four-Speed Automatic Gearbox

Like the Dual-Range Hydra-Matic on which it was closely based, the Rolls-Royce four-speed automatic gearbox had a two-element fluid coupling (able to transmit torque, but not to multiply it), three planetary (epicyclic) gearsets, two brake drums (one connected to the front unit sun gear, the other integral with the rear unit annulus), two servo-controlled contracting band brakes, two multi-disc clutches, and a single cone clutch, providing four forward speeds, reverse, and neutral.

The fluid coupling’s torus cover, which was bolted to the engine flywheel, drove a short input shaft, which drove the front oil pump and the annulus (ring gear) of the front planetary gearset at engine speed. Passing through the input shaft was the hollow intermediate shaft, which was integral with the front unit planet carrier; the intermediate shaft was splined at one end to the fluid coupling impeller and at the other end to the hub of the rear multi-disc clutch. Passing through the intermediate shaft was the transmission’s main shaft, splined at one end to the fluid coupling turbine (driven torus) and carrying the rear unit sun gear at the opposite end. The rear unit planet carrier was integral with the output shaft, which was also splined to the reverse unit planet carrier and drove the rear oil pump and centrifugal governor. The rear unit annulus was affixed to the reverse unit sun gear, while the reverse unit annulus was part of the cone clutch assembly, which when engaged would lock the annulus to the transmission case.

In this diagram (which is not remotely to scale and is not intended to represent the actual appearance of the transmission’s innards), each color represents a set of components that rotate together: the flywheel, torus cover, front pump, and front annulus (light green); the front sun gear and drum (orange); the front planet carrier, intermediate shaft, fluid coupling impeller, and rear clutch hub (red); the fluid coupling turbine and rear sun gear (dark blue); the rear annulus and drum and reverse sun gear (light blue); the reverse annulus (dark green); and the rear and reverse planet carriers, rear pump and governor, and output shaft (purple). (author diagram)

To create the transmission’s four forward speeds, the front and rear planetary gearsets were compounded as follows:

Front brake band engaged, front clutch released, rear brake band engaged, rear clutch released
Front brake band released, front clutch engaged, rear brake band engaged, rear clutch released
Front brake band engaged, front clutch released, rear brake band released, rear clutch engaged
Front brake band released, front clutch engaged, rear brake band released, rear clutch engaged

In neutral, all brakes and all clutches were released. In reverse, the front band was engaged; the front clutch, rear clutch, and rear band were released; and the reverse cone clutch was engaged. Although there was no separate Park position on the shift quadrant, there was a parking pawl, which engaged to hold the reverse annulus with the engine off and the transmission selector in Reverse.

Gear selections were hydraulically controlled through a series of spring-loaded shift valves — some manually operated and some capable of automatic operation — which applied oil pressure to engage or release the various elements. Automatic shift points were determined by output shaft speed (signaled by hydraulic pressure metered by two centrifugally controlled valves within the governor), throttle position (signaled by hydraulic pressure metered by a throttle valve controlled by a series of rods and links connected to the accelerator pedal), and the tension of the valve springs. Governor pressure acted to push the automatic shift valves in the upshift direction, opposed by spring and throttle valve pressure. In this way, the transmission would upshift at relatively low road speeds under light throttle, while wider throttle openings would delay the upshift until higher speeds. As road speed decreased, governor pressure would diminish until spring tension and throttle valve pressure moved the shift valve in the downshift direction. (The control system had many additional nuances, but these are the basics.)

Just like the contemporary Hydra-Matic transmission from which it was derived, the Rolls-Royce automatic gearbox’s unusual layout had several important effects. First, because the fluid coupling was driven by the intermediate shaft through the front gearset, the fluid coupling impeller actually rotated about 30% slower than engine speed in first and reverse gears, reducing the efficiency of the coupling to limit the tendency to “creep” at idle speed with the transmission in gear. In neutral, the fluid coupling normally didn’t rotate at all (although excessive friction in the bands or clutches might transmit a small amount of torque to the intermediate shaft, especially in a transmission that wasn’t yet fully broken in).

As we’ve discussed in detail in our article on split-torque transmissions and lockup torque converters, the intermediate shaft also served to split engine torque along two paths in third and fourth gears. The main shaft continued to drive the rear unit sun gear in third and fourth, but with the rear clutch engaged, the rear unit drum and annulus rotated with the intermediate shaft, which was not subject to hydraulic slip. This reduced the net effect of slippage in third and fourth by about 62% — not a full mechanical lockup, but much “tighter” than a conventional fluid coupling or torque converter arrangement, improving engine response and cruising efficiency in the higher gears. (Even so, fuel economy was still not a Rolls-Royce or Bentley strong point; contemporary road tests suggest that the best one could expect in all but the gentlest driving was around 17 miles to the Imperial gallon, equivalent to about 14.2 miles to the U.S. gallon or 16.6 liters/100 km.)

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Schurkey

September 7, 2022 at 12:18 am

1. Thank you for new content.
2. I appreciate the research undertaken to provide this new content. Entertaining and informative. Job well-done!
3. I learned that Rolls switched to Chrysler-style lifters for their OHV V8 engine; they did prototype work with Buick lifters and had camshaft lobe failures. Not surprising–Buick lifters (Nailhead/Nailvalve era) had no crown, the Buick camshaft had no taper, and the Nailhead lifter bores were not offset on the lobes. The Buick Nailhead lifters were not intended to rotate in their bores. I don’t know of any other V-8 OHV engine that intentionally does not spin the lifters. Lifter spin reduces wear.

Reply
1. Ate Up With Motor
  
  September 7, 2022 at 2:34 am
  
  The Chrysler lifter design is mentioned in the text, as it was another example of Rolls-Royce engineers considering a problem, deciding existing American technology offered the best solution, and then arranging to manufacture it in-house. It’s also clear that developing the V-8 involved a pretty steep learning curve in various respects.
  
  Reply
Jerry Thrush, MD

September 15, 2022 at 10:30 am

An excellent review. Thank you!

Reply
jiro

September 17, 2022 at 7:39 pm

Aaron, while automatic transmissions in luxury cars are not my jam, reading your wonderful scholarship is. Thank you!!!Jir

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Chris In Australia

September 21, 2022 at 3:47 am

Aaron, great to see new content here, and to the same high standard as ever. I wonder if RR’s decision to licence and build, rather than merely import was due to to government currency restrictions regarding Sterling and the US Dollar?

Reply
1. Ate Up With Motor
  
  September 21, 2022 at 11:53 am
  
  I had the same thought, particularly when I saw that Rolls-Royce arranged to pay for the imported transmissions used in 1952–1953 cars in U.S. dollars. My understanding is that in that period, for a British company to pay for a transaction in dollars would have been only slightly less politically fraught than arranging to pay in bone marrow! However, Robotham’s account makes no mention of political considerations like that when he went to the U.S. in spring 1946.
  
  Also, if the rationale for licensed production was political, I have to think GM management would have been less puzzled. Wilson, surely, would have understood the impetus for local production due to import restrictions, unfavorable currency exchange rates (although it’s notable that the trip took place well before the subsequent devaluation of the sterling), or the costs of importation. The only explanation Robotham’s memoir offers for not wanting to import the transmissions rather than build them is that it was “unthinkable.”
  
  Reply
2. Don
  
  January 2, 2023 at 11:24 pm
  
  I drove a Rolls for a wealthy fellow in the early 90s..it had the exact same radio/cassette player as my ‘86 Cheney S-10, albeit with gold plating.
  
  A nice ride. What I remember most was it was quiet, and there was little or no plastic in the interior
  
  Reply
Scott

November 27, 2022 at 7:09 am

Coming in a bit late..many years ago I read that when RR received the first batch of Hydra Matics, they were shipped dry (of course) and RR as was their procedure, decided to “improve” the finish of the internals parts by cycling walnut hulls through them. This caused problems (I think the walnut oil messed up the rubber seals) and GM told them to stop doing that.

Reply
1. Ate Up With Motor
  
  November 29, 2022 at 9:20 pm
  
  I’m just glad that anyone is still reading, having gone years without new content to speak of.
  
  With Hydra-Matic, it’s clear that Rolls-Royce made a variety of efforts to gild the lily, as it were, generally ending with the crew in Crewe having to concede that there was no good in fixing what wasn’t broken.
  
  The one really substantive change they made from a mechanical standpoint (prior to the aluminum-case version used in the early Silver Shadow) was eliminating the forced upshift from second to fourth in Low range. With the Dual-Range Hydra-Matic, GM had concluded that it was better not to let the driver override the automatic controls to the extent that it risked over-revving the engine; you could delay upshifts, but no further than the hydraulic controls would otherwise allow at WOT. Rolls-Royce apparently felt that if a driver manually selected “2,” they damned well wanted second gear and shifting up should be at their discretion. This might have improved performance a bit, because with the GM units, the automatic upshift in Low was to fourth, not third. Winding out in second and then shifting to “3” would probably give slightly better acceleration. How many Rolls-Royce drivers would have bothered, I have no idea, since it would seem likely to spill milady’s mustard.
  
  Reply
Al

December 8, 2022 at 6:31 pm

Echoing others’ comments, it’s great to see some new content and the older articles continue to be prime points of reference. A bit of a tangent but the mention of Robotham’s name being abbreviated to two letters in internal memos reminded me that when I joined the Rolls-Royce aero engine business in the mid ‘90s (which had been separated from the car company since 1971), the Derby part of the company was still using this convention to the great confusion of those of us who worked at other sites. I’m not sure when it died out, but not long after.

Reply
Paul

March 6, 2023 at 3:30 am

Still reading!

Reply
Chris

July 2, 2023 at 3:56 am

Another great read, and I thank you for it. Not that I’m in the market for one, but couldn’t R-R at least put at the column shift on the left of the column for RHD cars?

And that steering wheel (styling wise) is more suited to a bus.

Reply
1. Ate Up With Motor
  
  July 2, 2023 at 4:25 am
  
  And that steering wheel (styling wise) is more suited to a bus.
  
  Which wheel? The Silver Cloud wheel does seem rather bus-like, perhaps because until the Silver Cloud II in 1958, power steering was a pricey extra, and some allowance had to be made for the chauffeur on cars without power assist. (Cars with power steering did get a smaller wheel set closer to the dash, but the difference in diameter was about an inch, and so I think it would take a practiced eye to spot the difference.) The two-spoke wheel in the Silver Shadow doesn’t strike me as particularly bus-like, although I can’t say I find it particularly attractive or in keeping with the general ambiance. A detail, I guess, but at these prices, the details ought to be exquisite.
  
  Reply
Brian Addley

September 18, 2023 at 4:24 am

I only discovered this site about a month ago and it is now my favorite auto site along with Mac’s Motor City. I really appreciate your in depth dives into various models and technology. I hope you continue to publish more as I can’t get enough!

Reply
Leon Wittwer

December 11, 2023 at 2:27 pm

Thank you for a very informative amount of info on RollsRoyce/Hydramatic transmissions.
My daughter is interested in a 1956 Armstrong Siddely fitted with automatic transmission,when I queried the seller is it automatic or preselector he said someone told him it was the same as RR used.
That car is now a lot more interesting as a result of your information thank you.

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Don’t Call It Hydra-Matic: The Rolls-Royce and Bentley Automatic Gearbox

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