After mentioning underdrive as an integral feature of Chrysler’s Fluid-Drive semi-automatic transmission in yesterdays 1946 Town and Country, it might be appropriate to shed a bit of light on its over-achieving relative: Overdrive. For you younger readers, no, we’re not talking about the top gear in modern stick-shift or automatic transmissions. This is about epicyclic or planetary overdrive (not interstellar overdrive, which also has its joys), as pioneered by Borg-Warner in 1934, and available on various American cars and trucks with three-speed transmissions until 1972, when the last Ford pickup free-wheeled off the lines with one.
Borg-Warner’s “automatic” overdrive premiered on a car that was quite advanced and adventurous in other ways: the 1934 Chrysler and DeSoto Airflow. Allpar has this to say about the origins of the B-W OD:
The “fourth gear” using the (Rex) Keller clutch was invented by Chrysler, but to avoid $25,000 in tooling costs, Walter P. Chrysler chose to have Borg-Warner make and supply it. That meant that competitors could also buy the system (Breer book, p. 124),
It would be natural to assume that Chrysler chose to take of advantage of this new technology because of the Airflow’s other technical advances and presumed higher cruising speed, thanks to its aerodynamics. But there was also another one.
In the twenties through the forties, American cars (and their drive trains) were designed for the prevailing conditions, which were very different than more modern ones. Towns and cities were compact and dense. Everyone still knew how to parallel park, even. Trips were typically short, with lots of starts and stops.
And highways were narrow, often with steep grades and tight curves. Highway speeds were typically around 45 or 50. Higher speed parkways and throughways were just emerging in the largest cities. Cars were geared to these conditions, with very low (high numerical) rear axle ratios, typically between 4:1 and 5:1. Combined with very long-stroke engines that preferred to run at low rpm, most cars really weren’t happy above 50 mph or so, except for short bursts.
The other key factor in drive train design was that transmissions were balky. Cadillac had the first syncromesh transmission in 1929–only on the second and third gears–and it eventually spread throughout the industry. Even with syncromesh, down-shifting into first meant either coming to a full stop, or deftly double-clutching. That would be the norm until well into sixties.
Although a substantial improvement over the crash-boxes before, these early syncromesh transmissions were hardly a joy to shift, never mind the heavy mechanical clutches. Women in particular were none too thrilled with that whole aspect of driving. The search for clutch-less and automatic transmission solutions had a strong motivation.
There were many approaches to this problem, and the fully automatic transmission was the Holy Grail. GM’s Hydramatic of 1938 got there first, but it was complex, thus expensive. Meanwhile, other solutions arose, like Chrysler’s two distinctly different Fluid Drives. But even simpler and cheaper part-measures also abounded. This included free-wheeling, which allowed the car to “coast” whenever engine speed dropped low enough to not actively power the car. This also enabled essentially clutchless shifting. But the danger of cars outrunning their brakes on long downgrades soon put the kibosh to that. “Automatic clutches” had a vacuum servo that sensed when the driver was about to shift, and obliged him or her by disengaging the clutch–as long as it was adjusted properly. Both had a short life.
Borg-Warner’s epicyclic overdrive is typically thought of as affording only the benefit of lower engine speed on the highway, and that it did admirably, offering a 30% reduction. A 1934 Chrysler Airstream hustling along at 75 mph reduced engine speed from a near-redline 3780 rpm to a much more tolerable 2650 rpm.
But the other benefit was in essentially eliminating shifting altogether in town driving. Overdrive? How that? Because most cars had such low gearing, starting out in second was very doable on flat terrain. At above the minimum engagement speed of 20 to 32 mph (depending on manufacturer), a quick lift up on the gas pedal, and the overdrive kicked on, creating a gear ratio almost perfectly spaced between normal second and third, which made a perfect gear for cruising on typical city/suburban roads in the 30-35 mph range. A stab on the gas pedal resulted in a downshift to second-direct, for passing or hills. Or if the car slowed down below the governor speed of 20 to 32 mph, and the gas was eased up a bit, it would also shift back down.
And there’s more: if a rolling shift into (unsynchronized) first is necessary, the shift was a breeze without any clash, because the combination of the clutch at the front and the freewheeling on the output shaft meant that there were no forces acting on either shaft that had to be synchronized. And if one had a bit of timing, the shift to first could be made without pressing the clutch at all.
My ’66 Ford F-100 was originally built with Ford’s new all-syncro three-speed manual. When the transmission crapped out a couple of years after I bought it, I wanted an overdrive, and I knew a guy who messed with these things and had a few out back. For simplicity’s sake, we didn’t replicate the electric system for “automatic” functioning. Frankly, I preferred it that way, having full control of the OD in all gears. But I failed to understand how to make full and best use of it, and only activated (unlocked) it when I went out on the freeway, which alone was a big boon in dropping revs to a very comfortable 2000 rpm at 60 mph. But I was annoyed that the first gear was now unsynchronized.
It was only some years later when I read a very detailed article on the B-W OD (highly recommended for all the details I can’t fit into this article) and I totally changed my habits. Now I have OD enabled all the time, except when I want to avoid freewheeling while in a direct gear (there is no freewheeling when in any OD gear). And I only use the clutch for starting and stopping: all my other shifts, up and down, are made without the clutch, thanks to the freewheeling acting like a clutch, even if it is on the other end of the transmission.
Now my truck has five very nicely spaced gears, and I use them all regularly. Most of all, I use second-OD in town, as its perfect for the typical 25-35 mph zones. American three-speed transmissions tend to have an overly large hole between second and third; in town one is either revving faster than necessary, or chugging. And when I regularly hauled a load and a trailer between Oregon and California, second-high was perfect for the steepest sections of I-5, which my truck couldn’t take in third before.
The BW OD, typically R11 and R12 units in modern times, sits behind the legendary BW T-85 transmission. The round object projecting from the side is the speed governor/solenoid, that allows OD to engage at 28 mph (in the modern era). As such, OD is realistically available only on second and third. With my manual set-up, I can engage OD in any gear, although first-OD is so close in gearing to second-direct, I hardly ever use it, except to avoid shifting if I’m in stop-and-go traffic or one of those residential streets with a stop sign at every block.
I’m not going to do a detailed mechanical and electric explanation of the BW OD here. The article I linked to above is excellent in covering the basics of its design and operation. And if you want more detail, here is a pdf of Borg-Warner’s original manual. A great read, if you go for such things.
Another significant benefit of the B-W OD came into play during the early years of the high-performance era, prior to four speed manuals becoming readily available. Ford didn’t have a four speed until 1962. Ordering the super-tough BW T-85 with OD allowed one to specify a very low (high numerical) rear axle ratio for maximum acceleration, especially on the drag strip. But 4.11 0r 4.56 gears were almost impossible to live with on the freeway, so OD made that possible.
Overdrive was not consistently available on all cars; in fact, none of the GM divisions except Chevy offered it, and then only after 1955 after they abandoned the torque-tube driveshaft, which made OD essentially impossible to add. Same went for Ford, until they dropped their torque-tube in 1949. Somewhat oddly, Dodge and Plymouth shunned OD after the late fifties, perhaps because they were so invested in their excellent Torqueflite automatic. The independents were the most consistent and enthusiastic OD users, which played to their thrifty and practical sides.
Ford offered their last passenger car OD units in 1964, and kept it available on pickups until 1972. Chevrolet’s last year for cars was 1968. Maybe someone wants to peruse oldcarbrochures for the last American car to have it available. A Rambler, probably.
After the first energy crisis, everyone was in desperate rush to get gas-saving overdrives back, but now the solution was to use a four speed manual altered to have an overdrive ratio on top gear. Worked, but not as elegantly as the BW OD, and not nearly as much fun to drive, especially the part about having to use a clutch.
So how does this all relate to the Gear Vendors unit that is familiar to most modern hot rodders, RVers, and towing junkies?
The Gear Vendor unit is pretty much the modern decedent working in a similar fashion.
Never heard of Gear Vendor…I need to look that up.
The 1957 Chevy I attempted to restore in the mid-70′s had an overdrive tranny. Too bad I never got a chance to try it out, as I sold that car in boxes in 1979….
My 1980 Dodge truck and my 1976 Dart Lite both had three-speed sticks with 4th an overdrive. They worked well.
A fascinating piece. I had always known about OD, but never about the freewheeling/clutchless shifting functions of the BW unit. Of course, I never drove one of these.
Fords of the torque tube era could be fitted with a 2 speed OD axle manufactured by Columbia, which was a fairly popular accessory in the 1930s and 40s. Also, the guy with the 365 Days of A website (who drove a Model A as everyday transportation for a year) fitted his A with an OD unit that was basically a 2 speed transfer case that required cutting the torque tube and replacing the driveshaft. The unit mouned under the front floor and a control lever came up through the floor. But both of these lack the elegance and added functionality of the BW unit.
Now you’ve done it – you have gone and added one more thing to the already crowded list of things I want to experience.
Overdrive is a particularly popular upgrade to As. They have the power to cruise at 55-60, but need to keep the revs down. There’s a number of different ways to go, including splicing in a BW unit into the drive shaft.The A I shot for my CC had a OD unit too.
Here’s a BW OD as spliced into a Model A drive shaft:
I drove a Dodge colt at work many years ago that had a “super power shift” lever next to the normal shifter which was basically overdrive. If you were quick with both shift levers you basically had an 8-speed trans. Gear Vendors can achieve the same effect with some electronic gadgetry, doubling the number of usable gears of your current slushbox.
Yup- the Mitsubishi Twin Stick. I had an 83 Colt (a Plymouth Colt, no less) with this tranny. I used mine as a 5 speed – low range through all 4 gears, then shift the other lever to high range, which made for an OD 5th gear. I believe that some Ramblers used a similar setup in the mid 1960s. It seems like an effective solution, although is probably more expensive than just engineering another gear into a transmission.
The mitsu twin stick is a little different in that it engages two different final drive ratios making it closer to a 2sp axle in basic theory. It was best used as a 5sp and that is what it morphed into when installed in the Excel/Precis. A vacuum motor, switches and relays replaced the second stick. An internal modification was done that allowed that “5th” shit position that still engaged direct drive or “4th” and activated a switch that engaged the second axle ratio or OD if you prefer.
Another example of the concept is some of the modern 6sp manual transaxles like used in the Focus SVT which essentially was a 3sp transmission with 2 separate final drive ratios.
The basic OD set up is used in some modern ATs to achieve the 5sp and 6sp configurations. For example the Ford 5sp 5R55E gets it’s additional ratio vs the 4R44E by engaging 2nd and OD.
“Ford didn’t have a four speed until 1962. Ordering the super-tough BW T-85 with OD allowed one to specify a very low (high numerical) rear axle ratio for maximum acceleration, especially on the drag strip. But 4.11 or 4.56 gears were almost impossible to live with on the freeway, so OD made that possible.”
Lyrics to “Hot Rod Lincoln”, written by Charlie Ryan (his real car shown below):
It’s got a Lincoln motor and its really souped up
And that model A body makes it look like a pup
It’s got eight cylinders, uses them all
It’s got overdrive, just won’t stall
It’s got a four barrel carb, and dual exhaust
With four eleven gears you can really get lost.
It’s got safety tubes, but I ain’t scared
Brakes are good, tires fair
As a kid I had a record of the Johnny Bond version of the song from about 1961 or so. Still have it. I played it so much you can barely understand any of the words now.
Try the nine-minute Bill Kirchen version – you’ll be blown away!
I remember his version on KFAT from Gilroy. Ah, the good times.
Both of my British cars had electric OD on 3rd and 4th. The TR3A had an odd switch that seemed to offer a delay–couldn’t find any details on it, and it was a scary thought to open it up for checkout. The MGB used a vacuum cutout so the OD would stay locked out until you let up on the throttle (memory is a bit fuzzy, haven’t owned it for 39 years…). Both OD units were effectively integral with the transmission. I know the B’s was a planetary, and assume the Triumph was the same way.
Hard to imagine asking modern drivers to learn anything so complex. (Then again… Ford Sync!)
I used to have a Honda CB1000C, with a 5×2 transmission. I just used it as a 6-speed–no need to split gears when you have a 1000 cc engine. (Would’ve been much more appreciated on my earlier CM400T!). Anyway, a paragon of simplicity compared to this BW overdrive. I’m endlessly fascinated by how simple and standardized driving has become.
That Honda 2-speed gearbox was put in there because they had the space due to changing the rotation to the shaft drive. I knew a few people who had the bikes, almost all of them just put the x2 transmission into the high range and left it there.
Top photo – I’d recognize a Tri-Five anywhere.
Great story!
Fascinating article, especially given the fact that my first car a 1956 Plymouth Savoy had a three speed transmission with overdrive.
I’m not entirely sure , but I believe it was this same BW electric unit.
I distinctly remember letting off the gas at about 30mph ,then feeling the overdrive engage.
I also remember the “freewheeling ‘ aspect of the overdrive ,which I thought was cool.
As a 16 year old I thought I knew and had done everything possible with that car. I must confess though that I had no idea that you could use the overdrive as stated in the Borg Warner overdrive link .
my mom’s ’83 volvo 240 had a 4 speed automatic with overdrive. the overdrive was a little “od off” button & led on the shift knob. i asked my dad what it was and he said it’s like fifth gear on his fiat. i now know that is incorrect but please don’t test me on the material above. my mom never touched the button. i had a lot of fun using that button as if it were a 5th to 4th gear downshift control until one day it didn’t go back into overdrive. fortunately, my mother never noticed and my father didn’t drive the volvo. a few weeks later, i was greatly relieved to see that the car had somehow gone back into overdrive. i never touched the button again.
In a number of years the Volvo OD units were actually supplied by Gear Vendors and uses a similar design as the BW unit.
Ive been battling the high numerical diff ratio in my Hillman as being a 59 it was designed before England had motorways and 100kph cruising was painful on the low profile tyres Im using diff ratio 4.55:1. An overdriven Hillman box can be got reconditioned for $750.00 out of my meagre price range so Ive fitted a 3.89:1 diffhead and axles from a 65 Minx they were motorway capable like the recently featured Rapier and my car now cruises quietly at 100kph/60mph with the 205/60/15 tyres which give it cornering ability equal of most modern cars and the best part the parts were free from a friends wreck collection
Very good – and relevant to me – article. I used some elements of the three-speed with overdrive in my ’63 Galaxie back in the late ’80′s. Now I just have to get it going again which is another story in and of itself.
I used to think the three-speed manual was a real snoozer of a transmission, but I have been thinking otherwise the last several months.
I remember driving a mid sixties falcon convertible with a 289. It had the most satisfying clunk when the overdrive kicked in.
In driver training class (1975) we were admonished to always lockout the overdrive in wet or icy weather.
I have heard of these, but, never have gotten such a good explanation of how they worked. I’ve been looking at four speeds and a different rear end to solve a problem. I may make a few junk yard calls this weekend!
Thank you for the lessons in a type of transmission I’ve never had experience with. Sounds like an elegant solution for a problem that exists at one time. Nowadays manufacturers would just as well put a 6-speed tranny rather than a separate OD system, so unfortunately it’s not likely to come across this type of tranny in the future…
My sister’s ’75 Volvo 240 had a 4 speed manual with an electric overdrive switch on top of the shifter. It knocked about 400 rpm off the engine speed when it came on.
How does the Borg-Warner overdrive compare to the Bachman-Turner Overdrive?
Quieter, less bass.
+1
+2
+3 (:D
The British loved their overdrives. Up until recently GKN made an electric OD unit for Landrover Defenders, but the factory 6 speed seems to have killed it. You can still get the Rocky Mountain manually shifted planetary OD units for various Landrovers plus the old Fairey units with conventional gears are still easily found and fixed.
You know, Paul, I remember Massey-Fergeson tractors of a certain vintage had a “Multi-Power” transmission system that functioned similarly. In addition to the usual crash box, it had a low-high range lever that you could shift on-the-go, I believe with freewheeling in the low range. It was handy if you were hauling a load downhill in low range, and when the load started to “get away from you” rather than trying to engage both brakes equally (each rear wheel had it’s own pedal), you could switch to high range and engine braking would take over, without overreving the engine or putting too much strain on the transmission. I used it mostly when driving the baler tractor, as you could run along in the low range and switch to high without having to stop and clutch (and risk a comment from the stackers on the wagon, who didn’t understandably didn’t like being jerked around) when you hit an area without very much hay. I wonder if the “Multi-Power” system was another name for a Borg-Warner overdrive. I shall have to study up on this-thanks for a most informative entry.
Oh-and my vote for favorite version of “Hot Rod LIncoln” goes to Commander Cody and the Lost Planet Airmen, from the Hot Licks, Cold Steel and Other Truckers’ Favorites album.
Not sure about the Masseys, but a fair amount of tractors still have a high-low range after the transmissions. My little Deere 790 has 4 x 2, and can go from about 0.9 mph (in low 1st) to 11 (high 4th) with the dual ranges at max (nominal) throttle. International offered a “Torque-Amplifier” system, sounds a lot like the MF one.
Fergies and Ford 8Ns could be fitted with an aftermarket 3-range transmission. I was told by a Fergeson owner that he could go about 35MPH in top range. Fun if you like tractor racing, I suppose. I had the Deere on the road at 11mph once and it was more interesting than I ever wanted to experience again. (High center of gravity, very short wheelbase, and fast steering. Did I mention no suspension?)
The International “Torque Amplifier” appears to be very similar, except it worked as an underdrive, allowing “downshifts” without any clutching. That was areal boon, when things got a bit tough or steep in the field, instead of having to stop and shift down. And yes, they did freewheel with the TA engaged, as I found out in a very scary situation myself once, on a long downhill road with a steep turn at the end. Thought the TA would slow me down; it was quite the opposite.
Dang Paul, I never knew that stuff. I had a 68′ Volvo with manual trans and overdrive, never knew how to use it. Now I have an 87′ Volvo with auto trans and OD, I just leave it engaged. It’s not often I learn about some mechanical thing I should be familiar with that I’m not, thanks!
I know British automakers frequently offered various Laycock de Normanville overdrives, which in some versions could be used on everything but first (a TR4 with overdrive effectively had seven speeds, although whether they were all useful is debatable). What I confess I’m not sure about is how common overdrive was in other European markets prior to the 70s. I know some Volvos offered it, but I’m trying to think of a 50s or 60s German or Italian car that did: four- and five-speed gearboxes, sure, but not LdN/BW-style overdrive units.
It wouldn’t necessarily surprise me if overdrive was more popular in the U.S. and U.K., just because both markets had a fondness for engines with long strokes and lots of low-end torque. (The U.S. hadn’t fully embraced the oversquare format until the mid-50s and a lot of sixes were still pretty undersquare, with torque peaks in the 1,200-1,500 rpm range.) With a big 3-liter or 4-liter straight six, overdrive makes a lot of sense; with the tiny Italian engines that had no discernable torque below 3,500 rpm, perhaps less so.
One day I should go through the technical materials and get some sense of the differences between the Borg-Warner unit and the various Laycock de Normanville ODs…
Oh, side note about the Airflow; The reason Chrysler adopted overdrive was because the Airflow cars ended up substantially over their target weights and were actually quite a bit heavier than the conventional cars they replaced (comparing the shipping weights of a ’33 Chrysler or DeSoto with the Airflows is an eye-opener). Overdrive offered a way to keep fuel economy within reasonable limits without getting into a lot of expensive and time-consuming engineering changes.
Good point. I can’t think of any German or Italian cars with overdrive. They tended to design their engines to be more suitable for high speed use.
Aaron I hadn’t seen your comment before I’d written the one below – I think European makes typically had four gears in their gearboxes to start with, meaning less requirement for overdrive which they added as an extra (5th) gear from the early 60′s.
Often the English overdrive worked on 3rd & 4th gear, but the rally teams modified this to work on all gears.
The Hillman Imp runs a 0.85 fourth gear ratio, but the final drive ratio is 4.86:1 so it does 15mph per 1000rpm (4000 at 60mph). The close ratio set in my Imp puts it at closer to 5000, I have a couple of other gear ratios, but the Imp is quite rev-happy, my other Imp (also modified) sat on 6000rpm/90mph easily. I wouldn’t mind a fifth gear though to cut down the noise a bit.
It’s true that the European makes were more likely to have four speeds forward, although the British sometimes seemed to fall into the trap of making first gear so impossibly short that it was functionally useless much of the time — the same thing that doomed the handful of American four-speeds in the early 1930s. Perhaps the most notorious example is the Austin A90 gearbox, whose low ratio was so short that when the engine and gearbox were transplanted into the Healey Hundred prototype, the Healeys opted to just block off first and make the transmission into a three-speed (with standard Laycock de Normanville OD).
Very interesting Paul. I haven’t come across one of these, just the Laycock de Normanville type on Sunbeams etc plus other British makes, and 2 speed axles in trucks.
I love overdrives, and both of my cars have Laycocks- interestingly one of the first and one of the last. My p4 Rover has an overdrive that has (had) one of the most complex almost rube goldbergian systems. There was a lever on the column to switch overdrive on and off- as with most cars. Additonally, there was a kickdown button under the accelerator to drop the overdrive out for passing. Very sensible. Ditto the lockout switch so it would only operate in 4th gear to prevent ditzy owners using 1st or reverse with the overdrive and burning it up. Good idea.
However, the third contraption was overly complex and pointless. On the carburettor throttle shaft, there was a cam with two micro switches that would drop the overdrive in and out when the engine speed was too low. I could never figure out the purpose of this, as the overdrive kickdown switch would surely do the same thing. This was to prevent you from lugging the engine with the overdrive and stressing everything, but again- if you floored it in overdrive, you’d depress the kickdown button. When it was new, I’m sure this made the drive buttery smooth- that bit of overkill just to get the car to be that little bit more refined. Unfortunately, after a few miles and decades, the effect was exactly the opposite.
As the brass bushing on the throttle shaft wore, the cam would flop about temporarily disconnecting the switches and making the overdrive switch on and off every few seconds. My p4 guru bypassed it and said that even he had no idea why the thing existed, as it served no practical purpose.
Moreover, my ’79 Volvo uses the same (well actually the J type not the D type) Laycock overdrive. It doesn’t have any kickdown switches or funny sensors, but just a 4th gear lockout and a little button to shift in and out. Volvo kept with their trusty Laycock until the mid 90s on the euro market 900 series. Is the Volvo brick the last passenger car with an overdrive? I can’t think of any others.
For 2 lane highways, overdrives are much better than a 5th gear, simply because you don’t need to clutch and physically move the lever, but just flip a switch to pass, meaning you lose fewer revs downshifting. Whats more, when 4 speed gearboxes had 5th gears added in the 80s, they were found to make the gearbox less reliable- Volvo M47, Saab 900, Lada Niva, are some examples of very robust 4 speed boxes that became failure prone due to the extra idler shaft being crammed in at the top and not getting enough oil.
I’m pretty sure that Volvo was the only non- British car to use that style OD. My theory was that it was a purchasing bundle with the SU carbs – maybe the same sales rep handled both lines – and persisted even after Volvo switched to fuel injection.
IIRC, both Ferrari and Maserati offered the Laylock electric OD in the early 60s. The Wikipedia entry says “Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph’s TR sports car range, until the end of TR5 & TR250 production in 1969.” Maybe I’m wrong about Maserati.
Update: Some quick Googling indicates that the 3500GT went from a 4sp ZF to a 5sp ZF in 1961, so apparently no Laylock OD.
Great, now where is the history for the Laycock de Normanville overdrive system?
In the mid-90s I had a ’64 Rambler Ambassador 990H hardtop with the AMC “Twin Stick” which was a 3-speed manual + OD. Mine never worked, though, as the 327 V8 engine would die when I applied the overdrive lever. It ran fine with just the three speeds anyway, though I’ve read that it could be used to equate a 5-speed if it had worked.
I think Volvo was the last car maker to offer an overdrive unit.