Could anybody explain the principle behind the switchable 'overdrive' fitted to the Triumphs and others in the 60's? Was it an extra gear? If so why don't more manufacturers fit it - surely an effective way of lowering engine speed and thus improving MPG when cruising on motorways,etc.?
P.
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Peter,
Overdrive was an extra mini gearbox on the output of the existing gearbox. It was up to the designers as to how many extra gears were available because it was only the electrical interlock that prevented it working in all gears including reverse.
The reason the engagement was limited came down to the ability of the overdrive unit to handle power (torque?) in the lower gears under acceleration.
Sometimes it worked on top only, in the case of the Triumph range it was 3rd and 4th.
I liked the way (with the Triumphs) it allowed a gearlever switch to select a ratio just lower than top when in third and selecting overdrive, useful option in a long overtaking manoeuvre. Overdrive top was a very very high ratio for those days and really allowed the Triumph 6cyl cars to cruise with smoothness.
Even the old Land Rovers can have them but with these it is just another lever to engage the overdrive. This means you really can operate it in all gears, not reccomended as stated above though.
David
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Peter
Yes, overdrive was an additional gearbox behind the main one which provided a slightly higher gear ratio, although usually only on 3rd and 4th gears. This restriction was due to the limited ability of the overdrive box to handle the torque of the lower gears. It had the benefit of providing a much closer set of ratios from 3rd onwards (ie 3rd, 3rd OD, 4th, 4th OD). The other benefit was that the minor ratio changes into and out of overdrive were available via a switch, and didn't need operation of the clutch.
It was a proprietary item which tended to be fitted on the more expensive cars as an option (although it was available on Spitfires for example). Common wisdom at the time was presumably that 4 gears were enough, and I suspect most owners wouldn't have wanted a 5 speed gearbox.
Why isn't it more common these days - cost. It's place has been taken by the 5 speed gearbox - the Maxi was probably the first mainstream production car to use a 5-speed 'box, and these day even a 6 speed gearbox is being found on mainstream cars. A 5 or 6 speed 'box is a much lighter and simpler way of achieving the greater spread of gear ratios.
regards
John
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Essential on my MGB when using the motorway, not pleasant when it stopped working. This had a switch on the dash (as opposed to the gear lever) as Peter says, we use 5th gear instead now.
S.
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I seem to recall that HJ has commented, either here or in the DT, that with the power and torque of modern engines five gears are over the top and that four would be more than adequate if properly spaced.
I get the feeling that a lot of people miss out gears when shifting up or down. In some circumstances the left hand motion seems to be more akin to stirring a cake.
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Many thanks everyone for your thorough and lucid explanations!
Peter.
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Dad's Buick Electra had the ideal gearbox - 3 gears:
Gear 1. Pull the side of a house down.
Gear 2. Go like blazes
Gear 3. Do gear 1 backwards
Simple, really - as long as you had enough wedge for the liquid go!
Ian
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what you say makes sense, Brian.
Most modern Autos have 4 speeds (the biggies have 5) and they seem to be closing the gap on manuals re MPG.
I'm still amazed at the proliferation of SIX speed boxes.
Most folk i know battle with five...
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Peter
If you're really really hankering after a car fitted with overdrive various 80's Volvo 2&7 Series had an overdrive 5th gear. It was a 5 button on the top of the gear knob. These are about the latest cars I can think of which had O/D fitted.
Charles
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You're right Charles. I was trying to think of the most recent car I've driven with O/D.
It was a 1987 Volvo 740TD Estate.
David
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David
Volvo 740 TD with the straight six 2.4 VW LT van engine shoehorned in. Famous for head gasket failures. I used to have great fun making the demonstrator car rock like an American muscle car at the traffic lights just by blipping the accelerator!! Happy days working at a Volvo dealership.
Charles
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My 2001 F-150 truck (auto) has an overdrive
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It just so happens I was driving a Jaguar Mk II 3.8 this very day and it had overdrive. On the Jag it only (logically) works on 4th, is engaged by a column switch on the right of the column and switches in and out 'automatically' (you don't dip the clutch). Worked brilliantly and soon became instinctive to use.
HJ
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I had a Triumph 2000 in the 60/70s which I kept for ten years which was fitted with an overdrive. Great car and the overdrive was far superior to a 5th gear insomuch as you simply had to flick the column mounted switch (button on gear lever on later models) without lifting the accelerator to get a very smooth change of ratio. Mine used to go wrong occassionally with the solenoid on the gearbox sticking. Taking it off and cleaning used to put it right.
As stated previously it operated on 3rd and 4th only although there was a simple thing one could do which made it work on all 4 gears which some owners used to do. Can't remember what it was though. Also trying to remember the manufacturer of the overdrive. I recall they were made in Coventry and the name Laycock DeVille springs to mind. Can anyone remember????
Alvin
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Alvin,
Might thinking of something else, but I think the word Norman might have been part of the Laycock name. You could disconnect the inhibitor switch to allow overdrive to be selected on all gears. This meant that reverse could be selected whilst the overdrive was still engaged. Instant disaster. Overdrives can't work in reverse - much damage to internals.
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Thanks fella's, it would have been nagging away at me for hours.
Alvin
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Laycock de Normanville?
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Thats it, been racking my brain.
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