Can anyone explain to me how overdrive worked / works.
I know that many 60 / 70s cars had overdrive, some on both 3rd and 4th giving in effect 6 speeds.
Indeed I can remember seeing adverts for manual Volvos from the 1990s, stating the gearboxes were 4 speeds with overdrive, as opposed to the largely ubiquitous 5 speed boxes.
So what was 'overdrive' and why do we not see it anymore?
Ta
Some cars now have 6 speed manuals. At what point does the number of gears cause more problems, do cars really need 6 speed boxes ? I know that HGVs have 8 or 16 speeds, but that?s for the heavy loads that they carry.
Thanks again.
Alex
|
The Borg Warner overdrive was basically an extra gear which was engaged when the overdrive was switched 'in' by a small lever and the engine speed was allowed to drop by the appropriate amount (typically close to 30%), i.e. it was a bit slow to engage. They generally operated on 2nd and 3rd speeds in a 3-speed box. The overdrive was mounted on the back of the gearbox which had a longer mainshaft extending into the overdrive unit to drive it. It was fitted to some Fords, amongst others.
The Laycock de Normanville was much more complex and refined. Contained epicyclic (sun & planet) gears, hydraulic valves, solenoid, etc. A-type was very reliable, later J-type (mid-70s on) was almost unbreakable. My Triumph has the J-type and it operates on 3rd and 4th at the flick of a thumbswitch on the gear lever knob, to give a 27% step up or down almost instantaneously, even on full throttle. Great fun!
Disadvantages: Expensive when new, cumbersome and heavy. And if you forget to flick the Laycock out of overdrive when coming to a stop, you start off in 1st, 2nd and then an almighty jump to O/D 3rd. An extra gear (or two) in a non-overdrive box is probably a lot cheaper, lighter and fool-proof. But not nearly as much fun!
|
|
|
Alex,
The six speed box in the S6 I used to drive helped to keep the revs in the 'power band' - there was always a ratio available to enable the driver to extract maximum power. Even though the turbo engine is very 'torquey' even at low revs, that particular engine really takes off between 4,200 and 6,500 rpm.
Martin.
|
|
|
I've got a BMW Compact 320D sitting in the driveway at the moment and it's a real cracker despite just a 5 speed box. Top gear ratio seems to work out at about 32mph per 1,000 rpm. I know overdrive is worked out differently from this, but what is a sensible mph per 1,000 rpm to regard as 'overdrive'? Is it something like 23mph per 1,000 rpm, or a lot more than that?
HJ
(It costs a whopping £20,850, but the roadholding and handling are brilliant. And the engine is a chain cam diesel in the lowest CO2 VED bracket.)
|
'Over drive' is a term often used to mean a gear ratio lower than direct drive, when talking about gearboxes. So 1st gear might be 3:1, 4th gear direct at 1:1 and 5th 0.8:1.
I've got a 6-speed box in one of my cars and 6th is 1:1 - ie. direct so it's not an overdrive box.
An over drive is a unit stuck on the back of the gearbox to alter the gearing between the gearbox and the diff.
More gears = better is a difficult question. I suspect in most cases you will be best off talking to the marketing department to find the 'engineering' need for 6th.
Piers
|
|
Surely it's better to have the highest gear a direct drive, then fit a lower ratio "back axle". That way you have the fewest gears working at the toppest ratio - which must be the most efficient way to go about things.
|
|
|
|
What annoys me is that cars actually seem to be coming lower geared these days.
Go back a few years and mid rangers like Cavaliers etc were doing 25mph/1000rpm - Nice easy, economical and silent cruise at 70-80mph. Nowadays and presumably because people want "sportier" engines, all gears seem to be lower and cars rev a lot higher at motorway speeds.
I'm all for some flexibility but I'd still like a nice tall top gear/overdrive when I just want to sit at 80mph on the motorway, without the need for amazing acceleration, and when I get get at least a half decent fuel consumption
Dan
P.s. The best ratio I have found that suited me was 26 2/3 mph /1000 rpm in a 1.8i Cavalier. 3000rpm = 80mph. My 2.2 Vectra is at 4000rpm at that point which doesn't help the old juice consumption...
|
It's not as simple as lower revs = lower consumption.
You need x amount of power to maintain a certain speed. This comes from burning the fuel. Any more power and you accelerate, any less you slow down. As engines vary in efficiency at different loads and speeds it you could burn less fuel to produce X hp at 4000 rpm than to produce X hp at 3000 at the same road speed and with different gearing.
It's very tricky to simplify this kind of stuff, but basically although the answer appears to be obvious, it isn't.
Piers
|
|
|
Just shows we are all different the best top ratio which suited me was 32~/1000rpm in a Astra Belmont 1.8i. Very relaxed cruising.
Been a dieselhead for quite some years and noticed that manufacturers are gearing these so as to ease the culture/performance shock for petrolheads changing to diesel.
It has seemed for some time that the real need is, as Dan says, a 5 speed with the current ratios with a 6th "overdrive" rato so that it all settles down into a relaxed gait on the motorway. Something like 2000 @ 70.
But then when could you achieve such a thing for any distance on a UK motorway anyway unless you are up at sparrows.
|
|
|
|
Dan J
Is not lower gearing is a result of speed limits so that some cars are now built for faster acceleration rather than top speed?
|
Years ago I had a Mk2 Cavalier 1.6 4 speed which cruised beautifully at 80+, and I replaced it with a Mk3 1.6 5 speed which didn't feel anywhere near as relaxed on the motorway. (not a good car overall, really)
I'm sure that in many cases 5th is the same ratio as the 'old' 4th, so you just end up with a sort of close ratio 'box.
Diesels seem better. My A4 TDi cruise at 70mph @ 2,300 rpm, ie about 30mph per 1000rpm. (recent consumtion test: 52mpg mixed driving over 1200 miles)
I personally like a long-legged car, provided it's got the torque to prevent changing down on inclines etc.
Pat
|
|
|
|
The "top" gear of a properly designed four speed transmission gave the maximum speed of the vehicle at the rpm corresponding to peak power.
Overdrive was traditionally 25% higher.
provided the rest of the gears were arranged to give an equel rev drop when changing up,and ensuring that you were not dropped into a flat spot,that was and is the only way to design gear ratios.
manufacturers have long used cranky gear ratios to cover up manifold design idlenesses.
like low second gears rather than design the synchro properly,or to give an underpowered german box some Alp climbing ability.Audi still do it.
My 405 TD has a 5th which is low even for a normal 4th.,this covers up the inept turbocharging.My Hdi110 has a huge 5th ratio,but insufficient power to drive it because every vehicle its installed in is too damned heavy.
For any vehicle the correct gear ratios are calculable,on a fag packet,we are again the victims of cynical marketing.
|
|
|
