What happened to overdrive?
Discussion
Just wondering? I know gearboxes have got more advanced and you can probably get a 25 speed auto paddle thingy with 3 clutches if you want. But a lot of the cars I've driven, even with 6 gears, are (significantly, IMO) undergeared at motorway speeds. I understand why this might be in a 1.2 with 80hp, but why not stick a very long gear on the top of most cars, especially anything with a reasonable amount of power?
VW did that with the 4+E box in the early eighties. Most five speed boxes go 1 to 3 underdrive, 4 direct, and 5 overdrive. Or at least used to.
I suppose the disadvantages of using an extremely high top gear would be that the car would become unresponsive to the throttle. If the gearing was such that it could cruise at 70 mph whilst turning at, say, 1,200 to 1,500 revs then the car would need mighty torque low down to accelerate whilst remaining in that gear. That was fine in the old days of sixes and V8s but it wouldn't suit the modern turbocharged four cylinder petrol or diesel. Those would come off boost entirely and be left incapable of spooling up again.
Which leaves the option to change down to a direct geared fourth every time the driver wishes to gain speed. That would be annoying! In addition if the gap between the ratios is wide it would require a hefty rev match, possibly even double de-clutch, to get it in without a horrendous shock to the drivetrain. This is beyond the capabilities of the average driver.
Most UK dual carriageway driving, certainly in the South East, isn't constant cruising. It's more of a 60-85 (ish) mph oscillation based on traffic. Therefore a certain degree of responsiveness in top gear is needed.
Edited because I didn't read it!
I suppose the disadvantages of using an extremely high top gear would be that the car would become unresponsive to the throttle. If the gearing was such that it could cruise at 70 mph whilst turning at, say, 1,200 to 1,500 revs then the car would need mighty torque low down to accelerate whilst remaining in that gear. That was fine in the old days of sixes and V8s but it wouldn't suit the modern turbocharged four cylinder petrol or diesel. Those would come off boost entirely and be left incapable of spooling up again.
Which leaves the option to change down to a direct geared fourth every time the driver wishes to gain speed. That would be annoying! In addition if the gap between the ratios is wide it would require a hefty rev match, possibly even double de-clutch, to get it in without a horrendous shock to the drivetrain. This is beyond the capabilities of the average driver.
Most UK dual carriageway driving, certainly in the South East, isn't constant cruising. It's more of a 60-85 (ish) mph oscillation based on traffic. Therefore a certain degree of responsiveness in top gear is needed.
Edited because I didn't read it!
Deva Link said:
PanzerCommander said:
Some automatics still have them, the automatic in my 2006 Mustang GT has an 0/D switch.
It might have an O/D switch, but it doesn't actually have an overdrive unit. 
krissharm said:
ok... moron alert, whats the difference?
Fifth gear is a further ratio within the transmission case. An overdrive is a separate addendum which is bolted to the end of the gearbox. It is a drop gear, electronically engaged, and it is usually available on both third and fourth gear, effectively making a six speed transmission. Whether third overdrive is useful is a matter of chance, usually: get lucky and its right between third and fourth, less lucky and its too close to fourth to make the messing about worthwhile.Deva Link said:
krissharm said:
ok... moron alert, whats the difference?
Google it - it used to be a separate unit bolted on to the end of the gearbox.http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Dracoro said:
Deva Link said:
PanzerCommander said:
Some automatics still have them, the automatic in my 2006 Mustang GT has an 0/D switch.
It might have an O/D switch, but it doesn't actually have an overdrive unit. GC8 said:
krissharm said:
ok... moron alert, whats the difference?
Fifth gear is a further ratio within the transmission case. An overdrive is a separate addendum which is bolted to the end of the gearbox. It is a drop gear, electronically engaged, and it is usually available on both third and fourth gear, effectively making a six speed transmission. Whether third overdrive is useful is a matter of chance, usually: get lucky and its right between third and fourth, less lucky and its too close to fourth to make the messing about worthwhile.
There was an overdrive made for the MG Metro when it was in its hay day (prototype). The car was driven at the time by the head of BL who I think was Mr H Musgrove, he said it transformed the car and was brilliant, the deal wasn't done due to cost (pence). If this had gone into production things may have been different, but as it turned out they found out a fifth gear was cheaper to make.
The overdrive could in theory be made to run in all gears making it a sort of eight speed, although using it on the lower gears would be pointless.
Overdrives are still being refurb'd by the guys that made, assembled and tested the originals at Laycock Engineering.
One of the largest manufacturers to use the overdrives were Volvo and when they did the fifth gear this really killed the overdrive. Laycocks made overdrives until the early nineties when they shut, it's now flattened and is occupied by small companies.
The overdrive could in theory be made to run in all gears making it a sort of eight speed, although using it on the lower gears would be pointless.
Overdrives are still being refurb'd by the guys that made, assembled and tested the originals at Laycock Engineering.
One of the largest manufacturers to use the overdrives were Volvo and when they did the fifth gear this really killed the overdrive. Laycocks made overdrives until the early nineties when they shut, it's now flattened and is occupied by small companies.
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