"BATC42" (BATC42)
08/26/2014 at 09:05 • Filed to: None
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"BATC42" (BATC42)
08/26/2014 at 09:05 • Filed to: None | 1
| 23 |
Help a clueless french man. I had never heard of overdrive before Jalopnik and Oppo. What is it? What is the point?
Have a flying/rocket powered DS for your time.
Jake - Has Bad Luck So You Don't Have To
> BATC42
08/26/2014 at 09:07 |
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If we're talking about gears, the overdrive gear is defined as one where the wheels are turning faster than the engine itself. Generally it's the highest gear on the transmission, and is usually used purely for highway fuel economy.
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BATC42
> Jake - Has Bad Luck So You Don't Have To
08/26/2014 at 09:08 |
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Thanks. So basically it's a 5th or higher on a regular gearbox?
rhorizon
> BATC42
08/26/2014 at 09:09 |
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All I know is that it serves to confuse the hell out of my mother-in-law, who refuses to turn it on in their minivan for "economy" reasons.
Never mind that she's wrong
bob and john
> BATC42
08/26/2014 at 09:11 |
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not always. Over drive pends on the gear ratios.
RamblinRover Luxury-Yacht
> BATC42
08/26/2014 at 09:11 |
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Yes, if that 5th is a greater than 1:1 ratio (it isn't always, just usually). Some overdrives are a secondary unit added to a gearbox to raise the gear ratio instead of being a 5th or higher in a regular gearbox. Like this one on a Volvo:
BigBlock440
> BATC42
08/26/2014 at 09:12 |
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The 1:1 transmission gear (in old automatics was 3rd) was the drive gear. You put the car in drive, and it'll shift up to 1:1. When overdrive transmissions were developed, the gear ratios were higher than 1:1 (.8:1, now some cars have as high as .5:1 and multiple overdrive ratios). The new gear was higher than drive, literally over drive, hence the name.
*that's my theory on the name, but generally any gear over 1:1 is considered an overdrive ratio.
Edit: It allows the engine to spin at lower RPMs, usually increasing fuel economy. Also raising the top speed of the vehicle.
Cé hé sin
> BATC42
08/26/2014 at 09:12 |
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The word can have different meanings. An over driven gear is one where the output shaft turns faster than the input one which is usually the case in the higher gears on a car gearbox. You would have a ratio of say 0.75 to 1, that is the input shaft turns 0.75 times for each turn off the output.
An
overdrive unit is a two speed gearbox fitted to a main gearbox which provides normal and overdrive versions of one or more of the main gears.
TheBaron2112
> BATC42
08/26/2014 at 09:13 |
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It's any gear with less than a 1:1 ratio.
So like, 2nd gear or whatever may be 3:1, 4th may be 1.10:1, and anything like 0.99:1 or 0.85:1 would be 5th, 6th, etc, and overdrive.
Jobjoris
> BATC42
08/26/2014 at 09:13 |
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An overdrive is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear. Officially it's only an overdrive if the outgoing speed is higher than the incoming speed. Or something. Try wiki .
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RamblinRover Luxury-Yacht
> BATC42
08/26/2014 at 09:13 |
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Also, from your own country, the Panhard Dyna Z actually had an overdrive 4th gear, not overdrive 5th. It's very odd in some other respects too, but that's the one of interest here.
crowmolly
> BATC42
08/26/2014 at 09:14 |
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The point of overdrive is to lower engine RPM at higher speeds. You get better fuel economy and less wear on your engine.
Basically it is a gear with a ratio less than 1:1 (direct drive) that allows the engine to spin slower than the output shaft of the transmission.
If your gear ratio is 2:1, the engine turns twice for each rotation of the output shaft
If your gear ratio is 1:1, the engine turns once for each rotation of the output shaft
If your gear ratio is 0.5:1 (overdrive) the engine turns once for each two rotations of the output shaft
The more "gear" you have (higher numerical ratio) the more torque you can make at the wheels but the engine is spinning more.
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Cé hé sin
> BATC42
08/26/2014 at 09:15 |
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Not always. Some boxes have a direct top gear with no overdrive ratio although this is now uncommon.
Skater Dad
> BATC42
08/26/2014 at 09:15 |
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Yes, typically 5th or above. If you search the internet for "(your car) gear ratios", the gears with a ratio less than 1 are the overdrive gears (I'm at least 50% sure of that!).
In olden times, overdrive was a separate gearbox attached to the normal one, and had to be purposefully engaged. Wikipedia has some interesting info: http://en.wikipedia.org/wiki/Overdrive… )
Shoop
> RamblinRover Luxury-Yacht
08/26/2014 at 09:18 |
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like an inverse transfer case?
TwinCharged - Is Now UK Opponaut
> BATC42
08/26/2014 at 09:18 |
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Other people have explained it mechanically, so here's my input on how it's almost always used wrongly in media.
Shoving it into overdrive doesn't make the car reach a new level of high performance. If they do, they're basically saying they're shoving it into high MPG, smooth and comfortable cruising mode. Not power.
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RamblinRover Luxury-Yacht
> Shoop
08/26/2014 at 09:22 |
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Yes, except that a "transfer case" usually means a transfer of direction as well as featuring a step down. In fact, some vehicles have both - such as 60s Land Rovers fitted with a Fairey overdrive. The overdrive changes the input speed at the point of entry to the transfer case - a similar unit was available for some Jeeps.
GhostZ
> BATC42
08/26/2014 at 09:23 |
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Tl;dr: technical distinction about gear ratios that wasn't really used, until it became a marketing slang for fuel-saving low ratio gears. Refers to both the ratio, and the button that tells the transmission to use those ratios or not. Also refers to gears that aren't necessary to driving, but serve to just keep the RPM low.
Gear Ratios > 1:1 are "underdrive".
Gear Ratios < 1:1 are "overdrive."
In US cars, most cars were designed for acceleration and torque, since anything over 50-60mph was dangerous anyway and they were heavy. Who in their right mind would drive a 40s or 50s sedan that fast out of the box? So the highest gear ratio was usually 1:1, since it was easy and simple to do and when you have limited gears, that's about as high as you need to go. Anything higher, and the car wouldn't actually go faster . So a 0.8:1 gear was silly.
It made no sense to have more torque and an underdriven engine, when you could just have lower rear-end gear ratios. Therefore, transmissions rarely exceeded 1:1 as the lowest ratio.
Since most cars were limited by aerodynamics, engine power, frame stability and common sense, having underdriven gears wasn't an economic proposition, since adding a gear was far more expensive than changing the rear differential gearing.
The problem is that there's a certain limit to that, and you'll always be running at high RPMs since top speed was always the peak power at the top gear.
It only became sensible to add more gears of under 1:1 ratio when you take into account highway driving and engine RPM.
So "Overdrive" became a marketing technique for letting people know that they had an extra gear that didn't increase the top speed, but WOULD let them save fuel on the highway. Now it no longer meant "less than 1:1 ratio" but it explicitly meant "added gear for fuel economy that won't increase top speed"
This is one reason why a "true" 5 speed has 5 gears that are 1:1 or greater, since rear torque would be better at the rear wheels. The Corvette 4+3 was infamous for being a 4-speed with 3 overdrive gears, not really a "7 speed transmission" because the 4 main gears were just a vanilla 4-speed, and the 3 overdrives were just lower-rpm versions of gears 2,3, and 4 I believe.
So "overdrive" became a feature on automatic transmissions that could be activated on-demand, shifting into this low-ratio gear to maintain speed, though it would kill acceleration. That's the origin of the 'overdrive' button.
Turning overdrive off means your engine won't kick into higher gears while driving, giving you better acceleration.
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crowmolly
> RamblinRover Luxury-Yacht
08/26/2014 at 09:26 |
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Gear Vendors OD units are still popular. Back in the 60's it was the Hone-O-Drive.
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RamblinRover Luxury-Yacht
> Shoop
08/26/2014 at 09:31 |
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Here are the internals of a standard Series II Land Rover transfer case, with the input gear to far right missing. The input gear gets replaced with the overdrive gear when an overdrive is installed - it takes the power input, speeds it up, and then returns it to drive the gears in the transfer case.
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RamblinRover Luxury-Yacht
> crowmolly
08/26/2014 at 09:35 |
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We stuck a Gear Vendors on a P-chassis motorhome with a 6.2 diesel and TH. Works great, but currently has a gremlin or two due to water getting in the control box..
JEM
> rhorizon
08/26/2014 at 13:27 |
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That's both hysterical and sad at the same time.
davesaddiction @ opposite-lock.com
> BATC42
08/26/2014 at 13:52 |
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Gregory Haberek
> BATC42
09/02/2014 at 11:15 |
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The Chevrolet Cruze Eco has three - count 'em, three overdrive gears!
1 = 3.820
2 = 2.158
3 = 1.302
4 = 0.959
5 = 0.744
6 = 0.614
Notice the green circle on the "6" on the shifter, which indicates MAXIMUM OVERDRIVE !