Talk:Overdrive (mechanics)

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Hi: I need to know when would it be apropiate to use the OD on or off, I live in Mexico City a very crowded town but I really dont know when to use the OD on or off, should I turn it off inside the city or what.

THANK YOU I'll WAITING FOR YOUR ANSWER.

EMAIL: rafaelg@rocketmail.com

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Rafael, 99% of driving should be done with the overdrive enabled.

Posted some edits, forgot to give references. http://www.ohiovw.com/files/touaregfiles/6speedautotranny.pdf http://bandwidthmarket.com/resources/patents/apps/2002/7/20020086761.html

http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=2005-01-1020

http://techinfo.toyota.com/ Sorry, last two are pay sites, unfortunately that is where most of the good stuff, with lots of details is. 3sgte 16:35, 2 December 2005 (UTC)

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Coming at this as a clueless person, I didn't understand one thing after reading the article: How/why can overdrive be engaged without using the clutch? Good article otherwise.

I thought the same (my father had a Volvo 740 so equipped) the best answer I can offer is that the gear engages rather like the gears on an automatic box. So I can say why, but not how, sorry! Alastairward 11:22, 2 February 2007 (UTC)

When you start your car, many cars (such as the Nissan Altima) default to the state of the Overdrive being turned ON. You might find a button somewhere that takes it OFF, however. 198.177.27.23 (talk) 07:50, 19 October 2008 (UTC)

Bias Toward Automatic Transmission

When reading this artice, I notice that manual transmission was only mention once, with the remainder of the artice favoring OD in automatic transmission. I think this is Bias. --KB1KOI 15:32, 28 July 2006 (UTC)

Bias towards automatic transmissions

I do not believe there is a bias in this article, as in the manual transmission, the overdrive gear is treated like any other; while in the automatic transmission, it is usually electronic and switched on or off with the press of a button, but generally is not usually interfered with.

not bias

one of the reasons that this article may seem bias is becuase the automatic engine, switches to OD automatic-ly hence the name, where as manual you have to actually put it into OD

Rename Page?

I'm trying to find consensus for renaming "(mechanics)" pages, where "mechanics" refers to machinery, mechanisms, mechanical devices or mechanical engineering, rather than (e.g. classical) mechanics in physics and applied math. I would like to rename this page to "Overdrive (mechanism)" or "Overdrive (mechanical device)" or something similar. Any preferences or objections? Geometry guy 20:57, 21 February 2007 (UTC)

Perhaps Overdrive (automotive)? Publicly Visible (talk) 13:58, 4 May 2012 (UTC)

Drive train wear

I don't know a lot about mechanics, but the section in this article on drive train wear didn't make sense to me. Unless using an overdrive gear means you are going to drive faster than you have would otherwise, why would there be more wear on the drive train from using overdrive? i.e. isn't drive train speed fixed to vehicle speed? Interlope (talk) 20:20, 14 January 2008 (UTC)

I quess it has something to do with the speed and load of transmission. Lets say you drive an old car, equipped with four speed transmission, electric overdrive and a low rear end gear ratio for example. Remember, that the overdrive unit is basically an auxiliary two speed transmission, positioned right after the main transmission. If you run 100 km/h (~60mph) first without overdrive and then switch the overdrive on the main transmission speed (and the engine speed) is reduced while driving the same vehicle speed. Because power is relative to speed and torque (momentum), in order to keep the same vehicle speed (100 km/h or ~60mph) the main transmission must transmit the same power to the wheels with lower rotation speed. This means that the momentum affecting the transmission increases, and so increases the load and stress of its bearings and other mechanical parts. If the transmission is not build to withstand such increased load, it might break down. Or if the transmission speed while using overdrive is too slow for proper lubrication to take place, the transmission starts to wear at considerable rate and also becomes prone to fail.

Hopefully this was of assistance... —Preceding unsigned comment added by 80.223.93.188 (talk) 00:34, 7 January 2009 (UTC)

overdrive on a '98 Nissan Altima

I read the main article but remain confused as to the usefulness of the "overdrive" button on the stickshift of my '98 Nissan Altima.

The little overdrive button on the side is not the same thing as the big stickshift button on the top. The little one is located on the driver's side of the stickshift, about three or four inches down from the top. It's small, and measures about a quarter inch across, so you aren't likely to confuse it with the relatively larger stickshift button on the top, which measures about 2 by 3 inches. You change gears (like, going into reverse) if you push the big button down, and then pull the shiftstick back into the appropriate gear. But what on earth does the overdrive button do? I've heard gossip that it is useful when driving at high speeds.

In my case, the car is almost always in "overdrive" mode, but if you decide to press the overdrive button, it goes in a bit, so it is flush with the rest of the stickshift. Then a dashboard light comes on, and it says "O/D OFF" but what is even more interesting, the car's RPM will go up. This happened lately while I was driving in the 45 to 55 mph range.

Am I getting better miles per gallon if I am driving 50 mph on a relatively level road with the O/D button pushed in, so the overdrive is off? 198.177.27.23 (talk) 04:32, 16 October 2008 (UTC)

I have no knowledge of the'98 Nissan Altima and even less about the US version of it but overdrive works always the same way. Simply put: The overdrive works similarly to changeable differential. If you switch the overdrive off it is if you would cruise at smaller gears. It causes your engine rpm increase at the same vehicle speed which means your economy aka mpg drops but you gain more pulling power (torque to the wheels increases). I would assume that in this case overdrive should be switched off only when additional pulling power is required, for example when pulling loads or moving in difficult environment like climbing a steep hill... Hopefully this helped... —Preceding unsigned comment added by 80.223.93.188 (talk) 22:27, 4 December 2008 (UTC)

Cruise control versus Overdrive

At least on a '98 Nissan Altima, pushing the Overdrive Button in (so the O/D OFF light comes on), the rpm's go up. Does it make a difference if the car is being operated with the Cruise Control switch on? 198.177.27.16 (talk) 20:30, 16 October 2008 (UTC)

Cruise control is a separate system, a device with a sole purpose to hold the vehicles speed steady by adjusting engine rpm. It should have no effect to the overdrive: In other words driving with overdrive off your vehicle consumes more fuel wether you use cruise control or not. As stated before, in this case overdrive should be used unless extra pulling power is required... —Preceding unsigned comment added by 80.223.93.188 (talk) 23:56, 6 January 2009 (UTC)

Prevelance

I noticed something in the article that pretty much said overdrive is pretty much standard but I don't belive thats the case. The only modern cars I've seen with overdrive controls are nissans. Might this be a difference between UK/EU markets and other markets?(Morcus (talk) 03:27, 6 January 2011 (UTC))

I really need to write a decent lead for this article. "Overdrive" has four meanings.

The first is a high ratio (gerbox and final drive) such that the top speed of the car is less than it might be in a lower gear (although its fuel economy at the same speed will be better).

The second and third are mechanical means to achieve this. The older system was a separate "overdrive" gearbox attached to the output of the main gearbox, often electrically switched. Later systems (especially front wheel drive) merge this into an extra high ratio on the main gearbox.

This second form also gives the fourth meaning, again only applicable to rear-wheel drive, where the propeller shaft was turning more quickly than the engine.

As modern cars are now too fast to use their performance at all frequently, but fuel costs are astronomical, pretty much all cars are now "overdrive" by the 1st and 3rd meanings, although the 2nd and 4th have pretty much vanished. Andy Dingley (talk) 12:02, 6 January 2011 (UTC)

• I've rewritten the lead. Any clearer? Andy Dingley (talk) 14:35, 7 January 2011 (UTC)

That does make more sense, The way I read it when I posted that made it seem like it was refering solely to the old system where you had and extra bit of gearbox on the back, I'd assumed thats what this article was about for some reason rather than as you say when the final drive ratio is more than one.(Morcus (talk) 07:15, 8 January 2011 (UTC))

overdrive gear ratio

an overdriven ratio for cruising thus required a gearbox ratio even higher than this (1:1), i.e. the gearbox output shaft rotating faster than the original engine speed.

Isn't this backwards? An overdrive gear's ratio will be less than 1. Typically around 0.75.

20:19, 17 August 2011 (UTC) — Preceding unsigned comment added by 174.46.204.210 (talk)

That section (Introduction) is discussing the transmission overall, not a separate overdrive unit specifically. It's difficult to be unambiguous here, as the transmission reduces engine speed to final drive speed, thus the ratios are somewhat inverted between common discussion and their numerical values. 1st gear is described as "low" and top gear is described as "high", even though their ratios are probably 3:1 and 1:1 (approx) - so the "low" ratio is numerically higher than the "high" ratio. Presumably this stems from a natural tendency to describe ratios from whichever direction is greater than 1, not the fractional value. These words have far more common usage than the numbers do, so I've used them to describe the ratios. Overdrive might well become 0.8:1, but we'll still call this a "higher" ratio, not a lower one. Andy Dingley (talk) 23:14, 28 September 2011 (UTC)

Hugely popular mistake

There is an excruciatingly popular (and stupid) misconception that Overdrive Gear refers to any gear with a reduction ratio of less than 1. This is completely wrong. Overdrive Gears almost always have reduction ratios of less than 1 but not all such Gears are Overdrive.

I'm glad to see this article got it right. — Preceding unsigned comment added by 95.76.145.55 (talk) 22:51, 18 October 2011 (UTC)

Single source May 2012

I really like this article, but the beginning relies on one source from 1976, and every section but Overdrive (mechanics)#Overdrive in Europe is unsourced. More WP:VERIFY would take this from good to excellent :) Publicly Visible (talk) 13:56, 4 May 2012 (UTC)

Yes, but the source is L J K Setright! — Preceding unsigned comment added by Mr Tangle (talk • contribs) 14:55, 17 October 2014 (UTC)

Fairey Overdrive

It is a common misconception that the Fairey Overdrive should only be used in third and fourth gears. I have edited the photo caption to remove this misconception. Mr Tangle (talk) 15:09, 17 October 2014 (UTC)

It should only be used in the upper gears. Especially if you have a V8, the torque in lower ratios can damage it. It's not too bad if you leave the overdrive engaged then shift down from 3-2, but it's really not a good idea to engage the overdrive in low gear.

Also, why on earth would you want to? Andy Dingley (talk) 15:40, 17 October 2014 (UTC)

I agree that you wouldn't want or need to, but the overdrive will operate in lower gears and forgetting to disengage it when changing down shouldn't usually be disastrous if it is otherwise well maintained and the driver does not have leaden feet.Mr Tangle (talk) 06:24, 19 October 2014 (UTC)

The Landie gearbox of this era is already fragile enough. If you have a V8 (maybe a factory fit) and you're driving it as such, then you can break a well-maintained gearbox or overdrive by rough handling of it. Engaging the overdrive in the lower gears will do that. The vehicle is even placarded (if it's a factory overdrive) not to do this. Andy Dingley (talk) 09:44, 19 October 2014 (UTC)

The 4 cylinder engines produce less torque than the (comparatively rare in Series 3) V8. The placards sometimes seen appear to be after market. Fairey did not limit the gear range in its own product literature. Mr Tangle (talk) 13:10, 21 October 2014 (UTC)

Caption revised, but see discussion elsewhere on the Net:

http://www.landroverwriter.com/thelandrovers/faireyoverdrives.html

http://www.series2club.co.uk/forum/forum/index.php?topic=50897.0. (there are similar threads on various Landy fora).

More definitively, the manufacturer specifies the gear ratios when using the unit in low range. Low range third and fourth are quite torquey. Mr Tangle (talk) 13:23, 21 October 2014 (UTC)

Here's a replica plate, as was factory fitted and was included with overdrive kits: http://www.vin-plate.co.uk/land_rover_plates.html

Note the text, "IMPORTANT Use overdrive in 3rd and 4th gears only".

If you have WP:RS to indicate that Fairey and Land-Rover were wrong to placard it in this way, then I'm sure you know where to put it. Andy Dingley (talk) 13:31, 21 October 2014 (UTC)

Land Rover and Fairey did NOT placard the unit in that way. That plate is NOT a factory fitment. See the Fairey instruction manual. — Preceding unsigned comment added by Mr Tangle (talk • contribs) 06:31, 25 October 2014 (UTC)

Meaning of "overdrive"

I was always under the impression that it's called an "overdrive" because it has a gear ratio of OVER 1:1. Before that, the top gear ratio for automobiles was 1:1. Thus, adding a fifth, higher ratio (0.75:1, etc), makes it an "overdrive" transmission. I don't think it's because the car is "overgeared", especially as that depends on the specific car, the gearing of the final drive, etc. Many cars can make top speed in overdrive. I'm pretty sure the present explanation isincorrect. Also, could be a bit less muddled. For starters, I'm going to add a sentence explaining WHY it's better for a car to run in a lower gear and higher RPMS...i.e. because the engine generates more power at higher RPMS. Thats just for starters..45Colt 12:43, 12 January 2015 (UTC)

I wrote a clearer lead a few years back [1] It has of course been removed by now. Andy Dingley (talk) 12:50, 12 January 2015 (UTC)

Recent addition

Recent added [2]

"because it mimics the effect of having the driveshaft connected directly to the output of the engine, i.e. they both spin at the same rate. However, the gearbox creates this effect by having a 4th gear ratio of 1:1, meaning both gears spin at the same rate (both have the same number of gear-teeth) and the input and output RPM is equalized, although some power is lost as it is still being transmitted through a set of gears, in spite of the fact that they are of a 1:1 ratio. The final drive is a second set of gears, generally part of the differential in the rear axle, which then takes this output and multiplies it again before sending it to the wheels (it's called the "final drive" because it is the last set of reduction gears the torque is transmitted through before the drive "

"both allowing the car to run at a higher RPM at any given speed, giving more maximum horsepower, and acting to give mechanical advantage to the engine, helping to mitigate the low torque of the smaller motor. "

I've reverted this change as there are a number of problems with it.

• "because it mimics the effect of having the driveshaft connected directly to the output of the engine, i.e."

No-one wants to "mimic" anything. Having the optimum speed/torque combination to drive the final drive is useful, but that's not because it's similar to how it used to be with a direct-drive gearbox top ratio

• "However, the gearbox creates this effect by having a 4th gear ratio of 1:1, meaning both gears spin at the same rate (both have the same number of gear-teeth) and the input and output RPM is equalized, although some power is lost as it is still being transmitted through a set of gears, in spite of the fact that they are of a 1:1 ratio."

RWD gearboxes are direct drive in 4th, they don't use pairs of equal-sized gears.

FWD gearboxes are all-indirect (see layshaft), but they don't have 1:1 ratios. 1:1 is not some magically efficient gear ratio, it's just what you get from direct drive.

• "generally part of the differential"

The final drive is not part of the differential. They're both part of an overall mechanism that is commonly called "the diff", but that's not an accurate term.

Andy Dingley (talk) 13:35, 12 January 2015 (UTC)

It's better than the way it was before, when it claimed that a "conventional RWD car sends the power to the "transaxle"". Pretty sure that's NOT how conventional RWD's work. Second, by "mimic", I meant that a 1:1 ratio (whether in a 4 or 5 speed transmission) being gives the same effect as having no gearbox at all, i.e. same input and output shaft rotation. Except that to say that it eliminates friction losses is foolish, because it's STILL being run through a gearset...simply one that runs at a 1:1 ratio instead of 0.92:1. Thus, both shafts are turning at the same speed, which means the driveshaft is being driven at engine speed. But you don't save on gear friction as you would if you just bolted the driveshaft to to flywheel directly. It's always using gears to transmit the power. Third, I didn't say anything about 1:1 being "magic". I was referring to the previous statements regarding "direct drive", which to my knowledge refers to a 1:1 ratio transmission gearset. That is the most common one that I've seen for 4th gear, and it is the reference point for the term "overdrive" in the first place. If it's 1:1 or over it's "overdriven". And last, I'm pretty sure the most common method of obtaining a 1:1 ratio in 4th is to use 2 equally sized gears. That is the simplest way to make two gears with revolve at the same rate as each other. When one gear is smaller than the other, the smaller gear has to revolve more than one time to make one revolution of the larger gear. The opposite for the opposite. Thus, I find it reasonable to assume that in the vast majority of cases, a 1:1 ratio 4th speed gear will employ two equally sized gears, with an equal number of teeth, one attached to the input shaft, and one dog-clutched to the output shaft. Feel free to explain to me how that is incorrect, however. And yes, okay, part of "The Differential Assembly". Before it just referred to some mythical "Final drive" device, which may or may not have been a part of the "rear transaxle" in a conventional front-engine, RWD car. I didn't find any article about a "third member", so I used the simplest locating term I could to indicate whereabouts the final drive assembly is. It is integral with the differential assembly, much as how I would refer to the underdrive function on a 4WD truck as being "part of the transfer case", although in truth that refers only to the device which splits power to both axles. It combines this function with an underdrive (4 Low) gearbox, and the whole is referred to in common usage as a "transfer case". In all, I don't see anything incorrect about what I wrote before, and I don't understand your complaints. At the very least, I'm not sure you read what I was saying correctly. .45Colt 14:08, 12 January 2015 (UTC) Also, what the hell is wrong with saying "a lower final drive allows the engine RPMs to be faster at any given speed and therefore gives more maximum horsepower". That is exactly how lower gearing works.

"Transaxle" has gone from the RWD cars. I think we're agreed that it's excess detail as it doesn't change the overdrive issue.

"Second, by "mimic", I meant that a 1:1 ratio (whether in a 4 or 5 speed transmission) being gives the same effect as having no gearbox at all, i.e. same input and output shaft rotation."

It's a minor point, but I still don't like that term "mimic". It implies a deliberate choice made to emulate a previous situation. It might have the same end result, but it wasn't made that way to emulate the previous.

"Except that to say that it eliminates friction losses is foolish, because it's STILL being run through a gearset."

"It's always using gears to transmit the power."

It isn't. It's direct drive. In 4th gear of a 4 or 5 speed RWD gearbox (as used for decades) there is one, and only one, set of gears in the drivetrain, the crown and pinion bevel of the rear axle. The drive just isn't going through any of the other gears.

"If it's 1:1 or over it's "overdriven"."

That is one popular meaning, but it's not the original or the fundamentally defining one, which is about optimising the power band and road speed, such that the fastest gear is no longer the highest gear, but the one below it.

"And last, I'm pretty sure the most common method of obtaining a 1:1 ratio in 4th is to use 2 equally sized gears. "

No. Cite a WP:RS for this. It's not the case in RWD, it's not even the case in FWD.

"That is the simplest way to make two gears with revolve at the same rate as each other. When one gear is smaller than the other, the smaller gear has to revolve more than one time to make one revolution of the larger gear. The opposite for the opposite. Thus, I find it reasonable to assume that in the vast majority of cases, a 1:1 ratio 4th speed gear will employ two equally sized gears, with an equal number of teeth, one attached to the input shaft, and one dog-clutched to the output shaft. "

I assume. – Don't, cite.

Gearbox in direct-drive 4th

"Feel free to explain to me how that is incorrect, however. "

Try reading layshaft and the section on all-indirect gearboxes (and how they've been obsolete for RWD since some time around WWI). Otherwise look at diagrams like that shown here (although maybe find a better one from elsewhere) that explain a direct-drive top.

"it just referred to some mythical "Final drive" device"

There's nothing mythical about a final drive. The term has been correctly in use on road vehicles for about 150 years.

"how I would refer to the underdrive function on a 4WD truck as being "part of the transfer case""

I'm not even getting into that one, but I've never heard the low ratio of a transfer case as being termed an "underdrive" before.

"Also, what the hell is wrong with saying "a lower final drive allows the engine RPMs to be faster at any given speed and therefore gives more maximum horsepower"

Because it implies that changing the gear ratio changes the engine's maximum horsepower. It can't do this. All the choice of gear ratio can achieve is to optimise making use of whatever horsepower is available. Like "mimic", this isn't clearly wrong (in the way that "direct drive uses 1:1 gears" just is), but it does create a misleading impression amongst readers and should be avoided for that reason. Andy Dingley (talk) 15:02, 12 January 2015 (UTC)