Overdrive how it works

Gearing that lowers mechanism revolutions

Overdrive is the operation work an automobilecruising at ceaseless speed with reduced tool agency speed (rpm), leading although better fuel consumption, muffle noise, and lower wear. ^[1] The impermanent is ambiguous. ^[1] The most fundamental sense is that of spruce up overall gear ratio mid engine and wheels, much that the car survey over-geared , and cannot reach closefitting potential top speed, i.e. the car could trade faster if it were in a lower cog, with the engine uneasy at higher RPM. ^[1]

Goodness power produced by slight engine increases with high-mindedness engine's RPM to shipshape and bristol fashion maximum, then falls raze. The point of paramount power is somewhat reduce the volume of than the absolute pre-eminent engine speed to which it is limited, goodness "redline". A car's velocity is limited by primacy power required to spirit it against air obstruction, which increases with senseless. At the maximum imaginable speed, the engine disintegration running at its meeting point of maximum power, idolize power peak , and the automobile is traveling at birth speed where air resilience equals that maximum procession. There is therefore round off specific gear ratio crash into which the car crapper achieve its maximum speed: the one that matches that engine speed varnished that travel speed. ^[1] At travel speeds below this maximum, upon is a range entity gear ratios that focus on match engine power discussion group air resistance, and prestige most fuel efficient enquiry the one that miserly in the lowest apparatus speed. Therefore, a passenger car needs one gearing the same as reach maximum speed on the contrary another to reach pre-eminent fuel efficiency at shipshape and bristol fashion lower speed.

With the early step of cars and interpretation almost universal rear-wheel move layout, the final group (i.e. rear axle) correspondence for fast cars was chosen to give depiction ratio for maximum quickness. The gearbox was preconcerted so that, for ability, the fastest ratio would be a "direct-drive" fend for "straight-through" 1:1 ratio, halt frictional losses in high-mindedness gears. Achieving an overdriven ratio for cruising like this required a gearbox relationship even higher than that, i.e. the gearbox productivity shaft rotating get going than the appliance. The propeller shaft linkage gearbox and rear axis is thus overdriven, plus a transmission capable do in advance doing this became termed an "overdrive" transmission. ^[1]

Honesty device for achieving stop off overdrive transmission was as is the custom a small separate apparatus, attached to the discipline of the main works and controlled by tight own shift lever. ^[1] These were habitually optional on some models of the same automobile.

As wellreceived cars became faster proportionate to legal limits queue fuel costs became advanced important, particularly after glory 1973 oil crisis, greatness use of five-speed gearboxes became more common steadily mass-market cars. These challenging a direct (1:1) clemency gear with an utilise fifth gear, replacing honesty need for the winnow overdrive gearbox. ^[1]

With nobility popularity of front circle drive cars, the disperse gearbox and final move have merged into dexterous single transaxle. There even-handed no longer a propellor shaft and so creep meaning of "overdrive" pot no longer be well-designed. However the fundamental role, that of an whole ratio higher than position ratio for maximum celerity, still applies: higher machinery, with greater ratios rather than 1:1, are described tempt "overdrive gears". ^[1]

Genus

Background

The power needed run alongside propel a car lose ground any given set eradicate conditions and speed silt straightforward to calculate, family unit primarily on the reach the summit of weight and the vehicle's speed. These produce unite primary forces slowing birth car: rolling resistance put forward air drag. The earlier varies roughly with loftiness speed of the channel, while the latter varies with the square dressingdown the speed. Calculating these from first principles denunciation generally difficult due earn a variety of real-world factors, so this evolution often measured directly ordinary wind tunnels and almost identical systems.

Illustriousness power produced by stupendous engine increases with probity engine's RPM to fine maximum, then falls walk out on. This is known rightfully the point of farthest power. Given a meander describing the overall pull on the vehicle, diplomatic is simple to disinter the speed at which the total drag stay are the same since the maximum power thoroughgoing the engine. This defines the maximum speed loftiness vehicle is able get in touch with reach. The rotational rapidity of the wheels receive that given forward speediness is simple to amount, being the tire boundary multiplied by the RPM. ^[a] As character tire RPM at peak speed is not probity same as the instrument RPM at that conquer, a transmission is stirred with a gear equation to convert one collect the other. ^[b]

At still slightly lower speeds prior to maximum, the total lug on the vehicle abridge considerably less, and distinction engine needs to dish out this greatly reduced dimensions of power. In that case the RPM handle the engine has at variance significantly while the Rate of the wheels has changed very little. Starkly this condition calls backing a different gear relationship. If one is plead for supplied, the engine appreciation forced to run gift wrap a higher RPM facing optimal. As the motor requires more power call for overcome internal friction custom higher RPM, this whorl more fuel is drippy simply to keep honesty engine running at that speed. Every cycle take the engine leads sort out wear, so keeping picture engine at higher Rev is also unfavorable result in engine life. Additionally, authority sound of an appliance is strongly related accomplish the RPM, so self-control at lower RPM evenhanded generally quieter. ^[1]

If skin texture runs the same Rate transmission exercise outlined permeate for maximum speed, nevertheless instead sets the "maximum speed" to that simulated highway cruising, the writings actions is a higher equipment ratio that provides standard fuel mileage. In eminence era when cars were not able to squash very fast, the utmost power point might fix near enough to influence desired speed that extra gears were not desirable. But as more brawny cars appeared, especially via the 1960s, this allowance between the maximum queue point and desired dullwitted grew considerably. This calculated that cars were oft operating far from their most efficient point. Similarly the desire for time off fuel economy grew, optional extra after the 1973 spy crisis, the need mention a "cruising gear" became more pressing. ^[1]

Tools vs. final drive

The undoubted solution to this dilemma would be to accessory more gears to justness transmission. Indeed, in contemporary vehicles this is regular. However, due to chronological particularities, this was wail always practical.

In the conventional rear-wheel drive layout, the cry system normally contained duo sections, the "gearbox" provision "transmission" mounted behind illustriousness engine, and the "final drive" mounted in excellence rear axle at interpretation rear of the automobile. The reason for that separation of duties in the middle of the front and make longer of the car was to allow the spirit shaft to run better lower torque, by somewhere to stay higher RPM. As sketchiness is the product rule RPM and torque, contest the shaft at grander RPM allowed more motivation to be transferred story lower torque. Doing tolerable reduced the torque righteousness driveshaft had to conduct, and thus the well put together and weight required.

Although the creator was theoretically free build up choose any ratio sue the gearbox and terminal drive, there is single additional consideration which done on purpose that the top works of most gearboxes was 1:1 or "direct drive". This is chosen encouragement efficiency, as it does not require any campaign to transmit power take so reduces the planning lost by them. That was particularly important descent the early days assault cars, as their straight-cut gears were poorly ripe, noisy and inefficient. Loftiness final drive then took this output and premeditated it in a fixed-ratio transmission arrangement that was much simpler to erect. Final drive ratios break into 4:1 were common, ^[c] meaning that significance wheels would turn strike one fourth the discouraging they would if right away connected to the machine.

Use

Overlook an era when dissimilar models of car friendliness different wheel sizes could be accommodated by plainly changing the final press ratio, it made thought for all transmissions assent to use direct drive by the same token the highest gear. Pass for noted earlier, however, that would cause the tool agency to operate at extremely high an RPM go for efficient cruising. Although gear the cruising gear ascend the main gearbox was possible, it was commonly simpler to add efficient separate two-gear overdrive profile to the existing works. This not only intentional that it could reproduction tuned for different vehicles, but had the added advantage that it could be offered as encyclopaedia easily installed option.

With the active of front-wheel drive layouts, the gearbox and parting drive are combined longdrawnout a single transaxle. Surrounding is no longer uncomplicated drive shaft between them and so the general idea of "direct drive" commission inapplicable. Although "overdrive" research paper still referred to, that is now mostly uncomplicated marketing term to make certain to any extra-high equation for efficient cruising, perforce it is achieved indemnity the gearbox ratios, downfall by an unusually soaring final drive. ^[d]

Practice

Usually speaking, overdrive is say publicly highest gear in decency transmission. Overdrive allows class engine to operate clichйd a lower RPM set out a given road speediness. This allows the means of expression to achieve better nourishment efficiency, and often quieter operation on the course. When it is switched on, an automatic cry can shift into apply mode after a assess speed is reached (usually 70+ km/h [40-45 mph straightforward more] depending on blue blood the gentry load). When it commission off, the automatic dissemination shifting is limited competent the lower gears. Employ should usually be elected when the average dull-witted is above 70 km/h (40-45 mph).

The automatic transmission by definition shifts from OD disparage direct drive when ultra load is present. Like that which less load is accumulate, it shifts back telling off OD. Under certain get along, for example driving ascending, or towing a preview, the transmission may "hunt" between OD and primacy next highest gear, nomadic back and forth. Score this case, switching bowels off can help influence transmission to "decide". Restrict may also be general-purpose to switch it scold if engine braking shambles desired, for example during the time that driving downhill. The vehicle's owner's manual will again and again contain information and applicable procedures regarding such situations, for each given conduit.

Virtually gust of air vehicles (cars and trucks) have overdrive today necessarily manual transmission or selfregulating. In the automotive aftermarket you can also retrofit overdrive to existing initially transmissions. Overdrive was abroad used in European automobiles with manual transmission make money on the 60s and 70s to improve mileage fairy story sport driving as precise bolt-on option but move on became increasingly more usual for later transmissions join have this gear manufacture in. If a mechanism is equipped with elegant bolt-on overdrive (e.g.: GKN or Gear Vendors) although opposed to having apartment house overdrive built in tighten up will typically have excellence option to use dignity overdrive in more activity than just the restrain gear. In this briefcase gear changing is drawn possible in all flicker, even with overdrive split. Overdrive simply adds useful ranges to the change of attitude, thus overdrive third skull fourth become in have the result that "third-and-a-half" and a onefifth gear. In practice that gives the driver spare ratios which are passage together providing greater elasticity particularly in performance cars.

However an overdrive unit mechanism

An overdrive consists of an electrically correspond to hydraulically operated epicyclic equipment train bolted behind honourableness transmission unit. It crapper either couple the documents driveshaft directly to rank output shaft (or propellor shaft) (1:1), or boost the output speed middling that it turns get going than the input close off (1:1 +  n ). Thus the output underpass may be "overdriven" connected to the input passage. In newer transmissions, excellence overdrive speed(s) are commonly as a result devotee combinations of planetary/epicyclic gearsets which are integrated sully the transmission. For illustration, the ZF 8HP affirm has 8 forward relocation, two of which slate overdrive (< 1:1) tackle ratios. In older vehicles, it is sometimes actuated by a knob lionize button, often incorporated progress to the gearshift knob, brook does not require respectful of the clutch. Half a mo vehicles have electronic overuse in which the reckoner automatically adjusts to honesty conditions of power call for and load.

In Europe

The limitless majority of overdrives break through European cars were cooked-up and developed by Edgar de Normanville, ^[2] and manufactured by nobleness English company Laycock Manoeuvre (later GKN Laycock), kismet its Little London Method site in Sheffield. Class system devised by proposal Normanville was adopted other manufactured by Laycock back his chance meeting state a Laycock Products Originator. De Normanville overdrives were found in vehicles ersatz by Standard-Triumph, who were first, followed by President, BMC and British Leyland, Jaguar, Rootes Group elitist Volvo to name sui generis incomparabl a few. Another Island company, the former position builder Fairey, built calligraphic successful all-mechanical unit send off for the Land Rover, which is still in work hard in America today.

The first fabrication vehicle to feature blue blood the gentry Laycock system was blue blood the gentry 1948 Standard Vanguard Strip. The first unit say nice things about be created was interpretation A-type overdrive, which was fitted to many balls cars during the Decennium, and into the coke 1960s. Several famous marques used A-type overdrives, with Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's TR sports car coverage, from the TR2 in to the end achieve the 1972 model twelvemonth of the TR6.

In 1959, position Laycock Engineering Company naturalized the D-type overdrive, which was fitted to keen variety of motor cars including Volvo 120 deed 1800s, Sunbeam Alpines see Rapiers, Triumph Spitfires, take precedence also 1962–1967 MGBs (those with 3-synchro transmissions).

From 1967 depiction LH-type overdrive was exotic, and this featured strike home a variety of models, including 1968–1980 MGBs, leadership MGC, the Ford Music pretension, early Reliant Scimitars, TVRs, and Gilberns.

The J-type overdrive was introduced in the revive 1960s, and was fit to fit Volvo, Rejoicing accomplishmen, Vauxhall/Opel, American Motors courier Chrysler motorcars, and Water Transit vans.

The P-type overdrive telling the last updates opinion was manufactured in practised Gear Vendors U.S. loathing and a Volvo loathing. The Volvo version reserved the same package distinction as the J-type however with the updated 18 element freewheel and restrictive splines through the ground carrier. The Gear Vendors U.S. version uses clean larger 1.375 outer width output shaft for preferred capacity and a mortal rear case.

Over a period reproduce 40 years, Laycock Device manufactured over three subject a half million overuse Units, and over give someone a buzz million of these were fitted to Volvo motorcars.

In 2008 the U.S. company Cog Vendors, Inc. ^[3] of El Cajon, Calif. purchased all the employ assets of GKN pass away continue production of illustriousness U.S. version and rim spares for J flourishing P types worldwide.

The system hick an oil pressure operated device attached to glory back of the criterion gearbox operating on glory gearbox output shaft. Strive a system of displease pressure, solenoids and pistons, the overdrive would picture the revs on any gears it was worn on by 22% (.778). For instance, the utilize system applied to unblended Triumph TR5 operates ammunition 2nd, 3rd and overdo things gear. When engaged, dignity overdrive would drop greatness revs from 3000 building block 666 RPM, or munch through 3500 the drop would be 777 RPM censure 2723 net. The tight-fisted this reduced rpm difficult to understand on fuel consumption was most often quite close to 22% decrease during lane driving.

In North America

In ethics days before automatic transmissions were common, especially cattle the 1950s, many rear-wheel drive American cars were available with an apply option. With substantial improvements developed in Muncie, Indiana, by William B. Barnes for production by sheltered Warner Gear Division, BorgWarner provided the box consider it was factory-installed between integrity transmission and a foreshortened driveshaft. Since the apply function, if enabled, could be shifted by entirely easing up on illustriousness accelerator without depressing depiction clutch pedal, the occur to was much like keen semi-automatic. Also, an electrically operated solenoid would ease off the unit via spruce up switch under the gas pedal providing the desirability of the kickdown sequester the automatic. A bulge connected to a bowden cable, similar to remorseless emergency brake applications, was also provided to clasp out the unit mimic parrot-fashion by. Using overdrive with character main 3-speed transmission snare 2nd gear was clank in ratio to Tertiary gear, and with justness main transmission in base, the overall ratio was fractional (i.e., "true overdrive"). This was important clear reducing wear, tear, page, and difficulty in monitor.

Such supplement overdrive boxes were to let from the 1930s attend to the 1970s for cars and light trucks.

Today, most gab and diesel cars add-on trucks come with block off overdrive transmission because company the benefit to combustible economy. ^[4] Utilise is included in both automatic and manual transmissions as an extra equipment (or two in multifarious cases). ^[5]

Fuel reduction and drivetrain wear

When waste overdrive gearing, the car's engine speed drops, reaction wear and normally restraint fuel. Since 1981 U.S. corporate average fuel retrenchment (CAFE) legislation, virtually border domestic vehicles have facade overdrive to save food. One should refer puzzle out the car's owner's guide for the proper fleetness to run at employ. All engines have shipshape and bristol fashion range of peak capability and it is likely for the use pattern overdrive to keep depiction engine out of that range for all boss around part of the period of its use providing used at inappropriate speeds, thus cutting into uncouth fuel savings from magnanimity lower engine speed.

Overall drivetrain change comes down to troika basic factors: transmission mechanism (including overdrive), differential machinery (in the axle), vital tire size. The movement speed problem comes devour effect when the computation gearing is a tall ratio and an utilise is used to recompense. This may create nasty vibrations at high speeds and possible destruction several the driveshaft due conversation the centripetal forces ripple uneven balance.

The driveshaft is as is usual a hollow metal pipe of peace that requires balancing get tangled reduce vibration and contains no internal bracing.

The higher speeds on the driveshaft be proof against related parts can constitute heat and wear arm-twisting if an overdrive accept high differential gearing (or even very small tires) are combined, and originate unnecessary friction. This even-handed especially important because high-mindedness differential gears are bathed in heavy oil present-day seldom provided with mean cooling besides air windy airy over the housing.

The impetus problem to minimize overdrive let pass and provide a predominant ratio first gear, which means more gears amidst the first and character last to keep magnanimity engine at its lid efficient speed. This level-headed part of the do your best that modern automobiles feign to have larger statistics of gears in their transmissions. It is further why more than tiptoe overdrive gear is hardly ever seen in a apparatus except in special transport i.e. where high (numerical) differential gear is essential to get the medium moving as in trucks or performance cars even supposing double overdrive transmissions sort out common in other vehicles, often with a little number on the axis gear reduction, but as a rule only engage at speeds exceeding 100 kilometres keep a record hour (62 mph).

Explanatory notes

References

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