velryba

does anyone know what overdrive means???

something to do with a gearbox or a clutch ....

thanks for any help

robert

IanW

Isn't that the same as kickdown on an Autobox?

means it revs a bit higher when you put your foot flat to the floor?

velryba

i thought in automatics as you put your foot down it undergears but what about manuals ?
logicaly if you put your foot down the revs goes up
or am i mistaken a it goes only with automatics?

Ken E

I remember old cars used to have overdrive. These were pre 5 speed and used to have a button on the gear lever. When you pressed the button you went into overdrive, which was effectively a higher gear. I think in automatics it's the same sort of thing, although some autos have a high and low ratio selector (aka sport mode) which affects all the gears not just top.

mutant_matt

What Ken said. I think it was something like an extra gear in the rear diff which was electrically operated like Ken said, from a switch on the gear stick.

It was usually a much higher gear which was used to reduce the revs at sustained high speed, I think mainly for comfort and economy.

Matt

mutant_matt

P.S. Some of the "more mature" ( )readers on the board want to post (as I'm sure they've had cars in the past with this feature).

Mr Felstead/T-S?

Matt

patGT

Matt is right.

My dad's Volvo 240 GLT (1980) used to be 4-gear manual with Overdrive. I drove it myself in my youth ;-)

The four gears were normally switched by gear lever while the overdrive could be activated by a button on top of the gear lever - and only when in 4th gear.
Physically, the overdrive used to be an additional gear device flanged to the standard gearbox.

Big advantage: you did not need the clutch when switching overdrive on and off. Very fast downshifting from 5 to 4 indeed!

With these Volvo models, it provided a cost-effective solution to make a 5-gear car while using the standard 4-gear box.

Cheers,
Pat

BugEyed

Hmm, I wasn't one of the names mentioned, but might fall into that catagory.

Overdrive just means that the gearing is changed to raise it such that the engine revs less highly making for relaxed cruising. Traditionally cars were geared so that they were running at max revs when they hit their maximum speed (as governed by aerodynamics and other drag vs. power). Whilst Scoobs are generally geared like this, a lot of modern cars reach their max speed in 4th and 5th is effectively an overdrive.

For some cars in the 50's to 70's an overdrive could be fitted as an option to improve their cruising ability. The most common was a unit manufactured by Layc0ck de Normanville (I put in an 0 to avoid being censored) which was/is an electrically controlled epicyclic gear. If you turned on the solenoid (via a switch on the gearlever or the dash) it went from direct drive to a higher ratio via the epicyclic gears. Generally it was wired so that you could only use it in 4th (making a 5 speed box), but with a small mod you could add it to all the gears (at the risk of it breaking as it doesn't like too much torque, but making a 8 speed box ). These were OE on some Jaguars, MG's, Volvo's, Bentley's etc.

Hope that helps.

Duncan

PS. I've got one on a 1930 Bentley at present and it gives me 80 mph at 2,200 rpm - very relaxing on the motorways on long trips, just a pity the brakes don't like stopping from those speeds in traffic!

PPS. Edited to add that they are normally mounted between the gearbox (often bolted to the back of it) and the rear axle. Normally they are not part of the rear axle as they were mainly used on live axles and you wouldn't want to (further) increase the unsprung weight.

PPPS. Sadly they are now banned in most forms of historic racing - a downward gearchange on my car takes about 2 seconds and on tight corner changing out of overdrive was a lot quicker, particularly at Prescott or Shelsley Walsh.

[Edited by BugEyed - 5/21/2002 12:35:10 PM]

velryba

---------> bows down to the ground...

thank you very much

now i guess i can say with a confidence that i know what overdrive is

thanks guys
robert

RT

"Overdrive" in the original sense of the word, means that the total gear reduction ratio is less than 1. Thats all.

Put simply, one engine revolution would turn the wheels more than one wheel revolution. Manufacturers have often used the term to indicate the topmost gear on an automatic, which probably adds to the confusion.

BugEyed

RT

I don't quite agree!

This would imply that the back axle ratio has been overcome. However, you are basicly correct, it is where the ratio of the gearbox is less than 1:1 (ie. a step up).

Duncan

RT

Yes, I meant that the overall ratio is less than 1. Gearbox reduction ratio times final drive ratio (where-ever the final drive is...may be in the gearbox for a frontWD).

To my understanding, this is the original meaning of "overdrive". Of course along the way, many manufacturers have coined up different marketing interpretations.

BugEyed

RT

I guess that we will just have to agree to differ. Just for the record, I'm not aware of any car that has an overall ratio of greater than 1:1 as normally the final drive is 3:1 or greater. Even when working on very old cars with low revving engines (reline at 1,500 rpm anyone?) they still have an overall reduction.

Duncan

Dave R

Ford MT75 as in late Sierra's etc... has a 5th gear of 'over' 1:1, so is said to have an over driven 5th gear...

velryba

ok ok children

don't shoot each other over me

or if you are going to then i want to see it tho

RT

Duncan, actually I think you're right.

*stand corrected*


Its probably the gearbox ratio thats less than 1:1, without taking the final drive into account.

BugEyed

RT

No problem, but Robert's waiting for a fight, so we mustn't let him down, must we?

Duncan

carl

Mmmm, Laycock overdrive

You were supposed to lift off the throttle before flicking the switch, but you could (at your own risk) use the overdrive button to do full-throttle gearchanges. Also found on old Triumphs (Herald, Vitesse, Spitfire, GT6), usually on third and fourth gear. Earlier models had a switch on the dash, later ones on top of the gearstick. As has been said before, it's a 'sun & planet' system much like an auto box. What was cool was that the shift quality was lovely because the stick went straight into the gearbox (which was in the transmission tunnel) and then the overdrive sat behind it.

velryba

yes it's actualy for the Triumph Spitfire

my friend who's selling the Shogun was after one but didn't know what the overdrive ment so I've told him that here you find all your answers (and some blood bath too )

so generally speaking is that car a worth a look ?

glo-glo

carl

What, a Spit?

I was actually looking at getting one of these (when I was a student) so I've got a very good book on them. What version is it?Mk1 and Mk2 bloody impossible to get bonnets for, Mk3 had the most power (of the 1300s), Mk4 the least, 1500 had even more but if driven hard can need a new engine after 15k miles

Handling is 'interesting' -- my dad had a Vitesse (same chassis, but 2 litre straight 6) and the suspension is transverse leaf springs. If you corner too hard, the inside rear 'tucks under' (change in camber from full load to no load can be 30 degrees...) and then you're in an unrecoverable spin. Attempting to opposite lock it just results in it spinning the other way.

PS: Get one with an overdrive, otherwise the ratios are just too short for everyday motoring. As they sucked more and more power out due to emissions regs with the later versions, they kept shortening the ratios to make the acceleration vaguely acceptable.

[Edited by carl - 5/21/2002 7:35:58 PM]

BugEyed

Robert

The Spitfire looks nice, but isn't up to much in comparison to the equivalent Sprite/Midget. Beware the handling (as Carl says, although it can be cured with different linkage and bushes), and avoid the 1500 cc engined versions (the engine started life at 850 cc and has been stretched beyond its limits - too many revs (5,500) and bang). The Vitesse (4 seat Herald with a 2 litre six) or a GT6 (Spitfire GT with 2 litre 6) are better choices.

Duncan

carl

Pah, Spridgets
The book to read is 'Triumph Spitfire and GT6' by Graham Robson. Amazon show it here but it's on 3-5 week delivery. I still have a bit of a soft spot for Spits/GT6s. I was even considering running a GT6 as a company car (original list price was about 1500 quid, so I don't mind paying the tax on that )

BugEyed

You might be right about the "modern" Spridgets, but what about the real thing? 1936 Riley Sprite (1750 cc 6 cylinder, good handling decent brakes - nothing that can be said for Spitfires)


Duncan

velryba

i'll pass all the info to my friend

lol i think he's after something different for his commuting only so not sure about a pure veteran

carl

GT6 then. It's got proper wishbones instead of cart springs, and a much better engine.