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Technical Overdrive!!

Discussion in 'The Hokey Ass Message Board' started by Boneyard51, Oct 6, 2020.

  1. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    Why is there over drive transmissions? Where is the wisdom of gearing up in the transmission, then gearing down in the rear end? Going straight though a transmissions, instead of going through a set of gears saves power, ie horsepower and milage. I can understand overdrive on trucks, ( loaded vs unloaded) but why cars! Anybody?






    Bones
     
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  2. squirrel
    Joined: Sep 23, 2004
    Posts: 46,870

    squirrel
    Member

    Beats me....my new old car has 2.53 rear gears. Coupled to a powerglide with a 1.76 first gear, and a huge torque converter, it eventually builds up speed....and only shifts once.

    But seriously, if you have a 5:1 gear reduction in the transmission and 2.5 in the rear end, the rear end needs to be pretty stout to handle the torque. If you have less reduction in the transmission, and more in the rear, there is less torque in the driveline.
     
  3. Phil P
    Joined: Jan 1, 2018
    Posts: 298

    Phil P
    ALLIANCE MEMBER

    Good question. I've always thought it might be a sales gimmick.

    Phil
    .
     
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  4. BJR
    Joined: Mar 11, 2005
    Posts: 6,518

    BJR
    Member

    So you can have more pickup from stop lights around town, and still get good gas mileage on the highway.
     

  5. VANDENPLAS
    Joined: Dec 14, 2009
    Posts: 2,676

    VANDENPLAS
    Member

    Better fuel mileage and increased hwy speeds while still being able to get out of its own way at lower speeds.

    kind of like newer cars with 6-7-8 speed auto transmissions!

    I drive a Nissan cargo van for work with a 6 speed trans and between 60-80kmh ( 40. -60 mph) the transmission feels like it’s having a seizure up down up down up down up down :mad:
    I end up just manually shifting it into 4th as it drives me crazy.

    but beating down the hwy at 140 ( 90 mph) at under 2000 rpm with a fully loaded work van, the trade off is tolerable.
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 46,870

    squirrel
    Member

    Oh...I thought the question was why do they have overdrive and a transmission with a 3:1 first gear, instead of no overdrive, and a steeper first gear ratio.

    Was the question why we need more than 2 or 3 gear ratios in the transmission?
     
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  7. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    I don’t think folks read my question, or I worded it poorly. Lol.
    My question is why do they gear up in the transmission by adding a gear them gear it back down in the rear end? Why not a lower first gear and a higher gear rear end. That way when in high gear , you are going straight through your transmission instead of going through four gears. Going through gears creates more friction, eating power and milage.
    I understand adding an old overdrive to a car, I've done it. But why do modern car do it?






    Bones
     
  8. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,602

    seb fontana
    Member
    from ct

    My thought too.
     
  9. squirrel
    Joined: Sep 23, 2004
    Posts: 46,870

    squirrel
    Member

    So... Why doesn't every car have a truck granny gear transmission? Have you seen how big first gear is in them?

    Sent from my Trimline
     
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  10. grumpy65
    Joined: Dec 19, 2017
    Posts: 417

    grumpy65

    All to do with "conservation of energy" (read fuel saving) if using a standard-ish rear end ratio of around 3:1 or less.
    Higher ratio low gears simply make it easier to shift the load from a stand-still.
    More gears gives less rpm drop between gears, keeping the engine nearer the "sweet spot" during acceleration.
    Lower ratio high gears (overdrive) mean lower rpm at cruising speed.
    Combined, this allows manufacturers to use smaller capacity engines (yaaawwn :confused:).
    All of these factors contribute to fuel savings when set up as intended. In my view, overdrive transmissions are the biggest step forward for lower fuel consumption that we have seen, followed by computer controlled efi.

    Now, the fun part.
    Couple an overdrive transmission with a higher ratio rear end and the magic happens. You can get off the line like a stabbed rat, as long as you can get traction, and still have not overly high rpm at cruising speed. Sure, the fuel saving gig has all but disappeared, but the "wow factor" just got huge. Try it. I think you may like it. :D:D:D
     
  11. BamaMav
    Joined: Jun 19, 2011
    Posts: 4,430

    BamaMav
    Member
    from Berry, AL

    Bones, for a while in semi trucks the hot thing was direct drive with extremely low numerical gears, 2.50 or lower. It was said the straight through design robbed less power than an overdrive did. It was measured in the tenths of a gallon or maybe less. That thinking didn't last long because they didn't pull good unless you dropped a couple of gears, eliminating any fuel mileage gains made. Then the new line of thinking has been more ratios instead of less. Overdrive 13 speeds, 18 speeds, etc. With the new automated shifting ran by computer, they are getting lots better mileages than the straight through gearing ever could. That thinking has carried over into cars and pickups, automatics with 6, 8, even 10 speeds are common now.

    I think it was just a matter of what engineer spoke the loudest and got his ideas into production.
     
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  12. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    I understand the more gears the more efficient the car/truck. Some older cars only had two speed transmissions while huge trucks have 18 speeds to get the load moving and keep it moving.
    I’m just hard headed and just can’t see the advantage of gearing up then down. Seems redundant. I know modern power trains are built this way. It’s just hard for me to understand the advantage in modern power plants.








    Bones
     
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  13. VANDENPLAS
    Joined: Dec 14, 2009
    Posts: 2,676

    VANDENPLAS
    Member

    Would you rather a 2 speed axle ?

    not sure exactly what answer your looking for.

    my guess is the final drive is a trade off for good off the line acceleration and low rpm at hwy cruising speeds.

    easier to have all the shifting happen in one spot then 2 ?

    4 speed manual transmission have a 1:1 ratio is that what your thinking ?


    Tuna sandwich ?
    Ham on rye ?

    or just some rye on the rocks ?
     
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  14. grumpy65
    Joined: Dec 19, 2017
    Posts: 417

    grumpy65

    Most input energy is used to accelerate an object from a standstill. As the velocity of the object increases, the energy needed to maintain velocity decreases. Once cruise speed is acheived, minimal energy is needed to maintain speed. This being the case, higher ratio gears, and more of them, help to get you going efficiently, and a lower ratio rear end helps to keep you going more efficiently at cruise speed.
    Clear as mud, right ??? o_Oo_Oo_O

    Sorry if this has already been stated. Had to field a phone call in the middle of it all.
     
  15. fordflambe
    Joined: Apr 9, 2007
    Posts: 516

    fordflambe
    Member

    This question is like asking "why have a four barrel carb rather than one large one barrel?". Better yet, "why have eight pistons rather than one large one?".
     
  16. rpu28
    Joined: Jan 17, 2006
    Posts: 133

    rpu28
    Member
    from Austin

    This is how an old guy explained it to me:

    The original overdrive transmissions, in effect, added a gear that increased the transmission drive above 1:1. That meant the tires were rotating faster at cruising RPM than those of a similar car without overdrive. I suppose the goals were better gas mileage and/or higher top speeds.

    But those OD transmissions were also built to free-wheel when you let off the gas if you were in OD - it's like you put in the clutch and were coasting without the engine slowing the car. With the tires no longer coupled to the engine, you'd get better gas mileage when slowing down.

    I think today's OD transmissions may be misnamed; they simply have a final gear whose that increases the drive ratio. They don't free-wheel.
     
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  17. squirrel
    Joined: Sep 23, 2004
    Posts: 46,870

    squirrel
    Member

    They are called overdrive still, because the transmission output shaft is turning faster than the input shaft. That's the definition of overdrive.
     
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  18. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    I guess I’m just not clear. I understand in the old days it was cheaper to develop a gear box bolted to the original three speed transmission, use the same rear end gear and get a car that would cruise at the higher highway speeds and get better mileage. I actually have one and love it. I know why they designed them in the forties and fifties .
    And I understand that today engines like to run about 1800 rpm at about 70 mph or so. So the gearing needs to be right for that. And that more speeds usually make the power train more efficient. I get all that!
    What I don’t get is why they gear up in the transmission instead of making high gear straight through and gear the rear end accordily.
    I just can’t get through my thick skull why they gear up then down!






    Bones
     
    Last edited: Oct 7, 2020
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  19. Phil P
    Joined: Jan 1, 2018
    Posts: 298

    Phil P
    ALLIANCE MEMBER

    The Doug Nash 5+1 had a direct 5th so you could use a rear end in the 2.xx's and still have good low end acceleration, but no mechanical losses in top gear.

    Phil
     
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  20. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    Actually new cars do have one large one instead of four little ones. But then they are fuel injected. Lol






    Bones
     
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  21. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    There you go! At least Doug is on my side! I also have one of his overdrive transmissions for a C-6.






    Bones
     
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  22. grumpy65
    Joined: Dec 19, 2017
    Posts: 417

    grumpy65

    You need to think of it as two different jobs to be done. Get moving, then maintain speed.

    1. Acceleration (moving an object from rest to a specific velocity).
    This is where deeper transmission gears and more of them help. Think of it as having more leverage to move the load.
    First job done.... now you are motoring.

    2. Cruising speed (maintaining velocity at a set level).
    Shallow rear end gears mean lower rpm at cruise speed (usually around 1800). You are not making a lot of power at such low rpm, but you don,t need much to only maintain velocity.
    Second job done.... goin' with the flow.

    3. Lets throw this in while we are running hot. Now you want to overtake someone (acceleration again).
    Trans kicks back, deepening the gear, applying more 'leverage' through the system, go man go.

    If you did it the other way (shallow trans gears / deep rear end), you could technically get the same cruising rpm but your acceleration potential would be woefull. :oops: In the good old days this was offset by stuffing in more cubic inches, which worked but guzzled the fuel a bit. :eek: The new setup allows for smaller engines / less power input to acheive the required work, thereby conserving fuel and saving the planet.

    Now, once again, for the fun bit.
    Combine an overdrive transmission / deeper rear end with more powerful engines and enjoy the fireworks. Hey, you can't save the planet every day...........;););)
     
    Last edited: Oct 6, 2020
  23. It's to spread the torque loads around to certain extent.... Gearing is a means of torque multiplication. The more multiplication, the higher the loads. Ratio and physical gear size are some of the factors in how much torque a gear set can withstand. Tremec downrates the low-first-gear TKOs 100 ft-lbs for just that reason. Tacking a OD on top of a manual trans means lower torque figures in every lower gear compared to adding a lower first gear to a non-OD trans and running a taller rear gear. Another reason is once you get to 1:1, no multiplication is taking place and gear strength requirements for the OD is minimal.
     
  24. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,120

    fiftyv8
    Member
    from CO & WA

    I had a Panel Delivery years ago with 4.58:1 diff gears and a 3 speed tranny.
    I was always the first across an intersection at the lights but after that it was a dead loss.
    I found around town I was always seemed to be in 3nd gear at slow speeds and on highways it was screaming.
    The GM 6 cylinder engine wore itself out due to the amount of revs that darn thing had to produce to move any where.
    Positive side was that it would haul weight all day long.
    If it had an O/D it would have been a much nicer vehicle to get around in and the engine life would have been much longer.
     
  25. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,348

    Boneyard51
    Member

    Finally an answer! Thank you Steve. Now I understand. They can make the lower gears lighter and the rear end lighter due to overall reduced torque. This is an improvement over build bigger components to handle the higher torque, therefore saving power and fuel!
    I got it!
    Thanks Steve, you came through again!










    Bones
     
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  26. Ned Ludd
    Joined: May 15, 2009
    Posts: 4,073

    Ned Ludd
    Member

    I think I get you. You want to know why transmissions have direct drive in the (effective or actual) second-highest gear rather than the highest gear? It's a valid question.

    I suppose it hinges on what kind of driving is expected: in which gear is the car expected to spend the most time? The more highway driving is expected, the more you can assume that most of the driving will happen in the highest gear, in which case the greatest mechanical efficiency is indeed achieved where the highest gear is direct drive, and the final drive ratio is fairly tall. We'd have to ask ourselves if the difference is all that much, though. And then, having the drive shaft spin faster in any given gear means that it deals with less torque and may therefore be slightly lighter, which may offset the slight loss in mechanical efficiency. (Edit: that's basically what Crazy Steve said above.) I've had the idea of a gearbox with a direct first and several overdrives, driving an extremely short final drive, but bearing speeds and centrifugal forces on synchronizers become problems.

    An important factor is the machinery used to make gearboxes, which makes it worth the manufacturer's while to base new gearboxes on the architecture of existing gearboxes. That is why a lot of five- and even six-speed manuals are derived from earlier four-speeds, and so still have direct drive in 4th. It's not always the case, though where a lower first gear is added it often results in a dog-leg-first box.
     
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  27. grumpy65
    Joined: Dec 19, 2017
    Posts: 417

    grumpy65

    Sorry, seems I was barking up the wrong tree with your question. You were after answers regarding the physical strength and weight of the system. I was rabbiting on about the performance capabilities of the system.
     
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  28. squirrel
    Joined: Sep 23, 2004
    Posts: 46,870

    squirrel
    Member

    I'm glad Steve was able to explain it...I was giving you hints along the same line, but I guess they didn't get through. Sorry.
     
  29. Beanscoot
    Joined: May 14, 2008
    Posts: 1,587

    Beanscoot
    Member

    I've wondered the same thing for years. Most of the variables mentioned can be discounted in ordinary, non high power cases.

    Example: I have a Ford Toploader overdrive based on the '60s Toploader four speed, attached to a small block engine. The car has a 3.25 differential.
    In top gear, the power is going through the countershaft, causing a lot of wear on its bearings. This was a common problem with these transmissions.
    The basic toploader transmission was also used in Jeeps as a model 176 (and a couple more), but with much lower gears and 1:1 top gear. Theoretically these gears could be used in the toploader case, the diffy changed to a lower numerical ratio resulting in the total gear ratios (transmission ratios x diff ratio) remain pretty close to the same.
    There is no problem with case or gear strength. All gears will fit in the case. The diffy is plenty strong. The driveshaft sees more load so can be replaced with one of slightly larger diameter.

    I can't see why this wouldn't end up a better setup.
     
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  30. You're not actually reducing the overall torque loads, you're redistributing them. Any load 'saved' at the trans will be transferred to the rear axle, all else being equal. But the axle uses much larger gears than a transmission, so it's easier to add strength there.

    I suspect that if both combos were run across a chassis dyno with the same road speed/RPM in each gear, you'd see the same or nearly so torque numbers at the wheels.
     
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