Thanks Steve and Ned, both your answers combine to make sense to a question I've ponderd for a long time. Beanscoot I think that the 4 speeds converted to 4 speed overdrives (Ford and I think Dodge did it in their 6 cylinder trucks) were maybe done for marketing. Phil
Marketing or not, they are a real flexible transmission. I’ve got a couple of those too. I always was an over drive nut! I like the old ones. Bones
A lot of it had to do with long stroke motors. Long stroke motors don't like to run up near the peak hp rpm, and most of them made their torque at 2k or lower. So low rear ends to get to the hp peak quick then the or to drop cruising rpms back down c!oser to the peak torque band. My car came from the factory with a 1 to 1 high gear connected to a 4.11 rear end. It's pretty peppy of the line but not real happy at 3300 going down the road at 63 mph. I put the od in last year and it's much happier at 2400 at 65, and by being closer to the 1800 peak torque rpm still has plenty of grunt to pull highway grades without dropping out of od. The other deal is I can split second gear and with the od. I can drive in traffic almost like driving a two speed automatic and have a speed range of 15 to 50 mph on surface roads. Most of these engines put out 80 to 125 so more gear choices made them more flexible. Overdrive's fell out of favor when manufactures switched to short stroke motors that like to rev and didn't have their pistons nearly 5 inches from the cranks when they fired off. The reason modern multi speed transmissions are hunting for the right gear all the time as short stroke motors also make their peak at higher rpms. So the engine can maintain 70 in it's third over drive gear at 1500 rpm on the level, but throw in a small upgrade and a head wind and pretty soon it's looking for a gear to match load and drag with torque.
Gear Vendors makes an Underdrive unit as well as the more common Overdrive. You could build your car with a very tall rear gear and then use an Underdrive on the back of the transmission to give a better ratio for acceleration or for towing, switching to straight through once under way, or split shifting. The end result should be the same.
Most early cars with overdrive had gears around 4:11 - it was with Studebakers and why most posi Dana 44's are missing under those Studebaker trucks. If you ever own an early stock car with one you'll see the difference when you shift into it on the highway.
are you sure about that? Perhaps it's just some marketing jargon they made up....something about "splitting gears" by shifting into overdrive 2nd as an underdrive 3rd gear.
I’m just starting a project putting an early 50s mopar gearbox with overdrive(R-10-G-1) behind a B motor. Just looking for longer legs at higher speeds but still good gearing around town.
New transmissions do similar A 6L90 trans has a low 1st gear. But doesn’t have a 1:1 at all. Like most new 6spd manuals. 5th is a slight OD followed by another OD Why? Because the rpm range the engineers designed into it to match modern speed limits for optional mpg. By having a final ratio of 1:1 you limit options on your final drive rpm. The newer 8 and 10 speed autos maximize an engines torque curve to keep the engine at is most efficient rpm. My only question is why it took so long to make a decent trans over the standard 3 and 4 speed options.
Heck I always thought it was all about us crazies that liked shifting gears - the more the merrier Sent from my iPhone using H.A.M.B.
Having an overdrive, especially more than one gives the driver more final drive options. I guess it’s faster and more effective than constantly changing your rear gear on the side of the interstate. The answer to basically any “why did the factory do or do not” will always lead to two conclusions. Economics and government mandates. We get 8 and 10 speed autos, along with the previous ODs and electric/hybrid to get cars under the mandated CAFE standards. We had 3spds for so long because the manufacturers were cheap and stingy. logic is only a minor factor if any
I thought I was, LOL! But after seeing your question I went to their website and yeah, it looks like it is as you say, some marketing jargon they made up. Dang, it sure seems like I read about their product when it first came out decades ago that you could get either an under drive or an over drive, depending on the configuration of your rig. This was in the RV sector where I first heard about them, and I thought it was the market they first went after. Now I'm beginning to question everything I thought I knew about them.
I've spent some time talking with Rick, the guy who owns the company, on several Drag Weeks. Interesting guy...but he is pretty big into marketing....which I guess is why I have been able to talk with him. The unit they sell is the old Laycock D'Normanville that was made in England in the 60s-70s.
Not really. You can make any necessary adjustment to the final drive ratio to accommodate one gear being 1:1, and the result would be the same. The important thing is the overall ratios between the crankshaft and the driving wheels, and there it's about ratio intervals and ratio spread more than the actual ratios in the box — subject to the torque issues mentioned before. For performance you want short ratio intervals, and ordinarily you want a fairly constant interval, to limit the rpm drop at each shift. A classic road-race gearbox would be a close-ratio gearbox with about the same interval for each shift. The interval might be as low as 1.20:1, so for a four-speed that gives something close to: 4: 1.20º = 1.00:1 3: 1.20¹ = 1.20:1 2: 1.20² = 1.44:1 1: 1:20³ = 1.73:1That gives a very narrow ratio spread of 1.73, which is why classic road racers are sometimes not that quick off the line unless they have really short rearend gears. We could add another gear, be it a lower first of 2.07:1 or an overdrive of 0.83:1, but given a suitable adjustment to the final drive ratio, the result would be the same. For economy you want to slow the engine down, within reason, in top gear. That means either a very tall first gear, which may be impractical for daily use, or a much wider ratio interval, which increases the ratio spread but also increases the rpm drop at each shift. Wide-ratio gearboxes were traditionally found in low-performance cars, though they are practical performance gearboxes in very light, powerful vehicles. A Dad's-Oldsmobile three-speed might have a ratio interval around 1.60:1, giving something around: 3: 1.60º = 1.00:1 2: 1.60¹ = 1.60:1 1: 1.60² = 2.56:1That's already a far wider ratio spread, at 2.56 than the 1.73 of the road-race box. It might even be wider, because in wide-ratio gearboxes it is far more common to find a variety of intervals for various reasons, and sometimes for no apparent reason, as in weird combinations like one long interval between two much shorter ones, etc. The obvious way to have both a wide ratio spread and a small rpm drop at each shift is to add gears. We'd have to add three gears to the road-race box to beat the Dad's-Olds box's ratio spread. If we stick to the 1.20:1 interval, a seven-speed would have a spread of 2.99. The point is, it's the intervals and spread which matter. What the actual gear ratios mean for the crank-to-wheel ratios depends entirely on the final drive ratio. You have as many options for resolving that as you have gears. If your spread is, say, 2.75, you could have top gear direct and a 3.00:1 rear, or first gear direct and an 8.25:1 rear. In the latter case you'll have to ask yourself if the gearbox output is going to like cruising at 12000rpm or more, but in terms of overall ratios, both scenarios are the same. As a general principle, you'll want to multiply torque as late as possible along the drive train, within reason. (In line with that principle I've done some investigations into running a very-low-stall torque converter behind a gearbox. The effects of having progressively lower effective stall speeds in successive gears might be interesting.)
The whole paradigm is TQ multiplication based on the need for work. If you need less work from the engine it's more efficient. The only performance needs for OD I can muster is in things like speed trails (like Bonneville) or powersports, especially snowmobiles. Snowmobiles are CVT so I won't go there, but they end up about .85:1 and need it because no matter what they're small engines. As said above, dropping back to accellerate is a benefit too. Fuel efficiency is best attained by low RPM/high TQ output. Not everything can use OD either, hence the need for steep rear gears to get the end result. A 2 litre engine pulling a 3500lb car is common these days. OD is a near perfect plan there. OD on a slightly warmed flathead Ford? Probably perfect in "our world."
I don’t race. I just cruise or pull stuff I also like trucks. Pulling a trailer with a 3spd sux. My truck with the 370 rear gears pulled well but didn’t like the hwy and had issues on hills due to the ratios in the trans The old school granny gear is better but doesn’t utilize torque as well as it should. Thus the 2 speed rear for some. A new 6 or 10 spd auto is great.
Everyone stating that the gears multiply torque are absolutely correct. You also need to think of the overdrive trans as part of a larger system in concert with rear end ratio, not independent of it. The OD gear will allow you to run a numerically higher ratio in the rear. Here's an example: A non-overdrive trans with a final ratio of 1:1 and rear gear of 2.87:1 will cruise at the same speed and rpm as a car with a .7 OD and 4.11:1 gear. The difference will be that all things being equal, the car with 4:11s will accelerate much quicker because of the mechanical advantage of the lower gear. The seat-of-your-pants test will tell you that the car with the 4:11s is much more powerful, and it would be faster, but not due to more power. Because of the OD gear, OD transmissions also tend to have ratios that are gapped a bit further out to enjoy the benefits of both better acceleration, and higher cruising speeds. I'd also add some practical notes. Driving a non-OD car, even with a fairly tall 3.00 gear or higher in the rear, basically taps the car out at around 75, where it gets to be somewhat uncomfortable revving the engine at 3000 rpm or better for hours on end. Especially for those of us with milder engines, with big cubes and torque, you've already used up at least half of your powerband at your highway cruising speed. Simply put, it's totally unnecessary to rev the engine that hard to maintain that speed, since the engine makes sufficient power much lower in the power band to maintain that speed. You wind up spinning it that fast not because you need the power, but because you need the rpm. All that does is waste fuel and increase wear and tear on virtually every moving component in and on the engine, in addition to making for a less enjoyable driving experience where you need to constantly keep your foot on the gas simply to maintain highway speed. The (more) modern overdrive transmission is the key to truly making these obsolete engines we love so much more usable in modern driving.
Just picked up a Doug Nash OD unit. I’m running a th400. Not the best 1sr gear. I’m installing a 500 caddy, replacing a 350. The torque increase from the 500 should help the 1st gear issue. I didn’t use a 4l80 because it has the same 1st gear ratio as the th400. The best option is a 6l90. That trans has a much lower 1st gear but now we’re way of subject for the site. 1st Gear Ratio .... 4.02 2nd Gear Ratio .... 2.36 3rd Gear Ratio .... 1.53 4th Gear Ratio .... 1.15 5th Gear Ratio. .085 6th Gear Ratio..667 the 8 and 10 speeds have a lower first and much closer ratios in between. Their 1:1 ratio gear is 6th and 7th respectively followed by multiple close ratio overdrives. So if the question was "why an OD instead of more gears" or "better gearing" I would say the bean counters at the factory said no until the government raised CAFE standards. Why have 6-10 speed transmissions when gas was super cheap and emission standards low.
Exactly! Imagine the torque multiplication (force) that would be exerted on all aspects of a transmission to try and keep the final ratio 1:1 with a tall rear end gear. Anyone that's every ran a 3:50 first gear Saginaw 4 speed behind a V8 knows what the out come is most often.
" I think" what you are saying is "Why do they make the top gear "overdrive" rather than "straight thru direct drive" and then select a rear end ratio that gives you the equivalent of an overdrive with with a higher ratio (numerically) rear gear. In other words, the final result in either application would be the "same" overall ratio. My guess is that its torque multiplication and the ability of the components to handle what the engine throws at it. The gear in the rear end is many times larger and stronger than the gears in the transmission and can handle larger torque loads that have been multiplied by the gears in the transmission. The first gear in a transmission multiplies torque the most, and there are limits as to how big and how strong you can make it "economically". Since there are some reasonable limits to what this gear can withstand, its easier and cheaper to make up the wanted/needed torque in the much stronger rear gears. Then add an overdrive that doesn't need to be as strong in order to regain good mileage. I bought a Tremec TKO 600 a while back. Its a 5 speed (overdrive 5th) that will withstand 600 ft/lbs of torque. First gear ratio is 2.87. If I ordered the TKO with a 3.27 first gear, its torque capacity drops to 500 ft/lbs and they now call it a TKO 500. A simple change in that first gear ratio drops its torque capacity 100 ft/lbs. If you continue to follow that logic, dropping to an even higher numerical ratio like say 4.00 would lower the torque capacity even further and require more engineering and higher quality materials. It would probably still require the same or maybe an even stronger rear gear even though the ratio was numerically lower (2.50 ?). So no cost savings there for the manufacturer. Just recently I purchased another Tremec. Its called a Magnum and has 6 speeds and 700 ft/lbs of torque capacity. They offer two ratios. You can get 2.66 or 2.97 and they are both rated for 700 ft/lbs. I think the 2.66 is actually probably higher, but not enough to advertise a round number of 800 ft/lbs, so they under rate it and have some cushion. Notice that neither of these ratios are above the 3.00 ratio like the 3.27 in the 500 TKO. So they keep torque capacity at a certain level with gear ratio choices. The Magnum is also physically much larger than the TKO 500/600. I assume the stronger gears may be larger or made of higher quality materials. They offer some polishing/blueprinting/cryogenic treatment that will increase torque capacity to 1100 lbs $$$$. So if I understood your question correctly, I think the answer is that its easier and cheaper to increase the torque in the large rear end gear and compensate with an overdrive than it is to build a low enough first gear that can handle the torque of a strong engine.
Well put. And it's not just strong engines, either...on smaller lower power cars, keeping the drivetrain as light as possible keeps costs down.
A parallel to what Steve related about the strength of Tremec TKO trans with high numerical ratio in 1st gear can be found in the more traditional pre'49 Ford 3 speed. Include the Lincoln Zephyr trans since it was actually the same trans except for the ratios in indirect gears. Gearsets could be found with the front, or 1st gear in tooth counts of 25, 26, 27, 28, and 29 teeth on the cluster gear. The larger the cluster gear tooth count, the smaller the tooth count on the input shaft gear. So really the taller ratios of the Zephyr gears, you also had the benefit of a stronger gear on the input shaft! This interface between the cluster and front main drive on the input shaft was usually the point of failure in these transmissions.
Yep! Guys I have been pondering this for years! Why gear up and then down? Crazy Steve reviled it to me. Ekimmmero, got it, as did a couple of others. Maybe I wasn’t clear in my question! But what floors me , is was right in front of me all the time on the ranch! And I couldn’t add two +two! Lol Farm tractors for years had 550 rpm ptos. Worked great for years, but when the horsepower/ torque went up... pto u- joints started failing. Solution..... make the pto turn 1000 rpm! You can put twice as many horsepower through a 1000 rpm pto that is the same size as a 550! It was right there in front of me for years! I just couldn’t see it! Thanks, guys, for making my world round again! Well , maybe a little lop sided. Lol Bones
So the implements that the 550 rpm PTO drove now have to be changed to implements that NEED 1000 rpm; can't imagine running a 550 rpm Bailer at 1000 rpm!
The reason for the 1000 rpm pto was to transmit more horse power. You could transmit 1000 horsepower through a 3/4 inch pto..... if you turned it fast enough! You know... big and slow.... or small and fast! Edit: most new tractors have both pto shafts, so that they can run all equipment. But you put a 120 horsepower power tractor on a 550 rpm pto..... you are going to have trouble! Bones
Have had an overdrive equipped vehicle since my first high school ride in 1974 - my Studebaker truck had a 3sp OD and you could tell the difference in engine rpms - by sound - when you went into OD. Pretty much the same with a tach reading on my others - you can see the RPM's change when it goes into OD. My Studebaker truck now has the factory 5 speed overdrive and yes when I'm on the highway you can really hear/feel the difference.
drive one in town sometime. leave it in second. start out from a light in second direct, lift your foot and you're in second overdrive. saves wear and tear on the clutch and your left leg!
Lenco segments are indeed underdrives. Because they compound, i.e. segment A drives segment B drives segment C etc., their ratios might be roughly the same, and equal to the interval between consecutive gears. A four-speed Lenco would have three segments. If all the segments have a 1.30:1 ratio, the ratios would be 2.20:1, 1.69:1, 1.30:1, 1.00:1. To simplify construction the lower ratio of each segment does not have a clutch but is held by a sprag until the upper (direct) ratio is constrained to outrun the lower ratio. As a result the combined transmission will afford engine braking only in top gear, when all the segments are locked into direct drive. This is not a problem in the drag-racing context the Lenco transmission was developed for, but it is a limitation in other kinds of driving. I like to downshift a lot, so a Lenco wouldn't suit me without some radical modification. Theoretically it should be possible to build an underdrive out of overdrive parts, by turning the planetary gearset around and reversing the internal sprag relative to it, so that what is now the lower ratio is still held by the sprag. Overdrives generally have an internal sprag even where the lower ratio is held by a clutch, as in the case of the Laycock-de Normanville/Gear Vendors design, which has two cone clutches. This is to eliminate a neutral condition and is what makes the clutchless shift possible.
On my overdrive 1960 Econline van, you can start out in low gear and shift all the way to high with out the clutch, but I don’t have my hooked up all the way. And I haven’t driven it in 25 years. Lol Bones
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