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Back-To-Back Transmission

Like a transfer case or 21 speed bike
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Most modern manual transmissions have gears on two shafts, one connected to the engine and the other connected to the wheels, with gears meshing between the shafts to make different ratios. If a manufacturer wants to add more gears to a transmission, they lengthen these shafts to accommodate more gears.

My mountain bike essentially has two transmissions, connected by a chain drive. The front derailleur has three sprockets and the back wheel has seven, making for a total of twenty-one possible ratios. The only inconvenience is that you must operate two gearshifts to go through the whole range of gears.

A cheap and easy way to create more gear ratios in a transmission would be to simply stack two manual transmissions together, one behind the other. Mitsubishi did this on one of their economy cars in the '80s; it had a regular four-speed manual with an extra two-speed gearbox added on for a total of eight ratios. Imagine if modern car manufacturers revived this idea.

You could have ultra-low ratios for quick acceleration, and a super-overdrive for better fuel economy on the highway. The only problem is that you would have two gearshifts, or maybe a solenoid shifter activated by a push-button like some 4WD systems. Hot-rodders might also adopt this system over a quick-change rear end to allow for quarter mile acceleration and top speed runs at Bonneville with the same car.

discontinuuity, Mar 24 2007

Auto-Biography Pt. 8 http://www.thetruthaboutcars.com/?p=3336
Scroll down to the description of the "chore scooter" [discontinuuity, Mar 24 2007]

Plymouth Champ/ Mitsubishi Colt http://jalopnik.com...-on-ebay-232571.php
With the 8-speed transmission [discontinuuity, Mar 24 2007]

Something like this? http://www.gearvend...od/jan03/index.html
Baked in the hot rod world for a few years now. [Hunter79764, Mar 24 2007]

Bicycle Gears http://patentpendin...ears_for_drive.html
you may be interested in this... [xenzag, Mar 25 2007]

Tiptronic Shifting http://en.wikipedia.org/wiki/Tiptronic
One of the newer shifting methods designed to minimize shift time. [croissantz, Mar 25 2007]

Paddle Shift http://en.wikipedia...dle_shift#Operation
The other way to shift quickly. [croissantz, Mar 25 2007]

One of the many choices http://www.gearvendors.com/index.html
[elhigh, Mar 27 2007]

[link]






       How do lorries and tractors (which seem to have a huge number of gears) manage things?
MaxwellBuchanan, Mar 24 2007
  

       No idea. I've never driven anything with more than five gears (besides the bike). I think some 18-wheelers have 14 gears.
discontinuuity, Mar 24 2007
  

       The friction caused by using 2 gearboxes intertwined (at least cheaper ones) is a lot more noticeable on a car than on a comparitively small bicycle. Still gets a bun, though.
croissantz, Mar 24 2007
  

       Search "Gear Vendors Overdrive" or go to the link for a magazine write up on one being installed on a General Lee.
Hunter79764, Mar 24 2007
  

       But wouldn't a CVT be better anyways (plus weigh a lot less)?
acurafan07, Mar 24 2007
  

       The idea here is to make something much cheaper than a regular six-speed transmission or CVT. I'm not sure if this would rob much power or not, especially if you keep the size of the extra transmission small or incorporate it into the main transmission case.
discontinuuity, Mar 25 2007
  

       Four wheel drives often run two boxes, but only for a lower range. For most driving conditions this is not necessary, the range from a four to six speed box - reduced through your diff- is calculated to give good preformance and economy.
the dog's breakfast, Mar 25 2007
  

       The less gears you have, the lower performance/economy cambination you have. In the early days, 3 speeds were very common, then it moved to 4 speeds, because they got better performance without winding out the motor on the highway. Then it went to 5 speeds, then 6, now 7. Bottom line is the more gears you have, the better off you are. This is a good idea, and that's why its baked to a crisp and found in many cars already.
Hunter79764, Mar 25 2007
  

       //Then it went to 5 speeds, then 6, now 7.// Even 8 now with the new Lexus LS460. The only problem I see is that the more gears you have to shift to get to a certain speed, the more time it takes to shift overall. But since you yourself mention that the idea has already been baked, what exactly is new? Perhaps you could have a small switch on the steering wheel or shifter to change between the 2 gears more quickly?
acurafan07, Mar 25 2007
  

       That's the idea on the overdrive units for manual trans cars. You take off in first, just like normal, but in either first or maybe second (depending on other gear ratios and how well your car hooks off of the line), you shift electronically (clutchless, which is faster than a regular shift) then continue through the gears as usual, although each gear is overdriven from what it was. You basically gain a clutchless shift in the quarter mile, so where many street cars would barely get out of third and have to waste the time to shift to fourth, you now can go through the traps in 3rd/over and don't have to lose time with regular shifting. But on the automatics, the idea is more to put a gear between each gear, doubling the number of foward gears you have to keep the engine in the powerband (or the most efficient zone if mileage is a concern) the entire time. It works by itself or manually, and basically lets you have a modern overdrive transmission that can handle large amounts of power without failure. (a modern 4L60 4 speed overdrive transmission can usually handle up to 450 hp before failure, if its well built. A 2 speed Powerglide can handle 1500 or more if well built. These units handle 1200 in normal trim, but can be modified to handle 2000 or more.)
Hunter79764, Mar 25 2007
  

       acurafan07:   

       Good idea, but the 'small switch' for shifitng has already been invented. In two flavors, too. There's the 'Tiptronic' system (see link) and paddle shifters (see other link).
croissantz, Mar 25 2007
  

       No, what I meant was a small switch to change between the high and low.
acurafan07, Mar 25 2007
  

       The need for more speeds in a transmission is linked to the way in which the power is produced by the prime mover. If large lumps of torque happen at low speeds and the power output remains constant in proportion to the speed of rotation then we don't need any gears. (Steam engines, Electric motors)   

       The petrol engine develops very little of anything at low speed, and needs gears to get the weight of the vehicle moving. The modern petrol engine will have a rev range of say 2000 to 6000 rpm which is usable, and can manage adequately with 5 speeds, the lowest allowing you to pull a caravan up a 1 in 4 hill and the highest being overgeared to cut down on fuel consumption and engine wear.   

       Six speed manual boxes are needed for turbo diesels which have a more restricted rev range with a maximum of about 4500 rpm and therefore an overdrive sixth makes use of the better torque available at low speeds, while the high top means that you don't suffer from running out of revs and overspeeding the engine.   

       Seven or more speeds are always associated with some form of auto shift and would be bordering on unusable for a manual box.   

       The boys with the trucks have very short rev ranges say 1000 to 2500 rpm, and to get 40 tons rolling must use the whole of that range, hence the complex splitter boxes giving multiple ratios.   

       In my 40 years and 1 million miles on the road I have had from 3 speeds to 6 speeds and boxes with and without overdrive.   

       In my view, and supported by 125,000 miles of driving in the last six years and two cars, a diesel Golf Avant and a diesel A4 Avant, you can't beat a six speed manual.
john of England, Mar 25 2007
  

       I agree that a five or six speed manual is best for most cars. The main idea here is for a cheap and easy retrofit like the one Mitsubishi did with their Colt, allowing for a better range of acceleration, economy, and top speed out of a small engine.
discontinuuity, Mar 25 2007
  

       This idea is fully baked and old news in the RV and heavy duty pickups market. Google "aftermarket overdrive" and you'll be met with myriad options, whyfors and whynots. I added a link to Gear Vendors, but there's plenty of others.   

       I had a notion to post an idea that eliminated the gearbox entirely and made up the ICE's lack of low end torque with a large electric traction motor. There would only be a clutch and a straight-thru shaft coming from the ICE, completely eliminating all those nasty losses coming from the tranny. Above a certain speed, the motor drops out of the equation and the ICE does all the high speed work where its operational regime and the performance requirements coincide. It's a hybrid, yeah, and yet also less...
elhigh, Mar 27 2007
  

       As has been mentioned earlier, this is widely known to exist. It wouldn't get you ultra-quick acceleration: Firstly, you would have too much torque for your tyres to handle at low speed, and then what acceleration you did get would be lost to gearshifting. In the meantime, the losses which are unavoidable in gear systems have roughly doubled, so you're robbing more power and putting more heat into your oil. Finally, the second gearbox is going to have to be strong and heavy compared to the first, because it has to deal with much more torque.
david_scothern, Mar 27 2007
  

       In the early days of motor vehicles one of the major problems was gear changing. Many people just could not handle a non synchronised gear change and an in or out clutch and there were lots of wacky and not so wacky inventions to avoid or get round gears. A particular problem was found with buses, where a lot of the old drivers had been used to either horse pulled vehicles or electric trams and needed a simple solution.   

       An English oufit called Tilling-Stevens developed a petrol electric transmission where all the driver had to do was let off the brake and push the gas pedal to speed up the ICE which drove a DC dynamo to power an electric motor connected to the back axle via a cardan shaft. Add the refinement of a shunt controlled by a lever on the steering wheel and any non mechanical type could drive it.   

       This system gave the effect of an infinitely variable ratio between two points but although it was simple and reliable, it was all done and dusted by 1930, killed off by synchromesh or pre-selective boxes and more powerful diesel ICEs. Some of the old buses got a second life as showman's transport, the dynamo being used for lighting coconut shies and such, when the vehicle was stationary.   

       Add some trick electronics and a traction battery and you've got a Toyota Prius!   

       Students of the really weird might want to look up the Crown Magnetic, or Entz transmission.
john of England, Mar 27 2007
  

       You wouldn't believe me: I haven't looked in at the HB for a few days, but was thinking about this very problem only yesterday afternoon.   

       There is a way to arrange a single shifter, if one of the two transmissions is a two-speed. Let's say you use a Toploader and an adapted 4x4 transfer box. The thing would be to arrange a face-cam shifter like on a motorbike to shift the Toploader: not impossible with a side-shift box like that. That can be arranged that the four ratios are selected "in line" by moving the shift lever across the gate (that might mean a rather clunky cross-shift, though). The fore-aft movement would select the transfer-box ratios.   

       The choice of ratios shouldn't be a problem. Go for a wide-ratio set, as long as the proportion between ratios remains pretty constant through the range: about 1.6 is a safe bet. The transfer box would then need direct and 1.2649:1 (i.e. sq rt 1.6).   

       This comes up every time I consider more than five speeds and more than 500lb.ft at the same time...   

       Here's another, added 90 minutes later, and more in line with the "Back-to-Back" nomenclature. Use one Toploader or Muncie M22 or whatever with close-ratio gears, and another behind it BACKWARDS. Supposing you want top speed in 6th with an economy-overdrive 7th, use 2.024 / 1.600 / 1.265 / 1.000:1 gears in the front box, and 2.500 / 1.600 / 1.265 / 1.000:1 gears in the rear box. Because the rear box runs backwards the ratios would be 1.000 / 0.791 / 0.625 / 0.400:1. If the final drive ratio were then lowered by a factor of 1.6 (i.e. using 5.6:1 gears instead of 3.5:1, say) the ratios would be equivalent to 3.239 / 2.561 / 2.024 / 1.600 / 1.265 / 1.000 / 0.650:1.   

       Of course one gearbox's reverse train may be removed.   

       The rear box (the one running backwards) would need to be built to see 10 000rpm in short bursts (at the redline in 5th and 6th). One would need to remember it won't do that all day, every day.   

       The shift is simply the original two boxes' shifters! Shift 1st through 4th on the left shifter, leaving the right shifter top left; then shift 4th through 7th on the right shifter, leaving the other bottom right. It'll take a bit of getting used to, but I can imagine it becoming second nature soon enough.   

       And, I wouldn't worry about losing any notable power through an extra pair of gears, they're extremely efficient things (99%+).
Ned_Ludd, Mar 28 2007
  

       First of all, //you would have too much torque for your tyres to handle at low speed, and then what acceleration you did get would be lost to gearshifting.// is not true at all. you do gain acceleration, because there is alot more traction available in your tires than what it seems sometimes, and any car that is going to be using this is either an underpowered car that need the extra mechanical atvantage, a hot rod that already has the proper setup for traction, or trucks and rv's who couldn't spin the tires if their life depended on it. And the shift is very precise and quick, so it doesnt lose much time there. Most of the units available are relatively small (about 6" long and the width of the transmission tailhousing) and are very efficient. You would lose more power through not greasing your suspension mounts on a regular basis than by installing one of these.
Hunter79764, Mar 28 2007
  

       I'm familiar with the overdrive system; as fitted to sports cars, they were typically only usable on third and fourth gears. If they had been intended for use in first and second, they would have been unduly heavy to cope with the torque requirements. Your high-torque, low-rpm drag racing boxes are not made to be light in weight, or particularly efficient. You can get 99% efficiency from a gear pair, true, but that falls when you lubricate it - that's at least 1hp per meshing pair in a 100hp car, and an epicyclic such as is commonly used for an overdrive actually contains several gear pairs, so you're losing a certain amount on top of what your main gearbox is swallowing, not to mention bearing losses, sump churning losses...   

       Bottom line is, you're adding weight and complexity for a limited gain. It's not going to buy you more acceleration than your tyres can stand.
david_scothern, Mar 29 2007
  

       I'm not trying to be rude, but take the time to read up on things. From the Gear Vendors wbsite: The parasitic loss is .25% (or 1 hp in 400 as they put it) and it is an overdrive unit, meaning that it sends less torque to the rear tires than the gear preceeding it, just at a higher RPM. Weight is between 32 and 46 lbs based on applications. Stock units are capable of 1200 hp capacity.   

       Say you have an engine which produces 100 ft lbs at all RPM's. The car is equiped with a GM turbo 350 trans and a 4.10 rear end ratio (just because I remember the gear ratios of the 350 offhand). Off the line, your 100 ftlbs goes through the 2.52 first gear, then the 4.10 rear gear, making the overall torque 1033 ftlbs. 2nd gear, at 1.52, would take it down to 623. The overdrive gear in between them, at .78, gives you 806 at the tires. If the tires can handle 1033, they can handle 806 ftlbs.   

       But in real world applications, engines don't have a flat torque curve. But by giving more gears, you keep the engine in the area where more torque is produced. So instead of overshooting your peak torque then dropping down far below the peak in the next gear, you can operate in roughly half the RPM range, keeping centered on the peak numbers.
Hunter79764, Mar 29 2007
  

       I appreciate the benefits of more gears. However, the "ultra low ratios for quick acceleration" mentioned in the article are a no-go, because you hit another limit before you reach the massive acceleration you're looking for, namely, wheelspin.   

       I recognise that more gears bring you closer to the ideal torque/speed curve, but the real world isn't so simple; your acceleration relies on staying close to that curve _and_ on shifting quickly. More gears = more shifting time, period. While a band-braked epicyclic can shift without a break in power transmission, I'd warrant that you'd give it some severe punishment if you were shifting it up, down, up, down as you worked through your four gears plus overdrive four times.   

       And as for the efficiency of gears, trust me, they're not getting 99.75%. I know this. I design 1500hp, 30,000rpm aero gearboxes for a living. Admittedly they don't change gear, but I know what sort of efficiencies can be expected, and our boxes kick out _A LOT_ more than 3.75hp in waste heat.
david_scothern, Mar 29 2007
  

       Ex: A late model Chevy Camaro Z28 comes with a 3.23 rear gear (I believe. Possibly 3.56, but I'm pretty sure its a 3.23). With an aftermarket overdrive, you could run a 4.10 rear gear and have relatively the same final drive ratio, meaning the same fuel economy. But a 4.10 rear end is very useable on the street. Many guys run them even without an aftermarket overdrive (at the expense of higher RPM's on the highway) without any real traction problems. With a little work, 4.56 gears could run on the street, and there are many cars with even lower gears than that. A friend of mine runs 5.14 on a street '65 mustang.   

       Most cars are built with more economy in mind than performance, so there's room to work with before your traction is at its end. Another more extreme example is my own car, an '85 Chevy Monte Carlo. It has about a 2.14 gear for better mileage stock. If I had an external overdrive, I could replace the gear with something like a 2.73 and keep the same final drive ratio. Interestingly enough, the '85 Monte Carlo SS with a 305 instead of my measly 262 ci came with a 3.73. With more power, the OE's still chose to go with a lower geared rear end, because the overdrive tranny that these came with allowed for it. And I almost guarantee that even with a 4.10, my car wouldn't have traction problems, but it would accelerate much quicker, even with 50 extra pounds and 10% loss (to exaggerate any possible loss).   

       And the .25% seemed low to me too, but I was just relaying the website information. It can't be too high though. If it were, the company would have gone out of business long ago.
Hunter79764, Mar 29 2007
  

       Could it be that oil drag works like aerodynamic drag, i.e. increasing exponentially with speed? That would explain the impression we're getting that gears are extremely efficient at low speeds but notably inefficient at higher speeds.
Ned_Ludd, Mar 30 2007
  

       Gear Vendors wants to sell you a transmission, and most people have no idea what kind of parasitic loss to expect from a tranny. I guarantee you it'll take one hell of a lot more than one horsepower to run that tranny at speed under load. I'm betting the real number is closer to twenty, give or take depending on the design.
elhigh, Mar 30 2007
  

       Hunter, all you're really arguing is that overdrive is a good idea. Overdrive is widely known to exist and does not constitute a new idea.
david_scothern, Mar 30 2007
  

       I'm arguing against the idea that one of these units will not increase performance and mileage. They can and do, and that's my point. Overall, the benefits outweigh the drawbacks, and the only reason one of these isn't in every car is the price. At $2000 apiece, it's not worth it to alot of people, but that doesn't mean that it doesn't work.
Hunter79764, Mar 30 2007
  
      
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