This thread (together with something else which is completely unrelated) got me to thinking about a weird concept. It might be good for the Automotive Weirdness thread, but I certainly have no intention of building anything along such lines, nor would I suggest that all cars, or indeed any car specifically, ought to be built along such lines. I will probably spend the next few days background-daydreaming about what could be done with the concept, and doubtless thereafter conclude that it isn't really worthwhile. The diameter of the driven tyres has a bearing on the effective torque multiplication situation: the taller the driven tyres, the more torque multiplication you need. In terms of the torque capacity of components, as we've seen, the smaller the driven tyres, the lighter the drivetrain components could be. Of course the final drive ratio would need to be adjusted taller to compensate for shorter tyres. There would be further advantages, i.e. lower drivetrain height meaning less drivetrain encroachment into the interior, lower unsprung-mass CG height, rear packaging is simpler, etc. The disadvantages would be that smaller tyres have smaller contact patches, have lesser load capacities, and don't deal quite as well with road irregularities: apart from the arrangement looking really, really weird with normal-sized front wheels. Assuming that you could design around the weirdness, much of that could be remedied by using multiple tyres on the back. For instance, six classic Mini 165/70R10s have almost 40" of section width between them, and can carry about 4700lbs, but they are barely 19" tall. The first vision I had in my head was the six tyres spaced evenly across the width of the car a bit like some old semi-trailers, e.g. the military one below; but perhaps a sort of "triply" arrangement might be more practical. One advantage of the widely-spaced wheels is that more than two brakes could be used, useful when there is barely space for a 7" drum inside a 10" wheel. With six rear wheels, the middle pair could be driven directly off the ring gear, with the outer wheels fixed in two pairs driven through a differential, via CV-jointed shafts: which might be good for traction. Each pair could be on a little axle suspended independently from the other pairs, but without any roll stiffness between the two wheels of a given pair, which would give good camber consistency regardless of where the rear roll centre is located. I'm not sure that the aesthetics could be swung, though.
I think it could been explained like this: You and your buddy have to push a car up the driveway. To get it rolling you two have to really lean into it and make just little steps. Once the car rolls, your steps can be wider and you don´t have to push as hard anymore. Then , after you have reached the top of the hill and the car is running downhill pretty much effortlessly you could very well use some longer legs or you two out of shape slabs will be coughing your lungs out after running after the car for a few 100 yards...
All transmissions are 1 to 1 ratio in high gear so overdrive provides higher than 1 to 1 ratio to the rear end. But who cares, in the 50's everybody ran 820 x15 rear tires for the most high end so do the same and be happy.
The answer to the original question was answered by squirrel early in the thread. Output tq at the driveshaft would need to be higher if the rear ratio is numerically lower. That means every part between first gear to the ring gear would need to get progressively beefier to withstand the added load. Minimizing weight and materials are job one for engineering.
The best trans invented is the overdrive transmission in my opinion. In very new vehicles the multispeed transmissions have everything to do with peak performance, and mileage. Keeping the engine at it's optimum operating rpm all the time. My '39 Chev I built using a 700R4, and the low 1st gear, plus OD 4th gear make it a joy to drive! 1st gear is lower than most manual trans 4 speed cars, yet it cruises at 65-70 mph at around 2100-2200 rpm's. So it pulls hard off the line with my 3.73 rear gear, but still gets 17-19 mpg cruising at low freeway rpm's. Tough to beat the combination I think.
WTF? It doesn't matter if you have a OD trans or a 1:1 4th gear. The driveline and pinion speed is the same at 90-100 mph in either scenario if the rear axle is the same ratio. But with an overdrive the engine speed is lower, where 4.11 gears without an OD will have your engine and driveline screaming at 65 mph! I'd prefer not to have my car be a dog off the line with 2 series rear gears. Just seems silly to lose the takeoff performance just to get highway speeds, when an OD does it all.
Nope. I’m adding an OD to my ride. It will drop engine RPMs 20%. Meaning at 65mph I will be turning around 2240 rpm at 65mph instead of turning 2800 rpm. At the 2800 rpm, I should be running around 78mph instead of 65mph. Driveshaft rpm is not a factor to worry about. driveshaft failure is 99% lack of maintenance, poor engineering or excessive abuse. A well constructed hot rod will have provisions to handle driveshaft failure.
Wow! Guys, I think most everyone here understands overdrive! But I don’t think we understand each other! Or at least each other’s post! Lol! Especially Elcohaulic post! I finally got the answer to my original question as to why gear up in the over drive, then gear down in the rear end. I always thought it was odd! Why not gear the rear end high and add a lower gear to the transmission! I understand in the old days, it was easy to add an overdrive to the back of the transmission and the rear gears were already fairly low! I get that! What problems I had was why did the OEM’s continue to do this when the totally redesigned the cars and drivetrains! The basic answer was they were able to lighten the total drivetrain there fore overcoming the added friction inherent with gearing by saving weight and ultimately saving fuel. This has been a lively thread, Neds thinking is taking it to another step! Thanks guys for all the input! Bones
I will give you a great example.my 51 merc weighs about 3900 lbs the flatty stock is 112 hp 4.11 gear to get you rolling and overdrive so you don’t max your rpm and blow the motor.
Anthony, I don’t think you understood what Elcohaulic was saying. I believe he has now removed his post, so we can’t go back and reread it. I think he left out the part that with the overdrive, you usually have a lower geared rear end, therefore you WOULD be spinning the driveshaft faster to get the same speed. We all know, or at least I hope we all know that the drive shaft on a rear end turns the exact same speed at say 60 mph , no matter what transmission is in front of it. I think it’s a problem of misunderstanding what the other guy is saying! Bones
I use the 1st 4 gears for around town/towing to keep the engine in the power curve, where the torque is... 5th is for the highway.
Actually I was just messing with his grammar. But I will say his post was confusing. As far as why transmissions did not have more gears, I still say it comes down to the bean counters that wear the suits. And possibly the technology nor the demand at the time. We have those transmissions now. 6-10 speeds including multiple ODs.
I agree. And that was my original question as why gear up then gear down! Any time you go through a set of gears, any gears, you add stress and friction. To my understanding this would eat power and fuel! This is what couldn’t understand until Crazy Steve and Squirrel came forward with the answer. I now understand it. I knew the OEMs were doing it, so it had to be beneficial and correct. I just couldn’t figure it out with out help. Now it seems so simple! Bones
There is a very old saying that transmissions are necessary because engineers have yet been unable to build the perfect engine. Some also theorize that with modern fuel injection no motorized starter should be needed. All this, of course, is a theoretical conundrum faced with the shadow of another problem of even greater magnitude: the end of fossil fuels in pursuit of the same kind of purity of essence the completely insane general in Kubrick's Dr. Strangelove was seeking. Are y'all ready for that?
Well, like it or not, electric cars are coming and due to the nature of the electric motor, the transmission becomes less important. I problaly won’t see the complete take over, but some of you will. Being from Oklahoma, I’m kinda fond of fossil fuel! Bones
I'd also add that we're dealing with this issue of lower RPMs and "highway cruising speed" in current terms. The fact of the matter is that while these cars were used as daily drivers when they were new, they were not used as daily drivers in modern traffic conditions like we experience today. Those conditions have changed significantly. Nixon signed law into effect in 1974 establishing a 55 mph speed limit, and it wasn't until the 1980s that that limit was raised to 65. Then it wasn't until 1995 that Congress allowed speed limits to be governed solely by the states, some of which have adopted certain limits of 70 mph or higher. So for HAMB-relevant vehicles, "highway speed" was around 55-60, whereas highway speed now is around 65-80. Pretty much any car produced from 1949 to 1964, or drivetrain combination from that same era, could easily chug along at 55. But that makes you a stationary object on most major freeways now.
Elcoholic has a point when he talks about the speed of the rear end gears and the driveshaft at highway speeds.......if someone was driving at extremely high speeds. I think what is being overlooked is that for the most part, people will not be running their engines at extreme rpms when they are in overdrive, so the driveshaft and rear end will not see more rpms due to the overdrive. Look at it this way. The rear end will linearly increase in rpms as the speed of a vehicle increases. A 4.56 will be spinning faster than a 3.00. Depending on which ratio you have, the driveshaft will also turn at a greater/lesser rpm. It doesn't matter what gear the transmission is in, its simply the fact that Rear tire size, and rear end ratio will determine how fast the driveshaft spins at each desired speed. It doesn't matter if you are in first gear or overdrive the relationship of the tire...to the rear gear....to the driveshaft doesn't change. What does change is the rpm of the engine and the amount of torque you can apply at that speed. The speed of the driveshaft will increase either 4.56 or 3.00 times what the rear end increases. If it were possible to have your (non-race) engine turn 5000 rpms while in a .70 overdrive, your driveshaft would be turning 8500 rpms and so would your pinion gear. The thing is, other than Bonnieville, most of us aren't going to be anywhere near these parameters and the driveshaft will only be turning much slower rpms in overdrive at highway speeds. Driveshaft speed only increases with an increase in road speed (or wheelspin).
This is very true, unless you have the ultimate overdrive...... the Columbia! It changes the drive shaft speed! Lol! I just sold the one I had been hoarding for years, to buy my FE long tube cast iron exhaust manifolds. I understand the overdrive of the past. I actually own one, in a 1960 Econoline. I understand why the OEMs of the 1920s to the 1960s built them! Regular transmission for city/ slow speeds then stick a gear box on the back of the transmission for freeways. Cheap, easy solution to the problem! I get it! What my problem was.... why did they continue with overdrives when the OEMs total redesign the automobile ? Why not have top gear straight thought the transmission and have a high geared rear end and a lower first gear for take off? That way at cruising speeds you would only be going through one set of gears, not two sets of gears. To me this would reduce wear and fuel consumption due to less friction and stress. It was explained to me by Crazy Steve and Squirrel. I get it now! I appreciate all the input, it was a lively thread! And keeps on going! Lol Bones
I think ya only need to factor in drive shaft speeds if you’re doing this. (Or possibly for large trucks)
Well...... Anthony, the OEMs are factoring in drive shaft speeds and size! Of course they are a little different than our old drive shafts, but they are drive shafts, none the less. The faster a drive shaft turns, the more precise it has to be! That is a factor. The faster something turns, the faster it wears! That’s a factor. Losing a drive shaft on a rear wheel drive car, is a factor! Granted all these factor can be overcome! But drive shaft speed ,size, and weight factors are why OEMs today now have two and three over drives! Bones
https://spicerparts.com/calculators/driveshaft-safe-operating-rpm-calculator Here is this if anyone is curious
I have cut several and never balanced them. But I’m a low and slow cruiser. Or for a 4x4. Must be why I have never had an issue. We did balance customers drive shafts.
It's like $50 around here to have a driveshaft shortened and balanced. My buddy actually used to collect Imperial and full-size Chrysler driveshafts because he'd just get those mammoth shafts shortened for a fraction of the price of what a new shaft would cost.
Adding a DNE2 OD to my th400. I have a 2 piece drive shaft. The first shaft will only be around 8 inches long with the OD. May move the hanger bearing and shorten the 2hd shaft.
Actually, prior to the 55 limit established in 74 states were able to set their own limits, and many had no limit at all. I can recall many trips across the country with family where typical speeds were as high or higher than today.
I think that a lot of builders (the ones not influenced by hype) use an OD so that they can have a deeper gear for a hole shot and still have decent highway gearing. Think about it most performance oriented people would like to have a 4.11:1 gear for instance and still be able to take a road trip. I had an OT mini truck back in the '90s that came stock with a 5 speed, 5th being .7:1. Stock was a 4.10:1 gear. It was a 4 cylinder truck, and needed the 4+:1 rear just to get out of its own way, but the OD 5th gear made it so that on the flat on the interstate you could still drive it.
Beaner, I get the reason for old overdrives. I have one and readily enjoy it’s benefits! And I under stand why the OEMs did it back then. Cheap and they didn’t have to redesign the car! This I understand. My problem was , why did the continue, with the over drive. When the total redesigned the automobile? Gearing up then down. Why not just go straight though the transmission and have a higher geared rear end and add a lower gear in the transmission? But I got the answer, back in this thread. Bones
They do it because it works best. Car manufacturers are mandated by big shots in Washington DC to have a certain MPG average number for the combination of cars they sell each year. They have to be at or under the target. The mentioned "savings" of cruising at a 1:1 ratio is barely a blip on the radar compared to the huge savings of dropping the RPM and barely cracking open the throttle on the highway. My daily has a factory 400hp V8 and 6-speed trans and none of the gears are "direct". Fourth gear is still slightly underdriven, then it skips 1:1 with an overdriven fifth and overdrives sixth another notch. 1:1 doesn't appear to be a magic panacea for efficiency in real-world cars.
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