Explain To Me: Axle Ratios

"ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)" (adabofoppo) 11/18/2013 at 16:59 • Filed to: School Me Oppo

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As mentioned in the !!!error: Indecipherable SUB-paragraph formatting!!! post, you can spec a truck with different ratio rear differentials/gears/turning thingys.

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What does it all mean? I assume those numbers represent better or worse acceleration or economy.

School me Opponauts, I require knowledge.

Yowen - not necessarily not spaghetti and meatballs > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:01

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lower numbers = better fuel economy, slower acceleration

higher numbers = better towing of heavier loads, worse fuel economy, faster acceleration

Arch Duke Maxyenko, Shit Talk Extraordinaire > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:03

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Larger number shorter gear- meant for acceleration and engine can rev faster.

Smaller number higher gear- meant for fuel economy and lower rpms at cruising speed.

Nibbles > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:04

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To add on to what Yowen said: The first number is rotations of driveshaft, the second is rotation of axle(s). Therefore, 3.73:1 is 3.73 rotations of the driveshaft for every 1 rotation of the axle shafts.

MtrRider Just Wants Doritos > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:05

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A large number (3.73 or 4.10) is a short gear. That's good for acceleration, but hurts top speed and fuel economy. If you're drag racing or towing heavy trailers, towing in hilly areas you want the short gears. The long gears (3.15) are good for high speed driving and fuel economy because you run at lower RPM, but you accelerate slower. So you were exactly correct.

I hope that helps.

Textured Soy Protein > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:07

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So each gear in your transmission has a ratio. A gear ratio of 1:1 means that for every 1 RPM of the engine, you will get 1 RPM of the output shaft of the transmission.

Lower gears will typically be more than 1 RPM of the engine for 1 RPM of the output shaft. So like 1st gear could be something like 3:1 or whatever. 3 RPM of the engine = 1 RPM of the transmission output shaft.

A gear is an "overdrive" if you get more than 1 RPM of the output shaft for 1 RPM of the engine, i.e. anything like 0.8:1 is 0.8 RPM of the engine = 1 RPM of the output shaft.

The lower the gear, the more turns of the engine to make the wheels turn.

Now the thing is, the leg bone's connected to the knee bone, and the output shaft of the transmission is connected to the differential, which has more gears in it, which also have a ratio. So the final drive ratio is talking about the fixed gear ratio of the differential.

The "shorter" the final drive, the higher number it will be, like 4.8:1, or a "tall" final drive is 3.3:1. The higher number final drive, which makes it shorter (even though it's a bigger number) is like being in a lower gear - more acceleration, more RPMs, more noise on the highway, and lower MPGs.

Arch Duke Maxyenko, Shit Talk Extraordinaire > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:08

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Think about your transmission and it's the same principle.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Textured Soy Protein 11/18/2013 at 17:08

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That made my head hurt.

Joe_Limon > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:12

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Some good responses so far. So here is an extremely literal/unhelpful answer to mix it up. The ratio refers to the relative number of teeth on either side of the differential.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Arch Duke Maxyenko, Shit Talk Extraordinaire 11/18/2013 at 17:12

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I don't think about my transmission, other than to curse the ridiculously heavy clutch return spring that ClutchMasters supplies with the replacement clutch kit I had installed on my car a couple months ago.

It's so heavy that I can't even feel the engagement point on my wife's car anymore.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Joe_Limon 11/18/2013 at 17:14

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Relative number?

So are there also imaginary numbers of gears as well??

Joe_Limon > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:15

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Were you thinking of real numbers?

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Joe_Limon 11/18/2013 at 17:16

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No.

...yes.

I tried to make a math joke. I have a History degree. Math and I have an understanding that I won't bother it, and in turn it won't make my head hurt.

Textured Soy Protein > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:18

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Ok, let me put it another way:

Engine ——-> Transmission ——-> Differential ——-> Wheels

Both the transmission, and differential, have a gear ratio.

The transmission lets you choose from one of several gear ratios.

The differential has one, fixed, gear ratio, called the final drive ratio.

To see how many RPMs of the engine it takes to get one RPM of the wheels, you multiply the ratio of whatever gear you're in, by the final drive ratio.

So 4:1 1st gear * 4:1 final drive ratio = 16 RPM of the engine for 1 RPM of the wheels.

Let's say you shift up to 2nd gear which has a 3:1 ratio, now it's 12 RPM of the engine for 1 RPM of the wheels.

So, by changing the final drive ratio, you effectively change all the other gear ratios in the transmission.

A numerically larger final drive ratio means that you get faster acceleration, higher RPM on the highway, and use more gas. Because for a given speed, you're spinning the engine faster.

Joe_Limon > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:19

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Fair enough, I passed all my math classes, but I don't know how. Near the end I was sleeping in the front row so I could get out of advanced boundary problems sooner.

Arch Duke Maxyenko, Shit Talk Extraordinaire > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:19

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Well the low gears are for getting you moving while the high gears are for speed/economy

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Textured Soy Protein 11/18/2013 at 17:20

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Thanks.

I get the general concept.

I have a History degree. Math and I agreed to leave each other alone a long time ago. Ha ha.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > Joe_Limon 11/18/2013 at 17:23

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Yeah. I recall having to take calculus in high school.

I've never understood how knowing how to find the are under a curve was beneficial to me in anyway when I just wanted to work in a car museum.

Also, there are plenty of other people to like to do that, and are actually good at it, so I might as well apply my energy to something I was good at. Which, it turns out, was BS. Which is what a degree in History entails.

Pages and pages and pages of BS.

Demon-Xanth knows how to operate a street. > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:31

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Axle ratios aren't as important now that transmissions have a much wider spread and massive overdrive ratios. When you only had two or three gears, they were very important in the balance of excessive highway RPMs vs. getting off the line. In trucks the most useful purpose of getting something outside of the 3.5:1 range is to balance out oversized tires so the transmission isn't overstressed.

In the oversize tire department it works like this:

A 30" tall tire with a 3:1 ratio looks the same to the transmission as a 40" tall tire with a 4:1 ratio.

So when you swap out the stockers for some super swampers and are running that badass chipped diesel, if you don't adjust that gear ratio you're constantly putting extra stress on the trans and start toasting them right and left.

If you're going to stick to stock, not really worth sweating much when you're packing an 8 speed transmission.

Joe_Limon > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:31

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If it makes you feel any better. The engineering equivalent to what you are describing comes in the word assumption. We make assumptions for a living. Without them, we would be physicists! Assuming pi=3 is close enough, then I can do the rest of this problem in my head while getting an answer that is close enough!

Laird Andrew Neby Bradleigh > Joe_Limon 11/18/2013 at 17:35

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You should read Going Postal by Terry Pratchett.. Then you'll see what happens if pi = 3 :P

Paul, Man of Mustangs > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 17:48

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The final drive ratio is the ratio of teeth on the pinion and ring gears, and the difference in input and output speed. The pinion (the one on the shaft) is driven by the driveshaft and transfers power to the ring gear, which is bolted to the differential. The pinion nearly universally has fewer teeth than the ring, so it takes more turns of the pinion to turn the ring. The more turns the pinion makes per turn of the ring gear, the more mechanical advantage there is. Think of it as a rotary lever. A higher numerical ratio (4.10 versus 3.31, for instance), the bigger the lever. A bigger lever means more force, or torque. However, this also means that the speed is reduced.

There are always trade-offs like this. You can use a short-handled wrench and push for 3 inches with a great deal of force, once; or you can use a long-handled wrench and push for 3 inches with ease, several times. You're using the same total energy, but it's spread out over a longer period of time (the effort put into the lever at any given point is power. You're using more power on the shorter wrench for the same end result).

It's no different with gears. You can use a smaller gear (say, 3.08) to increase your speed for the same engine speed, but this will be at the expense of available torque, just like using the smaller wrench. Or you can use a bigger gear, like a 3.73, which will give you the longer wrench. You'll be able to use more of your engine's torque, but it will turn the engine faster.

Then there's differentials. Here's a gif:

This is an example of an open differential. The two output axles can spin independently of each other. We see the results of this in a one-wheel-peel. All the power gets sent to the wheel with the least traction. Naturally, this is undesirable. So we invented the limited-slip differential. The most common version uses clutches to reduce relative motion, but the better ones use worm gears as very large levers to actually send power to the highest-traction wheel. That's a Torsen differential, if you care to research further. There are other types of limited-slip diffs out there, as well, but they're less common.

lucky's pepper > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) 11/18/2013 at 18:14

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Imagine the set of sprockets at the back wheel of a multiple-speed bicycle. The little sprockets that you want the chain on when you are starting off would be the equivalent of short differential gears (something like 4.10s). They are good for accelerating but bad for cruising at speed. The bigger sprockets would be the equivalent of taller gears (something like 3.08s), they make it harder to get moving but are better at cruising speed.