Rear end gearing help.

"Bandit" (2bandit) 12/02/2014 at 11:15 • Filed to: None

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I'm writing some matlab stuff for my engineering class that will analyze the performance and economy of different rear end ratios in both cars and trucks. What kind of gear ratios are wanted in trucks? Lower = more torque so more off road? Higher = ? And what values are you looking for for offroading (4.11? 3.08?) Basically I have two functions, one speaks about performance (probably offroad in this case) and the other speaks of fuel economy (will probably be the same as a car).

Nonster > Bandit 12/02/2014 at 11:18

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depends on the desired purpose of the truck, but usually the gears are set so you get a certain engine speed at cruising speeds for fuel economy purposes. Lower gears (numerically higher) are used when you are correcting for larger tires or just want more torque at low speed

As for values, common ones are 3.31 up to 4.56. 4.10 are really common

KusabiSensei - Captain of the Toronto Maple Leafs > Bandit 12/02/2014 at 11:21

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Lower numerical numbers mean that the drive is turning slower relative to the input. This increases fuel economy and top speed.

Higher numerical numbers mean an increased mechanical advantage (i.e. more torque), so you can pull more load, but you increase fuel consumption and lower the top speed.

Party-vi > Bandit 12/02/2014 at 11:21

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Early Jeeps used 5.38 gears, which are great off-road and for farming applications but just terrible everywhere else.

Bluecold > Bandit 12/02/2014 at 11:24

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If only you could change gear ratios while you are driving...

Aaron James > Bandit 12/02/2014 at 11:26

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Lower gears (numerically higher) are also used in towing applications. Higher gears are mostly for highway economy.

HammerheadFistpunch > Bandit 12/02/2014 at 11:27

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I think this is covered here by others but typically quick response and high torque multiplication happens with numerically higher (larger) ratios to one, and better economy happens with lower ratios. For a personal example, my land cruiser comes with a ridiculous 4.10 stock axle ratio front and rear (since its full time 4wd) but when I got a lift and bigger tires I had trouble pulling a trailer over hill and dale at elevation so I re-geared to 4.56. Now my highway rpms are up and I basically have shifted my powerband so that my peak hp and torque is active at lower speeds for more powerful towing. it took a hit on my economy but provided the gearing i needed. as a bonus, its increase my crawl ratio as well for slower and more controlled off roading.

HammerheadFistpunch > Bandit 12/02/2014 at 11:29

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preload spring type clutch lsd.

Bandit > Nonster 12/02/2014 at 11:31

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Thanks! So 4.10 would be a good base line of the higher end of stock gearing and 4.56 would be ridiculous?

Bandit > Aaron James 12/02/2014 at 11:32

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Ahh perfect! Towing is good.

Bandit > Party-vi 12/02/2014 at 11:32

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Thanks! That's an absurd ratio!

nermal > Bandit 12/02/2014 at 11:34

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Just the axle gearing is only one part of the equation.

You need to also figure transmission ratios, transfer case ratios (low range), and tire size.

On top of that, you need to figure where the engine makes power. Some are tuned for lower end power vs higher end.

The goal is to get everything working together and playing nice with each other for optimum performance for the intented job. As a general rule, the higher the number, the more times the engine spins to turn the wheels a certain amount, so the more power is actually transferred to the wheels. The lower the number, the less number of times the engine needs to spin to turn the wheels a certain amount, so generally the less fuel used. This can go the other way, because if you bog the engine it will use more fuel.

If you're talking trucks as in pickups, Ford and GM offer 3 ratios and Ram 2, at least in their half tons. The highest (lowest #) gears are the *fuel economy* ones, which means they're basically used to game the EPA tests, and provide maximum economy while lightly loaded. The middle of the range is a compromise, and the lowest (highest #) is generally combined with "max tow" packages to provide the highest towing capacity.

camaroboy68ss > HammerheadFistpunch 12/02/2014 at 11:34

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it's a 64-72 (depending on width l) Chevelle 10 bolt. The big rubber bushings for the trailing arms are the big giveaway

RamblinRover Luxury-Yacht > HammerheadFistpunch 12/02/2014 at 11:34

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My Ranchero was special ordered with a 4.10 rear. 13" wheels + 4.10... idiotic for any kind of road use.

HammerheadFistpunch > camaroboy68ss 12/02/2014 at 11:37

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good eye!

HammerheadFistpunch > RamblinRover Luxury-Yacht 12/02/2014 at 11:37

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HAHA 13 inch wheels and a 4.10 with a V8?! hilarious.

RamblinRover Luxury-Yacht > HammerheadFistpunch 12/02/2014 at 11:39

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That was the original diff I'm talking about. 144 six + 4.10 + three-on-tree + 13"s with tiny tires. No way in hell I'd go for that + V8. The Jag is a 2.78 or some such.

As to why some loon did that? Forest service - 2wd dirt road climbing.

Bandit > nermal 12/02/2014 at 11:40

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Yeah... but this is just suppose to be a simple code exercise. I didn't want to complicate it too much.

bhardoin > Bandit 12/02/2014 at 11:42

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Both economy and performance will be a function of the rpms you're running at a desired speed. If you are going 10 mph offroad or coming out of a highspeed turn on a racetrack at 40 mph, you want your motor to be at the beginning of its powerband so that you can accelerate forward. If you are aiming for fuel economy, you want the motor to be at the lowest revs possible without lugging. So if you're on the freeway and you want your bigass V8 turning only 1700 rpm, you'd gear it so that 65-70 mph matches that.

RamblinRover Luxury-Yacht > Party-vi 12/02/2014 at 11:42

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A Land Rover Series is 4.7:1, but the transfer case is an underdrive 1.15:1 as well. Equates to a 5.4.

RamblinRover Luxury-Yacht > Bandit 12/02/2014 at 11:42

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Old Land-Rovers too.

Party-vi > RamblinRover Luxury-Yacht 12/02/2014 at 11:44

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Strange. Is there a reason they didn't go with a 1:1?

You can tell a Finn but you can't tell him much > Bandit 12/02/2014 at 11:45

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TL;DR Common truck ratios that I've seen are: 3.31, 3.55, 3.73, 4.10 with 3.31 being the economy end and 4.10 being the towing/off road end of the spectrum.

3.73 seems to be a pretty common ratio on trucks for tow packages. It used to be the go to on GM's and is the ratio in the max tow package (11,300 lbs towing) on the F-150 as of a few years ago. 4.10 is less common, but is the only option on the Raptor. The 4.10 seems to be more common on off road focused trucks, though I think that is what the Tundra uses for everything. Now a lot of trucks are coming with 3.55 and 3.31 rear ends to help with economy. I've got a 2011 F-150 with the 3.5EB and even with the 3.55 it is rated for 9,800 lbs towing.

RamblinRover Luxury-Yacht > Party-vi 12/02/2014 at 11:50

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The lulz: the only reason the British do anything. More specifically, because it has offset output for both axles, there's no particular reason to go with 1:1, so they decided on 1.15:1 based on... I dunno.

Below is a transfer case from one in high, with the input spur missing. You will notice that the intermediate shaft rides on a roller bearing about 2" across. This is because the older design with a small shaft sometimes got oil-starved and overheated, so the engineers said "Never Again" and over-engineered the fuck right out of it.

There's a transfer case conversion available that ups the output and preserves your low range (mostly), but it's expensive due to being three new gears.

Aaron James > Bandit 12/02/2014 at 11:50

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a 3.73 gear is a great compromise for towing while still able to get decent highway mileage with an overdrive transmission. Also an ideal street performance gear for anything with an overdrive.

Party-vi > RamblinRover Luxury-Yacht 12/02/2014 at 11:51

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That looks a lot like the Spicer 18 in my Willys, which is also offset-output.

Bandit > You can tell a Finn but you can't tell him much 12/02/2014 at 11:57

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Thank you!

RamblinRover Luxury-Yacht > Party-vi 12/02/2014 at 11:57

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It's very similar, and the overdrive kits work exactly the same way. As do the PTOs, of course.

KusabiSensei - Captain of the Toronto Maple Leafs > RamblinRover Luxury-Yacht 12/02/2014 at 12:07

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Was that a Ford 8.8 4.10 ring gear? Or did that come with the Ford 8 inch diff?

Because I have reasons to want to rebuild the rear end of my truck. 8.8 3.55TL to 3.73 Torsen T1 from a Mustang. Because silly.

Racescort666 > Bandit 12/02/2014 at 12:18

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Not sure how much this will help you but Summit sells a bunch of ring and pinions that will give you an idea of what's available: http://www.summitracing.com/search/departm…

RamblinRover Luxury-Yacht > KusabiSensei - Captain of the Toronto Maple Leafs 12/02/2014 at 12:18

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7.25" Ford. The PeeWee of the two axles in this picture. Sane ratios can take 180hp, but the 4.10, I imagine I'd be lucky to get up to 140 before the pinion and bearing would say "lol fuck this" let alone the axle shafts.

uofime > Bandit 12/02/2014 at 12:27

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ignoring all practical constraints, for max fuel economy you want maximum theoretical overdrive, ->0:1 and for max torque you want the inverse max underdrive, -> inf :1

BJ > Bandit 12/02/2014 at 12:28

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Towing AND cargo carrying (same thing, really). Multiplying your torques makes it easier to overcome inertia without straining your engine and/or clutch.

KusabiSensei - Captain of the Toronto Maple Leafs > RamblinRover Luxury-Yacht 12/02/2014 at 12:33

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The noise on my 8.8 inch diff is the pinion bearing, so since I'd have to pull the whole damn thing anyway, I figured moving from the Traction-Lok to a Torsen might make sense, along with going from a 3.55 ring gear to a 3.73 ring gear and pinion set.

XJDano > Bandit 12/02/2014 at 12:35

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I really only know a few things about it that I have used in my own Cheorkees.

Stock auto gets 3.55

stock manual gets 3.07

Adding 31" tires to the manual XJ & 3.55 gears kept the same tooth speedo gear

Adding 33" tires to the auto XJ would have required 4.10 gears to keep speedo same tooth, but I went down to 4.56 for a tad more torque, I could run 35" tires with the 4.56 for highway & street use but tires are expensive.

tromoly > Bandit 12/02/2014 at 12:44

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What is different in your code between cars and trucks?

Also, if you don't already have it, Thomas Gillespie's "Fundamentals of Vehicle Dynamics" is awesome for giving you acceleration equations and whatnot.

Bandit > tromoly 12/02/2014 at 13:09

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It shoots out different fprintf statements and the tolerances (gear ratios) are different. Here's the code for the "performance" part. I didn't know a car with stock 3.73 gears so I just picked my brothers mustang that had a swap done.

function performance_two(gearratio,type)

%—————-This part down is for cars———————-

if type ~= 0 %Sets this segment of code to run if the vehicle is a car

amount = numel(gearratio);

running = true;

counter = 0;

while running == true; %Tells loop to stop

for x = gearratio > 3.36 %Restricts performance analysis to run if the ratio is greater than 3.30

if gearratio > 3.36 & gearratio<3.78

fprintf ('This ratio is not great for fuel economy.\n')

fprintf ('This ratio good for performance driving.\n')

fprintf ('Ratios greater than 3.30 and less than 3.78 are common on many modern automobiles.\n')

fprintf ('One example of a car with a ratio greater than 3.30 and less than 3.78 is the SN-95 Mustang with a gear ratio of 3.73.\n')

fprintf ('\n')

elseif gearratio > 3.78

fprintf ('This ratio is horrible for fuel economy.\n')

fprintf ('This ratio would be ideal for racing.\n')

fprintf ('Ratios greater than 3.78 are common on race vehicles.\n')

fprintf ('Examples of a car with a ratio greater than 3.78 are street race vehicles commonly with a ratio of 4.11\n')

fprintf ('\n')

end

end

counter = counter + 1;

if counter == amount

running = false;

elseif counter ~= amount

running = true

end

end

%———————This part down is for trucks————-

elseif type == 0 %Sets this segment of code to run if the vehicle is a truck

amount = numel(gearratio);

running = true;

counter = 0;

while running == true; %Tells this loop to stop

for x = gearratio > 4.00 %Restricts performance analysis to run if the ratio is greater than 4

if gearratio > 4.0 & gearratio<4.50

fprintf ('This ratio is not the best for fuel economy.\n')

fprintf ('This ratio okay good for occasionsal off road driving.\n')

fprintf ('Ratios greater than 4.0 and less than 4.50 are common on many modern trucks.\n')

fprintf ('One example of a car with a ratio greater than 4.10 and less than 4.50 is the Ford Raptor with a gear ratio of 4.10.\n')

fprintf ('\n')

else gearratio > 4.50

fprintf ('This ratio is horrible for fuel economy.\n')

fprintf ('This ratio would be ideal for off road applications.\n')

fprintf ('Ratios greater than 4.50 are common on custom built off road trucks and farm use vehicles.\n')

fprintf ('These ratios provide the torque that is required for offroad driving.\n')

fprintf ('One example of a car with a ratio greater than 4.50 is the original Willys Jeep with a rear end ratio of 5.38.\n')

fprintf ('\n')

end

end

counter = counter + 1

if counter == amount

running = false;

elseif counter ~= amount

running = true

end

end

end

tromoly > Bandit 12/02/2014 at 16:32

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Oh, so it's comparing an input to a given set of output responses. Gotcha.

Bandit > tromoly 12/02/2014 at 17:20

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Yeah, like I said, it is very basic. It's our 1st year engineering class so I still am learning. Next semester I have to take a CS class and learn Python.