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Old 25th July 2008, 17:24   #1
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Default 4x4 Vehicles and Various Gearing Ratios

In 4x4 vehicles, we all would have come across the various ratios applicable to the different parts of the vehicle. Modifying these can significantly alter the performance of one's 4x4 for various conditions.

Many a times, on this forum itself, we come across terms like ' I am currently running 4.27:1 which is better suited to the higher power .... and give more relaxed freeway cruising. ..... Sometimes I wish that I could go to a 3.73 ratio.'

"....for axle ratios 3.92:1... "Terms like diff ratio, final drive ratio, ring & pinion ratio, crawl ratio etc. are things that an off-roader will encounter and must know to fully understand their machine and it's capabilities.

Though there are many technical & elaborate definitions and explanations of what each ratio is about, can we pool in our know-how and demystify the various gearing parts, their role in a 4x4 and how it impacts the other gear parts and their ratios etc in a simple and comprehensible language that is understandable by most folks.

Some of the terms i have come across are
  • Transmission Ratios:
  • Transfer Case (T-Case) Ratios:
  • Diff ratio:
  • Crawl Ratio:
  • Axle Ratio:
Looking for the collective wisdom of the 4x4 experts here.
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Old 25th July 2008, 19:18   #2
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Final drive ratio (a.k.a. incorrectly called diff ratio, correctly called R&P or ring and pinion ratio, axle ratio etc.)

It is typically stated as 5.375:1, or 5.375, or as X/Y.

It means for one revolution of the drive wheels, the prop shaft takes 5.375 turns.
A lower ratio (e.g. 5.375:1 is lower than 4.88:1) means greater torque multiplication and speed reduction.

To measure the final drive ratio of your vehicle, jack up one of the rear wheels with the vehicle in neutral and rotate the wheel. Note the number of revolutions of the propshaft for two revolutions of the wheel. The axle ratio will be <no. of revolution of the propshaft>:1. This will be a bit complicated for vehicles with an LSD.

Differential ratio

This usually applies to vehicle with an interaxle differential with a fixed torque bias.

e.g. The Ural-375 6x6 truck has a planetary central diff with a torque split of 1:2 (F:R)

Crawl Ratio

It is the ratio between engine speed and wheel speed.

i.e <no of revolutions of the engine>:<1 revolution of the wheels>

For a typical 4x4 vehicle, it can be calculated as Axle Ratio x TCase ratio x Gear Ratio.
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Old 26th July 2008, 09:37   #3
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Alfa, thanks for the inputs.

I think that we should simple terms while explaining these terms so that most newbies can also understand what this all means. It would also help if we could use pictures of where these are in the vehicle.

For example: When we talk of Transmission Ratio, we are talking about the 'normal' gears that we use in the 4x4 (or in regular cars also) while driving. Each gear will have a ratio :

Say for example (Gearing Ratio/Transmission Ratio for Gypsy):
-----------
1st 3.652
2nd 1.947
3rd 1.423
4th 1.000
5th 0.795
Reverse 3.466

Between the engine and the wheels, this is also the first area (among a chain of parts) that play a part in translating engine energy into 'motion'. Hence the gear ratios of the transmission play a huge role in how your 4x4 will perform.

To be continued..
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Old 27th July 2008, 16:22   #4
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To continue with the 'Transmission Ratio's/Gearing Ratio' part, let's take another example and then analyze the numbers.

NOTE: It is ONLY a comparison of the specs and NOT a statement on ANY kind of capability of any vehicle. The objective here is to learn better on how to interpret the numbers objectively.

Gearing Ratio/Transmission Ratio for Classic:
----------------------------------------
1st 3.986
2nd 2.368
3rd 1.473
4th 1.000
Reverse 5.315:1

So what can we interpret from the the above numbers (Right now in isolation, without taking into account the T-Case Ratio & the Differential Ratio):

1. The pattern of ratio distribution is similar for both the Gypsy & Classic. Both get their 'respective' power in the lower gears.
2. Classic has the largest ratio for the 'Reverse' gear. Why? How does it help?
3. Notice the overdrive on the Gypsy's 5th. This helps on the highway and is an attempt to balance off-road/highway driving
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Old 27th July 2008, 22:16   #5
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Can you tell me what can we make out of the following.
Name:  landy gear ratio.jpg
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Old 27th July 2008, 22:38   #6
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Quote:
Originally Posted by khan_sultan View Post
Alfa, thanks for the inputs.

I think that we should simple terms while explaining these terms so that most newbies can also understand what this all means. It would also help if we could use pictures of where these are in the vehicle.

I agree. Please give more details in lay mans terms. Also pics would surely help
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Old 27th July 2008, 22:50   #7
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A critical element we all need to remember in automotive design is there's always a trade-off with every move. You enhance one aspect, another aspect suffers.

So here too with the subject of gearing its very complex as the tradeoff must be clearly understood before modifications are attempted
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Old 28th July 2008, 00:30   #8
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@SirAlec

1) The clutch bit is quite simple, the vehicle uses a single dry 230mm dia clutch plate. Most vehicles except bikes and bike derived ones utilise single plate planar (i.e. flat, not conical) dry friction clutches. The friction lining is in the form of a circular band on the face of the disc.

The outer diameter indicates the size of the clutch plate. The inner dia of the friction lining indirectly indicates the total area availalbe for power transmission between the engine and the GB

2) The vehicle has a four speed main GB (4F+1R). It is a constant mesh unit; meaning that the gears in each pair (there should be 4 pairs, one for each forward ratio and an extra one for reverse) are always engaged (with the possible exception of reverse, which could be sliding mesh). The 1st and 2nd are non synchromesh, requiring double declutching for proper operation.

All gears are helical cut, so the teeth are at an angle to the axis of the gear.

3) The two prop shafts are open, not enclosed as found in some serious offroad vehicles like the Tatra, Pinzgauer and the Unimog.

4) The final drive ratio is 4.7. i.e. the prop shaft turns 4.7 times for one complete revolution of the wheel.
The front axle has CV joints for steering, rather than relatively primitive universal joints.

5) Overall gear ratio = [main GB ratio for the selected gear]x[TCase ratio]x[final drive ratio], so 84.581 means that the engine turns those many times for one revolution of the wheel.
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Old 28th July 2008, 00:39   #9
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Quote:
Originally Posted by khan_sultan View Post
To continue with the 'Transmission Ratio's/Gearing Ratio' part, let's take another example and then analyze the numbers.

NOTE: It is ONLY a comparison of the specs and NOT a statement on ANY kind of capability of any vehicle. The objective here is to learn better on how to interpret the numbers objectively.

Gearing Ratio/Transmission Ratio for Classic:
----------------------------------------
1st 3.986
2nd 2.368
3rd 1.473
4th 1.000
Reverse 5.315:1

So what can we interpret from the the above numbers (Right now in isolation, without taking into account the T-Case Ratio & the Differential Ratio):

1. The pattern of ratio distribution is similar for both the Gypsy & Classic. Both get their 'respective' power in the lower gears.
2. Classic has the largest ratio for the 'Reverse' gear. Why? How does it help?
3. Notice the overdrive on the Gypsy's 5th. This helps on the highway and is an attempt to balance off-road/highway driving
Most vehicles have a 1:1 ratio for the 4th gear because the engine is directly connected to the output shaft. This is possible only with gearboxes with three shafts. Many modern GBs have only two shafts as the cruising gear is on overdrive.

The Classic has a lower reverse because in any GB, an additional intermediate gear is required to reverse the direction of motion. So the reverse ratio has a set of three gears rather than a pair for forward ratios. It is practically difficult to match the reverse ratio to the 1st ratio. The reverse being taller or shorter than 1st depends more on design convenience than anything else.
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Old 28th July 2008, 10:59   #10
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Default Transfer Case/T-Case/Transfer Gear

Ok, now let's move on to the next part in this series -- the 'Transfer Case Ratio'. This is also known as the 'Transfer Gear Ratio or the T-Case Ratio'.


Name:  tcdiagram.jpg
Views: 3422
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From the inside/driver's perspective, Transfer Case is the part that the driver uses to engage the 4wd of the vehicle. It has 2 (infact 3, but we will focus on 2 only here) important movements -- 'High' and 'Low'. (We will NOT go into detail of what these mean and will stick purely to the gearing ratios)

What does the T-Case do?
Transfer Case is usually found attached to the transmission(Gear Box) and it gets it's input from the transmission(Gear Box)

T-Case directs power to both the 'axles' (wheels) of the 4x4 through the front/rear drive shafts.
  • In 2WD mode of the T-Case, power is transferred to only the rear wheels (axles) by the T-Case
  • In '4-High/Low' mode, the front driveshaft is engaged and power is sent to both rear and front wheels (axles).
Taking our previous example of the Gypsy/Classic, here are their numbers:

AGAIN NOTE: It is ONLY a comparison of the specs and NOT a statement on ANY kind of capability of any vehicle. The objective here is to learn better on how to interpret the numbers objectively.

Gypsy/Classic
---------------
High: 1.409/1
Low: 2.268/2.46

Interpreting the above numbers:
---------------------------------
High: In Classic, the T-Case does NOT do any speed reduction/torque multiplication in 4-H mode (Notice the Ratio of "1"). In a Gypsy, there is speed reduction/torque multiplication happening in case of 4-H mode (Ratio of "1.409").

Low: speed reduction/torque multiplication happens for both the Classic/Gypsy. The lower ratio helps where torque multiplication is required for steep hills or more engine power is required to pull the vehicle.

Summary in Simple Terms: (Considering both the Transmission & T-Case now)
---------------------------------------
The T-Case is sort of a "middle man/broker" sitting between the 'Gears' (as we know in normal world :-)) and the wheels/axles. This broker decides if only the rear wheels/axles are powered or the front wheels/axles are also powered. The 'High/Low Ratio' of this broker decides how the power/torque multiplication happens in the High/Low range.

Source:
Transfer Case

Last edited by Samurai : 29th July 2008 at 11:37. Reason: source added
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Old 28th July 2008, 11:50   #11
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How is the transfer case attached to the transmission ? When talking about engine replacement, we usually hear that you might lose the 4 wheel drive function.
So, this connection becomes important. What if the transmission (gear box, i.e., everything after the clutch which is this side of the flywheel) is also replaced along with the engine ? Will 4 wheel drive use become easier then ?
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Old 28th July 2008, 17:14   #12
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Default Axle Ratio (Final Drive Ratio, Diff Ratio, R&P Ratio)

The last part of the gearing ratios is the 'Axle Ratio'. This ratio is also known as:
  • Final Drive Ratio
  • Diff Ratio
  • R & P Ratio (Rack & Pinion)
The axle/differential gears are located in the 'solid spherical' thing that you see on the front/rear wheels of a 4x4.

Name:  Axle Ratio lead.gif
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This is how the axle ratio works:

Name:  ar.jpg
Views: 2763
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Source:
AXLE RATIOS
Axle Ratio on carlist.com


The 4:1 axle ratio means the drive shaft (coming in from the T-Case) turns 4 times for every one time the tires spin.

A higher axle ratio is good for low speed pulling power. A low axle ratio is good for fuel economy.

Continuing our earlier example of the Classic/Gypsy, here are the Axle Ratios.
----------
Gypsy: 3.73:1
Classic: 4.88:1

An axle ratio of 3.73 is considered a reasonable compromise between pulling power & fuel economy.

As is evident from the numbers, the Classic has it's axle ratio very apt for pulling power -- great on rocks and inclines.

NOTE: Till now we have mostly looked at the gearing ratio's in isolation. We will soon see what happens when we combine these all together.

Last edited by Samurai : 29th July 2008 at 11:37. Reason: source added
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Old 28th July 2008, 18:11   #13
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Shahnawaz, when you are posting info and images from other sources, please mention the original source/link at the bottom.
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Old 28th July 2008, 19:31   #14
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@KS

R&P means Ring and Pinion when we are talking axles, Rack and Pinion is for a steering.
Diff ratio is different from axle ratio.
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Old 28th July 2008, 20:10   #15
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Quote:
Originally Posted by SirAlec View Post
Can you tell me what can we make out of the following.
Attachment 32928
Thanks! Its from my land rover manual. I know them yaar, but how they were calculated thats mystery for me.
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