Introduction to Differential Gearing

Gear Design – Hypoid versus Non-Hypoid

Differential gears come in two different designs – hypoid and spiral bevel. Spiral bevel differential gear sets are also referred to as non-hypoid. In a nutshell, with a spiral bevel design, the pinion gear meshes with the ring gear at the center line of the ring gear. The first automotive differentials were spiral bevel non-hypoids.

A hypoid differential has the pinion gear meshing with the ring gear either above or below the center line.

If it is above, not surprisingly, this is referred to as a high pinion. The amount that a pinion gear is off from the center line of the ring gear is referred to as the hypoid offset. In this day and age, most differentials are low pinion hypoids with a moderate hypoid offset. There are advantages and disadvantages to both designs. Many people assume that hypoid differentials are inherently stronger than a non-hypoid but this is not necessarily correct. Both designs can be very durable.
Large hypoid offsets have fallen out of favor because they inherently generate a substantial amount of friction resulting in heat, wear and inefficiency. With the advent of hypoid differentials, the usual lubrication had to be upgraded to “hypoid rated” to deal with the sliding action between the gears and the resulting increases heat and wear.
The first hypoid differential was produced by the American automobile company, Packard in the 1920’s. The original motivation for the designing of the hypoid differential, contrary to popular belief, was surprisingly to lower the driveshaft tunnel in passenger cars.


Regular versus Reverse Rotation Gears

Differential gears have directional teeth. This results in the individual teeth having both a drive side and a coast side to the teeth.They can run in either direction but ideally you want the drive side to be taking the larger load under acceleration. This is primarily because the drive side of the tooth is in the range of 20 to 30% stronger than the coast side of the gear. This is referred to as a regular rotation gear set.

You would logically assume that all differentials are designed to drive on the drive side of the tooth but this is not the case. The vast majority of rear differentials do run on the drive side of the teeth. Most front differentials run on the coast side of the teeth. An example is Land Rover.

Many Land Rover front and rear differentials are interchangeable, i.e. you can switch them back and forth. The rear diff is running in the correct direction. The front differential is actually running backwards compared to the rear! This is not normally a big issue but if you want to maximize the durability of the ring and pinion gears, you can accomplish this by reversing the cut of the front gears. These are known as a reverse cut or a reverse rotation design. These will make the backwards running, front differential gears mesh properly, i.e. drive on the drive side of the teeth. As noted earlier, the drive side of the gear teeth are substantially more durable so you increase the durability of the front gear set by 20%+ by this change alone. We were the first aftermarket specialty Land Rover gear supplier to incorporate reverse rotation designs into our differential gearing.
A secondary but significant advantage of front reverse cut gears is longer bearing life. This is because the larger, better lubricated, inner pinion bearing is taking the load under acceleration.


Differential Gear Ratios

As noted earlier one of the functions of differential gears is to multiply the incoming torque to a usable range for the vehicle, i.e. within the designed power band. It does this thru two gears, the pinion gear and the ring gear.

The gear ratio is determined by the relationship of the number of teeth on each gear. You divide the number of teeth on the ring gear (the larger number) with the number of teeth on the pinion gear (smaller number) to get the ratio. Example: 46 divided by 13 = 3.54. This is the differential gear ratio.

This is also the stock ratio for the vast majority of coil sprung Land Rovers. The tooth count is usually expressed in this format: 13 X 46.

Now that you know how to calculate the ratio, what does 3.54 actually mean? It means that the pinion gear will rotate 3.54 times for every time the ring gear rotates once. Or in other words, the engine will rotate 3.54 times for every revolution of the tires. This assumes a one to one output ratio of the transfer case, which is not always the case but we will make that assumption for this discussion at this point. Another way to look at this is one engine revolution will propel the vehicle X number of feet during one revolution of the tire.

Why is this important? First let’s have a quick discussion of some terminology. Under geared vs over geared, low gearing vs high gearing and raising or tall gearing. It is easier to use examples to explain this so here goes:

As an example: you put your transmission in 1st gear and accelerated away from a standing start but do not up shift into another gear. The vehicle would accelerate fine but you would quickly get to a point where your engine was screaming along and your speed would be very limited. This is an under geared situation! Also known as low or lowered gearing. If using the same example, you shifted to to 2nd gear this would be known as raising your gearing.

Conversely if in the same situation say you put the transmission in 4th gear and attempted to do the same thing i.e. start from a standing start. The acceleration would be very slow but eventually as the vehicle came up to speed, it would cruise along just fine at a higher speed. The initial part of this example would an example of being over geared. Also known as high or tall gearing.

Why this terminology can be confusing is that if you change your differential gearing from say 3.54 to 4.11, this is known as lowering your gearing even though the numeric ratio is getting larger. A 3.54 ratio is also a taller gear than a 4.11. If you go the opposite direction (4.11 to 3.54) you are raising your gearing. To compare this to another example, say a 5 speed transmission, first gear is lower than 5th but it has a higher numeric number for the ratio.

Anyway, back to the original question, why are differential ratios important? The major reason is that internal combustion engines do not make a uniform level of power at all engine speeds. They inherently work most efficiently within a certain rpm (revolutions per minute) range also referred to as the power band. An example is a Rover V8. At idle (600 rpm) it is producing almost no power, which is quite evident if you abruptly let out the clutch on a manual transmission equipped vehicle. The power increases slowly with increasing engine speed until you hit a certain engine speed at which point the engine power increases substantially and proportionally at a much greater rate. This is because you have entered a zone, the power band, where your engine is operating most efficiently. The power produced increases rapidly until the design and tuning limitations of the engine start coming into play. At this point the power curve starts to flatten out again and eventually starts to decrease even though the engine is spinning faster.

Again, using a stock Rover V8 as an example, most of them work efficiently in the range of 2300/2400 rpms until about 4500 rpm. Do not confuse the higher number with the red line on the tachometer. The red line is maximum speed that the engine can operate safely. Exceed the red line for any length of time and the engine blows up!

So what does this have to do with differential gearing? The manufacturer of the vehicle designed it (with stock differential gearing and stock tires) to operate most of the time within this efficient and ideal power band. The issue is that many off road modifications effectively “raise” the gearing of your Rover. One of the most common and best understood but certainly not the only modification that effectively raises the overall gearing is taller tires. Think of the earlier example we used, a 3.54 differential gear will still rotate the tires once for every 3.54 times the engine goes around but the taller tire will roll a longer distance. So at the same engine speed, your miles per hour or road speed will go up, hence to maintain, say the legal speed limit, you need to drop the engine speed. This is where the gearing problem starts. When you drop your engine speed below 2300/2400 rpm, you exit the power band and start suffering the consequences of it.

Conversely, when you are accelerating from a standing stop, it takes you longer to reach the power band. So what are the real live consequences of being geared incorrectly? Before we get into that, there is a characteristic of most late model Land Rovers Range (Rover Classics, all Discoverys and NAS Defender 90s) that is important to know. That is that they are geared a bit on the tall side, meaning that in stock form they are geared correctly but you don’t have much reserve gearing. The result of this is it is easy to go from being geared correctly to being over geared. One way to do this is to install taller tires and presto – over geared.

Anyway, back to the consequences of being geared too tall or over geared. First, your stop and go acceleration will be adversely affected, i.e. the vehicle will feel more sluggish. At higher speeds, particularly with an automatic transmission, you can end up in a situation known as hunting for the correct gear. What this means is that your Rover will not be able to maintain high gear at normal highway speeds. So the vehicle gets very “shifty” between 3rd and 4th gears. In 4th gear, the Rover starts to slow down until it downshifts into 3rd gear, at which point it speeds up and then shifts to 4th gear and starts the process all over again. It is essentially hunting for the correct gear but never stops because a correct gear no longer exists! This can get maddening if you have to drive a long distance. Bear in mind that the above discussion is just about on road performance. We haven’t even gotten to off road yet.

Ironically, many people inquiring about different differential gearing do so with the off road benefits in mind. The ironic part is most people get far more benefit from them on road!

The off road benefits of lower gearing are as follow
1) Your very slow speed performance is improved by increasing the engine speed thru torque multiplication. The purpose is to keep the engine within the power band that we talked about earlier. This allows you to negotiate difficult obstacles slower and hence with more control. If your gearing is not low enough in these situations you compensate for it by using throttle and momentum. These may be your only choices but they are not very good ones. Using excessive speed increases the chances of damaging other components of your vehicle such as the suspension and undercarriage and can literally be unsafe. Using excessive throttle increases the likelihood of drive train failures.

2) Increased compression braking. Compression braking is what allows you to descend steep obstacles slower and with more control. It can minimize your use of the brakes, which can be fraught with risk. Unfortunately modern engine management systems don’t allow engines to develop much manifold vacuum under deceleration so this technique is not as effective as it used to be in the good old carbureted days! But nevertheless, lower gearing is some improvement. Other than Hill Descent Control (HDC), which is a computer controlled braking system, lower gearing may be your only option for controlled slow speed descents. To be honest, it takes massive reductions in gearing to make significant improvements in this area. If you are going to attempt this thru differential gearing this requires very large drops in ratios. This can frequently involve a trade off in your on road performance i.e. high speed freeway driving, because you may end up at this point under geared. The other options to accomplish this are a combination of differential gearing, lower transfer case gearing or adding a third transmission (under-drives).




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