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WHY CHANGE YOUR DIFFERENTIAL GEARING?
Differential gearing changes are frequently called for to compensate for vehicle modifications such as installing larger tires and/or specialist applications such as towing, heavily laden expedition vehicles, aggressive off roading and improved off road performance. For more detailed information go to:
Introduction to Differentials
An Overview and history of Land Rover Differentials
Introduction to Differential Gearing
Introduction to traction enhancing differential carriers
ARE ROVER DIFFERENTIALS INHERENTLY WEAK?
The short answer is NO. More detailed information landroverweakno . (note to COLE: have to find where this is supposed to go)
DOES BEING A NON-HYPOID DESIGN HAVE ANY IMPACT ON THE STRENGTH OF YOUR DIFFERENTIAL?
This a question that is occasionally asked but unfortunately it is frequently answered incorrectly. The answer is absolutely – No. Rover differentials are quite durable especially after some common upgrades are added to them.
Let’s explore the differentials durability argument in more detail. Rover diffs are considered to be “weak” and in “stock” form they do have their issues. If you aggressively off-road a completely stock Land Rover until something major mechanical breaks, 95% of the time you will, in fact, break one of the differentials. Based on this you might jump to the erroneous conclusion that they are “weak”. But let’s look at what is actually breaking. By running a Land Rover differential remanufacturing facility and having disassembled hundreds of broken Land Rover differentials, has given us unparalleled experience in determining the causes. Our observation is that 99% of Land Rover differential breakages are the result of the cross shaft in the differential carrier breaking, not the ring and pinion gear. So in reality, the issue is that stock Rover differentials have “weak” cross shafts in the differential carrier and not weak ring and pinion gears.
Picture of cross shaft Cross shaft with spider gears Broken cross shaft
A stock Rover carrier is a two pinion design like most stock differentials of any brand. It has one cross shaft that is relatively small (and brittle) and two pinion gears. This is what breaks. The failure point is usually the stress riser created when you machine two opposing lubrication flats for the spider gears.
Here is an example of why the cross shaft breaks. When you are aggressively off roading close to or at the vehicle limits, you frequently compensate for the lack of traction by using extra throttle and momentum. It is not unusual in this situation for one of the tires to be momentarily pulled off of the ground. When this happens the torque in the drive train will immediately follow the path of least resistance i.e. the tire in the air with no traction. The result of this is that particular tire will speed up and will actually spin at twice the speed it would have been spinning if it had remained on the ground. Imagine what happens when it comes back in contact with a traction surface. Assume it is a high traction one. It will immediately slow down and possibly stop. Guess where all of this torque that has been converted to momentum goes! It attempts to shoot back across the axle assembly and whether the opposite tire has traction or is in the air, this creates a tremendous shock load. Guess what the weakest part of the axle assembly is? The carrier cross shaft and not surprisingly it is what breaks 99% of the time.
Picture
The solution to this problem is quite simple – install any aftermarket traction enhancing differential carrier and you have solved the problem! Better yet, if you are upgrading the vehicle for enhanced performance this is something you would probably do anyway! Examples of aftermarket differential carriers are ARB’s and Detroits. Both Detroits and ARBs are 3 to 4 times stronger than stock carriers. At a minimum, you have just tripled the durability of your Rover differential! Hence the end of “weak” Rover differentials since you have eliminated the failure point in 99% of Rover differential failures. At this point you have a very durable differential that will hold up very well to aggressive recreational off roading. At this point they are essentially comparable in strength to most conversion differentials, many of which are mistakenly marketed as substantial durability upgrades.
DOES BEING A NON-HYPOID DESIGN HAVE ANY IMPACT ON THE STRENGTH OF YOUR DIFFERENTIAL?
Another erroneous argument that Rover differentials are “weak” is that they are a non-hypoid design hence the cross section of pinion gear is smaller compared to a hypoid design. Folks promoting Toyota differential swaps compare the size of the pinion gear. The Toyota pinion gear has an overall larger cross section although the cross section of the pinion gear is not a major factor in the overall strength of the gear set. The ring gear teeth are actually smaller. Pinion gear teeth are always larger than ring gear teeth no matter the design or brand. If you have two teeth meshing together and shock load the gear set and something breaks – which one is going to break? The bigger tooth or the smaller tooth? Simple physics – the smaller tooth. So the reality is that the size of the ring gear teeth is a much more important factor in the overall durability of the gear set compared to the cross section of the pinion gear or the size of pinion gear teeth. Rover ring gear teeth are larger than Toyota ring gear teeth. A Rover ring gear diameter is larger than a Toyota ring gear. The Rover is 8.625 and the Toyota is 8 inches. They don’t call them Toyota 8 inch differentials for nothing. . For a more in depth comparison between Rover vs Toyota differentials see out section on Toyota differentials. (Link)
Picture from above showing diameter
HOW DOES GEAR RATIO AND TOOTH COUNT IMPACT DIFFERENTIAL DURABILITY?
The tooth count is another large factor in ring and pinion durability. A coarser tooth count means larger teeth. Here is a comparison of a Toyota 4.10 ring gear and a Rover 4.11 and a Rover 4.14.
Picture from the side show tooth size
Not much explanation required here! Obviously the thicker tooth is going to hold up better to shock loads loads and torque multiplication.
Another misconception about ring and pinion gear strength is that the higher numeric ratios are weaker than lower numeric ratios. Again this is not true. Following is a listing of the ring gear tooth counts of the various ratios of interest with the durability increasing as you descend down the list:
47 teeth / Series Rover 4.70 – least durable
46 teeth / Rover 3.54
41 teeth / Toyota 4.10
39 teeth / GBR 3.90
38 teeth / GBR 4.75 now discontinued
37 teeth / GBR 4.11
33 teeth/ GBR 4.71
29 teeth/ GBR 4.14 – most durable
This chart is very easy to understand when you understand the importance of the size of the ring gear teeth.
Something you frequently and erroneously hear about Rover differential gears is that the 4.75/4.71 ratio is a weak gear compared to a stock 3.54. This is simply not true. We think that part of this misconception is that 4.75 or 4.71Rover differential gears are frequently mistaken for Series 4.70 gears. As you can see from the above listing both stock Land Rover gear sets (3.54 & 4.7) are the least durable gear sets based on ring gear tooth size.
Also as you decrease the numeric ratio, an example being 4.70 to 3.54, the pinion gear diameter gets larger. Logically, you would think this results in a stronger gear set but remember it is the ring gear tooth size that is more important not the cross section of the pinion gear.
HOW IMPORTANT IS CORRECT ASSEMBLY TO THE DURABILITY OF A DIFFERENTIAL?
The answer is very simple – it is critical!!!!!! Again this based on our extensive differential remanufacturing experience and processing the occasional warranty claim. If we sell you a differential gear set and you or someone else assembles the differential and you break the gear set and you call us inquiring about what kind of warranty we have on our gears. Our answer is – “it depends”. If we didn’t build the differential ourselves, our procedure is that we must see the set up. We require you to return the entire third member to us for inspection. Our experience is that conservatively 50% of the gear failures we see are the direct result of improper assembly! You wouldn’t believe the issues we have seen.
If the differential is incorrectly built, logically we cannot warrant the gear set. Another observation we have made is that the world has a lot more people that claim to know how to build a differential than the number that actually can!
ASSUMING YOU HAVE UPGRADED YOUR ROVER DIFFERENTIAL, WHAT IS THE MOST LIKELY THING TO BREAK AND IS THERE ANYTHING YOU CAN DO ABOUT IT.
DIFFERENTIAL HOUSINGS
Rover housings are manufactured from a very high quality nodular cast iron. Folks that know Ford 9″ diffs know the significance of a nodular iron housing vs standard cast iron. Most differential housings are made from a lower quality cast iron. A stronger housing means less housing flex and hence less mis-alignment of critical components. Other advantages are that nodular cast iron is easily welded so upgrades such as thrust bolts can be easily done.
RING GEAR SUPPORT SYSTEMS
Now that you understand what usually breaks in a ring and pinion set i.e. the ring gear teeth, the question is – so what usually breaks ring gear teeth in high torque & shock loads? The answer is commonly – ring gear deflection. When you apply torque to a ring gear it cause two things to happen – one good and the other one bad. The good thing is that it causes the ring gear to rotate which results in vehicle motion. The bad thing is that it puts a lateral force on the ring gear i.e it is trying to push it sideways. This is due to the angle and cut of the teeth. In normal usage this is not an issue as differentials are designed to resist this deflection. As an example, this is why differential carriers have such heavy ribbing. This is to help them resist flexing. This is also why building a differential correctly especially carrier bearing preload is so important to durability of the diff because this reduces defection. The thickness of the ring gear is also a factor. As the gear numeric ratio goes up the pinion gear gets smaller but the ring gear gets thicker. A 4.71 ring gear is much thicker than a stock 3.54 gear.
Picture of gear thickness
In high torque and shock loads what happens is that defection can occur. The result is that the contact pattern between the ring gear tooth and the pinion gear tooth starts to ride up the face of the ring gear tooth. If you look at a side profile of a ring gear tooth it is triangular shaped, in other words it is thick at the root and gets thinner and weaker at the top.
Picture
You are also increasing the leverage on the ring gear tooth. The occasional result is – Boom, broken ring gear tooth. An increasingly popular way to prevent this and substantially increase the durability of the ring gear is through the use of a ring gear support system. Common names for the system are Thrust bolts, slipper blocks, pegging, thrust screws etc. Regardless of the name the principal is the same – preventing ring gear deflection. The concept is to install a support system behind the ring gear opposite of the pinion gear so that when the ring gear tries to deflect, the support system prevents this from happening. Most differentials cannot utilize this system because you need a smooth surface on the back of the ring gear or carrier. Both Rover and Rover P38 differentials have this design.
Pictures Rover /P38/ Toyota
In the USA in the recreational 4×4 market, the first folks to start utilizing this system were Ford 9 inch crowd. We started thrust bolting Rover diffs in early 2008. Some people are claiming increased durability as the result of Thrust Bolting, of up to 50%. To be conservative, we use the figure of 25%. It is not surprising that the technology to increase the durability of a Ford 9 inch has revolved around the carrier and the ring gear.
For more information on Rover Thrust Bolting, follow this link. Cole – have to figure out where he wants this to go.