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We must first understand what are gears to understand Gear Ratio. Gears are wheels with teeth that transmit power between shafts. The teeth may be on the edge of the wheels , on the side, or inside the wheel. To transmit power, a gear on one shaft is meshed with a gear on the other shaft. To mesh means that the teeth of a gear fit into the spaces between the teeth of another gear. When one gear turns, Its teeth then force the other gear to turn. The gear that does the turning is the driving gear. The gear that is forced to turn is the driven gear.

Before we discuss the difference between Differential Full Spools and Mini Spools, it is very important to understand What are differential spools and why at all one needs to use spools in differentials?

When a car takes a turn, the outer wheels of the car have to a travel a greater distance as compared to the inner wheels in the same time. Therefore, a mechanism called as differential is provided in the rear end of the car, so that the speed of the inner wheels reduces and the speed of the outer wheels increases and the car can take turn easily and does not skid. At the same time, differential helps in maintaining equal speed of the wheels when the car is going straight.

Various vehicles use different options to maintain the torque required to run the vehicle in different given situations, e.g. straight drive on street, driving in dirt, driving on ice, driving on rocks etc.  Therefore the type of differential required will depend on the situation where we want to run the car.

When the vehicle is moving off-road i.e. situations like rock crawling, steep hills, mud-blogging, sand dragging etc  it would require such a mechanism, which delivers equal power and traction to both the left and right side rear wheels. Such a mechanism which is available in the market is known as locker, A Locker is mounted on the differential and thus controls the speeds of the rear wheels, as required, but the locker is heavy and expensive. Therefore, to minimize the cost of the equipment used and to make the vehicle lighter, a spool is used.

A differential spool is known as the best alternative in place of a locker both in terms of value and weight. A spool is basically a piece of steel that the ring gear bolts to and the Rear Axle Shaft splines fit into the internal splines of the spool. The spool does not have any internal gears. The simplicity of the spool is also the reason for its tremendous strength. No moving parts = brute strength. Thus a spool, completely replaces the differential center with a solid piece, but retains the Rear Axle Shafts. The basic function of a differential spool is to permanently lock both the left and right axles of the rear wheel together, so that they move together and there can be no loss of power due to one wheel spinning faster than the other wheel.  The Differential Spools are also used as a support for supporting the ring gear much better than most other differentials. Spool is only recommended to be used in such vehicles where the vehicle has to remain most of the times, off the road. Full Spools are recommended to be used in the vehicles where a higher horse power is required.

Advantages of using differential spools:

1. A differential spool is less costly and light weight as compared to Locker.
2. A differential spool provides guaranteed traction at all times,
3. As differential spool is a single unit and has no moving parts, which means that it has got lesser tendency to break.
4. A differential spool does not have clutches, therefore, neither a question of wearing out of clutches arises and nor this will need adjustment. 

Disadvantages of using Spool:

1. Spools should not be used on the vehicles which are run on street, as this will cause the tyres to wear out prematurely and one of the tyre will skid while turning, as the speed of both the tyres is maintained constant.
2. Using spools on street vehicles also reduces the rear axle shafts life considerably.

Difference between Differential Full Spools and Mini Spools

Full Spools:

The differential spools as explained above are commonly known as Full Spools. The full spools can further be characterized as Light weight full spools and Ultra Light Weight Full Spools. As mentioned above, one of the reasons, apart from saving money, why people go for  full spools is to reduce weight as a result of which industry drilled some extra non functional holes in them to shed their weight and these full spools started being called light weight full spools. The growing demand of racing enthusiasts forced the industry to think and further and come out with a further lighter version of full spools wherein the extra area close to the holes, fitting the ring gear, was scalloped. These spools are known as ultra light weight differential spools.

Mini Spools:

While the full spool replaces the differential case, the function of a mini spool is to replace the spider gears and side gears / pinion gears in the differential case. These mini spools also come in two different designs. For Ford applications, the mini spools normally come as one piece but in the case of GM applications, they come in four pieces i.e. two parts with internal splines ( to replace spider gears) for axle shaft splines to fit and two slider blocks to replace the pinion gears. The purpose of full spool and the mini spool is the same i.e. to lock the axles together. In simpler words a mini spool is something which is inserted in the stock carrier to replace the spider gears and lock up the differential.

A mini spool drives exactly like a full spool does, but they are not as strong as a full spool and do not add any extra support to the ring gear like a spool does. In terms of price also mini spools are far cheaper than full spools.

Emmbros is manufacturing a comprehensive range of Full Spools and Mini Spools. Pls. do check for your requirement in product catalogue section.

Noise is usually the first sign of trouble in the drive axles or differential. The kind of noise you hear can help you determine what is the exact cause of the trouble. But we should how to determine the same. The noise can be a humming noise, a growl or a knocking noise.  Also we have to make sure that which activity causes the noise. i.e. The noise is produced when the vehicle runs straight , or noise is produced when the vehicle is taking a turn. You have to note whether the noise is louder when the engine is driving the vehicle or when the vehicle is coasting. Ring and pinion gear noise changes with vehicle speed and is usually unaffected by loads and turns. It is difficult to diagnose the drive axle and the differential noise by running the vehicle in the shop with the wheels raised, road testing is the best method to diagnose such noise in the vehicle, sometimes the noise can be heard while running the vehicle on a chassis dynamometer.

Spline Count:

Splines are teeth which are made on the drive shaft / axle, which matches to the teeth in the propeller shaft. The no of the splines is same so that the axle shaft does not slip, when the vehicle is moving. The larger the spine count and larger will be the diameter of the axle shaft

Pressure angle

The pressure angle is defined as the angle on which the splines are made on the axle. The better the pressure angle the better is the spline and the durability & use of the axle. The splines are made of 45 Degrees angle which means that the base of the spline is wide the cut of the spline is also shallow. Had the spline been cut at 30 Degrees angle it would have meant a deeper cut and a narrower base, which means that the spline is not strong, and thus the resulting axle’s performance will not be better. The axles which are made with an spline angle of 45 Degrees are comparatively much stronger and better in performance for race cars.

Involute and Square cut splines:

The splines are made of involute design which means that the splines are slightly curved so that the contact is good and the pressure distribution is equal when the axle rotates. Some axles are however made with Square Cut Splines, the disadvantage of having a square cut splines is that it creates a high stress , and results in premature spline failure.

Spline Length:

The splines of the axles are neither too short and nor too long, as it effects the durability and the function of the axle shaft. To cater to the needs of racing car industry, the concept of cut to length axle shafts is also quite prevalent. Cut to length axles means the spline length is made quite long say anywhere between 4” to 11” so that the axle can be cut to the desired spline length and used in vehicles accordingly. Pls. do ask for the range of cut to length axle shafts from Emmbros.

Cut splines Vs Rolled Splines:

Traditionally the splines of axle shafts used to be cut on spline hobbing machine but nowadays the latest technology is of cold rolling of splines. Cutting splines cuts the grain flow thus making the axle weaker whereas cold rolling the splines means there is no cutting of steel. The splines are simply pressed thus making them 35% stronger than cut splines and hence increasing the life of axle shaft. Cold Spline rolling does have its limitations and that is why all axle shaft splines cannot be cold rolled. Emmbros proudly makes axle shafts with cold rolled splines since more than last one decade.

The differential gears transmit torque received from the transmission output shaft to the differential. The gear set is made up of a smaller driving gear or pinion gear and a larger driven gear or ring gear. The smaller gear in a gear is always the pinion  gear.

Rear drive axles use hypoid gears. Hypoid gears have teeth cut in a spiral form, with the pinion gear set below the centerline of the ring gear, this lowers the driveshaft, which allows a lower floor pan and driveshaft tunnel. It also allows more teeth to be in contact to carry the load.

The ring gear is three to four times larger than the pinion gear. When the pinion turns the ring gear, it reduces the speed of the axle shafts while increasing the torque applied to them.

The pinion gear connects to the rear end of the drive shaft and is assembled in to the front of the axle housing or differential carrier. The ring gear attaches to the differential case. The differential side gears are splined to the inner ends of the axle shafts. Rotation of the ring gear rotates the differential case.