Technical Part 1 - Open Differentials & Locked Axles
Before describing how useful limited-slip differentials can be, and how they differ from open differentials, we should start by describing how open differentials work and their limitations. Open differentials are fitted to most road cars and allow a difference in wheel speed when cornering. They do this by splitting the torque evenly between both wheels at all times. In a straight line this means both wheels have even amounts of power applied to them. During cornering the outside wheel of a car has further to travel than the inside wheel, which means that more power is applied to the outside wheel.
A few things to remember:
An engine produces torque, not power.
Power is measured by how many times per minute (RPM) the engine is capable of producing that torque. HP = (Torque in FtLb * RPM) / 5252
Power in HP = Torque in FtLb at 5252 RPM
Gearing is used to increase the amount of torque available at the wheels. Power at the wheels remains the same regardless of the selected gear.
For a vehicle to move from stationary there must be enough torque at the wheels to overcome its ‘moment of inertia’ (resistance to movement).
In order to accelerate there must be enough torque at the wheels to overcome rolling resistance, air resistance and the moment of inertia.
You can only put down as much torque as the road surface allows.
Wheel spin occurs when torque at either of the wheels exceeds the grip available.
Open differentials transmit torque evenly, and they can only transmit as much as the wheel with the least amount of torque. They do allow for differences in speed between the two wheels. On perfect surfaces with unlimited levels of grip this works well.
For example, if the axle speed is 700 RPM and there is 100 FtLb of torque at the axle, there will be 50 FtLb at each wheel. When turning, the outside wheel is at 800 RPM and the inside wheel is at 600 RPM. This means that the power split is 57% outside and 43% inside.
In this same scenario the inside wheel lifts in the air under cornering, this effectively means the only resistance to that wheel is from drivetrain drag. Let’s assume that this wheel is only capable of transferring 1 FtLb of torque, which also means that only 1 FtLb of torque is able to be transferred to the outside wheel securely on the ground. In this situation the excess torque will only increase the engine speed, and in turn it will increase the speed of the axle. The outside wheel is unable to increase its speed as it cannot apply enough torque to the ground to speed up. However the inside wheel can speed up as the torque being applied to it overcomes the resistance. At this point the outside wheel remains at 800 RPM, axle speed has increased to 1100 RPM and the inside wheel is at 1400 RPM. The power split between the wheels is at 36% outside and 64% inside. With 1 FtLb of torque being transferred to each wheel, there is still minimal power to each wheel and not enough to maintain speed. Without the ability to put power down, the vehicle’s weight gets transferred forward, creating even less grip at the rear of the car. On the road this can hinder safety, and on the track this leads to increased lap times.
The simple explanation is that in an open differential, power will always go to the wheel with the least resistance. So when a wheel is in the air, or on ice, mud, gravel or any low friction surface, the power gets put through the spinning wheel and not the one with grip. With enough power, wheels can spin even with good grip, resulting in the power getting sapped through the spinning wheel. During cornering, the weight transfers off the inside wheel and to the outside wheel. With less weight on the inside wheel, that wheel has less grip and is more likely to spin when applying power.
Locked differentials on the other hand work by forcing both wheels to spin at the same rate, but they do allow for an uneven distribution of torque. 100% of the torque can be transferred to either wheel, which allows for the maximum amount of torque to be put down at any one time. However it has severe effects on handling.
With both wheels being locked together during turning you experience hopping, skipping, and screeching from the tyres as they fight to maintain traction with the road. This causes heavy tyre wear and puts strain on driveshafts. During braking you get more stability in a straight line as both wheels are going at the same speed.
Locked centre differentials are commonly used in 4x4’s, but when driven on the road this puts strain on the drivetrain, as there should be small differences in speed between the front and rear axles. Without these differences the chassis experiences ‘wind up’ and can cause damage.
Fully locked differentials are never used in front-wheel drive vehicles due to the negative effects on handling. They will suffer from severe understeer on early to mid-corner turn in. Torque steer can also be a problem, even on low throttle inputs.
Rear-wheel drive vehicles have issues with understeer when turning, as both wheels want to stay in a straight line and resist making a turn. However when you turn sharply enough, both wheels will lose traction and start sliding, causing oversteer. During acceleration a locked differential will make the car more likely to oversteer, which can be unpredictable and hard to control.
Locked differentials are made by either using a spool, mini spool, or welding an open differential. Spool differentials are made from a single piece of metal, making them lightweight, and very strong. Mini spools replace the gears in an open differential, however they are not as strong and have a similar weight to the open differential they are fitted to. Open differentials that have been welded weigh more, and are only as strong as the weld. If the weld fails it can cause expensive and extensive damage to the rest of the differential.
When compared to open differentials, locked differentials are able to put far more power down. As mentioned earlier, open differentials are only able to transfer twice the amount of torque which the wheel with the least amount of grip is capable of transferring. Locked differentials can transfer all the torque from both the left and right wheels combined. In certain conditions this can be a significant increase in torque transferred to the ground, and will make the difference between moving or not when in low traction conditions.
We would not recommend locked differentials as limited-slip differentials can achieve the same level of lock under power. However they are much more controllable and allow for an increase in cornering speed. To find out more about the different types of LSD’s please click here.