A differential is a wonderful thing. It allows two wheels on the two sides of the car to receive torque continuously, even if they are turning at different speeds. It is self adjusting, maintenance free, simple, and in normal driving, very effective.
However, nothing in the world of vehicle dynamics is perfect. Take the same differential off the tarmac, and put it on some very bumpy terrain, and a critical flaw arises. The same mechanism that allows a diff to perform so well on road becomes its Achilles' heel off road. An open differential is designed to distribute torque between the two wheels depending on the resistance each wheel is facing. When driving around a corner, the wheels on the outer side of the car travel a path that is longer than the path traveled by wheels on the inside of the turn. This requires a higher rotation on the outer wheels, than on the inner wheels. A differential allows this to happen while still distributing torque. However, it can be "fooled". While traversing uneven terrain, it is possible to encounter a scenario where one driven wheel is in contact with the ground, while the other is in the air. Now the differential distributes the torque based on the resistance to rotation. The wheel that is in the air has practically no resistance compared to the wheel on the ground. An open differential will, in this case, spin the unladen wheel with maximum torque, and no torque will reach the wheel with grip. In other words, you're stuck.
Of course, there are many solutions to this problem. More complex differentials exist that do not allow one wheel to receive 100% of the engine's torque. Such a diff, called a Limited Slip Differential, is common on off-road as well as track cars. It improves traction overall, and allows the vehicle to move forward even if one wheel is in the air. Another solution, that is cheaper and possible lighter, is a locking differential. It is an open differential that, on demand, can lock the left and right wheels together, resulting in a 50-50 torque split. Most purpose-built off road vehicles featuring one or more locking differentials.
An even simpler solution exists. The differential itself can be removed and replaced with a solid shaft, or a "spool". Such a system has its drawbacks, but also has a number of benefits. Firstly, not having a differential can be much lighter than having a diff, along with its casing and the necessary drive train to transmit power to the diff. Secondly, in a vehicle that is only designed to be driven off-road, losing the differential is a compromise that can be justified. While its true that the handling of the car changes significantly, it is possible to create a setup where the overall package gives higher performance compared to a car with a open differential.
That is where tripod-ing comes into picture. In case you're wondering what that is, here's a picture to explain it:
The car in the picture appears to be a Volkswagen Golf Mk3 VR6. Since this car has an All Wheel Drive system, it will not significantly benefit from lifting a wheel, especially on tarmac. However, if we imagine a rear wheel drive car with a spool system doing the same, the advantages can clearly be seen. Since both wheels always receive the same torque, lifting a wheel will make no difference to the amount of torque reaching the laden driving wheel. Further, the resistance to turning that happens due to the lack of a differential is also removed. And since dirt is a low traction surface, the quickest way around a corner is often to oversteer around it. Lifting a rear wheel can be used to help induce more oversteer, since the inner rear tyre is not providing any traction.
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| A differential with the drive shaft. Picture courtesy http://www.freewaytransmissions.com/ |
Of course, there are many solutions to this problem. More complex differentials exist that do not allow one wheel to receive 100% of the engine's torque. Such a diff, called a Limited Slip Differential, is common on off-road as well as track cars. It improves traction overall, and allows the vehicle to move forward even if one wheel is in the air. Another solution, that is cheaper and possible lighter, is a locking differential. It is an open differential that, on demand, can lock the left and right wheels together, resulting in a 50-50 torque split. Most purpose-built off road vehicles featuring one or more locking differentials.
An even simpler solution exists. The differential itself can be removed and replaced with a solid shaft, or a "spool". Such a system has its drawbacks, but also has a number of benefits. Firstly, not having a differential can be much lighter than having a diff, along with its casing and the necessary drive train to transmit power to the diff. Secondly, in a vehicle that is only designed to be driven off-road, losing the differential is a compromise that can be justified. While its true that the handling of the car changes significantly, it is possible to create a setup where the overall package gives higher performance compared to a car with a open differential.
That is where tripod-ing comes into picture. In case you're wondering what that is, here's a picture to explain it:
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| Lifting the inner rear tyre when cornering Picture courtesy http://www.stealthtdi.com/ |
There are multiple ways to lift the inner tyre when going around a corner. In essence, the rear of the car should roll less than the front of the car. This causes a diagonal weight transfer to take place, with the maximum weight being on the outer front tyre. As a result, the minimum weight acts on the inner rear tyre. If this effect is exaggerated, the inner rear tyre can lift off the ground. Adding a anti-roll bar to the rear wheels can help maximise this effect, by preventing the inner wheel from going into droop. This effect can also be achieved by designing a suspension geometry that doesn't allow as much roll in the rear wheels, as it allows in the front.
It is a package I am trying to develop for our SAE Baja team, so I thought I'd write a bit about it. Our ATV is designed to work in very specific conditions, and we can afford to compromise predictability for some speed. Of course, this may not work for mass market products.
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