© 1997-2008 H. Pietschmann
|4WD only works good on level ground
why you find me here using the word torque rather than power
Remember: On level equally conditioned ground traction on all 4 tires is equal.
2 components are mainly involved in creating traction:
1.) A specific friction coefficient between two materials (rubber and asphalt, rubber and dirt, rubber and mud, rubber and mud, etc. - that number varies from 1.0 to 0.1 - 1.0 is best) -
2.) the weight pressing the tire to the ground (more weight is better).
More rubber on the ground can be an advantage, but generally more weight per suqare inch is better.
On pavement and other level ground all 4WD systems generate equal torque at all 4 wheels as long as the entire surface is identical (because traction on all 4 wheels is equal). All four tires rotate equally. Full time 4WD, part time 4WD, 4Hi, 4Lo - doesn't matter they all do the same thing.
The biggest challenge for a 4WD is when the terrain gets substantially uneven (off-road of course but also when approaching a steep driveway diagonally from a level roadway).
Lets say one wheel (left front) is driven into a 1 ft deep ditch or depression. Down travel of that wheel indicates that the spring is relaxing. A relaxing spring leads to the assumption that that wheel carries way less than 25% of the vehicle weight. Right? The opposing wheel will carry the difference = its spring compresses more. Less weight = less traction (resistance) - more weight = more traction (resistance). The differential will react to the situation and will make the wheel with less resistance (traction) slip and spin. The front left wheel. The torque generated is limited to twice the amount that can be generated at the wheel with less resistance. That will be very little torque. See previous page for more detail.
Unfortunately in this case, no matter whether you have a part time 4WD or a full time 4WD with locked center diff, the rear axle will not be able to help you with more torque.
When the left front of the vehicle moves into a ditch or depression (with a diff action described above) the left rear of the vehicle moves up and the right rear tire down. Again: A relaxing spring leads to the assumption that that wheel carries way less than 25% of the vehicle weight. The opposing wheel (left rear) will carry the difference = its spring compresses more. Less weight = less traction (resistance) - more weight = more traction (resistance). The differential will react to the situation make the wheel with less resistance (traction) slip and spin. The right rear.
And the front left is already spinning. Two slipping/spinning tires = you are stuck.
There is a rule here: Any suspension action on one corner of a vehicle will trigger a similar action on the diagonally opposed corner. No matter what suspension system you have. This is most pronounced in short wheel base vehicles (Jeeps) and less serious on long wheel base vehicles (pickups). A somewhat flexible frame will also lessen the effect - but can't avoid it completely.
So again, the rule is:
Compress front right - rear left will react similar.
Relax front left - right rear will react similar.
You are stuck because the amount of torque that can be generated at both axles is each limited to twice the amount of torque generated at the wheel with less resistance. So you end up with four small amounts of torque. Combined they are just not enough to move the car.
A limited slip in the rear axle diff can help avoid getting stuck a little. LS somewhat postpones the start of the right rear wheel spin (by adding a pre load to the diff more torque can be generated at the wheel with less resistance). So, as the name says, limited slip limits the slip but it does not prevent it.
The only way to maintain uninterrupted forward movement in unequal traction situations, is by using axle differential locks (factory on some, like Jeep Wrangler Rubicon, Mercedes G500, Dodge Powerwagon, Hummer etc - or aftermarket units like Detroit Locker, ARB etc.). Axle diff locks are disabling both differential funtions: To allow unequal wheel/shaft speeds and to distribute torque equally. After applying the lock, the differential will have both wheels/shafts rotate at equal rpm and the wheel/shaft with the most resistance (traction) will generate a higher amount of torque than the one with les traction. Torque distribution is not equal any longer. This can result in very high torque loads on connecting parts (up to 100% of the entire vehicle torque). To deal with the added stresses, vehicles with factory lockers have very strong axles and drive line components. If an aftermarket locker is added, it is highly recommended that the axle components are upgraded.
Several automatic traction management systems are available to manage diagonal wheel spin . There are mechanical (Torsen), mechanical/hydraulic (Gerodisc) or electronic (ETS, Haldex) traction management systems. However, they are not as capable as a manually locked axle diffs. That is why the best 4x4 on the market today offer a combination of automatic as well as driver activated traction management (Porsche Cayenne, VW Touareg, Mercedes G500, Jeep Wrangler Rubicon, Mitsubishi, Toyota etc.)
But wait, it can get more difficult than this
| 2WD use of traction | 4WD layouts | crawl ratio | 4WD components |