Traction changes on uneven ground Diagonal traction loss Whenever one of the wheels on a car leaves its level position (up or down) the diagonally opposed wheel will react similarly. This is most pronounced on vehicles with short wheel base - the longer the wheel base the less drastic the diagonal reaction. All axle and suspension designs react the same way. It has more to do with the rectangular vehicle platform than with suspension systems. However, vehicles with long suspension travel are in less danger to lose traction. Here is a basic vehicle model displaying what happens when one tire would drive up a rock. In relation to the frame (and the body) the front left drops down. It carries less than 25% of the vehicle weight (note the gap between axle and frame) resulting in less traction. Remember the weight factor of traction. The right rear, diagonally opposed from front left, has also dropped down. Not quite as much as the front left but clearly visible. It carries less than 25% of the vehicle weight, resulting in less traction. Both front right and left rear now carry each more than 25% of the vehicle weight. You would get an identical reaction if the front left would be driven down into a ditch. Moving down as an individual translates to carrying less weight resulting in less traction. The right rear wheel will also move downward - in fact it just stays on the ground but rather the body moves up. But the result is the same. It carries less weight resulting in less traction. Both front right and left rear will then carry the bulk of the vehicle s weight. Here is the graphic display of the situation:
Since the front left wheel is down it carries less weight resulting in less traction - 25 units. Following our initial example, requiring 140 units of torque to move the vehicle, each wheel received 35 units. Now with one wheel up on the rock 2 wheels developed less traction. In fact not enough to support the torque. Accounts overdrawn! Front left and right rear are spinning. As bad as it sounds, but spinning tires could be good. Read why. Since the tire is still on the ground it still has some traction (only a tire up in the air would have no traction). 25 units - which could support 25 units of torque. So, even though there are two wheels spinning there is still some torque available. Since the differential always distributes torque equally the two wheels with good traction also get only 25 units of torque each. Total torque generated in this situation is 100-140 are needed, so the vehicle is not moving anymore. If the vehicle is equipped with a differential lock, and the driver activates it, the differential s ability to split torque equally is disabled. Now the available traction of 100* at the left rear wheel can be used. It is sufficient to support the 65 units of torque needed. The vehicle will move again. Even better, the driver could activate the diff lock proactively and avoid to get stuck in the first place. An additional front lock would be bliss. An differential unlocked differential is called an open differential. *) Actually traction on the wheel carrying more weight increases - but I wanted to keep it simple here. If the vehicle is equipped with a limited slip it won t be able to master this particular situation. A limited slip differential creates additional resistance so a little more torque can be dealt with. 35 units in this case. Creating 35 units of torque on each wheel at the axle with LS. The total of 120 units of torque is not sufficient to move the vehicle. If the rock would have been less high,traction would not have dropped as much and the LS may have been able to keep the vehicle moving.
Here is another method to deal with the situation: Just move a second rock, or a pile of small rocks in front of the left front tire and have both wheels climb equally, keeping the vehicle level - thus keeping traction equal. If no building material is available - use your spare tire as stepping stone On level ground traction on all 4 tires is equal. On pavement and other level ground all 4WD systems send equal power to all 4 wheels as long as the entire surface is identical (then 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). 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) spin. The front left wheel. 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. Why? When the left front of the vehicle moves into a ditch or depression (with a diff action described above) the right 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 and make the wheel with less resistance spin. And the front left is already spinning. Two 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 vehicles (Jeeps) and less serious on long wheel 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. 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). So, as the name says, limited slip limits the slip but it does not prevent it. The only way to maintain uninterrupted forward movement, when the suspension is responsible for traction changes, is by using axle differential locks (factory on some like Jeep Wrangler Rubicon, Mercedes G500, Hummer etc. - or aftermarket units like Detroit Locker, ARB etc.) Several automatic traction management systems are available to avoid 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, Mercedes G500, Jeep Wrangler Rubicon, Mitsubishi etc.)
How weight shift changes traction Torque applied is 70 Each tire carries 100 units of weight Friction coefficient is 1.0 (asphalt - perfect) As long as the vehicle travels on level ground a traction safety of 65 units remains Torque applied is 70 Right tire carries 25 units of weight Left tire carries 175 units of weight (it has to carry the difference) Friction coefficient is 1.0 (asphalt - perfect) Once one of the tires drops into a depression or rut or ditch it will carry less than 1/4 of the vehicle weight. Since weight is an important component of traction, traction goes down. in this case so much that it can not support the amount of torque. The tire will start spinning. The perfect traction on the left tire (it has actually increased with the weight shift) can not be put to use because the differential always splits torque equally. The maximum torque value is determined by the wheel with the least traction. The weakest link determines the outcome of the torque/traction challenge. Only traction management, like differential locks or limited slip can remedy the situation.
Here is a variation of the theme You are coming up a steep side trail to enter a main trail going right. The main trail is not very wide. So, you decide to enter at an angle. You might lose traction by doing so. Your front left as well as your right rear tires might lose traction due to weight shift. With an axle diff lock no problem of course. I used my trailer to give you a better visualization Gaps between axle and frame at front left and right rear indicate less weight and traction loss. This front view shows the interaction between frame and axle very well. However, there is one advantage of the diagonal approach: The frame stays clear of the edge. The ramp angle is less than if you would go up straight
For perfect traction you would need to go straight up All four wheels are level, all carry an equal share of the vehicle weight. Perfect traction. The front view demonstrates that axle and frame are horizontal. good for traction. Disadvantage of going straight up is that the frame might scrape the edge - you might get high centered. If you try momentum you might get stuck right in the center of the vehicle. Creating too much additional drag so that your tires can not pull you free. Since now much of the vehicle weight is supported by the frame resting on the crest - the tires carry less weight and have thus less traction. You are stuck. Don t use momentum. You may have to find an angle betwen straight and your first approach. Maintaining enough traction but also giving you clearance under the frame.