FROM: Benjamin Lee SUBJECT: Re: 4runners and differentials DATE: Sat, 20 Mar 1999 15:18:05 -0800 ORGANIZATION: IBM Global Services - Remote Access Mail & News Services NEWSGROUPS: alt.autos.toyota,rec.autos.4x4 Andy Voelkel wrote: > My first reaction to all of this was to question why anyone would make a 4x4 > without at least a limited slip rear differential. After all, wouldn't you be just > as well off with 2wd and a locking differential? But after thinking about it, it > make a little more sense, because of weight distribution. > > If you had a rear wheel drive with a locking differential, your traction would > be comprised on level ground because there is much more weight at the > front of the vehicle. Backing up uphill would be particularly difficult. On the > other hand, if you had front wheel drive with a limited slip, traction would > be compromised when driving up a very steep grade, since weight will > transfer to the rear wheels. So maybe the car makers are as stupid as I > first thought. At least one of the two driven wheels is always guaranteed to > be where there's some weight on it. Car makers cannot be that stupid for them to stay in business for so long. Your reasoning makes sense, and also in a 4x4 with open differential, the non spinning wheel does put out some force to pull the car forward. Just that it is not pulling hard enough to move the car so you see one wheel spin while the other stays still. To equate the two vehicles you mentioned, 4WD with locked center diff. and open axel diff. compare to locker 2WD, lets assume one side of the vehicle is on ice and the other on pavement. If the traction on the spinning wheel is at least 1/3 that of the non spinning wheel then 4x4 with 2 wheels spinning will pull harder than a 2wd spinning 2 wheels with a locker. Here is the prove: A third the torque of the high traction wheel times 4 wheels for a 4WD is 4/3 the torque of the high traction wheel. This is because an open differential has a 1:1 torque bias meaning both sides will pull equally hard. Compared that to the locker 2WD which has 1/3 traction on the spinning wheel and complete traction on the wheel on pavement. This also give your vehicle the tractive power of 4/3 the torque of the high traction wheel. Now if all wheels are on equal traction surface, then a 4WD will get 4 times the traction of each wheel whereas the locker 2WD will only get 2 times the traction of each wheel. Going back to the original question. for a vehicle with open center, front and rear differential with one side sitting on ice, for the front axel, the total tractive torque is 1/3 + 1/3 = 2/3. Since the front axel is getting 2/3 the torque, the cent. diff. will proportion 2/3 of the torque to the rear axel also (1:1 torque bias remember). Now here comes the problem. No matter what traction you get in the rear even if both wheels are on pavement, the total torque for the rear axel will still be 2/3 that of the high traction front wheel because of the torque bial of the center diff. The total torque is still 4/3. Conclusion: 4WD w/ 3 open diff.: total tractive torque = 4 times that of the lowest traction wheel. 4WD w/ 2 open diff: total tractive torque = 2 times torque of the lowest front traction wheel + 2 times torque of the lowest rear traction wheel. 2WD with locker: total tractive torque = tractive torque of left + tractive torque of right wheel. Excuse the long reply, but I've been researching differentials lately trying to decide which one is the best.