As I was trying to answer a question about diff oil yesterday, I started to realize that my knowledge in this particular area is quite lacking. Although I understand the basic principles of what increasing and decreasing the diff oil weight does (see afm
's post here
), I really don’t fully understand the mechanics and physics behind what is happening with the car itself. I hope you guys can help me get it straight and help educate everyone else as well.
I found some very helpful information that explains how an open diff works, but it didn’t cover everything:
As I understand it, the diff helps deal with the fact that when the car turns, the outside wheel has a farther distance to go than the inside wheel and therefore the outside wheel rotates more than the inside wheel while turning. The diff also controls how fast or slow the inside and outside wheels rotate relative to each other and how much power is applied to each wheel. It is also important to understand the weight transfer to the outside wheel and how much traction is at each wheel as a result of this weight transfer.
Selecting the proper diff oil has to do with traction of the track, tire shore, track layout (sharp turns and 180s vs flowing), body roll, being on-throttle and off-throttle and probably a few other things that I've left out. Diff oil selection also impacts how the car performs during corner entry, mid-corner and corner exit – and corning speed.
Lighter diff oil provides less internal resistance within the diff, so the gears inside the diff spin faster. Heavier diff oil provides more internal resistance in the diff, so the gears inside the diff spin slower. In a turn, using a lighter oil causes the outside wheel to rotate much faster than the inside wheel. As you increase the weight of the oil, you slow down how fast the outside wheel rotates relative to the inside wheel. When you use very heavy oil, the diff starts to act like a solid, so there is less "differential action" between the outside and inside wheel and they rotate closer to the same speed. As the diff oil gets heavier, you also start to lose some corning speed. With lighter oil, if one wheel loses traction, you get less power to the remaining wheels than if you use a heavier oil.
Fill in the blanks…
BTW… If you use the term “power to the ground”, please explain what that means relative to the rotational speed of the tire and the level traction.
On throttle, on a track with medium traction, when the rear diff oil is too heavy, the rear of the car may break loose and come around because (fill in the blank). Another reason the car may break loose is because the rear tire shore is too high.
If the diff oil is too heavy, you want to use lighter diff oil because (fill in the blank).
On throttle, on a track with high traction, when the rear diff oil is too heavy, the car will start to push because (fill in the blank). In this case, you want to use lighter diff oil because (fill in the blank). Another reason the car may push, is because the rear tire shore is too low.
If the diff oil is too light, (what happens?) because (fill in the blank).
The reason why you start to lose cornering speed with heavier diff oil is because (fill in the blank).
- Would you ever want the front and rear diff oil to be the same? Why or why not?
- Would you ever want the front diff oil to be lighter than the rear diff oil? Why or why not?
- Why would you want to use a front diff with very heavy oil instead of using a solid? Why not just use the solid?
- When the track temperature reaches a point that the tires start to lose traction, can changing the diff oil improve traction?
- Should you ever change your diff oil to deal with a traction-rolling problem?
Feel free to add any other info that you feel would be helpful. Ask questions as well!
Thanks for the help guys!