One of the most overlooked and abused parts on your car is most likely the differential. On most cars it is not easy to remove and simply rebuilding it doesn’t always make it better. If you are having problems with traction in corners, a good place to start finding some more grip is your differential.
Wondering what the diff does? In short, a differential allows your wheels to rotate at different speeds. When making a turn, your outside wheel has a further distance to travel than the inside wheel. The diff makes it possible for the wheels to rotate independently and your car to rotate instead of skid around corners.
There are five major components to a ball diff, the gear, diff rings/balls, thrust assembly and outdrives. Each part is very important to the diff function. One bad component can and will make it function less than optimally. The most important parts are the diff rings/balls and thrust assembly. When replacing these, make sure to replace both the balls and rings at the same time. A bad set of rings will make a brand new set of balls not function properly and vice versa.
The most confusing part about a diff is adjusting it properly. There aren’t any set-in-stone rules for adjusting a diff. Different surfaces and driving styles will get optimum performance from different adjustments. First off, lets get one thing very clear. Running your diff too loose or “barking” it is very, very bad! I cannot stress this enough. Sometimes barking your diff just once, will ruin it. If you are unsure of your adjustment, err on the tighter side. If you are starting with a freshly built diff make sure to break it in properly before adjusting. Lets get down to the good stuff…
Build or Rebuild:
- Start with brand new components if possible.
- If re-using parts be sure to clean them thoroughly with motor spray. Do not leave old and dirty grease in your fresh diff.
- Visually inspect your diff and thrust rings. If you see any grooves or pitting its time to replace them.
- Sanding your rings is good practice. I like 400-600 grit sand paper. Use your outdrive to hold the ring in place and sand on a flat surface, preferably glass or granite countertop. For thrust rings just use your finger. This is a good method to bring used rings back to life. Make sure to remove entire groove from the rings.
- Use plenty of grease in your diff. I like to pack the thrust assembly with Team Associated black grease so the balls are completely encased in grease. I put a thin coating of Team Associated diff lube on each side of the diff gear once balls are in place. (note: DO NOT use black grease on the diff balls. This does not supply enough friction and will cause your diff to slip.)
- re-assembly and wipe off any excess lube overflowing from the outdrives
- Keep your diff a little on the loose side for break in. Too tight may cause premature wear. Be sure to not drive your car or adjust your slipper with diff loose.
- With diff installed back in your car, grab your radio and turn you car on.
- Hold one wheel off the ground and apply 5% throttle. No need to get the wheel spinning very fast, just enough to keep it rolling.
- Hold this for 20-30 seconds, switch wheels, and then repeat one more time for a total of 2 spins on each wheel.
- Now it is time for diff adjustment
Everyone has little tricks on how to adjust their diffs. Ryan Cavalieri showed me how he does his a while back and it stuck. The two things I like about his method are ease and consistency.
- Make a mark on the inside bead of your rim (many racers have directional arrows already. These work great for both purposes)
- Hold your spur gear with one hand and give one wheel a spin with the other. If you are wondering how much, well, this is hard to describe but the best thing to do is to be consistent with the amount of muscle you use. Its not a contest to see how many rotations you can get so a quick flick will work great.
- Keep an eye on the mark on your wheel while doing the above. A good starting point is one full rotation. Look for that mark to start and stop in the same area.
- Adjust as needed and repeat until you get to your desired adjustment
Once you get this method down it will take a matter of seconds to get your diff properly adjusted and more importantly, keep your tune consistent from race to race. Slipper adjustment would be the next logical thing after you get your diff set up. Make sure to start with your slipper loose and work your way tighter. A locked slipper can make a diff bark no matter how tight it is.
Tuning options (2wd vehicles):
Wondering whether to run your diff tighter or looser and how does it affect the handling? Good, you are asking the right questions.
Looser diff- Allows car to rotate easier around corners and will give you more low speed steering. If your car is sliding sideways in corners or you just cant seem to get through the 180’s very good, loosening your diff might be a good idea.
Tighter diff- More forward bite and traction coming out of corners. If your car is spinning out easy under acceleration or going around corners, a tighter diff may be what you’re looking for.