Quote:
Originally Posted by Timee80
Why is everyone using really thick oil in the centre diff? Surely at 100k you aren't getting any proper diff action anyway
|
Its a great question. Partially the answer is people are familiar with a slipper (locked diff equiv) so a tight centre diff gives an expected behaviour.
For a more detailed answer you need to appreciate what a diff does (forgive me if you know this already)
Diffs on one axle are there to allow the wheels to rotate at different speeds to allow them to track different radius curves without locking /skipping. In a 1:1 car these are set quite loose so you don't wear your tyres out. The downside is that if you are in low grip or accelerating very hard a loose diff sends the drive to the wheel with the least resistance (grip) meaning you spin that wheel and the wheel with grip gets no drive. This is often known as diffing out.
Racing cars (and therefore our RC cars) are not bothered by tyre wear so much and prefer traction so we can increase the tightness of the diff so the wheels are more locked together and resist diffing out. The downside is that the wheels are less keen to rotate at different speeds and the whole car resists turning corners. This is why a rear diff often has a thinner oil so it still turns the corners well. The front has the steering to force the turning so can have a thicker oil or in the case of on-road cars a spool - or locked diff.
With a centre diff, under acceleration, the least traction is on the front axle because the front lifts so if you had a loose diff, all the power would go to the front, spinning up the front axle and not driving the rear wheels. This is a bad thing for fast acceleration so most centre diffs are run close to lockup to maintain the drive to the rear wheels but loose enough to put a little bit more drive to the fronts to pull the car out of corners. A bit like the overdrive pulleys that you can/could get from belt drive cars.
I have not driven the centre diff yet but on mega grip I would expect it to be run almost locked. I have heard in the states, some are using a million weight oil for high grip! On lower grip or a bumpy track, more front drive makes the car easier to drive so a lighter oil such as the 60k would be a good starting point