Differential Preload

rogdini

Hello all, not many physics-related threads so far :p

According to default hdv files, Differential Preload is a preload torque that must be overcome to have wheelspeed difference.

Here's how I understand it: When torque resistance difference between wheels increase from 0 and keeps below DiffPreload, both sides will remain at same speed, so the differential is working like a rigid axle. At this point, I'm assuming this torque difference is transferred from one wheel to other, instead of simply "braking" the lower torque wheel, which would result in an actual loss of input torque.

When torque difference reaches DiffPreload, speed difference occurs and when it's just above 0, my guess is that torque transfer between sides is kept at this DiffPreload, rather than falling to 0. So that, DiffPreload is applied at whole range of speed difference as a constant factor, instead of only at 0 rad/s, (it only takes into account the sign of this difference). Then, the other speed- and input-dependant torques are added to this.

As I'm much better in explaining things using drawings rather than english language, check out these graphs.
View attachment 133

I may be misunderstanding this behaviour, is it B or C? (any D?). B case makes sense for me since DiffPreload is usually a consequence of a friction and, although it may be lower when it's kinetic than when it's static, they should be close anyway.
 
I'm surprised no one ever replied to this. There are loads of wrong explanations all over the net about diff settings and especially preload on sims. This was an opportunity to clarify things without falling in two extremes: that either it's "the speed of transition between power and coast" (which is well spread but afaik completely wrong), or picturing by complicated clutch formulae etc.

Your graph B is the best repr. I've seen in quite a while since it's simple yet it presents a simplified picture of how the preload works (i.e., fixed axle until wheel speed difference is > x ).
 
It should look like figure B, but the x-axis should be input torque. And the torque transfer should have the same sign across the axis, for a given speed difference sign

For input torque>0 (engine on power) the curve should start at preload and continue with a slope corresponding to the "power" setting.
For input torque<0 (engine braking) the curve should start at preload and continue with a negative slope corresponding to the "coast" setting.
 
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