Differential type & ratio for Regen braking

[kevpatts]

Hey,

I have to replace the small case diff in my E30 as it doesn't handle much above 150bhp. The medium case diff handles much higher, and has LSD, but comes in two ratios: 3.91 & 4.10.

I'm looking to use Regen braking with a GS450h. Does the diff ratio make a difference to Regen? My engineering brain says it shouldn't, that your still recovering the same number of watts of kinetic energy, but I thought I'd ask cause an extensive google gave me no results.

Kev

[EV_Builder]

Well efficiency is the thing and top speed / acceleration choice.

Did you calculate top speed from GS450h rpm to wheel speed and tires? I would first focus on that and then on regen / inertia..

[kevpatts]

Well efficiency is the thing and top speed / acceleration choice.

Did you calculate top speed from GS450h rpm to wheel speed and tires? I would first focus on that and then on regen / inertia..

I believe the attached calculation is correct. Based on unlocked MG1 & MG2.

[EV_Builder]

150km/h i think you found your answer then regarding the diff right?
Because if you mount 4.1 it will be lower and the rpm's used are the maximums and you also need voltage in your batteries to reach rpm's. So i would guess 3.91 is your ratio.

Maybe other can jump in with their opinion too.

[kevpatts]

Thanks, but I wasn't thinking mainly about the max speed, but I was more thinking of mechanical stresses through the diff. My thinking was that most diffs are designed to transfer significant load from the prof shaft to the axles in the forward direction. Regen will put significant load in the opposite direction (will stress the teeth of the diff gears the other way), and the ratio of the diff will determine how this gets transferred back to the regen motor.

I'm just wondering if this is something I should be concerned about or are diffs fine to handle this load? I guess I'm over thinking this

[rstevens81]

I think you are over thinking it.
Even in racing diffs don't normally get replaced as the stock ones are normally good enough, the only exception being when particular models have problems in stock form. (Early MK1 1.6 mx5 are am example where stock diffs would just sometimes just blow up in road use, however people still went racing with them as well).

The price of a replacement diff is normally scrap metal values as they basically don't break. (There are exceptions normally relating if they have LSD centres).

Torque and shock are what kills things... At the diff you are probably applying similar levels of torque, you wouldn't be dumping the clutch so less shock load.

As for regen you will be applying it in a controlled manner i.e no shock the torque levels are are alot lower than in forward direction.

download.jpeg (5.78 KiB) Viewed 1463 times

If you look at the ring gear stolen from Google ...
Although the teeth are directional for. Best contact area the ratio of the contact areas between the forward and reverse size will probably still be favourable compared to the torque in the forward reverse direction from the motor.

The main failure mode for a differential gear will be shearing the tooth off (tearing in the direction of rotation) which will have the same area in both forward and reverse directions as it is the area at the bottom on the gear tooth.

Replacing the diff oil will probably do a huge amount in terms of extending the life of the diff, as it's probably never been changed and the oil isn't expensive or hard to change.

[kevpatts]

Thanks for the detailed answer. I'll stop looking for things to worry about, I have enough already in this project!