Regenerative Braking and RWD...

[danielt1263]

Considering that some 80% of braking power comes from the front wheels and Slate is RWD, I can only assume that the regenerative braking capability will be rather minimal when compared to this. capability for AWD or FWD vehicles. Also, because of this, single pedal steering is unlikely. Is my understanding correct?

 

[cadblu]

You mean regenerative braking? lol.

I think the Slate team made it clear that regen braking will be “non-adjustable” and one pedal driving is totally possible for most driving situations. Of course, this doesn’t include evasive maneuvers, avoiding potholes, road hazards, other bad drivers, or those red lights that come out of nowhere 🙂 where you will need to rely on those 4 wheel disc brakes.

While it’s true that for most conventional vehicles, front brake pads tend to wear out faster than rears, I’m not sure this will be the case with the Slate. Given the ~ 50:50 weight distribution, I suspect that front / rear pads will wear quite evenly. We’ll need some real world driving experience to see how that goes. Keep in mind that EVs generally don’t require brake pad replacements until 70-100k+ miles.

 

[KevinRS]

There are a couple of reason that front braking is usually where most of the power is.
One is most of the weight is there on most ICE vehicles, that's where the engine and transmission are. Unloaded trucks even more so, they are designed to be balanced when loaded.
The second is once you start braking, that front end is pressed down into the road even more by momentum.
So manufacturers traditionally put heavy disc brakes at the front, and lighter brakes at the rear.
With EVs with regenerative braking, the batteries are towards the rear anyway, and by design, the regen takes most of the load, the disc brakes only come into play to finish the stop or when you need to stop fast. Unless you are doing a lot of stopping fast, there shouldn't be much wear.

 

[Johnologue]

There is still far more braking power in the front wheels due to forward weight transfer when braking (think of how a car "dips" forward as it slows down to visualize this).
50/50 weight distribution isn't actively contributing to that imbalance, but it's also not counteracting it.

Regen isn't for hard/immediate braking. Think of it as being more like "engine braking", where your car slows itself down when you aren't accelerating, especially when you shift into a lower gear. I think the Slate's rear wheels will have plenty of grip for that.

 

[ElectricShitbox]

If you wanted to fully maximize the amount of energy you can regenerate, then you would want to regenerate from the front axle in addition to the rear. But even with just a rear motor, it can provide enough regen braking to reduce friction brake usage while also recovering a useful amount of power. There's plenty of RWD EVs out there for reference.

 

[beatle]

Hopefully the Slate has a slightly more rear-biased weight distribution. The RWD Model 3/Y is 48/52. This extra weight in the rear will allow for more regen strength.

 

[Kopsis]

Considering that some 80% of braking power comes from the front wheels

That's at threshold (the point where any additional braking will cause the tires to lose traction). That doesn't mean you can't slow down significantly by braking just the rear (as most bicycle riders know from first-hand experience).

My RWD EV6 only has rear regen. It's perfectly capable of one-pedal driving. Keep in mind that one-pedal still uses the brakes because regen drag drops off as speed approaches zero. On a drive from Sequoia Nat'l Forest to Visalia I actually had a higher SOC at the end than the beginning from all the rear-wheel regen slowing for curves on the descent.

 

[agrobotics]

I'll have to get used to the feel, I'm sure. I am used to coasting to a slow speed in my ice cars.

 

[RagedCarrott]

Our RWD Ioniq 5 does just fine with regen and haven’t had any issues. It’s for sure not as aggressive as my dual motor R1T.

 

[Johnologue]

Hopefully the Slate has a slightly more rear-biased weight distribution. The RWD Model 3/Y is 48/52. This extra weight in the rear will allow for more regen strength.

If rear traction was a limiting factor for Slate's regen, 2% rearward weight distribution would not make a notable difference.

 

[Daemoch]

the axle suppling drag is where youll do ~99% of the 'braking' until you apply actual brakes. the last ~1% is air resistance and nominal drag any rolling assembly will have, braking or otherwise. The weight bias F/R plays into handling. It effects the breaking force needed to over come the drag coefficient of any individual wheel (ie - friction of the rubber on the pavement) of the tires (and technically the brake itself, but realistically youll already be locked and sliding before a disc brake gives out normally). It matters THEN because weight (and weight transfer) changes the breaking point for the friction formula. This applies to acceleration as well; same basic formula. Its why dragsters use larger tires (more contact area to divide the friction force product into, so more 'grip'). And as long as your wheels are all rolling (no lockups) during RB, the only axle providing any resistance in effect is the one with the motor(EV)/engine(ICE) attached, which in the EV here is the part normally equipped with the RB (though you could design all 4 as RB setups too technically...).

front/rear weight bias is more relative to handling and ACTIVE braking behavior, not RB efficiency. The math formulas for RB efficiency dont even care how many motors, which axles, or where they are located; its all irrelevant to 'how well it works' as long as nothing is past the rubber/pavement friction break point (locked up/skidding).

Adding more RB motors just changes how much rolling resistance the drive assembly can apply before it would start (literally) dragging tires while the other ones free-spin, like a semi with a blown air brake line, a car with a locked up hub bearing, or a car on ice.

https://www.sciencedirect.com/scien...n energy can,SOC) and the ambient temperature.

 

[Daemoch]

This one does less science and more real explaining. It also does touch on tire friction, weight transfer, and motor/RB aggressiveness effecting efficiency.

https://chargedevs.com/features/reg...oser-look-at-the-methods-and-limits-of-regen/

 

[beatle]

If rear traction was a limiting factor for Slate's regen, 2% rearward weight distribution would not make a notable difference.

A better (rearward) bias for weight will certainly give you more grip and more stability. You also want to tune regen to be consistent in all conditions, not just dry pavement. Having higher regen all the time benefits efficiency and (arguably) drivability. I have a ton more regen in my R1T than I did in my 2015 Model S (also dual motor). I almost never have to touch the brakes in the Rivian since it has such strong regen, where the weaker regen of the Model S meant I had to use regen a lot earlier and switch to friction brakes more frequently. This is less about weight balance though and more a point that doing everything you can to maximize regen is important.

 

[Daemoch]

Just curious because I have no idea - are those AWD? I could see the AWD being able to adjust the amount of RB being applied per wheel, which would explain the handling effects (just like TC or ABS basically). It could be done either mechanically or electronically.

Weight bias is situational. There is never a "best" design.

 

[Daemoch]

Adding more bells and whistles is also very much counter to KISS and low cost. I'm not surprised they didnt go that route at all.