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Comment by Kirby64 | Hacker News Reader

How does FWD vs RWD matter for energy recapture in regen breaking? As long as you're not losing traction on your rear wheels during breaking, there's no difference in terms of energy captured in FWD vs RWD.

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"As long as you're not losing traction on your rear wheels" is the key part.

The more you brake, the more weight shifts forward, which results in less traction available to use to recapture.

People vastly overestimate how much traction is available at the rear during braking, and running out of traction at the rear typically results in the car spinning (especially in an EV with low rotational inertia with most of the weight between the axles) so regen tends to be very conservatively tuned to not cause rear lock up, thus using even less of the little traction available.

This is obvious to anyone who's changed the brakes on their car. Front brakes are massive compared to the rear.

Or used any vehicle with independent front/rear braking like a motorcycle or bike. The rear is for fine tuning while the front is for actually stopping.

They're both for stopping. It's why they're biased to have more power in the front than the rear. You can take advantage of this with the rear to "tune" in some situations, or to maintain stability at low speeds, but it should also be fully used when braking.

Also anyone who has ridden a bicycle.

In that case, leaning back makes a surprising amount of difference.

Also people who ride dirt bikes- the back will slide out fairly easily compared to the front (but using the front exclusively comes with some pretty big tradeoffs)

...or to anyone who ever rode a bicycle or motorbike with front and back brakes. Pull (bicycle) or press (motorbike) the back brake hard and the rear wheel tends to skid. Pull the front brake hard and the thing wants to somersault. The back brake is needed to stabilise the vehicle but the front brake does the real work.

Not on more modern cars. In an emergency stop situation the front brakes still do the work, but in normal more gradual stop situations traction control biases to the back wheels as much as possible so the front wheels can steer.

Regen breaking never comes anywhere near the traction limits though. I don't follow the point.

Rear-wheel energy recapture is surely fine so long as you are braking normally. Similar to driving a rear wheel drive car and just using engine braking.

Yes, when braking strongly rear wheels alone certainly don’t have enough traction: signs of strong front wheel braking are the nose of the car starts to dip and your passengers are complaining! Similar issue with slippery surfaces and no ABS (regenerative traction control)?

I would be interested to see some real-world figures of how often rear-wheel-only regenerative braking hits limits and wastes power.

In addition, are there limits to the amount of power regenerative braking can push into the batteries? I presume that even on front-wheel regenerative braking, when heavy braking occurs the front calliper is engaged and energy is lost. The theoretical difference between front and rear regeneration might not matter in practice?

Disclaimer: I don’t know much about this, and I suspect you are technically correct, but my guess is that it doesn’t matter in reality, and other constraints/costs/benefits matter more for deciding on front vs rear wheel drive.

Rear wheel regenerative braking (with a decent traction control system, which is table stakes) is fairly effective, but the down side is reduced life span of the rear wheels.

The front brakes do most of the work. It is not the case, as you assume, that both front and rear contribute equally to braking. If you didn't use the front brakes, or even if you tried to have them apply the same force, the rear wheels would absolutely skid.

If you want an extreme visual example, check out stunt cyclists who brake so hard the rear wheel comes off the ground. Pretty easy to see that the rear wheel isn't contributing to braking when it's in the air.

Bingo. When I was a kid, I once flipped my bike because I was only braking with the front. I learned not to do that, but to an extreme — I then braked almost always with just the back.

It wasn't until I got older and faster that I realized that braking just in the back is not as effective, since the braking action makes your nose dive and puts more weight over your front tire. I now brake with both sides, but always first in the back.

My commuter bike only has a front brake. You can shift body position to make it pretty much impossible to go OTB. Meanwhile my DJ bike only has a rear brake, which at best "strongly suggests" you slow down.

Why would a commuter bike (or any bike) have only a front brake?

Having only front brake is almost common for fixed gear bikes because they can break rear by retention and fixie counts as a commuter bike.

Also it might be a decent config for learning some BMX tricks because if you want to be able to do a barspin you can not have both brakes.

And if you have a bicycle with a coaster hub and a front brake and your chain is off than you are effectively having only front brakes.

Ironically the law pretty much everywhere in north America mandates that bikes have at least a rear brake.

The trick is to lean your body backward while braking.

Unless you are in a big rush to stop, you can keep the balance pretty even between wheels.

In cars that only have rear motors, unless you apply the brakes (or they have some blended braking setup) then ALL regen is via the rear motor.

Obviously, front brakes are capable of more braking, but the limit at the end of the day is the regen of the battery itself and how much the inverter can handle regen. RWD-only EVs are already capable of ~75kW of regen. Moving to FWD to try to gain regen is only marginally going to give you better efficiency, if anything. It only matters for harder braking.

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