What effect does electric-vehicle regen braking have when you’re hotlapping on a racetrack?
One of my readers watched the Porsche 911 Track Mistakes video:
and posed this question:
Nice video! It brings up an interesting question for EVs. I have a Tesla EV and wondering about leaving regen on when on a track. It is essentially like engine braking and activates as soon as you let off the accelerator pedal. And in a Tesla it is pretty strong to allow for one-pedal driving. I guess you could maintain some accelerator pedal and do left-foot braking, but that seems touchy as far as how much accelerator to give. In my car I can also turn off regen in “track mode” but all the videos I have seen leave it on because you burn up so much energy at the track. Let me know your thoughts and whether you have discussed this with any EV folks at the track.
An interesting question, so I’m answering here in a blog post after I’ve had a think about it and chatted to an automotive engineer with regen calibration experience.
“Regen” is short for regeneration – when an EV slows down, the electric motors turn into generators and create electricity which chargers the battery. I’ve got up to 2% coming down a hill!
Now when regen works it provides a braking effect. The reader is concerned that this would potentially cause a loss of control, as seen in the Porsche 911 video above.
The short answer is – there’s nothing to worry about.
The longer answer – the concern comes from something called ‘compression lockup’ in ICE vehicles which can be seen in the video above, and I plan on explaining the fix which is the heel’n’toe technique in future.
Here’s the scenario; you have a rear-wheel-drive ICE car, in 3rd gear, braking hard in say into a 2nd gear corner. The driver changes into 2nd gear at high speed, and lets the clutch up. The engine revs are too low for the car’s speed in 2nd gear, so the engine is forced to rev up quickly to the appropriate rev count. This provides a braking force, and that can be both significant and abrupt, enough to lock the rear wheels and spin the car especially as there’s less grip on the rear of the car due to braking. You can see an example of that in the video above.
With regen you don’t get the equivalent of ‘compression lock’. The reason is that regen comes in a lot more progressively as it is computer-controlled, and doesn’t lock the wheels up…unless maybe you’re on ice or similar. Also, regen only brakes the car at around 0.3g which isn’t much. A car can typically brake at around 0.8 to 1.0g depending on its design and especially brake/tyre combination. So if you’re driving your EV on a racetrack you’ll certainly be using the friction brakes to achieve deceleration, so the regen won’t matter overmuch. There will be ABS so there won’t be any wheel lockup, and if you do need to reduce brake pressure you can do simply by reducing the friction brake effort.
If your EV is all-wheel-drive then the regen is typically split with a greater regen (braking) on the front of the car in order to maintain stability.
I haven’t tracked an EV or a Tesla – although I have instructed in one, that’s me in the title photo! – but if I did track an EV, which is something I hope to do soon, then I might leave regen on for additional braking and to conserve energy unless I’m going quick with the stability control off. And there’s a reason for that.
Thinking further, there is an argument that regen could be problematic in recovery situations; let’s say that you need to correct oversteer (see how here) and in certain situations you’d come off the throttle and brake entirely, so as to maximise the tyre’s traction for lateral grip. With regen operating at off-throttle you’d still be slowing down and therefore using some tyre traction for that which wouldn’t be ideal.
One could argue the same is true of a ICE car engine-braking but it’s not, regen at 0.3g would be more significant and you wouldn’t get engine braking to that degree unless you really use a low gear. So let’s say you’re in a fast sweeper and need to correct oversteer; you’d come off the throttle, do your steering, and sort it out. With strong regen you’d also need to add throttle to balance the car…which wouldn’t feel right. Nevertheless, it’s a skill that could be learned – adding throttle helps with oversteer in front-drive cars, it’s deadly for rear-drive.
Another interesting point about EVs is there’s no longer any need to manage gears, which is something a quick driver needs to do even in an automatic if you really want to get the best out of the car. Another skill gone, which is a shame as the whole point of track driving for me anyway is to master the skills required to drive a car fast, not merely sit there as a semi-passenger while the car does all the work.
We’ve not yet seen a proper track EV yet, and maybe Hyundai will be the first, but when we do I feel it’ll require different skills to that of an ICE, in the same way that a skilled driver will use different techniques for front-drive, rear-drive, all-wheel-drive, mid-engined, manual and auto. Let’s see what the future brings!