Marty,
Interesting discussion.
The nerd and physics teacher in me had to put numbers to your description.
If my calcs are correct and assumptions are reasonable, at a speed of 45 mph
with an ‘accelerator to brake’ time of 0.2 seconds, the stopping distance is 7
ft shorter (110 vs 103 ft) and the speed change is 1.5 mph. This assumes a max
stopping ‘friction coefficient’ of 0.7, and that regen generates 50% of that.
The spreadsheet below should allow anyone to play with the assumptions.
https://docs.google.com/spreadsheets/d/1JOveST4hfFlzAdRzr8PGpAM3ehCwaIBtxuSI3-B4f2A/edit?usp=sharing
Verified the non-regen distance with
https://www.omnicalculator.com/physics/stopping-distance
Will
From: knoxev-bounce@xxxxxxxxxxxxx <knoxev-bounce@xxxxxxxxxxxxx> On Behalf Of
Marty Young
Sent: Saturday, January 16, 2021 12:48 PM
To: knoxev@xxxxxxxxxxxxx
Subject: [knoxev] Regenerative Braking
[External Email]
I have seen some debate recently on various EV websites concerning regenerative
braking and one-pedal driving. It also came up this week on a podcast I listen
to called EV News Daily. I shared my thoughts about this with the program's
host, Martyn Lee, and I wanted to share this with you, too:
Hi Martyn,
You recently discussed regenerative braking on your podcast, and I just wanted
to share a slightly different perspective on the topic.
I have been a paramedic instructor for more than twenty years. Like everyone
else who works in a public safety profession, I have seen more than my fair
share of injury and death. The most difficult calls always involve children,
and I have responded to several incidents where a child has been struck by a
vehicle, usually after the child ran out into the road without looking.
It is not an exaggeration to say that the difference between life and death in
these instances is measured in milliseconds. If you are driving a typical car
and a child runs out in front of you, your car will not slow down until you
apply significant pressure to the brake pedal. During the time you are lifting
your foot off the accelerator, moving it across from the accelerator to the
brake pedal, and beginning to press on the brake pedal, your car is not slowing
down in any significant way. Those are milliseconds that are lost at a time
when you have none to spare.
However, if you are driving an electric vehicle with maximum regenerative
braking, the moment you begin raising your foot off the accelerator your car
starts to slow down. It continues to slow as your foot moves from the
accelerator to the brake pedal. It continues to slow as you begin to push on
the brake. By the time you have depressed the brake pedal enough to pressurize
the brake fluid in the lines and force the calipers and pads to begin squeezing
on the rotors, the speed of your car has already slowed significantly due to
the regenerative braking of the magnets in the electric motor.
And this can make an enormous difference. Why? Because physics tells us that
the energy at the moment of impact is most dependent upon speed. In the
formula for kinetic energy (the energy of motion), velocity is squared. In
other words, by reducing the speed at impact by half, you reduce the impact
energy by a factor of four. And that can be the difference between life and
death.
For this reason, I always advise everyone with an EV to drive with the
regenerative braking at its maximum. This is a superpower that your car
possesses! Use it!
You can do this and still drive smoothly. You simply have to slightly change
the way you think about driving. You will soon learn that lifting your foot
off the accelerator is the same thing as pressing on the brake. Therefore, to
stop smoothly, you simply lift your foot slowly and smoothly off the
accelerator. With just a little practice, your passengers won't feel any
difference.
Martyn, as you often say, EVs are just better in every way. And this is just
one more way in which they are better.
Please encourage your listeners to learn to drive with maximum regenerative
braking. Please don't turn off one of the most important safety features of an
EV just so it will behave like an ICE car.
Thank you,
Marty Young
