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The brake pedal
- Thread starter Donkasan
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- Driving Dynamics Model S regen
Yes, but so?
AWD would allow stronger regen, in theory. FWD, I'm not so sure. But all of the regen can be tied to the accelerator pedal, no matter how much regen is available, and many of us think it's better that way.
I'm curious why you think that an AWD or FWD setup will require using the brake pedal for regen. It can still be done all with the accelerator as is currently done. Frankly, I probably wouldn't have bought a Model S if it were FWD as it would take away some of the fun. I would have still considered it if it were AWD, depending on price.
Well this isn't going to be close to true. Lets do some math!
A Prius weighs ~1400 kg.
60mph is ~ 27m/s.
20mph is ~ 9 m/s.
Knowing this we can calculate the amount of energy that a Prius has at both speeds. Using E =1/2 M V^2
E60 = 0.5 * 1400 * 27 * 27 ~= 500,000 J
E20 = 0.5* 1400 * 9 * 9 ~= 55,000 J
So the amount of energy that needs to be scrubbed to do this deceleration is ~445,000 J.
Given that a W = J/s we can figure out how much regen is required to accomplish this deceleration.
W = 445,000 / 2 = 222,500 W = 222kW of deceleration. Which the Prius is obviously not producing via regen! I think a real number would be more like 20seconds of nothing but regen. Obviously wind resistance helps a lot here, but 2 seconds is a full order of magnitude off.
Doing this calc for a Tesla and estimating seconds using 60kW of regen gives us the time of ~11 seconds.
2100 kg * 0.5 * 27 m/s *27 m/s =750,000 J
2100 kg * 0.5 * 9 m/s * 9 m/s = 85,000 J
Energy dissipated = 665,000 J
60,000 W = 1,330,000 J / s -> ~11 seconds to decel. This seems about right to me getting off the interstate.
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@elSupreme, What are these jewels you speak of? And I'll let them eat fruit and cake on this debate; rather than my fig newton 
That's not been my experience (and I have 150,000 miles on my 2004 Prius). I hardly ever touch the brakes on the Model S. They will likely last longer than the Prius brakes because the Prius almost always uses some friction braking when your foot is on the brake pedal. From the Prius New Car Features manual:
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That is true for the Prius and many other cars because there is a lot of weight transfer. With the Model S the low centre of gravity (lower than most sports cars) minimizes weight transfer so the rear brakes are very effective.
On Prius chat there are loads of Prius with untouched brakes at 100 and 150k miles. It is common for those who drive efficiently.
The point is that most slowing force comes from the front wheels for any car. Without antilock the rear tires will lock on strong braking long before the front tires lock. The amount of slowing force from the front tires is much greater. The amount of regen energy from the rear tires will always be less than you can get from the front tires simply because the rear tires will lock long before the front tires.
And in any case a regen system that uses the front wheels can capture much more energy than any regen system that only uses the rear wheels.
Of course, it's common. Mine haven't been touched either and last year, before the Model S, I averaged 69.2 mpg. But the friction brakes are used--just not that much. ElSupreme has it right--if you're slowing from 60 to 20 in 2 seconds you are going to use the friction brakes on the Prius.
I have driven over 40000 miles in both a Prius and a Tesla.
As a good driver I have no problem using regen for the vast majority of my level deceleration in either car - except for panic stops.
In the Prius I have many times filled the battery on a descent so that regen is no longer available and had to use friction - have never yet needed that in a Tesla - on the exact same roads.
There is a very steep hill with twisty 25mph max turns near my home. Descending that hill in the Prius requires friction brakes, the Tesla does not.
Therefore i can confidently say that I get significantly more value ( energy captured ) in the Tesla.
In the Prius it is difficult to find the pedal position which yields maximum regen without adding friction braking.
The Tesla is far superior when it comes to regen. Drive one and you will agree.
As a good driver I have no problem using regen for the vast majority of my level deceleration in either car - except for panic stops.
In the Prius I have many times filled the battery on a descent so that regen is no longer available and had to use friction - have never yet needed that in a Tesla - on the exact same roads.
There is a very steep hill with twisty 25mph max turns near my home. Descending that hill in the Prius requires friction brakes, the Tesla does not.
Therefore i can confidently say that I get significantly more value ( energy captured ) in the Tesla.
In the Prius it is difficult to find the pedal position which yields maximum regen without adding friction braking.
The Tesla is far superior when it comes to regen. Drive one and you will agree.
Theoretically. However, no existing regen system is that strong. The Model S has by far the strongest regen of any existing EV, and it's far from the limit of rear wheel braking.
Thanks for the hard numbers and other insights, folks. As an interested "bystander" (I own a Model S and never had any interest in a Prius), it's interesting to get the fuller picture from the numbers, graphs, etc. I didn't think what bhzmark wrote added up, but now I can actually see why versus just second-guessing.
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(BTW, sorry, I mean no disrespect to bhzmark.)
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(BTW, sorry, I mean no disrespect to bhzmark.)
The OP was about using the brake pedal to control the regeneration. And my point was simply to suggest that also using the brake pedal and the front wheels would be a better way to capture and control regen. Doing high school physics (with incorrect data on the prius) showing only that MS has a larger generator and battery charge capacity doesn't change the more general point about the design.
The limitations of controlling and capturing regen ONLY from releasing the accel pedal and ONLY from the rear wheels are the following:
1) It wouldn't be acceptable to release the accel and have the car effectively slam to a halt. So the accel release can only generate so much slowing effect before it becomes too much slowing effect. For some people Standard may be too much, so they only use low. A usable driver interface will have to trigger the strong braking only from the brake pedal and thus that limits the accel release regen.
2) The brake pedal never triggers any regen so all that braking energy whenever called for by the brake pedal is forever, inescapably lost to heat and friction and brake dust. It can never be captured by regen. That is inefficient.
3) It wouldn't be acceptable to get all the braking from the rear wheels. Even with a low center of gravity and even weight distribution, strong braking needs to come from the front wheels, especially when traction is limited or when going around a corner. See http://www.youtube.com/watch?feature=player_embedded&v=qhRBI8mQ3Oo#at=82 at 1:07 and 1:16 and explained at 7:20 for a demonstration of the problem with getting strong regen braking only from the rear wheels. So the rear wheels can only generate so much slowing effect, before it becomes too much and the front wheels must be used for braking. But the front wheels will never generate any regen so all the braking energy from the front wheels is forever lost to heat and friction and brake dust. It can never be captured by regen. That is inefficient.
The toyota/Lexus HSD design doesn't have these inefficiencies because it can use the brake pedal and the front wheels to control and capture regenerative energy.
I will be surprised if the Model X or even the AWD drive MS don't use the front wheels and the brake pedal to capture and control more regen from the front wheels and for braking forces above what merely releasing the accel provide.
edit to add: By the way I have driven a MS and it occurs to me that the Tesla person that was showing me the car very likely put the regen in Low as they might commonly do to accommodate people who they think aren't use to braking from accel release. I did not notice a particularly strong braking effect from releasing the accel and I used the brakes as much as I normally would. And the whole time, I thought what a waste that all that braking energy wasn't being captured for the battery as it would be in Toyota/Lexus hybrid.
The limitations of controlling and capturing regen ONLY from releasing the accel pedal and ONLY from the rear wheels are the following:
1) It wouldn't be acceptable to release the accel and have the car effectively slam to a halt. So the accel release can only generate so much slowing effect before it becomes too much slowing effect. For some people Standard may be too much, so they only use low. A usable driver interface will have to trigger the strong braking only from the brake pedal and thus that limits the accel release regen.
2) The brake pedal never triggers any regen so all that braking energy whenever called for by the brake pedal is forever, inescapably lost to heat and friction and brake dust. It can never be captured by regen. That is inefficient.
3) It wouldn't be acceptable to get all the braking from the rear wheels. Even with a low center of gravity and even weight distribution, strong braking needs to come from the front wheels, especially when traction is limited or when going around a corner. See http://www.youtube.com/watch?feature=player_embedded&v=qhRBI8mQ3Oo#at=82 at 1:07 and 1:16 and explained at 7:20 for a demonstration of the problem with getting strong regen braking only from the rear wheels. So the rear wheels can only generate so much slowing effect, before it becomes too much and the front wheels must be used for braking. But the front wheels will never generate any regen so all the braking energy from the front wheels is forever lost to heat and friction and brake dust. It can never be captured by regen. That is inefficient.
The toyota/Lexus HSD design doesn't have these inefficiencies because it can use the brake pedal and the front wheels to control and capture regenerative energy.
I will be surprised if the Model X or even the AWD drive MS don't use the front wheels and the brake pedal to capture and control more regen from the front wheels and for braking forces above what merely releasing the accel provide.
edit to add: By the way I have driven a MS and it occurs to me that the Tesla person that was showing me the car very likely put the regen in Low as they might commonly do to accommodate people who they think aren't use to braking from accel release. I did not notice a particularly strong braking effect from releasing the accel and I used the brakes as much as I normally would. And the whole time, I thought what a waste that all that braking energy wasn't being captured for the battery as it would be in Toyota/Lexus hybrid.
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Well you did say that regen should be like the Prius. And then stated that regen only could slow the car 60-20mph in 2 seconds. I was just pointing out that this is off by an order of magnitude. And that infact the regen in the Model S is about double as strong in deceleration. So doing the basic HS Physics seemed prudent.
Well maybe, but this is speculation and opinion. But I will give you the fact that this would probably be more dangerous. But not necessarily dangerous by itself.
This problem is just purely not true. If your car is capable of 60kW of regen it does not change whether or not it is on the brake or accelerator pedal. You still have to use friction brakes for anything over 60kW. This could be the exact same issue as #1 in that you don't think 90kW of deceleration is acceptable if you let off the accelerator. But it isn't a separate point at all. It has nothing do with what pedal controls what.
In fact it is MUCH easier to prevent the friction brakes from grabbing, because they are on a completely separate pedal. If there was some regen on the brake you would have people pressing the brake and possibly dragging the friction ones every so slightly, trying to hit that 'max regen' spot. When putting it on a single pedal I can hit 100% regen, and 0% friction every time instantly without even thinking.
I disagree. With straight line threshold braking sure you would be correct. But you use LOTS of the front wheel's traction while turning. And locking the fronts is WAY more dangerous than the backs.
Take it from people who drive the Model S every day. You DON'T use the brakes. Really you almost never press the brake pedal. It is all regen.
The two exceptions are quick stops (emergency, or barely not making a red light). And low speeds where the real limit is the nature of induction motors, and their reduced power capacity at low motor speeds.
And you would have to produce a vehicle that is capable of decelerating faster than the Model S with pure regen to claim more efficacy. Otherwise no matter what the combination brake pedals, wheels impacted, and anything else, the Tesla will be able to recover more energy. And the inability of accidently/unknowingly using friction brakes allows the Model S owner to recover maximum energy.
roblab
Active Member
This seems to be the problem. Unless you drive one, you can't get your head around it. But yesterday I was showing off the car, let the guy drive. In two miles, he says, "I LOVE this power recapture!" It sold the car. I had spent a little time telling him about one pedal driving, and how regen is controlled just like acceleration, with gradual pedal change. Push more for more power. Release slowly, slowly regen. Even pulling the foot off the pedal immediately, the regen comes on as a gradual curve: you don't get whiplash, unlike with the Prius, if you brake in B, the car slows way faster than you anticipate, unless the battery's full, and then, Surprise! No Regen.
I wish all these arm chair physics professors would just go out and drive one. It is immediately apparent. Tesla got it better.
This is purely not true:
Being on the verge of, or actually in, understeer is inherently more stable than oversteer. Braking from the rear pushes a car towards oversteering (fishtailing and spinning around) - braking from the front pushes it towards understeer (just plowing ahead).
Here is a nice summary of why it is better to brake with your front wheels instead of your rear wheels -- even with good anti-lock which can still permit momentary locking that can destabilize your car:
from http://www.yildiz.edu.tr/~uzunsoy/brake.pdf
Front tires braking and locking:"The situation where the front tires of the vehicle lock up is represented in Figure 5.1. Subject to a yaw disturbance, the heading of the vehicle has rotated slightly, while the resultant path of vehicle travel is still directly forward, in the original direction. The locked up front tires only create forces in a direction opposite the direction of motion. These forces create only a small yawing moment about the CG. The rear tires, which have not locked up, still generate significant lateral forces, perpendicular to the rear wheels. These forces act in a direction to oppose the yaw disturbance, and will restore the vehicle to the original heading.
Rear tires braking and locking:
"The situation where the rear tires lock up first is represented in Figure 5.2. Subject to the same yaw disturbance, the vehicle is rotated slightly from the absolute direction of travel. The locked up rear tires generate forces only in a direction opposite the direction of vehicle motion. These forces create only a small yawing moment about the vehicle CG. The front tires, which remain rolling, generate lateral forces perpendicular to the wheels. The lateral forces at the front tires will create a yaw moment that increases the yaw disturbance. Thus, the vehicle will most likely spin out of control. The net effect is that if the rear tires lock up first, the front lateral forces alone produce a vehicle that is unstable in yaw. The tire forces then tend to increase the effects of the disturbance and cause the vehicle to lose control.
"Conversely, [when the fronts are locked] the rear lateral forces acting alone produce a vehicle stable in yaw that tends to create yaw moments to reduce the disturbance. The phenomena itself is akin to that seen in caster wheels. If the caster wheel lies forward of its pivot it will always swing around to trail the pivot pin. A caster wheel lying ahead of its pivot resembles an unstable vehicle with its rear tires locked. "
end quote:
Or most simply, if you still don't agree, find an old car with a hand parking brake and pull the hand brake locking the rear wheels at speed. With even the smallest deviation from a truly straight line course, you will invite the rear of the car to switch places with the front.
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Thank you. This is my point. Merely releasing the accel DOESN'T give the FULL 60kW of regen braking. That would be too abrupt. So they have to design in giving you less (probably much less at slow speeds) than the full regen so that the driving experience doesn't have you screeching to a halt every time you release the accel.
But sometimes you do want to decel and slow faster than the accel pedal release curve, and for that you are forced to use friction braking.
In cars that trigger stronger regen from the brake pedal (instead of being limited to the accel release curve) you can capture more of that energy as regen.
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