Model S regen

[Doug_G]

Finally, unless it's different than Roadster, regen braking is not anti-lock controlled. On Roadster, if you're in heavy regen and the rear wheels lose traction you'll lose all braking and the car will feel like it's lurching forward (even though it's only going into coasting).

Actually that coasting sensation is caused by the Roadster's traction control triggering while you're braking. Even while using the friction brakes the regenerative braking is providing a fair chunk of the deceleration force. When the traction control triggers it momentarily reduces the braking force and that feels like the car surges forwards.

On the other hand, if you're driving on a slick surface and you lift off the accelerator, the car decelerates smoothly using the traction control. So it has its pros and cons.

 

[Vger]

I emailed a well-placed contact within Tesla about this issue last week. Here is his response:


"Regarding the level of regen, I checked last night with the powertrain team. The Roadster and the Model S are calibrated to the same level of deceleration. ...
*
We cannot really increase the regen amount, unfortunately, for a variety of reasons: vehicle stability and charge acceptance being the primary ones
*" [bold emphasis mine]

I, for one, am satisfied with this explanation. As smorgasbord points out above, the Roadster regen gauge is calibrated to 40 kW and the S gauge to 60kW, very close to the ratio of their respective masses. So rate of max deceleration should be similar. I guess when I get the car, I can verify this with my iPhone accelerometer. :wink:

 

[J in MN]

I guess when I get the car, I can verify this with my iPhone accelerometer. :wink:

I had already planned on doing that on my test drive in Chicago to compare against the Volt. Numbers don't lie - no room for opinion or perception.

 

[smorgasbord]

I can verify this with my iPhone accelerometer. :wink:

I don't think that will work. As I said, Roadster regen comes on abruptly while Model S regen ramps up more slowly, so even if max braking force is the same, the transition in Roadster will be greater and so have a higher acceleration change. It would be like using acceleration to measure differences in top speed.

Doug, thanks for the TC explanation of Roadster regen effects. It's a very disconcerting feeling. When moving forward having TC cut out power is OK, but cutting out regen seems wrong to me. It certainly feels wrong, but maybe Tesla is trying to avoid the rear wheels from catching when they regain traction. I know that's a cause of some motorcycle accidents - if the rear wheel loses traction and locks up, you may lose control and "high side" it when the wheel grabs the pavement again.

 

[VolkerP]

I once had the rear wheels lose traction on a descending slope in icy conditions, when driving an ICE with manual trans. I used engine compression braking on that slope routinely but this time the rear wheels turned slower than the car moved. The cure was to press the clutch. Luckily the wheels regained traction before the car could swing around. But phew that was fearsome.

Under no circumstances regen on the rear wheels should make them lose traction. IMO it's a must for TC & ABS to cut back on regen as well as friction brakes to maintain traction. Imagine you must come to an emergency stop on snow or ice. You slam on the brake pedal and hope for TC & ABS to stabilize the car. But you let go of the accelerator doing so, triggering max regen. Now if regen makes the rear wheels break lose while the friction brakes bite into the front wheel rotors, nothing would keep you from swinging the car. Not good :scared:.

I am sure the gearheads at Tesla took proper care of regen behavior.

 

[smorgasbord]

IMO it's a must for TC & ABS to cut back on regen as well as friction brakes to maintain traction. Imagine you must come to an emergency stop on snow or ice. You slam on the brake pedal and hope for TC & ABS to stabilize the car. But you let go of the accelerator doing so, triggering max regen. Now if regen makes the rear wheels break lose while the friction brakes bite into the front wheel rotors, nothing would keep you from swinging the car. Not good :scared:.

I am sure the gearheads at Tesla took proper care of regen behavior.

There's are differences between anti-lock and how TC kicks in when Regenning. Anti-Lock releases and reapplies the brakes many times a second. TC releases and then reapplies maybe a second or two later. You're probably right that during hard braking disabling regen when traction is lost is the right thing. However, it does create a scary feeling when you lose traction when only regen braking.

In case anyone with a Roadster in the SF South Bay wants to try this out, I have a reproducible example. Get on 280 North and exit on Alpine Road in the right lane. During that off ramp the pavement changes. If you exited at freeway speeds and are regen braking only in the off-ramp, when the pavement changes you'll lose traction, which will temporarily disable regen.

 

[strider]

There's are differences between anti-lock and how TC kicks in when Regenning. Anti-Lock releases and reapplies the brakes many times a second. TC releases and then reapplies maybe a second or two later. You're probably right that during hard braking disabling regen when traction is lost is the right thing. However, it does create a scary feeling when you lose traction when only regen braking.

During my Model S drive yesterday I observed that Model S is MUCH faster at recovering from traction loss both during acceleration and during regen. The Roadster takes as smorg mentions a second or two before it recovers. Model S recovered in a fraction of a second. They're definitely learning from the Roadster (and perhaps increases in computing power allow them to sample and recover more quickly).

VolkerP, what you describe is correct. Neither the Roadster nor Model S will allow the car to swap ends due to regen slowing the rear tires. In fact, Model S should be even better than the Roadster as it has proper stability control in addition to simple traction control like the Roadster.

 

[J in MN]

I don't think that will work. As I said, Roadster regen comes on abruptly while Model S regen ramps up more slowly, so even if max braking force is the same, the transition in Roadster will be greater and so have a higher acceleration change. It would be like using acceleration to measure differences in top speed.

No, this is not correct. Force = mass x acceleration, so if the force is the same between Roadster and Model S, the deceleration in Model S will be less only because of its larger mass. The "ramp"-up has nothing to do with it.

You are confusing acceleration (rate of change of speed) with jerk/jolt (rate of change of acceleration).

 

[smorgasbord]

In my earlier post, I talked about how the Roadster regen was about 40Kw while Model S was about 60kW, and talked about the weight (mass) differences. So, I was really saying that the force per mass was about the same. Sorry if I wasn't being technically correct with the casual words I was using.

It is very possible that I've confused change of speed with change of acceleration effects. I do believe, however, that ramp-up of regen is a factor in deceleration readings. I can do this experiment on my Roadster:

1) Drive at 65 MPG and lift off the go pedal abruptly. Measure g-force.
2) Drive at 65 MPG and take about a second to lift off the go pedal (this simulates the Model S ramp-up). Measure g-force.

Are you saying the two g-forces will be identical?

 

[jkirkebo]

It is very possible that I've confused change of speed with change of acceleration effects. I do believe, however, that ramp-up of regen is a factor in deceleration readings. I can do this experiment on my Roadster:

1) Drive at 65 MPG and lift off the go pedal abruptly. Measure g-force.
2) Drive at 65 MPG and take about a second to lift off the go pedal (this simulates the Model S ramp-up). Measure g-force.

Are you saying the two g-forces will be identical?

Yep, they should be. After a second at least, when the ramp-up is complete. Under ramp-up the g-force will also ramp up, to the same max value as the non-ramped one.

 

[derekt75]

I agree with jkirk: deceleration or g-force is measured with an accelerometer (such as on an iphone) and is independent of jerk (third derivative of position wrt time).

I would think that if the limiting factor is battery charge acceptance that a bigger battery could handle more regen than the smaller battery. Will the regen scale with battery size?

 

[smorgasbord]

Yep, they should be. After a second at least, when the ramp-up is complete. Under ramp-up the g-force will also ramp up, to the same max value as the non-ramped one.

I don't believe you're correct. My prediction is that what's going to happen is that the abrupt lift-off will initiate full regen at the full speed while the ramp-up will get a lower full regen since the speed after the ramp-up is somewhat less. Therefore, the (de)accelerations will be different.

I've got a Roadster - what iPhone App should I use? I'll get a passenger to video the thing and post the results. After all, any excuse to nerd out in a Roadster is welcome.

 

[ElSupreme]

I don't believe you're correct. My prediction is that what's going to happen is that the abrupt lift-off will initiate full regen at the full speed while the ramp-up will get a lower full regen since the speed after the ramp-up is somewhat less. Therefore, the (de)accelerations will be different.

I've got a Roadster - what iPhone App should I use? I'll get a passenger to video the thing and post the results. After all, any excuse to nerd out in a Roadster is welcome.

You don't really need an iPhone app to do this simple equivalence test. In fact a pendulum and a marker will probably be more precise.

 

[smorgasbord]

You don't really need an iPhone app to do this simple equivalence test. In fact a pendulum and a marker will probably be more precise.

Hmm. What about car pitch changes as the load on the shocks change?

I'll probably just wimp out and put a camera on the VDS.

 

[J in MN]

what iPhone App should I use?

I use http://itunes.apple.com/us/app/g-tac-free/id305409405?mt=8

Turn off cellular data while using it.