RE: Rimac Nevera unveiled as 258mph sensation
Discussion
herebebeasties said:
chrisironside said:
re33 said:
Sounds impressive. 0-60 in 1.85 seconds is not possible on currently available road tyres though so maybe that's on track tyres.
Bizarre claim.But for road cars, it stands to reason that you can't accelerate from 0-60 faster than you can brake from 60-0. You're traction limited in either case, and when braking air resistance is augmenting the force provided by the tyres, whereas when accelerating it's obviously working against it.
The very best road cars stop in just under 27 metres. Handily, 60 mph is approximately 27 metres per second. If you assume a constant rate of deceleration, the average speed over the distance from 60-0 will be half that, or 13.5 metres per second. 27 metres divided by 13.5 metres per seconds is 2 seconds. So especially if you factor in the drag too, it's basically impossible to get from 0-60 in under two seconds on normal road tyres.
Does this now seem less bizarre to you?
You can cheat by including the "foot roll-out" that drag strips take off, caused by their laser beam timing gear, which may get you down to ~1.9ish, but you're then measuring something like 4-60 mph, which is different.
@thegreenhell
@Dave Hedgehog
@NFC 85 Vette
@herebebeasties
I take it back.
chrisironside said:
herebebeasties said:
chrisironside said:
re33 said:
Sounds impressive. 0-60 in 1.85 seconds is not possible on currently available road tyres though so maybe that's on track tyres.
Bizarre claim.But for road cars, it stands to reason that you can't accelerate from 0-60 faster than you can brake from 60-0. You're traction limited in either case, and when braking air resistance is augmenting the force provided by the tyres, whereas when accelerating it's obviously working against it.
The very best road cars stop in just under 27 metres. Handily, 60 mph is approximately 27 metres per second. If you assume a constant rate of deceleration, the average speed over the distance from 60-0 will be half that, or 13.5 metres per second. 27 metres divided by 13.5 metres per seconds is 2 seconds. So especially if you factor in the drag too, it's basically impossible to get from 0-60 in under two seconds on normal road tyres.
Does this now seem less bizarre to you?
You can cheat by including the "foot roll-out" that drag strips take off, caused by their laser beam timing gear, which may get you down to ~1.9ish, but you're then measuring something like 4-60 mph, which is different.
@thegreenhell
@Dave Hedgehog
@NFC 85 Vette
@herebebeasties
I take it back.
Max_Torque said:
Worth noting that because an Electric powertrain is very smooth torsionally (hardly any torsional oscialtions) and because it can modulate torque at high speed (10,000 times a second for example), and because it is intrinsically bi-directional (it can provide +ve and -ve torque at the same magnitude) this furnishes a larger effective friction condition than for an ICE or via brakes (ABS). By keeping tyre slip in a the very narrow window at the highest value of mu, electric cars can out perform ICE cars in terms of producing a higher tractive effort for their mass.
Indeed. I'm curious how that actually works - what's the feedback mechanism to keep it at optimum µ? Wheel speed sensors and it reduces torque to any wheel that is overspeeding relative to the others? Are ABS sensors sufficiently sensitive to do a good job of that?wst said:
Max_Torque said:
"There is nowhere to charge them"
If HoofHeads had been around when Rudolf Diesel and the other smart chaps were making stuff in the 1800's I imagine they would have complained about the lack of refuelling infrastructure.herebebeasties said:
Indeed. I'm curious how that actually works - what's the feedback mechanism to keep it at optimum µ? Wheel speed sensors and it reduces torque to any wheel that is overspeeding relative to the others? Are ABS sensors sufficiently sensitive to do a good job of that?
Firstly we need a reliable velocity vector for the vehicleThis is a data fusion using Kalman filtering from a number of sources, but typically including, wheel speeds, chassis accelerometers, and GPS / NAV data, and that data is validated using a physics model so sensible rates of change are included as limit values
Then we need reliable wheel speeds
This is again a mixture of data sources, with moden ABS sensors have pretty decent resolution but also the electric motor position encoders having a very high resolution, enough to give us the average of the driven wheel speeds on that axle, then relying on the ABS sensors to estimate the cross axle variation.
Then we use a dynamics model to calculate the optimum wheels speeds, taking into account yaw and slip
Then we work out the tyre slip, by taking away the measured wheel speeds from the target speeds. Usually, we will also be looking at the rate of change of slip, and not just the pure slip, because that value can be more effective in allowing us to forecast slip at some future moment, and so intervene early enough.
Ultimately, we end up with a torque demand modulate to the eMachine driving the wheels, and that is extremely fast, because current is modulated at over 1KHz on a typical control inverter (voltage is modulated at around 10 to 20 kHz).
In order to aboid excessive latencies, the torque control is increasingly done within the inverter itself, and the much slower (relatively) slip calc done by an external controller, typically within the ABS or chassis control system.
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