RE: Rimac Nevera unveiled as 258mph sensation

RE: Rimac Nevera unveiled as 258mph sensation

Author
Discussion

chrisironside

339 posts

130 months

Thursday 3rd June
quotequote all
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.
Valid claim. Slicks and 168kg weight gets you down to 1.53s (see https://ethz.ch/en/news-and-events/eth-news/news/2...

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.
@re33
@thegreenhell
@Dave Hedgehog
@NFC 85 Vette
@herebebeasties

I take it back. smile

Max_Torque

16,923 posts

185 months

Thursday 3rd June
quotequote all
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.
Valid claim. Slicks and 168kg weight gets you down to 1.53s (see https://ethz.ch/en/news-and-events/eth-news/news/2...

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.
@re33
@thegreenhell
@Dave Hedgehog
@NFC 85 Vette
@herebebeasties

I take it back. smile
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.


herebebeasties

496 posts

187 months

Friday 4th June
quotequote all
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

3,414 posts

129 months

Friday 4th June
quotequote all
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.

otolith

45,079 posts

172 months

Friday 4th June
quotequote all
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.
I'm not buying a car until I can feed it by tying it up in a field.

Max_Torque

16,923 posts

185 months

Friday 4th June
quotequote all
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 vehicle

This 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.

fblm

18,911 posts

231 months

Friday 4th June
quotequote all
^Fascinating. I'll say this; my Taycan pulls off feats of grip on regular cold road tyres on less than perfect roads that I've only ever experienced with hot semi slicks on track before.

mclwanB

465 posts

213 months

Saturday 5th June
quotequote all
Re grip limitations on traction limiting acceleration- how about the thrusters on the tesla roadster point upwards to produce significant downforce from a standstill, like the Murray fan car but more so? Had The idea about 35 years ago!

bigothunter

1,437 posts

28 months

Saturday 5th June
quotequote all
258mph top speed will come in handy on the M25 hehe

Roundm

128 posts

86 months

Monday 7th June
quotequote all
and this thing even looks nice (not true for all milk flat powered vehicles!)

APontus

1,001 posts

3 months

Tuesday 8th June
quotequote all
bigothunter said:
258mph top speed will come in handy on the M25 hehe
There'll still be somebody in a 320d tailgating you.

bigothunter

1,437 posts

28 months

Tuesday 8th June
quotequote all
APontus said:
There'll still be somebody in a 320d tailgating you.
320d is all you need biggrin