I3 acceleration question

[LWHDurham]

Physics/engineering question. Could a 2015 i3 accelerate from a rolling start (near 0 kph) to 73 kph in less than 100 meters? Deciding whether to dispute a traffic ticket, since I don't know if the alleged violation is accurate or even possible. Not an aggressive driver and the speedometer only showed 54 kph before the cop turned on his lights.

 

[mojo]

With a few rather crude assumptions (e.g. that acceleration will be linear, which it won't), it seems likely that it should easily be possible. According to my maths, an i3S BEV should (in theory) make it to 73kph in around 52m. A non-S with REX for example would stretch that out to approx. 60m.

But it is, of course, academic and there's no point in trying to dispute what the police officer says! But I'm sure you know that anyway...

 

[LWHDurham]

With a few rather crude assumptions (e.g. that acceleration will be linear, which it won't), it seems likely that it should easily be possible. According to my maths, an i3S BEV should (in theory) make it to 73kph in around 52m. A non-S with REX for example would stretch that out to approx. 60m.

But it is, of course, academic and there's no point in trying to dispute what the police officer says! But I'm sure you know that anyway...

Well, I would dispute it, if I thought the cop was wrong. In this case, I don't know. Was focused on the road more than the speedometer, while accelerating out of a curve. But I'm still astonished that the car could hit that speed so quickly.

Thanks for your analysis. Most likely I'll just shut up and pay the ticket. First one in 18 years. Impressive little car. Bad grandpa.

 

[mojo]

One of the things that makes the car (and any EV for that matter) so dangerous for points on your licence is that there is so little fuss and noise when accelerating hard. Most of us will come from "performance" ICE vehicles where you can accelerate hard but it involves a lot of noise and gear changing. An EV transforms that - so I have to be careful at all times!

 

[Rossifer]

Well, I would dispute it, if I thought the cop was wrong. In this case, I don't know. Was focused on the road more than the speedometer, while accelerating out of a curve. But I'm still astonished that the car could hit that speed so quickly.

Thanks for your analysis. Most likely I'll just shut up and pay the ticket. First one in 18 years. Impressive little car. Bad grandpa.

Your original question asked about a "rolling start, near 0 kph" then you say you were accelerating out of a curve.
I'd say you were a the 73 kph the police caught you at since these awesome little cars have a very sneaky way of getting up to speed without the roar of a v8.
Pay the ticket.

 

[ksnax]

Seems plausible to accelerate that fast. I frequently notice that there is a substantial delay in the display updating when doing it, with it always lagging by at least 2-3 MPH until I let off - even noting that it can go higher if actually dipping into regen quickly enough.

I don't know why BMW built it into the system like that. Obviously, being digital, it could have been much more responsive for accurate display.

 

[Ollie]

Since you say "18 years" ask if you have a traffic class/deferment option. Don't know your system and how it can affect you insurance rates. You can also try the best story a judge has ever heard or filibuster with such BS they throw you out. I don't recommend the last one.

 

[johnheddy]

It sounds faster as 73 KPH, but that is only 45.4 MPH. I have a 2017 REX, and it can hit 45 MPH in very little space and time. But from what I have heard and experienced... BMW has tuned down the response of these cars. The story I heard was that when they first came out they were the fastest cars from 0-30 MPH that BMW made period. They also had issues with the power applied for a quick start off the line ripping the motor mounts loose (over time) because of excessive torque.

It is a simple matter to translate the pressure on the accelerator to a slower curve in software, and that was the cheaper solution (possibly safer as well). Just switching from Comfort to Eco Pro to Eco Pro+ does that already.

Just imagine what "Bad Grampa" would do if they had gone the other route and made the motor mounts stronger and optimized the acceleration off the starting line.

I would really like to have one of these cars optimized for acceleration off the line, not for everyday driving, but maybe in a "burnout" mode where you know you are going to burn through half your battery SOC in 4 or 5 seconds. Every now and then we all want that thrill of acceleration. It is best to avoid indulging in front of a speed trap if you can though.

 

[alohart]

The story I heard was that when they first came out they were the fastest cars from 0-30 MPH that BMW made period.

When that claim was made, it was easily debunked by comparing acceleration times published by various auto magazines.

I bought a 2014 i3 new which had its original July, 2014, system software (integration level). After the left motor mount bracket mounting bolt sheared in several i3's which could cause significant collateral damage, BMW issued service action SI B22 04 15 which included replacing this bolt with a stronger version and programming the traction control system to not allow a wheel that lost traction while accelerating from spinning up too fast (e.g., over a speed bump or when turning out of a driveway onto a road with the inside rear wheel dropping off a curb). The sudden jolt when the spinning wheel touched down on the road was apparently what caused the bolt to shear.

After this service action was performed, I don't recall noticing any difference in acceleration from a stop unlike some i3 owners. I don't recall actual acceleration tests that showed a difference. The narrow i3 tires don't provide enough traction for really rapid acceleration without spinning which traction control prevents by reducing torque.