[colourmecarbon]

I've just discovered this forum, I had no idea modding EVs was so popular!
I'm looking to improve the i3's performance both in power (and ideally range), my plan was to add a second i3 motor in the big space in the 'engine' bay where the ReX version has its generator.

But having browsed this forum I realise I'd be dumb not to ask the community of more experienced modders here, what they would do to improve an i3.

 

[Duncan]

Hi
Range is more batteries - nothing else will work

More power
Depends on what you are willing to do and what you are wiling to give up

The BMW is a production car - 200,000 miles and very low failure rates

Going by other vehicles you can probably drive that motor twice as hard - but you will be increasing the risk of failure

All of these systems have a "Brain Board" - that drives the heavy duty electrics - that drive the motor

The "brain board" does all of the controlling - you could replace this with your own (a number of people are working on that) - and simply dial in the required power

Use the old Hot Rod approach - identify the limiting bit and replace it - then continue replacing the bits you break

 

[brian_]

I'm looking to improve the i3's performance both in power (and ideally range), my plan was to add a second i3 motor in the big space in the 'engine' bay where the ReX version has its generator.

That would be a substantial mechanical project. To start, you would need to decide whether you would feed the output of the second motor into the same transaxle, or would have a separate motor and transaxle for each wheel. Either way, you would need a second inverter and coordinated controllers.

How about adding battery capacity in that space instead (for both range and power availability), and pushing the motor harder as Duncan suggested?

 

[colourmecarbon]

thanks, two good answers, and the hot-rod methodology is the way I like to work.
The firmware/software route is the one that'll bring immediate results (and I could call on the warranty if I 'brick' its brain.

I've a lot of research to do on the motor's capability.

 

[psron]

Very old thread, but my reading on the motor design indicates that it does not lend itself to substantial output increases... their limited magnet usage and design seems to limit the "overhead" that can be used. I would be surprised if more than a 25-30% gain is achievable.


HOWEVER... that being said, the low-end limitations are strictly imposed for traction limitations... by now you surely know that the acceleration is much stronger once you get above 25-30 MPH, because the traction limits are not there.



This is to say that a "simple" re-mapping of the torque curve should provide a VERY distinct "seat of the pants" improvement... but of course you would have to eliminate the reason for this factory limitation... the small tires.


There are plenty of tips and tricks for this... even using all "front" (narrow) i8 wheels with spacers and add fender flares to cover the rubber.


Beyond that, one reason BMW backed-off of the original prototype horsepower (185) was due to bearing wear indications. If you do some reading, the i3s has different bearings (I would assume to resolve this weakness) and allow somewhat higher RPM for higher top-speed.

 

[brian_]

... that being said, the low-end limitations are strictly imposed for traction limitations... by now you surely know that the acceleration is much stronger once you get above 25-30 MPH, because the traction limits are not there.

That makes no sense to me. The amount of traction available does not depend on speed. Of course at higher speed the same force (simply proportional to motor torque in this single-ratio drivetrain) corresponds to more power, but the rate of acceleration depends only on the drive force (and drag and inertia).

I assume that BMW didn't enforce traction control limits in the control logic only at low speed. That's the opposite of a rational approach.

 

[colourmecarbon]

Thanks for adding more useful commentary to my thread, one of the first mods I did to my i3 was to use 7.5x19” rims with 225/45 tyres. This did improve the 0-30 because traction control wasn’t kicking in so early.
The ‘safe’ torque curve is still obvious in the seat-of-pants accelerometer. If I could tune that out, it would be a much quicker car.

I’ve moved on to more aerodynamic improvements and already upped the range by 20% by reducing weight, swapping side-mirrors for tiny drone-cameras, a front air-dam/splitter and tidying up the underside.

I saw a wrecked i8, and measured up the front subframe and motor as a ‘mechanically possible’ conversation- but I lack the electrical knowledge to make this idea a reality.

 

[psron]

That makes no sense to me. The amount of traction available does not depend on speed. Of course at higher speed the same force (simply proportional to motor torque in this single-ratio drivetrain) corresponds to more power, but the rate of acceleration depends only on the drive force (and drag and inertia).

I assume that BMW didn't enforce traction control limits in the control logic only at low speed. That's the opposite of a rational approach.

It's not really about the amount of traction available relative to speed... it's all about the ability to apply your motor power towards acceleration - relative to speed.

Several ways to approach this... but the ability to effectively apply torque is relative to your current momentum (vehicle speed).

Take the 184 lb-ft of the i3 electric motor torque, multiply by 9.70 (final drive ratio) you have 1784.8 lb-ft of torque at the drive axles. My i3 has 175/55R20 rear tires, that's a radius of 1.15 feet. 1784.8 divided by 1.15 = 1552 lb-ft of torque applied at the road at full rated motor torque.

If you apply that full torque at zero forward speed, you have to be able to have that much tire grip to resist spinning the tires.

Remember, as electric motor speed increases from zero (maximum torque), its torque begins to drop off and horsepower begins to climb [HP = torque x RPM / 5252 (radians/sec)]... and horsepower is easier to effectually apply as acceleration.

SO... once you have any forward speed, the resistance to accelerate is reduced by your forward momentum... your tires are now applying less torque to the road, but more horsepower.

Then there's this little matter of Newton's first law... An object (car) at rest wants to stay at rest (has the greatest resistance to being accelerated)... because you have to overcome the inertia of the car. This resistance decreases as speed/momentum increases.

 

[psron]

Thanks for adding more useful commentary to my thread, one of the first mods I did to my i3 was to use 7.5x19” rims with 225/45 tyres. This did improve the 0-30 because traction control wasn’t kicking in so early.

Did you use i8 wheels, or aftermarket?
I'm at that point RIGHT NOW... before winter kicks-in here in KC... since nobody makes winter or all-season tires for my 20" i3 wheels.

 

[colourmecarbon]

I tried i8 wheels second, first I tried 18”x7J with 215/45 tyres, but that caused unclearable DSC /ABS error (had to replace the dsc module.
Then I tried the i8 wheels, but after the amazing handling and acceleration of the 18” setup, the 20” i8 wheels made the car feel slow and didn’t improve handling.
I scaled down to 19” which is better (no errors), but not as agile as the 18”.

 

[colourmecarbon]

if you’re shopping for aftermarket wheels, look for mercedes fitment with a low offset 32 or 34mm so you only need 12mm spacers on rear and 18-20mm on front.

 

[psron]

What wheel offset and what thickness spacers do you use?
Tire rack doesn't offer many i3 options, did you just pick a bolt pattern and offset you wanted?... or did you end up with used wheels from another vehicle?


Any help you can offer is greatly appreciated... this will be my first "full winter" with my 2015 BEV... and with a 40 mile each way commute, I need to get something reliable... my rear tire tread is already getting thin in less than 1 year.


I also plan on getting the lowering springs... too much gap above the tires.

 

[brian_]

It's not really about the amount of traction available relative to speed... it's all about the ability to apply your motor power towards acceleration - relative to speed.

Several ways to approach this... but the ability to effectively apply torque is relative to your current momentum (vehicle speed).

Take the 184 lb-ft of the i3 electric motor torque, multiply by 9.70 (final drive ratio) you have 1784.8 lb-ft of torque at the drive axles. My i3 has 175/55R20 rear tires, that's a radius of 1.15 feet. 1784.8 divided by 1.15 = 1552 lb-ft of torque applied at the road at full rated motor torque.

If you apply that full torque at zero forward speed, you have to be able to have that much tire grip to resist spinning the tires.

Close... accepting your calculations without checking them, you would have 1552 pounds of force (not lb-ft of torque) applied to the road.

That's less than half the weight of the car - not a huge traction challenge for a rear-heavy rear-wheel-drive vehicle, but I do realize that this one has skinny tires, and so the traction control needs to act, especially if the surface is less than perfect.

The real issue with very low-speed acceleration of the i3 appears - from a chart on a a BMW web page - to be that BMW has chosen to limit torque, presumably to help people drive smoothly: torque ramps up linearly until it reaches the peak. It isn't even really trying until 1500 rpm at the motor, which is 155 rpm at the wheels or 20 km/h (13 mph); this is way short of the 20-30 mph where a change is felt. Of course, this same chart also claims constant torque output to 7000 rpm, while we know it is only sustained at that level until 4800 rpm, so it's presumably distorted by some marketing idiot.

The transition from nearly constant acceleration in the constant-torque region of the motor and controller (within the limits of traction, and reduced a bit by aero drag as speed increases) to smoothly decreasing acceleration in the constant-power region of the motor and controller should be at 4800 rpm, or 65 km/h (40 mph).

Remember, as electric motor speed increases from zero (maximum torque), its torque begins to drop off and horsepower begins to climb [HP = torque x RPM / 5252 (radians/sec)]... and horsepower is easier to effectually apply as acceleration.

SO... once you have any forward speed, the resistance to accelerate is reduced by your forward momentum... your tires are now applying less torque to the road, but more horsepower.

Then there's this little matter of Newton's first law... An object (car) at rest wants to stay at rest (has the greatest resistance to being accelerated)... because you have to overcome the inertia of the car. This resistance decreases as speed/momentum increases.

Oh, you were so close. All you had to do was stop after you realized that acceleration is proportional to torque. After that, you go completely off track.

Inertia does not change with speed (or momentum). "F=ma" does not contain a velocity term; a=F/m (that's the acceleration of an object equals the net applied force divided by the object's mass), at any speed. As the car speeds up in the constant-power regime of the motor and controller, the force decreases and acceleration decreases. It's even worse than that, since drag increases so the force decreases even more.

These are BMW's specs for acceleration:

• 0-60 km/h: 3.7 s
• 0-100 km/h: 7.2 s
• 80-120 km/h: 4.9 s

so

• 0-60 km/h: 3.7 s -> 16.2 km/h per second, or 4.5 m/s2
• 60-100 km/h: 7.2-3.7 = 3.5 s -> 11.4 km/h per second, or 3.2 m/s2
• 80-120 km/h: 4.9 s -> 8.2 km/h per second, or 2.3 m/s2

The faster you go, with constant power, the lower the rate of acceleration. That's expected, and it's okay.