Why does an I3 need vented front discs?

[Zwerius]

Just wondered: why does an i3 need vented front disc brakes.
The brakes are rearly used and if so, only for short moments.
Non vented discs would be cheaper and lighter. Less mass means lower electricity consumption! And less mass in the wheels means increased riding comfort! (ratio sprung / unsprung masses)
Is there a point I'm missing?
I'm living in the Netherlands, so no hills what so ever. Maybe you need them on mountain roads?

 

[alohart]

Maybe while fully loaded on a long, steep descent with a full and/or hot battery pack where regenerative braking is limited, having ventilated front discs would be necessary to prevent serious brake fading. Having ventilated front disc brakes seems intended to prevent brake fade under very rare circumstances. They are certainly overkill in normal circumstances, but BMW probably doesn't want to take a chance that something bad might happen.

Or maybe for those who want to take their i3's to the track…

 

[i3atl]

Because some owner's driving styles require use of the brakes much more often than others.

Plus alohart's point about the track (and auto-x). The brakes wouldn't hold up well at all if the front rotors weren't vented.

 

[psc]

Probably cost ? - both front and rear brake rotors are the same diameter (but different thickness ventilated vs solid) - extra capacity on the front via ventilation.

 

[Boatguy]

Probably because they are an existing part used in another car. While much of the i3 is brand new, I'm sure the product managers tried to use a few things off the shelf.

Or to support those who autocross on the weekends!

 

[i3atl]

Probably because they are an existing part used in another car. While much of the i3 is brand new, I'm sure the product managers tried to use a few things off the shelf.

Good thinking, but it looks like the rotors are not used on any other BMW models. They could still be shared with one of the Mini models, since that's the case for some of the other parts on the i3.

 

[stumbledotcom]

I'm living in the Netherlands, so no hills what so ever. Maybe you need them on mountain roads?

There absolutely are situations where you need the friction brakes. Here in SF there are plenty of hills steep enough that regen braking alone doesn't slow the car. In fact, I use routes several times a week where the i3 actually speeds up going down hill even when I'm off the accelerator pedal completely in full regen.

 

[barrychan]

Its a BMW ....

 

[Zwerius]

There absolutely are situations where you need the friction brakes. Here in SF there are plenty of hills steep enough that regen braking alone doesn't slow the car. In fact, I use routes several times a week where the i3 actually speeds up going down hill even when I'm off the accelerator pedal completely in full regen.

But still, the friction brakes have a lot less work to do than on a conventional car (without regen braking).....

 

[PluviaPlumbum]

Ventilated disc are lighter than solid discs -on account of the holes - Just like Massdammer cheese

 

[i3atl]

Ventilated disc are lighter than solid discs -on account of the holes - Just like Massdammer cheese

Sounds like you're describing drilled rotors - not the topic of this thread.

Vented rotors are heavier - see this comparison: http://www.dcperformance.co.uk/images/brake_disc_types.jpg

 

[stumbledotcom]

Any automotive engineer will tell you that vented rotors are superior to solid. Shorter stopping distances. More resistant to fade. Less likely to warp. Any weight or cost difference is minuscule and not worth the reduced performance and increased maintenance.

 

[PluviaPlumbum]

Ventilated disc are lighter than solid discs -on account of the holes - Just like Massdammer cheese

Sounds like you're describing drilled rotors - not the topic of this thread.

Vented rotors are heavier - see this comparison: http://www.dcperformance.co.uk/images/brake_disc_types.jpg

Yep your right, I was thinking of cross drilled discs/rotors which I realised when I was washing the car today

 

[imolazhp]

The i3 is an EV (or hybrid if you have a REx) but its still a BMW, some people will drive it like a typical BMW and will need good braking performance from their i3.

Why stop at the brakes? Why do they even offer 20" wheels? Why not this, why not that? Why buy a BMW at all?

I enjoyed the braking performance when I took it to an Autocross event. The few times I've had an emergency braking situation I enjoyed the shorter braking distance too.

 

[m6rk]

To the all reasons pointed out until now, I would like to add one more: Offering solid discs would be a weakness against other EVs. The cost & weight difference is negligable compared to the braking performance gain & marketing strength...
Vented disc brakes are standard in front of nearly every modern automobile. Zoe does not have them but the Leaf has...

 

[eneka]

I can tell you they stop well...went to the BMW ultimate driving event where you got to do acceleration and braking tests. It was non stop 0-40, 40-0 and didn't notice any fade at all! Even after like 10+ runs. Same with the 640i they had there which was able to hit 60mph

 

[prettig]

..... go into the mountains and decent with an i3 from 3000m.....then you know why (your foot will be on the brakes all the time).....carbon would be better.

 

[Zwerius]

Sorry,
Maybe I shouldn't have asked.
I live in a flat country. We don't have mountains.
I guessed that regenerative braking would be enough on long downhill slopes, but it semms that I was wrong.

 

[jadnashuanh]

When first released, the i3 (at least in the USA) had a higher regeneration capacity. Some people complained that they could not drive the car smoothly, so within the first year, BMW changed the software to reduce the maximum regen and later tweaked it again to change the ramp up/down curve and tip-in. Personally, I didn't have any issues with it as original, but even then, on a steep hill, it couldn't slow the car - maybe hold a speed, but not slow, depending on the slope. There is a maximum regen value, but I've not worked out how close the car ever gets to the value the batteries can sustain (nor do I know that that value is - 7400W is the max the system is capable of during acv charging, but not sure if the circuit could handle more during regen). The maximum amount of regeneration possible would also depend on the total weight of the vehicle, so that could get tricky.

 

[alohart]

There is a maximum regen value, but I've not worked out how close the car ever gets to the value the batteries can sustain (nor do I know that that value is - 7400W is the max the system is capable of during acv charging, but not sure if the circuit could handle more during regen).

With regards to the drive motor/generator, the 06 I01 High-voltage Components manual states:

"The maximum power of 125 kW can only be made available for a maximum duration of 30 s. Otherwise, the components of the drive train would be damaged through overheating – this affects not only the electrical machine, but also the high-voltage battery and the electrical machine electronics. The maximum power applies for the motor operation – in theory it could also be used in the alternator operation during brake energy regeneration. However, in practice only a fractional part of this maximum value is used in alternator operation. As a result, the braking torque at the rear axle is restricted so as not to affect the driving stability by the brake energy regeneration."

During regen, the Electrical Machine Electronics (EME) converts ~400 V of AC power from the motor/generator to DC power for the battery pack. The EME also converts up to 3.7 kW of 120V - 240 VAC power to DC power to charge the battery pack. A different controller, the Comfort Charging Electronics (KLE), optional in some markets, converts an additional 3.7 kW of 120V - 240 VAC power to DC power to charge the battery pack. The EME charging circuit must not be the same circuit used during regen because maximum regen power feels greater than only 3.7 kW / 125 kW as powerful as maximum acceleration power. I haven't found any specification for the maximum regen power actually available.