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i3 and i8 drive ratio

Posted: Fri, 14 Sep 2018, 10:24
by HeadsUp
I have been watching an assembly video on the drive train for the i3 and i8

They appear to have reduction gears in the transmission with around a 1:4 and 1:6 reduction ratios ( 1 : 24 total )

If the drive tyres have a diameter of 27" ( 20 inch rims ) , that means the tyre circumference is around 2155 mm

wheel RPM must be 860 RPM to provide 110 kmhr speed.

860 RPM at the wheel / 1:24 ratio means the motor would have to do 20640 RPM

I cant see anyone going higher than 6000 - 8000 RPM in a drive motor.

Whats wrong in the numbers ?

scroll forward to 1:50 on the video to see the gear ratios.


Re: i3 and i8 drive ratio

Posted: Fri, 14 Sep 2018, 12:59
by Richo
2018 i8
Fourth Gear Ratio (:1) : 1.14
Final Drive Axle Ratio (:1) : 3.68

Total 4.1952:1
851 RPM wheel -> 3570 RPM Motor

Re: i3 and i8 drive ratio

Posted: Fri, 14 Sep 2018, 13:12
by HeadsUp
thanks Richo

The i8 has its petrol engine and 6 speed transmission if i am interpreting their spec page correctly whereas the i3 has only the option of the range extender petrol engine generator.

The i3 spec still states it has a single speed transmission , and the video supports that although the video referred to both i3 and i8 i think they showed the single speed version.

Perhaps what they are showing in the youtube video is "not entirely accurate" to sidestep theft of intellectual property while its still early in its production ?

Re: i3 and i8 drive ratio

Posted: Mon, 17 Sep 2018, 12:42
by Richo
The i3 says 9.665:1.
There is a nice graph of the power/torque graph.
Peak at 4775 RPM and max RPM 11,500.

I'd always trust info on a data sheet over a promo video.
The old Azure dynamics AT1200 was about 10:1 so nothing really outlandish from BMW.