m6rk
Posts: 13
Joined: Sat Dec 26, 2015 5:51 am
Location: Turkey
Contact: Website

Speed sensor...

Tue Aug 04, 2020 3:35 am

Hi everyone,
Does anyone knows how i3 detects its speed? From a single wheel, two front wheels average or two rear wheels average? The reason I am asking is that to learn what happens if I have different wheel diameters at the front & rear axles. I know lower diameter will show higher number on screen, but what happens if -let's say- we keep the original diameter at the front & increase the diameter on the rear?

Thank you very much,
Casual XTRIathlete
47.492 km on a '14 i3 (got separated)
44.580 km on a '18 i3s (still together!)

alohart
Posts: 2121
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Speed sensor...

Tue Aug 04, 2020 1:16 pm

I would expect the traction control system to freak out if rear tires have a significantly different circumference than the front tires.
Aloha,
Art

2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE

Neil
Posts: 45
Joined: Sat Sep 06, 2014 10:11 am
Location: Davis, CA USA

Re: Speed sensor...

Tue Aug 04, 2020 3:02 pm

alohart wrote: I would expect the traction control system to freak out if rear tires have a significantly different circumference than the front tires.
A while back when I replaced my rear tires only due to a front tire availability situation, the traction system starting engaging on curves on my commute where it had never done so before. This was somewhat dangerous in one location where the freeway on ramp was a gentle down sloping curve and the car was prevented from accelerating. This problem went away once I was able to get the new front tires. So yes, it can freak out if the circumferences and therefore the rotational speeds are not what the system limit expects.

jadnashuanh
Posts: 5036
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Speed sensor...

Tue Aug 04, 2020 3:50 pm

I don't know on the i3 how it determines road speed, but it does use the wheel rotation sensors to check for uneven rotation rates to detect what it determines to be slippage. I do know that there's a computer in the loop, as BMW has chosen to increase the indicated speed by 2-mph across the range to account for slight differences in tires and to meet the German requirement that the speedometer can't read lower than your actual speed, but it appears that the odometer is accurate.

On most systems today, the speed sensor is now an electronic sensor in the drive-train, often the transmission, not at a wheel, since with the differential, and going around curves, the individual wheels' rotation will vary except if you're going perfectly straight, and the tires are all like new.

I did a quick search on a parts breakdown page (www.realoem.com), but it was not obvious where the actual sensor is from there.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above

m6rk
Posts: 13
Joined: Sat Dec 26, 2015 5:51 am
Location: Turkey
Contact: Website

Re: Speed sensor...

Wed Aug 05, 2020 7:11 am

alohart wrote:
Tue Aug 04, 2020 1:16 pm
I would expect the traction control system to freak out if rear tires have a significantly different circumference than the front tires.
I am thinking to keep original size at the front (175/55R20) and change the rear tyre's side wall by only 5" (55, instead of 50). This is because the 195/55R20 has more alternatives, less difficult to find, cheaper & more confortable -I assume-. The circumference increase is 20mm. (703 / 723).
Casual XTRIathlete
47.492 km on a '14 i3 (got separated)
44.580 km on a '18 i3s (still together!)

jadnashuanh
Posts: 5036
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Speed sensor...

Wed Aug 05, 2020 1:08 pm

That difference is nearly 3%...normally, they expect the tire sizes to be within 2%, so you may have some logic issues for the traction control, not counting the speedometer and odometer readings.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above

m6rk
Posts: 13
Joined: Sat Dec 26, 2015 5:51 am
Location: Turkey
Contact: Website

Re: Speed sensor...

Wed Aug 05, 2020 3:04 pm

jadnashuanh wrote:
Wed Aug 05, 2020 1:08 pm
That difference is nearly 3%...normally, they expect the tire sizes to be within 2%, so you may have some logic issues for the traction control, not counting the speedometer and odometer readings.
Hmm... Thanks a lot for that info... With a short net search I saw plenty of people who messed up with this issue on other brands...

So, another alternative that comes to my mind is... to put 195/55 R20 tyres both rear & front axles. Front wheels are 5.5J and rear ones are 6J.

This way I'll have same front-rear circumferences, a bit more flex on tyres, slower 0-100 kph and probably less mileage due to extra rubber weight & rolling resistence... And will have a max speed of (167.9 kph - 3 kph) 164.9 kph instead of 160 kph...

Where I live, the price of one set of Michelin Primacy or Nokian Wetproof is less then half price of stock Bridgestone EP500s...

First I will buy only the rear set. In case of having the issues you mentioned then I will buy another two for the front wheels....

Thanks again, I feel enlightened! :)
Casual XTRIathlete
47.492 km on a '14 i3 (got separated)
44.580 km on a '18 i3s (still together!)

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