engineeringDynamo
Posts: 4
Joined: Mon Aug 07, 2017 8:36 am

Looking for smaller rims for my i3

Wed Aug 09, 2017 10:40 pm

I want to get 16, 17, or 18in rims for my i3, but I am not able to find any.
IMO big rims are a triumph of marketing over engineering.

alohart
Posts: 930
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Looking for smaller rims for my i3

Thu Aug 10, 2017 3:31 am

engineeringDynamo wrote:I want to get 16, 17, or 18in rims for my i3, but I am not able to find any.
IMO big rims are a triumph of marketing over engineering.

In the case of an i3, large diameter wheels were an engineering choice to allow the use of narrow tires with less aerodynamic drag and rolling resistance. The total contact patch area is independent of tire size and depends only on inflation pressure and the weight supported by the tire, so narrow tires don't reduce the contact patch area. To have the necessary contact patch area to support the weight of an i3, a smaller circumference tire would need to flex more to create the contact patch which would increase its rolling resistance. Narrow tires also allow the wheel wells to be narrower to maximize interior space.

The only advantage of a smaller diameter wheel that I can think of would be a higher aspect ratio tire to keep the circumference the same which would provide a more comfortable ride, especially compared with the 20" tire.
Aloha,
Art

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

engineeringDynamo
Posts: 4
Joined: Mon Aug 07, 2017 8:36 am

Re: Looking for smaller rims for my i3

Thu Aug 10, 2017 12:49 pm

I am not trying to put a wider tire on my i3.
I am looking for a wheel+tire combination that is the same bolt pattern, tire width, and tire outside diameter as the original setup.
It must weigh less, and have a lower moment of inertia about the axis of rotation than my current 19in setup.
Because the wheels are pretty unique, and BMW does not offer 17 or 18in rims, I think they may not exist.

Vehicle dynamics is a complicated subject.
It tends to take sophisticated simulation to get the results you want in a car.
I am not going to ever be allowed to see BMWs discussion, modeling, or experimentation about 18 vs. 19in rims.
In this forum we can go back and forth about how a wheel+tire combination affects dynamics (and every statement we make should be qualified with "generally" or "usually" unless we talk about specific rims and tires).
I am glad that we agree that a smaller diameter rim with more tire sidewall usually provides a more comfortable ride.
But I disagree that this is the "only advantage".
Another advantage would be that the tire and wheel would be more likely to survive an impact with a pothole.
Given the same tire width, and tire outside diameter, and the same style of forged rim by the same manufacturer, in general:
larger rims with less sidewall are heavier.
larger rims with less sidewall have a higher moment of inertia about the axis of rotation. This is true despite the fact that smaller rims with more sidewall need more material in the tire sidewall to achieve the needed sidewall stiffness.
There are detailed discussions of this on sports car and racing forums. You can read many stories about miata drivers tracking their car on larger rims and low profile tires, only to be disappointed with a higher lap time.
Even formula 1 has considered bigger rims, but "it's not an attractive direction performance-wise":
http://www.autosport.com/news/report.php/id/119331
(the pic in this article is not a Formula 1 wheel, this is a Formula 1 wheel http://jalopnik.com/5922489/the-wheels- ... rmula-one/)

I think the 18in rim and tire that I want does not exist.

alohart wrote:In the case of an i3, large diameter wheels were an engineering choice to allow the use of narrow tires with less aerodynamic drag and rolling resistance. The total contact patch area is independent of tire size and depends only on inflation pressure and the weight supported by the tire, so narrow tires don't reduce the contact patch area. To have the necessary contact patch area to support the weight of an i3, a smaller circumference tire would need to flex more to create the contact patch which would increase its rolling resistance. Narrow tires also allow the wheel wells to be narrower to maximize interior space.

The only advantage of a smaller diameter wheel that I can think of would be a higher aspect ratio tire to keep the circumference the same which would provide a more comfortable ride, especially compared with the 20" tire.

Hypermiler
Posts: 2
Joined: Mon Jan 09, 2017 12:27 pm

Re: Looking for smaller rims for my i3

Thu Aug 17, 2017 7:16 am

Am having similar thought process. Also wondering could I improve acceleration by running smaller circumference? Am not using top speed anyway... .

Suggestions:

A: did you try this:

https://tiresize.com/calculator/

B: Let us see what the i3s version will have in terms of tire sizes.

Best,
HyperMiler

jadnashuanh
Posts: 3450
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Looking for smaller rims for my i3

Sun Aug 20, 2017 3:05 pm

FWIW, the wheels used on the i3 are forged, and are quite light compared to most any cast wheel you can find. Not sure what you'd find. Going to a smaller OD tire would make the odometer and speedometer less accurate, and if you were on a lease, you'd be putting more indicated miles on the vehicle, which may cost you money or decrease the resale price if you bought it. IT would also affect when your warranty expired on the main car and the battery. People have said they had damage when using the 5.5" rim from the rear on the front...It does not appear that there's very much clearance, and a wider tire and wheel combination would need to be carefully selected, if available at all. Some cars lend themselves to lots of tire/wheel choices...not sure the i3 is in that group. The EV has maximum torque at zero, so lowering the gear ratio with a smaller OD probably wouldn't buy you much of anything...it would appear as you reached your target speed faster, but the speedometer would be inaccurate. It's not like an ICE where you want to build revs so your power and torque peaks are reached faster...they start out high with an EV.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

engineeringDynamo
Posts: 4
Joined: Mon Aug 07, 2017 8:36 am

Re: Looking for smaller rims for my i3

Wed Aug 30, 2017 8:29 am

According to Engadget, the i3s will come with 20in rims.

https://www.engadget.com/2017/08/28/bmw ... bocord-ev/

jadnashuanh
Posts: 3450
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Looking for smaller rims for my i3

Wed Aug 30, 2017 3:00 pm

ANd, the track is wider plus the fenders flared to allow them to fit.
Jim DeBruycker
2011 535i x-drive GT, 2014 i3 BEV

bdjuju
Posts: 3
Joined: Wed Sep 20, 2017 4:29 pm

Re: Looking for smaller rims for my i3

Thu Sep 21, 2017 9:03 am

Mercedes and VW/Audi also make wheels in this 5x112 bolt pattern. Whether you can find something with the backspacing you need is another story, but youre not limited to just BMW wheels. I would be interested in seeing the fattest tires one could fit inside the fenders, as well.

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