Joff
Posts: 76
Joined: Wed Jan 02, 2019 2:44 am

Tire pressure and miles per Kwh

Mon Apr 01, 2019 5:32 pm

It is accepted that higher tire pressure results in lower rolling resistance. Has anyone measured the difference in in raising the i3's tire pressure over stock spec? I would think that if higher pressure can give a measurable increase in mpg in an ICE car, the difference should be higher in an electric that isn't using most of its fuel making waste heat. On the other hand it seems like an easy thing to measure, and I haven't see anyone posting about it, so maybe the improvement is insignificant. Thanks for any insights!

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Tire pressure and miles per Kwh

Mon Apr 01, 2019 6:37 pm

It probably makes a little difference. Raising the pressure will also tend to wear out the center of the tread, shortening the life of the already low life tires. The rubber compound and tread and carcass are all designed for low rolling resistance.

Running the tire at the extreme limit also could subject the tire to damage if you hit a large road imperfection...the sidewall acts like a spring, and as you raise the pressure, it can't flex as much, which puts more stress on the carcass and the springs, shocks, and passengers.

Going to the extreme can also lengthen your stopping distance as you'll end up with a lower tire contact patch. It can also affect your cornering performance.

The springs and shocks are tuned for the specified pressure. It's your car, change the settings if you want but be aware of the consequences.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

alohart
Posts: 2431
Joined: Sat Nov 01, 2014 7:36 pm
Location: Honolulu, HI

Re: Tire pressure and miles per Kwh

Mon Apr 01, 2019 7:34 pm

For the first 5k miles of i3 BEV ownership, I inflated all tires to 10 psi over the recommended pressure which put the pressure of the rear tires at the sidewall maximum pressure of 51 psi and the front tires at 43 psi. After 5k miles, I measured the tread depth at 3 locations across the width of each tire, inside, center, and outside. Tread wear was uniform across the width of each tire, so I did not measure any increased wear in the center of the tread. I think the wear pattern of radial ply tires is less sensitive to inflation pressure than that of old bias ply tires.

I then reduced the inflation pressure to the recommended pressures. After another 5k miles, I will measure the tread wear again, but that hasn't happened yet. I immediately sensed that the rolling resistance of the tires was significantly greater. Our i3 did not roll as freely and was more difficult to push by hand. However, I did not quantify this difference by measuring a difference in efficiency. iRemote/BMW Connected reported a 5.5 mi/kWh average during the first 5k miles. After driving ~3k miles at the recommended inflation pressure, my average efficiency has dropped to 5.1 mi/kWh. However, I can't claim that all of this decrease was due to the reduced inflation pressure. For example, I probably used climate control more often because during the first 5k miles, I did not drive our i3 during the summers as I began doing during the current 5k mile test period.

Traction and ride quality are almost certainly reduced at higher inflation pressures. Neither of these was a problem for me However, had I needed to make an emergency stop, I might not have been able to stop in as short a distance at higher inflation pressures.

I think that higher inflation pressures protect wheels against damage when driving over a sharp pothole edge because the tires can't collapse as easily thus exposing the wheel to damage from the pothole edge. Higher inflation pressure does transfer more of the bump absorption to the suspension, but so do the aftermarket lower-height springs that some owners install. I haven't read about any suspension damage caused by these springs, so I'm not concerned about higher inflation pressure damaging the suspension.

If I don't measure any tread wear advantage running the recommended inflation pressure, I'll probably increase the inflation pressure again as I did for 15 years driving our Honda Insight without any tire or suspension damage.
Aloha,
Art

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

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Tire pressure and miles per Kwh

Mon Apr 01, 2019 9:09 pm

The i3's wheels are forged rather than the more conventional alloy wheel which makes them both lighter and stronger. But, an impact puts more stress on the wheel and suspension if the pressure is higher. The i3's 20" wheel option is more prone to wheel damage than the 19" one because the sidewall is shallower. That's true for any vehicle. Excessive pressure can cause damage. Now, whether you'll experience damage, it takes a fairly severe impact to cause it, so probably not. To me, the cost/benefit does not add up to the potentially improved efficiency.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

Joff
Posts: 76
Joined: Wed Jan 02, 2019 2:44 am

Re: Tire pressure and miles per Kwh

Tue Apr 02, 2019 12:02 pm

[quote="jadnashuanh"]It probably makes a little difference. Raising the pressure will also tend to wear out the center of the tread, shortening the life of the already low life tires.

That effect is more pronounced on wider tires. One hypermiler (different car) found he got more miles out of his tires over inflated 10 psi. He surmised that less flex resulted in lower temperature and cooler rubber is harder and longer lasting. I have seen rims being damage from underinflation but not over

Joff
Posts: 76
Joined: Wed Jan 02, 2019 2:44 am

Re: Tire pressure and miles per Kwh

Fri Apr 05, 2019 2:50 pm

I raised my tire pressure up 10 psi higher than the spec. I haven't done the most scientific of comparisons yet, but I have gotten the highest miles per Kwh that I ever got. 9.3 mi/kwh according the bmw connected app. I think around 6 was my best before and my average (not trying to conserve amps) is 4. Don't know how accurate it is or why the trip odometer gives me a different number. It was a 12 mile round trip so going down hill was not a factor. After a recharge, the guess o meter (range) showed 7 miles more than the previous charge 5 miles higher than I have ever seen on it. so that seems to confirm a measurable improvement. For a more accurate measurement of that improvement, I found a road loop circling a reservoir that I could drive around at a steady slow pace, I'll do a few laps at higher pressure, the laps at stock pressure then back to higher pressure.

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Tire pressure and miles per Kwh

Fri Apr 05, 2019 7:15 pm

FWIW, the tires do heat up as driven, so you'll need relative pressures unless you let things cool back off to ambient in between runs.

The roads where I live are tough with the stock pressure...adding 10 more pounds wouldn't be worth it to me.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

ksnax
Posts: 21
Joined: Sat Nov 10, 2018 8:02 am

Re: Tire pressure and miles per Kwh

Sat May 25, 2019 11:37 am

The easiest way to measure the difference in such a change is with 'coast-down' testing.

Using the same piece of road with no traffic, repeated runs are timed from a set speed down to a predetermined low speed while coasting. The i3 would need to be placed in roller mode to do this, but multiple runs over the same stretch of road could easily be done in a very short time to assess the difference.

Since this is about rolling resistance instead of aerodynamics, it would make sense to skew the test speeds to the lower end, perhaps even to a complete stop.

ksnax
Posts: 21
Joined: Sat Nov 10, 2018 8:02 am

Re: Tire pressure and miles per Kwh

Sun Jul 21, 2019 10:55 pm

I recently conducted some coast down tests on my i3. And contrary to what I stated before, roller mode is not necessary, as neutral works just as well.

That said, what I noted was that the recommended pressures for the i3 are a little on the soft side and are soft enough to hurt rolling resistance slightly. I was able to determine that 45 psi is about where efficiency returns flatten out however. Anything over that doesn't do much but stiffen up the ride. The BMW spec pressures seem to knock about 2-3% off of roll-out distance. At 65 MPH, that is 4-5 miles on a 120 mile range being sacrificed. At 30 MPH, that's 12-15 miles (but you are obviously going to go a lot further at lower speeds anyway with minimal aerodynamic resistance in effect).

I've started running my tires at 45 front / 50 rear and I honestly like the way the car drives and handles there as well. The higher pressure should also help substantially against pothole damage to sidewalls and rims. I really don't have need for additional range 99% of the time, but it's nice to know another way to get it.

Joff
Posts: 76
Joined: Wed Jan 02, 2019 2:44 am

Re: Tire pressure and miles per Kwh

Sun Aug 25, 2019 7:28 pm

jadnashuanh wrote:
Mon Apr 01, 2019 9:09 pm
an impact puts more stress on the wheel and suspension if the pressure is higher.
Well that sounds reasonable, but it is only true if the tire never "bottoms out" Hit a big enough pothole, and low profile tires absolutely will. All photos of i3 rim and tire damage from potholes that I have seen, show a tire that did not have enough pressure to keep the rim far enough away from the sharp edge.

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