Coast down testing

[TexaCali]

Anyone know of some good software for logging speed and time off a GPS for coast down testing? I figured there should be a nice iPhone app for something like this, but even a PC with an external GPS would be fine. All I need is a log of GPS speed at about 1 second intervals.

For the curious, the idea is to accelerate to a known speed on a flat course (with calm wind) and then place the car in neutral and log speed vs time until the car stops. Obviously you log multiple runs in both directions to average out any slope or wind. The resulting curve can be used to compute aerodynamic drag and rolling resistance. But I'm not really interested in actual drag numbers - the real goal is to compare various modifications. First up will be lighter and wider wheels/tires (there is a claim wider tires reduce rolling resistance, so lets find out). I also have some aerodynamic tweaks I want to test. If a change makes the car coast longer then I'll know it works.

Cheers!

 

[jmueller065]

You could use one of the bike tracking Apps (I had one on my iPhone but don't remember what it was called). It would record your route and then let you export it as a comma-separated file.

Here is one, for example:
https://itunes.apple.com/us/app/map-my-ride-gps-cycling-riding/id292223170?mt=8

 

[hybridbear]

Ski Tracks might also accomplish what you want.

 

[TexaCali]

Thanks. There are also some car speedometer apps, but what isn't clear on any of these (until you install the app) is will it export a log file with readings of at least once per second? Guess I'm just going to have to try a few.

 

[michael]

It's probably more than you want, but a device called GPS Tracking Key (http://www.landairsea.com) does what you want. It's a standalone logging GPS tracker with an app that shows everything the car did. It exports CSV logs.

P.S. I'd be very surprised if wide tires have less resistance. Bicyclists try to go for the narrowest tires at the highest pressures.

 

[unplugged]

P.S. I'd be very surprised if wide tires have less resistance. Bicyclists try to go for the narrowest tires at the highest pressures.

Wide tires have more wind and friction resistance. See BMW i3 for an example of going in the weirdest opposite direction as the FFE.

 

[TexaCali]

Clearly wider tires would have more wind resistance, but the rolling resistance is a bit more complicated.

Wider tires are designed to carry a heavier load, so if the load does not change, a wider tire should deform less, and thus less energy will be used to flex the tire (and less heat generated). If you do some internet searches, you can find some data (for both automobiles and bicycles) that supports the claim that wider tires do in fact reduce rolling resistance, and that that reduction is even greater than the increase in wind resistance. Probably the most scientific discussion of the subject I've found is this - http://www.barrystiretech.com/rrandfe2.html

But since bicycles were mentioned, there are plenty of articles like this:
http://velonews.competitor.com/2012/03/bikes-and-tech/technical-faq/tech-faq-seriously-wider-tires-have-lower-rolling-resistance-than-their-narrower-brethren_209268

All that said, I still remain skeptical, and that is why I intend to perform coast down tests. Unfortunately I wont be able to eliminate all the variables in my tests, so my data will still not be conclusive, but at least I'll know if the changes had a net positive or negative impact.

 

[unplugged]

...you can find some data (for both automobiles and bicycles) that supports the claim that wider tires do in fact reduce rolling resistance, and that that reduction is even greater than the increase in wind resistance. Probably the most scientific discussion of the subject I've found is this - http://www.barrystiretech.com/rrandfe2.html

I just betcha that BMW engineers would disagree with that suggestion since the i3 has such obviously thin tires as to be comical.

But in any case, it would be interesting to see the difference between wide and thin tires on the same car in a coast down comparison.

 

[TexaCali]

...you can find some data (for both automobiles and bicycles) that supports the claim that wider tires do in fact reduce rolling resistance, and that that reduction is even greater than the increase in wind resistance. Probably the most scientific discussion of the subject I've found is this - http://www.barrystiretech.com/rrandfe2.html

I just betcha that BMW engineers would disagree with that suggestion since the i3 has such obviously thin tires as to be comical.

But in any case, it would be interesting to see the difference between wide and thin tires on the same car in a coast down comparison.

There are other reasons for going with thin tires other than rolling resistance:

1. Less material = less environmental impact = more ecco friendly. The "ecco friendly" aspect of electric cars seems to be a big marketing push for most manufactures. We get leaves, butterflies, green badges, and marketing bits about recycled and sustainable materials, etc. etc. Thin tires certainly "look" more green.

2. Lower weight. Reducing mass, esp rotating mass has to be an important goal in EV design, and the thinner the wheel the better in this respect.

3. Reduced material cost. Simply put, less rubber to build the tire means less cost in building the tire. Of course BMW will negotiate that with the tire mfg, but when you and I purchase replacements they will charge us more because it is an odd sized tire....

I agree, I would love to see an apples to apples coast down test. The only meaningful test would involve using the exact same tire compound for the different width tires. Compound probably matters more than width, so switching compounds makes the results meaningless.

All that said, I now have about 60 miles on my wider tires. I don't have any coast down data yet, but based on my daily trips, I'm not seeing any significant change in watt hours/mile. I should point out:

1. My new tires are not the same compound as the originals. While I have no hard data on the various compounds, I suspect this new compound (Primacy MXM4) would yield higher RR than the stock (Energy Saver A/S). That is if you could test the identical size tire in both compounds, I bet the Energy Saver would win. Too bad they don't make an Energy Saver in 235/40R18 or even 235/45R18.

2. Tires have higher RR when they are new and take a while to break in. In other words, once I get a few hundred miles on them the RR should decrease slightly.

Once these tires are broken in I'll try to collect some hard data - even if the compounds are different.

 

[EVA]

And I bet you just smile every day you drive with those new tires, wheels, and paint.

 

[TexaCali]

 

[TexaCali]

Indeed, and they both have Michelin's "Green X" rating. But I figured a score of "10" wasn't a very high resolution measurement and there could conceivably be a big gap between two "10's". Also, if you look at Tire Rack's tests (where they measure fuel economy) the Energy savers beat the tires they compared it against, but the Primacy's (different test, different car and different set of tires to test against) didn't win in their test. Kinda hard to extrapolate between the two, but it does lead me to believe the Energy Savers are more of a "10" than the Primacy's.

All that said, I installed the new wheels and tires on June 4th, and take a look at my consumption:

Now my driving varies a lot from day to day - I'm not always making the same trip at the same time. Traffic can vary a lot, sometimes it is more freeway, sometimes more surface street, sometimes I'm cruising at 60, other times I'm crawling at 15-20, Sometimes AC is blasting, sometimes not, etc, etc. So I wouldn't read too much into the above results other than the swap to wider tires of a different compound certainly did not kill my range.

 

[twscrap]

3. Reduced material cost. Simply put, less rubber to build the tire means less cost in building the tire. Of course BMW will negotiate that with the tire mfg, but when you and I purchase replacements they will charge us more because it is an odd sized tire....

$140 isn't too painful...

http://www.tirerack.com/tires/tires.jsp?tireMake=Bridgestone&tireModel=Ecopia+EP600&partnum=57QR9EP600&vehicleSearch=false&fromCompare1=yes

 

[TexaCali]

Wow - that is a lot better than I would have guessed for an "unusual sized" tire. Or is that size used on other vehicles we just don't know about (sold overseas, industrial use, etc)? I remember when I got my E36 M3 the tires were (at that time) an unusual size and they were silly expensive. Pretty inexpensive now, but back then they were out of sight.


Back to RR on my new wider tires - I just had to make a quick freeway dash (~14 miles) to a meeting and a quick dash back to another meeting (another ~13 miles). Lets just say I was in the left lane and moving as fast as I dared. The AC was also blasting. MyFordmobile claims I used 251 wh/mile on the way up and 287 wh/mile on the way back. I honestly thought it was going to be over 300! Once that was done I went back to my normal driving and saw sub 200 on a short freeway stint. I still can't say for sure if there was any change from the old tires/wheels, but I certainly don't feel I have anything to complain about.