Steve Dallas

A word about using a lap timer app on your smart phone.

A friend and I often do track days together, and we are very competitive with each other. He and his Golf R put up very similar times to me and my 8. On one occasion, we were baffled by how Harry's Lap Timer could be reporting the lap times it was. For example, in one particular session, his LT showed faster lap times than mine, but I chased him down and passed him that session. It didn't make sense.

While bored at work the next week, I plugged our data files into Google Earth, and this is what I found after plotting the GPS coordinates on the track.

His iPhone:



My Galaxy S5:



My Galaxy's GPS is better than his iPhone's but they both show us off in the grass and in strange places on the track. The truth is, when a smart phone is maxed out on satellites (they often are not), the most precision you can expect is about 50 feet, and that translates to fractions of a second in lap times. If you want accurate lap times, you need an accurate GPS.

With that in mind, we both bought DUAL SkyPro XGPS160 external GPS units. We both run them under the rear window. It is accurate to about 6 feet and updates 10 times per second. We are seeing about 300 data points per lap vs. about 90 with the phone GPSs.

Here is a different track onto which I overlaid our data. You can see the improvements in accuracy for both cars. You can also see why my lap time was faster than his.

Our respective lines are much closer to what we actually drove.

SkyPro GPS:




In summary, if you want accurate lap times, you need accurate equipment.

pcs

iTrader: (2)

Did you guys ever run both the phone and the standalone timer to see what the variation is between the two?

We did that in my car with an Aim Solo and another unit that logs at 10hz as well, was pretty impressed at how close they read. On a 2:10ish lap, they were within .05s of each other IIRC

Steve Dallas

The phone GPS was that close to the Aim and the external GPS, or the Aim and external GPS were that close to each other?

Steve Dallas

Here is a plot from my car from last Saturday on the same track. This time I made it look the same as the first two plots for better comparison. It is not perfect in depicting the line I drove, but it is much closer (seems to be universally shifted about 6' to the right). And you can really see the 10Hz resolution by how close together the sample points are.

kbrewmr2

s/f line location & accuracy is important also - moving that a few feet can start to cause 0.00X range differences even on the same equipment, move it to even more and it can reach the seconds range. You're not taking every corner and accelerating down every straight perfectly the same each lap, nevermind any changes in how the car performs differently at different times in the session. Small differences but stack them up and it matters.

One time we had a guy end up with two transponders in his Viper at TWS, one in the front bumper and the other from his buddy's car in the glovebox. Approx 5-6 feet or so apart. Laps were 0.00X to 0.01X different (one faster some laps, other faster on others) even though everything was measured by the same AMB system with the loop embedded under the track and everything, most accurate system known. (well, maybe TAGHeuer and/or other transponder-based systems but you don't see those at club level events).

Also VERY possible for him to have one single lap faster, but for you to still run him down and pass (happens in WTW all the time all over the place). Running down & passing is driven more by average pace over many laps, not one single lap time. I got alot better race pace when I made myself chip away at gaps & focus on good average through whole race versus trying to string together Time Trials sorta flyers.

Be careful about getting to competitive/racey in an HPDE enviornment also please

(former T&S guy for several years, also many years after that watching over their shoulder as race official )

blu3dragon

iTrader: (1)

I think PCS was comparing transponder timing with the aim solo (which should have similar accuracy to your 10Hz gps timing). EDIT: actually from the other thread, he was comparing qstarz external gps with aim solo.

My own limited testing with the aim solo and a transponder timing system are that they are pretty close as well.

Aim solo vs my phone's internal gps? Not so close, easily 1s out. That phone was a few years old though. My new phone might be better, have not tried. Location matters too. Phone in your pocket is not going to get as good a view of the satellites as one mounted below the windshield.

pcs

iTrader: (2)

oohh, forgot that i had posted that previously... it was indeed the qstarz vs aim solo

hufflepuff

When doing Time Trials, my AMB transponder and my RaceChrono android app usually report within a tenth or two of a second. Not terribly accurate, but enough for me to do some basic comparisons.

Steve Dallas

We both mount our phones to the windshield in similar fashion, so location should not be an issue for either of us. I can't think of a good way to compare the phone GPS to the SkyPro directly. They will both log on their own, but lining up the data in the logs could prove problematic. What I can say is that our lap timers are much more closely reflecting our respective performances on the track. One of these days, I will spring for the real thing, but I have too much to do in the short term to do it now. Must keep feeding the car brakes and tires.

hufflepuff

Good call... racechrono *barely* worked when my phone was in my pocket, and then improved to a few tenths accuracy when mounted on the dash.

j9fd3s

this is a great point. lap times, and position are related, but not the same.

if the is a B, its that the phones are hooked to the network, and the network depends on signal, which depends on carrier, and how many people are on the network at the same time.