dbbarron

After my first DE with a SoloDL/SCHD I have been digging into my data.
One thing I though I'd take a look at is wheel slip. I am using a math channel which is individually taking the difference of GPS speed - each wheel speed over GPS speed and then converted to a %. Thus, I get 4 values for slip and if negative it shows actual wheel speed > GPS speed.

For instance: 100*(GPS_Speed-SM_WHSPD_FL)/GPS_Speed

I looked through my laps and found typically slip is below 6-7%, nominally 4-5% through corners and braking.

I did find one example I wanted to get some feedback on. This is a lap from Palmer. There are two peaks; a positive peak on the entry into turn 9 (a relatively sharp and banked on entry (but not exit) right hander. Slip goes positive on the entry indicating wheels speed is slower than GPS speed. I think this is just a GPS error during hard braking, but I'm not sure.

The more interesting is the negative peak for slip exiting the corner. Both the Rear Right and Front left wheel appear traveling 11-12% faster than the GPS speed. I'd call this slip on power out, but why the front? Again, I'm going to GPS error and not actual wheel slip.

Thoughts?
db

ProCoach

Have you validated your GPS Speed versus your wheel speed measurements? I assume you have measured the difference between Porsche's OEM tire fitment and your aftermarket performance tire circumference to take this into account?

dbbarron

GPS_Speed and individual tire speeds seem very close, I'd say within 2%. Hard to tell since no straights at Palmer.

dbbarron

Since there are no straights at Palmer, hard to validate, but I have looked over the data of ECU wheel and vehicle speed data seem very close to GPS speed when it should be (straighter sections). I'd say within 2%.

ProCoach

What are the sample rates of each? 10 Hz is the GPS, what are your wheel speeds sampled at? I'd even them up and check again, for the sake of your math...

dbbarron

All 10Hz.

ProCoach

I wouldn't do a slip against GPS speed, that's for sure. You could do it against Ground Speed, Drive Speed or something crafted from the wheel speeds, but not against GPS speed.

http://www.motec.com/forum/viewtopic.php?f=11&t=1352

dbbarron

Against SM_VEH_SPEED, which I think is derived from the individual wheel speeds, no outliers at all. Everything within +/- 4-5%. Of course, this is somewhat flawed in that that the wheel speeds directly contribute to VEH_SPEED. Nonetheless, if a wheel spun up, I think it would be noticeable.

Matt Romanowski

I've seen lots of different ways to measure slip, but never by using GPS speed. It sounds like you have really good correlation for GPS speed to wheel speed, but the rolling radius changes with torque (braking or accel), speed, and vertical load. Most people compare the wheel they are interested in against either an average or a max. For instance, RR would be compared against either the front average or the faster front.You can also do the same for the rear versus the front (average or max).

Take a look at the data from my car. The FL wheel speed sensor wasn't working, but compare the FR to the GPS speed and you'll see that while very close, they won't match. You'll also notice that the GPS doesn't pick up the wheel locks, carrying a wheel (I don't think your car does that though), or other actual wheel slip.

The GPS at 10Hz may not be fast enough for this sort of analysis. I would look at how GPS slip compares to actual wheel speed slips. Also look to see if you lose any satellites or GPS accuracy at those points. I had a couple of spots at Palmer where I was losing some satellites.