Harry's GPS Suite Forum

Ah, so your car is pretty heavily modified, but still streetable. Technically, so is mine, but without A/C, it's pretty brutal in the Texas summers. And since the spherical bushings aren't sealed, they shouldn't be driven in the rain, so my car's street use is pretty much relegated to driving to and from the track, and picking up a few groceries on the way home.

I'm running a lot of negative camber: -3.5 degrees up front, and -2.7 degrees in back. The entire suspension set-up was done per the recommendations of one of the best Porsche track prep guys around, and the g-forces I'm seeing, on relatively narrow wheels (I would love a set of 17 x 10.5's all around, but given my car's only-for-Porsche bolt pattern, this would mean going custom, which would lighten my wallet by nearly $6K - ouch), seem to indicate that he knows what he's doing.

Interesting observations about tire pressures. I was running 36 psi cold in the Rival S's, and they seemed pretty happy there, but when I started with this value in the Nitto's, my times were abysmal. So, on the advice of the lead instructor, I dropped the pressure to 36 psi hot (equivalent to about 31 psi cold), and my times dropped by about a second and a half on the 0.92 mile track. So your observation about more race-oriented tires needing less pressure seems to be spot on. I will experiment with this going forward. And no, I haven't checked my tire temps with a pyrometer yet, although I plan to do that when I take some private lessons next month. My wear patterns have always been very uniform across the tire face across multiple sets of tires, so I assume the camber, and the pressures I'd been running, were pretty reasonable.

I travel fairly long distances to events and don't have a trailer and tow vehicle, so being comfortable inside the car is important, i.e. AC, cruise control, radio, etc.

The general rule is that if you need more grip, you lower the tire pressure if the tire isn't rolling over on the sidewall. But you would get more accurate information with a pyrometer.

Harry gave a great explanation of the variance that may come frm the data, but at first-order, i buy those numbers. Maybe its really 1.15-1.2, but let's start with: your car was on hot Nittos. C&D was on cold street tires. That's a gigantic difference right there.

Using my Solo , I routinely see 1.1G and above, and i may be consistent, but i'm not all that brave!

G

FYI: G-Force readings are different with a good mount versus a shaky mount

I recorded videos with two identical phones at the same time a few weeks ago. One of the phones was on a shaky mount and one was on a rock-solid mount.

Cornering G's with a solid mount = 1.0
Cornering G's with a shaky mount = 1.3 (same turn at the same moment as above)


The only difference is mount rigidity - the 2 phones measure G's equally on good mounts and both phones were installed level so positioning would not affect G-Force readings.

Unrelated, These two screen shots were taken at the same moment, but the speeds were printed at 62 and 64 . . I think this happens when speeds are changing rapidly during braking or accelerating.

When I calculate lateral g from GPS speed and yaw rate, I get pretty much the same numbers as with the iPhone accelerometer. I have a RAM mount with a short arm and a cradle, not the X-grip. It's pretty solid.