What kind of cornering g forces are you guys recording?

[Cloud9...68]

I have a fairly decent car - a 1992 Porsche 968 with a full race suspension (all rubber bushings replaced with solid/spherical units, camber plates, track-level spring stiffness, Moton Clubsport shocks/struts, lowered, 17 x 9" wheels at all four corners with Nitto NT01's, track alignment, all done per the recommendations of a very successful Porsche race prep guy), plus I've taken about 250 pounds out of the car. So, I expect it to be capable of quite a bit more than the 0.90g cornering Road & Track recorded when they tested the car back in 1992, but the numbers I'm seeing in Laptimer are pretty eye-popping - I routinely exceed 1.2g in the fastest corners on the track, and the highest value I've recorded so far is 1.32g. I let a pro racer drive the car for a few laps a little over a week ago, and he hit a max of 1.34g. These numbers are higher that those recorded for for any car in the history of Car & Driver's Lightening Lap test at VIR, so I'm a little suspicious that they're pretty optimistic. It's all relative, so the numbers themselves don't really matter, but I'm curious what others are seeing. Thanks.

[Harry]

Accelerometers deliver quite noisy data. Ideally, LapTimer will smooth this data to an extent you get reasonable max values. The picture below shows the G distribution (both lateral and lineal) of a lap in Spa Francorchamps with 1.12 G max acceleration. The data has been recorded by a fast and consistent driver (not me). I have highlighted the areas of max acceleration. What can be seen is these areas show a high concentration / condensation (in German: Häufungspunkt) and will include the real max value. In case your data looks a lot less clustered with spots to the left and right - ignore them, they are noise.

Screenshot_20160914-174016.png (247.25 KiB) Viewed 6364 times

Typical accelerometer precision for smartphones seems to be 0.01 to 0.02 G (see https://www.uni-weimar.de/kunst-und-ges ... ensors.pdf)

- Harry

[Cloud9...68]

I'll check the friction circle data when I get home (out of town for a few days) to see if the high readings I'm seeing are likely to be outliers. I've been monitoring the data by slowly dragging my finger around chart of the track, and I can see the g's gradually building as I go around the fastest corners (all the corners, actually, lol), and the >1.2g values I see don't appear to be sudden spikes - they usually appear where I'm applying power at or just past the apex. Oh, and I'm using a Dual XGPS160, not a smartphone's built-in accelerometer. But I agree that it's hard to imagine that my car is actually capable of much more than 1.1g or so.

[Harry]

That's how smoothing using a rolling average works... 1 1 5 1 1 may result in 1 2 3 2 1. LapTimer has better algorithms for generating acceleration from GPS, but the measured acceleration will look more like the sample. You really need to identify the condensed areas.

[gplracerx]

Road test lateral g for cars and tires is usually done by driving on a circular skid pad with known diameter and measuring the lap time. Sticky tires are pretty much guaranteed to overheat and lose traction in those conditions. I routinely see over 1.2 g lateral autocrossing my 2011 Mustang GT with either Bridgestone RE-71R or BFGoodrich Rival S tires. I would expect to see higher with NT-01's, especially if they've been shaved to 3/32 to eliminate most of the tread grooves. For autocross, Hoosier A7's are better than the Nitto, but remove you from any street tire class. Those require a treadwear rating of 200 or higher.

I've seen comparison tests that included laps around a race track and peak lateral g in each corner. Those numbers are always higher than the skid pad numbers.

[Cloud9...68]

Yes, I understand cornering forces reported by magazines are taken by driving around a skid pad and converting the measured speed to g's, but more recently, the magazines have been reporting the g's measured in specific corners on road courses. They tend to be a bit higher than skid pad numbers from the same cars, but not by all that much. Case in point is the Lightning Lap article in the current issue of Car and Driver, which summarizes ten years worth of recorded cornering force through Turn 1 at VIR. The highest value they ever recorded was lat year in a Corvette Z06 on Michelin Pilot Sport Cup 2 ZP tires. I'm not sure how they're making these measurements. But I'm sure Harry's right - I'll take a look at the friction circle data before concluding I created a world-beating car in my garage from a lowly 968 .

Oh, and I just switched from BFG Rival S's to NT01's, and was surprised, and slightly disappointed, to find that the Nitto's were no faster, nor did they generate higher cornering forces, than the Rivals, although I like they way the NT01's are wearing - the Rivals started chunking after the very first session, while the Nitto's are wearing much more evenly, and very slightly at that. I didn't get the Nitto's shaved.

[gplracerx]

The magazines use OEM tires for their tests too.

I think of the Rival S and RE-71R as autocross tires mainly. They turn on at fairly low temperature, fairly quickly. They have to, given that an autocross run is usually less than 60 seconds. If I were going to try to run either of those on a track, I'd probably shave and heat cycle them. Chunking is usually a sign that the tire needs to be shaved. I remember back when SCCA Showroom Stock class was introduced. Someone I knew took a brand new set of radial tires with full tread out on an RX-7 at Road Atlanta and pretty well destroyed them in one session. Meanwhile, other people with the same tire shaved to 4/32 were doing just fine.

The same thing happens to rain tires in the dry for the same reason. The tread blocks flex too much and overheat.

[gplracerx]

That's a lateral g plot from a recent autocross in my CAM-C (Classic American Muscle - Contemporary) 2011 Mustang GT with 285/35/19 Bridgestone RE-71R's on 19/10.5 inch wheels on all four corners. The blue line is calculated from GPS fixes and the purple line is accelerometer data (90 Hz setting). Peak lateral g is clearly over 1.2, and not just from noise, on several occasions, the peak just past 0.2 km, for example. The near 1.5 g readings are probably artifacts though. That's my calculation of lateral acceleration from GPS data, not the one from HLT.

My car is rather far from stock, but several similar models in the Street class, where modifications are more limited, were turning similar times at this event.

[Cloud9...68]

It's true that the magazines use OEM tires for these tests, but on the high-end exotics, that means barely-streetable models like the Pilot Cup Sport 2 ZP, which are in the same category as the Rival S and RE-71R. I agree that the Rival S is a good autocross tire, but if Grassroots Motorsports is accurate, the Rival S was developed at least partially in response to autocrossers' complaints that the Rival wasn't that great of an autocross tire. GRM has done extensive and very frequent tests on tires in the extreme summer category, on both autocross and road coarse venues, and has found the Rival S and the RE-71R to be the top (affordable) tires (with the RE-71R slightly edging out the Rival S) in both types of driving. I do agree, based on the wear patterns I've seen on my car, and a couple of friend's cars, that both of these tires would benefit from shaving, and probably heat cycling as well. In contrast, my Nitto's are just showing a faint, orange-peel like appearance, with no sign of blistering or chunking, after their first session. So, in that respect, I'm happy with them, but in terms of lap times and recorded g forces, they're no better than the Rival S's. Hopefully that will change as I acclimate to them. One thing the track pro found is that they like to run at significantly lower pressures that the Rivals, so there's probably still room to optimize there.

As far as recorded g forces, I wonder how much of it has to do with the fact that my (and from your description, it sounds like yours as well) car is pretty much a track-dedicated toy. No radio, no AC, most sound-deadening insulation removed, no rubber bushings to absorb the blows delivered through the 700/1000 lb F/R coilovers, etc. We should be able to generate some pretty respectable cornering forces, though probably not quite at the levels being recorded.

[gplracerx]

I haven't done anything to the interior of my car other than put in a better seat, a Sparco Chrono Road. It's still my daily drive car. I haven't gone to coilovers, but the springs are stiffer and the car is about two inches lower. The front suspension bushings are still stock. There's a possible problem with the electric assist power steering if you go to stiffer front suspension bushings. My mechanic races an S197 body Mustang and he uses the stock front control arms too. I have replaced suspension bits like upper and lower control arms on the rear axle and added a Watts Link. The control arms have spherical bearings on the axle end, but polymer bushings on the body end. The shocks are single adjustable Koni Sport (yellow) and I have camber/caster plates on the front struts. Currently I'm running an adjustable bar on the rear, but the stock bar on the front. It pushes too much with a stiffer bar.

The Rival S tires are reported to be sensitive to camber setting and the sidewalls aren't all that stiff compared to the RE-71R's. I was using about 40 psi in my Rival S tires, but closer to 30 with the RE-71R's, which have a substantially higher load rating.

Did you ever measure temperature profiles with a needle type pyrometer? Too little negative camber will overheat the outer edge quickly. If you didn't, where did it chunk and how much camber were you using? Race tires actually tend to be less sensitive to camber and need less pressure.

And I don't think your lateral g readings are too high.