Larry Herman

The 100 marker, on the back straight? C'mon Dave, tell it to the newbies. Unless your car was only going 110 mph, there is no way you were able to brake that late.

jgrant

He never said he made the corner.

jgrant

That rate of decel is mind boggling! How heavy is it, exactly? What size are the front tires?

sbelles

Is that necessarily a constant bias % though? Not all braking zone / corners are created equal. Do you guys with adjustable bias change at all going from a long, high speed course to a short tight one or visa versa?

Premier Motorsp

At the time that data was logged, it weighed about 2360 lbs. Add to that some fuel and a 240 lb driver.

Front tires are Yokohama 250/650 18 soft compound.

I don't think it is fair to compare this against a 997 Cup. The racing ABS that my car has allows the driver to get the max out of the brakes with almost no risk.


Chris Cervelli
Spline Technologies

2BWise

Its not exactly at constant percentage but awfully close. If you've tuned the bias for brake lock on all four wheels at the same master cylinder pressure (which will provide maximum braking) then any changes away from that would most likely be solely due to driver preference. Personnally after I find the spot I like I hardly ever change it except when the surface doesn't allow like rain, or if you're at a track that has recently been resurfaced or is well worn.

Chris,

Why kind of average decel do you get in a straight braking zone. Peak accel is cool for bragging rights but doesn't show the entire picture, especially for a non aero car. If your peak shows 1.8, but can only sustain an average of 1.4 and my peak shows 1.7 but can keep my average at 1.5 then I can out brake you every time. What type of accels are you running? I've found that the mid-low range accels are suseptible to more noise than the high end ones.

As a note, I have seen average decel of over 2g for a non aero car, but it was a very small open wheel car.

jgrant

Exactly.

Our Daytona data showed that Ross was driving our Cup Car with a 3.7% bias setting, to the rear. I'm almost thinking that it might have been due to the much stiffer than normal rear springs we were running, as well as driver preference.

Bryan Watts

With a driver, our cars are at similar weights and we run the same size front tire. I suspect you run wider rears (we run 280's) and obviously you have more weight on the rear. Your braking numbers just sound really high. Then again, comparing G's between different data acq systems in different cars may be as fruitless as comparing dyno numbers between different dynos...I don't know enough about them to be sure. Perhaps they are good for comparing relative to themselves, but not for comparing to others.

Ultimately, it sounds like we need to go into 10A side by side and see how big the difference is.

mark kibort

Why would down hill add to your "recorded Gs"? It should reduce it actually.

Premier Motorsp

Well for sure the 911 is going to beat the BMW under braking based on weight bias and CG height alone. The BMW ought to potentially have an advantage in fast corners and in acceleration because it does not have a stupid air cooled engine.

I also run a 280/680 18 rear tire. What lap times are you running? This lap was 1:27.3. I mention that because comparisons between cars that are more than 3 seconds apart are usually misleading.

The G sensors should be reasonably close if both sensors are calibrated right (which is a big assumption).

My car cannot do 2 G under braking consistently. It appears to have hit that number in this brake zone on this lap on that day. I would expect any reasonably optimized 2400 lb race car on slicks to hit 1.4-1.5 almost every time. I am assuming a decent ABS system.

I think in this case I am getting a little help from the 155mph approach speed and the good pavement conditions at this corner. As I mentioned the downhill orientation is going to add a little negative G to the reading as well.

Chris Cervelli
Spline Technologies

Bryan Watts

I, unfortunately, have missed Road Atlanta the last few years for various reasons. Dad turned a 1:29 there in 2007, but the car has has VANOS added back with a new Motec ECU and I tend to be faster than him at RA over the years by about a second, so I suspect I'd be right there with you in the 1:27's.

Our cars sound fairly comparable...would probably be an interesting data overlay to see where each car is stronger. Would be nice to race with a well driven and setup Porsche. Seems like all of the Porsches that run "similar" speeds as us in NASA or PBOC are much faster cars (700+ hp modified Cup cars or GTS RSR's) being driven rather poorly, so I spend all of my time being held up in the corners and losing 10-15+ car lengths on the straights.

We could definitely benefit from ABS. It's the next improvement on the list.

Bryan Watts

A little thing called gravity comes into play. For that matter, body roll, brake dive, and acceleration squat prevent you from getting a totally accurate G reading from most of the data acq. systems, again making numbers between cars hard to compare.

mark kibort

I would think since you are going down hill, the road is falling away from you, reducing grip and subtracting from the cars ability to stop. If you are positive that downhill adds to the g-meter's output, Ill have to think about that, or read up on it as to why. sure, roll, dive and things that upset the orientation will change the output I imagine.

I havent played with any of the high tech gymos yet. just video, track markers and stopwatches.

mk

Rassel

Don't want to make anyone upset, so this is an "opinion".
But it should be "add negative" as mentioned above.

Bryan Watts

Dive changes the orientation which changes the accelerometer output. So does driving down a hill. If you park the car at a standstill pointing downhill, the accelerometer isn't going to read 0. If you hang the car with a chain from the rear bumper, even though the car isn't moving, much less accelerating, the accelerometer isn't going to read 0. If you drive a car down the hill at a constant rate of speed (i.e. no acceleration), the accelerometer isn't going to read 0. The accelerometer is zeroed based on a stationary car on a flat surface.

No one is arguing that, all other things being equal, a car won't take more time to stop in a downhill braking zone than it will to stop in a flat braking zone. That's a given. That will obviously lower the rate of deceleration. But, because the car is pointing downhill (and experiencing dive), your accelerometer is going to read a higher rate of deceleration than the car is actually experiencing.