Flat Top

I have noticed that GPS data on our 911 RSR indicates that I pull slightly less max G's under braking compared to the G's I pull whilst cornering.

Is this a standard phenomena that affects all non aero cars or is there something else to it? (Don't say brake harder - we are already on the limit. Any more pressure would just lock the wheels up).

I have also noticed that in rain conditions the longitudinal grip suffers less than cornering grip. Meaning that although the grip for braking is impaired it is not as severely affected as cornering grip. Why?

Johan
www.almost.co.za

Nordschleife

sounds like a measurement/conversion error. You will pull greater G under braking than you will under cornering.

as far as the difference between wet and dry driving conditions goes part of it may be psychological, '... its wet, be careful....'

In the early LMP900 cars, there was a problem with grip when driving in downpours (1000KM Nürburgring 2000 comes to mind), the gap between the underbody and track surface became completely saturated with moisture in some places, when this happened, all downforce was lost and cars pirouetted off the track, mostly at quite low speeds. Bill A came up with a very smart solution to this problem make the rain tyres taller. Check it out, it works.

R+C

bobt993

Either math error, car setup or braking more immediately to set the front end weight balance (possibly larger tires up front). I can't see any data that the braking does not exceed the cornering g loads. From a physics view consider your straight line braking as balancing on all 4 tires (with added load forward of course) and cornering will always favor the 2 outside tires. Now on a heavily cambered track, you can exceed straight line braking g's. I have seen 1.6 plus g's on a banked turn just with R-compound tires on an almost stock 993.

stevehay

Johan - it's basic, but be sure you have "zeroed" your g sensors prior to logging data if you have them. I see that you are using a GPS based logger which I'm not very familiar with so all caveats apply there. You'll want zero your sensors on completely level ground if possible. You may be starting with a non-zero value when logging begins. I agree with Nordschleife that the load under braking (provided driver is using all or near max braking capability of the car) should be much higher than cornering. That said, are you having trouble with braking? If that's the case, and for some reason the brakes on the car are not set up or working properly, it's entirely possible that the cornering G's would be greater than those under braking. It's also not surprising that your logitudinal grip suffers less than lateral in wet conditions. The weight shift and aero forces of the car are pushing the car down directly into your path of travel. This is not the case when cornering (also why, in dry conditions, the g load under braking should be greater than when cornering). Also, as bobt993 mentions above, you have more grip surface to utilize because you are not loaded to one side of the car any more than the other.

If brake setup is good and g sensors have been zeroed you'll want to make sure that your g sensor (or GPS) is working and calibrated properly. There are many reasons why you might be seeing this condition in your data, but it's always best to start with the easiest solutions. I won't go into the details of why I know this so very, very well now. ;-)

Apologies if I'm stating things that you have already checked or that you inherintly know.

What data logging system are you using? Keep us posted on your analysis. I'm curious to know what's causing this condition. Good luck!

steve

Flat Top

Steve

Thanks for a well formulated response. I shall be checking whether the gps is zeroed.

Thanks guys for the insight. Sometimes our belief systems are tested and what you believe to be true (more g's in cornering as opposed to braking) may not be.

Johan

SundayDriver

How come I always seem to take the contrarian view?

I think what you are seeing is pretty much real and normal. I have seen that in most cases, and not only checked data but also down the math to verify the g data vs. speed under braking. The question is why would you think that braking will give higher g's than cornering?

The starting point is the tires. You only have so much grip and the tire doesn't much care if it is cornering or braking. There is one aspect that increases braking forces and that is drag. If you are looking at threshold baking at 140mph, then there is enough drag to make a difference. In a high downforce car, that drag is ~1.0g without ever touching the brakes.

Ignoring that aspect, you would think it should be the same without drag. When you brake, there is going to be massive weight transfer to the front tires. Most production cars will not let the platform settle under braking, so it is even more extreme. Now you have the (typically) smaler front tires taking much higher loads (weight) that at rest or even under cornering. Tire grip falls off with higher loads, so the grip capacity (max g's) are lower.

Summary - without the drag aspect, you shove most of the weight on the 2 smallest tires and your 1.2g capacity r compounds fall to .9 g capacity. Sidebar - You guys see why I think that using traction circle and combined g's have very limited value in driver development?

Premier Motorsp

Sunday,

You need to rethink this one. You have it all backwards.

Chris Cervelli
Premier Motorsports

Nordschleife

I couldn't put it better Chris

R+C
PS perhaps there are some very CONSERVATIVE brakers out there!

seriously guys, something is WRONG if you are seeing more lateral Gs than decelleration Gs, no ifs ands or buts, either the measurement system (maths, calibration, sensor alignment, whatever), the car or the driver are wrong or all of them. I suggest you check with Claude Rouelle for the maths.

bobt993

I looked further into your query on some of my runs last year: There is some truth to your question, but it is related to track terrain: I have both analog and gps based data so here is what I see: Many of the East Coast tracks have heavy braking zones on a declining turn entry. This has a huge affect on grip and it also would seem a cambered turn follows ie: Glen turn 1, turn 6 in the boot, Summit turn 1 drop off, chute major drop off, turn 10 moderate drop off. Shenandoah and Jefferson have some of this also. Pocono, yes North and South course using the Nascar track. Lime Rock pretty flat going into T1 some drop on the back straight(foggy memory). VIR, back straight great uphill breaking zone, but trailing into the downhill essess. Not sure if this helps, but my point is to not over analyze the data and pay more attention to the segment/rpms at track out along with interval times.

SundayDriver

I am open and always thought that until the data (at moderate to low speeds) showed otherwise. Starting from the premist that you can not exceed the g's that the tires are capable of, how can you get more lat g's than long g's (again, ignoring drag which you can do except at higher speeds)?

As a cross-check, how about some of you posting speed vs. time data from your data aq systems. We can easily calculate average g's from that info and see if it exceeds corneing g's.

Nordschleife - if you understand this and have data from Claude, how about sharing it. Again, I am open and always thought I agreed with you guys - how about some proof - again IGNORING aero drag.

Premier Motorsp

wheelbase>track

fore/aft weight transfer<side to side weight transfer

less weight transfer = more grip

fatbillybob

I guess I'm lost...this means more grip higher g. Thus Higher long g than lat g. Wasn't the original post a problem of more lat G than long g? This supports Sunday Driver?

stevehay

FYI..I asked an engineer friend of mine (Daytona, Le Mans, Sebring winner)about this and he said that "every Porsche 996 RSR I have seen was delivered with the G-sensor oriented incorrectly. He should manually test the sensor by holding it with each of the three axis facing down while looking at the readings in " monitor active channels" and see if they correlate."

steve

Nordschleife

All my telemetry data is from actual race cars in developing, practice or racing, so I cannot publish it - would you tell the competition that? But it is fairly easy to get hold of data which shows GT3 type cars, whether on slicks, intermediates or rain tyres consistently achieving greater braking G forces than cornering forces. The Claude Rouelle course material discusses this. Even F1 cars which benefit from huge downforce have bigger retardation numbers than cornering numbers. My road car manages somewhere between -1.4 and -1.6 on super sticky rubber, but the brakes are after market specials and cost about as much as the annual budget of a TPLAC.

To achieve high negative G-force numbers when braking, you have to nail the brakes very hard indeed as early as possible in the braking window, even if you are travelling at over 300 km/h. This is when you record the (absolute) big numbers.

SundayDriver

Thanks. I understand and had not considered that. OTOH, the weight shift to the front is going to provide greater overloading than side shift. 911 type car from wider to narrow tires. On a sedan, there is big wieght already on the front and the shift will further overload the tires.

Unless we know the exact loading and characteristics of the tire, we can't even begin to identiry whether that is a significant aspect. What I do know is that data from quite a few street cars, with drivers who are quite good on the brakes, show lower brakig g's than cornering. It is usually only .1-.2 g's but in those cases the data did not support my preconceived notion of what was supposed to happen.

Agian, your point is very valid and one I had not considered so I certainly concede on that part. Though I have seen data, that I have cross checked, that shows otherwise on quite a few cars so I want to understand why.