CofG in car design

[DaveW]

Apologies for the sanitisation, but I thought this might help. Nothing dramatic, but the Porsches do appear to have deen ballasted....

[gixxer_drew]

I was also wondering; do they design parts (save for ballast) close or on the floor with heavier materials on purpose?

Do that all the time. Thicker gearbox casing, higher amperage alternator, thicker cage tubing, wherever you know you will throw ballast later you get leeway in your engineering margins on components in that area.. You will trade them for chassis stiffness, reliability, driver comfort or anything else.

[Lycoming]

Would you still do that to the same degree if you were in a series with no minimum weight? Or would you just go straight for lowest possible weight then?

[silente]

whitout lower weight limit, i would always go for lighter. In my opinion, together with Aero Load and tyres, this is the parameter which is paying more in terms of lap performance.

[andylaurence]

Would you still do that to the same degree if you were in a series with no minimum weight? Or would you just go straight for lowest possible weight then?

Lowest possible weight, especially in aero formulae. The ratio of downforce to weight has a strong bearing on your ability to corner, so by reducing weight and keeping downforce the same, you generally corner quicker ... and stop better ... and go better too! I've no idea what my CoG is, but I do know the corner weights and thus the overall weight of my car. It performs quite well for a 999cc engine, mainly because it only weighs 421kg.

[Tommy Cookers]

about the same weight as the Tyrell F1 cars when they had disposed of their 'brake coolant' (water ballast) ?

[gixxer_drew]

Rules can interfere though....

Sometimes the weight distribution wants to go the opposite direction of where you can make downforce....or the basic layout is resulting a bad handling car, one that destroys tires, etc. Springs, bars, tuning tools have their limits of what they can correct when the basic parameters are not in harmony with each other. The rules can sometimes make that impossible to achieve and force you to having the weight distribution follow. So that will mean the rules favor one platform or another and often in ways that the performance 'equalization' doesn't recognize.

[DaveW]

Designers with experience (i.e. those who have made mistakes) tend to have key stiffness targets when designing a vehicle structure. Arguably, those who don't have been truly fortunate (or unsuccessful).

[gixxer_drew]

Designers with experience (i.e. those who have made mistakes) tend to have key stiffness targets when designing a vehicle structure. Arguably, those who don't have been truly fortunate (or unsuccessful).

I only ever experienced this with one car. It was my own car during University days. Oddly I think I learned the most from this car. At several points during the progression it became apparent and I was forced to address it. After that I just invented a rule of thumb but I wondered how this manifests itself for others? It is unusual to run into this with GT cars. The minimum weights, safety and aero rules mean the chassis tends to be ahead of the stiffness requirements.

The first time it hit me over the head was when I removed a chassis stiffening bar and the ride comfort improved!

This was at a very low level of stiffness, I had no data for flex but to give you some idea it was in the neighborhood of driving over a speed bump with one wheel meant you can feel the center console change relative to the body.

The second time I thought it was a problem and improved the stiffness it felt like a general improvement in responsiveness and consistency through the corner. Just felt "confidence inspiring". Looking back later I could actually see the flex in photographs if I looked carefully enough. No data for chassis stiffness was collected back then, but I started after that and came up with a rule of thumb for the minimum stiffness. Before this version, you could put your finger between a cage tube and the roof of the car and feel a few millimeters of movement relative to each other when loaded in a corner.

Final time I ran into the problem the stiffness was over 4000lb/deg (not that I put much stock in an absolute value though I could measure slightly different and get a big swing). As the aerodynamic loads exceeded 800lbf/axle and it was using higher grip tires. It was described as "wallowy" by the driver. I drove it myself, although I have difficulty describing it I wouldn't have agreed with that driver's description. It acted like that classic 'bad handling' car where you cant look up the problem in any of your if/then statements list for setup. Certain corner behavior were contradicting others, even if they were similar. To me it felt like the car couldn't really get settled and did not respond as expected to setup changes. It was impossible to gauge heavy aero load corners for me... but I'm not much of a driver and I could never push there.