Group C Car Downforce Balance

[Newyorkkopter]

Group C cars made like 5,000lbs of downforce @200mph but those 5000lbs were mostly towards the back right?

Is that why they had understeer even with maximum downforce?

[whiteballz]

Forgive me if I am wrong, But with too much downforce on the front tyres wouldn't it theoretically cause understeer too?

[culver]

Group C cars made like 5,000lbs of downforce @200mph but those 5000lbs were mostly towards the back right?

Is that why they had understeer even with maximum downforce?

I don't believe they were mostly on the back. I suspect they were placed such that the center of pressure and the CG were reasonably close together.

[Newyorkkopter]

why do you say that?

[culver]

The underbody aero effects occur over the length of the underbody, not just at one end.

[Newyorkkopter]

But Group C cars had rear diffusers as well as big rear wings.
I read at Mulsannescorner.com that sports cars have historically had a problem with front end grip.
Then theres this qoute by Costas Los (also at Mulsannescorner.com)
"On all the Group C cars I drove, except the Allard, if you loaded both ends to the maximum you would get an understeering car.”

So thats why I'm thinking that most Group C cars had more downforce at the back than at the front.

If I'm wrong then please correct me, I'm no expert at this.

[wwgkd]

Forgive me if I am wrong, But with too much downforce on the front tyres wouldn't it theoretically cause understeer too?

If forza is at all accurate, then yes. Though really a car that predominantly oversteers rather than understeers would be pretty scary to drive, so a little correctable understeer is a good thing.

[culver]

It's possible that the group C cars have balance issues because they run a wing at the back but not the front.

Contrary to popular belief, diffusers don't generate downforce. What they do is generate air flow. They help suck air from under the car. The air that rushes from the flat under section of the body to the diffuser tunnel is what actually produces the grip. So your grip is produced under the flat section of the belly. The diffuser helps the car move more air under the flat section. I know it isn't intuitive at first and my explanation is abridged but that's the scoop.

Also, we need to make sure we are clear about what you mean by more downforce in the rear. Like most race cars the Group C cars had a static rear weight bias. To maintain this bias under cornering the rear should have more downforce. I think what you mean is the center of load on the tires shifts rear. That is the CG+the center of downforce pressure shifts rearward.

[Alastor]

It's possible that the group C cars have balance issues because they run a wing at the back but not the front.

Contrary to popular belief, diffusers don't generate downforce. What they do is generate air flow. They help suck air from under the car. The air that rushes from the flat under section of the body to the diffuser tunnel is what actually produces the grip. So your grip is produced under the flat section of the belly. The diffuser helps the car move more air under the flat section. I know it isn't intuitive at first and my explanation is abridged but that's the scoop.

It should also be noted that diffuser can significantly improve the performance of the rear wing.

[Newyorkkopter]

I think what you mean is the center of load on the tires shifts rear. That is the CG+the center of downforce pressure shifts rearward.

I'm not sure exactly what you mean by the above statements.
Do you mean that the center of downforce and the center of gravity can shift rearward like for example, during cornering?

[culver]

What I mean is are you saying the rear generates more downforce or the percent load on the rear axle increases with increase in downforce?

For instance, statically a race car might have 400lbs on the front wheels and 600 on the back (40-60 weight distribution). When the car is stationary we have no aero so the total percent of the load on the rear axle is 60%.

Aero loading will add say 1000lbs to the car. If that 1000 is split 40:60 between the front and rear wheels then we still have a 40:60 load split on the tires. Yes, the rear tires saw more of the total aero load, 600lbs vs 400lbs in front. However, as a percentage the load on each axle went up the same 100%. So the question is what did you mean by more downforce in back? Certainly they have more in back. The question is does the percent load on the tires change with an increase in downforce and would designers prefer to move it forward if they had the choice (all else equal).

[Newyorkkopter]

What I mean is are you saying the rear generates more downforce or the percent load on the rear axle increases with increase in downforce?

yep

Certainly they have more in back.

Thanks. Thats what I wanted to know. So there is more downforce at the back. Now is that why most of them undesteered even if setup for maximum downforce?

The question is does the percent load on the tires change with an increase in downforce and would designers prefer to move it forward if they had the choice (all else equal).

Thats another question whose answer I'd like to know. So would they want to move it forward?

[RacingManiac]

Nothing is as cut and dry as that though. The handling balance you want at a 60mph corner is not the same balance you want at 160mph mph corner. If a car is very neutral at 60 and below, it can be quite reactive and twitchy at faster speed. And that would not be good at all. There are ways of modifying the handling of a car, use aero and suspension setup in unison. From something like setting up toe with your ackermann steering to running suspension packer or ride damper(3rd spring) such that your car may assume a different rake at high and low speed based on your aero load(which shifts your center of pressure). It is historically more difficult to have more "built-in" front downforce on a sports car like Group C cars, since unlike an open wheel car, it is difficult to make a front wing that works well without actually resort to making one(and in the later era of Group C, they do actually make front wing on cars like XJR-14 and 905 Evo). And that is why cars like R15's concept is so controversial. For most sports car, because of the size of their aero platform, they can develop a lot of ultimate downforce at speed, but to get enough of that at lower speed especially with the right balance, thats what most are fighting for....

[NSXType-R]

So right now shifting aero packages are banned right?

Why not use it? I mean, cost goes up considerably, but I don't see why carmakers shouldn't go for absolute speed.

[RacingManiac]

Aside from F1 last year(not sure if they still have it this year), no series lets you do active aero anymore....

Even active suspension in the form used in early 90s in F1 is used more or less as an aerodynamic aid, since you can rake your car to adjust aero balance as well.