Balance Issues

[Eagle7]

My ITS '86 RX-7 (RWD) front/rear balance is noticably different in low speed corners than in high. For example, at Mid-Ohio I have to lift to get it to turn in the carousel or the keyhole, but I'm fighting oversteer in turn 1 (with the go pedal on the floor) - last time there I ran out of talent on the way to the tire wall.

• Is this a "normal" characteristic, or should I be able to tune it for more consistent balance?
• If it's not "normal", is it likely mechanical or aero? I do run a splitter (ISC) for cooling reasons.
• If it's mechanical, what should I be adjusting?
• If it's aero, since I'm now developing my own custom splitter design, how should I adjust it to minimize downforce while retaining effective cooling?

TIA,

 

[Greg Amy]

Is this a "normal" characteristic, or should I be able to tune it for more consistent balance?

Yes, it's normal. And it's very likely aerodynamic.

This is why "real" race cars run rear wings. It's much more pronounced on front-wheel-drive cars, but any car will exhibit lift at speed. The design of the car will determine where that center of lift is, but in general most cars, when setup for understeer at low speeds, will exhibit oversteer at higher speeds. To accommodate this you would tune the mechanical grip for the lower speed stuff, and the rear wing for the higher-speed stuff.

Unfortunately, since you really have no rear aero opportunities, there's not a whole bunch you can do about it. The way this is done on "real race cars" is by tuning the rear for optimal rear downforce, then tuning the front to match it for balance. You can't do that. And, in fact, by adding in there a front airdam and splitter you're making the higher-speed problem worse.

Managing aero balance with only one end is a losing proposition. About the only thing you can do is try to tune the front addition to not reduce lift at speed, either by eliminating the splitter or by trying to allow more air to pass underneath the car. I'd suggest starting with *NO* front splitter at all, simply use the airdam to improve cooling, see how that works out. If you find you still have understeer at speed then BRAVO, you win. At that point you start tuning the length of your splitter to suit your fancy. If, however, you still have oversteer at speed then you're kinda hosed, and you either have to accept some oversteer at speed or adjust the mechanical grip balance to accept slow-speed understeer so you can keep your foot in it at speed. And, of course, that will vary by track...

At least you're in a good place to start. now it just takes testing and tuning. - GA

 

[C. Ludwig]

Marty,

Something else to think about in your specific example is the cross-weight versus direction you want the car to turn. T1 at M-O is the only left hand corner that matters at all, everything else is about getting the car to turn right. Don't be afraid to wedge the car to make it turn one way better than the other. A good lap time at M-O is all about getting off the Keyhole and getting through the Carousel quickly. By altering cross weight to free the car up going right you will tighten it going left. In your specific example that would be a win-win.

 

[Andy Bettencourt]

And as a combination of both those posts, tune your car to be fast in the most important corner(s), then deal with the rest with your driving.

I would rather be really fast in the stuff that is important and slow in the 'throw away' areas than average everywhere.

 

[Eagle7]

Marty,

Something else to think about in your specific example is the cross-weight versus direction you want the car to turn. T1 at M-O is the only left hand corner that matters at all, everything else is about getting the car to turn right. Don't be afraid to wedge the car to make it turn one way better than the other. A good lap time at M-O is all about getting off the Keyhole and getting through the Carousel quickly. By altering cross weight to free the car up going right you will tighten it going left. In your specific example that would be a win-win.

Trying to put me into China Beach eh?

Thanks guys. Very helpful comments.

 

[Z3_GoCar]

IMHO the aero rules of IT are backwards, they should allow aero on the back and minimize it on the front. I know the historical presidence from production circa '60 allowed air dams and nothiing else. But this was the dark ages of race car aerodynamics.

 

[seckerich]

Need to pay attention to the ride height as well Marty. The second gen is sensitive to rake angle and can be balanced better with small adjustments. I will assume you run the 89 rear spoiler. Openings in the front spoiler will change the balance more than you would think.

 

[John Herman]

One other thing to keep in mind, is this a steady-state issue you're dealing with or is it an initial turn-in kind of issue? If it's steady-state, then the main things like springs/sway bars along with corner weights should be looked at. However, if it is a problem with the initial turn-in, then you want to look at shock tuning. The shocks (or shocks and struts for us FWD) are what is going to affect your transcient behavior. Turn 1 at M-O is definately a balancing act. If I can't turn-in with confidence, it's game-over. I used turns like the carousel or keyhole to tune my steady state handling, but turn 1 is where I focused the tuning of the shocks/struts. For example, if the rebound on the rear shocks was too "stiff", the car would want to overrotate (i.e. oversteer) immediately, even though the car wanted to understeer in normal situations (or even later on in corner 1). So depending on "when" you're fighting the problem, you may need to adjust the shock rebound/damping to get the balance right. Actually, because corner 1 is high speed, to gain a little more confidence to carry the speed, you may want or need to dial in a little bit of understeer at turn-in.