Handling set up Q

[mwizard]

Until recently I have been attempting to set up my car to have neutral handling and been pretty successful. Last wkend the course, on concrete, had many tight turns(and no slaloms), so I adjusted my rear bar for some oversteer and it worked out for faster times. Yesterday we ran on the same concrete site with the same bar setting and a course with 3 slaloms. I kept the same bar setting and the car was a bit looser, a bit harder to control, but also had some useful oversteer, when I got it right So, I am wondering if I should work on getting the oversteer to help me or go back to neutral?
How are the rest of you setting up your cars?
Mark

[F five hunj]

Until recently I have been attempting to set up my car to have neutral handling and been pretty successful. Last wkend the course, on concrete, had many tight turns(and no slaloms), so I adjusted my rear bar for some oversteer and it worked out for faster times. Yesterday we ran on the same concrete site with the same bar setting and a course with 3 slaloms. I kept the same bar setting and the car was a bit looser, a bit harder to control, but also had some useful oversteer, when I got it right So, I am wondering if I should work on getting the oversteer to help me or go back to neutral?
How are the rest of you setting up your cars?
Mark

A loose car is usually slower through the slaloms for us. I shoot for as neutral as possible, with heavy rear brake bias. This allows us to drive something like an offset cone to cone, rotating the nose towards the next one with the brakes, rather than having to give some up and backside the cones.

[mwizard]

A loose car is usually slower through the slaloms for us. I shoot for as neutral as possible, with heavy rear brake bias. This allows us to drive something like an offset cone to cone, rotating the nose towards the next one with the brakes, rather than having to give some up and backside the cones.

Sounds like something to try. Someone locally w/ a BM car said he was doing it that way. I am currently running about 50/50 split on brakes.
Mark

[S Lathrop]

Rear ARB helps with transition into a corner. Increasing the rear spring rate and lowering the rear ride hight helps with mid corner and corner exit. Spring rate is a very important variable.

More rake and softer rear spring will help transition in but may tend to make a car more pushy mid corner to exit.

Unfortunately things are not quite as simple as your description suggests.

[RudeDude]

Rear ARB helps with transition into a corner. Increasing the rear spring rate and lowering the rear ride hight helps with mid corner and corner exit. Spring rate is a very important variable.

More rake and softer rear spring will help transition in but may tend to make a car more pushy mid corner to exit.

Unfortunately things are not quite as simple as your description suggests.

Interestting.... Coming from the builder of my car (I bought Jim Garry's 85 Citation), I'll have to give it a try. The car manifests mid corner and corner exit understeer when the tires are either hot or past their prime. I already lowered the the rear ride height, I will consider increasing the rear spring rate a bit.

[mwizard]

Rear ARB helps with transition into a corner. Increasing the rear spring rate and lowering the rear ride hight helps with mid corner and corner exit. Spring rate is a very important variable.

More rake and softer rear spring will help transition in but may tend to make a car more pushy mid corner to exit.

Unfortunately things are not quite as simple as your description suggests.

So, how much rake are we talking about, 1/4 in, 1/2 in?
Mark

[S Lathrop]

So, how much rake are we talking about, 1/4 in, 1/2 in?
Mark

Mark:

I can't tell you what it takes. Some cars and setups I worked with only takea a 1mm change to make a difference. Other cars take much more.

You need to figure this out during testing.

[chrisw52]

So, how much rake are we talking about, 1/4 in, 1/2 in?
Mark

Mark,

for what its worth, the factory setup sheet I have for my car calls for 40mm ride height in front (measured off the front skids) and 100mm in the rear measured off the rear suspension beam.

I can't tell you how it performs yet, because as I go through the finer details of my current setup, I see unequal length drop links, crazy amounts of preload on the springs and a host of other issues that need to be resolved before I can tell you if the car handles.

[Dick R.]

More rake and softer rear spring will help transition in but may tend to make a car more pushy mid corner to exit.

Steve (or anyone else who knows ),

I understand that depending on the car (and the driver for that matter) noticeable rake changes may be "barely" to fairly large fractions of an inch from some initial value.

1) Other than relative F/R cg height and suspension angle impacts of changing ride height at one end, is there any other significant factor for autocrossing that rake impacts in order to influence handling grip/balance? Note that under car aero downforce/drag effects are not relevant to autocross FF's even though I understand that rake and ride height are significant for track racing FF's.

2) Is actual "rake" very "design specific" and the real issue is simply changing the relative ride heights?

3) If not, is there any sort of typical rake initial value for FF1600's with different front/rear tire diameters? If so, measured from "where"?. I.e. under the floor pan at the front axle and under the bellhousing at the rear?

For example:

The initial VD and importer/racer ride heights for my 85 VD FF1600 were about 1 3/4 inch front and 2 1/2 inch rear resulting in "rake" of 3/4 inch. Since a projection of the floor pan is about 3/4 inch HIGHER than the bottom of the bellhousing measuring to the floor pan "projection" would result in "rake" of 1 3/4 inch.

Thanks!

Dick

[mwizard]

My car is about 1/8 in lower in the back measured off the skid plate.
Mark

[S Lathrop]

Rake is only a way to express the relationship between the front and rear ride height. With a ground effects car, it is a variable that tells where the center of pressure for the under body might be.

When you change ride height, you move the roll center by close to the same amount as the ride height changes. The effect of a ride height change is made up of a change in roll center and a the center of gravity. In short, the change you feel is equal parts CG and roll center movement.

Balance is determined by how fast you load the tires at one end of the car relative to the other. Push or loose is simply the point where you reach the maximum cornering potential of the tires at the front or rear of the car.

The loading of the tires is directly related to the position of the roll center and the CG at either end of the car. Up means faster loading.

Aero balance also changes with rake. More rake generally implies a shift forward of the center of pressure. Change the rake and you get three variables moving in the same direction.

Tire compounds are very sensitive to the rate that they are loaded.

Why the roll center position makes thing happen the way it does is the subject for a day long lecture.

Rake is what makes the front and rear setups work in a cordinated fassion. Change the setup at one end and the rake will have to change if you want to maintain the same balance.

And I am not sure that this is the answer you were looking for.

[Dick R.]

And I am not sure that this is the answer you were looking for.

Actually it is what I was looking for . . . I just didn't know how to ask it.

This statement answers what I asked.

"Rake is only a way to express the relationship between the front and rear ride height. With a ground effects car, it is a variable that tells where the center of pressure for the under body might be."

The additional information then explains why the ride height and rake changes do what they do. Great info!

Thank you VERY much!

Dick

[mwizard]

I tightened up the rear low speed compression on Sat by 2 clicks and added a bit more rear brake bias for the concrete surface. The car was a bit too loose, so I turned it back the 2 clicks and got FTD by 1/2 sec
Mark

[LanceKTM950]

Hey Steve,

With Carrol Smith is no longer with us it's time for you to write the book for us on setting up small formula cars both from scratch and when we're looking to change/improve handling.

I can't wait!

Lance

[Dave SanF 50]

I see a lot of measurements are being given for different FF ride heights but I don't see the parameters for the measurements. When I measure and set my ride heights I use the following set of criteria:
1) weight of the driver has been added
2) tires are set to hot pressures
3) surface is relatively level
4) the car has been "bounced" and rolled on it's suspension to remove some of the static friction
5) a repeatable location is used-not a low hanging bolt that can be ground off on track

Anyone have any other parameters they would like to share?

[Neil_Roberts]

Anyone have any other parameters they would like to share?

1. A known and consistent fuel level.
2. A measurement location that is very close to the axle center in the fore-aft direction. If it isn't, changing one ride height will change both measurements.
3. Either left and right side measurements or a bubble level to monitor tilt (static roll angle of the frame).
4. Nose and tail bodywork either on or off, as long as it's the same every time.
5. Steering rack locked straight ahead.
6. Four measured tire diameters.

[Dave SanF 50]

Steering rack locked straight ahead.

Thanks Neil, I forgot that one along with the others you mentioned. I cut a piece of aluminum tubing (about an inch ID) in half lengthwise to place onto of the steering shafts where they exit the rack housing. They keep the front wheels pointed straight ahead. The Alum pieces fit between the rack housing and the large outboard washers. Great for setting toe and camber. I mark each piece because the two sides are not quite identical.

And, of course, if you are using worn tires, no flat spots on the bottom. LOL

A measurement location that is very close to the axle center in the fore-aft direction. If it isn't, changing one ride height will change both measurements.

This shows up when you drop the nose and the rear rises up.

[Modo]

No Stick.....U LOSE 1:13