The "Latest" Rig

The "Latest" Rig
Bodnar Wheel w HPP Pedals (Added Rift in Summer 2017)

Wednesday, September 23, 2015

What does the 3rd Spring Do?

On the IndyCar in iRacing, there is a "third spring" in the front and rear.  Pictures are worth a lot of words so:

Pic of Front Suspension


Pic of Rear Suspension

Here is a link to article that provides some interesting data. Pics are from that article.


Article
http://www.lolachampcar.com/3rdSpring.html

Bottom line is that the "3rd Spring" essentially provides resistance to pitch and squat relatively independent of the four "corner" springs.  Note the rods connected to the suspension rockers that go to third rocker supported by the "3rd Spring" assembly.  If the car rolls to one side in a turn, with one suspension rod going up while the other moves down, then the third rocker simply rotates and the "3rd Spring" does not move.  If the car dives while braking or is lowered by aero downforce, than both suspension rods move closing the "gap" and then compressing the "3rd Spring".

Keep in mind that IndyCar aero downforce can nearly double the wheel loading at very high speed, so with the "3rd Spring" it is no longer necessary to carry that aero load with the "corner" springs, allowing the car to handle with more compliance in turns.

The "gap" is the distance that the rocker will travel before engaging the "3rd Spring".  Choosing that setting is an important part of chassis setup.

The actual "3rd Spring" may be a form of dense foam bump-stop progressive rate spring rather than the coil spring in the photos, but the principle of action is almost the same. 


Thursday, September 17, 2015

Caster Angle-The Most Misunderstood Front End Setting

Caster Angle is an important setting to achieve desired handling characteristics.

Race cars almost universally employ positive caster.  The steering axis is "leaned back" at the top, which places the tire contact patch behind the point where the steering axis intersects the road.

Positive Caster create Five (5) effects:

1)  Steering Effort and Straight-Line Stability.   Because the contact patch is behind the steering axis's intersection with the road, increasing positive caster increases the force necessary to drag the tire sideways. This occurs on both front tires and is greatest at beginning of a turn when the wheels are starting out in the straight ahead position.


2) Camber Change.  With positive caster, the negative camber of the outside tire is increased while the negative camber of the inside tire is decreased, or the positive camber of the inside tire is increased. This is usually beneficial as it compensates for chassis roll and generally increases the grip of the front tires.

3) Cross Weight.  Because the steering axis in inclined, in a turn, the outside tire is lifted away from the road, and the inside tire is pushed down toward the road.  When the inside front tire is pushed down, it also increases the weight of the outside rear tire, essentially creating a negative change to cross weight. The more steering input, and the more positive the caster, the more effect.  This essentially makes the car have less understeer or more oversteer as steering input is added.


4) Auto Steering.  The same principle described regarding Cross Weight change also can be used to make the car turn more easily on oval tracks. On oval tracks, the inside tire is usually set to have less positive caster which actually causes the car to turn left with little or no steering effort.

5) Effect of Steering Corrections. The same principle regarding Cross Weight when turning produces the exact opposite effect when making a steering correction to an excessive oversteering situation. Turning the steering the opposite direction of the normal turn will push the outside tire down, placing more weight also on the inside rear. The more positive caster, the more exaggerated and rapid this change in handling occurs--sometimes large positive caster settings results in the driver over-correcting.

So, on a race car, caster essentially is a tool to change the handling characteristics of the car during turning. More positive caster makes the car looser in mid corner and more stable in a straight line.

Many, many years ago, my Father taught me about caster by using a pencil and a pin. He took the pin and stuck it into the side of the pencil at a 90 degree angle.  Then he leaned the pencil back and rotated it. The pencil was the steering axis (king pin in the old days) and the pin was the wheel axle.  Rotating the pencil counterclockwise with the pin on the right illustrated the outside tire in a left turn--the pin went up. Rotating the pencil counterclockwise with the pin on the left illustrated the inside tire in a left turn--the pin went down. My first lesson was 56 years ago when I was 8, but I have used the concept ever since.