John Stranahan

iTrader: (27)

I noticed that the setup I posted did not have caster settings. I also noted the caster setting on the car were not what I expect. Kind of the reverse. This is an easy mistake for me make. I have seen others do it as well. Upper arms forward gives you zero caster. You adjust it in 2 degree increments by moving backward the white shims one at a time. See the photo. I have made the addition to the above setup.

Caster
So what does caster do for you. Well three things actually. Firstly as a wide flat bottomed tire rotates around the angled kingpin it will start to ride on the edges a little. If you watch the front of the car with 4 degrees caster on both sides and use the radio to turn the wheels left you can see the left front of the chassis lift. You are adding load to the left front and right rear. This is increasing wedge. It is giving you more steering the more you turn the wheel from increased wedge (you also of course are using more slip angle, but this by itself is somtimes ineffective like when you crank the wheel more and the car stays on its present course or pushes). I believe this is the primary and strongest benefit of caster. This lets you crank in more steering by just turning the dual rate wheel on the radio to give more steering throw even when you are not using full throw. You don't get much of this benefit if your left front wheel is at 0 caster.

Secondly Caster improves the camber of the outside front wheel as you turn and the car rolls more. It gives you negative camber gain. It improves midcorner steering at the expense of corner entry steering or turn in. The negative camber gain is only helpful after the car has rolled a little in the turn.

Thirdly, and of little benefit to us, caster actually lifts the front end of cars with wide flat bottomed tires. The natural tendency is for the car to settle back down and drive straight. Our servo steered cars are little affected by this tendency.

Caster Experiments
I have been told that caster causes scrub on the oval so I try to use low settings.
I ran the car with several caster setting today.
2 right and 6 left. Now this was opposite of what I inteded. I intended 4 right and 0 left. The car steered great but there was some possible scrub mid corner.
0 right and 0 left. I lost some late corner steering. The car now stumbled on itself on corner entry. You could hear it and see the car slow. It was helpful to steer extremely gently into the turn. I just did not like this at all.

2 right and 0 left. This reduced turn in so that it was very smooth. I still lacked just a little late corner steering. I was out very close to the bumpers a lot.

2 right and 2 left. I like this a lot. I got my late corner steering back. I had smooth turn in. I had little scrub. Jeff saw the car and said it looked good. He had complained of scrub the day before. I use the same setting on-rioad.

Inner Hinge Pin spacer
The picture shows the little device CRC sells (as do many other oval companies) to raise that inner upper A-arm hinge pin. The CRC device is just a spare top cap with longer screws. It fit well. So what is it supposed to do. Most of the oval writing I see concentrates on its effect on negative camber gain. Upper arms more parallel to the ground gives you less negative camber gain. I put it on the car on the left side. I took it off today. The effect on the flat oval was small. I could not tell the difference. It is important to reset toe in after you take it off as this changes a little and gives the appearance of more steering if you just take it off. You get some toe in. Camber has an effect on tuning but it is not a large one. We mostly adjust camber to wear the tires flat. (I do sometimes remove camber to get some better straight line stability on fast road cars). So what does less negative camber gain do to the left front tire. Well I think that it moves the tire the wrong way as the car rolls to the right. Negative camber gain on the left side when rolling left means positive camber gain when rolling right if there is a little left droop available. I would think you would want to keep this positive camber gain when rolling right on the left front.

Note the roll center is also changed a little by this part, but not very much. The roll center stays kind of high. I wish for a lower one. Experiment to come. I plan to bend a left side axle up at 5 degrees and remachine it round on the lathe. I hope it bends 5 degrees before it cracks (T6 temper is not know for flexibility). (Rick I have spares already). This will give me a inward incined kingpin angle and low roll center along with positive camber on the left side.

The car was hooked up. 170 F motor temp after 10 minutes 85/36 2.320 inch rear tire.

I welcome comments on caster. Personal observations are prefered.

John