Tex Powell Driveline Specialist...
Tex Racing Enterprises
Another question I have is about roll center and camber gain. I notched the top of the frame for the upper control arms about 31/44 of an inch and laid in a flat plate. My control arms are heims at each end with screw-in ball joints. The problem is I can only get about 16 degrees on them and fear my roll center will be a little high. I have a five-inch ride height and the lower control-arm points are at equal heights. Should I concentrate a little more on camber gain or rework what I have done and get shorter front springs?
Name Withheld Upon
A: I would suggest the angle of the drag link and the angle of the right-side tie rod in the top view will have an effect on your ackerman. I assume you have bump-steered the front end after the changes, and if you haven't, you should do that first. Then do a test with the front end on steer plates. If you zero them out before you start, record the steer angle on the left wheel while steering the right wheel in two-degree increments. You can then look at what kind of ackerman you have. Plotting the angles on graph paper with the left-steer angle on the Y-axis and the right- steer angle on the X-axis will show you if you have parallel steer, ackerman steer or anti-ackerman steer geometry. You want to have something close to parallel steer from zero to 10 degrees. Another factor to watch is the overall steering ratio. The idler arm is a different length than the steer arm. This means the arcs they follow will be different for a given input from the pitman arm (the length of the pitman arm was not given). The steering system has an overall ratio between the steering-wheel angle and the right-front wheel steer angle. The ratio comes from the steering box ratio as well as the linkage ratio due to pitman arm, idler arm and steer arm lengths. You should measure the overall ratio by recording the steering-wheel angle to get five degrees of right-front wheel steer. If it took 45 degrees at the steering wheel, you would have a 9:1 ratio. If it took 50 degrees at the steering wheel it would be 10:1. Normal short-track ratios will be around 12:1, which would be 60 degrees at the steering wheel.
Regarding the upper control arm, I am assuming this means the upper-arm angle is in the front view. You said you could get only 16 degrees. If you were able to get more angle, the roll center would actually be higher, while the camber change rate would be quicker. As for the question, about whether you should rework what you have done or get shorter front springs, there's really no way to answer that because you didn't say anything about how it performs. How do you know that what you have will not work? If you need more camber gain, use a shorter upper-control arm. The shorter front spring would lower the ride height and change the upper-arm angle, giving more camber gain, but we don't know if you have enough ground clearance for this or if the rules let you lower your car any further. However, if you raise the spring rate by cutting off the spring, you might make the car push (because of the stiffer spring) more than you gain in front grip from the increased camber.
Front-end geometry is a complex problem. There are many interactions of the geometric properties, so it is very difficult to change just one factor. The best method is to measure all of the front-end components and then analyze the geometry in a good three-dimensional computer program. Lowering the inner pivots of the upper-control arm will change the camber gain, the bumpsteer, the roll-center height, the roll-center lateral position, the tire scrub and the anti-dive. Which one of these factors that changed can the driver attribute to a different feel in the car? The answer is not necessarily obvious. A method for evaluating these factors is difficult, which is why even the top Winston Cup teams continue to work on front-end geometry to get the right combination of these many factors working together.
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