Answers provided by Senior Tech Editor Bob Bolles (unless otherwise noted)
Q When making measurements for designing the roll centers, is the "centerline" the chassis centerline, or the centerline measured from the center of the left front wheel to the center of the right front wheel? Also, when designing the upper control arms, should the same length be used for both sides? I'm using the same size now for each side, but didn't know if there was a benefit from running two different sized arms. I thought they would change angles at different rates, therefore shifting the dynamic roll center left or right. Am I correct in figuring this?
A For dynamic analysis, we always want to know where the roll (moment being a better description) center is located in relation to the two front tire contact patches. That is where the car "feels" the moment center. I always find the distance between the front tire contact patches and use half that distance, measured in from the center of the right front and the right rear tire contact patches, to establish a centerline of the car. It is always suggested that you line up the right side tires' contact patches when you are doing your basic chassis alignment so the centerline you are establishing is parallel to the right side tire contact patches, as well as being in the center of the front tire contact patches.
One of the primary reasons we use different length upper control arms is so we can adjust the angles of the arms to locate the moment center. Some cars have no means to adjust the angle of the upper control arms and so length changes, which change the arm angles, help accomplish this. The moment center will almost always move to the right (when turning left) after the car dives and rolls in the turns and whether the arms are equal or not does not necessarily affect that.
The two critical front-end design goals are:
1) location of the moment center after the car dives and rolls as it is in the middle of the turns. This dictates the stiffness of the front end and is a factor in balancing the setup.
2) establish a right upper control arm angle that will yield zero camber change after dive and roll. This means that the right front tire will keep the same camber angle relative to the racing surface throughout the entire lap. This has been found to be very beneficial after analyzing many different front-end designs. We know that if the camber continues to change on entry and through the middle, the right front tire will not grip as well and the car may develop a push regardless of how well-balanced the rest of the setup is.
Forward Bite For '80s Monte Carlo
Q I run a mid-'80s Monte Carlo (Hobby Stock with no weight jacks or racing shocks) and I have a big problem getting forward bite. It wasn't as big a problem with a sway bar and 31/44-inch of preload but the car handles much better through the corners without it. The car weighs 3,200 pounds with driver, with 43 percent rear weight, and I'm guessing 55 percent crossweight now without the sway bar. My springs are as follows: RF 1,300, LF 1,000, RR 200, and LR 250. The car is awesome in the middle of the corner and pretty good going in. What do I need to do? I've been told to reduce the split in the rear, and someone else has told me I need softer springs.
A From my experience and listening to others who know, we know the sway bar does help you get bite off the corners. It doesn't affect the middle handling balance as much as some racers think. That means you may have been close to balanced with the old setup. What you need to do is make changes to improve the setup with the sway bar installed. Not knowing the banking angle of the track, or other pertinent information, I cannot specifically address your setup. Here are some general trends that will help your car in the middle of the turns.
Reduce the front spring split by softening the right front spring. If the track is flat (less than 10 degrees), you can probably reverse the front springs or soften both sides with the left front stiffer than the right front. You can also reduce the rear spring split to, say, 25 pounds of difference to free up the car in the middle. I would reduce the left rear spring rate to 225 pounds to accomplish this. With the metric four-link rear suspension that I assume your car has, the rear roll center is very high. If the 25-pound rear spring split makes the car loose in the middle, go back to the 50-pound split.
By reinstalling the sway bar, you have a tool to tune the bite off the corner. Remember, speed through the middle is worth a lot and helps the speeds all the way around the track. If you have a choice, you should maintain the middle balance and the speed that goes with it and live with the bite problem. With a little refinement in the spring rates, you just might be able to have the best of both.
Lowering The Metric Spindles
Q I have a problem, (and) may not have any choices. I have a metric chassis-Hobby Stock dirt with not-so-stock rear weight jacks, 3,000 pounds, and 355 cubic inches. We can no longer run the aftermarket 2-inch lowering spindles. Is there any known method for getting the stock spindles lowered? I heard others talking about different bottom ball-joints and moving the top joint to the bottom of the A-frame (is this safe?). Could you please help with this, if possible? I have the front end disassembled and am looking for solutions now. Also, S-10 spindles are not allowed.
A I assume you are trying to take some of the angle out of the lower control arms. You can use the adjustable mono-ball type of lower ball joint and drop the lower ball joint up to one inch. I have heard of teams drilling and tapping the spindle to accept a machine-head bolt the same size as the mono-ball hole to further lower the ball joint.
The lower ball joint takes a lot of the cornering forces, and I would strongly discourage you from using a top ball joint in the bottom of the spindle. The mono-ball type of joint uses a much stronger shaft for the lower ball joint than the upper ball joint, and I have not heard of any problems with them breaking.
Another thought would be to raise the front of the car to help take angle out of the lower control arms. We have done that with the dirt Late Models in order to help the design of the front roll center. The roll center placement can improve the car to turn in the middle and promotes performance, more so than raising the center of gravity hurts. On some dirt tracks, a higher center of gravity is actually beneficial in getting more weight transfer for dry, slick conditions.
Ford MacPherson Strut Street Stock
Q I am interested in getting back into racing. It seems like everywhere I go, all Street Stocks are Chevy. I want to be different so I would like to build a mid-'80s Thunderbird. How does someone make a front strut race car turn on a tight radius dirt track? I have seen a couple attempts, but none of them made their car work well. The camber is always wrong, and the strut does not allow enough adjustment, so the car pushes too much. Does Ford offer an upper/lower A-arm car similar to the Chevy? Is this the impossible conundrum?
Christopher C. Arnett
A The strut systems (I assume you mean the MacPherson Struts) are very limited when it comes to performance. The camber change is not right and the roll center design is terrible.
What you might do to help the situation is similar to what Street Stock and road racing teams do-order aftermarket upper mounts for the strut. These mounts allow you to put much more camber into the front wheels and also to adjust caster settings. There is some cutting required on the fender around the mount, but not enough to cause structural problems.
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