Volume II, Issue 2, Page 4

SUSPENSION GEOMETRY 101

Hope everyone had a great holiday season. As we begin a new year we need to take time to look back and think about our family, cars and all the things we have. It is also time to start thinking about the fact that it’s winter, probably cold where you are and your car is put away for the season. And now is the perfect time to update that ride of yours, and add all the new things and do all the changes you thought about doing last summer. Yes, it is tear-down time.

You may have some plans for your car that include basic bolt-on parts. It may only be what you can afford this year, or possibly you can’t do a major teardown where you have it stored. Some of you, however, are just aching to tear into that engine or chassis to really up the performance for this summer. There are some really cool bolt-on chassis parts that will make your car handle like a slot car. Control arms, dropped spindles, big brake kits, all of these components can transform your car into a high performance machine that feels like it is unstoppable. What a way to drive home from work! There are, also, some major chassis modifications that involve cutting off brackets and welding on new control arm mounts. These are all designed to enhance the geometry of the suspension, and they very well may do a great job. It is, however, quite a sizable undertaking for the average guy who does not have a welder in his garage, so we will limit ourselves to bolt on products here.

The overall geometry of the entire chassis is what really makes the car handle well. Plotting suspension geometry is easy and interesting at the same time. For purposes here, I’ll limit this discussion to basic theory, and try to keep it really simple to understand, and ask that you just try to visualize the diagrams as they are described.

The concepts described here are the important thoughts. For example, to actually plot the front suspension geometry, you must first know exactly where the chassis mounting points and ball joint locations on the spindles are. To begin, you would draw a horizontal line on paper, which is the pavement, and a vertical one that is the center of the car. Then measuring out from the center of the car, draw another short line starting from the inner pivot on the lower control arm, to the ball joint. You will also have to decide at this point what size tire you will be using, to position the chassis and the lower arms at the proper height up from the pavement. Again, it may be easier just to visualize this. You will also note, and think about the fact that these lines, which represent the control arms, are straight lines. It makes no difference what shape the control arms are when studying geometry. The radically bent upper arms on the Camaros and Chevelles have absolutely no effect on the geometry. Also beware of control arms that claim to change the geometry by some radical shapes or other mysterious means that even the manufacturers cannot explain, but really do not move the pivot points. This is kind of like saying that you can change the lift and duration of a cam by changing the diameter or length of the lifters. Again, the pivot points are determined by the mounting points on the chassis and the spindles, not by the control arms, just as the lift and duration of the cam is designed by the shape of the lobes on the cam. Unless you physically move these locations on the chassis or spindles by changing to a taller spindle, you have not changed the geometry. Period. Geometry is nothing more than a study of pivot points.

Moving on, do the same for the upper arm. In a true high-performance suspension system, typically the upper control arm has the ball joint higher than the inner pivots, so the upper arms slant downward from outer to inner point. This will yield a great “camber curve”, or camber change under hard cornering, which will keep the tires square and tire patch flat on the pavement. You then project the lines out until they intersect. Again, if it is a truly high-performance design, the intersection of those lines will be on the opposite side of the car, and they may be really far away. Yes, you may need a really big piece of paper. You will also need to have measured the ball joint and wheel location on the spindle, as it and the actual wheel and tire needs to be added next. Once all the components are in place, then you can finish the plotting. Now draw a third line from the intersection of the first two back to the center of the tire patch on the ground.

This third line will cross the vertical centerline of the car. The place where this third line comes back and intersects the centerline is called the “Instant Center”, or roll center, in chassis design terms. This is the theoretical pivot point of the front suspension during the beginning of body roll. Now simply measure the dimension from the pavement up to the instant center (you have been drawing all this to scale, haven’t you!). In a further analysis, this point is then compared to the instant center of the rear suspension to determine the roll axis of the car, but that is another chapter. If you do all these diagrams, even if you just roughly sketch them, you can see how the angle of the upper and lower control arms can really affect the location of the intersecting lines, and therefore the instant center. In general, basic handling rules would like the front instant center to be low. If both upper and lower control arms are going downhill towards the center of the car, it is possible to have the instant roll center so low that it is below the pavement. Since it is only a theoretical point, that is OK, but it really does not help the overall roll of the suspension. When compared to the rear instant center it will generate a very steep roll axis, which will understeer excessively. Again, this for another chapter.

After you have finished redesigning your car, you can now complain to the big three on how they could have done a much better job designing your ride from the get-go. But they probably won’t listen, so save your breath, and get to work on your car. You can bolt on some cool parts now and wait until you can find a place to work on your car this spring and really do some major mods. 


 









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