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Making your car go faster, Part 2.1: Suspensions (long)

(part 2 sliced in half to fit on bmw digest)
>In our last episode we discussed how to make the car go faster by improving
>the driver (you!). In this part we'll delve into some technical details about
>suspensions: how and why they make your car go faster.
>
>One of the reasons we bought BMWs in the first place instead of Camaros or
>Corvettes is that handling is important to us. At least, I hope so. And one
>of the things that BMWs do really well is handle. If so, why is it people
>like me  and Carl Buckland are forever modifying our suspensions to get
>better handling? 
>
>Two reasons: to go faster and to have more fun. While it's true that races
>are won on the straightaways, it's also true that all other things being
>equal, the car that can get through the twisty bits faster will get lower lap
>times. As for fun, many of us like the responsiveness that a taut suspension
>gives our cars. We find them simply more  fun to drive. 
>
>What's wrong with the stock suspension on my M3, you ask. Good question,
>Bucky. The engineers that designed your car had to factor in many goals in
>their design. First, the car must be safe. Since BMW can't make you pass
>adriving test before you get to buy your shiny M3, they have to assume that
>not all their customers are excellent drivers. Thus they design in some
>understeer so if you get in trouble the car will be more recoverable. Second,
>the car must be comfortable, so relatively soft springs and shocks are fitted
>to keep the ride quality high. Third, the car has to be competitively priced,
>so they have to select a suspension design and components that are
>affordable. Many other factors, from the size of the engine compartment, to
>ground clearance, ease of maintenance, and tire longevity have to be figured
>into the final design. Needless to say, that design, sterling as it is, is a
>compromise. 
>
>This year BMW will sell 100,000 cars, and  most of them will still be running
>on a stock suspension 10 years from now. So BMW must be doing something
>right. For some of us, however, we want to optimize the design towards some
>specific goals. That's where modifications come in, and that's where it gets
>complicated. 
>
>For every modification to your suspension you make to improve handling,
>there's some tradeoff that comes along. Reduce the ride height and there goes
>your ground clearance. Pretty soon you'll rip some overpriced hunk of plastic
>off the bottom of your car as a result. Increase spring rates and ride
>quality goes out the window. Five miles of bad road and your tailbone will
>ache and you'll have more rattles than a 58 Nash American. 
>
>The other thing to consider when upgrading suspension components is exactly
>how the car will be different after you install them. And it will be
>different. It may handle better, it may handle worse. Read any article or
>posting about cars that have been modified and the authors universally rave
>about the better handling, turn in, predictable oversteer, etc. Rarely do
>they tell you that the car rode like a truck, darted all over the road, or
>plowed like a pig. All these things are possible and even likely if you start
>adding random suspension pieces without understanding the how and why.
>
>One of the fundamental things that happens when a car corners is weight
>transfer. Basically the outside tires get heavier and the inside ones get
>lighter. Tires can handle lateral loads in proportion to their weight up to a
>point. So weight transfer to the outside means those tires can handle more
>lateral load (ie cornering force). Unfortunately, the inside tires give up
>more traction as they get lighter than the outside tires gain as they get
>heavier. So weight transfer results in deceased lateral resistance for the
>tires. This is a Bad Thing. 
>
>Although many people don't believe it, there are only two things you can do
>to alter side-to-side weight transfer. You can increase the track (width at
>the wheels) or decrease the center of gravity (CG). Weight transfer is a
>function of a triangle drawn between these three points at each end of the
>car. (Thus either end can have more or less weight transfer than the other
>end; this is an important point we'll come back to later.) You can increase
>track by going to wider tires to a certain degree, but typically body
>clearance prevents much change here. (This is why Porsche went to "fat" 911s
>awhile back, and why Ferraris are so wide). An easier way to improve the
>golden triangle is to lower the car, thus lowering the CG. 
>
>How do you lower the CG? Normally you put on shorter springs. That in turn
>reduces the suspension travel, ground clearance, and, on some suspension
>designs, increases negative camber, which is a Good Thing, unless it's
>excessive, and then it's a Bad Thing. Also, since the wheel travel has been
>reduced, the springs need to be stiffer to prevent the shorter suspension
>from bottoming out more often. 
>
>By reducing weight transfer, we have increased the total lateral loading our
>tires can accept, thus we have increased the maximum stead-state speed we can
>carry through a turn. For the purposes of this discussion, consider a
>constant-radius sweeper. The less weight transfer, the greater the speed
>possible before the car starts to slide. The car is more balanced side to
>side.  (to be continued in part 2.2...) 
>
>John Browne
>BMW CCA
>BMW ACA Puget Sound Region
>M3 LTW (PeeKay)
>Suburban 2500 (Godzilla)
>326 iX (Spunky the brave little car)
>
>copyright (c) John Browne; all rights reserved
>