The second post in this series looks at the challenges of the M3 front suspension.
The front suspension has some particular challenges: travel (always) and camber change. The BMW engineers solved both by putting a really well designed bump stop in the front suspension. At static (sitting there waiting to go) the both the front and the rear suspensions are compressed by 4 inches (100mm) at the wheel. Static is 355mm from hub to fender edge, full droop is 455mm. By limiting the travel in the front suspension design, they controlled the camber change. Your tires are very happy about this.
The front suspension moves the damper at virtually the same rate as the wheel (motion ratio is 96%, which is essentially “one” for all practical purposes). That means the damper compresses 100mm (4 inches) from full droop (lifted into the air on a jack or a lift) to sitting on the ground ready to go. The front damper has a total travel of 150mm, or 6 inches. That means that there is 2” travel left when the car is sitting on the ground. The question is, what do we do with the two inches?
The factory chose a 167 pound-per-inch spring, so weight transfer in a corner or a bump of 334 pounds that pushes the wheel up into the wheel well by two inches would run out of travel. The factory put a cleveryly designed bump-stop in place that augments and raises the spring rate smoothly and rapidly so that the suspension doesn’t run out of travel suddenly with a metal-on-metal collision. Travel just comes to a rubbery end at about 1.5 inches rather than a hard bang at 2”.
So, how does this play out in suspension mods? Well, if you just put “lowering springs” in place, the suspension has to sit lower than stock (duh) so the spring has to be SOFTER than the stock spring. You've also used up some of the two inches available travel. The four inch rise of the suspension has to become five inches to get a lowering of one inch, and the remaining travel is reduced to one inch. One inch of travel is "not much" in anyone's language.
Now, along the way, the BMW engineers gave the spring companies a “gimme” by pre-loading the 167 pound spring by about 133 pounds so that it's producing 800 pounds of force at 4" of compression (133# + 4"x167#/" = 800#). A spring with no pre-load that sits just-barely slack in the OEM spring seats can have a 200#/inch rating (four inches of compression with about 800# sprung load) so it CAN be stiffer than stock at the STOCK height. If the spring lowers the car by allowing five inches of drop instead of four, it has to be 160 pounds per inch (160#/inch times five inches drop = 800 pounds) so you get a lowering spring with the same rate as the stock spring.
What about handling, though? You’ve got the stock spring rate, but instead of two inches of travel (334 pounds of weight transfer) you only have one inch, or 160 pounds. Needless to say, your car’s handling will suffer – too little travel left to soak up bumps before you’re into the bumpstop. The car will be fine on smooth roads, choppy on rough ones.
I'm going to stay out of the "progressive spring" discussion until I get to Type Two suspensions. Most Type Ones look like linear-ish springs, let's just leave it at that. Progressive springs change the game, but they have their own problems to cope with.
The Dinan kit dodges this bullet with two important design choices: First it limits the drop to about ½ inch. Second, it increases the suspension travel (at the front) by the same amount, keeping the same travel as stock. Travel remains the same as stock, rates are slightly higher (I’m guessing) and overall the performance of the suspension doesn’t change that much.
My opinion (I promised to keep them out of this, but it's just too tempting) is that Dinan springs and suspension parts matched with Bilstein's Sport dampers would work really well together.
When you do the same analysis for Type Two, Type Three and Type Four suspensions, the situation improves somewhat. After a brief discussion of the rear suspension, I’ll post about Type Two’s next.