“Body roll is your friend, because you want to make the transition from no lateral grip with the tire to maximum grip in a smooth consistent arc without spikes,” says Kline. “The only way you can make that transition is for the chassis to absorb some of the load. If the tires have to do it all, it feels like a go-kart but will also skate, understeer and oversteer under power and become unmanageable at the limit.”
In racing, Kline explains, the secret is to run softer springs with more shock damping.
“If you talk to the best suspension engineers at the highest levels of motorsport, they’ll all tell you that you want to run a suspension that has tremendous shock control but is still compliant,” he says. “If you put too much spring on the car, the spring is forcing a rubber deflection instead of allowing a progressive change from straight to maximum cornering lean. Optimal spring rate is determined by the tire. The more grip a tire has, the more spring rate you can run.”
From the bushings on up
Bushings also play a major role in suspension dynamics, and Kline says that the 1 Series’ rear subframe bushings—parts it shares with the mass-market 3 Series—are its biggest shortcoming. Kline says they’re too soft and have too much vertical movement for enthusiastic driving.
“If you put the 128i or 135i on a lift, you can make the rear subframe move an inch using only a pry bar because the bushings are so soft,” says Kline.
The subframe bushings must be matched to the spring rate, he says, and the car simply won’t handle as well as it should if a 1 Series owner decides to upgrade the shocks and springs without also replacing the stock subframe bushings.
“BMW did a fairly good job of matching the spring rate to the bushings for ride quality and going over bumps, but as soon as you put anything close to reasonable spring rates in the car for more performance, the bushings are totally inadequate,” Kline says. “If you put a stiffer spring on an E90/E92 3 Series or a 1 Series, you overpower the bushings and make the back of the car pogo up and down.”
Kline says this unwanted movement made development of his 1 Series suspension more difficult than expected, leading him to reduce the spring rates repeatedly until a comparison with the M3 revealed the problem.
“When we got the M3, we measured the spring rates and found that they were much higher than the 1 Series (with a 580-lb. rear spring rate compared to the 1 Series’ 350-lb.), and we knew that BMW had done something different with the bushings,” he says. “The rear end on the stock M3 doesn’t bounce up and down at all, so we tried the M3 rear subframe bushings with our suspension package and got rid of most of the pogo effect.”
Kline wanted to see how well he could make the 1 Series handle, and he was especially keen to see what could be done with the 128i. At 3,208 lbs., it’s the lightest BMW sold today, weighing 165 lbs. less than the 135i and carrying 120 fewer pounds over its front axle.
He installed rebound-adjustable Koni shocks/struts, M3 rear subframe bushings and his own camber/caster plates, set to provide around two degrees of negative camber up front and around one degree at the rear. Where the stock 128i rides on springs that measure in the neighborhood of 125 lbs. up front and 350 lbs. at the rear, Kline installed 350-lb. springs up front and 500-lb. springs at the rear. (Kline says he tried 600-lb. springs that were a little stiffer than the M3 springs, but they proved too stiff on the lighter 128i.) The rates he chose are much stiffer than stock, but they suit the car’s intended purpose as a combination track tool/daily driver.