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History User's Guide Contact |
It all started after I took
the delivery of the brand new 2007 BMW 335i Coupe in
December 2006. My passenger complained that the ride was
jerky. It had been 10 years since I last touched a stick
shift. We, as drivers, usually don't get bothered by shift
shocks. But it is painful to watch your passenger lurch back
and forth. So I spent the next few weeks trying to figure out the perfect "shift", swift and jerk free, like with an automatic transmission. I tried to clutch in and out slowly and gently when shifting. It would reduce the shift shock but not eliminate it. It would also wear out the clutch sooner. Most importantly, I don't want to waste half of a second when shifting. When trial and error does not work, I went on to read the articles about the clutch from How Stuff Works.com. I started to understand the gear ratios, the number of turns of a gear for each turn of the engine. Say the gear ratio of the 2nd gear is 4:1 and the gear ratio of the 3rd gear is 2.4:1. That means for each turn of the 2nd gear, the 3rd gear will turn 2.4 / 4 = 0.6. Say if you are traveling at 30 mile per hour in 2nd gear at 3000 RPM, when you shift to the 3rd gear, the matching RPM would need to be 3000*0.6 = 1800. My next step was to calculate the typical shifting values in a table. When I am not racing, I usually shift when the engine reaches 2000 RPM to save gas. For example, I am on the 1st gear, and I clutch in at 2000 RPM. Then I let the engine to drop to 1182 RPM before clutching out. Similarly, if I am on 2nd gear and clutch in at 2000 RPM, I'd let the engine to drop to 1317 RPM before clutching out. The following table shows the relative RPM needed when shifting from adjacent gears at three thresholds 2000, 2500, 3000 RPM respectively:
"A picture is worth a thousand words." In my case, a graph with all the possible ways of shifting finally took shape on March 6, 2007. This later became the Shifting Path. Time line:
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