Why can't I rest my hand on the shift knob?

Ever wonder why people say that its bad to rest you hand on the shift knob while driving?  Well, there is a reason, and good thing you listened!

Within the transmission itself, BMW has a specification of 0.004"-0.006" gap between each gear and the shifter rail in the transmission (for reference, a human hair is approximately 0.0035").  This gap allows for the transmission lubricant to acts as a barrier so that the shifter rail and gear are not directly touching, causing premature wearing of the gear (not syncros), shifter rail, or both.

When you make a 0.001" change at the shift knob and maintain pressure (like resting your hand on the shift knob while driving), the required 0.004"-0.006" gap is closed, preventing the lubricant to ride between the gear and shifter rail.  Premature wear of the gear or shifter rail may hamper transmission shifting performance, gear engagement, or worse, require rebuilding or replacement of the transmission.

A recent popular fix for short shifter notchiness is the use of an excessively heavy, metallic shift knob.  At only 3 oz. (approximately 100 grams), the factory shift knob is relatively light, in comparison with the available aftermarket knobs.  We have analyzed the additional mass of the Rogue Engineering shift knobs (approximately 6 oz.) and have found that they're additional mass was not enough to close the 0.004"-0.006" gap.  However, as we began adding more weight to the shift knob area, we did find that when the mass increased to 8 oz., the gap began to close.  Using anything heavier than this would be the equivalent of driving with your hand resting on the shift knob.  When considering an aftermarket shift knob, consider how much mass it threatens to add.  Too much is a BAD thing for your transmission!

A more elegant solution to addressing the effort is the Rogue Engineering Weighted Selector Rod.  While stationary, its additional mass does not change the internal dimensions of the 0.004"-0.006" gap, since it's below the leverage point of the shifter.  Its additional mass aids in improving the shifter feel without the potentially negative drawbacks of weighted moments on other parts of the shifter mechanism.