Join Date: Jun 2002
Location: The Deep South of Vermont
Re: engine noise
K bike rattle:
I posted about this years ago. Yeah, BMW mechanics just say "its normal". Well, it is, but that doesn't explain much.
Other's say it is the clutch, or alternator.
Excluding exhaust, plastic, and hosts of other sources of rattle, the K motor does (can) have an inherent rattle. The interesting thing is that it varies from bike to bike.
Here are some ways to test if your rattle is classic K-bike rattle:
1. Pull in the clutch: the rattle goes away. This doesn't mean that it is the clutch that is rattling.
2. Turn on a heavy electrical load like your driving lights: the rattle goes away. This doesn't mean that it is the alternator that is rattling.
3. If pulling in the clutch doesn't change the rattle, try this: put the bike in gear, hold the brake, and slip the clutch: the rattle goes away.
Look at the picture kindly posted above and reposted here with a photoshop added circles and arrows. Note that the crankshaft has a helical cut gear. The alternator shaft and the output shaft also have helical cut gears.
Here's the thing with helical cut gears: they create axial pressures on the shafts that they are attached to and the shafts that they drive. When the engine loads the alternator and output shafts the shafts will move a little in an axial direction. When there is no load on the drive train within the motor, the shafts can oscillate back and forth in an axial direction creating the rattle. Any shaft with a helical cut gear will do this. As soon as you load the drive train, the back and forth movement of the shafts will stop and the ratttle will stop. You can load the motor by turning on electrical stuff thus loading the alternator, the alternator drive shaft and the crankshaft, or by slipping the clutch thereby loading the output shaft and crankshaft.
The limit of axial movement in these shafts is determined by shims. There are maximum and minimum limits for movement specified in the BMW Service Manual. Some shafts will have more axial play than others. THIS is why some bikes have more K-bike rattle than others.
On my 2000 the crankshaft clearly is the source of rattle. I have listened with a mechanic's stethoscope with all the tupperware removed and it is clearing coming from the "bottom end".
In the attached picture, see the helical cut gears circled. The arrows show the direction of axial movement that the shafts may experience when there is no load on the bike and the shafts oscillate back and forth. Again, as soon as you put a load on the motor the oscillation stops and the rattle goes away.
It is a nice rattle, it can sound like rocks in a can, (remember the old trick of putting rocks inside someone's hubcap?), enjoy the K-bike rattle.... it adds character to the bike.
My bike rattled when new to the point of my distraction. Now at approx 60K miles it still rattles. Maybe it rattles less, or maybe I am just more used to it, who knows?
Last edited by CharlieVT; Jul 15th, 2008 at 8:13 pm.