Pulsing in 4th

[ChazzyB]

My sub-2k mile 1200 does it. As did the three previous engines (over two bikes).

 

[Tyzer]

I never noticed it on my 2 previous GS's ,I just thought it must be a characteristic of the 1250 !

 

[CPJS]

As the 'thrumming' is engine speed related it can be one of three things engine,clutch or gearbox.
1. Engine, if an engine thrummed like that it would be on it's way to throwing a rod. so I don't think so.
2. Clutch. Mmm not sure on that one, pulsing vibration from a clutch, dunno, just doesn't feel right.
3. Gearbox. The vibration happens at a specific rev range and primararly in 4th with lesser incidence to a lesser degree in 3rd and 5th, happens under light load and dissapears if the gearbox is loaded or unloaded.
This to a non engineer me seems like a harmonic resonance.
I cannot believe that BMW don't know exactly what it is, or what is causing it as it has been around for ages, So why have they not engineered it out?

 

[beaver]

As the 'thrumming' is engine speed related it can be one of three things engine,clutch or gearbox.
1. Engine, if an engine thrummed like that it would be on it's way to throwing a rod. so I don't think so.
2. Clutch. Mmm not sure on that one, pulsing vibration from a clutch, dunno, just doesn't feel right.
3. Gearbox. The vibration happens at a specific rev range and primararly in 4th with lesser incidence to a lesser degree in 3rd and 5th, happens under light load and dissapears if the gearbox is loaded or unloaded.
This to a non engineer me seems like a harmonic resonance.
I cannot believe that BMW don't know exactly what it is, or what is causing it as it has been around for ages, So why have they not engineered it out?

Dont forget the transfer shaft front of enging to back. Shaft in a shaft as the outer is the cam drive, the inner the gearbox.. not sure what the bearing set up is here but this area has potential also IMO

 

[CPJS]

Dont forget the transfer shaft front of enging to back. Shaft in a shaft as the outer is the cam drive, the inner the gearbox.. not sure what the bearing set up is here but this area has potential also IMO

Surely then the vibration would be present in all gears.

 

[beaver]

Surely then the vibration would be present in all gears.

Maybe.. but is it resonance at given loads.. why are some worst in 3rd and others in 4th.. and what's the tapping in 5th???

 

[ChazzyB]

My sub-2k mile 1200 does it. As did the three previous engines (over two bikes).

I only ever seem to experience the 'problem' in third. I reckon it's two separate, almost identical frequencies of vibration, which periodically the peaks of the 'waves' from each coincide and are experienced as a much stronger vibration.