[RC65]

I'm just curious: Anyone know why BMW changed the final drive position from the right side for the previous 1150 and 1200 platforms to the left side for the new 1250 (and, of course, the flip of exhaust side followed suit)? Was there some expressed benefit/improvement in performance from the change, or was it just necessitated by some factor related to the new Shift-Cam design?

 

[R1200N8]

I'm just curious: Anyone know why BMW changed the final drive position from the right side for the previous 1150 and 1200 platforms to the left side for the new 1250 (and, of course, the flip of exhaust side followed suit)? Was there some expressed benefit/improvement in performance from the change, or was it just necessitated by some factor related to the new Shift-Cam design?

“In contrast to all previous BMW Motorrad boxer models, the swingarm with integrated cardan shaft drive and EVO Paralever (in its third generation and designed using the very latest calculation methods) is now positioned on the left-hand side. This avoids any contact between the rider and the hot exhaust system when pushing or getting on and off the machine. Another pleasing effect relates to the appearance of the vehicle: when the motorcycle is on the side stand there is now a clear view of the redesigned rear wheel.”

 

[Pale Roadster]

I had heard it had to do with the front mounted wet clutch from the old traditional position dry clutch. They also rotated the intake/exhaust location on the heads which I presumed was because cooling the exhaust port was no longer reliant on air flow. I haven't researched but also think the shift cam has a counter rotating balance shaft so I would guess the engine case is getting crowded.

 

[RC65]

I had heard it had to do with the front mounted wet clutch from the old traditional position dry clutch. They also rotated the intake/exhaust location on the heads which I presumed was because cooling the exhaust port was no longer reliant on air flow. I haven't researched but also think the shift cam has a counter rotating balance shaft so I would guess the engine case is getting crowded.

“In contrast to all previous BMW Motorrad boxer models, the swingarm with integrated cardan shaft drive and EVO Paralever (in its third generation and designed using the very latest calculation methods) is now positioned on the left-hand side. This avoids any contact between the rider and the hot exhaust system when pushing or getting on and off the machine. Another pleasing effect relates to the appearance of the vehicle: when the motorcycle is on the side stand there is now a clear view of the redesigned rear wheel.”

Interesting, thanks. With both in mind, sounds like it could then be a mix of aesthetics and engineering.

 

[arfingdog]

Well, I've just now learned that I have an extremely rare 1200LC that has the shaft drive on the left. I'm open for bids on this possibly unique bike! ;-)

 

[Panzermann]

Shifting the drive from right to left means the drive shaft is spinning in the opposite direction. The LC engine rocks to the bike to the left when you blip the throttle, which I understand is the same as earlier models (correct me if I’m wrong) so the engines spin in the same direction. I suspect then that swapping the side is an outcome of the drive train arrangement in the LC engines (1200 and 1250) - primary drive, clutch and input/output gear shaft arrangements (rotation direction swaps with each gear interface) in what is a completely different gearbox to earlier models. The outcome of shifting the silencer to the right then becomes a very convenient result of the engineering - and who hasn’t melted their wet weather pants on a silencer. Incidentally, the dry-clutch/separate-gearbox R18 has the shaft on the right like the air/oil cooled Boxers that precede the LC.

Rather than cooling considerations, placing the inlets above the head and exhausts below achieves a downdraft effect, which makes use of gravity to assist gas flow and hence aid cylinder filling during the inlet phase.

 

[Pale Roadster]

I agree with the advantage of the downdraft effect, what I meant was they had not used that orientation on air cooled engines due to cooling the exhaust port taking priority over any othe consideration.

 

[Panzermann]

Well, I've just now learned that I have an extremely rare 1200LC that has the shaft drive on the left...

Are you sure you haven't just been looking at the bike (and yourself) in the mirror, arfingdog?

 

[zer0netgain]

This avoids any contact between the rider and the hot exhaust system when pushing or getting on and off the machine.

That sounds like a personal problem to me. Doesn’t matter what side the exhaust is on if you’re prone to hit it.

 

[Panzermann]

That sounds like a personal problem to me. Doesn’t matter what side the exhaust is on if you’re prone to hit it.

Most of us get on and off the bike, and push it around where necessary (or to a fuel station ...), from the left. But if you swing to the right, or are ambidextrous, Zer0netgain, more power to you!

 

[MikeHSG]

Short answer - single sided swingarms look pretty, so move the pretty side to the right

 

[RC65]

Short answer - single sided swingarms look pretty, so move the pretty side to the right

I think I'll go with that reason.

 

[arfingdog]

Are you sure you haven't just been looking at the bike (and yourself) in the mirror, arfingdog?

Are we talking about perspective here? From the bike's perspective, it's on the left. Much like Pale Roadster's picture of his 1250.

 

[Panzermann]

Are we talking about perspective here?

Just checking, Arfingdog. In fact I went out to the garage this morning, and sure enough, mine’s on the left, too! We’re both unique. 😄

 

[Mr. Badger]

K bikes had a very similar design on the left side for a long time. When they upped the power with water cooling it was probably faster and cheaper to recycle a design that they knew worked instead of designing a new higher capacity final drive for the right side.

 

[zer0netgain]

Most of us get on and off the bike, and push it around where necessary (or to a fuel station ...), from the left. But if you swing to the right, or are ambidextrous, Zer0netgain, more power to you!

Well, I’ve always been cognizant of the exhaust pipe when it’s hot, and I’ve had bikes with dual side exhaust. Never been an issue for me. I suppose if you’re used to a one side exhaust system....

 

[JanK]

downdraft effect, which makes use of gravity to assist gas flow

Seriously? I am quite likely ignorant of the conditions in and around the engine, but I would expect this effect to be negligible.

The mass of the 600ccm of air is 0.5g at 20 degrees C. On the other hand, assuming perfect vacuum in the cylinder and 2x40mm diameter intake valves with a total of 25cm^2 area, the force drawing the air into the cylinders is around 25kg - 50,000 times higher. Even for vacuum of only 0.2bar, the factor is still 10,000. IOW, an increase of around one hundredth of a horsepower.

What am I missing?

And I prefer the exhaust on the right. Both me and my partner have managed to singe our pants while moving the F650CS around in the garage after coming from a ride.

 

[Pale Roadster]

My understanding is the downdraft effect has more impact on fuel atomization than a concern about the gravitational impact due to the mass of air. In any case it seems the Germans felt there was an advantage worth changing a layout they had used since the early 1920s. YMMV

 

[r0ckrat]

Moving the exhaust port to the underside also makes accidental burns on the headers a little less likely.

Sent from my SM-G960U using Tapatalk

 

[Panzermann]

What am I missing?

When releasing the LC, BMW stated 'The flow through the cylinder heads is vertical for optimum charging'.

I understand it's related to the mass of air being drawn into the engine being given a boost by gravity. At peak power, rpm is 7,750, drawing ~5.5 kg of air into the engine every minute (assuming, incorrectly, that each induction cycle draws 1170 cc, but OK for illustration - more likely ~85% of this). Presumably making use of gravity assistance to shift this air is worthwhile.