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Discussion Starter · #1 ·
one of the guys who works for us has a bmw. nice bike but i can't get use to the horizontal cylinder configuration. i know every manufacturer has there own twist on this. how does the position affect the behavior of a twin motor in real world terms. i realize there are probably very technical answers, but does the beemer have less torque, more torque, run higher rpms, those sorts of things. things i think about when i ride... ??? ::)!

julie
 

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I dunno about the BMW specifically. But, I was always told horizontal "boxer" engines put the weight of the engine lower in vehicle and thus improve handling.
 

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The only thing you really notice, or so I hear, is that since the torque is perpendicular to the line of the bike, then you get a tendency for the bike to 'twist' to the right under hard throttle. It's easily compensated for, but can be unnerving for someone not used to it.
 

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Another advantage of the boxer configuration that BMW uses is that the pistons reciprocate each other, i.e. are both moving out or both in at the same time, which significantly decreases vibration. This is not possible in a V or L-twin, although good counterbalancing can help. I assume that they don't combust at the same time, so the sound is nice and even also.
 

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One more advantage is even cylinder cooling, though it doesn't need to be 1800, just the east/west orientation. I suppose you could also count as an advantage the fact that the valve covers double as frame sliders (jk).
 

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BMW was originally an airplane motor builder(the logo is a propeller spinning). so I believe they took there engine design and made it work on a bike. That's my story and I'm sticking to it.
 

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Other than the improved cooling that howie mentioned, there's almost no power benefit to the boxer configuration.

There is a *slight* benefit from less power loss due to air being pumped around inside the crankcase, but at typical BMW twin rpms, it's not much.

The balance of the boxer engine, as far as vibration is concerned, is good, but our 90 degree v-twin is good also.
 

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superjohn said:
The only thing you really notice, or so I hear, is that since the torque is perpendicular to the line of the bike, then you get a tendency for the bike to 'twist' to the right under hard throttle. It's easily compensated for, but can be unnerving for someone not used to it.

'specially in the middle of a corner! [laugh]
 

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Discussion Starter · #10 ·
this all seems to make sense, so then what about engines that take the l-twin and flip it sideways? am i dreaming this...is there such a thing? or engines that vary the angle of the cyclinders?
 

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al vento said:
this all seems to make sense, so then what about engines that take the l-twin and flip it sideways? am i dreaming this...is there such a thing? or engines that vary the angle of the cyclinders?
It already exists,

just go check out a Moto Guzzi MGSO1



and also, as a previous BMW boxer cup owner, I can tell you that the boxer motors, while unbreakable, and easy to service, are actually limited to a very short rev-range, shifting is often, and the power is never truly felt while on a boxer.
 

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Discussion Starter · #14 ·
if we are purely talking about looks-the guzzi looks better to me than the bmw, but all in all if performance isn't markedly different i just don't get it...guess i'm an l-twin duc girl!!! ;)
 

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I really like BMW's boxer motors. I don't think I would buy a non boxer BMW. Its what makes it a BMW, IMO.

The torque moment to the right is very slight, you can really only feel it at a standstill. Rev the bike & you get a bit of a pull, but its not much. I do suppose it makes a BMW harder to balance at a standstill, something I still have not mastered for more than a few seconds.

The boxer 1200 motor puts out about 85 ft lbs of torque IIRC, and riding a bike with the boxer is all about torque. The weight is very low, and the bikes actually corner well. The heads are not the first thing to touch down, its the pegs followed by the center stand followed by the heads. I've not come close to the heads scrapping.

A big advantage to the boxer motor is the lower leg protection they give. You see very few BMW riders with amputated legs, not so true of other riders who are hit by cars.

All in all, yes, there are more modern engine designs, but I appreciate a link to history (first BMW boxer goes back to the 20's I believe), and I like the torque of twins, which are both reasons that I am looking at a Ducati, another brand that lives/dies by its engineering history.
 

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Discussion Starter · #16 ·
is the boxer motor different than say the motor on the gs as far as cyclinder placement? i thought the configuration is the same. the lower center of gravity really does make sense though...
 

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AFAIK, BMW has had 3 configurations of the 'boxer' motor since WWII.

Not *hugely* different, but significant changes to the architecture of the motor.
 
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Speeddog said:
<snipped>

There is a *slight* benefit from less power loss due to air being pumped around inside the crankcase, but at typical BMW twin rpms, it's not much.
I thought the opposite was true: Since both cylinders went in and out at the same time there was a more significant pumping of air than on other motors. Enough so that a few folks have converted the crankcase to a mild supercharger using a set of reed valves.
 

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With both pistons going in and out at the same time, the air in the crankcase just gets compressed and expanded, which consumes very little power.

With an inline 4, there are 2 pistons going up as the other 2 are going down, so the air doesn't get compressed, but it *does* get pumped back and forth between cylinders, going past the crank and rods and other stuff in the bottom end.
On an I-4 literbike, that's 250cc of air that gets pumped back and forth 200 times a second at 12k rpm.
The peak airspeed is about 70 mph, not huge, but it's lost HP all the same.
 

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I've never understood the principle of a lower center of gravity on a boxer-engined bike.

In a car, this is clear. The engine lies flat, as opposed to having cylinders sticking at various upward angles, but either engine configuration has the same lowest point, so the horizontally-opposed engine keeps its Cg lower.

In a bike, an engine with a longitudinal cylinder configuration can keep the pots from sticking out the sides, so it should theoretically be capable of being placed lower than a boxer engine, which must stick the pots to the sides, and must raise them so they don't drag in a lean. Yet, if you look at an S2R and a typical BMW, the vertical position of the pots is higher in an S2R than in a BMW, or on some models equivalent. Why aren't the L-twins's engines lower?

What gives?
 
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