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Old 11-03-2012, 11:52 AM   #3541
markjenn
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Quote:
Originally Posted by DannyZRC View Post
This slight misalignment is what causes the "rocking couple" of the boxer, which is a back and forth oscillation in the yaw axis of the motorcycle, it's the primary source of felt vibes.
I've always wondered why BMW hasn't tried eliminating the offset (and presumably the rocking couple vibration) by using a forked conrod on one side with bearings on either side of the other side's bearings so that the two cylinders could be aligned.

- Mark
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Old 11-03-2012, 12:03 PM   #3542
DannyZRC
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Quote:
Originally Posted by markjenn View Post
I've always wondered why BMW hasn't tried eliminating the offset (and presumably the rocking couple vibration) by using a forked conrod on one side with bearings on either side of the other side's bearings so that the two cylinders could be aligned.

- Mark
sort of a not quite knife and fork? I'm not even remotely an engineer, but either you'd have a really big fork to make way for bearings, or you have no bearings and one of the conrods is supported by webbing only on both sides?

I imagine the big fork that allows bearings would make for a heavy conrod to deal with the bending of that conrod, and the small fork would have a heavy crankshaft so the webbing is beefy enough to support all that load?
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Old 11-03-2012, 12:13 PM   #3543
norseXL
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Quote:
Originally Posted by markjenn View Post
I've always wondered why BMW hasn't tried eliminating the offset (and presumably the rocking couple vibration) by using a forked conrod on one side with bearings on either side of the other side's bearings so that the two cylinders could be aligned.

- Mark
It´s what Harley´s been doing for over a hundred years, so it must be bad
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Old 11-03-2012, 12:19 PM   #3544
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Originally Posted by norseXL View Post
It´s what Harley´s been doing for over a hundred years, so it must be bad
not quite, a knife and fork conrod arrangement on a horizontally opposed engine doesn't yield a boxer, the pistons move back and forth together not in opposition.

DannyZRC screwed with this post 11-03-2012 at 05:20 PM
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Old 11-03-2012, 12:20 PM   #3545
ragtoplvr
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you guys do not visualize the boxer correctly. A knike and fork like you suggest would have one piston moving in while he other is moving out. In a boxer, both move out together, an d in together. Knife and fork would not do this.

Perhaps this will help

http://www.animatedpiston.com/BMW.htm

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Old 11-03-2012, 12:28 PM   #3546
DannyZRC
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Originally Posted by ragtoplvr View Post
you guys do not visualize the boxer correctly. A knike and fork like you suggest would have one piston moving in while he other is moving out. In a boxer, both move out together, an d in together. Knife and fork would not do this.

Perhaps this will help

http://www.animatedpiston.com/BMW.htm

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Old 11-03-2012, 01:11 PM   #3547
marchyman
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Quote:
Originally Posted by markjenn View Post
I've always wondered why BMW hasn't tried eliminating the offset (and presumably the rocking couple vibration) by using a forked conrod on one side with bearings on either side of the other side's bearings so that the two cylinders could be aligned.

- Mark
The wethead reduces the cylinder offset according to this:

Quote:
The crankshaft is now much more compact, thanks to the reduced width of the connecting rods and consequent reduced parallel misalignment of the opposing cylinder. This was possible thanks to the enlarged diameter of the crank journals, from 48mm to 50, which increased the load capacity of the related bearings, thus reducing their width. The main journal diameters were reduced from 60mm to 55. The new crankshaft is more rigid and lighter. The reduced misalignment of the cylinders causes less rocking imbalance and related vibrations, and favors the much-higher rotational speed needed to produce the new level of power.
http://www.cycleworld.com/2012/10/03...gs-first-look/
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Old 11-03-2012, 02:03 PM   #3548
markjenn
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Originally Posted by DannyZRC View Post
not quote, a knife and fork conrod arrangement on a horizontally opposed engine doesn't yield a boxer, the pistons move back and forth together not in opposition.
Yes, I'm not talking about putting the two conrods on a common crankpin, I'm talking about having one conrod "forking" and being supported on two crank throws, each flanking a single throw for the other conrod.

The following illustration uses two conrods rather than a forked one and uses counterweights as bearing containers, but it illustrates the concept:



Other interesting stuff at this site:

http://hildstrom.com/projects/boxer/index.html

- Mark
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Old 11-03-2012, 04:39 PM   #3549
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Nice idea and it does solve the rocking couple but looking at it I can't help but think the crank would either be overly heavy or not very strong. It might just be because of the simplistic way it is drawn though that gives that impression.
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Old 11-03-2012, 05:27 PM   #3550
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Originally Posted by Marki_GSA View Post
Nice idea and it does solve the rocking couple but looking at it I can't help but think the crank would either be overly heavy or not very strong. It might just be because of the simplistic way it is drawn though that gives that impression.
Plus, two con rods weigh twice as much as one = out of balance reciprocating + rotating masses.
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Old 11-03-2012, 05:28 PM   #3551
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Originally Posted by Beemerlover View Post
Plus, two con rods weigh twice as much as one = out of balance reciprocating + rotating masses.
look closer, the "single" is a double. ;D
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Old 11-03-2012, 05:35 PM   #3552
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look closer, the "single" is a double. ;D
You're right, I missed that. Two con rods per piston - what a deal.
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Old 11-03-2012, 06:27 PM   #3553
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So it's rear wheel only braking for you then?

lol

silly wabbit.......i was trying to say that while on the gas or trying to maintain throttle and reach the signal cancel is slightly irritating. While writing that comment I was also thinking how retarded my Harley is since the cruise control is also on the right hand....which is REALLY silly to try and hold throttle and then reach over for the "Set" button.

Sorry - guess I should have given more attention to English class in school....i ain't not no good with writin my thought's and whatnot into werds and so forth...
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Old 11-03-2012, 09:57 PM   #3554
markjenn
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Originally Posted by Beemerlover View Post
Plus, two con rods weigh twice as much as one = out of balance reciprocating + rotating masses.
There is no need to have two con-rods - you could have a single one until getting to the limit of the crank throw, then forking to a "Y".

And two con-rods would not weigh twice as much as a single one as each would be downsized considerably as each would see much less stress.

I'm sure there are lots of tradeoffs, but I'm still surprised BMW hasn't tried the concept (or perhaps they have and have rejected it). The opposed twin's Achilles Heal from a vibration standpoint is the rocking couple vibe problem.

- Mark
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Old 11-04-2012, 04:28 AM   #3555
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Too many problems with a double conrod on one side.

If BMW wanted to cut vibration way down, then they would have had to install two balance shafts.
But that must be paid for with extra weight, cost, and frictional drag. Admittedly, they might have re-designed and simplified the new & complex multi-stage transmission in order to come out with an overall "cost neutral" set-up ~ but obviously that decision was nixed at some early stage (and for reasons we will probably never hear of).

Whatever, we are just about to receive the ultra-new model, and there's a snowball's chance that it will get a major redesign any time soon.
Both the new and the old system, with a single balance shaft, reduces the amplitude of the engine's out-of-balance vibrations ~ and it may well be that the new engine has less noticeable vibes than the oldie . . . but we'll just have to wait and see.
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