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Old 11-03-2012, 12:52 AM   #3541
cug
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Yeah, I was right. Checked the side pictures again, sizing by the front tire and the bike are pretty much the same length. Must be a slightly different perspective or lens creating the different length. But the seat is definitely very different. Looking forward to hopefully seeing the bike at the San Mateo IMS.
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Old 11-03-2012, 10:57 AM   #3542
Montauk
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Question Engine alignment

Never seen engine heads from the top before, look at it seems like right side engine is a tad bit behind while left is tad bit forward. Is that an accurate observation?

Obviously it does work, but why like that? Maybe my current oil head s like that too.
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Old 11-03-2012, 11:13 AM   #3543
DannyZRC
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Quote:
Originally Posted by Montauk View Post
Never seen engine heads from the top before, look at it seems like right side engine is a tad bit behind while left is tad bit forward. Is that an accurate observation?

Obviously it does work, but why like that? Maybe my current oil head s like that too.
your oilhead is also offset. The crankpins are opposed 180*, if they were in line the conrods would collide, so they're offset by a little bit. 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.
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Old 11-03-2012, 12:52 PM   #3544
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, 01:03 PM   #3545
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, 01:13 PM   #3546
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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, 01:19 PM   #3547
DannyZRC
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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 06:20 PM
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Old 11-03-2012, 01:20 PM   #3548
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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

Rod
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Old 11-03-2012, 01:28 PM   #3549
DannyZRC
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Quote:
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

Rod
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Old 11-03-2012, 02:11 PM   #3550
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, 03:03 PM   #3551
markjenn
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Quote:
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, 05:39 PM   #3552
Marki_GSA
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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, 06:27 PM   #3553
Beemerlover
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Quote:
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, 06:28 PM   #3554
DannyZRC
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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, 06:35 PM   #3555
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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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