Curious on why a drive shaft with 2 U-Joints - BMW Luxury Touring Community
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post #1 of 14 Old Oct 17th, 2018, 5:51 pm Thread Starter
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Curious on why a drive shaft with 2 U-Joints

Ok,,I'm sure this is an old question for most of you riders of new BMW stock, but I've spent my whole life with airheads, more specifically my '73 R75/5. In those days, the drive shaft had 1 U-Joint --right at the swing arm to tranny. A few months ago I treated myself to the next best touring machine, yes, the K1200LT. Now that I'm going over many of the engineering changes from the old days (surprisingly many things haven't changed that much in design), I see where the drive shaft has 2 joints, one at the final drive, and another up at the tranny. My main question for you engineers is why is that? I'm guessing it maintains wheel base better as suspension moves up/down, but maybe not. In earlier days the suspension was ,..well, way less technical

Thanks

Bob
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post #2 of 14 Old Oct 17th, 2018, 6:19 pm
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Re: Curious on why a drive shaft with 2 U-Joints

Hey Bob, I am not the biggest BMW expert so I went looking for the R75 images to have a look. If I found the right bike, it looks like the only pivot point is up at the trans on the R75 and the shaft housing is bolted to the final drive. The LT has 2 pivot points, one at the trans and the other just in front of the final drive with a torsion strut. Not being an engineer, I am not sure what benefits having 2 pivot points has other than the FD remains in a fixed rotational position as it moves up and down rather than rotating along with the wheel. I suspect it allows the entire drive train to remain more stable as the suspension is exercised. Lets see if I figured it out by others comments. There are many engineers on this site.

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post #3 of 14 Old Oct 17th, 2018, 7:46 pm
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Re: Curious on why a drive shaft with 2 U-Joints

Quote:
Originally Posted by Bob.K1200LT View Post
Ok,,I'm sure this is an old question for most of you riders of new BMW stock, but I've spent my whole life with airheads, more specifically my '73 R75/5. In those days, the drive shaft had 1 U-Joint --right at the swing arm to tranny. A few months ago I treated myself to the next best touring machine, yes, the K1200LT. Now that I'm going over many of the engineering changes from the old days (surprisingly many things haven't changed that much in design), I see where the drive shaft has 2 joints, one at the final drive, and another up at the tranny. My main question for you engineers is why is that? I'm guessing it maintains wheel base better as suspension moves up/down, but maybe not. In earlier days the suspension was ,..well, way less technical

Thanks

Bob
The type of swing-arm + Rear-drive we have on K1200LT is called "ParaLever".
The ParaLever design is NOT unique to K1200LT and represents an evolution that BMW introduced many years ago on much earlier boxer models in 1988 on R80GS and R100GS.

Anyone who has accumulated mileage on older boxer models having a single joint (almost all "R" boxers before 1988) is familiar with the so-called "jacking of the rear" effect when power was applied on acceleration. Even worse was the opposite effect when htrotlle was cut a bit abruptly when entering a curve (reducing ground clearance further and modifying the whole chassis stability).

To explain the reasoning and advantages of having 2 joints in the rear-drive / swing-arm, I would suggest to read this article by a well-known long-time mechanic and guru for BMW motorcycle:
Paralever rear suspension

By the way, the home-page of his web-site is full of technical articles for older K-bikes and Boxer models:
www.largiader.com
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post #4 of 14 Old Oct 17th, 2018, 7:52 pm
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Re: Curious on why a drive shaft with 2 U-Joints

Newer shaft drive bike have two U-joints because they have two points that bend, older shafties had only one because the drive line only bent at a single point. Look at the drive and linkage on the LT, you'll see the drive pivots both near the trans and near the final drive. It isn't that complicated.

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post #5 of 14 Old Oct 18th, 2018, 9:29 am Thread Starter
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Re: Curious on why a drive shaft with 2 U-Joints

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Originally Posted by sailor View Post
The type of swing-arm + Rear-drive we have on K1200LT is called "ParaLever".
The ParaLever design is NOT unique to K1200LT and represents an evolution that BMW introduced many years ago on much earlier boxer models in 1988 on R80GS and R100GS.

Anyone who has accumulated mileage on older boxer models having a single joint (almost all "R" boxers before 1988) is familiar with the so-called "jacking of the rear" effect when power was applied on acceleration. Even worse was the opposite effect when htrotlle was cut a bit abruptly when entering a curve (reducing ground clearance further and modifying the whole chassis stability).

To explain the reasoning and advantages of having 2 joints in the rear-drive (including swing-arm) I would suggest to read this article by a well-known long-time mechanic and guru for BMW motorcycle:
Paralever rear suspension

By the way, the home-page of his web-site is full of tecnical articles for many older K-bikes and Boxer models:
www.largiader.com

Thanks Sailor for that link. My engineering hunch was centered around suspension and wheel base, but you bring up a very good point about acceleration that I hadn't considered. Under very hard acceleration, I imagine the rear joint is free to flex, in effect allowing the wheel to move up since it's not attached to the frame like in the older boxers of old ( by attached I mean bolted to the two rear shocks in older bikes). If I'm right on that, the 2 U-Joint design can be summarized as the rear joint is to compensate for accelerating forces and the front joint is for your simple ups and downs on the road. So,,...do we have a verdict...or is the jury still out ...

thanks guys,

//Bob

Bob Johnson
Knoxville TN
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post #6 of 14 Old Oct 18th, 2018, 9:46 am
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Re: Curious on why a drive shaft with 2 U-Joints

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Originally Posted by Bob.K1200LT View Post
Thanks Sailor for that link. My engineering hunch was centered around suspension and wheel base, but you bring up a very good point about acceleration that I hadn't considered. Under very hard acceleration, I imagine the rear joint is free to flex, in effect allowing the wheel to move up since it's not attached to the frame like in the older boxers of old ( by attached I mean bolted to the two rear shocks in older bikes). If I'm right on that, the 2 U-Joint design can be summarized as the rear joint is to compensate for accelerating forces and the front joint is for your simple ups and downs on the road. So,,...do we have a verdict...or is the jury still out ...

thanks guys,

//Bob
Not exactly. Two joints are needed due to the final drive rotating around a virtual pivot point that is not located at the same point as one of the u-joints. This virtual point is well ahead of the swing arm pivot point which is which lessens the jacking force. The final drive essentially thinks it is on a much longer swing arm.
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post #7 of 14 Old Oct 18th, 2018, 10:46 am Thread Starter
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Smile Re: Curious on why a drive shaft with 2 U-Joints

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Not exactly. Two joints are needed due to the final drive rotating around a virtual pivot point that is not located at the same point as one of the u-joints. This virtual point is well ahead of the swing arm pivot point which is which lessens the jacking force. The final drive essentially thinks it is on a much longer swing arm.
Thanks Voyager. I'm trying to virtually visualize that in my head

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post #8 of 14 Old Oct 18th, 2018, 5:13 pm
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Re: Curious on why a drive shaft with 2 U-Joints

One of the few things I recall from undergrad mechanical engineering classes: a single u-joint will not pass a constant angular velocity, whereas 2 u-joints will. This may not be pertinent to the current discussion; if not, forget I ever posted this.
- Bob
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post #9 of 14 Old Oct 18th, 2018, 8:29 pm
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Re: Curious on why a drive shaft with 2 U-Joints

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Originally Posted by bob_menton View Post
One of the few things I recall from undergrad mechanical engineering classes: a single u-joint will not pass a constant angular velocity, whereas 2 u-joints will. This may not be pertinent to the current discussion; if not, forget I ever posted this.
- Bob
Well, a single u-joint will pass a constant angular velocity if the angle between the two shafts is zero.

And two u-joints only pass a constant omega if they are properly phased.

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post #10 of 14 Old Oct 19th, 2018, 11:03 am
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Re: Curious on why a drive shaft with 2 U-Joints

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Well, a single u-joint will pass a constant angular velocity if the angle between the two shafts is zero.

And two u-joints only pass a constant omega if they are properly phased.
..... and if they are deflected the same angle.
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post #11 of 14 Old Oct 19th, 2018, 5:02 pm
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Re: Curious on why a drive shaft with 2 U-Joints

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..... and if they are deflected the same angle.
Yep.

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post #12 of 14 Old Oct 19th, 2018, 11:52 pm
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Re: Curious on why a drive shaft with 2 U-Joints

ummmm German Engineering.

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post #13 of 14 Old Oct 20th, 2018, 9:38 am
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Re: Curious on why a drive shaft with 2 U-Joints

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ummmm German Engineering.
Nothing to do with Germany. Physics is the same everywhere.
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post #14 of 14 Old Oct 20th, 2018, 4:07 pm
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Re: Curious on why a drive shaft with 2 U-Joints

The reason for brake torques to cause suspension extension, isn't just to minimize shaft jacking, but to also minimize the suspension compression (front) /extension (rear) under heavy brake-induced torque. This keeps the front suspension extended so that nearly all suspension travel (called jounce) is available during and after braking for a pothole etc. It is definitely a safety feature.

Contrast that to an telescoping front suspension about to hit a pothole.
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