Concentric swingarm pivots. Why not?

[Hoofhearted]

I'm no expert but the idea of the swing arm pivot being on the same plane as the sprocket is to maintain a constant chain tension. I have read, but don't remember the details, that the position of the swing arm pivot in relation to the sprocket can affect the attitude of a bike as the power is applied. Depending on the position it can cause the bike to squat or rise when the throttle is opened. If you go search you can find a two bar (on each side) that formed a parallelogram and produced zero rise or squat. It was also supposed to reduce wheelieiing out of corners It was experimented with in the late 70s or early 80s but what became of it I don't know. The longer the swing arm the less the varying of wheelbase over bumps. Supposed to make for better handling.

 

[4eyes]

The guys on the Husky forum say that the new bikes maintain traction better through the rough stuff. I haven't yet had the pleasure of trying one, but it makes sense that without chain slack whipping about, it would put power down smoother.

 

[hillsy]

Changing the angle / length of the swingarm is commonplace in racing to fine tune rear suspension. No doubt about that.

Having the swingarm pivot on the same axis as the sprocket DOES eliminate chain stretch with swingarm movement. Totally.

Why don't most manufacturers do it? Cost and complexity v's no real gains.

 

[teazer]

IF the front sprocket is on a separate C.L that is true. With the front sprocket essentially on the same shaft as the front pivot of the swingarm, the wheel doesn't rise and fall in relation to the front sprocket. It pivots.
Make a quick 2D paper model to help visualize it, it becomes obvious.

We are saying different things. You are saying if they are in the same pivot plane, tension remains constant. I'm saying that you don't want that and so the tension will change.

 

[two-smoker]

Just take into account that the SB2 or SB3 was made by about the best MC chassis designers,welders and the best materials available at the time.You are not Bimota,remember that.Some HD's and Honda choppers use jack shaft double chain setups to run massive tires.It is a job for a serious professional engineer.It's not feasible,period and not just for mass production .

not feasible "period" is a little strong. I've seen it, and I know it can be done. It's just a problem on a large scale. I asked the question because I saw it in print, where it was not understood by the person the question was for, and also because if I had such a suspension, I would not haver a chain trying to saw through my swingarm. Mainly, curiosity. Obviously, there are advantages and drawbacks. Just like anything, else.

 

[4eyes]

You don't know what you are talking about.Mert Lawill or Werner relied on swingarm geometry changes to change suspension dynamics for their racing XR750's for years.
Of course they did.
Some Road race bikes have adjustable swingarm pivots that can go up and down to change suspension leverage curves and also it changes the way the suspension reacts under power and the way it reacts with the throttle off.
Which is more important than the insignificant amount of chain tension change in the small range of suspension movement.

Husky has built great bikes for decades.It the latest low volume bike with the next great suspension idea works,great.Don't you think if it was such a great idea Chad Reed would have it on his race bikes?

Because not everyone that rides off-road is racing. There are shaft driven dual sports (BMW), that doesn't mean it is best for racing.
The O.P. asked if any manufacturer built bikes with concentric drive sprocket and swingarm pivot, and I gave one example, and gave the reasoning why. I also stated that I haven't ridden one and don't know if it works as intended or not.
But either way, the chain tension WILL remain stable over the range of motion.

 

[teazer]

The point here is that most things in motorcycle design are a compromise between at least two competing things. In this case, it can be done, but it is not an advantage from a handling perspective as it turns out. Bimota tried it and dropped it because the cost wasn't justified and chain tension advantages were not so much of an advantage after all and it is advantageous to have swing arm droop and the sprockets not in line with the swingarm.

I have no idea what the original question was or what the answer was, or what made it less smart, but overall it's not a great design idea.

 

[Flugtechnik]

Apparently the Shinden, the new electric race bike from Mugen is using this concept.

http://hellforleathermagazine.com/2012/03/john-mcguinness-rides-honda%E2%80%99s-electric-race-bike/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+HellForLeather+(Hell+For+Leather)

"The Mugen also uses that motor to connect the swingarm to the twin-spar frame, with the swingarm pivoting on the motors center. "

 

[teazer]

I didn't read it that way. I read it that the swingarm pivots in the rear of the engine casing. They didn't mention the pivot being concentric with teh sprocket - at least that's how I read it.

 

[Flugtechnik]

Since it uses an electric motor, I read it that the swingarm pivots on the motor's centerline. Most electric bikes don't use transmissions so the sprocket would be mounted directly to the motor shaft, which is of course at its centerline.

However, looking at the pics it doesn't look like it is. Perhaps they were confused since it's close cousin, the Honda RC-E had a concentric swingarm pivot.

Shinden

RC-E

 

[crazypj]

Cost is a major issue, it is much more difficult to get engine in and out of frame .
It was easier to use 'virtual' pivot and 2 smaller swing arms with drop link at rear. (make a parallelogram)
There are a few pics of early 70's bikes with conversions but the concept has been around for a LONG time
Scroll down on the second link to see some virtual pivot designs
http://myclassicmotorcycle.blogspot.com/2011/03/dondolino-debut-classic-moto-guzzi.html
http://ozvmx.com/community/index.php?topic=6427.0
Rear suspension is now so advanced that the pivot point /torque reactions are much better controlled than 40+yrs ago