Concentric swingarm pivots. Why not?

[two-smoker]

I was reading an issue of a certain popular motorcycle magazine the other day, and noticed a question that was not understood by the editor. While I knew what the writer of the letter was after, I could easily see how the question could go misunderstood. But I digress. The question pertained to swingarm pivots on motorbike frames being "in line" (meaning concentric-author's words) with the countershaft sprocket. It was such an interesting question, I had to post something here. I knew of one custom XV920 in Illinois that was built with said feature, but I have never seen a production motorcycle so equipped. So..... why not? I realize changing the countershaft sprocket would become nearly impossible without removing the swingarm, but are there any other reasons nobody would mass produce a motorcycle with a concentric swingarm pivot? I think the advantages (constant chain/belt tension, longest possible swingarm length in a given wheelbase, etc.) would make it worthwhile. So what do YOU think?

 

[Hoofhearted]

Bimota did something like that in the 70s I think it was. The swing arm pivot point was in line with the sprocket and the theory was the chain stretch would remain constant throughout the movement of the swing arm. I think you named the reason why manufacturers never embraced such a design.

 

[SONICJK]

Im dense, what exactly are we talking about? ???

 

[Sonreir]

Having the swingarm mount in line with the front sprocket rather than to the frame a few inches behind.

 

[SONICJK]

AAAAAH gotcha.
That does seem like it would alleviate a lot of chain tension issues, but really unless you're utilizing a LOT of suspension travel (washboard roads?) I don't see the angle change having a lot of effect on the tension.
Am I wrong?

 

[AgentX]

Several variations of this were tried with mountain bike suspension, but none really stuck...most were simple swingarms with a pivot concentric to the bottom bracket (crank/pedal axle). No chain growth was one major selling point, but none actually performed very well. One small company, however--Rotec-- went to using the concentric pivot with a Lawwill-pattern swingarm and it was pretty good when I rode it once. Other companies used the lawwill design but with the pivot behind the bottom bracket. Also worked well but Rotec claim they have Mert's original design. Now most designs seem to be "virtual pivot point" setups.

Here's the Lawwill design:

But the dynamics of a bicycle (light, with a relatively large proportional mass moving slowly in a big circle to give it power, and the rider's weight being many times that of the bike) and a motorcycle (heavy, with (a) miniscule piston(s) moving a relatively tiny distance back and forth with awesome force) are totally different, and I don't know a thing about engineering. So there's no point to my post!! Ha. Suckers!!

 

[Hoofhearted]

I had to go look. I tend to forget the details anymore and retain just a hazy memory. It was the Bimota KB2 that had the concentric swing arm. Not a great photo but you can see where the swing arm bends out to clear the crankcase.

By weslake at 2012-03-19

 

[Sonreir]

I wonder what effects the extra width have on the torsion of the swingarm when corning hard. I would expect the additional leverage to result in a bit of twisting; even when well supported. It seems like it may be very difficult to execute without introducing problems more serious than the one being resolved.

 

[Hoofhearted]

Interesting question. I never heard how it worked but I am guessing Bimota wouldn't do something that didn't work. But on the other hand no one is perfect (except Manx Nortons).

 

[hillsy]

Interesting question. I never heard how it worked but I am guessing Bimota wouldn't do something that didn't work.

cough....V-Due.....cough....... :

Seriously though, the idea is sound but the cost and practicality penalties far outweigh the advantages of totally negating chain stretch. Get the pivot as close as possible behind the sprocket and it's "good enough".

 

[Nebr_Rex]

Look up Rickman.

 

[two-smoker]

The key word here was "production". Ie.: CBR's, Ninjas, GSXR's, etc. I don't see why it would cost any more to produce, obviously the longer swingarm is good for handling or the R1 wouldn't exist, I'm just curious why it hasn't caught on at a larger scale than certain low-volume or custom motorcycles.

 

[hillsy]

The extra complexity involved is not worth the effort. You foul the front sprocket which makes maintenance a PITA and you have to run narrower swing arm bearings to keep the whole she-bang narrow which means they are not as durable.


Yamaha stacked the gearbox shafts on the R1 which made the gearbox shorter and allowed them to run a longer swing arm in a conventional manner. Everyone else followed suit and now they all have nice long swing arms with short wheelbases.


Some ideas just don't make it in the long run. Like hub steering, anti-dive forks, 16" front wheels, etc.

 

[two-smoker]

Alright, that's satisfactory in my eyes. I still like the idea, mainly because of my current dilemma involving a chain and swingarm trying to occupy the same space, but I see why it's not feasible for mass production.

 

[cowboysculptor]

It sounds like a great idea until you really get into the details.

The longer a swingarm, the less stable in corners. Long gangly parts have more room to flex. Shorter, more compact swingarms are more rigid. Your idea presents a complicated solution to a problem that isn't really much of a problem.

If you have trouble with chain slack on bumps, an easier solution might be a spring-mounted idler sprocket about midway down the chain to take in the slack. But is it really causing trouble?

 

[4eyes]

I'd say that the small amount of suspension travel makes it more trouble than it's worth, on a street bike.
Husqvarna calls it their, "Coaxial Traction System" on dirt-bikes and dual-sports.

 

[teazer]

In the real world the swingarm pivot point, engine sprocket and rea wheel sprocket are not supposed to be in line. As Swivel pointed out earlier, modern bikes are designed to have a small amount of squat. At the time that Bimota made those beautiful bikes, that aspect of suspension was not well understood.

If you lengthen the shocks too much and have too much droop in the swingarm, the chain will try to saw through the arm and there will be massive changes in chain tension/free play. Sometimes it's advisable to move the engine position to change chain geometry.

 

[4eyes]

In the real world the swingarm pivot point, engine sprocket and rea wheel sprocket are not supposed to be in line. As Swivel pointed out earlier, modern bikes are designed to have a small amount of squat. At the time that Bimota made those beautiful bikes, that aspect of suspension was not well understood.

If you lengthen the shocks too much and have too much droop in the swingarm, the chain will try to saw through the arm and there will be massive changes in chain tension/free play. Sometimes it's advisable to move the engine position to change chain geometry.

I call BS.
If the swingarm pivot center line, is the same as the engine drive sprocket center line, no matter where the swingarm pivots the chain tension will be the same.

 

[teazer]

Balderdash. As the wheel rises (or falls), they are no longer in line and tension changes too.

Good try though

 

[4eyes]

Balderdash. As the wheel rises (or falls), they are no longer in line and tension changes too.

Good try though

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.