Why did Bultaco give up on this idea?

[David Lahey]

This is a post on Facebook:

Michael Kohlleppel‎Bultaco
4 hrs ·
The Date on this US Patent is Dec 17th 1973, why did Bultaco Not continue with this design? It is an interesting Design, did Bultaco Purchase it so no one else could use it who knows! If any one out there has any information on this design please share it with us. Because Frankly in a few years time the people who were in the Know are now elderly and important information on our much loved motorcycles may be lost forever, Please share this information with us. Thanks Michael.

I know they did try it on some works bikes. Does anyone know why they dropped it?

[Julz]

This is essentially how a 'Horst link' works on a mountain bike. Significant traction benefits a regained by having the virtual pivot point of the rear suspension ahead of the front wheel. There are many variations on how to arrange the spring and shock absorber via linkages so as to keep the weight low and centred, but. They all do essentially the same thing.

[David Lahey]

I was wondering if it was also a benefit in maintaining a more constant chain tension.

[Bully fanatic]

I can remember reading somewhere that the reason why Bultaco stopped working on this idea was the money. It was too expensive to keep going with the idea. It might have been in The Bultaco Story. They also tried a short chain to another sprocket on the swingarm pivot to stop the chain changing tension. The JJ Cobas bikes also had this. There is a very interesting article in the new Classic Trial mag on the JJ Cobas bikes.

[JC1]

David, there were many reasons:

- it was based on pie-in-the-sky theory that had serious limitations (or at least the theory was misapplied/misunderstood)
- Bultaco had no idea how to develop it, nor did anybody else at the time, including its inventors it seems!
(Kawasaki also tried it on both road-racers & mXers in DuHamel & Weinert's time)
- Bultaco's "dual-link" version added about 10kg & considerable complication for no gain
- test riders reported that it caused the nose to dive under acceleration off jumps, & rear suspension was either "too hard" or "too soft" (couldn't get it set-up right)
- it consumed precious funds & development time which caused Pursang development to stall & fall behind the competiton & they never really recovered

One could say that Bultaco got conned by the 'inventors' claims. It was supposed to overcome (or at least greatly minimize) chain-pull torque & its effects including squat/anti-squat, stiffening/softening of rear suspension etc, & also overcome/minimize variation in chain-slack allowing long suspension travel, but.....

Nobody has ever developed it successfully with good reason it seems.

Perhaps the best comments are by frame guru Tony Foale (Motorcycle Handling & Chassis Design; the Art & Science):

"Double swing-arms
Often the descriptions that accompany such designs indicate that even their designers don't fully understand the characteristics of such layouts. The claims made would require the use of an alternate form of physics to have validity. However a realistic evaluation of such designs is little more difficult than with a standard swing-arm...

"The anti-squat & movement characteristics are identical to those obtained with a single conventional arm as shown, pivoted at point 'A'. Press descriptions of such dual-arm designs are very often highly flawed."

Parallel swingarms JPG.JPG (22.58 KiB) Viewed 5507 times

[JC1]

Bultaco had 4 versions. I've seen pics of 2 of them & they both had an extra crossover jackshaft with 2 chains (as Graham inidcated above). I have no idea why Bul did that, cos if the claims for the system were true they wouldn't have needed it. Perhaps it was only on their last 2 versions they did that when the 1st 2 versions didn't work!? (Kaw's versions used conventional single chain. They also had several versions, so they both tried considerably)

That jackshaft is above & behind the usual position of a conventional swingarm pivot on the modified Pursang frame (between the 2 front pivots of the dual swing-arms, but much closer to the top one.) The geometry is horrendous & seems to indicate (with the benefit of hindsight) that they didn't have much of a clue what they were doing!


Looking at that Fig 9.15 above, if the rear plate(between the 2 swing-arms) was reversed, so that the rear axle was in front of the rear pivots of the two swingarms (not behind them), then the front pivot point of the equivalent single arm could be made very close to or even coincident with the front sprocket. But it is probably not worth the extra complication/weight/cost etc over the modern system of long-ish swingarm pivoting very close to the front sprocket.

[Guy53]

JC1 is right, Kawasaki tested it on some road racer with Yvon Duhamel, they called it here in America the FooBar system ( I gess for four bar system ) .

Guy

[JC1]

Here's a couple of small pics I found on the web of one of the Pursang prototypes

Bul pursang dual-link proto.jpg (12.97 KiB) Viewed 5409 times

Bul Dual-Link proto.jpg (16.3 KiB) Viewed 5409 times

Some of the Kaw versions looked a good deal more refined.

[David Lahey]

I see those bikes have the cross-bike drive shaft. What a way to add weight. I can't help thinking that the Bolger rear end also would add quite a bit of weight to an OSSA Phantom

[Bully fanatic]

I have also seen a couple of photos of a Bolger rear end on a MAR. I think the Bolger rear was a rising rate suspension system though and supposedly worked very well.