rising rate vs falling rate? - YZ monoshock

mooch

mooch

PR Elite
I've posted this question on a couple of old bike forums as I need to figure out real soon whether I'm going to keep the shock in question.

I bought a new shock for my 1981 YZ250 from one of the larger suspension companies that deals with vintage/post vintage bikes. I had an issue with the way the shock fit on the bike and was talking with an engineer at the company about it. Turns out I do need to send the shock back to fix the issue I originally called them about.

During that conversation he asked me to check out the rebound dampening and sag before returning it and said it should rebound fairly quickly due the the 81 YZ250 having a falling rate suspension. I checked the sag and rebound dampening after I got off the phone and I could only get about 3 inches of sag when setting on the bike so the spring that came on it is too stiff. The rebound dampening is also wrong because at it's lightest setting it returns very slowly and on it's stiffest rebound setting you might as well get a calendar out to time it.

At this point I'm having second thoughts on whether this company knows how to set up a shock for this bike since nothing seems right. In addition to being told the bike had a falling rate suspension, the engineer also said my bike was fairly rare because he knows of only about 5 or 6 shocks they've ever made for an 81 yz250. That seemed very odd to me as I thought it was one of the more popular PV bikes around.

After all that....my question IS, does anyone know if the 81 YZ250 rear suspension is a rising rate or falling rate (gets softer as it gets further into the travel). Since I'm not real happy with the way the shock came set up, it would pretty much put me over the edge if it's not actually a falling rate suspension as the engineer indicated.
 
The monoshock would probably considered a falling-rate set-up. There's no linkage, so there is no rising-rate. The farther the rear wheel travels as the shock compresses, the more leverage you get against the shock. That's what I discovered when I laid down the shocks on my '74 MX, way back when, the stock springs were way too soft with the new leverage. The travel would begin nicely but then just continue until it would bottom. The monoshock can use a huge spring to compensate for this.
 
I'm really not sure on the 81 YZ's, but just because there is no linkage does not mean falling rate, pds KTM's Husabergs, Atk's etc with no linkage have rising rate suspension. The first uni trak Kawasakis with a linkage, had falling rate suspension. Sorry I'm no help on the YZ though.
 
Thanks for your thoughts on this. I did quite a bit of googling and there's not a lot on the subject it seems. I came across one motocross action article on rising rate suspensions (see link) and in that article they had said this...

"Yamaha bought Tilkens' idea and the monoshock was born. Although the monoshock was a successful marketing idea, it wasn't the greatest working suspension set-up in the world (nor would it classify as a rising-rate linkage)."

http://motocrossactionmag.com/Main/News/DEATH-OF-THE-RISING-RATE-1327.aspx

I also saw another site that had claimed the early monoshocks were falling rate, then, on the 1980-81s the first half of the travel was not falling rate and the second half was. ??
 
I think??? If Jeff isn't sure, I don't know who would be. He's a like walking dirt bike encyclopedia.

I was thinking that the geometry of the monoshock approximates old laid down falling-rate twin shocks and the only way to modify that would be with some kind of linkage. I think the '82 YZ had a kind of hybrid monoshock/linkage that truly was a rising rate set-up? Jeff???
 
DEATH OF THE RISING RATE


Well the whole premise of that article was wrong. Doesn't the new KTM have a linkage?

I sure like the ATK rear suspension. But I'll never attempt a super cross jump either.
 
Mooch, Tim Shepard seems to have the monoshock figured out as it has been his PV bike of choice. He might be a great and local source concerning your bike.
 
jeffro667 said:
I'm really not sure on the 81 YZ's, but just because there is no linkage does not mean falling rate, pds KTM's Husabergs, Atk's etc with no linkage have rising rate suspension.
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But the no link bikes like KTM, etc still have rising rate based on the fact the mounting points create a scalene triangle. That old monoshock setup basically just pushed straight on the shock.

Does the RM Full Floater produce a rising rate?
 
KTM went without the linkage and with the "PDS" system, which stands for progressive dampening system, and essentially added a needle type apparatus into the shock reservoir that engages an oil passage more and more depending on the where the shock is in the stroke............ essentially increasing the resistance as the shock stroke increases, ie... rising rate. The idea of rising rate is to increase the shocks resistance as it travels through the stroke, therefore increasing bottoming resistance the furthur the shock is compressed. Linkage does this by changing the length of shock / angle of the mounting points as it is compressed, giving the changing location as it compresses to increase resistance to bottoming. There are many different ways to accomplish the same sort of goal as there are many differenent suspension set ups out there that work the same by doing things differently........

How that relates to certain failed monoshock designs is beyond me, but you can assume that since Yamaha abandoned it after a few short years......... it must not have worked the way it was intended. Now, the science behind all of this has evolved that someone may know how to make it work properly now by re engineering the way the shock works or whatever.... but you would need a real suspension expert that knows what happens to the shock as it compresses and a knowledge of valving and how it works to make it work correctly.
 
How that pertains to the monoshock is just as I said in the post, no linkage does not always mean falling rate. Does not get any more simple than that.
 
RocketRobin said:
How that relates to certain failed monoshock designs is beyond me, but you can assume that since Yamaha abandoned it after a few short years......... it must not have worked the way it was intended.
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The monoshock suspended Yamaha production MX bikes from '75 through '81. Seven years was actually a very long run for this pioneer during the long travel revolution.
 
Yep, hard to call the monoshock a failure based on the wins put together by the likes of Hannah and Glover.
 
jeffro667 said:
The first uni trak Kawasakis with a linkage, had falling rate suspension.
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I stumbled on this article a couple years ago.
http://www.motorcycle-usa.com/290/3617/Motorcycle-Article/Memorable-Motorcycles-KDX-175.aspx

Kawasaki was the first manufacturer to use a mechanical leverage to provide rising rate rear suspension in a production motorcycle. In simple terms, rising rate suspension is where the rear shock is not compressed consistently so that it gives initially soft compression which then becomes harder towards the end of the travel. This had been a holy grail of suspension designers right from the early 1920s and had been achieved by having two, or more, sets of springs on one suspension device. The first, soft ones, provided the initial travel and then they combined with a stiffer second set to deal with bigger loadings.
The problem was that multi-rate springs did not provide the linear rising rate which the designers sought – and riders begged for. The Uni-Trac, with its purely mechanical compression of the rear shock, promised to be the solution. For an off-road bike, this was a dream. The rider could float along with the rear wheel maintaining grip and the front end compressed - typically when braking or going downhill - and the stiffer element of the rear suspension would give stability over big jumps. Keeping the rear wheel constantly in touch with the ground also vastly helped traction under acceleration and braking.

With Kawasaki’s design, this was achieved by making the shock move not directly by pushing it from the swinging-arm but rather by making the swinging-arm move a lever which then compressed the shock. If the engineers got their maths right, the mechanical ratios could be designed such that initially the shock only moved a little bit, and so the suspension was soft, and then was made to accelerate so that the shock action became stiff towards the end of its stroke. So far, so good - primarily because all engineers love math, and the more complex the better.

At the time, all off-road Kawasakis were manufactured in Lincoln, Nebraska. When the engineering drawings arrived from Japan for the new motocross machines a zealous, enthusiastic American engineer saw that his Japanese colleagues seemed to have made a fundamental mistake. They had designed the rocker arms for the rear suspension so that the travel was inconsistent. In fact, they had built rising rate suspension. Now, how dumb was that?

So, the legend has it, the mistake was corrected and all the bikes now had FALLING rate rear suspension which actually worked completely contrary to what was intended. It’s easy to sneer at this mistake but you have to understand that a lot less was known about engineering almost 30 years ago - and the lines of communication between Japan and the US were vastly less sophisticated than they are today.

The only bike which escaped the redesign was the KDX 175 which, from the outset, had been conceived with neutral suspension. In the little enduro bike’s case, the Uni-Trac rocker-arm suspension was purely a marketing gimmick since the shock had a linear movement directly related to the rear wheel. Ironically, because the KDX was left out of its bigger brother’s birthday treat it worked - whilst the cleverer systems on the motocross bikes most certainly didn’t!
 
When I say that Yamaha failed in it's design and abandoned it after a few short years........... I merely was saying that the concept behind the monoshock was not as ahead of it's time as was marketed as......... the benefits of rising rate suspension were not utilized in the "monoshock" design and therefore it was abandoned in later year to a more linkage type design. I'm not bagging on Yamaha for all you Yamaha lovers...... rather pointing out that their genius marketing dept used a design that essentially did nothing different that the twin shock designs of the same time period into making the public think it was better! Genius Yamaha marketers......... not so genius Yamaha Engineers!

I, by know means, claim to know what is really going on with suspension, but through the years have gathered an appreciation for the the different types and what actually works on a machine. Over the years, I've ridden Honda's with dual shocks, Husky's with dual shocks, Kawasaki's with Uni - Trak, Honda's with Pro - lever, Yamaha's with Monoshocks, and KTM's with ProLever's and linkless set ups. I've tested suspension for both Pro Action and KTM America and can only offer my opinion of what works and what doesn't.......... I don't understand the actual science behind the systems, but grasp the concept of what they are trying to do.

It's really cool stuff to think about, just wish I was smart enough to understand and change it for the better.
 
AHRMA361 said:
I assume you didn't get a Race Tech modified shock?? I like mine!
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Mike, did you send your shock to Race Tech in California for the modification or did one of their dealer affiliates on this side of the country do the work for you? When I originally restored the bike, I sent the shock to Mickey Kessler for just a rebuild and he let me know the shock was in good shape, so hopefully it'll hold up for awhile if I get it modified with that dual spring and gold valve setup that RT does. I'd imagine you also went with the stiffer spring an gold valves in the forks?
 
RocketRobin said:
When I say that Yamaha failed in it's design and abandoned it after a few short years........... I merely was saying that the concept behind the monoshock was not as ahead of it's time as was marketed as......... the benefits of rising rate suspension were not utilized in the "monoshock" design and therefore it was abandoned in later year to a more linkage type design. I'm not bagging on Yamaha for all you Yamaha lovers...... rather pointing out that their genius marketing dept used a design that essentially did nothing different that the twin shock designs of the same time period into making the public think it was better! Genius Yamaha marketers......... not so genius Yamaha Engineers!
Click to expand...

Yep, Yamaha must have sold the single shock system well. One shock of course became the norm. Even Husky eventually got into the act!:D
 
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