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Linkage vs. PDS rear suspension?


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I am getting back into riding dirt bikes after many years away and as m not familiar with the acronym P.D.S for rear suspension. I have gathered that it is a non-linkage type of suspension like used on KTM XCW models. While P.D.S. intuitively seems simpler, I am curious as to what benefit comes from a linkage type rear suspension. Thanks for your help. I'm really learning a lot from the TT forums.

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With most linkages, the geometry causes the rate at which the shock is compressed as the swing arm moves from extended to compressed increases as the arm swings upward.  This makes the whole spring and damping picture more progressive than it would be otherwise.  The effect is primarily due to the holes in the shock relay arm being laid out in a right angle arrangement.

 

Then again, because of the acute angle the shock in the PDS setup takes to the swing arm, it's also progressive, just not as much, and on a slightly different curve.

 

Without the linkage, the shock has to be placed at an angle that produces the desired rate increase.  With a linkage, the shock can be put more or less where you want it, then the rising rate can be dialed into the linkage.  The rate of increase can also be better controlled, and even modified without moving the shock, also.

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  • 2 weeks later...

in addition to what grayracer said

pds is progressive dampening system

they use 2 shock pistons the top one with a bypass through the shaft, a large needle enters the shaft towards the end of the stroke bringing the secondary piston into effect

 

some use 2 pistons and a piston cup at the end of the stroke

normal-shock-and-a-pds.jpg

pds needle on right in pic and small superimposed pic.

 

pds cup style

ohlinspds.jpg

 

 

in my mind linkage is superior, increase spring rate and dampening through the linkage

the linkage moves the shock further in per inch of wheel travel at the end of the stroke.

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  • 2 weeks later...

I don't have a PDS system but here are some negatives for a linkage system:

The linkage requires annual removal for cleaning and greasing with waterproof grease.

Aftermarket bearing and seal kits are usually just under $100 and will be required if frequent cleaning and regrease is not performed.

Most of the linkage systems put the linkage under the swing arm where it is subject to damage, usually not a problem for MX but can be for trail riding.  I have a Trials bike and the link rod has become so damaged from rock hits that it will soon need replacing.  The link rod on my CRF250X has also suffered damage. As a result I'm looking for shielding solutions.

Honda XRs put the linkage above the swing arm so don't have the same problem. 

 

As the chart in the above link shows the difference really begin during the last third of travel.  

With the RT mods the PDS system approaches the CR250R.

 

The rising rate is nice for those unexpected hits while trail riding.

For the type of trail riding that I do I want high speed compliance for absorbing tree roots and rocks without deflecting the bike and because of the shocks used out of the box linkage systems seem to be better for that than PDS. YMMV

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  • 2 weeks later...

The PDS is a awesome design that works great. It is simple, easy to maintain, not prone to damage, and all around a better option in a trail bike. I will take the PDS every time.

 

The PDS is a awesome design that works great. It is simple, easy to maintain, not prone to damage, and all around a better option in a trail bike. I will take the PDS every time.

 

yeah must by why ktm has gone to linkages

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yeah must by why ktm has gone to linkages

exc, xc-w and xcfws still come with pds. All their "eastern" style woods bikes still are using pds. I would have to say, for the style of riding that I do, pds can be a great thing. Granted, it has helped to switch to a progressive rate spring and have the shock revalved, as well.

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Coming from mountain bikes, this is interesting to see. On bikes, there's a whole list of linkage designs, and what the KTM has would be called single pivot, the rest of the bikes seem to be a linkage driven single pivot set up. It's been shown that based on pivot position and shock angle, you can get just about any leverage curve you could want with a single pivot, if you can't, you add a link to drive the shock. What KTM is doing by using extra damping to get the ride they want is exactly what the bike industry is getting away from, too many compromises. From that stand point, I feel like they either need to revise their pivot layout, or go to a linkage driven design. Still interesting non the less. 

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