In my case, a 3 stage setup in the rear shock as per RT recommendations. My thinking is, however, that when you have the shock with about half of its travel in sag, how do you get full performance out of your shim stack? I know there is supposed to be some correlation between shaft position to the shim stack performance to some degree. I'm sure that all compression control in a rear shock is not accomplished by the spring, as you well know. And it strikes me that if half your shock stroke is taken up with sag, you lose a good bit of the potential for the damping to dissipate some of that compression energy. My rear shock's damping in the shim stack setup is meant to work through the entire shock's designed stroke, minus the recommended amount of sag at about 95mm. Some bottomout control has to be compromised in my thinking if you're not using as much of that damping control as you should due to the shock starting at much reduced length. Right?

 

yeah mostly on rough trails im not carrying as many tools and such with me as i would be going on a long trip but i did some messing with it some more today preloading the spring a bit more but i m not really a tall guy 511 but i have short legs i guess lol i like to be able to put at least one foot flat on the ground and now its borderline on doing that now so i just assume let someone other than me who knows what there doing take a lookat it but ill be going to to the shop a few town over this weekend and ill talk to the supension guy up there he is a genious when it comes to this stuff ill just let him deal with it lol thanks again guys

 

The best explanation I've ever seen regarding preload (front or rear) and it's effect on your ride can be found at brucessuspension.com. He goes into the actual physics.

I followed his recommendations and made my last KTM far more plush and bottom-resistant at the same time. Bottom line - you want as little preload as you can get away with and still keep your sag numbers within spec. That preload is stored energy within the system and any hit that imparts less than that stored amount of energy will cause the bike to ride over it with no longitudinal deflection of the shock. That is it will bounce over the obstacle rather than absorb it. Most of us actually need a heavier spring than we think.

The stock springs on my 07 make it feel much harsher than it really is. The front is so soft that it rides very deep in the stroke so I can't even use the plush (initial) portion of the travel. The rear has enough proload to put it in the situation described above. It'll be much smoother with the right springs.

 

dj, that's a good site for some plain language on suspension issues. That issue of rear spring preload is one that's hard to find good explanations about. The "stored energy" aspect is not often mentioned, and finding good explanations about how much preload creates how much stored energy is almost never mentioned. Most sites give you some simple numbers...like 95-100mm race sag and 25mm free sag...but that does little to tell you how and why those numbers are achieved and how they affect stored energy. Frankly I had not found a source that referenced the 1/2" relative optimum preload for linkage bikes like ours. He also goes into KTM-style non-linkage designs too. I was basically shooting for the 95-100mm race sag with 25mm free sag. Most experts say if you can achieve that, you have the correct spring...but no explanation about the stored energy issue. I guess they don't want people's heads to explode...LOL!

Well, what do you know. As I stated in an earlier post here, I achieved 95mm race sag with 25mm of free sag by applying 13mm of preload on my 6.0 Eibach spring. With 12.5mm being that magical 1/2" preload amount that he mentions, I unwittingly got pretty close at 13mm of preload. My test ride yesterday pretty well confirmed I'd hit the right spot, but after reading his description on the process, I think I fully understand why it worked so well.

I'd still like to pick the brains of some experts about how too much sag also bypasses a portion of your damping influence. In other words, if you have excess sag, your shock piston and shim stack has a reduced amout of time and distance in the shock fluid to dissipate the energy from a compression hit.

Thanks for the link, dj. That 1/2" rule is a good guide. Obviously it could change a bit depending on the specific linkage design on a given bike, the compression piston and shim stack design, and some other factors. In my case it seems to have worked out exactly like he described.

 

Glad you liked it TNC. That's my go-to site whenever the suspension stuff starts feeling like a black art. Bruce usually makes it understandable.

There's another gem in there regarding fork stiction. His alignment process has one more step that I had never seen before. He tightens the left side fork at whatever height he wants it but leaves the right fork loose in the triples. Then he inserts the axle through both sides with the wheel off and moves the right fork up or down in the triples until he finds the position in which he can rotate the axle with the least resistance.

The point being that the forks aren't usually the exact same length due to spring differences, etc. Making them even up top (like we usually do) sometimes induces a binding action at the axle level which adds to the natural stiction in the sytem.

I used to hate riding my husky because I couldn't get the stiction out of those forks and they beat me to death. With Bruce's help I added that one simple step into my alignement procedure and it made a world of difference in the feel. Also got rid of the chronically leaking seals.

I don't know if Bruce is still working or not due to health issues, but he used to have a special interest in KTM's and a special setup for the KLX/KDX.

 

Glad this thread has stimulated a good discussion. I knew that too much pre load on the spring would affect performance. A spring is simple. F=KX. F=force, K=spring rate, X=distance spring compressed. Hypothetical. On a normal bump a 300 lb spring compresses 2 inches. Now add pre load. Say 1/2" of spring travel. That means you already have 150 lbs of stored energy in the spring. Its going to take more than 150 lbs to move the suspension. To get to that original 2" of travel is going to take 750 lbs vs a no pre load of 600 lbs.
In my research I found that QA 1 makes a "high travel" spring. Info here. Less coils, 28% less weight, and more travel. I ordered one. $47 at Summit Racing here. I'll report back on how it goes.

 

Say you have a weaker spring with more preload putting your sag as desired.
the amount of force input needed to bottom out must be less than with a stiffer spring with same sag. Both compress the same distance to bottom but the higher spring constant will require more force to get to the same bottom of the shock. Preload will have no effect assuming that the spring constant is constant, not a function of spring length, etc. If you bottom out with a given spring and damping settings, no amount preload will cure the issue. Now someone will pipe in that it works for him. What do I know?

 

But figuring it isn't. Seems the makers of the springs give you that information... Otherwise you'd have to buy an unknown spring and do the calculations, wouldn't you.

That is what I pointed out - you don't need to do that, the sellers have it in the specs. Too soft a spring, no calculations - buy a higher rated spring. Do the research to find what others' experience is then apply it to one's situation.

I'm sure there are methods to calculate what the proper rating would be, but it isn't that formula. It would have to be specific to the bike design to include all leverages altering the forces at given points. Kind of a statics problem like I did in the Statics and Strengths of Materials, translating forces along beams and trusses. Probably an app somewhere on line that could do it for me, plug in some weights (forces) and some speculation on riding style to get a ball park in-lb spring rating.