Jason Williams,

You think that is over your head try to read the "YZF suspension midvalve" post from about a week ago! Now that can be a confusing post (but very informative when you understand the terms...it took me a few reads to completely understand what everyone was talking about)! Later,

Garrett

what is the proper amount of sag for the front end?? i like my rear at 100mm.. but the front seems to be sagging a little more then it should... this is on a 02 426...and i weigh 190lb... i think the stock spring rate is good... but maybe i need a little more preload in the front?? thanks for your help...

Okay...here we go.

When setting up a bike, (or car, or your favorite couch) the thickness (or "weight") of the spring is always determined first.

What determines the size of the spring is the weight of the rider, the weight of the bike, and how much down force will be applied to the bike when riding based on your application of riding.

Down force is applied when the weight of the rider and bike are pressed harder against the suspension due to other forces, such as centrifugal force in a corner, or the inertia of the weight coming back down when landing a big jump.

The tricky part with springs, is that the more you compress them, the harder they push back.

Suspension tuners like this, since it’s good for a suspension to get tighter as a generated force, like landing a jump, is applied against it.

However, most suspension tuners find that the first 5 to 20% of spring compression provides a rate of resistance that is too slow, and not as linear as the remaining 80 to 95% of compression.

To fix this, they “pre-load” the spring. Pre-load is the difference between how tall the spring was before you installed it in the bike, and how tall the spring is now that you have put it in the fork, or installed it on the shock. It’s noted in mm or inches, but for serious designers, it’s done as a unit of force.

Sag” is controlled by pre-load, or better put, is the result of a pre-load setting. For example, 10mm of pre-load may result in 120mm of sag.

Sag is part of that ready-for-action concept, but it’s also used to balance the distribution of weight. Make the rear go up high, (less sag) and you apply more weight to the front wheel. Apply more weight to the front, and your front wheel sticks a bit better. And vice versa.

So springs and spring settings (weight of spring, pre-load and sag) are done in accordance with an application spec. Ie, a 160lb SuperCross rider.

Now the issue of oil.

Oil in a fork or shock controls how quickly a spring is compressed in addition to the resistance to the spring. When the fork or shock is not moving, the oil does nothing. Zippo!

When a fork or shock compresses or decompresses, oil is moved from one reservoir to another. To move both back and fourth between the reservoirs, the oil has to flow through a collection of holes. The smaller the holes, the harder it is for the oil to move, and therefore more resistance. When you turn in those screw clickers on your forks, you are in a sense, making the oil transfer holes smaller. (well, not really, but for now pretend that’s how it works).

Also covering some of these holes, are things called "shims". They look like very thin flexible washers. They cover the hole, but deflect away when the oil pushes against them when moving about. When your fork moves abruptly, say when you hit something like a curb, the oil wants to exit very rapidly, therefore pushing the shim out of the way with great force. Slower suspension movements cause the shims to barely lift, but still allow oil to flow, (umm...move).

The difference between these movements is called “high compression” (fast moving oil) and “low compression”, (slow moving oil). Suspension guys sometimes refer to this distinction with a value to "rod speed". Which just means that the rod that pushed the oil is pushed fast and hard, or slow and easy.

In all cases, a shim never works along. They are stacked on top of each other, providing support and greater resistance. This is called a “shim-stack”, (sometimes called a “valve-stack”, and sometimes called a “piston-stack” – or maybe not).

Suspension tuners vary the number of shims and the thickness of the shims to control how quickly the oil will flow when a bump is hit. This issue of how many shims and of what thickness is a science that a lot of people pay very good money for.

By the way, turning the screw-clickers on your fork and shock actually causes more oil to flow through the shim stack, or by-passing the stack and taking an easier route. For the science majors among us, you are “by-passing” the “effects” of the stack.

Onward.

Now, one may confuse too many shims, or too many thick shims, with a spring that is too stiff.

I mean, if you go off and ride someone’s bike and come back and say “gee, that’s a stiff bike”, do you mean, the rider has too stiff of a spring, or does he have too many shim washers installed?

Well...it takes a good rider to ride a bike and tell the difference. This is why most of us should rely on setting the height of the bike via the spring weight, then adjusting stiffness via the clickers, and or shim stacks...afterwards.

Was that long or what?

Now before things get too silly, understand that there are plenty of variables that I left out. Like, “nitrogen charging”, and “air or oil level” issues.

Also, keep in mind that just because I can explain this stuff, doesn’t mean that I don’t spend half of my day at the track scratching my head in total confusion. I can get this stuff only so right before I find more problems.

Variables.

DaveJ

Awesome post, Dave. After reading that I may not be so sure that I need a heavier spring. However, I do want to try one so, back to the original post; does anyone have a 5.6 rear spring they want to unload?

Whoeee,

Dave you are my Hero!!..Actualy I was allready pretty familiar but your post was an exelent refresher course..

Thanks...do you think you could help me with my spelling?

Does anyone have a 5.6 rear shock spring that they found to be too stiff or that they just didn't like?

I weigh 220 and would like to try one (std is 5.4). I already found a guy who lives near me that found his .48 fork springs (std is .46)to be too stiff (he weighs 160); bonus buy for me! Thanks.

Pilot,

I'm 170 and I'm running a .47 and 5.6. All is well, however I think the factory valving together with a .47 could be a bit stiff for many.

This is when I start to push my theories on front pre-load and sag. Mainly that many of us fail to understand the height setting of a fork to that of what we weigh, to the issue of the fork being too stiff, due to the valving.

In other words, consider how much sinking is proper in the front end, adjust via spring and pre-load shims, then adjust out a hard ride via the valve stack.

How's that sound?

DaveJ

426 Pilot -

There is no doubt that you need to run a 5.6 spring on the rear. Don't hesitate to go buy one. You won't be disappointed.

However, you can't change the rear without changing the front. IN this case, you'll need a .47.

This is where you may be disappointed, since the .47 spring with the factory valving creates a high-compression arm killing madness. So the trick is to get the right springs, then remove or thin out some of the shims that control high-compression.

Eh?

DaveJ

Pilot,

You may want to run the numbers via Race-Tech again. For 220lbs SuperCross, I'm getting .48 front and 5.8 rear.

Or just give them a call.

As for the oil, I would stick with the KYB spec. However, in theory a thinner oil would remove a high compression edge.

I'm not sure how you are doing your oil changes, but with the right tools, you can make changes to the valve stack quicker and easier than you do most oil changes.

I can give you some tips and send over some charts if you like. I may even have some old videos about the place that show how to do this.

But in either case, don't jump to conclusions. Install the springs first, then decide what you do and don't like about the ride and performance.

Fair?

DaveJ

More than fair. I'll have the springs installed over the holidays when I won't be riding much and go from there.

Thanks for all the great input!

Dave -

Thanks, I just picked up the springs for the front that I mentioned before; they are 48's. I hope they aren't to stiff. I know nothing about valves and shims (at least that I'd be comfortable doing myself) and unfortunately I don't have the cash to do it all.

On Race Tech's web site, I ran my profile and they give some recommendations (full blown revalve but also no revalve). Believe it or not, the site recommended the 5.4 (std) for me for the rear. It also recommended the.48's for the front along with oil height, clicker settings, sag, etc.

On my low budget, I will try the RT non-revalved settings with a 5.6 rear and see what happens.

DaveJ, in your opinion, would running a lighter oil help with the heavier spring?