Offroad Suspension Basics and Settings
Most experienced riders agree that the best value, dollar for dollar to go faster, is properly set up suspension. Good suspension will give you the confidence and ability to use the power you have fully. All the horsepower in the world does you no good if you can’t put it to the ground or if you have to slow down to keep from being thrown off.
The first step in any suspension project is springing the bike properly for your weight. Something to keep in mind while working on your suspension is that springs hold up the bike, and valving controls the feel. This is the “first axiom” of suspension tuning.
An under sprung bike blows through the stroke too quickly, rides too low, bottoms often, and will be harsh due to the stroke and bottoming issues. An over sprung bike rides too high, doesn’t use the full suspension travel, and beats you to death.
One of the first things to do is to set your rear sag. Proper sag gives you the correct steering head angle that combines straight line stability and cornering feel.
Most bikes don’t come from the dealer with the correct sag, or even the correct springs for your weight. Generally bikes are geared towards the “average rider” who weighs about 165 pounds. Well, I don’t know about you, but I sure don’t classify as the average rider weighing in at about 205 manly pounds (or maybe thats porky pounds)………..
Setting your sag properly can mean the difference between flowing around the track, and fighting the bike. A small change in the bikes balance can make large changes to its handling. One thing to remember, though, is that every rider is different! It’s important to spend a little time experimenting to see what works best for you!
What to look for?
Too Little Sag: The bike tends to knife in on corners (turns in too fast), kick in the whoops and under braking, want’s to put you over the bars going down hill, and fishtails while accelerating.
Too Much Sag: The bikes tend to push in corners (turns in too slow), won’t hold a line and deflects, and feels “choppered”, but feels really stable at speed.
Japanese bikes tend like about 100mm (+/- 5mm either way), and KTM's like 115mm (+/- 5mm either way) of rider sag.
To set your sag, you need a sag measuring tool (really handy) or a tape measure (it’s easier if it’s marked in metric measurements), and an assistant. This will look funny, but you need to have ALL the gear you wear to ride (boots, pads, tools, hydration system, etc.). You need to set your sag with the proper weight on the bike. For many folks this can make a difference of 25 to 35 pounds!
The first measurement is made with the bike on a stand with at least the rear wheel hanging off the ground. Measure from the rear axle straight up to a point on the rear fender (for example, I use the edge of the swing arm and the center of a rivet on my silencer). Set the sag tool to zero or make a note of this measurement. This first measurement is your starting point or baseline. Write this measurement down so you don’t forget it.
Next, sit on the bike as you do when riding (if you are mostly a stand-up rider, stand; if you are mostly a sit down rider sit). Balance the bike with the very tips of you toes (if sitting), or by reaching out to touch something lightly like a wall (if standing) for balance. Putting all of your weight on the bike bounce it a few times and measure from the same two points as before. Subtract this number from your baseline number. This result is your rider sag. Write this measurement down too so you don’t forget it.
Hopefully this measurement is close to your target sag (100mm +/- 5mm for most bikes, 115 +/- 5mm for KTM), if not, loosen the lock ring (to increase sag) or tighten the lock ring (to decrease sag) as needed. Repeat the rider sag measurement procedure above, until you hit your target sag.
When the rider sag is set it’s time to check static sag. This will determine if your spring is correct. Measure the sag without the rider on the bike. Bounce it a few times, and measure from the same two points as before! Subtract this number from your baseline number. This result is your static sag.
Write this number down too so you don’t forget it (see a pattern here?).
Now you should have three numbers, your Baseline, your Rider Sag, and your Static Sag.
More than 45-50 mm static sag means that your spring is too stiff. Less than 15-20 mm means that your spring is too soft.
Most riders who can do their own “normal” maintenance can change out their own springs, both on the front and rear of their bikes. Many suspension companies have spring calculators to get you the correct spring from their inventory, try some of these for example:
MX Tech - http://www.mx-tech.com/?id=spring_generator
Race Tech - http://www.racetech.com/evalving/menu/searchdirt.asp
Front sag is a little harder to measure and set due to the “stiction” of the fork seals, the angle of the forks, etc. The spring generators do a good job of predicting what you need. The old “rule of thumb” for front sag is 35 to 45mm static sag, and 65 to 75mm for rider sag. You adjust the fork sag by either adding or subtracting the spacers under your fork caps, or adjusting the preload bolt on your fork caps if they are adjustable. Adding more than 10mm of preload is a good sign that you need the next step stiffer fork springs.
Now that you have your bike sprung properly (remember the line: springs hold up the bike, and valving controls the feel ?), this may be all you need to be happy, with just a little clicker adjustments.
Clickers:
Most forks will have compression and rebound adjusters on the bottom, top, or both ends of the forks. Here’s an example with my fork and shock adjusters:
Compression damping affects how fast the suspension collapses (sucks up a hit), while rebound damping affects how the suspension extends (gets ready to take the next hit). Too much compression damping can make the bike harsh, but not enough lets the bikes suspension “pack down” and not extend in time to take the next hit with the full travel. Too much rebound can cause packing too, or not enough makes the bike feel “bouncy”. Packing happens when each hit drives the suspension a little deeper into it’s travel, continually, until you run out of suspension travel.
This is usually referred to as a crash………..
To adjust your clickers, go to one of your “usual” riding areas that you know well. Hopefully it will have a small jump you are comfortable on, and a few whoops or a ditch, or a log about 8” to 12” across that you can run over. You want to be able to land flat off a jump, take a rolling hit, and take a sudden big hit to your suspension while testing.
As an example, lets start with the jump; it doesn’t have to be a big one. Make a few leaps to get comfortable, and pay attention to the bike on landing. Try to land flat, with both wheels touching down at the same time, and staying on the throttle lightly all the way through (chopping the throttle drops the front end, and rolling on a little more raises the front end). Does it bottom out? Does just one end bottom out? Does it bounce back up like a ball? Does just one end bounce back too fast?
Let’s say it lands flat, stays flat, and you think it feels pretty good, well………
Here’s the “second axiom” of suspension:” the best suspension you have rode so far is the best suspension that you know”. Some of us aren’t ever satisfied, and are always looking for a little more. So, lets make a few adjustments and see if we can make it better.
Lets try taking about 2 or 3 turns of compression damping out of each fork, and the shock (look at your bikes manual, but most bikes add compression damping by turning the adjuster clockwise, and subtract by turning the adjuster counter-clockwise). Go take a couple more passes at the jump, and see how it feels. What we are looking for is taking compression damping out until both ends of the bike bottom out lightly, and then add back a couple clicks of compression damping. This lets you use most of your suspension travel most of the time, as it was designed to.
Let’s say it lands flat, but only one end bottoms out. Add a couple clicks of compression damping until that end stops bottoming, then maybe try reducing the compression damping on the opposite end until it bottoms out lightly, then add back a couple clicks at a time until it stops.
Let’s say it lands flat, but then the front end bounces back up like a ball, causing it too hobby-horse. This means you don’t have enough rebound damping. Add some more rebound until the “bounciness” goes away. As the opposite, you can reduce the rebound 2 or 3 clicks at a time until it feels a little bouncy, then put a couple clicks back in. This lets your suspension set up for the next hit quickly and not pack down. This should also improve the “grip” of the front end when cornering.
After a bit, you should have the bike landing smoothly, recovering rapidly, and accelerating out away from the jump in a straight line. This should get you in the ballpark for some finer tuning. Now you want to go to the whoops, ditch, or log to fine-tune a little bit more.
The idea is to get a feel for the way the bike is working through the obstacle. Is it getting a little sideways on acceleration through it (needs a touch more rebound damping in the rear), does it shake it’s head on impact (too much compression damping up front), does it bounce back too quickly on a sudden hit (not enough rebound damping), etc.
See, this isn’t magic! It just takes a couple hours concentrating on what you are doing, and repeating a few sections testing as you do, not just out rippen’ it up……………..you will be surprised how valuable the time spent will be!
However, if you still want a little more……………..
Here’s the “second axiom” of suspension rearing it’s ugly head again, remember “the best suspension you have rode so far is the best suspension that you know”? Some of us aren’t ever satisfied, and are always looking for a little more.
Once the bike is sprung correctly for your weight, you may want a bit more. The valving can then be optimized for MX / SX (stiffer to take the deep whoops and jump landings), more towards sand tracks (stiff, but not as stiff as for MX / SX), desert (varied but higher speeds, g-outs, and sudden big hits, but softer still), trails (generally plush, and for a sit-down style), or maybe for rocks (lots of travel, very compliant, and generally the softest type of setup).
Most suspension is set up as a compromise between two categories, and you can compensate somewhat with the clickers. It’s not perfect any where, but pretty good most of the time………… To do this, it’s time to go inside, since they are easier for the “average mechanic” lets start with your forks……………..
Fork Components:
The common single chamber fork on most bikes will be similar to what I cover here. Since I’ve got it, these will all be KTM / WP pictures, yours will be close but different.
The fork consists of the fork lower (the chrome tube on the bottom of the fork where the wheel attaches) and the fork upper (the gold, black, or silver top part of the fork that is held by the triple crowns).
Contained inside the fork is the cartridge assembly, consisting of the basevalve, the cartridge itself, the rebound / midvalve rod, and the springs.
The “feel” of your suspension is controlled by the flow of oil through a metered orifice and a stack of flexible shims (think of a carburetor; instead of changing jets to tune your engine, you change shims to tune your suspension). The use of different diameter and thickness shims, the different weight of the oil, the height of the oil, and how much the clickers are open determine what your forks will feel like.
For starters, here is a basevalve, with the shims piston, and such attached. The basevalve sits at the bottom of your fork, and attaches to the bottom of the cartridge that rides inside the fork leg. The oil flow paths are marked. The green arrows show the flow when the forks compress (compression damping). For the oil to flow, it has to bend the stack of shims to open the port. The blue arrow shows the flow as the fork extends (rebound, but this is just a check valve, with no damping, that happens somewhere else……). The basevalve flows freely in rebound to quickly fill the lower part of the cartridge with oil. The red arrows show the flow through the bypass ports of the clicker. This is a bleed that will flow oil on both rebound and compression stroke (remember this, large clicker changes can overlap from one circuit to another).
And, here a basevalve disassembled, with the shim stack broken down.
Here is a simplified drawing of a basevalve, with the two basic kinds of shim stack layouts. The first drawing is a “single stage” stack. Note the gradual taper.
The next drawing is a ”two stage” stack. The idea is that the first set of tapered shims (low speed stack) will deflect easily on small hits, and the second set of tapered shims (high speed stack) will move progressively together on larger hits. In suspension we refer to wheel travel speed, not bike speed. For example, hitting a square edge initiates more suspension speed than a rounded edge.
The shims set on top of a piston, here is an example of a high-flow aftermarket piston on the left (A Race Tech Gold Valve) and a stock piston on the right. Note how the square edges have been rounded off and smoothed out.
Traveling through the middle of the cartridge is the midvalve and rebound assembly. It moves through the cartridge much like a piston moving through a cylinder. As it moves downward the oil flows through the midvalve piston and past the midvalve shims. The oil that moves through the basevalve is the amount of oil displaced by the rod that enters the cartridge, that the assembly attaches to, which in turn attaches to the top of your fork at the fork cap. Remember the midvalve does not push fluid through the basevalve, it just flows through the oil. The rod that enters the cartridge displaces the oil through the basevalve. Here’s a picture of the assembly and the oil flow through it.
And a midvalve / rebound assembly disassembled and the stacks broken down.
Just as the basevalve closes the bottom of the cartridge, the bottoming cones and spring guide close off the top of the cartridge. As the spring guide enters the bottoming cone, oil is displaced around the area between the inside of the cone, and the outside of the spring guide. This keeps you from having metal-to-metal contact on bottoming (like the forks did in the old days) and softens the sudden stop at the end of your suspension travel. Here’s a picture with the oil flow paths.
The last thing that comes into play in tuning your fork is the amount of oil you add to the assembly. The amount of unfilled space above the top of the oil, with the springs out and the fork leg compressed, is called the air chamber. Most bikes run somewhere between 100mm and 150mm of airchamber. Air compresses, so it acts as a secondary spring, and is referred to as airspring. Generally, you lower the airchamber (reduce the airspring) to make a fork softer or more compliant, and raise the airchamber (increase the airspring) to firm a fork up and to resist bottoming. Additionally. the weight of the oil can add one more tool to tune with. Just like in an engine, a lighter weight oil will flow faster, while a heavier weight flows slower.
I’m not going to get into the many valving changes that can be made, because each and every fork brand is different, each rider is different, and the type of terrain all effect the decisions you make for the valving. Companies like Race Tech sell kits, that come with shims and valving charts with recommendations for different rider weight, skill levels, and type of riding.
This is where I started at, but after a while I moved on to experiment with many other setups. I’m looking for a “do-all” setup, that I can adjust for different situations with my clickers. I can firm it up for sand, and soften it up for rocks, in just a few minutes with nothing but a small screw driver and my fingers.
Is it perfect everywhere? No. Is it perfect anywhere? No. But, it’s pretty close everywhere, and I’m happy with it! Plus, the journey itself has been fun……………………..
If any one is interested, I can do something similar with shock internals, but they are usually beyond the tools and abilities of the “average” mechanic. Shocks are pressurized to anywhere between 150psi and 200psi. If you don’t know what you are doing, they can seriously hurt you. Also, shocks must have all the air properly bleed out of the assembly or you can easily (and expensively) damage you shock. They do work much like a fork, with pistons, shims, and the assorted adjustments possible. Race Tech and many others make kits for these also. I’ve had good luck with their Gold Valve kits in my bike. But just like in my fork, I started there, and went down a different path eventually.
I hope this helps you guys out, and gives you a little bit better idea what goes on in your suspension.
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