Have you ever noticed that some suspension forks will come with adjustable rebound damping, but not adjustable compression damping? But there aren't any that come with adjustable compression damping and not adjustable rebound damping. This is for a good reason.
Rebound damping is directly related to spring rate (air pressure). Spring rate is directly related to rider weight, so any fork that has an adjustable spring rate needs to have adjustable rebound damping. Yet whenever the conversation turns to suspension tuning it seems like it's always focused on compression damping. We often see fancy charts like these for different compression shim stacks. A good suspension tuner would know which shims to arrange in what order to make a fork more supple at the beginning of the travel yet resist bottom-out for a light weight rider. Or they could arrange a shim stack that makes your fork resist pedal bob but allows you to use all of the travel. It's really quite fascinating stuff, but honestly, you'd almost need a physics degree to really understand it. Plus I've never owned a fork or rear shock where I had the option of tuning it with shim stacks. I have been content with setting my air pressure and turning the knobs to get my suspension set up correctly.
Like most mountain bikers I like to fix and adjust things for myself. I am also stubborn, in the sense that I have figured out my suspension settings by trial and error instead of looking up the manufacturer's recommendations. It takes me a few rides to get it just right, but I feel like I do a good job despite my lack of formal suspension tuning training.
Now as a bike mechanic I get to see a lot of others peoples bikes and talk to them about their set-ups. I'll be honest (and maybe a bit judgmental), I think a lot of people would be benefit from trying the manufacturer's recommendations. Okay, that was downright condescending, I know, but I am a bike mechanic after all. This is why I am quite excited about the new suspension set-up systems from various manufacturers like Trek and Fox.
Trek came out with their Suspension Set-Up Calculator last year (I believe) and I happened to have been riding a 2011 Fuel EX 9.9 for a couple months. So in the name of professional curiosity, I decided it was my duty to try this new tool. After all, it could make things easier when we sold a mountain bike. No more asking the customer to hop on the bike, then asking him or her to get off without bouncing around, then measuring the o-ring height and trying to calculate 15 % off 110mm (remember, I don't have an engineering degree). So while driving down to Buck Hill for a Thursday night race, I pulled out my smart phone and started punching in some numbers.
When I got there I began setting up my bike according to what the Trek Suspension Calculator said. I was quite impressed, both with myself and the calculator, that the only change I had to make was to back off the rear shock rebound damping by two clicks. Turns out that all my other setting were exactly where the calculator said they should be. So I went out for a warm up lap. The one thing I really noticed was that on one small g-out (okay, it was a little dip on the trail) I kept having to stand up or I would get bucked off the saddle when the rear tire came to the face of the dip. So I turned the rebound knob back in two clicks and voila! I could stay seated through that dip and I actually noticed that the rest of the trail didn't feel as rough.
I had another similar experience recently on my Cannondale Flash Carbon 3. I was riding at Elk River with Tyson Acker from Shockspital, and I kept feeling like I couldn't hang onto my bars very well. I had recently removed a few headset spacers so I thought maybe I just needed to put them back in. But that didn't make sense, because I had ridden it a dozen times since then and I had liked the way it felt. Then I remembered talking to Chris Ames about this same thing when we were riding Red Wing a week or so ago. He had noted that my handlebars were "frowning" and suggested that I rotate them so that the sweep was up in more of a smiling position. So I kept fixating on my handlebars. I have also felt that the stem was too long so I started wondering about the cost of a new OPI stem. But as we kept riding I knew it wasn't any of these things because I have always loved the way this bike feels. What the heck was going on?
Then as we got the the last quadrant of the Hillside trail I started to realize that it was the suspension. As I would descend over stutter bumps my wrists want to buckle and it was starting to get concerning. Then I remembered that Tyson had just come back from a tuning class at Cannondale in Pennsylvania and he kept talking about being able to change out the compression damping shims to fine tune these Leftys. So, I thought to myself, maybe I will have to let someone else work on my bike for the first time since I became a professional bike mechanic.
But that didn't sit well with me.
So I racked my brain as the trail racked my body. What could be different about my bike that was causing this problem? Most of you have probably figured this out by now, but you probably have engineering degrees and stuff. Besides, I was riding my bike and feeling a bit oxygen deprived so my brain wasn't firing on all four cylinders at this point. Then it hit me out of the clear blue! I had used my bike to demonstrate on while giving a lesson on Leftys at the shop recently. For crying out loud! I had turned the rebound knob in a couple turns to demonstrate what is does. IT SLOWS DOWN THE REBOUND, YOU MORON! I backed it out a couple twists while riding and quickly noticed a huge improvement. The trail didn't feel as rough. My wrists didn't want to buckle. My upper body relaxed. I remembered why I loved this bike so much.
Rebound damping is quite possibly the most sensitive of the three suspension tuning facets. Air pressure can vary by 10-15 psi and the rider might not care all that much. High tech compression damping tuning is complicated. Plus, the compression dampers are really well tuned by the manufacturer from the get go so there's very little reason for most of us to spend money getting a custom tuning job. Rebound damping, on the other hand, has to be spot on. Two clicks off and the ride quality can go suffer tremendously. The weird thing is when the rebound damping is off, the suspension feels harsh.
Here are some links for more information on how to set up and diagnose your suspension.