Suspension Tuning – Part 1: Rider Sag, Free Sag, and PreloadPosted: January 12, 2010
Consider this Miss Busa’s version of CliffsNotes of Andrew Trevitt’s book “Sportbike Suspension Tuning – How To Improve Your Motorcycle’s Handling And Performance”. All the stuff online is confusing, and some of it is downright wrong. Buy the book, it’ll clear up any misconceptions and it’s actually easy to understand. It’s best to read it once all the way through to get a general idea of what you’re getting yourself into. Reread, break it down into parts, make notes or highlight the important stuff, get things straight in your mind, formulate your game plan, invite a couple of friends and just do it. I believe that this is just like any other thing a m/c newbie finds confusing at first, although it is way more involved that most stuff! Do it, and it will slowly become clear. It will start falling into place, and eventually there’ll be nothing to it anymore. I’m not afraid to screw things up and I have a game plan. Let’s get cracking:
SPORTBIKE SUSPENSION TUNING
- Make one change at a time.
- Document everything!!!
- Understand in technical detail how a suspension works internally and be better equipped to understand how adjustments affect the machine and why.
- Suspension setup is a compromise between several factors.
- Different situations (ideally) require different suspension settings, i.e. drag racing, track days, playing in the twisties, commuting, etc.
- Rider sag or sag is the amount the fork and shock compress when the rider sits on the seat.
- Free sag is how much the spring compresses under the bike’s weight.
- Rake is defined as the angle of the steering head relative to a line drawn perpendicular to the ground (usually between 20 and 25 degrees).
- Trail is the distance from where the steering axis intersects the ground to the front tire’s contact patch directly below the axle (usually between 80 and 110mm).
- Steep front-end geometry (with small rake and trail numbers) gives light, quick steering at the expense of stability.
- Relaxed front-end geometry (with high rake and trail numbers) results in heavier steering but more stability.
- Steep, aggressive geometry may lead to headshake when going over bumps or when accelerating out of a turn.
- Increasing ride height results in steeper rake and reduced trail.
- Raising fork tubes in the triple clamps results in steeper rake and reduced trail.
- Preload does not increase the spring rate, or the spring’s stiffness, for that the spring has to be replaced.
- Preload is a measure of how much a spring is mechanically compressed when the fork or shock is fully extended.
- Preload is mostly used to change the suspension’s range of operation within the total travel available.
- Topping or bottoming out the suspension will cause a loss of traction and upset the chassis.
- An ideal suspension setup uses almost, but not quite, the full range of travel.
- F = k * x (Force exerted = spring constant * distance) <~ linear spring rate
- The spring rate of a coil-bound spring is infinity.
- Damping controls the speed of the wheel’s movement; the spring controls the distance the wheel moves.
- Compression damping controls how quickly a spring can compress.
- Rebound damping controls how quickly a spring can extend back to its original length.
Part 1: Rider Sag, Free Sag, and Preload
Measure FRONT RIDER SAG:
- Have two friends lift the front of the bike by the clip-ons.
- Measure the amount of exposed inner fork tube, between the slider and the axle casting (for inverted forks). Call this measurement L1.
- While one friend supports the bike from the rear, sit on the bike in full gear, in your usual riding position.
- The other friend gently lifts up the front of the bike and lets it slowly settle on its suspension. Measure again. Call this measurement L2.
- Gently push down on the front of the bike, and let it slowly rise up. Measure once more for L3.
- Sag = L1 – (L2 + L3)/2
- Front rider sag should be no less than 25mm for a race bike
- Front rider sag should be no less than 30mm for a street bike
Below these numbers the fork is in danger of topping out on acceleration, which hinders traction and can cause tank slappers as the wheel continuously contacts the ground.
If your measurements fall out of range, adjust your preload: stiffer (clockwise) for less sag, softer (counter-clockwise) for more sag.
Measure REAR RIDER SAG:
- Have a friend hold the front fairing to steady the bike.
- Measure the distance from the axle directly up to a solid point on your bike’s subframe or bodywork. Call this measurement L1.
- While one friend supports the bike from the front, sit on the bike in full gear, in your usual riding position.
- The other friend gently lifts up the rear of the bike and lets it slowly settle on its suspension. Measure again. Call this measurement L2.
- Gently push down on the rear of the bike, and let it slowly rise up. Measure once more for L3.
- Sag = L1 – (L2 + L3)/2
- Rear sag should be 25-30mm for the race track
- Rear sag should be 30-35mm for street riding
If L3-L2 is more than 25mm for the front end, or more than 5mm for the rear end, there is too much friction. Investigate.
If your measurement fall out of range, adjust your preload: stiffer for less sag, softer for more sag.
ADJUSTING PRELOAD DIRECTLY CHANGES THE BIKE’S GEOMETRY!
- Adding preload to the fork springs will raise the bike’s front by the same amount. As long as the suspension isn’t topped out, dialing in more preload will not compress the spring more, but raise the bike on its suspension and equal amount, raising the ride height. Likewise, taking preload out will lower the bike on its suspension. Same goes for the rear shock.
- Add preload equally, front and rear, so that the front and rear sag numbers change by the same amount.
- Bumpy surfaces need more sag and less preload.
Adjusting REAR PRELOAD:
- As a measure of preload (on most Suzukis) record the distance from the top of the threads to the top of the two rings.
- Mark one of the tabs of the bottom ring with a Sharpie, then tighten or loosen it in one-turn increments as needed.
- Measure your setting after you lock the top ring in place.
Before riding attach a small zip tie to the shock shaft, if you can access it. It may be difficult do to the spring and bump stop. This will give you an indication of how much travel you use.
Adjusting FRONT PRELOAD:
- Some adjusters will move in relation to the cap as you turn them, you can record the number of lines showing on the adjuster as your measurement. Otherwise, turn the adjuster all the way in, clockwise, counting the number of turns until it lightly bottoms. This is full-stiff.
Before riding, put a zip tie around an inner fork tube and snug it up so that it doesn’t move on the tube. Slide it up against the outer tube’s dust seal. The zip tie will indicate how much suspension travel you use in each riding session. You can keep track of this number by measuring from the zip tie to the axle casting (inverted fork).
Checking FREE SAG:
- Take the measurements the same as for rider sag, but measure L2 and L3 with no rider on the bike.
- Free sag should ideally be between 5-10mm.
I’ve Got Her Number!
Here are my numbers. Everything is still at stock settings. What is painfully obvious is that The Fat Lady’s a little bit weak in the knees… or would that be her elbows? Something needs a little adjusting here. And I wasn’t even riding her all that hard. :/ No time now, however, but I will post my progress as I continue on my quest to wreck a perfectly acceptable setup. ;P I still need to measure the threads showing above the top lock ring on the monoshock, I procured myself a metric ruler, just for that purpose and I couldn’t get the damned zip tie around the thing, I’m gonna try again with a longer and skinnier one, maybe I can get it in there, somehow, someway. I’m just too curious to let that one rest. *sigh*
Linkage to the entire series:
- Suspension Tuning – Part 1: Rider Sag, Free Sag, and Preload
- Suspension Tuning – Part 1: Plan ‘A’. Plan ‘B’. Plan ‘C’ It Is.
- Suspension Tuning – Part 1: Exploratory Surgery
- Suspension Tuning – Part 1: Let’s Go Shopping!
- The (preliminary) results: OMG! OMG! OMG! The Fat Lady Can Dance?
- Suspension Tuning – Part 1: The Fat Lady’s Got Slammed
- Suspension Tuning – Part 1 (Results): There’s a first time for everything…