Suspension Part 2 Sag and Spring Rate pt1

To start off, I wish this post could have been shorter, but there is quite a bit of information to be covered on this one subject. Entire books have been writen on this one subject alone.  This post will give you a healthy head start understanding springs and how to setup suspension sag.
Race-Techs-Motorcycle-Suspension-Bible-Motorbooks-Workshop-0 Sportbike-Suspension-Tuning-0
We are not all created equal.  We and our bikes all weigh a different amount. So why would one spring rate and sag settings for a 130lbs rider be the same for a 250lbs rider? It will not be. Not only will the preload be different but the spring rate might be different as well depending not solely on weight, but on suspension linkage setup of the make and model.  To top it off an advanced rider will prefer their suspension to have a different sag adjustment or a different spring rate than your average novice or street rider.

Suspension sag is the first adjustment that should be looked at and adjusted after the preventative maintenance steps have been taken to ensure the bike is in proper working condition. This is covered in the previous suspension post. Setting the rider sag will determine if a stiffer or softer spring is needed. Spring sag is the amount the springs compress between fully topped out and fully loaded with the rider with riding gear on board in the riding position.

If you can use a measuring tape and do basic math, the first timer setting sag can do it in 15 to 30min.   This adjustment is important and will get your suspension into the sweet spot so it can work correctly. Sag allows the wheel to drop into a dip, maintaining contact and pressure with the pavement. On a sportbike rider sag is about 1 to 1.375 inches or 25mm to 35mm.  Some riders like a plush ride which would result in a 35mm sag setting, and others like a firmer ride (25mm) that will give them more feedback.

CYCLEPEDIA Online Manuals

Definitions

  • Free SAG = This is the amount the bike settles under its own weight, from fully extended.
  • Rider SAG = This is the amount the bike settles when the rider is in full gear and on the bike in position, from fully extended (0 weight on the suspension)
  • Prelaod = The length the spring is compressed when it is installed into the fork or onto the shock from its free length before installation.  EX: my fork springs measure 315mm long outside of the fork before installation.  When the fork spring is installed, it has an installed length of 295mm.  The spring has 20mm of preload when installed.
  • Spring Rate = Measurement of the force it takes to compress a spring a certain distance.  lb/in, N/mm, or kg/mm.
  • Sprung Weight = This is the portion of the motorcycles weight that is being suspended by the suspension. (It is NOT the total weight of the motorcycle EX: wheels and a portion of the swingarm would not be factored in)

So what is spring rate? How is spring rate measured? Spring rate is the distance the spring will be compressed with a specified weight. EX: The spring rate on my motorcycle is approximately 20kg/mm. so If I want to compress the spring 1mm 20kg will need to be applied to the spring and if I would like the spring to compress 2mm 40kg would need to be applied and so on.   This rate is linear (on most bikes) and the suspension linkages provide the progressive compression through the travel. On occasion a two stage or progressive rate spring will be used. Racing suspension uses a linier rate suspension.

Linear Spring Rate Example
Notice the free length is 150mm, and then when 10kg is applied to the spring it shortens by 10mm. This is a 1kg/mm spring rate. So if 25kg were to be applied the spring will compress 25mm and so on. This is a linear rate spring and is the type of spring most of us are dealing with.

Increasing spring preload DOES NOT increase spring rate. Spring preload and Spring rate are two completely different things. Spring rate can only be changed by changing the spring. Adjusting the preload only adjusts how much force is needed before the spring begins to move. When the spring is installed it will have a compressed length.  Using the image example above. If we have this same spring, and the installed length is  140mm it will take an additional 5kg on top of the 10kg being applied by the preload to compress it 5mm, so a total weight of 15kg. This is because we already have 1omm of preload (10kg is already loaded in with the installed length) and 5 more kg is needed to get it to move 5 more mm.

When setting rider sag and the max or minimum adjustment is met on the shock adjusting nut or the fork preload adjuster on the fork, it is a good sign that the spring rate is incorrect and should be swapped out accordingly. For example: if the rear sag measures 40mm  and the adjuster is set to the maximum preload adjustment, the spring rate is to low and the next stiffer spring should be installed. Installing a stiffer spring allow for less spring preload, and the adjust will not need to be cranked down as far, and will put the rider sag into the correct range. Setting rider sag is your first step to determining that the spring rate that is currently on the bike is correct.

Measuring free sag also helps to determine if the spring rate is correct. Free sag on street and roadrace bikes is measured at 0 to 5mm, but should not “top out” or hit its fully extended limit hard. On the front, expect to see 5 to 10mm of free sag. It should barely have enough force to top out without the rider on board. If the rate is to stiff the bike will be extremely hard to compress from the fully extended position, and will top out hard. If the spring preload adjuster is backed off (making the springs installed length as long as possible) and the suspension still tops out hard, indicates that the spring is to stiff.

How do you set rider sag?  There is a plethora of videos on Youtube talking about rider sag, but the best one and simplest is the video by Kieth Code on his Twist of the Wrist II. The first things needed are the tools.

  • Notebook and pencil
  • Tape measure
  • Calculator
  • 16, 17, or 18mm T-handle
  • Spanner wrench for the rear shock preload adjusting nuts, or a soft drift punch and hammer.
  • A lovely assistant (The job is easier with two)

A spanner for the rear shock adjusting nuts will not always fit and a soft drift punch and hammer may be used to adjust the preload of the rear shock.

What about suspension stiction and friction? Race Tech has a methodical way to check rider sag, and takes stiction into account.  Stiction is the friction caused by the seals on the shock linkage, shock shaft and piston or the friction on the fork seals or bushings against their mating surfaces.

Setting Rear Suspension Sag (The Race Tech Method)

  1. OK we have our tools rounded up and we are ready to get started. The First measurement we need is the fully extended length. So zero weight on the tire. The bike should not be resting on the tire or end that this measurement is being taken from.  Measure from a known point on the rear of the swingarm up to a known point on the body work.  This measurement must be near perfectly vertical. This measurement is our completely extended measurement or L1.
  2. Next we measure the rear ride height on the lower limit.  At this point the bike should be back on the ground and the rider should be in full gear on the bike. With the rider on and in riding position an assistant should hold the bike upright, and another assistant should firmly push down on the bike and let it rise slowly (DO NOT BOUNCE). Measure between the same two points as in step 1.  This measurement is L2.
  3. Next with the rider in the same riding position on the bike as in step2. Have the assistant that pushed down on the bike slightly lift up on the tail section and let it settle slowly (DO NOT BOUNCE).  Take another measurement between the same two points as in step1.  This measurement is L3. At this point we should have 3 measurements L1, L2, and L3.  If we subtract L2 from L3, this will be the amount of stiction in the rear suspension linkages and various components.  When we average L2 and L3, and then subtract that average from L1 we get our rider sag number.
  4. At this point we know if an adjustment needed to be made, and the Preload of the spring needs to be increased or decreased. Adjust the spring collars accordingly.
  5. If an adjustment needed to be made repeat steps 2 – 4.  We do not have to do step1 again, because the fully extended length will not have changed and we can use this measurement again.
    20160328_154859[1]
    This shock doesn’t use a spanner wrench, it uses a rod to turn the preload collar. the rod is inserted into the holes in the collar and turned.

Setting Front Suspension Sag (The Race Tech Method)

Adjusting front sag is very similar to the rear.

  1. The First measurement we need is with the fork fully extended. The bike should not be resting on the front tire.  Measure from the dust seal down to the casting on the lower portion of the fork (Upside down forks).  Or measure from the dust seal up to the bottom of the lower triple tree (conventional forks).  This measurement is our completely extended measurement or L1.
    Measure the forks fully extended.
  2. Next we measure the front ride height on the lower limit.  At this point the bike should be back on the ground and the rider should be in full gear on the bike. With the rider on and in riding position an assistant should hold the bike upright, and another assistant should firmly push down, compressing the forks and let it rise slowly (DO NOT BOUNCE). Measure between the same two points as in step 1.  This measurement is L2.
  3. Next with the rider in the same riding position on the bike as in step2. Have the assistant that pushed down on the bike slightly lift up and unload the forks and let it settle slowly (DO NOT BOUNCE).  Take another measurement between the same two points as in step1.  This measurement is L3. At this point we should have 3 measurements L1, L2, and L3.  If we subtract L2 from L3, this will be the amount of stiction that the forks have (fork seals generate more friction/drag than the rear suspension components).  When we average L2 and L3, and then subtract that average from L1 we get our rider sag number.
  4. At this point we know if an adjustment needed to be made, and the Preload of the spring needs to be increased or decreased.
    Fork preload adjuster “Blue”

    20160328_155355[1]
    17mm wrench on the fork preload adjusters. A t-handle works best for this type of adjustment.
  5. If an adjustment needed to be made repeat steps 2 – 4.  We do not have to do step1 again, because the fully extended length will not have changed and we can use this measurement again.

Dampening will be covered in Part 3 of this series.

Chad

Riding motorcycles and wrenching since my preteen years, I have moved from motocross to street bikes. Being a teen back in those days it was tough to get me off of the bike. Now days even though I am very busy being a dad I still have my weekends and go to the track to race and on occasion will do a track day or two.

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