Free sag vs ??

[Admin]

Since I'm going from an FZR to a GSXR 600, I've got all of these fun little nobs to play with on my forks and shocks.  I recently had Traxxion do my forks, and I have a Penske shock.  Traxxion recommended that I set a 20 mm free sag on the forks and an 8 mm free sag on the shock.  

What I am curious about is why I would want to do free sag (I guess this is without a rider) vs. sag with my weight on it.  Is that because the springs should all be setup for my weight so if I get the free sag right everything else should be right too?

Obviously, I don't know alot about this suspension stuff.  My FZR was on rails from the day I bought it and I never touched the suspension.

[Gixxer124]

Free? Did someone say free? Where do I sign up? Max, maybe you should answer this one. :-[

[Super Dave]

Max gets you set up where, yes, if you get the free sag right, you're pretty much there.  Did you get the numbers for the length of the shock too?  And how high the fork tubes should be?  Get those numbers.  Changing your gearing will effectively make the swingarm longer or shorter, too.  That will cause the swingarm pivot's location to move higher and lower.  

And of course that will change the weight distribution and the center of gravity.  So, you might find that when you change gearing, you might be changing the rear ride height to make it work the same way all the time.

[Admin]

Yea, I got the shock length too, although its kinda hard to measure, so I'm probably just in the ball park on that one.  But, there was no talk about gearing and how that changes the shock length...  ???

[Super Dave]

It doesn't change the shock length...

Think about this....

You go from a 45 rear sprocket to a 47.  So, you would have to change your chain adjusters because you're using the same chain.  To fit, you'll move the adjusters so that the rear axle ends up being closer to the front of the bike.  Additionally, the distance between the rear axle and the swingarm pivot will be closer to each other.

The rear axle and the front axle will always remain the same distance from the ground, unless you use a different tire.  The swingarm pivot becomes lower, and proportionately in a different location in comparison to where it was from the front and rear axles, because the swingarm has a slope from the pivot.

So, lets imagine that your bike handles great with the 45.  The bike's center of gravity (CG) is located somewhere the swingarm pivot.  When you ride, the CG aids in weight transfer from the front and rear, causing the front and rear suspensions to work a certain way.  Ok, now remember, it handles great with the 45.

Now you put on the 47, the rear axle is now closer to the front axle and the swingarm pivot.  The swingarm pivot, and the bike's CG have moved closer to the rear.  The same weight transfers that you had with the 45 cannot be expected.

Chain torque is another issue, but we'll leave that one alone.

[Jeff]

What I am curious about is why I would want to do free sag (I guess this is without a rider) vs. sag with my weight on it.  

The slight left after the bus-stop at BHF is a perfect example why.  There is a small dip, and you're hitting while under hard accelleration.  Under hard accelleration, your shock can top out since the force of the rear tire is pushing the rear end UP.  When this happens, if you hit a dip or any other anomaly in the track surface, your shock is already topped out, and has no more travel, thus you start spinning/sliding.

Free sag is a very important item...

(I'm no suspension expert, but I did stay at a Holiday Inn Express last night)

[tzracer]

It doesn't change the shock length...


So, lets imagine that your bike handles great with the 45.  The bike's center of gravity (CG) is located somewhere the swingarm pivot.  When you ride, the CG aids in weight transfer from the front and rear, causing the front and rear suspensions to work a certain way.  Ok, now remember, it handles great with the 45.

Now you put on the 47, the rear axle is now closer to the front axle and the swingarm pivot.  The swingarm pivot, and the bike's CG have moved closer to the rear.  The same weight transfers that you had with the 45 cannot be expected.

I have been thinking about whether I should respond to the above statement or not. As an LCR instructor it may look as if I am picking on an instructor from another school. As a physicist and a professional instructor it is very difficult to not reply. Therefore this reply is coming from the physicist. Dave I hope you will take this reply as intended.

What you are saying is essentially correct, I disagree with how you state it and your conclusion.

For this discussion I would use CM (center of mass) not CG. They are not the same thing. Inertia (weight transfer) acts through the CM. It is true for objects near the surface of Earth, the CG and CM are in essentially the same place, as long as the object's size is much smaller then the radius of Earth, but they are not interchangeable terms, just like speed and velocity are not interchangeable terms (the moon's CM and CG are not in the same location, that is why the moon does not rotate freely about its rotation axis, that is, the same side of the moon always faces Earth).
BTW Rick also likes to use CG, and I tell him the same thing.

In the second paragraph you seem to be using 2 reference frames. One seems to be the bike, "the rear axle is now closer to the front axle and the swingarm pivot.", the other would be the rear wheel, "The swingarm pivot, and the bike's CG have moved closer to the rear". To avoid confusion it is best to stick to one reference frame.

When the rear wheel is moved forward in the frame, the wheelbase and the swingarm pivot to rear axle distance are both reduced. The CM moves forward in the bike (closer to the front axle). The rear axle moves forward more than the CM moves forward so the rear wheel ends up closer to the CM.

I am not sure the weight transfer will change much at all. I ran some numbers to get an idea of the amount the CM could be expected to move. I will use US units, but will include the correct units (metric) in parenthesis afterward. I used a bike with a total weight (including rear wheel) of 400 lbs (1779.2 N), a mass of 12.4 slugs (181.4 kg), and a rear wheel of 30 lb (133.4 N), a mass of 0.93 slugs (13.6 kg) with a wheel base of 55 in (1397 mm) and a rear axle angle of 20 degrees as measured from the horizontal. I placed the CM at the center of the wheelbase and 24 in (609.6 mm) from the ground. I moved the rear wheel 1 inch (25.4 mm) along the swingarm, which is a 0.940 inch (23.9 mm) horizontal movement and a 0.34 inch (8.6 mm) vertical movement. I found that the CM would be moved forward 0.071 inches (1.8 mm) and upward 0.026 inches (0.66 mm). The distance between the rear axle and the CM will change by about 0.87 inches (22.1 mm).

It would seem that the small change of the CM forward and upward would have little effect on braking, it may be large enough (combined with the movement of the rear wheel) to be noticed on acceleration. It may even help by putting more weight on the rear wheel, but this depends upon how easily the bike wheelied before the change. One thing that has not been metioned is the rider. By moving on the bike, the rider can have a more profound effect of the CM of the bike than the moving of the rear wheel. That is, the rider can counter any changed the moving of the rear wheel has caused by body positioning.
Also, not being consistant with body positioning can easly mask any small geometry changes.

[OmniGLH]

---snip---

It would seem that the small change of the CM forward and upward would have little effect on braking, it may be large enough (combined with the movement of the rear wheel) to be noticed on acceleration. It may even help by putting more weight on the rear wheel, but this depends upon how easily the bike wheelied before the change. One thing that has not been metioned is the rider. By moving on the bike, the rider can have a more profound effect of the CM of the bike than the moving of the rear wheel. That is, the rider can counter any changed the moving of the rear wheel has caused by body positioning.
Also, not being consistant with body positioning can easly mask any small geometry changes.

GREAT reply Brian... at least, for me  ;)  I'm kind of a maff and physics geek to begin with... THIS is the kind of explanation I am always looking for.  With this, I now understand how a gearing change can effect things.. and from this, I can use to understand other stuff.  It's another missing piece to the puzzle in my mind that is motorcycle suspension.  

[KBOlsen]

 ??? I think I get it... just as long as I don't have to 'splain it.

Thanks, Jeff, Dave and Brian!

[Jeff]

That is, the rider can counter any changed the moving of the rear wheel has caused by body positioning.
Also, not being consistant with body positioning can easly mask any small geometry changes.

Amen to that!  When racing, the most tunable component relating to suspension is the rider.

[RandyD]

Free (or static sag) essentially compensates for the weight of the bike on the forks and rear shock.  Its the first thing you set when you put in different springs.

The important measurement is rider sag, which makes sure that springs have the correct pre-load for your weight.  You need the free sag in order to deteremine the rider sag.  

Ultimately, free sag is just a tool to get you to the correct rider sag.

[Admin]

Free (or static sag) essentially compensates for the weight of the bike on the forks and rear shock.  Its the first thing you set when you put in different springs.

The important measurement is rider sag, which makes sure that springs have the correct pre-load for your weight.  You need the free sag in order to deteremine the rider sag.  

Ultimately, free sag is just a tool to get you to the correct rider sag.

Now I'm confused again.  So rider sag is really what I should be looking at, not free sag?  Obviously (to me), if I set free sag, and then rider sag, my initial free sag is overridden by whatever changes I make with rider sag, since pre-load determines the sag regardless of rider or not.

So, let me ask the question another way...should I set free sag or rider sag (costly sag?).  If I should set sag with rider, what should it be?