Free sag vs ??

Started by Admin, December 12, 2002, 11:39:26 AM

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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.
Super Dave

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.
Super Dave

Jeff

QuoteWhat 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)
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tzracer

QuoteIt 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.
Brian McLaughlin
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OmniGLH

#7
Quote---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.  

Jim "Porcelain" Ptak

KBOlsen

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

Thanks, Jeff, Dave and Brian!
CCS AM 815... or was that 158?

Jeff

QuoteThat 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.
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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

QuoteFree (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?

tzracer

QuoteNow 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?

You should ask Max. Since you are setting free sag (no rider) I would think that Max has compensated for your weight.

Brian McLaughlin
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Eric Kelcher

Oops

the relationship between free sag and total sag tells you the correct spring. If you have the correct spring you can set either and the other will be correct. You compare the two to sort out spring rates. not enough free sag with proper total sag then you have to light of a spring,too much free sag with proper total sag=too heavy a spring. I compare the two as a spring wears out over time and should be replaced and this is easy way to check it.
Eric Kelcher
ASRA/CCS Director of Competition

Jeff

QuoteNow 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?

You need to be concerned with both.  If you end up getting your rider sag set, and there is no free sag left in the bike, your spring rate is not correct.

Find a suspension TUNER who can work with you over the season to manage this for you.  
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Admin

QuoteOops

the relationship between free sag and total sag tells you the correct spring. If you have the correct spring you can set either and the other will be correct. You compare the two to sort out spring rates. not enough free sag with proper total sag then you have to light of a spring,too much free sag with proper total sag=too heavy a spring. I compare the two as a spring wears out over time and should be replaced and this is easy way to check it.

Ahh...now this makes sense to me!  Thank you!

Super Dave

QuoteI 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.

Brian, I have always been polite to you.  I'm pretty darn polite, in general.

However, I find you, and this statement to be condesending, in addition to the other posts that you make when I'm trying to help.

I will be doing some work over next several days, and I will make a post on this thread.
Super Dave

max@traxxion.com

QuoteOops

the relationship between free sag and total sag tells you the correct spring. If you have the correct spring you can set either and the other will be correct. You compare the two to sort out spring rates. not enough free sag with proper total sag then you have to light of a spring,too much free sag with proper total sag=too heavy a spring. I compare the two as a spring wears out over time and should be replaced and this is easy way to check it.


A+ for Mr. Kelcher.  Absolutely correct.

Both are very important, and interrelated.

I need to add that we have gone to telling people to set their free sag and ride it.  We never, ever, find properly set sag when people try and do it with a rider.  It sounds easy, but apparently it isn't.  The thing that screws everybody up is stiction in the suspension system.  You get different numbers every time, with little accuracy unless you know how to account and compenstate for it.

Invariably, people set their springs too stiff.  So set it up for free sag, and then leave it alone.  I use damping and geometry adjustments after that.

max@traxxion.com

Mr. Physicist has made common mistakes that most engineering types make by analyzing the static properties of an object that only functions dynamically.

I can assure you that changing your sprocket has a massive and noticeable effect on the chassis of your bike.

There are two noteworthy items here, one of which SD touched on.  The first is the "wheel rate" and the other is the mechanical "anti-squat" properties of your bike.

If you simply adjust your chain, you have altered all of this.

In SD's example, moving the wheel forward shortens the length of the lever that is applying force to the spring.  This affects the bike  only slightly in a static situation (which is what left the physicist confused), but still enough that it will alter your sag.  You will have to loosen the spring up in this example to restore the free sag.  

Note also that changing your swingarm angle (rear ride height) has this same effect.  As the swingarm gets steeper, it will have less leverage on the spring...).

Anyway, subtle changes in wheel rate in the pits make big changes on the track.

Depending on a combination of factors, you the result will be a nearly infinite number of outcomes, many unexpected.  The factors are swingarm angle, chain angle (dictated by sprocket choice), torque applied by the motor, spring rate, linkage ratio, wheel rate, distance the swingarm pivot is from the centerline of the countershaft sprocket, and distance from the swingarm pivot to the center of the rear axle.

As you can imagine, there are already millions of possibilities with just those factors.


The "Angle of Anti-Squat" is an imaginary line "roughly" located between the centerline of the swingarm, and the line of the drive chain.  This angle is measured from horizontal.  So your angle of antisquat will be less than your swingarm angle, for this discussion.

So, it may be possible to go to a bigger sprocket which would move the wheel forward reducing leverage, making a higher wheel rate.  But, at the same time, the angle of anti-squat is reduced, potentially dynamically cancelling out the effective change in sprocket.

So grab your slide rules and fire up your calculators, but you are not going to predict accurately what will happen with all of this.  I have been around the top teams in the world, and while they have estimates of what will happen, the way they find out is by putting a rider on the bike and sending it around the track for a half an hour.

Motorcycle R&D has not evolved to the point where engineers can predict on paper what will happen on a racetrack.  It will someday, when robots drive bikes.  Until then, as long as humans drive the machinery, no scientist will ever predict anything accurately.


tzracer

Seems I have stepped on some toes with my reply in this thread.

Dave I am sorry if I insulted you in any way. The reason I replied was because I found your post confusing, it was somwhat unclear exactly what was happening to the CG. I then did a calculation to see what did happen.

Maybe I am just not a good enough rider, but on my TZ, I don't notice a change in the handling when changing the rear sprocket by a tooth or two. What should I be noticing? What changes should I make to restore the handling? Does it differ significanly from bike to bike (is this perhaps why I don't notice it)?

Max I do like the switch to static sag vs sag with the rider. Can stiction be a problem when measuring static sag or is it so little that it is not significant?
Brian McLaughlin
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Eric Kelcher

To measure correct sag you MUST account for stiction. To measure correctly you take bike and press down then slowly allow bike to raise. Measure. Then you lift bike and slowly allow it to settle. Measure. Take the two measurements and find average. You also want to record what the differnce is between the two is, this is your stiction. There will be more in the front(you have two "dampner rods" to one in the rear) The exact amount of stiction acceptable varies from machine to machine and brand of fork/shock before rebuild is needed.  

Yep Max I read and remember your series in RRW from couple years ago. I wish I could find some of those articles. Are they on the net anywhere?
Eric Kelcher
ASRA/CCS Director of Competition

Eric Kelcher

this works for measuring free or total sag
Eric Kelcher
ASRA/CCS Director of Competition

max@traxxion.com

Eric, you got it 100% right except for one detail...

The reason there is more stiction in the fork, is because there is WAY more sticky dragging things in the forks.

There are two huge rubber seals, two huge rubber dust seals, four huge teflon bearings (that are frequently no parallel with each other exacerbating this situation).  Then, the two opposing tubes are held at an angle so they cock and load against each other.

In the rear, there is only one small shaft, and a small rubber seal.  There is greater pressure on this seal due to the nitrogen charge, but it is also has 2-3 times the leverage on it, so stiction is way less.

In forks, 3-8 is ok, 10mm stiction, you have a problem you need to find.

In the shock 2-5 is normal, outside of that, you need to find a problem.

For TZ racer, note how the bike leaves the turn.  That is where the biggest change will take place.

I measure with static sag, because if I let people measure by rider sag, they will end up with no free sag, particularly in the shock.  They will have no free sag.  So when I say, set the free sag to 10mm and ride it, that is pretty hard for anybody to F up.  We run 20mm in the fork.