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.

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.

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.  

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!

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.

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.

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.

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?

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?

this works for measuring free or total sag

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.