Significance of static sag?

[stopintime]

I'm in the process of tuning my new fork (ST4 Showa with GSXR internals).

Quite rough measurements at this point.... ~20mm static/free sag (from topped out to bike only weight) and ~37mm with me on it.

Other than ride height/geometrical concerns, how does this somewhat generous free sag affect performance?

[ducpainter]

Just me talking, so take it with a half grain of salt...

what does it matter if the static sag is excessive as long as the rider sag is correct? Are we concerned how the bike will handle after we fall off?

Now as to your numbers...you want rider sag to be less...maybe 30 in my experience.

[stopintime]

Salt Man 

My rear rider sag is just a little less, so there's balance front/rear to concider as well as individual numbers.

I'm used to the fork compressing generously, never bottoming though, and have been ok with that. Even if the bike would be 'better' with smaller numbers, it might be wise to change things a little at a time?

This smells stiffer springs - unless I find a way to reduce the compressing forces 

[Speeddog]

You're fine, run it.

[ducpainter]

Salt Man 

My rear rider sag is just a little less, so there's balance front/rear to concider as well as individual numbers.

I'm used to the fork compressing generously, never bottoming though, and have been ok with that. Even if the bike would be 'better' with smaller numbers, it might be wise to change things a little at a time?

This smells stiffer springs - unless I find a way to reduce the compressing forces 

If you're comfortable with the way things feel then...

You're fine, run it.

[EEL]

So i'll start with ye old YMMV but based on my experience and research on suspension, the free sag levels are a gauge for accurate spring stiffness. The sag of the bike without rider needs to be measured AFTER the desired rider sag is set.

generally between 5 - 10mm of static sag is acceptable for the front forks. Essentially what i've gleaned from my research and discussions is that springs (even linear ones) dont necessarily act in a perfectly ideal linear manner. They have a range where they function linearly. You want to select a spring rate that sits in this range.

If you have too light a spring, you will have to wrench down on the preload to get proper rider sag out of the forks. The side-effect of this is your free sag level is small (low) with more preload added.

If you have too stiff a spring, your level of preload for rider sag will be conversely significantly less. In turn, the free sag will be higher.

20mm of free sag suggests that the spring rate is too stiff for your bike. I dont know what the weight of a GSXR is vs a ST4 but L-Twin configuations tend to have less weight on the front than in line 4's

This may explain the difference

I've added a link below for a PDF for some good information and a table you can use to set your suspension better.

Hope this helps.

[EEL]

Also a couple of other things:

1) Did you account for stiction? 20mm free sag needs to be the average of the forks when they spring back from being pushed down or pulled up and subsequently released slowly back to their resting state.

If you did it simply by dropping the forks down from their unlaiden state and measuring, then you might have a value that's too conservative.

2) Did you take your measurements after a healthy dose of riding and with gear on? The suspension needs to be heated. Viscosity values and change results sometimes when the suspension is cold vs hot.

[stopintime]

I have that document (maybe it's in the book)

It's clear that I have little added preload. With ~37mm rider sag and little preload I'm close to a good spring rate though. The 20mm could indicate stiff springs, but the 37 and the fact that I use 80+% of my travel points towards a soft'ish spring. The springs are 10.4 N/mm, so even if I'm a big guy they should be ok.

I'll measure properly later - stiction is not accounted for and both my 20 and 37 are very rough.

The GSXR springs are 20mm longer than Öhlins, the rod is thicker than ST4 Showa and the damping adjustment range is wiiide.

I hit a pretty good street set up today. I had way too much compression damping and reduced it by a full turn (=4 clicks I guess) which made it 'comfortable', planted and predictable. Made a few adjustments on the shock and have good balance between front and rear dynamics now.

I'm still amazed by how much cold vs warm suspension components means. I've heard Dave Moss talk about 2-3-4 clicks changes from cold to warm and thought 'come on', but there are big differences that even I can feel.

I have a track day next weekend, so I'll be able to evaluate under full load  

[Speeddog]

The free sag and rider sag are indeed a gauge for proper spring rate, but in my experience, there's a large slice of reality that intrudes  rather rudely on that.

Firstly, getting *consistent* sag readings on multiple attempts is very difficult.
There's enough stiction in even good forks that you're going to get variation.
That's the reason for pulling the bike up, slowly letting it settle, then measuring, then push down, and slowly letting it rise, then measuring, and then averaging those two numbers.
That helps eliminate some of the error due to stiction.

The other issue is measurement accuracy itself.
If you use a millimeter scale and a good naked eye, and are very careful, it's possible to measure a distance to +/- 0.5mm of the actual value.

I'll show how disastrous that measurement inaccuracy is.

I'll use Lars' numbers of 20/37 sag, and assume that when Lars climbs aboard, it adds 50 kg of load to the front suspension.
I'm going to neglect the rake of the front forks in this calculation, as that's a geometry factor that doesn't change for this comparison.

Measuring dead-on correct, with 50 kg load, the fork travels 17mm (37-20).
50 kg / 17 mm = 2.94 kg/mm

If you miss each measurement by 0.5mm, measuring free sag a little short, and rider sag a little long, the fork travel is 18mm (37.5-19.5).
50/18 = 2.78 kg/mm

Flip the measuring error the other way, and you get 16mm travel.
50/16 = 3.13kg/mm

So, due to just the measuring error alone, the numbers are +6.4% / -5.4%.
Taking those errors, and applying them to a 0.90 spring, you get 0.96/0.85.

If you get 'sloppy' measuring, and only get them +/- 1mm, then the errors are +13.4%, -10.5%.
That results in 1.02/0.80.

That's nearly the *entire* range of spring rates available.
So, I don't put a lot of stock in determining spring rates from sag numbers.
Predominantly, it's verifying that the springs aren't a mile off or something haywire inside or binding due to misalignment or bent parts.

I do feel significant difference in suspension performance due to temperature, and that's both oil viscosity changes and tire compliance.
My GF's 848 still has the stock shock on it, and it's virtually rigid when at room temperature.
Run the bike around a bit, stop and let it sit a few times, and get the shock smokin' hot, and it starts to have some compliance.

I also notice a very significant difference in how confidence-inspiring the fork is, depending whether I've got a full tank or nearly empty.
Feels a lot better on a full tank, as Monsters are a bit light on the front.

[stopintime]

You're fine, run it.

Who said that? 


Your math shows how difficult this is  Maybe actual dive in actual circumstances is better, but again personal preferences, geometry requirements and surface conditions will send us off in all directions.

I remember a local coach who told me how he sometimes helps riders... they complain about suspension issues, get told to leave the bike with him and go for a coffee, get out again on a tuned chassis, come back in with seconds faster lap times and be told 'I did nothing, it's mostly in your head'.

[ducpainter]

It's all in your mind.

If you don't bottom, and the suspension is compliant, and you're confident in the bike it's like Nick says...

Run it.

As I said before, I like a little less sag/stiffer spring with damping tuned to remove the harshness from our roads.

I wish the vet would chime in here. He's pretty good with suspenders.

[MonsterHPD]

I agree with most that has been said here. Sag measurements are wildly inaccurate and a guide at best.
I aim for 15 to 20 mm difference between empty and loaded sag, set the damping too lose and then ride and adjust until I like it. I think it's easier to get it right from too lose, since it is more obvious than too stiff. Might help the tires as well.
I use thin oil (around 20 cSt/sae 5 to 7) since it changes less with temperature.

Mostly, it's a matter of finding a reasonable setting that one feels confident with.

[SpikeC]

I am finding this very fascinating. My bike feels very stiff riding around town, but I'm thinking back to last summer blitzing the twisties on roads that sort of parallel 101 at the Oregon coast. The faster I rode the smoother the bumpy roads became. My friend on his BMW K1200rS was pregnant doging about bumps and craters and all I could think of was how much fun the road was and I could not remember the bumps! (Of course, his bike had 70k on the suspenders!)

[BK_856er]

I can't add anything not already stated by the far more experienced posters above, especially wrt measurement accuracy and repeatability.  Just remember that almost ANY spring will provide a suitable rider sag if the adjuster is cranked one way or another.  But that could be a really sucky and wildly inappropriate spring rate.  The added factor of free or static sag helps to assure that the spring is in its sweet spot.  Carefully measured spring pre-tension to achieve a particular rider sag is also an indicator.  Forks are kinda easy and generic across brands and applications - shocks and their various rocker geometries not so much.

BK

[stopintime]

Track day today. Suspension guy available.

We added two rings ( the rings on the preload stanchions(?) ), from fully out, and I got loads more contact with the front wheel   

Very pleased so far.

(there are possible issues with oil level, bouncing while extreme braking, but I'll adress that later)