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Discussion Starter · #1 ·
Why is static sag so important?

AFAIK static sag consist of a certain amount of travel or stroke calculated with the difference between the motorbike suspension being fully extended and then dropped with its own weight.

I saw a lot of videos from Dave Moss and he always pull up the rear of the bike to determine if there's a correct amount of static sag or it's almost non existent.

The problem is that for heavy guys with the proper spring the motorbike weight will always be pulled up, and the stiffer the spring is, the less the bike will come down, reducing the amount of static sag.

But all this is is totally independent from rider sag, because in order to reach proper rider sag numbers with the proper spring and preload, you will ways sacrifice static sag. If somebody tells me I have almost no static sag and tries to take out some of my preload, it will obviously increase my rider sag.

So I would like that somebody explains me this kind of dilemma, and where is the trick.
 

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If you have the correct spring, it'll all equal. Correct rider sag with no static sag indicates a too soft spring requiring the preload to be cranked down too far in order to get the needed loaded sag to allow the proper static sag..
 

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Discussion Starter · #3 ·
If you have the correct spring, it'll all equal. Correct rider sag with no static sag indicates a too soft spring requiring the preload to be cranked down too far in order to get the needed loaded sag to allow the proper static sag..

How can two different rear shocks with different spring rates, from fully extended to only compressed through the weight of the bike, produce the same valid free / static sag for the same bike?



I am just trying to understand it.
 

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How can two different spring rates produce the same valid free / static sag for the same bike?

I am just trying to understand it.
They won't if measure correctly and all other inputs are equal..
What year bike do you have? What shock are you using? How much do you weigh? What spring type/weight are you using?
 

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Some static sag or bike sag (some call it top out) is necessary so that when you come off the power into a corner and are under hard braking the rear wheel still maintains contact with the road/track. Otherwise the shock 'tops out' (has run out of extension) and it starts lifting the rear wheel instead.

If you've got the same bike and shock but two different springs to try out then the one with the higher spring rate will need less preload wound on to get your desired static sag (10 - 15mm) than the spring of softer rate. Clear as mud? :giggle:
 

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Just FWIW, static sag is handy when you encounter a pothole or any surface anomaly that is the reverse of a bump. If there wasn't any sag, the entire bike would dip or the front tire would lose contact for some amount of time.
 

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Just FWIW, static sag is handy when you encounter a pothole or any surface anomaly that is the reverse of a bump. If there wasn't any sag, the entire bike would dip or the front tire would lose contact for some amount of time.
If you're encountering a pothole, you're probably riding, so the 'rider sag' value would be relevant.
 

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You're right, GeorgeRYoung (I can't remember how to quote). After I thought about it, I realized that I have only measured static sag to determine stiction, and that was a differential comparison. The only sag I really pay attention to is the rider sag.

So pay no attention to what I said. I really don't have any meaningful input on the subject. I just tried different spring rates until everything felt right without bottoming either end.
 

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Discussion Starter · #9 ·
If you're encountering a pothole, you're probably riding, so the 'rider sag' value would be relevant.

That's my point.

Once a rider is on the bike, rider sag will "compress" the bike, there's no need, IMO, for a predefined static sag value. If braking forces or potholes are enough to lift the rider out of the seat, static sag will do nothing or very little to help. We see constantly on racing how the rear end goes up in the air when hard braking. 10 or 15mm top out margin is for me irrelevant because of the nature of riding is simply unpredictable. Nobody can guarantee that those 15mm will not be used.
 

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Until I got my first SV650 in 2015, pretty much all of my riding experience was on 1970s bikes, with their skinny tyres, poor suspension, flexible frames etc. I got so familiar with letting the bikes wander, weave and twitch around corners (and in a straight line too) that when I rode an SV, to me the handling and roadholding were amazing.

Maybe if Dave Moss set up my current SV's suspension for me, I might think that was an amazing advance compared to the stock set-up, but until then I'll just ride and not worry about it :)
 

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Once a rider is on the bike, rider sag will "compress" the bike, there's no need, IMO, for a predefined static sag value. If braking forces or potholes are enough to lift the rider out of the seat, static sag will do nothing or very little to help. We see constantly on racing how the rear end goes up in the air when hard braking. 10 or 15mm top out margin is for me irrelevant because of the nature of riding is simply unpredictable. Nobody can guarantee that those 15mm will not be used.
Only at the very top level like Moto GP riders and the likes of Toprak Razgatlioglu and Jonathan Rea in World SBK's. Most riders cant control a bike under heavy braking once the rear wheel leaves the ground. Generally it comes round on them trying to swap places with the front wheel at which point the rider gets off the brakes and it drops to the ground at a weird angle and flicks them off. What's more common to see though is a racers bike fish tailing into corners on the brink of out of control because the rear end has gone light under brakes, possibly because of a lack of static sag but not always. However, your in a much better position if you do have a good amount of static sag dialed in to help maintain rear wheel grip.
This observation is based on seven years of racing SV's. If you choose to think this info irrelevant then so be it.
 

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Discussion Starter · #12 ·
Only at the very top level like Moto GP riders and the likes of Toprak Razgatlioglu and Jonathan Rea in World SBK's. Most riders cant control a bike under heavy braking once the rear wheel leaves the ground. Generally it comes round on them trying to swap places with the front wheel at which point the rider gets off the brakes and it drops to the ground at a weird angle and flicks them off. What's more common to see though is a racers bike fish tailing into corners on the brink of out of control because the rear end has gone light under brakes, possibly because of a lack of static sag but not always. However, your in a much better position if you do have a good amount of static sag dialed in to help maintain rear wheel grip.
This observation is based on seven years of racing SV's. If you choose to think this info irrelevant then so be it.

I don't find this information is irrelevant per se, I'm just looking for the technical explanation. When you answer "rear end will have more grip margin because it stays on the floor", I answer "that value is already included in rider sag."
A rear shock does not understand about rider sag or static sag. It can be fully extended, partially compressed, or fully compressed. Static sag is no second adjustment like lo or hi rebound or lo-hi compression.
Its just the first part of the rider sag stroke.
That's the way I see it.
 

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As I understand it, if you can't set static sag to about 10mm and rider sag to about 35mm, the spring rate is wrong for your weight and should be changed. And it's setting the spring rate appropriately which improves the traction and ride.
 

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I just wrote a little article on the subject:


OK, there seem to be plenty of myths and misunderstandings around suspension sag. I’ll try to explain it as thoroughly as I can while still keeping it simple:

First of all, a few definitions:


Sag = How much the weight of the bike + the weight of the rider compress each suspension, front and rear, from fully extended.

Dynamic sag = The sag of the suspension as the bike is moving. Because it’s virtually impossible to measure dynamic sag — not only the bike is moving but the suspension compression is always changing due to bumps, dips as well as acceleration and deceleration — we need to measure it while the bike is motionless. This brings us to…

Static sag = The sag of the suspension with the bike stopped, measured with the rider on the bike (fully geared), the motorcycle being held upright and the rider in riding position. More on how to measure it a little later.

Free sag = How much the suspension sags without the rider. In most situations, there is no need for measuring the free sag as you will never let your bike go without you on top! However, there are some occasions in which knowing the free sag is helpful, especially when you suspect the spring is simply too stiff... but more on that later.


To understand why you need sag and how much you need, ask yourself this question: What are you more likely to find on the road, bumps or dips?
A: BOTH!

Remember, the main function of your suspension is to keep your tires firmly and uniformly pressed against the ground so you always have good traction. Therefore, in a perfect world, you should aim to always have your suspension right in the middle of its available travel, this way it can be able to compress over bumps and extend over dips, maintaining this ideal traction at all times.

Then why is it that most suspension experts say that an ideal static sag is in the 35% vicinity when what we really want is a dynamic sag of 50%? Well, ask yourself another question: Where in the road do you want to have the best traction available?
A: In the curves!

… and, because in the middle of a curve both you and your bike effectively weight more (due to the centrifugal G force pushing you in the lean) it is expected that your suspension will compress that additional 15%, putting it in that happy middle and providing you with the best traction possible.

BAM! Now you understand sag.



Of course, the 35% recommended static sag is not set in stone as there may be other factors you may want to play with. For example, if you ride very fast in the corners (spirited canyon chaser, track rider, etc.), you will be adding more centrifugal force while cornering than a regular street rider, so maybe you want to set your static sag to 30% or even 25% so you can achieve the 50% —dynamically— in the corners.

Another fun factor to play with is having different sag measurements, front and back. For example, you can have the bike sag a little more in the front to make it dive into turns faster or vice-versa to make it more stable and less twitchy. I recommend playing with this only after you have ridden a while with equal sag front and back, so you understand better the attitude of your bike in the corners.


Anyway, I have father’s day to celebrate so I will write about the right way of measuring sag (and the importance of proper springs) a little later.

Ride safe!
 
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Discussion Starter · #15 · (Edited)
I just wrote a little article on the subject:

"Free sag = How much the suspension sags without the rider. In most situations, there is no need for measuring the free sag as you will never let your bike without you on top!"

And that's exactly my point.

I don't understand why should be incorrect when a rear shock set up for a 300lb guy doesn't allow the motorbike to sink down by its own weight, or only very few mm, if that shock is the one he needs to have a proper rider sag. Almost top out limit, almost fully extended, and very little free sag, it's everything the same consequence of a high spring rate
 

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"Free sag = How much the suspension sags without the rider. In most situations, there is no need for measuring the free sag as you will never let your bike without you on top!"

And that's exactly my point.

I don't understand why should be incorrect when a rear shock set up for a 300lb guy doesn't allow the motorbike to sink down by its own weight, or only very few mm, if that shock is the one he needs to have a proper rider sag. Almost top out limit, almost fully extended, and very little free sag, it's everything the same consequence of a high spring rate
Correct. The best approach for any rider —but especially for those in the extreme weight ranges— is to find a reliable spring rate calculator (both Race Tech and Sonic Springs have good online calculators although Race Tech's tend to recommend on the stiffer side, in my experience), install the right springs and set your static sag in the 30%-35% range as a starting point. If your spring rate is correct, you shouldn't have to worry about free sag at all.
 
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