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Old 19-06-2019, 09:14 PM   #1
Uncle Bob
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I still think it's worth checking the static sag in conjunction with dynamic sag. If the static sag is insufficient, you could experience excessive topping-out even if your dynamic sag is within the recommended range.

This is what happened when I first fitted an Öhlins DU737 with a 115N/mm spring to my Evo. The sag figures recommended by Öhlins were minimum 10mm static and 25-40mm dynamic. Out of the box, there was virtually zero or 1-2mm static sag but I didn't adjust it because the dynamic sag with me on board was within the recommended range (about 26mm). The ride was not very smooth and I subsequently backed off the preload to give 10mm of static sag while still staying within the recommended dynamic sag range.
Erm, what's the difference? I mean, all you've actually done is set the rider sag to 36mm, no?

What does it matter what the bike suspension does without a rider on it?! Why is it even relevant? That's what I still don't get.
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Old 19-06-2019, 09:45 PM   #2
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Erm, what's the difference? I mean, all you've actually done is set the rider sag to 36mm, no?

What does it matter what the bike suspension does without a rider on it?! Why is it even relevant? That's what I still don't get.

Simples really, if you have no static sag your bike will top out with a Bang in some instances, for example if you hit a small crest at speed, your suspension will top out, not good.
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Old 19-06-2019, 10:14 PM   #3
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Simples really, if you have no static sag your bike will top out with a Bang in some instances, for example if you hit a small crest at speed, your suspension will top out, not good.
Well done, Chris! You managed to explain in four lines what I took half-a-page to achieve!

I'm often accused of being a windbag - here's the proof!
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Old 19-06-2019, 10:30 PM   #4
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Well done, Chris! You managed to explain in four lines what I took half-a-page to achieve!

I'm often accused of being a windbag - here's the proof!
Bless you kind sir
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Old 20-06-2019, 08:22 AM   #5
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Simples really, if you have no static sag your bike will top out with a Bang in some instances, for example if you hit a small crest at speed, your suspension will top out, not good.
With respect, I think you might be incorrect there. Static sag alone will not stop the suspension topping out. The suspension can still top out regardless of what your preload is set to, it's not a 'top out prevention device'.

One of the reasons for setting 'sag' is so that with your weight on the bike the forks\shock have enough travel\range to decompress as well as compress. But they can both still technically top out.

If preload, and therefore (rider) sag, is set correctly, there will be enough travel in the fork\shock for the suspension to move up and down, then compression\rebound damping controls the rate at which that happens.

If all that is setup properly (in combination with the correct spring) then top out\bottom out shouldn't happen, but none of that really has anything to do with static sag. As far as I can tell anyway.
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Old 20-06-2019, 09:32 AM   #6
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With respect, I think you might be incorrect there. Static sag alone will not stop the suspension topping out. The suspension can still top out regardless of what your preload is set to, it's not a 'top out prevention device'.

One of the reasons for setting 'sag' is so that with your weight on the bike the forks\shock have enough travel\range to decompress as well as compress. But they can both still technically top out.

If preload, and therefore (rider) sag, is set correctly, there will be enough travel in the fork\shock for the suspension to move up and down, then compression\rebound damping controls the rate at which that happens.

If all that is setup properly (in combination with the correct spring) then top out\bottom out shouldn't happen, but none of that really has anything to do with static sag. As far as I can tell anyway.

Sorry Bob, but having been involved in racing myself and helping others with bike setups, builds etc, you NEED static sag, it suspends the weight of the bike before the rider gets on.
If you have no static sag on the front of the bike it will be nose high, will not corner correctly etc etc, really it will just run you off the road.
Don’t take my word for it give Maxton, K-Tech, Ohlins, Wilbers etc a ring and ask them.
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Old 20-06-2019, 10:06 AM   #7
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If you have no static sag on the front of the bike it will be nose high
Are you sure? What if there's no static sag on the shock either? Will the nose still be high then? ;-)

As per one of my last posts, I think what's happening here is that people are hanging on to snippets of information they've got about elements of suspension setup without necessarily considering the role they play in the overall setup, or the effects that other parts of the setup have on them.

It was just a question though (about why people think static sag is relevant). It'll be interesting to get the views of all those people you mention, but no doubt there will be differences of opinion from them too.

EDIT: Plus it's important to note when we're discussing this, I refer to 'static sag' as the amount the bike suspension sinks under its own weight with no rider, and 'rider sag' as the amount it sinks with you on it. I get the impression that the terminology is being mixed up.

Last edited by Uncle Bob; 20-06-2019 at 11:41 AM..
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Old 20-06-2019, 11:45 AM   #8
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I'm in no way (and certainly wouldn't profess to be) a suspension expert. I rely on proven experts like the two I've quoted above if I want to know anything. So a few words on their qualifications...

Niall Mackenzie is a successful former racer with 7 500GP podiums and 3 BSB championships. He has a wealth of experience on a wide range of machines under a wide range of conditions.

Tony Foale is a professional engineer, Bachelor of Technology and Master of Engineering Science. Sometimes, when a person is an acknowledged expert on something, people say "Oh, he wrote the book on that". Well, when it comes to motorcycle suspension and chassis design, Tony Foale actually DID write the book on that! (Check out his website https://motochassis.com/ there's a lot of useful information and interesting articles there.)

When people with that experience and those qualifications say the same thing, I tend to believe them.

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Yeah but that's the STATIC i.e. without rider position! It's the suspension's position WITH a rider that's way more important! Unless the bike is going around on its own it's a bit irrelevant.
Not actually irrelevant because, although the suspension may be compressed with the rider on board, that is not a constant state. There are times when the rider's weight is effectively removed from the equation. For example

• (on the rear shock), under heavy braking when all weight is transferred to the front
• (at both ends), going over a rise in the road like a hump-back bridge
• (at both ends), dropping into a hollow or pothole

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As for the second point, it's kind of true, but that's what rebound damping is supposed to control.
Up to a point, but if you're increasing rebound damping to control the results of insufficient static sag, you'll just be creating other problems elsewhere. For example, on a washboard surface, the suspension may tend to "pump down" over a series of bumps as the spring fails to return to it's static position before hitting the next bump, perhaps eventually even bottoming out.

Better to minimize the topping-out by setting the correct sag than masking the problem with excessive damping.

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Static sag alone will not stop the suspension topping out. The suspension can still top out regardless of what your preload is set to, it's not a 'top out prevention device'.
Correct, any suspension set-up can top- or bottom-out given the right extreme conditions. But, by having the correct static sag, you will minimize the times that happens. That's what Tony Foale wrote:

"• The static loaded position of the suspension is closer to the rebound stops, and so there is less suspension movement available before topping-out. So that occurs more often."

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If preload, and therefore (rider) sag, is set correctly, there will be enough travel in the fork\shock for the suspension to move up and down...
Also correct. But how do you know when it's set correctly? With my DU737, the "correct" setting was between 25-40mm so my original 26mm was "correct" according to the book. It wasn't until I also checked the static sag that I was able to arrive at the correct setting for my weight. Correct rider sag will, by definition, also include the correct static sag.

While suspension experts like the one Mossleymonster visited will no doubt be able to set the correct dynamic sag without also checking the static sag, the fact that they haven't checked it doesn't mean there isn't any or that it's not important to the overall set-up. I would still recommend that the home twiddler, without the experts' training and experience, still checks both figures to ensure that the final set-up is correct.
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Old 20-06-2019, 03:50 PM   #9
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I'm in no way (and certainly wouldn't profess to be) a suspension expert. I rely on proven experts like the two I've quoted above if I want to know anything. So a few words on their qualifications...

Niall Mackenzie is a successful former racer with 7 500GP podiums and 3 BSB championships. He has a wealth of experience on a wide range of machines under a wide range of conditions.

Tony Foale is a professional engineer, Bachelor of Technology and Master of Engineering Science. Sometimes, when a person is an acknowledged expert on something, people say "Oh, he wrote the book on that". Well, when it comes to motorcycle suspension and chassis design, Tony Foale actually DID write the book on that! (Check out his website https://motochassis.com/ there's a lot of useful information and interesting articles there.)

When people with that experience and those qualifications say the same thing, I tend to believe them.



Not actually irrelevant because, although the suspension may be compressed with the rider on board, that is not a constant state. There are times when the rider's weight is effectively removed from the equation. For example

• (on the rear shock), under heavy braking when all weight is transferred to the front
• (at both ends), going over a rise in the road like a hump-back bridge
• (at both ends), dropping into a hollow or pothole



Up to a point, but if you're increasing rebound damping to control the results of insufficient static sag, you'll just be creating other problems elsewhere. For example, on a washboard surface, the suspension may tend to "pump down" over a series of bumps as the spring fails to return to it's static position before hitting the next bump, perhaps eventually even bottoming out.

Better to minimize the topping-out by setting the correct sag than masking the problem with excessive damping.



Correct, any suspension set-up can top- or bottom-out given the right extreme conditions. But, by having the correct static sag, you will minimize the times that happens. That's what Tony Foale wrote:

"• The static loaded position of the suspension is closer to the rebound stops, and so there is less suspension movement available before topping-out. So that occurs more often."



Also correct. But how do you know when it's set correctly? With my DU737, the "correct" setting was between 25-40mm so my original 26mm was "correct" according to the book. It wasn't until I also checked the static sag that I was able to arrive at the correct setting for my weight. Correct rider sag will, by definition, also include the correct static sag.

While suspension experts like the one Mossleymonster visited will no doubt be able to set the correct dynamic sag without also checking the static sag, the fact that they haven't checked it doesn't mean there isn't any or that it's not important to the overall set-up. I would still recommend that the home twiddler, without the experts' training and experience, still checks both figures to ensure that the final set-up is correct.
Your absolutely spot on in my opinion , makes the most sense to me .
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Old 19-06-2019, 10:08 PM   #10
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Erm, what's the difference? I mean, all you've actually done is set the rider sag to 36mm, no?
Yes, but in increasing the dynamic sag, it also resulted in adding static sag that hadn't existed before.

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What does it matter what the bike suspension does without a rider on it?! Why is it even relevant? That's what I still don't get.
As for its relevance, I can't do any better than quote the two bullet points from Tony Foale's explanations above:

With insufficient static sag (my words):

"• The static loaded position of the suspension is closer to the rebound stops, and so there is less suspension movement available before topping-out. So that occurs more often.
• The preload force means that when the suspension does top-out, it does so in a harder fashion."


Niall Mackenzie describes what that feels like to the rider and his experience tallies exactly with the recorded behaviour of the wheel in those circumstances as shown in Tony Foale's graph.

As I mentioned when I was describing my own experience, it's quite possible to set your dynamic sag within the recommended range and yet still have no or minimal static sag so I always now check the two and make sure they are both within their recommended ranges.

I would add that the same amount of factory set preload that gave me zero static sag on the Öhlins DU737 with the 115N/mm spring, gave the perfect static sag with a 105N/mm spring.

Not sure I can add any more so I hope that helps.
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Old 19-06-2019, 11:13 PM   #11
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Yes, but in increasing the dynamic sag, it also resulted in adding static sag that hadn't existed before.



As for its relevance, I can't do any better than quote the two bullet points from Tony Foale's explanations above:

With insufficient static sag (my words):

"• The static loaded position of the suspension is closer to the rebound stops, and so there is less suspension movement available before topping-out. So that occurs more often.
• The preload force means that when the suspension does top-out, it does so in a harder fashion."


Niall Mackenzie describes what that feels like to the rider and his experience tallies exactly with the recorded behaviour of the wheel in those circumstances as shown in Tony Foale's graph.

As I mentioned when I was describing my own experience, it's quite possible to set your dynamic sag within the recommended range and yet still have no or minimal static sag so I always now check the two and make sure they are both within their recommended ranges.

I would add that the same amount of factory set preload that gave me zero static sag on the Öhlins DU737 with the 115N/mm spring, gave the perfect static sag with a 105N/mm spring.

Not sure I can add any more so I hope that helps.
Just for my own satisfaction I'm going to pass all of this on to Daz at MGR and see what he thinks. He is Ohlins trained.
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Old 20-06-2019, 08:33 AM   #12
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"• The static loaded position of the suspension is closer to the rebound stops, and so there is less suspension movement available before topping-out. So that occurs more often.
• The preload force means that when the suspension does top-out, it does so in a harder fashion."
Yeah but that's the STATIC i.e. without rider position! It's the suspension's position WITH a rider that's way more important! Unless the bike is going around on its own it's a bit irrelevant.

As for the second point, it's kind of true, but that's what rebound damping is supposed to control.

Reading this thread it seems that emphasis is perhaps being put on certain elements of suspension setup and not considering the combined effects of the actual equipment and how it's setup as a whole.
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Old 20-06-2019, 09:26 AM   #13
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Yeah but that's the STATIC i.e. without rider position! It's the suspension's position WITH a rider that's way more important! Unless the bike is going around on its own it's a bit irrelevant.
.
Sorry, but I disagree completely, if you have no static sag you are compromising the bikes ability to use it’s suspension safely, believe me, you do not want your bikes suspension topping out when the suspension goes light when you are at speed etc.
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Old 20-06-2019, 09:53 AM   #14
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Sorry, but I disagree completely, if you have no static sag you are compromising the bikes ability to use it’s suspension safely, believe me, you do not want your bikes suspension topping out when the suspension goes light when you are at speed etc.
Having no static sag does not mean your bike will definitely top out, unless the bike is riding itself around. Is it?
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