What is Suspension Sag?

Courtesy of @Andyb (y) for what he calls a 'proper' suspension article, and posted on his behalf (please direct all questions to AndyB, not me, by tagging him in your post).

As so many questions return again and again about suspension sag i thought id add my non expert six penneth on the setting of the rear sag on my track bike.

Before you go wasting any time twiddling with damping adjustment, the spring and sag settings need to be correct and correct for YOU!

It is well documented around types of spring and spring rates that are suitable both for your kitted weight and the type of suspension set up you may have, so do your homework first or pay someone to advise you. The ohlins site has a very good description on it summed up in these 3 pictures

1 is the measurement of full travel of the shock and forks. You can also increase and decrease this on most ducatis with the ride height adjuster ( a common fault is where people measure the tie rod length or measure the ride height with weight on the springs!) This is where the ducati ride height datum tool gives good consistency, but you could also do this to a fixed point on the seat or subframe, ideally directly above the rear axle centre. As long as you use the same points for all three measurements you will get your numbers.

2 is the weight of the bike alone acting on the springs

3 is the weight of the bike and rider acting on the springs.

So in theory you get 3 measurements

AndyBOhlinsScreenShot.png

This is how it worked out on my bike. Given my gentle weight and the type of RS flat/linear suspension rocker and length of S/arm i was using i already knew my rear spring needed to be around 105/110
So a 110 was fitted to my ttxGP shock.

So measurement 1

AndyB_IMG_3323.jpg


Weight off the rear spring, tyre clear of the floor. This is also where I can set the rear ride height adjusting the tie rod in this instance to 245mm.

AndyB_IMG_3325.jpg

Next bike down on the ground under its own weight. This is the point you see suspension gurus lift the rear lightly and press it down lightly They are looking to eliminate Stiction where a light drag on the moving parts either way can affect the measurements. To be really accurate you can take both upper and lower number add and divide to get the average.

AndyB_IMG_3326.jpg
AndyB_IMG_3329.jpg

So my free sag, the weight of the bike compressing the spring. Down to 234 giving me 11mm of free sag.

Now to rider and bike or static sag, so with me on the bike (ideally in normal sitting position fully kitted) it's down to 214mm.

AndyB_IMG_3331.jpg

So to recap the numbers

ride height 245
free sag 234
rider and bike static 214

that gives me 11mm of free sag and a total of 31 static sag.
these are good number for my weight and set up the free sag means I'm not the second kid on the trampoline every time it goes over a bump and the static sag is correct meaning the spring is the right rate for this set up.
 

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Alan Williams

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@Andyb, apologies if this is a stupid question.

How does one know what the ‘correct’ values are for free and static sag ?

Is it dependant on what type of riding, ie road or track and does it go further than that, ie touring, commuting etc ?
 

Andyb

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So its fair to say that the use of the bike and the area its used would play a part in deciding how to best set up the suspension. A cruising bike say multi would be a softer ride than an all out track Panigale. The multi has the unevenness of the road to have to deal with far more than say the Panigale on a lap of Donington smooth tarmac. Hence there is the potential for softer springs and more suspension travel on the multi.
But for both set ups its always a compromise.

So the first measurement is the full available travel of the swing arm (or fork leg fully extended on the front) This can get confused if the shock has a top out spring but that's for another time. So wheel off the ground full extend and it is what it is.

The second measurement is about the weight of the bike on the front or rear. This isn't a straight 50/50 split though it could be as the weight in the bike can be played backwards and forwards to say 52/48 even 54/46.... as a generalisation the front is heavier than the rear. On a Ducati raising the rear ride height or lowering the yoke on the forks can move the weight too to achieve where you want that feel.
So where the percentage of weight is will have the effect of how much the spring is compressed front and rear, along with the strength of the spring and a few other variants. This second measurement is important as the suspension need an amount of free play beyond the rider. NO measurement here would mean the spring is too stiff.

The third measurement is with the fully kitted rider on the bike in a normal riding position. Remember your gentle 17 stone is an awful lot of weight moving around on the bike. So to answer your question this number should be around a third of the full shock or fork travel So with most sports bikes Ohlins front as an example these have 120mm travel which is where the rider sag of a third ie 40mm comes from for a starting point. The same rule could be applied to the rear..
 
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Alan Williams

Elite Member
Subscriber
So its fair to say that the use of the bike and the area its used would play a part in deciding how to best set up the suspension. A cruising bike say multi would be a softer ride than an all out track Panigale. The multi has the unevenness of the road to have to deal with far more than say the Panigale on a lap of Donington smooth tarmac. Hence there is the potential for softer springs and more suspension travel.
But for both set ups its always a compromise.

So the first measurement is the full available travel of the swing arm (or fork leg fully extended on the front) This can get confused if the shock has a top out spring but that's for another time. So wheel off the ground full extend and it is what it is.

The second measurement is about the weight of the bike on the front or rear. This isn't a straight 50/50 split though it could be as the weight in the bike can be played backwards and forwards to say 52/48 even 54/46.... as a generalisation the front is heavier than the rear. On a Ducati raising the rear ride height or lowering the yoke on the forks can move the weight too to achieve where you want that feel.
So where the percentage of weight is will have the effect of how much the spring is compressed front and rear, along with the strength of the spring and a few other variants. This second measurement is important as the suspension need an amount of free play beyond the rider. NO measurement here would mean the spring is too stiff.

The third measurement is with the fully kitted rider on the bike in a normal riding position. Remember your gentle 17 stone is an awful lot of weight moving around on the bike. So to answer your question this number should be around a third of the full shock or fork travel So with most sports bikes Ohlins front as an example these have 120mm travel which is where the rider sag of a third ie 40mm comes from for a starting point. The same rule could be applied to the rear..
Thanks Andy.

One more, potentially silly, question.

The previous owner had ‘played’ with the set up of my bike ( a 2000 900ssie). One thing that stands out is he had raised (is that the correct term) the forks through the top yoke.

Anyway my question is, what’s a good starting point, by this I mean should I try adjusting the front / rear to achieve say 52/48 weight distribution and then set the sag ?
 

Andyb

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go back to standard if that's described anywhere.

I would document measure a few things as is eg

height of fork top through yoke
rake of forks (if adjustable)
wheel base
rear ride height
weight bias
seat height

then measure full extension front and rear of forks and shock ( centre of rear spindle straight up to a constant point above)
find out how much full travel of forks and shock for the bike.

then go back to std

compare notes and feel
 

Alan Williams

Elite Member
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go back to standard if that's described anywhere.

I would document measure a few things as is eg

height of fork top through yoke
rake of forks (if adjustable)
wheel base
rear ride height
weight bias
seat height

then measure full extension front and rear of forks and shock ( centre of rear spindle straight up to a constant point above)
find out how much full travel of forks and shock for the bike.

then go back to std

compare notes and feel
Thanks again Andy.
waiting for a stand to turn up, then will make a start.
I will probably be back for more advice. 😬
 

Andyb

Elite Member
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I found using a front wheel transport chock good, and a undertake stand. Axle stands for the foot pegs clears the rear off the ground.
I would also do the three measurements where your bike is now with you.

Its quite interesting seeing what difference a small change makes.
 

Andyb

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I thought I'd make a video of how a man in a shed with no ruler can do it 😁. If I can, you can (y)


ps. The old sheets stretched across the shed are so you can't see all the crap I can't get rid off until the charity shops open again :(.
How do I set my rear ride height with the slack bladder?
 

West Cork Paul

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How do I set my rear ride height with the slack bladder?
You don’t, but you know that anyway. It’s used to measure and help set your sag not your ride height and not all bikes have ride height adjusters, but you knew that too.
 
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Andyb

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You don’t, but you know that anyway. It’s used to measure and help set your sag not your ride height and not all bikes have ride height adjusters, but you knew that too.
I notice your bike has a SSSA which of course has an eccentric hub to adjust the chain and by the very nature how this works, every time you adjust your chain you are changing the rear ride height which can have an impact on the geometry of the bike up or down. How do you keep that constant, how do you measure it to confirm the constant or datum?

but the starting point on a lot of Ducatis is at rear ride height,,,,,so if you have a ducati with an adjustable rear ride height or of course a ducati that you can change a connector to make the rear ride height adjustable.

then you need another tool....or get a tool that can do both......I wonder if Halfords sell anything?
 

West Cork Paul

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I notice your bike has a SSSA which of course has an eccentric hub to adjust the chain and by the very nature how this works, every time you adjust your chain you are changing the rear ride height which can have an impact on the geometry of the bike up or down. How do you keep that constant, how do you measure it to confirm the constant or datum?

but the starting point on a lot of Ducatis is at rear ride height,,,,,so if you have a ducati with an adjustable rear ride height or of course a ducati that you can change a connector to make the rear ride height adjustable.

then you need another tool....or get a tool that can do both......I wonder if Halfords sell anything?
I understand your point as you covered it in some detail in your current build thread. However, I reckon that for the average road user, which I include myself in and which most likely covers the majority of those on here and in the wider world, the slight changes in rear ride height caused by the eccentric hub altering position as the slack in the stretched chain is taken up would have minimal, if any, effect upon the ‘feel’ of the suspension in use. For those who ride to the limit on track days it’s a different matter.

The important thing (and I’m not preaching to you here just setting this out for other readers) is to be in the right ballpark for rider sag ie to get it to be 1/3rd, +\- a few mm of the spring’s travel.

Certainly as the chain is adjusted on an SSSA the ride height will alter (and I never realised that until your article, thank you). If one were a perfectionist then ideally, after adjusting the chain, one would recheck (and if necessary adjust) the rider sag which can be done very quickly, on one’s own, with the Slacker. Most road riders won’t bother though, most will set it once and then forget it; blimey, I’d wager most haven’t even checked the suspension on their bikes ever.
 

Andyb

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"Certainly as the chain is adjusted on an SSSA the ride height will alter (and I never realised that until your article, thank you). If one were a perfectionist then ideally, after adjusting the chain, one would recheck (and if necessary adjust) the rider sag which can be done very quickly, on one’s own, with the Slacker."

two things are different ride height and sag... I noticed in your video you never measured the free length just went with manufactures stuff easier on the front, but with all the linkage on the rear I would of started with wheel off the ground...

anyway my point is, I lengthened the swing arm pivot to wheel centre by rotating it as far as is could go back with a little for future adjustment modifying the chain length to suit. I gained a lot of mm, not quite in the Pierobon extended arm 35mm, but a good 12mm. Why you ask, well if Ducati Corse and Pierobon clearly say longer is the way to go who am I to argue. The fact the corse stuff is unobtainable atm for mere mortals like me, and the pierobon mod id like to use a spare arm, which are rare atm. ive maximised what iv got.

now the purpose of the datum tool is just that, a constant number you can refer to. ( would question with something like that slacker getting that hook or magnet in the same place every time so you can be consistent with your numbers. and of course the critical ride height thing is a non stater. Sure its a start for a lazy way to get somewhere near.)

anyway back to a datum, so I can now compare the effect of longer position of eccentric and the fact it is higher or lower and the impact that has on my other numbers. rotating the eccentric, ie adjusting the chain, I agree to a larger extend than just taking up a bit of slack, made a difference to the static sag number. Thats levers and moments for you I guess but my numbers are all comparative.
 

990Glen

Active member
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Agree about datum from where all things can be measured from. With so many variables, if you really want to tweak, then all values will need to be
monitored and effects correlated. As an extension of Andyb's ruler and bolt on frame, it is possible to get more accurate measurements using something like Ulf Hansen's product from his Uhr-Tec business.
D3975902-1B87-45F6-AB43-D36B79C5208E.jpeg

A6E5154A-DC27-4A04-AD55-ED373D04D10A.jpeg


For the enthusiastic layman, I think the Motool device is great. While it may not be able to give a finite value for rideheight etc, it can be used to keep it monitored/constant when chain adjustment is made, by merely fitting it, zeroing it, adjusting chain and noting change in measurement, then dialling the rideheight adjuster back to an inducated zero again before removing the device. I'm sure this would be more than the majority of owners would ever need.

Lets face it, a handful of new bikes off the production line will have small variations in "base settings", the rest is where you have gone since delivery. Was it 5mm higher last time, and 3mm more today etc and did you like the effect.
 

Andyb

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I was thinking wow that's a bit of kit.........then I saw its the V2 Panigale...worth a look at though (y) :unsure:
 

990Glen

Active member
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He's a mate of mine from when I lived in Denmark/Sweden. Don't think he has moved on to any V4 products as yet, though I'm sure you could make up a basic adapter plate to allow its fitting.
 

Andyb

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im thinking mountings are so different more than an adapter.

just needs a new main plate designed and cut, which I know is harder than just saying. get him on it......
 

West Cork Paul

Elite Member
Subscriber
"Certainly as the chain is adjusted on an SSSA the ride height will alter (and I never realised that until your article, thank you). If one were a perfectionist then ideally, after adjusting the chain, one would recheck (and if necessary adjust) the rider sag which can be done very quickly, on one’s own, with the Slacker."

two things are different ride height and sag... I noticed in your video you never measured the free length just went with manufactures stuff easier on the front, but with all the linkage on the rear I would of started with wheel off the ground...

anyway my point is, I lengthened the swing arm pivot to wheel centre by rotating it as far as is could go back with a little for future adjustment modifying the chain length to suit. I gained a lot of mm, not quite in the Pierobon extended arm 35mm, but a good 12mm. Why you ask, well if Ducati Corse and Pierobon clearly say longer is the way to go who am I to argue. The fact the corse stuff is unobtainable atm for mere mortals like me, and the pierobon mod id like to use a spare arm, which are rare atm. ive maximised what iv got.

now the purpose of the datum tool is just that, a constant number you can refer to. ( would question with something like that slacker getting that hook or magnet in the same place every time so you can be consistent with your numbers. and of course the critical ride height thing is a non stater. Sure its a start for a lazy way to get somewhere near.)

anyway back to a datum, so I can now compare the effect of longer position of eccentric and the fact it is higher or lower and the impact that has on my other numbers. rotating the eccentric, ie adjusting the chain, I agree to a larger extend than just taking up a bit of slack, made a difference to the static sag number. Thats levers and moments for you I guess but my numbers are all comparative.
Andy, I’m not contesting anything you say and yes ride height and sag are two different things. The thing is, we don’t always have a friend/family member to hand to assist in taking the readings whilst sat on the bike. In my case I have a few bikes, I don’t want to continually impose upon Mrs P’s time and my nearest suspension specialist is 120km away. For me the Slacker makes economic sense to enable me to make sure my bikes are set correctly and to experiment with different settings all on my lonesome.

ps. Where does one purchase the datum tool you use?
 

Andyb

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ps. Where does one purchase the datum tool you use?
the older the bike the more readily they are available....eg the 748 etc ones are a copy of THE Ducati Corse hence the numbers can be used unilaterally eg a ride height of 230 on the tool even up to 245...... of course the numbers are meaning less unless the datum tool is the same.
the 748 one fits the 999 too....
1098/1198 one is different
and you can see that one for the Panigale Dan Kyle makes a good one too.

there is not one readily available for the V4 .. I saw Barni racing had something but they wanted silly money for one....and in any case I thought it could be improved on.....hence I made mine... (y)
 
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