What Is Suspension Sag....

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



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

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


next to picture 2

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.

So my free sag, the weight of the bike compressing the spring. down to 234 giving me 11mm of free sag.
Now to picture 3 rider and bike or static sag So with me on the bike (ideally in normal sitting position fully kitted)

its down to 214mm

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 im 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.



Now ideally i wanted 30 mm not 31 so i need to adjust the preload on the spring up by turning this silver adjuster in the centre if i count how much i add see what it does to the numbers i can then add or take away some to achieve my desired number!
Simples!
Now for the front....
Maybe ill add why i'm at 245 ride height at some point....

 

To add on some tell tale signs
Too heavy a spring and there will be very little free sag even with no pre load wound on to the spring
Too soft a spring and there will be a lot of free sag and static sag. Of course you could wind up the pre load trying to get the sag down to your requirements but you are coil binding the spring and getting outside it’s working range time to go up a spring... or two

 

Tyres pressures and temperatures hot and cold are a whole new ball game..which on there own will screw up the tyre add in the existing spring or suspension setting problems and it also screws the tyre..