I have asked here before about spring preload and I think I understand what thats for, to make it higher so it doesnt bottom out.

But what is the rebound damping for?

I just read my clymer manual and it said the xt600 has rebound damping adjustment too,at the bottom of the rear shock you can turn it manually by hands on different clicks and 4-5-6 setting means "stiff".

I thought XT only had 1 adjustment on the rear shock/spring preload. But I guess this is good news Though I dont know yet what its for it must be good for something.

Im 123KG now and the rear bottoms out on some bumps also i can feel that it gets much lower when i sit on it..lower rear the front.

Combined adjustment of spring preload and rebound needed? (compared to stock setting+my weight)

Damping is the act of slowing down movement !! It stops your rear shock acting like a pogo stick.

Oil is forced through small holes which creates resistance.

I don't know what model XT you have, but my XT600E certainly didnt have damping adjustment.

If you're bottoming out the rear, you could try tightening up the preload adjustment but you will find that you will probably throw out the "Static sag" and "race sag"..

That is the movement the spring compresses through general riding.

At 123kg (in your gear ???) You're heavier than the bike was designed for so you should be looking to replace the spring with a stiffer one.

__________________
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Rode some bikes.
Fix them for a living.
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123kg without gear
im also 185cm tall and the bike feels a bit low.

When I sit on the bike I can just tap the feets on the ground and push a tiny bit with my foot and the front sags up and down very easy..is that normal?

I looked at the rear shock yesterday and didnt see any knobs or anything on the bottom of the shock..so guess it doesnt have rebound setting, but i could atleast try to change the spring preload since its still stock.

When I started riding, the perceived wisdom was you should be able to put both feet flat on the ground whilst seated and be able to 'paddle' about with your feet outside of the footrests. You should also be able to stand aside your bike and shake the tank between your legs. Also kick start it whilst astride. Many modern bikes are way too high for me. You will discover the wisdom in this when you pull up at a crossroads on a slope or aside a pothole.

You need to separate in your mind the spring and dampening effects. Ignoring dampening for the moment assume your front forks and rear swinging arm both have six inches of travel.
When you sit on your bike the suspension should have used up about 2 or 3 inches of travel each end. IE (just under less than half the travel) You and your bike are now 'floating' on the springs.
Riding along at 50 mph the road suddenly dips 2 inches. The springs will propel the forks downwards to make contact with the bottom of the dip. As you come out of the dip the springs compress. The wheel will have moved down and up but not your headlight or bike. If the suspension was solid or the spring setting had your forks topped out, you would have to travel over three feet before your bike fell enough for your front wheel to make contact with anything. During that time ( 1/20th of a second) you would have no brakes or steering. The springs act faster than gravity so you get to stop flying and touch land much sooner.
Now imagine you hit a hump that is two inches high. Ideally the wheel should compress the spring and the bike should not pitch. Then when you come off the hump the spring will again propel the wheel down over the rear slope of the hump faster than gravity.
This is why I like lightly damped fast acting suspension. In my view many people set their suspension too high and too hard.

Try going to a quiet bit of road that is not perfectly smooth. look at your lower fork leg. It should be jiggling up and down all the time. If instead it is your headlight that is bouncing either
your forks are seized/binding ( happens a lot).
spring too strong.
over damped.

The damping is there to slow the return so that the wheels don't bounce up and down and make your bike act like a pogo stick. It also slows the return enough so that on roads with small close together irregularities the suspension actually 'sinks' a little allowing you to run on a harder part of the springs range.
Often they also will damp on the compression stroke This can easily lead to a harsh ride.

So first check out the springing, most bikes have adjustable rear rates.
Be very careful about adding shims or spacers to 'strengthen' fork springs. Often you will get to a state where the coils bind and thus limit the travel.
Once the spring rates have been sorted then delve into dampening.
If the ride feels harsh either your tyres are over inflated or your suspension is hard through over springing or over dampening.
It is your front wheel that should follow the road contours not your headlight.

lol... Im 182cm and 80kg and I was on the balls of my feet. I would of expected the bike to feel lower with your weight but it seems that the pre-load must be wound up pretty stiff if you're still on tippy-toes !

Have you got adjustment left in the rear for preload ? Where abouts is the adjustment between the threaded area ??

As for the front, it's very soft and saggy by nature. Putting some heavier oil in the front will improve it a little or you could get stiffer, more progressive springs.

__________________
Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.

The manual doesnt say how to measure the preload on the threads, it just says max spring length and minimum.

Anyone know how many millimeters the threaded area can be max when increasing the preload? i think now its about 1cm-1.5cm of the threaded area visible..