XRAY - Model racing cars

It sounds like something is binding. Your truck shouldnt be acting that way. Can you take some photographs?

When you drop the truck from a small height, it should settle in the middle of its travel (outdrives parallel with ground).

If you are referring to where the ball ends connect to the lower arms, then you may have identified the issue.

The ball ends shouldn't have hardly any resistance and should move very freely.

If yours bid at all it is an indication that you may have over-treaded the piston rod into the ball end, which will slightly deform the ball end leading to additional and unwanted binding.

I will try to explain better what I noticed.

If I put the truck on the ground gently its ride height is, lets suppose, 10 mm.
I push it till the bottom touches the ground, then release it: the shocks will lift the truck till 4-5 mm of
ride height but it will not return to 10 mm. It seems that the force due to the remaining spring compression is not enough
to win the weight. If I help them pulling the truck to the air the suspensions travel to their limit before the tires
leave the ground and It is OK.

The same behavior in another context leads to a smaller than expected ride height after a jump.
I think there is nothing wrong with my truck and I'd like to know if it was noticed by other people.
When XRAY will release stiffer springs I will try them if they solve the problem.
Finally what I'm looking for is more RELAIABILITY for the ride height. I need the maximum ride height (I have 3 + 1 preload) but as stated before after a jump it may become much smaller.

What happens when you drop the truck from a short height.. Does it settle into this 10mm ride height you mention?

I cannot understand very well what you mean. I will try to answer anyway.
When I refill the shocks with oil and they are without the springs they rebound a bit, till mid travel.
When the spring are added they rebound nicely to full travel, but they have very little force in the last mm
despite the preload. After a long ride if unmount the shocks and I remove the spring they don't rebound any more
as they do when they are refilled.

I made some measures and executed the test suggested by Mini_me.
Here are the results:

Pre load : 3 + 2 mm front and rear

If you put the truck gently on the ground the ride height is about 23mm which is near the maximum because
the suspension are close to their lower point.
Dropping the truck from about 100mm it settles with the bottom at 16mm from the ground.
Pushing the truck to the ground it returns to 12mm.
I would like those 4mm that are lost. Increasing the preload further will decrease the shock travel because the spring
is very close to its maximum compression.

Bye
Filippo

Remember also that you are testing the truck in a purely static situation.

Thisis not how it will react under power. The acceleration and deceleration of the truck will cause the weight to transfer towards the front or rear of the chassis and will effect the amount of force that the springs need to overcome to lift the truck.