I've got a Kindshock Dropzone seatpost with a single bolt clamp.
Even if I screw the bolt in hard the saddle will always slide to the back.
Any way to prevent it? It's pretty annoying.
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I have never had a problem getting single bolt tight enough to hold.
Most likely is seat rail diameter - 7mm is common road, 9mm common MTB but there are others. (refer Wikipedia).
Is there something assembled wrong, a piece upside down?
Although seat post bolts need to be tight, what ever you do, don't be 'that' "gorilla with a spanner" - it is never the correct approach when assembling bicycles.
There's a reason why most high end seatposts use the two bolt design.
You could use a torque wrench to tighten the bolt to spec (which is tighter than most people would use without one), and grease the bolt to allow more tension.
I presume it's assembled right.
Make sure the bolt threads and shoulder are clean and greased.
Putting a layer of fiber grip (aka carbon prep aka Liquid Torque, etc) on whatever area is slipping may help.
If I understand your question correctly, what you could do is add a thin piece of rubber underneath the rail clamp before you tighten it down.
That should have the same effect as using a rubberband to help remove a stripped screw.
The rubber will fill between the cracks that have been worn down, previously causing your saddle to slide back.
It's worth a shot considering it will cost you nothing or next to nothing.