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themachinestops in wrenches

Hey, I just joined this community! Sorry to be a jerk who asks a question straight off, but I've been really struggling with my bike for the past few months and none of my RL friends know anything about bikes, most can't even ride.

A few months ago I bought the girly version of this bike on Amazon because it had hit an amazing price. It was fairly easy to put together, all by myself, I only needed to borrow a few tools from my boyfriend's dad and have him adjust the seat/handlebars while I sat on the bike, etc. Everything was going great until I took my first ride... and the seat started to slip, both forwards and backwards, unless I held myself perfectly level. A couple of seat wedgies later I went back home and tightened the seat with a crescent wrench, which held for maybe a half hour until... it slipped again. Subsequent adjustments with the crescent wrench have had the same effect, it holds for awhile but any strong pressure (like that of a human ass) will make the seat list forward or bend backward, which obviously does not make for good riding. I've had "big strong men" adjust it too, it's no better. I am wondering if I am missing an important seat component, but I know I didn't lose anything from the bike box. Any idea what it might be, bike mechanics? I don't see any screw holes under the seat or on the seat post. besides the bolt that holds the seat to the post.

Thanks in advance for any help you can give me. I really want to fix this myself, in fact one of the reasons I bought a disassembled bike instead of a ready-made one (besides the price) was so that I would be forced to learn how my bike works instead of just treating it like some toy. If I do have to take my bike to a shop, though, how much will it cost to fix?

I can post pictures tomorrow if you all need to see the seat to diagnose it.
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It sounds like you are missing a piece or have a washer between the teeth plates. Usually, the stacks go like this:

Nut | Washer | rail grip | U grip to the seat post | Rail grip | Washer | Nut.

If there are washers between the rail grips ( that attach to the seat ) then you will get that pivoting, unstable seat. If you have pictures, we can give a better idea what is wrong. I would suggest taking the seatpost out of the frame (if it comes out easy ) and take a picture of it upside down.
Each railgrip may be two pieces, by the way.
Going on your description of the problem and the picture on Amazon, here's my diagnosis:

The saddle looks very much like it uses a nonstandard clamp. That is, instead of two round rails (like on the Koobi saddle seen here), it has two slender approximately 1" tall rails.

These are known, in the bike industry, as "cheap." (Technical term, I promise.)

The solution: Get a set of metric wrenches (because everything on a bike is metric - in this case the most likely fit: 14mm, possibly 13mm) instead of using a crescent wrench. Remove the nuts from both sides of the bolt going through the rails (one at a time if you want - doesn't particularly matter). Grease the threads on the bolt (I suggest using Phil Wood grease in general, though any bicycle-specific grease will work; Pedro's might be easier to find and cheaper), then reattach the nuts. Tighten using the box wrench.

With that style clamp, it's very difficult to position the saddle in a place where the clamp will have a tendency to slip, but check to make sure. The rails under the saddle are not parallel all the way to the front of the saddle, so make sure the clamp is in the middle of the parallel section.

If the seat still slips after that, I'm gonna need pictures.
Check for a washer or incorrect ordering of parts first, following the other suggestion. You should still get grease and a proper set of tools, but first checking the problem that has a free fix is always a good idea.
Heck, white lithium grease will work, and your boyfriend's dad (if he's your source of tools) is pretty likely to have some lying around.
Seconded.

There's really nothing that special about bicycle grease. A least nothing so special that a non-wrench should go out of their way to use it. Most generic greases from the HW store are cheap and excellent. I prefer trailer bearing/marine grease for many uses here in soggy Seattle.