I just got a new supercycle bike that came with everything, including the saddle rail clamp (the part connecting the double-railed saddle to the shaft beneath). After riding about 20km on it, the bike seat began tilting backwards randomly, without me adjusting it. I took it apart and found that the ridges on the insides of the saddle clamp had been ground down, and that the whole thing was useless. So far, I've only found one rail clamp that fits with both my saddle and the shaft underneath, and that's the exact same clamp as the first one. Is there another better clamp that won't wear down as fast as my initial one did?
Those style of saddle clamp only slip like you described if they’re loose. Make sure you tighten the side bolts really good (it’s hard to break those things) and use threadlocker.
A better option is to buy a 2-bolt (also known as “infinite adjust”) seatpost. Those use a better design where that slippage is not possible. Simple aluminum posts are not too expensive. Make sure you get the right diameter and length for your bike.
Is there another better clamp that won't wear down as fast as my initial one did?
No, there isn't.
High quality seatposts use an entirely different design with an integral clamp. Thus, you need to change the entire seatpost, not the clamp. Almost all clamps sold separately are complete and total garbage, failing very easily and being impossible to adjust in the perfect position.
You should only consider infinitely adjustable seatposts. Some cheap seatposts with integral clamp still may have the groove-based mechanism that allows only some positions. The grooves are not durable enough!
When selecting the seatpost, prefer designs that have two bolts where the failure of a single bolt does not cause the seat to become loose. This prevents a dangerous failure mechanism where the failure of the only bolt drops your butt onto the rapidly rotating rear wheel that tears your backside in a such horrible manner you will never want it to happen to you. Of course, if your bicycle has something to cover the rear wheel like a pannier rack, this failure mode is not as important to protect against. Still, I would prefer two-bolt seatposts where the failure of a single bolt is not dangerous.
Of the seatposts available in the marketplace currently, I have identified at least the following two as having a safe clamping mechanism (this list is subject to change as time passes):
- Ritchey Classic
- Procraft Classic
When selecting a seatpost, be sure to have sufficient length (too much never hurts, unless it's ridiculously long which no seatpost I know of is), and the correct diameter (use a caliper to measure your old seatpost), and least importantly, the correct setback amount (there is some adjustment room in the saddle but too much error in the setback and you can't adjust the saddle front/rear position to the desired one).
Not all seatposts with two bolts have safe clamping mechanism. For example, I had a hard time finding a good quality 25.4mm diameter seatpost with a safe clamping mechanism. The best I could find is Ergotec Skalar, but its clamping mechanism fails when one of the two bolts fail.
As others have suggested, an aluminum-alloy seatpost with an integral clamp is a better solution. The clamp you describe is made of steel, is hard to adjust, and most probably sits on a steel seatpost (the "shaft" you described) which together weigh a good deal more than a quality alloy seatpost. The ones with the integral clamp are easier to adjust and hold up well. Good-quality seatposts start at around $20-25 (US), and if you can find a shop with used parts, you may be able to score a seatpost for about half that. I don't believe seatposts wear out -- one of my bikes is 37 years old, and the seatpost is one of the few original parts. Be sure you get the right diameter and that the seatpost is long enough so that you have about 5 cm of it in the frame. Good luck!