Rollers will work just fine. If I interpret the numbers here (http://dwarffortresswiki.org/index.php/Minecart#Numbers_behind_the_scene) correctly, your cart needs to be faster than 3000 to skip on magma, (Or is it 3000 to move through magma? I'm not sure.)
and a one-tile roller on highest setting gets you to 50000. A 6-tile long roller should get you to the highest possible speed, so you can definitely do some good damage without exploits.
Oh crap, you're right about the rollers... Wow, I never noticed that.and a one-tile roller on highest setting gets you to 50000. A 6-tile long roller should get you to the highest possible speed, so you can definitely do some good damage without exploits.
Doesn't work like that: rollers do not accelerate, but set the speed of the cart to roller speed. So a one tile tile roller is the same as a 6 tile roller in the end, it will never accelerate past 50k speed, which is 2ticks/tile...in fact it's 49990, 10 speed below the speed needed to skip a corner. I'm not sure if it's the speed necessary to skip the cart on fluids too, but I'm sure it's not less than 50k. My guess would be 100k, I know that's a "trigger number" for some minecart quirks..
I can't comment on skipping magma, but I can admit that rollers generally suck for minecart traps. I made a circular hallway with several max powered rollers. The problem wasn't lethality, but that the minecarts never had the momentum to continue to another roller after hitting something. So the trap had to be reset after any injury.
If you can power the minecart enough, it will be a good design, but I'm skeptical that you can do it with rollers alone.
outside insideDon't know about rollers but a setup like that but changing the rollers for stops and having a dwarf pushing the cart instead would have the cart crash down the gap, you need at least one tile before the gap for it to jump over (if powered by dwarf power)
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T~==========~T
Key:
. is spike trap
= is minecart track
T is roller
~ is a channel filled with 7/7 magma