i play top a lot but i pretty much only use jayce. almost gold 1 and i only know 1 champ for my main role. i want to expand before i rise any higher and it becomes difficult to learn new champs without losing elo. so i've tried rumble and renekton before, i did terrible. i felt like rumble never did enough damage. am i suppose to always keep heat in danger zone and harass with enhanced flamethrower? same deal with renekton, do i go in for harass when i've got enough fury?
Renekton has really high bases and has a kit that lets him trade huge chunks for free. Abuse that. Beyond that, I don't know the champ well enough to comment much.
Rumble is all about heavily punishing anyone who cannot outdamage your Q. Basically, before level 4, you never want to start trades; only take free damage with Q (which I grab at level 3 unless I'm being pushed hard into tower or am against a Nasus, in which case I take it at 2). Free damage being something like when your opponent just used his skills and is backing out. Rumble does pretty much full continuous damage without ever slowing himself down, so when your opponent walks away they continue to take Q damage, and if they turn around you can E them and kite until Q comes off cooldown and use it again. If they all-in you, you can kite with W and E while minions hit them provided their burst is insufficient.
Once you hit level 4 or 5, you will outtrade almost everyone in the game except dedicated all-in champions like Darius and Riven, who you are capable of kiting and punishing (though Riven can be iffy because of her mobility). Lee Sin is an exception, and I'm actually of the opinion that he's an impossible lane if he doesn't outright feed now that Rumble's ult does literally nothing to him (lost the initial burst and he can always get out instantly). Lee can stay in lane forever and his full combo will kill you from nearly full health once he has his ult, if you ever miss a single E when he tries it. For everyone else, just W Q once they get too close, then E into an (ult and) overheat if they stick around or into a Q if it hits as they run away.
Managing heat is kind of important, but it really turns out that as long as you don't stay at zero heat, you will naturally get some danger zone skills off in your combos (W Q, danger zone E; if it hits, danger zone (W) Q overheat). It's more important to focus on taking advantage of your chances to trade advantageously. The only real downside to overheating is when you need your ult or when you do it before you use all your basic skills; you can be kited during overheat too, but not by most top laners.
"That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia
Well first off you should probably practice newer champs in normal draft before going into ranked if you want to preserve your elo. Though that depends somewhat on what your normal elo is, since if it's much lower than your ranked elo the practice might not mean much.
Renekton - mostly about fury management + kiting/burst. The classic combo is E in, W stun, Q, then E back out, which is very efficient given that your combination of mobility and CC prevents the enemy from retaliating effectively. If you can manage to do so, weave in as many autos in between your spells as possible, keeping in mind that W is an AA reset. Standard spell maxing order is Q > E > W, though early on you'll want to use your fury on Q or W over E. Your continuous damage output is not so great due to your cooldowns/fury costs, unless you have your ult.
Late game Renekton is a bit tricky, since the aforementioned costs really keep you from being that effective. Many Renektons build almost pure tank, relying on their base damage + Sunfire cape to pressure the enemy carries while soaking up as much damage and attention as possible, though you can also build damage to increase your threat. In general it's probably better to dive as Renekton than it is to peel, since your peel isn't that good. Though a fair amount of your utility in teamfights comes from landing empowered Dices on multiple opponents to cut their armor, in which case targeting tanks for your ADC to target wouldn't be too bad an idea. If you can get away with it during lane Ghost is overall stronger than Flash for late game sticking to carries.
"We all wear masks, and the time comes when we cannot remove them without removing some of our own skin." - Andre Berthiaume, Contretemps