View Full Version : Implications of implementing a bell curve to DnD?

2009-08-16, 10:03 PM
My group and I have always been big fans of the idea to use a bell curve for DnD, rolling 3d6 instead of the d20. Now, such a conversion would obviously not go as smooth as one could hope, so some adaptions would have to be made to the system.

The most obvious ones to me are the attack rolls. AC would have to be scaled down to account for the fact that the attack roll would fall within the range of 8-13 roughly half the time, and since I've been wanting to try out this injury system (http://www.fudgefactor.org/2004/05/non-linear-wounding-system.html) I've come to the conlusion that the best way to approach this would be by converting armor into damage reduction, and adopt the scaling AC bonus by class that I've seen in the Wheel of Time D20 books. Alas I can't remember how they scaled by class and level, anyone who could fill me in?

Other things I'm thinking might need to be readjusted is save DCs, specifically spell save DCs, though I'm not entirely sure about this one. The game will take place in a low magic E6 campaign so there will be no standard magic items the characters can rely on to boost their saves beyond their stat, feat and class bonus, so characters will be more likely to save against spells "below" them and less likely to save against those "above" them. I'm not sure whether this is a good or bad thing yet.

Lastly, and about this I have absolutely no freaking idea, is whether some numbers will be too great with a bell curve. Things like size bonus to grapples/bonus to combat tricks (like the feats for improved trip/disarm/sunder etc.). I get the feeling the penalties might be too severe and the bonuses too great, though this is kind of the same as the DCs. Not sure whether it's a good or a bad thing. Leave as is or reduce the numbers? Oh, and where might I find the converted crit ranges for weapons? I know I've seen someone link them from here before.

Anyone have experience with changes of this kind and know of anything I've overlooked that might need tweaking to make the balance "good enough" to play?

I'm going to bed now, and wish all of you getting out of yours a pleasant day. Cheers!

2009-08-16, 10:17 PM
A bell curve will also change crit frequency. A greatsword hits 19-20 x2 and a greataxe hits 20 x3. They're considered balanced, or pretty close too it. The greatsword hits exactly twice as often as the greataxe.

If you change things for 3d6 so that the greatsword crits on 17-18 and the greataxe crits on 18, you'll unbalance the two weapons, because 17 will come up more often than 18. 18 only happens on 6, 6, 6 (1/216) but 17 happens on 5, 6, 6; 6, 5, 6; and 6, 6, 5 (3/216). The greatsword will end up critting 4 times as often as the greataxe instead of twice. Anything that's normally 18-20 will do even better.

I don't know how you should address this.

2009-08-16, 10:20 PM
These rules already exist and are elaborated upon.

I'd suggest using Mutants & Masterminds' Toughness save system over the UA Injury variant, though - the former is more detailed.

Bell curves (http://www.d20srd.org/srd/variant/adventuring/bellCurveRolls.htm) and Injuries. (http://www.d20srd.org/srd/variant/adventuring/injury.htm)

2009-08-16, 10:26 PM
AC is already very low in the system. Switching to DR and giving level-based AC bonus would increase the overall AC on higher levels slightly and lower it on lower levels (which is why I'd consider using DR-variant, but not reduce the armor-bonus to AC - acquiring AC without magic is extremely difficult to start with).

But yeah, standard d20 D&D has a virtual cap of 10+8 Armor+5 Armor Enhancement+3 Dexterity+2 Shield+5 Shield Enhancement+5 Deflection+5 Natural Armor+1 Insight = 44 (Tower Shields aren't very good) for item-based AC on standard characters, making hitting trivial on higher levels (one can expect ~20 BAB+12 Strength+5 weapon+1 speed+1 competence+1 luck pretty easily as just attack roll bonuses and that's already +40 without accounting for Flanking, Higher Ground, Charge, etc.), with most characters not spending that much resources on AC being left around 40 or under, and most monsters clocking under 40. This won't come up in your game, but is an easy comparison for how AC measures up to attack bonuses.

The bell curve would make hitting extremely consistent; if you can assume to usually roll 8-13, you won't practically ever miss your first or second attacks on higher levels, nor your additional attacks from Haste and whatever (and on level 1 too, your +4-5 to attacks will be hitting opponent's AC 13-14 with decent consistency). It would also make bad saves fail rather consistently on low levels with good saves succeeding; in general, it would emphasize the dynamics that already exist in the game.

Good base values lead to nearly autosuccess, and the chance of rolling the mythic 18 for autosuccess on something you shouldn't succeed? ~0.5%. This definitely requires rewriting the Critical-rules if you want them to still be a relevant part of the game (maybe make hits exceeding opponent's AC with 10 crit? And increase crit range to ~16-18 or so?). Unfortunately I don't have links handy.

Also, saving throws would be much more consistent with autofails basically never happening.

Overall, this kind of a system would make the game much easier for PCs as natural 1s on saves, random low roll sequences in combats and random opponent's 20s would just happen far less.

Also, skill checks where you don't have "succeed on 1"-ranks (indeed, minimum being 3 helps a ton in all of them) would also be very safe with trap disarming and such in particular becoming much safer.

Save-and-X spells would be more efficient as long as you target saves properly since opponent is very unlikely to roll in the 15-20 range he needs to succeed. Opposed checks would have the smaller differentials emphasized especially due to the ruling that "bigger modifier wins a tie", meaning if you had even a +1 on someone in Grapple, you'd actually be a notable favorite since he effectively needs to roll 2 higher than you to win.

This part probably favors monsters, actually, due to their high Str, HD-based BAB and size. Though focused trippers are probably slightly improved by this.

Biggest changes you'd probably want to implement:
-Modifiers on opposed checks could be scaled down slightly to make up for the smaller range of possible numbers and lesser likelihoods on extremes. I think +3 per size category (over present +4) would be pretty good.

-Acquisition of AC needs to be made easier if you want not-getting-hit to be a relevant character option. Something like BAB to AC could definitely help, although it won't really make up for the loss of armor-based AC much before level 6 (which obviously hurts in an E6 system).

-You need to ensure that characters have means to pump their weak saves as being hit in your weak save is practically an autofail. Adjusting DCs is like to do little especially since the system is very hard to alter (only thing you could really do is make Ability Focus only give +1 to the DC), so just ensuring people have good enough saves to make the checks with decent consistency seems like the best road.

2009-08-17, 02:38 AM
With 3d6 very high and very low rolls become more unlikely. That makes things more predictable and that usually helps the pcs, while more randomization helps the monsters.

Kurald Galain
2009-08-17, 06:04 AM
Rolling 3d6 or 2d10 is one of the easiest fixes for the malfunctioning skill system, actually. And adjusting crit numbers is just a matter of math.

2009-08-17, 08:37 AM
Thanks all for the replies.

The matter of really specialized characters (you mentioned that focused trippers might benefit more than other) is not really a big problem since none of my players are really inclined towards super specialized builds. I just need it to be relatively balanced for use with base characters, perhaps who have spent a feat or at most two on the action in question.

How about letting both parts roll for things such as Attack vs. AC, or spell DC vs Saves? Basically replacing every 10 in 10 + modifier + whatever with 3d6 + modifier + whatever. Ought to make it a bit more random in terms of hitting with attacks and the like.

Need to move

EDIT: Spelling

2009-08-17, 10:21 AM
Check out the d20 SRD (http://www.d20srd.org/srd/variant/adventuring/bellCurveRolls.htm) for exactly this sort of thing.

As for criticals, a greataxe becomes a threat on 16-18 on 3d6, while a greatsword becomes a threat on 15-18. The odds of rolling a 16 or greater are 4.63%, the odds of rolling a 15 or greater are 9.26%, so it's fairly close to the old probabilities (5% and 10%).

It's been mentioned that a bell curve exaggerates the difference between your level, so tough monsters become much tougher, and weaker monsters become much weaker.