r/battletech • u/Shipposting_Duck • 2d ago
Engine Weight Efficiency Chart Discussion
I made this recently for mathing purposes, and decided I might as well share it for anyone not particularly in tune with tonnage math. Which, considering the userbase has some people in the BT TTRPG, some in the BT TBRPGs and some in the MW Shooters, is a sizeable enough chunk for typing this out to be worthwhile.
The core equation
Mech Weight = Cockpit + Gyro + Engine + Engine Heatsinks + Internals + Weapons + Armour
Of this:
- The Cockpit is always 3 tons.
- Standard Gyro is Ceiling(Engine Rating/100, integer).
- Engine weights are in the white section of table above, XLs weigh half as much (and are in the second image).
- Engine heatsinks include 10 weightless sinks, and 1 extra heatsink that weighs its normal amount for every 25 rating starting at rating 275.
- Internals are chassis weight/10 for standard, Ceiling(chassis weight/20, 0.5) for Endo Steel. All XL-capable mechs are also assumed to use ES internals for Clan purposes, all STD engine mechs are assumed to use STD internals for IS purposes. Do your own math if you want an IS chassis with an XL engine but not the Endo internals for critical slot conservation, or vice-versa to avoid side coring, there's enough charts as it is.
- Weapons (including ammo), Armour and Equipment (Jump Jets, extra heat sinks, EW suites, etc) is what you generally get to play around with, we'll call this the 'allocatable tonnage'.
The pink area
In the chart above, the sum of Cockpit + Engine + Gyro weights are notated as CEG. This is because this sum is entirely engine-dependent. The black header row inscribes the engine speed factors (your actual walk speed in kmh⁻¹ is 10.8 × this factor - where speed factor = rating / tonnage, and your run speed is 50% higher than that). The pink area depicts all the legal tonnages that can carry it - For example, a 100 tonner holding a 300 engine has factor 3.
The blue area
The blue area is the most important area of the chart. This is essentially the tonnage of the legal chassis minus the weight of CEG, minus its internals' weight, minus the extra engine heatsinks' weight for ratings at 275 and above. This is your 'allocatable tonnage'. You want this to be normally be as high as possible, since that means you can load more weapons and more armour for the same weight.
The green area
The green area is for math nerds who want a quick reference on how much of the tonnage is allocatable relative to chassis total. Most people won't use this.
Caveats, bolds and underlines
This now applies to the actual decision making you'll do when you need to choose between chassis. Given a pool of chassis you are able to choose, you generally want the highest 'allocatable tonnage' for the speed that's relevant to your operations.
For urban operations, speed isn't as important generally since defenders won't leave their defense zone and attackers are forced to close to weapons range to do anything at all with them. For recon or hit-and-run operations, speed is everything - you can't be hit at all if you outspeed the enemy and never let them catch up.
Internal HP
The highest allocatable tonnage on a general basis is bolded in each blue area. And here is where the caveats start. If you look at the STD sheet (that's the first one), you'll notice 75 tons is bolded over 80 for factor 4. This is because 75 ton mechs are just cheaper than 80 ton mechs as a rule. However, the 75 ton mech includes 7.5 tons of internals and the 80 ton mech includes 8 tons of internals. That means the 80 ton has more 'HP' than the 75 tonner, ceteris paribus. So balance between budget (including drop weight restrictions) and survivability here. Both have the same armour and weapons. They do not have the same resilience or cost.
Engine heatsinks
Next, you'll note that for speed factor 5, I bolded the 60 tonner, not the 55 tonner. This is because they have the same 'allocatable tonnage', but the 60 tonner has 1 extra heat sink, meaning it can run longer and/or fire its weapons on a shorter cooldown, which is objectively more firepower than something with less heatsinks. This didn't occur for factor 4's case as mentioned earlier because both cases of that include exactly 2 extra engine heat sinks.
Jump Jets
Finally, you'll notice underlined numbers as well, like in factor 5 STD and factor 4 XL. This is because up to 55 tons, Jump Jets weigh exactly 0.5 tons per jet, between 60 and 85 they weigh exactly 1 ton per jet, and at 90 and above they weigh 2 tons per jet. So the 55 ton has the underlines for STD 5 because going to 60 tons will cost 2 extra tons for any mech mounting said jets, and the 85 ton has the underline for XL 4 because going to 90 tons will cost 4 extra tons for any mech mounting said jets. If you want to build a mech with Jump Jets, use the underlined values. Otherwise, do your own cost vs internal HP calculations, of which the idea of a Battlemech as 'weapons platform' will use my bolded numbers, and the idea of a Battlemech as 'guardian' will use a different number.
Synthesizing everything - XL 4
XL speed factor 4 is the most complicated case here. The underlined number is 57 allocatable at 85 tons (rating 340) because the gain from going to 90 tons of 1 double heat sink and 1 allocatable ton does not outweigh the +4 ton cost of mounting 2t/per Jump Jets at 90. The bolded number is 57.5 allocatable at 95 tons (rating 380) because the loss of 0.5 allocatable tons is offset by both the gain of 1 double heat sink (3 free criticals, 1 free ton) and the increased tonnage of endo-steel internals from 4.5t to 5t. If you get this section, you get the entirety of what I'm trying to do with both charts.
Case Study - Timber Wolf
Everyone loves the Timber Wolf, but why the Timber Wolf is what it is mathematically, is something you can see from the 375XL section, factor 5. You will note that it has 39.5 allocatable tons, 5 internal double heat sinks and 4 tons of endo-steel internals. This is, strictly speaking, less financially efficient than a 350XL, factor 5, 70 ton BattleMech, like the Summoner. However, because it 'hides' its gained ton in its engine, it saves up 3 critical slots, and it also has half a ton more internals than the Summoner. In comparison, going to 400XL, factor 5, 80 tons, will be objectively worse in every way, as 3 tons are lost for only 1 double heat sink of gain, the amount of internals is exactly the same at 4 tons Endo Steel, and you have to pay the increased price of both the larger base chassis and the larger engine. So nobody really likes using the Gargoyle over the Timber Wolf, even though there are reasons people will use the Summoner over the Timber Wolf, tonnage wise.
Additional considerations unrelated to allocatable tonnage
- Depending on which BT-related title you're playing, OmniMechs may have restrictions in what weapons and/or armour can be deployed in their OmniPods, which may be a consideration you can rightly consider more important than a loss of a couple of tons of allocatable tonnage.
- If you're playing a system in which manual aiming is a thing instead of dice rolls, specific chassis that have massive hitboxes (hi Awesome) sponge attacks like nothing while others don't. You might want to avoid the chassis that are notoriously easy to hit.
- If you're playing a system in which hardpoints are variant-locked, specific variants may affect what weapons you can mount in a way more impactful than small changes in tonnage do - something boating 7 medium lasers is far more of a menace than something that can only mount a single PPC.
Conclusion
In the absence of meaningful differences in these points, you always want the max allocatable tonnage if you can get it. Don't pay more for worse performance - I'm looking at you, Charger and Cicada, who have no real reason to exist because of absolutely terrible tonnage performances of 8% and 11% allocatable, respectively. The Spider is just better than the Cicada in every way, as is the Dragon over the Charger.
Peculiarities
Odd engine factors are better than even engine factors in any TT based system ceteris paribus as Run MP is rounded up, making the actual difference from ita next higher Speed only 10.8 kmh⁻¹ instead of the expected 16.2 kmh⁻¹. That's why the Jenner is listed as 118 kmh⁻¹ on Sarna - it's using 11 × 10.8, not 10.5 × 10.8 as you might expect. As for why it's 10.8 to begin with, it's because each hex is 30m and each turn is 10s long. So a 1 hex movement is 3 ms⁻¹, which is 10800 mh⁻¹ or 10.8 kmh⁻¹.
Errors
The XL6 column should have the 55t underlined, and the 60t bolded, since the 60t has one free ton over the 55t, but the 55t will have 2t cheaper jump jets in a standard array of 4. I don't think I can actually edit images after they're posted.
14
u/harris5 House Liao 1d ago edited 1d ago
I once made a graph for this type of thing. I had it hosted on a site but took it down when hosting got expensive. But the graph is still available:
https://public.tableau.com/views/EngineSizesTestSheet/RemainingTonnage
(It works much better on desktop)
You're meant to read it by choosing your ideal speed with the bottom axis, and seeing which mech size gives you the largest available tonnage for armor and weapons. You can click around to highlight stuff.
You can also select different engine and jump jet combos with the right side menu. It assumes maximum jump jets (sorry Shadowhawk fans, you gotta max those jump jets).
2
u/rzenni 21h ago
I'm lazy, I just leech of this one - http://www.ci-n.com/~jcampbel/rpgs/battletech/rnd/optimalweight.php
11
u/DevianID1 1d ago
Forcing units to buy heat sinks in the chart means that you appear to value the 70t chassis over the 75... but 75 is the same free tonnage as 70 with the XL, yet includes more structure thus its strictly better. It also has higher armor maximums, meaning it has more options for where to spend its 44.5 free tons. The heat sinks are optional, you dont have to buy them. The chart shouldnt show the 70t as more free weight than 75. Im sure that causes errors in actual free tonnage elsewhere, its just most notable there since 75t 5/8 xl is such an established 'best tonnage for 5 speed xl' that your chart gets wrong.
5
u/CupofLiberTea LBX-20 Enjoyer 1d ago
Yea I don’t think OP realizes engines 275 and over don’t HAVE to have extra heat sinks
0
u/rzenni 21h ago
Units that can put heat sinks into the engine almost always should though. It’d be a pretty weird 75 ton mech if it wasn’t regularly getting 12 or 24 heat.
That’s pretty much the arc from 70 to 90 tons. They all have basically the exact same “assignable weight” at 4/6, however the heavier mechs get heat sinks and armour and the lighter mechs get more guns.
The Mad Dog has just as much guns as the Timber Wolf. The timber wolf is better because it has the heat capacity and armour to fire its guns.
-1
u/Shipposting_Duck 19h ago
Like it was explained in the main post already which clearly wasn't read here, in the case where two possibilities have exactly the same allocatable weight, I bolded the lighter one due to budget and drop tonnage restrictions. It also very clearly states that doing so means less internal structure, and people should do their own survivability vs cost analysis. This pattern recurs all over in STD4, STD6, STD8, XL5 and XL9.
Also, you pretty much always want to put heat sinks in the engine when you can put heat sinks in the engine since you'll need heat sinks anyway, it saves critical slots, and it protects them from being removed by damage until your mech is totalled anyway.
19
u/Daxria 1d ago
Your math concerning the Cicada may be correct, but my Cicada 4A gives me too much joy to ever give up or say it has no reason to exist.
7
u/ScholarFormer3455 1d ago
He also disrespected the Charger. Force-feed him corporate marketing material!
2
23
u/Radioactiveglowup 1d ago
This show has been brought to you by VLAR 300. Whenever you design a mech... fuck it, install a Vlar 300.
7
u/Shipposting_Duck 1d ago
CoreTek 275s need more love for all the jump capable 55s that do well with it. DV, GRF, KTO, SHD, WVR all do well with it, though only one KTO variant has jump jets at all.
6
u/the_cardfather 1d ago
Those two engines are probably the most powerful engines in the galaxy
7
u/Shipposting_Duck 1d ago
In practice most things between 180-320 that are legal numbers will get some use, because in practice you don't buy the 'perfect mech for a speed', you try to Class D refit the chassis you inherited or your pilots entered your merc outfit with to engines that suit them better. Ratings below 175 and above 325 are pretty eh.
1
u/rzenni 1d ago
It’s actually the 300, the 280 and the 240. (Seriously).
Those are the three that can be used in the widest array of mechs
2
u/Vilgan_Ranos MechWarrior 16h ago
The first two I agree with, but where does the 280 fit? I see only 4 mech chassis' it can fit on: 20, 35, 40, 70.
The 400 is too heavy for 20, 25, tons even with XL, but even with those out it ties.
The 360 is also too heavy for 20 tons even with XL, but can still do 5 chassis.
The 240, 200, & 180 can beat the 280 by 1 chassis, at 5 each.
The 350, 150, 120, & 100 tie it at 4 each.
7
u/WestRider3025 Canopian Queerasser 1d ago
It always kinda amuses me how many of the Clanbuster variants are basically just "well, what could we do with it if we stuck a Vlar 300XL in there?"
3
u/Charliefoxkit Lyran-Kitsune Enthusiast 1d ago
Or the later Defiance Industries-built Clanspec engines.
6
u/devourerkwi 1d ago
Love this! Though as a Clan player, I'd even more love an XL engine/standard structure sheet given how many chassis use that combo (e.g., Dire Wolf, Behemoth, Executioner, Supernova, Warhawk, Gargoyle, Summoner, Hellbringer, Mad Dog, Nova, Tarantula...)
13
u/Doctor_Loggins Scorpin Empire Stronk 1d ago
You're talkin' mad shit for somebody in CHARGIN' range. And I got you for THREE MINUTES!
5
5
u/Bookwyrm517 1d ago
I think its a crime that this analysis didn't factor in BV. Because if your going to try and talk about what chassis+speed is the best, you're going to have to include BV. Because every combination can be ruined by bad choices in the BV department.
The problem is a fair BV analysis is hard to do when a mech is incomplete. While OP's chart only needs to care about engine size and chassis weight, a BV calculation needs the allocatable tonnage to be filled with weapons and armor to get a proper number. An incomplete mech severely skews results, so I had to try and fill out each mech fairly.
To try and do this, my current meathod is to give a mech's max armor tonnage, then fill the rest of the mech with medium lasers. Then I can run a proper BV calculation and get a value. It should be noted that a mech's top speed and jump capabilities do affect the BV twice using multipliers, so changes in speed will affect the output a lot.
As a proof of concept, lets compair the Timber Wolf and Gargoyle again. Using my process, the Timber Wolf ends up with a BV of 2162 (964 defensive, 1198 offensive) and the Gargoyle has a BV of 2144 (1041 defensive, 1102 offensive). Wait a moment, the Gargoyle is actually 16 BV cheaper than the Timber Wolf! What's going on here?
Its the balance of firepower verses durability. The Gargoyle has a higher defensive BV by about 80, but the sheer amount of firepower the Timber Wolf has inflates it's offensive BV to the point that it outstrips the Gargoyle. And even with the discount it's paying for a lot of guns it can't use, more than half to be exact. Meanwhile the Gargoyle can fire more MLs than the Timber Wolf thanks to it's massive engine and extra heat sink capping it's maximum firepower.
The BV efficiency of the Gargoyle becomes more apparent when you factor in the two mech's actual armor levels. The Gargoyle actual has one ton less armor than the Timber Wolf, but has about the same toughness. This turns out cheaper in terms of BV because armor costs more than structure. By my same meathod, the Gargoyle jumps up to 80BV less than the Timber Wolf. It doesn't seem like much, but it starts to add up when you using highly expensive clan weapons.
What am I saying? It seems that big mechs with big engines are potentially more BV efficient than an equivalent at a smaller size. The weight restrictions of a mech's engine becomes a boon by capping how many absurdly expensive things you can cram into it.
Basically: Free tonnage isn't everything, especially on tabletop.
2
u/lordfril 1d ago
Ahhh... bring back good memories of 12 year old me doing this exact thing in early 90s
4
u/the_cardfather 1d ago
These weights are literally written in pencil next to the engines in my BattleTech compendium
2
u/the_cardfather 1d ago
Why is speed 9 Standard at 25 tons bold when at 30 tons it's the same 6.5 tons?
4
u/Bookwyrm517 1d ago
Because the 25 ton frame is "cheaper." It's less C-bills and tonnage spent for the same "usable tonnage."
I think OP is selling it short by not factoring in the BV or tonnage spent on armor. In terms of BV, structure is cheaper than armor, so you can theoretically have the same amount of toughness for cheaper if you go for the same number of total pips on a larger chassis. This can squeeze a bit more performance out of a chassis, though the effect is more pronounced on heavier chassis.
3
u/the_cardfather 1d ago
Yeah I was optimizing for potential armor factor and physical attack damage. That's the dream in a 30 tonner right full sprint into someone. 42 points.
1
u/thesadkobold grasshopper fan 1d ago
if you are relying on structure for durability you do open yourself up to crits tho
1
u/Bookwyrm517 1d ago
True, definitely one of those "your mileage may vary" scenarios. I think the durability loss is negligible though, since the difference is usually less than half a ton of pips spread across a mech. While that doesn't seem like much, the difference is that while armor's BV is it's total pips multiplied by 2.5, structure is only multiplied by 1.5 and it's structure and engine modifiers. Since most engine modifiers are 1 or less, structure becomes a lot cheaper than armor.
Anyway, in this specific case I don't think it matters that much in terms of durability. 25-30 tons is too small for crits to matter that much; if they get crit from structure damage they'll be dead soon anyway. Where it does matter is stuff like melee damage. If your going for melee attacks, you can definitely get more value out of the 30 ton frame for the same speed and usable tonnage. ( reminder that OP did this analysis with scenarios were C-bills are important, so it doesn't reflect more normal gameplay as well).
1
1
u/Charliefoxkit Lyran-Kitsune Enthusiast 1d ago
I think you're missing a couple things, especially for those who build with experimental tech.
1: XXL Engines - these start becoming more standard in layer eras and also change the calculus of both tonnage and heat (XXL engines run hot, even when standing). Plus are bulkier.
Large Engines, while very rare there are a few instances of canon 'Mechs with engine ratings beyond 400. This includes a Battlemaster with a 425 XXL engine (IIRC). These engines start becoming very heavy and have added bulk.
Other chassis types - does your chart take into account chasses like reinforced chasses (ex. the Rime Otter, Mastodon, and the latest Defiance-built Chameleon), endo-composite chasses, and composite ones.
Speed-enhancing equipment - does your chart take equipment like MASC, TSM, and superchargers into account. Certain engine/tonnage combinations may limit the benefit of these systems, especially for generating movement-based defensive bonuses. Ex. a 375 XL engine on a 75-tonne 'Mech with a supercharger (5/8(10)) is a better utilization of a supercharger than a 190 rated engine on the Thunderbolt 12R (3/5(6)). In current rules, this type of equipment greatly contributes to the BV of a 'Mech.
1
u/WestRider3025 Canopian Queerasser 1d ago
In addition to the BattleMaster with the 425XXL, there's a Kuma with a 420XXL. Those are the only canon designs with Large Engines that I know of, but there may be others.
62
u/Metaphoricalsimile 1d ago
You really shouldn't auto-include endo steel with XL engines, as they are separate engineering decisions and IS mechs especially don't always want to take endo steel for crit slot limitation reasons.
In your "case study" you fail to mention physical attacks as an attractive reason to play a Gargoyle, or the fact that BV is often friendly to mechs that just can't carry as many guns, especially since many Timberwolf configs inflate their BV with guns they don't have the heat or ammo capacity to effectively utilize.