I'd be happy to hear about experiences with locking-on, especially that I was told that low Thermal Signature vehicles can be difficult to target.

Won't it be too difficult to target them? Especially if they manage to cause some penalties to lock-on rolls? Is this intended to encourage to melee?

Actually, now that I think about it, there isn't anything that would prevent the internal game logic from being ported over to physical targeting versus thermal targeting (this applies to the Steamtech vs High Tech thread as well).

Thermal Signature = Targeting Signature, based on Size.
Tech Modifier is due to size of power plant.
Thermal Dampers = Technical Developments that allow the same technology to be fit into a smaller frame, i.e. micro-sizing.
Stealth System = Stealth System
Ballistic and Energy Weapons increase TS because they are simply physically bigger than kinetic weapons.
Weapon Mod's that increase or decrease TS are likewise due to space requirements for the equipment.

Due to the % math of the "Thermal Dampers", using full Size instead of half Size for the TS would only raise the final TS by 1-2, but it would make the balancing of TS and equipment and weapons harder, and would doubtless make for a bloodier game.  I think there might be a "wait and see who runs out of smoke" effect, but that's already built in to the system to a certain degree.

that was one of the things I was thinking about - base the Signature on Size for the settings where your targetting would be based more on vision or sounds than on special devices. for example at the 'clockwork' tech level, where thermal signatures tend to be low but also machines should be so big and clanking that they would be easy to spot and target.

and what was your idea? use Size as the base Targetting Signature and modify it in the same way as Thermal Signature would be modified?

Or modify the Size in some way and then use it as the base for Targetting Signature?

I agree. This works for all the settings where you use some kind of devices to spot targets. But when spotting is based on what you see or hear, size or noise become more important. This is more important in the early tech levels (clockwork - which also has low Thermal Signatures, and steam). I believe it should be much easier to see a large metal clockwork walker (which has very low thermal signature) than a squad of infantry).

So I think that for such settings I would use Size as base for Targetting Signature, and then modify it in the same way as Thermal Signature would be modified.

Moreover, you all discuss optimal and suboptimal vehicles/walkers - ok, I agree creating them is a lot of fun, but sometimes you want to stick to a number of predefined vehicles so that the players have to use what they are allowed to choose from and cannot create their own vehicles.
In this case it's only the tactical aspect of the game that they will be able to experience and use for their advantage.
Not that there's anything wrong or right about it - just the way it can be (and will sometimes be in my case).

The cumulative +1 is actually a pretty good idea, and would provide some interesting tension.  I would say that a failed Stability roll would cancel the accumulated bonus.

The overall issue is one of stacking modifiers.  There are four classes of modifiers, which can be combined to great effect: 1) tech that reduces TS, 2) tech that provides cover, 3) movement, and 4) cover.

Tech that reduces TS is in the form of Thermal Dampers (up to 67% TS reduction but pretty expensive), Stealth System (straight -2 but pretty cheap), Ceramic-composite Armor (straight -2 and very cheap, but +1 to Dmg is tough), and Organic Armor (straight -1 and regenerating armor for only 2EM and no downsides ... obviously needs campaign consideration).

Tech that produces cover are Cratering Charge (+1, but canceled out by the -1 mod for standing still), Smoke Dispensers (+3), and Decoy Flares (+5/+3/+1).

Movement modifiers are -1 for 0-2 moved, +1 for 6-8, +2 for 9-12, and +3 for 13+.  The -1 for not moving means that most units won't sit still unless they have cover to cancel out the penalty.  Most medium and even heavy mechs can get the +1, and most light mechs can get +2.  It's possible to design very light mechs that get +3.

Cover modifiers are +1 for partial cover and +2 for forests (which I suppose accounts for the biomass making the TS fuzzy).

Enhanced Sensors can give bonuses to hit, but they can be canceled out by Electronic Countermeasures, which more or less produces an arms race where everyone will try to buy max levels of both if possible.

So, just by using movement and cover you should be able to get anywhere from +2 to +4 in average situations, meaning that mechs with a TS of 2-4 are going to be very hard or even impossible to hit.  A TS of 2-4 is easy to achieve with the use of Thermal Dampers by themselves, even for Fusion powered mechs with multiple hot weapons systems.  Given this math, the use of Smoke Dispensers and Decoy Flares essentially represent turns of being invulnerable to targeting, but they are limited in number (and you might want to save some in case your Thermal Dampers get hit).

Some possible variants to consider, depending on what flavor campaign you want to run:

1) Don't allow Thermal Dampers and Stealth Systems to stack, i.e. Stealth System is just a variant of Thermal Dampers and you have to choose one type or the other.
2) Reduce the effectiveness of Smoke to +1, and Flares to +3/+2/+1.
3) Reduce the number of Smoke charges to 3 and Flares to 1.
4) Don't allow Smoke and Flares to stack with Thermal Dampers or Stealth System, the "fluff" behind this being that they create a negative-effect cold halo that can be targeted just as easily as a TS.  If you use this one, you probably don't have to use #2 and #3!
5) Don't allow Electronic Countermeasures, which will allow Enhanced Sensors to cancel out a lot of bonuses, but will result in a bit of an arms race with everyone wanting L3 Sensors.
6) Use full Size instead of half size for calculating base TS.  Sounds radical, but given the math of the Thermal Dampers this will only result in an increase in final TS of 1-2.  However, it might be enough to make targeting easier and still leave the rest of the systems in place as is.

As for my campaign, I think I'll go for #1 and not mix Dampers and Stealth.  I like the idea of Smoke and Flares being "oh shit" resources that you use to save your shiney ass after your Dampers get taken offline, so I'll probably use #4 but not #2 or #3.  I'm not sure if I'll use #5 or #6.

now this seems to be the answer to many issues mentioned so far!

including my idea of settings not relying on thermal targetting - now you can use size-based targetting and moving doesn't make target acquisition more difficult but causing serious damage becomes harder when the target is moving fast.

good one for me.

which means send infantry or your own light units in to take and hold advantage terrain or close to short or point blank range and attack.

Don't panic yet...there's one other minor modification I'm pondering on as well to finish addressing the issue...but it doesn't include the "1" always locks because I believe that someone should be able to generate just such a situation if they are playing tactically.  However...it shouldn't be so easy to do as it is presently.