How exactly Semi-ablative DR works?

[BaHalus]

I am not sure if I understood how Semi-ablative DR works. I see two possible interpretations.

1 - All damage received counts towards elimination of semi-ablative DR, so, if you have 1 point of semi-ablative DR it will protect you only twice against a 5-point damage attack (stoping 1 point of damage twice).

2 - Only damage stopped by the DR counts againt towards elimination of semi-ablative DR, so, if you have 1 point of semi-ablative DR it will protect you ten times against a n-point damage attack (stoping 1 point of damage ten times).

The first interpretation let me think it is not worth acquiring this kind of DR, nor reliable, being better to buy normal DR ou ablative DR (both will absorb more damage for the same cost in majority of situations), the second interpretation seems a fair and reliable trade, but I am not sure if is the intended interpretation.

[gilbertocarlos]

Example:
You have DR20.
If a guy keeps attacking you and doing 10 damage, you keep your DR.

Example 2:
You have ablative DR20
If a guy keeps attacking you and doing 10 damage, after 2 attacks you have DR0

Example 3:
You have semi-ablative DR20
If a guy keeps attacking you and doing 10 damage, you start losing 1 point of DR each time, after 10 attacks you have only DR10.

[Varyon]

Here's a previous discussion on the topic.

From the looks of things, semi-ablative continues protecting you indefinitely so long as you never take 10 or more basic damage from any single hit, but takes into account all damage, not just what was stopped by the DR (so if you have Semi-Ablative DR 10 and get hit by a 100 basic damage attack, it's all gone).

Houseruling semi-ablative to be cumulative but only apply against what the DR stopped seems logical and balanced. Otherwise, Semi-Ablative may well be too good at low TL, where 10+ basic damage isn't particularly common. That thread also has a houserule for allowing various levels of ablation (instead of Basic's case of Non- vs Semi- vs Fully-Ablative), which may or may not be of interest to you. At first glance, it seems rather fair and balanced.

[BaHalus]

My problem is not when the damage is lower than the DR, but when the damage is higher than the DR,

Lets say you spend 20 points in DR

1 - Normal. You have 4 DR
2 - Semi-ablative. You have 5 DR
3 - Ablative. You have 20 DR

If all damage counts. For 11 turns you receive 10 points of damage, you would have:
1 - 66 of total damage
2 - 95 of total damage (no DR since 6th turn)
3 - 90 of total damage (No DR since 2nd turn)

If only blocked damage counts
1 - 66 of total damage
2 - 80 of total damage (no DR since 10th turn)
3 - 90 of total damage (No DR since 2nd turn)

In most simulations I have done, semi-ablative DR doesn't seems to be worth the price if all damage counts, but seems to be in a fair price if only blocked damage counts. If all damage counts, than semi-ablative DR is only worth the price for High DR versus low damage, because, in this case, almost all damage received will be blocked by DR.

[Varyon]

Originally Posted by BaHalus

My problem is not when the damage is lower than the DR, but when the damage is higher than the DR,

Lets say you spend 20 points in DR

1 - Normal. You have 4 DR
2 - Semi-ablative. You have 5 DR
3 - Ablative. You have 20 DR

If all damage counts. For 11 turns you receive 10 points of damage, you would have:
1 - 66 of total damage
2 - 95 of total damage (no DR since 6th turn)
3 - 90 of total damage (No DR since 2nd turn)

If only blocked damage counts
1 - 66 of total damage
2 - 80 of total damage (no DR since 10th turn)
3 - 90 of total damage (No DR since 2nd turn)

In most simulations I have done, semi-ablative DR doesn't seems to be worth the price if all damage counts, but seems to be in a fair price if only blocked damage counts. If all damage counts, than semi-ablative DR is only worth the price for High DR versus low damage, because, in this case, almost all damage received will be blocked by DR.

Of course, under RAW, if that were instead 9 damage, you'd be looking at totals of 55, 44 (DR retained), and 79 (DR reduced to 2 after turn 2, gone thereafter), in which case Semi-Ablative works out to be the best deal. In fact, for any damage greater than 4 but less than 10, Semi-Ablative is better here.
Also, in GURPS, DR is primarily there for when your defense fails, which if you've built a character intending to see a lot of combat should be fairly rare. If you only get hit with that 10 damage attack twice in a combat, Normal is actually the worst DR to have (you end up with a total of 8 damage for normal, 7 for Semi-Ablative, 0 for Ablative).
EDIT: Add to this the possibility of wearing additional armor, and the ablative DR's get even better. Let's say your character with natural armor is at TL 4. Give him some medium plate (DR 6), and now normal DR renders you immune to damage up to 10, semi-ablative renders you immune to damage up to 11 (and doesn't start losing effectiveness until you get hit with 16+ damage), and ablative renders you immune to damage up to 30 (although it loses effectiveness rapidly against attacks that deal 7+ damage).

As I said above, it would probably be balanced to have damage to Semi-Ablative DR be based entirely on what the DR actually blocks, provided you also make said damage cumulative.

[BaHalus]

Originally Posted by Varyon

Here's a previous discussion on the topic.

From the looks of things, semi-ablative continues protecting you indefinitely so long as you never take 10 or more basic damage from any single hit, but takes into account all damage, not just what was stopped by the DR (so if you have Semi-Ablative DR 10 and get hit by a 100 basic damage attack, it's all gone).

Houseruling semi-ablative to be cumulative but only apply against what the DR stopped seems logical and balanced. Otherwise, Semi-Ablative may well be too good at low TL, where 10+ basic damage isn't particularly common. That thread also has a houserule for allowing various levels of ablation (instead of Basic's case of Non- vs Semi- vs Fully-Ablative), which may or may not be of interest to you. At first glance, it seems rather fair and balanced.

Ok, I looked there:

Originally Posted by Maz

No for semiablative you need 10 damage in one hit. so 5*2 dam wouldn't reduce DR. You would need 1*10 DR. And it doesn't 'carry over'. If you take 9 dam in one turn and 13 in another, you do not lose 2 DR. Only 1 from the 13 damage.

You could houserule that you add all damage in a single turn to determine how much the DR is reduced, but that would be a houserule.

This seems to be simple and functional and would surely make Semi-ablative DR worth the price, but it seems to contradict what is written in Basic:

Originally Posted by Basic Set Page 47

When an attack strikes semi-ablative DR, every 10 points of basic damage rolled removes one point of DR, regardless of whether the attack penetrates DR. Lost DR “heals” as for Ablative (and you cannot combine the two). -20%.

But this house rule of levels to the limitation seems cool to me, I liked it.

Originally Posted by Maz

Personally I also use more 'levels' of ablativeness so that you can have DR that is reduced per 5 damage taken or 15 damage taken:

New limitation. Levels of Ablative.
Ablative (7): 1 DR per 1 dam = -80%
Ablative (6): 1 DR per 2 dam = -60%
Ablative (5): 1 DR per 3 dam = -40%
Ablative (4): 1 DR per 5 dam = -30%
Ablative (3): 1 DR per 10 dam = -20%
Ablative (2): 1 DR per 15 dam = -15%
Ablative (1): 1 DR per 20 dam = -10%

[Mr_Sandman]

Originally Posted by BaHalus

This seems to be simple and functional and would surely make Semi-ablative DR worth the price, but it seems to contradict what is written in Basic:

But this house rule of levels to the limitation seems cool to me, I liked it.

I think you may have missed the significance of a word that did not get included in your quote.

When an attack strikes semi-ablative DR, every 10 points of basic damage rolled removes one point of DR, regardless of whether the attack penetrates DR.

The way I read that, 10 points of damage have to be done by an individual attack to remove a point of semi-ablative DR.

[BaHalus]

Oh. That makes sense now. Thanks. I was focusing in the "every 10 points of basic damage rolled removes one point of DR", not in the "When an attack strikes semi-ablative DR" part.