Hard Rock (HR) mod icon
For the full list of all mods, see: Game Modifier
Everything just got a bit harder...
The Hard Rock mod is a game modifier that aims to increase the difficulty over most aspects of a beatmap. Generally, it achieves this goal by raising the circle size (CS) by 30%, the approach rate (AR) by 40%, the HP drain (HP) by 40% and the overall difficulty (OD) by 40%, though the details vary for each game mode.
In osu!, (along with the increase in difficulty settings) the Hard Rock mod flips the beatmap on it's X-axis (shown below). This can sometimes make certain patterns feel unnatural, which can increase difficulty.
Additionally, if a player has memorised or become familiar with the movements and patterns of the beatmap, switching to the Hard Rock mod can break that memorisation and cause a sort of artificial difficulty.
Comparison of a regular play (left) vs a play with the Hard Rock mod enabled (right) in osu!
In osu!taiko, the Hard Rock mod increases the slider velocity as well as the overall difficulty (OD). This mod is widely regarded as the most commonly used mod in osu!taiko due to it being the only difficulty increasing mod that does not require some sort of memorisation or extremely fast hitting on harder difficulties.
The hits required to finish a denden is increased due to the increase in OD. Therefore, the maximum possible score for osu!taiko difficulties with the Hard Rock mod on is larger than that of the Hidden mod.
However, this effect is insignificant in most cases because each hit in a denden gives a static score of 300. Which is even smaller than a typical score difference between a GREAT and GOOD hit (550 points at 100x combo or above).
In osu!catch, the Hard Rock mod has several effects. In addition to making the catcher and fruits smaller and increasing how fast they fall, this mod can cause ordinary fruits to veer slightly to the left or right from their normal position. This change requires players to make more chaotic movements and to use hyperdashes more frequently, which can also increase difficulty.
Due to this much larger spike in difficulty compared to other game modes, the Hard Rock mod is worth 1.12x multiplier as opposed to 1.06x on all other game modes.
Comparison of a regular play (left) vs a play with the Hard Rock mod enabled (right) in osu!catch
Notice: Using this mod on osu!mania will result in an unranked play.
In osu!mania, the HP drain is increased by 40% while the timing windows are about 29% stricter. Although the AR value is displayed as increased, it does not affect the gameplay.
To calculate the timing windows after HR is applied, the original timing window needs to be divided by 1.4. The timing window of PERFECT is fixed regardless of the OD, which means the timing window of a PERFECT with HR will always be ±11.43ms.
Note that the calculation method is different in ScoreV2.