diff --git a/src/doomdef.h b/src/doomdef.h
index 69faba1a0..844be1261 100644
--- a/src/doomdef.h
+++ b/src/doomdef.h
@@ -607,6 +607,9 @@ typedef enum {
 #ifndef max // Double-Check with WATTCP-32's cdefs.h
 #define max(x, y) (((x) > (y)) ? (x) : (y))
 #endif
+#ifndef clamp
+#define clamp(x, y, z) (((x) < (y)) ? (y) : (((x) > (z)) ? (z) : (x)))
+#endif
 #endif
 
 // Max gamepad/joysticks that can be detected/used.
diff --git a/src/k_kart.c b/src/k_kart.c
index 6e88dc3da..ac2663e2c 100644
--- a/src/k_kart.c
+++ b/src/k_kart.c
@@ -12573,12 +12573,7 @@ void K_AdjustPlayerFriction(player_t *player)
 	// Friction was changed, so we must recalculate movefactor
 	if (player->mo->friction != prevfriction)
 	{
-		player->mo->movefactor = FixedDiv(ORIG_FRICTION, player->mo->friction);
-
-		if (player->mo->movefactor < FRACUNIT)
-			player->mo->movefactor = 19*player->mo->movefactor - 18*FRACUNIT;
-		else
-			player->mo->movefactor = FRACUNIT;
+		player->mo->movefactor = P_MoveFactorFromFriction(player->mo->friction);
 	}
 }
 
diff --git a/src/k_terrain.c b/src/k_terrain.c
index 261ffb9d5..872d94067 100644
--- a/src/k_terrain.c
+++ b/src/k_terrain.c
@@ -703,9 +703,7 @@ void K_SetDefaultFriction(mobj_t *mo)
 	if (mo->terrain != NULL)
 	{
 		fixed_t strength = mo->terrain->friction;
-
 		fixed_t newFriction = INT32_MAX;
-		fixed_t newMovefactor = INT32_MAX;
 
 		if (strength > 0) // sludge
 		{
@@ -731,18 +729,7 @@ void K_SetDefaultFriction(mobj_t *mo)
 
 		if (isPlayer == true)
 		{
-			newMovefactor = FixedDiv(ORIG_FRICTION, newFriction);
-
-			if (newMovefactor < FRACUNIT)
-			{
-				newMovefactor = 19*newMovefactor - 18*FRACUNIT;
-			}
-			else
-			{
-				newMovefactor = FRACUNIT;
-			}
-
-			mo->movefactor = newMovefactor;
+			mo->movefactor = P_MoveFactorFromFriction(newFriction);
 		}
 	}
 }
diff --git a/src/p_local.h b/src/p_local.h
index 11bfa5aed..4d2bbdb07 100644
--- a/src/p_local.h
+++ b/src/p_local.h
@@ -589,6 +589,7 @@ void P_CheckGravity(mobj_t *mo, boolean affect);
 void P_SetPitchRollFromSlope(mobj_t *mo, pslope_t *slope);
 void P_SetPitchRoll(mobj_t *mo, angle_t pitch, angle_t yaw);
 void P_ResetPitchRoll(mobj_t *mo);
+fixed_t P_MoveFactorFromFriction(fixed_t friction);
 fixed_t P_ScaleFromMap(fixed_t n, fixed_t scale);
 fixed_t P_GetMobjHead(const mobj_t *);
 fixed_t P_GetMobjFeet(const mobj_t *);
diff --git a/src/p_mobj.c b/src/p_mobj.c
index 0a6d5c9fa..fd3628d4e 100644
--- a/src/p_mobj.c
+++ b/src/p_mobj.c
@@ -1371,6 +1371,25 @@ void P_ResetPitchRoll(mobj_t *mo)
 	mo->roll = 0;
 }
 
+//
+// P_MoveFactorFromFriction
+//
+fixed_t P_MoveFactorFromFriction(fixed_t friction)
+{
+	fixed_t ret = FixedDiv(ORIG_FRICTION, friction);
+
+	if (ret < FRACUNIT)
+	{
+		ret = 19*ret - 18*FRACUNIT;
+	}
+	else
+	{
+		ret = FRACUNIT;
+	}
+
+	return ret;
+}
+
 #define STOPSPEED (FRACUNIT)
 
 //
diff --git a/src/p_slopes.c b/src/p_slopes.c
index f5e4bd3ef..f13c60d7b 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -25,6 +25,7 @@
 #include "w_wad.h"
 #include "r_fps.h"
 #include "k_kart.h" // K_PlayerEBrake
+#include "m_easing.h"
 
 pslope_t *slopelist = NULL;
 UINT16 slopecount = 0;
@@ -1202,8 +1203,34 @@ void P_ButteredSlope(mobj_t *mo)
 	// Let's get the gravity strength for the object...
 	thrust = FixedMul(thrust, abs(P_GetMobjGravity(mo)));
 
-	// ... and its friction against the ground for good measure (divided by original friction to keep behaviour for normal slopes the same).
-	thrust = FixedMul(thrust, FixedDiv(mo->friction, ORIG_FRICTION));
+	if (mo->friction != ORIG_FRICTION)
+	{
+		// ... and its friction against the ground for good measure.
+		// (divided by original friction to keep behaviour for normal slopes the same)
+		thrust = FixedMul(thrust, FixedDiv(mo->friction, ORIG_FRICTION));
+
+		// Sal: Also consider movefactor of players.
+		// We want ice to make slopes *really* funnel you in a specific direction.
+		fixed_t move_factor = P_MoveFactorFromFriction(mo->friction);
+
+		if (mo->player != NULL)
+		{
+			if (mo->player->icecube.frozen == true)
+			{
+				// Undo this change with ice cubes, because it is insanity.
+				move_factor = FRACUNIT;
+			}
+			else if (mo->player->tiregrease > 0)
+			{
+				// Undo this change with tire grease, so that
+				// springs and spindash can still overpower slopes.
+				fixed_t grease_frac = clamp((FRACUNIT * mo->player->tiregrease) / greasetics, 0, FRACUNIT);
+				move_factor = Easing_Linear(grease_frac, move_factor, FRACUNIT);
+			}
+		}
+
+		thrust = FixedMul(thrust, FixedDiv(FRACUNIT, move_factor));
+	}
 
 	P_Thrust(mo, mo->standingslope->xydirection, thrust);
 }
diff --git a/src/p_spec.c b/src/p_spec.c
index 6303b84ae..dbddeead1 100644
--- a/src/p_spec.c
+++ b/src/p_spec.c
@@ -9164,11 +9164,7 @@ static void P_SpawnFriction(void)
 		if (friction < 0)
 			friction = 0;
 
-		movefactor = FixedDiv(ORIG_FRICTION, friction);
-		if (movefactor < FRACUNIT)
-			movefactor = 8*movefactor - 7*FRACUNIT;
-		else
-			movefactor = FRACUNIT;
+		movefactor = P_MoveFactorFromFriction(friction);
 
 		Add_Friction(friction, movefactor, (INT32)(s-sectors), -1);
 
diff --git a/src/r_data.c b/src/r_data.c
index ff38f888f..e4993ffad 100644
--- a/src/r_data.c
+++ b/src/r_data.c
@@ -95,7 +95,7 @@ UINT32 ASTBlendPixel(RGBA_t background, RGBA_t foreground, int style, UINT8 alph
 		}
 		return output.rgba;
 	}
-#define clamp(c) max(min(c, 0xFF), 0x00);
+#define clamp_rgb(c) max(min(c, 0xFF), 0x00);
 	else
 	{
 		float falpha = ((float)alpha / 255.0f);
@@ -104,30 +104,30 @@ UINT32 ASTBlendPixel(RGBA_t background, RGBA_t foreground, int style, UINT8 alph
 		float fb = ((float)foreground.s.blue * falpha);
 		if (style == AST_ADD)
 		{
-			output.s.red = clamp((int)(background.s.red + fr));
-			output.s.green = clamp((int)(background.s.green + fg));
-			output.s.blue = clamp((int)(background.s.blue + fb));
+			output.s.red = clamp_rgb((int)(background.s.red + fr));
+			output.s.green = clamp_rgb((int)(background.s.green + fg));
+			output.s.blue = clamp_rgb((int)(background.s.blue + fb));
 		}
 		else if (style == AST_SUBTRACT)
 		{
-			output.s.red = clamp((int)(background.s.red - fr));
-			output.s.green = clamp((int)(background.s.green - fg));
-			output.s.blue = clamp((int)(background.s.blue - fb));
+			output.s.red = clamp_rgb((int)(background.s.red - fr));
+			output.s.green = clamp_rgb((int)(background.s.green - fg));
+			output.s.blue = clamp_rgb((int)(background.s.blue - fb));
 		}
 		else if (style == AST_REVERSESUBTRACT)
 		{
-			output.s.red = clamp((int)((-background.s.red) + fr));
-			output.s.green = clamp((int)((-background.s.green) + fg));
-			output.s.blue = clamp((int)((-background.s.blue) + fb));
+			output.s.red = clamp_rgb((int)((-background.s.red) + fr));
+			output.s.green = clamp_rgb((int)((-background.s.green) + fg));
+			output.s.blue = clamp_rgb((int)((-background.s.blue) + fb));
 		}
 		else if (style == AST_MODULATE)
 		{
 			fr = ((float)foreground.s.red / 256.0f);
 			fg = ((float)foreground.s.green / 256.0f);
 			fb = ((float)foreground.s.blue / 256.0f);
-			output.s.red = clamp((int)(background.s.red * fr));
-			output.s.green = clamp((int)(background.s.green * fg));
-			output.s.blue = clamp((int)(background.s.blue * fb));
+			output.s.red = clamp_rgb((int)(background.s.red * fr));
+			output.s.green = clamp_rgb((int)(background.s.green * fg));
+			output.s.blue = clamp_rgb((int)(background.s.blue * fb));
 		}
 		// just copy the pixel
 		else if (style == AST_COPY)
@@ -136,7 +136,7 @@ UINT32 ASTBlendPixel(RGBA_t background, RGBA_t foreground, int style, UINT8 alph
 		output.s.alpha = 0xFF;
 		return output.rgba;
 	}
-#undef clamp
+#undef clamp_rgb
 	return 0;
 }