PA28 STUDENT PLT AND INSTRUCTOR PLT HAD THE R MAIN GEAR SCISSORS BREAK ON LNDG. THE ACFT HAD A RWY EXCURSION INTO THE MUD.

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I AM A CALM AND MATURE; STUDENT PLT. I OWN A PA28; THE PLANE I WAS FLYING UNDER DUAL INSTRUCTION WITH A CFI PERFORMING TOUCH AND GOES AT STS. ON MY 8TH LNDG OF THE DAY I NOTED A MUCH STRONGER THAN PREVIOUS XWIND DRIFT TO THE R WHICH I ARRESTED JUST PRIOR TO TOUCHDOWN ON THE R HALF OF THE RWY. THE TOUCHDOWN WAS REASONABLY GENTLE; AS HAD ALL PREVIOUS LNDGS OF THE DAY. (THESE TOUCH AND GOES WERE EXPECTED TO BE THE PRACTICE FOR THE NEXT DAY'S FIRST SOLO.) SEVERAL SECONDS LATER; DURING A ROLLOUT WITHOUT BRAKING; THE PLANE BEGAN TO 'SHIMMY' THEN VEERED UNCONTROLLABLY TO THE R; EXITING THE RWY ONTO VERY SOFT MUD. OUR FIRST THOUGHT WAS THAT WE HAD A TIRE BLOWOUT; BUT INSPECTION REVEALED THAT ALL 3 TIRES WERE IN GOOD SHAPE. THE L MAIN SHOCK WAS NEARLY FULLY DEPRESSED AND THE R MAIN SHOCK WAS FULLY EXTENDED; THE SCISSOR TORQUE LINK MOUNT ON THE STRUT WAS BROKEN OFF. ARPT EMER CREW AND AN FBO CREW HELPED PULL THE PLANE FROM THE MUD AND TOW IT. THE SHOCKS WERE RETURNED BACK TO NORMAL DURING THIS OP. IT IS OBVIOUS THAT THE BROKEN STRUT ALLOWED THE R MAIN TO TURN. THE WHEEL ASSEMBLY THEN CASTORS; SO THAT AT SLOW SPD; NO PROB WOULD BE OBSERVED. BUT AT HIGH TAXI SPD; WITH A XWIND SIDE LOAD; THE WHEEL BEGAN AN OSCILLATION THAT BUILT UP UNTIL IT HAD TURNED FAR ENOUGH TO DRAG TURN THE PLANE. ONCE ONTO THE MUD; THE OSCILLATION ABRUPTLY STOPPED; ALLOWING THE PLANE TO ROLL SAFELY TO A STOP. THE ABRUPT TURN PROBABLY SHIFTED THE WT OFF OF THE R SHOCK; ALLOWING IT TO EXTEND; WHILE DEPRESSING THE L SHOCK. ON CLOSE INSPECTION OF THE BROKEN ALUMINUM STRUT CASTING; THE INBOARD LUG SHOWED CLEAN; BRIGHT FRACTURE. BUT THE OUTBOARD LUG SHOWED DARKENED FRACTURE; SUGGESTING THAT IT HAD LONG AGO OXIDIZED. I SUSPECT THAT THIS PART HAD BEEN PARTIALLY BROKEN LONG BEFORE I PURCHASED THE PLANE IN SEP/03. THE PLANE HAD ITS ANNUAL INSPECTION JUST LAST MONTH. AT THAT TIME; THE R INBOARD WHEEL PANT MOUNT WAS REPLACED; AS IT WAS FOUND TO HAVE BEEN BROKEN SOMETIME IN THE PAST AND REPAIRED USING RIVETS TO HOLD 2 BROKEN PIECES TOGETHER. THESE RIVETS WERE COMING LOOSE. ONE CAN SPECULATE THAT AT SOME TIME IN THE PAST; AN INCIDENT HAD BROKEN THIS MOUNTING PLATE; WHICH MAY HAVE ALSO CAUSED A FRACTURE TO FORM IN THE SCISSOR TORQUE LINK MOUNT ON THE STRUT ABOVE IT. I BELIEVE THAT HAD THE STRUT BEEN INSPECTED BY REMOVING PAINT; THE EXISTING CRACK MIGHT HAVE BEEN DISCOVERED PRIOR TO THIS INCIDENT. I WAS UNABLE TO FIND AN ENTRY IN THE LOG REGARDING THE REPAIR OF THE WHEEL PANT MOUNTING PLATE. LOOKING BACK AT THE INCIDENT; I MIGHT HAVE BEEN ABLE TO ARREST THE ABRUPT TURN HAD I RECOGNIZED THAT EXCESS DRAG ON THE R COULD HAVE BEEN BALANCED BY MATCHING BRAKING ON THE L. GIVEN THAT A TIRE BLOWOUT DURING LNDG IS A COMMON OCCURRENCE; PERHAPS TRAINING FOR THIS KIND OF SIT; USING A SIMULATOR; MIGHT INCREASE SAFETY. I ALSO RECOMMENDED THAT ALL ENTRIES IN ACFT LOGBOOKS BE COMPUTER PRINTED ON STICKERS; AS SOME ALREADY DO; SO THAT ALL ENTRIES ARE LEGIBLE (MANY IN MY ACFT LOGS ARE NOT). FURTHER; ALL INCIDENTS THAT CAUSE EVEN MINOR DAMAGE SHOULD BE FULLY DOCUMENTED IN THE LOGS. (I CERTAINLY WILL DOCUMENT THIS INCIDENT IN THE LOGS!) BECAUSE THIS INCIDENT WAS VERY MINOR; THE TWR DID NOT REQUIRE A WRITTEN RPT. I FEEL THIS WAS AN ERROR. HOW ELSE IS THE NTSB AND FAA TO KNOW ABOUT SUCH MINOR MECHANICAL PROBS DEVELOPING IN OUR AGING GA FLEET? IT IS MY UNDERSTANDING THAT THIS INCIDENT DOES NOT REQUIRE NTSB NOTIFICATION UNDER NTSB 830.5 SINCE IT WAS DAMAGE ON THE GND TO THE LNDG GEAR NOT EXCEEDING $25000 AND NO ONE WAS HURT. GIVEN THAT THIS STRUT WAS LIKELY TO HAVE BEEN BROKEN SOMETIME IN THE PAST; I BELIEVE THAT SOMEONE WAS NEGLIGENT IN NOT INSPECTING THE PART AFTER AN INCIDENT SEVERE ENOUGH TO HAVE CLRLY BROKEN ANOTHER PART. I FEEL THAT I MYSELF WAS PARTLY TO BLAME FOR NOT REALIZING THAT THE EVIDENCE OF THE BROKEN WHEEL PANT MOUNTING PLATE AND ITS REPAIR SHOULD HAVE TIPPED ME OFF TO THE POTENTIAL PROB WITH THE STRUT ABOVE IT. THE PLT/OWNER OPERATOR IS ULTIMATELY RESPONSIBLE FOR PROPER MAINT. (ALTHOUGH INTRUTH; HAD THE MOUNTING BEEN REPLACED; RATHER THAN REPAIRED; WITH NO LOG ENTRY; I WOULD NOT HAVE HAD EVEN THIS HINT.)