ACR WDB TXWY EXCURSION; MAIN GEAR TOO WIDE FOR TXWY AT SPI.

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I WAS CAPT OF SJU TO ORD THAT WENT TO ALTERNATE SPI DUE TO WX DELAYS AT ORD. SPI IS AN APPROVED ALTERNATE FOR THE ACFT AND I'M SURE THIS WAS THE 1ST EVER TO USE THIS ALTERNATE. ON LNDG RWY 22; I MADE A 180 DEG TURN ON THE RWY AT A RWY INTXN TO COME BACK TO TXWY D (A 75' WIDE TXWY). TWR AND I DISCUSSED USE OF OTHER TXWY DESCRIBED AS CAPABLE OF HOLDING OUR ACFT WEIGHT AND 50' WIDE. I HAD THE COPLT CHK THE APPROVED FLT MANUAL AND THE WIDTH OF OUTBOARD TIRE TO OUTBOARD TIRE IS NOT LISTED. STRUT CENTER TO STRUT CENTER IS LISTED AS 31'6'. I ESTIMATED OUR GEAR WIDTH TO BE NO MORE THAN 38'. ON DEP WE WERE GIVEN TAXI INSTRUCTIONS TO USE ALPHA TXWY TO 22. I HAD THE COPLT QUESTION THE GND CTLR ABOUT OUR WEIGHT AND WAS ASSURED IT WAS USABLE. OUR RELEASE NOTAMS CONTAINED NO RESTRICTIONS FOR OUR ACFT REGARDING TXWYS. AFTER QUESTIONING THE TAXI INSTRUCTIONS; I FOLLOWED THEM. WHEN I CAME TO THE ARC TURN FROM A TO A9 TO GET TO THE RWY; I STOPPED AND FLT CREW AGAIN DISCUSSED THE TURN WITH GND CTL BUT HAD NO OPINION NOW ABOUT THE TURNING RADIUS NEEDED. I FELT SOMEONE MUST HAVE CONSIDERED ALL THIS WHEN SPI WAS APPROVED FOR OUR USE AND IF I KEPT THE NOSE WHEEL JUST RIGHT OF ENTER; I COULD BE ON THE SAFE SIDE BY JUST A COUPLE OF FEET. ABOUT THREE QUARTERS OF THE WAY THROUGH THE TURN; I FELT A DRAG AND THERE WAS A SLIGHT BUMP. I ADDED A LITTLE POWER AND THE ACFT ACCELERATED SMOOTHLY OUT OF THE TURN. THE ACFT NEVER CHANGED ATTITUDE AND TAXIED NORMALLY. THE COPLT SAID; 'WHAT WAS THAT?' AND I ANSWERED; 'I DON'T KNOW;' NOT WANTING TO CONSIDER THE PROBABLE CAUSE. IT WOULD HAVE ONLY BEEN AN AVERAGE BUMP ON THE TXWY AT JFK. ON LINE UP ON THE RWY I COULD NOT SEE ANYTHING WRONG ON THE SIDE OF THE TXWY BUT DID SEE A CAR HAD BEEN FOLLOWING US OUT. ABOUT 5 MINS AFTER TKOF; I RECEIVED A MESSAGE THROUGH DEP TO CHK THE LEFT GEAR ON ARR FOR POSSIBLE MUD. AFTER LNDG AT ORD IN THE RAIN AND TAXIING THROUGH STANDING WATER TO THE GATE; I INSPECTED THE LNDG GEAR. THERE WAS ABOUT A 3' RING OF DIRT ON THE OUTSIDE OF THE TIRES ON BOTH THE LEFT AND RIGHT GEAR. I DISCUSSED THE RPT AND SITUATION AND A COMPREHENSIVE CHK OF THE LNDG GEAR AND TIRE WAS DONE AT THE HANGAR BY MAINT; AND NO DISCREPANCIES WERE FOUND. I RECOMMEND TXWYS BE LIMITED BY WIDTH OF LNDG GEAR AND NOSE WHEEL TO MAIN GEAR SPREAD TO ASSURE ABILITY OF AN ACFT TO MAKE TURNS AS WELL AS WEIGHT BEARING LIMITS. ALL APPROVED FLT MANUALS SHOULD HAVE THE GEAR WIDTH OF OUTBOARD TIRE TO OUTBOARD TIRE. I SPOKE TO THE ASSISTANT ARPT MGR WHO FOLLOWED US OUT. I HAD ONLY 3' ON EACH SIDE TO THE TXWY EDGE INSTEAD OF THE 6' I BELIEVED. TAXIING OF WIDE BODY ACFT IS NOW LIMITED BY THE SPI TWR TO ONLY THE 75' WIDE TXWY D. OTHER ALTERNATE FIELDS MAY HAVE A SIMILAR TRAP.