As the end approaches for the on-site phase of the National Transportation Safety Boards probe into last Saturdays I-55 motor-coach disaster, chief NTSB investigator Gary Van Etten reported few new findings in a Thursday morning briefing. Van Etten did issue a revised estimate of tire marks left on the pavement by the doomed vehicle, a tour bus which was headed for the casinos of Tunica, Miss., when it left the Interstate and capsized early last Saturday, taking 14 Chicagoans to their deaths and leaving 15 others hospitalized in the Memphis, Tennessee-West Memphis, Arkansas area. Investigators have been handicapped somewhat by intermittent drizzle and continuous overcast conditions, but an extended burst of sunshine on Friday permitted the charting of a 47-foot skein of tire marks in addition to the 17 feet of late-stage skid marks disclosed on Thursday. Relating the marks to the vehicles steering, Van Etten stated no conclusions but said, Normal driving doesnt leave tire marks on the roadway. The chief investigator said his 6-member team had completed the mechanical part of its investigation and would be decamping on Saturday but would be returning periodically to the area as other aspects of the probe -- including interviews in Chicago, home base for the bus company and the passengers -- continued. Commenting wryly on some of the surviving passengers apparent preference to talk to the media rather than to investigators, Van Etten asked the media to issue its own appeals to survivors to cooperate with the NTSB. FROM 10-13 DEFECTIVE BUS? OR A DRIVER IN DIFFICULTY? The NTSB begins to finger some of the problems leading to last Saturday's I-55 bus disaster Some unusual mechanical conditions on the tour bus that crashed in Arkansas last weekend predominated among the initial findings reported by National Transportation Safety Board investigator Gary Van Etten in a Wednesday-morning briefing. Those findings include the discovery, through laser analysis, that the roof of the 1988-model motor coach -- separated from the body in the single-vehicle Interstate-55 accident that killed 15 Chicagoans on their way to the Tunica, Miss., casinos and severely injured 14 more -- had been completely coated over with a new aluminum skin, which added a total weight of 700 pounds. Van Etten said the vehicles manufacturer, Motor Coach Industries, were puzzled by the fact and said, We have yet to determine why somebody would put that on. Reasons could range from the cosmetic to the safety-related, he said. Van Etten said that other pieces of the puzzle unearthed by investigators included the fact that the vehicles rear two axes had suffered some measurable displacement and that there were cracks in the frame rails which Arkansas State Highway Police concluded may have pre-existed the accident. While stressing that it was too early for the NTSB team to hazard any conclusions, Van Etten called the cracks a significant defect -- enough to have sidelined the vehicle if discovered in one of the roadside inspections called for by federal regulations. The vehicle had been acquired in the mid-90s by its operator, Walters Charter and Tours, from another Chicago-area touring company, American Sightseeing, and the renovation work was done subsequent to that,Van Etten said. Other relevant information released by Van Etten on Wednesday had to do with the history and habits of the vehicles driver, Hubert Walters, who was among those killed in the crash. Walters, brother of touring company owner Roosevelt Walters, worked part-time for both his brothers operation and another touring line, said Van Etten, who reported, among other facts, that Walters had kept extremely incomplete logs and had been unused to overnight driving. Information gained from four of the crash survivors indicated that Walters had paused for 15 minutes at an I-55 rest-stop eleven miles north of the crash site. Van Etten said the vehicle had left 17 of skid marks on the road pavement from the rear tires, indicating that the vehicles front wheels were already off road when the brakes were first applied.