clive@druri.UUCP (StewardCN) (07/09/85)
line. I'm considering myself very fortunate only to have lost a little skin. Would like to have others' consideration on the cause of this mishap. Last night, went for a ride up a canyon on a friend's 82 or 83 Sportster. Wasn't going fast at all, and had finished with the curves, looking for a good place to turn around. At <= 60 mph, geared to third, and got on the brakes for upcoming turn. Going straight, dry rough (1" pebbles showing) slightly uneven asphalt. I might have pulled the brakes a little hard; had been trying to get used to their being harder pull but less action than my Honda. Was going noticeably uphill, which surely made everything easier. Without warning, the front end went completely wild, oscillating from side to side very rapidly. I hung on enough to tear up a foam handgrip on the right side, which didn't contact. At relatively low speed, bike went over, and I off. Bent fender, torn off gearshift and misc lights. Bounced and torn but functional rider. Very fortunate. Skid marks verify my description, right down the middle of my side of the road, well before the curve. The bike is stock except for Fat Bob tanks and a seat. Disk front brake. One suspicious item was a newish (400 miles) Dunlop Elite on the front end. Guy at the bike shop said he saw my friend nearly drop it riding away, from the grabbiness. Also, my friend himself said he had noticed he had to radically change his corner entry; set up sooner (he's no racer, though). I thought the same thing in the curves before the crunch; needed early setup (I'm no racer, either), but just put it down to being a heavier bike than I'm used to, more inertia and gyroscopic reaction to get initiated. Being of engineering background, I suppose I'm schooled enough in basic dynamics as they ought to apply to the fork. Though don't know where the torsional damping of the system on modern bikes would come from except tire patch friction, which throws the tire with its spring coefficient and dynamically nonlinear behavior squarely in the middle of the situation. The bottom line is this. I'll pay to have his bike fixed, etc., of course, including a first line inspection of the front end. But as far as I am concerned, I don't think this should have happened, and I'm a little worried about him riding the bike (20 year old). I see no difference between what I did, and what any one would, for instance, if a prairie dog ran out in front. Hit the brakes, and the worst that should happen is a locked wheel, which could result in a spill if you're turning or initiate one through woggles. I'd admit to any lack of skill or screwup cheerfully, but what occurred here was a very severe dynamic instability which was essentially uncorrectable (unless someone has a hint, which would be very welcome). At this point, I'm for getting that tire off and replaced with whatever the original equipment was. On cars, I know about the oscillatory tread action on some radials and semi-racing technology belted tires (Michelin XAS, for example). If the bike were mine, this is what I would do, and so will offer so to him. But of course, what I'm after is any combination of advice from others who may have experience and/or specific knowledge here, and possibly other avenues to suggest. Please help. Mail or net; I wouldn't mind seeing a discussion of this at all. Incidentally, we will have a look a damper oil and the head bearings. Anything else which should be checked/replaced outright? Thanks, Clive Steward ...ihnp4!drutx!druri ATT Information Systems Westminster, Colorado