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Old January 18th, 2020, 01:07 PM   #2
DannoXYZ
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Name: AKA JacRyann
Location: SF East Bay
Join Date: Dec 2011

Motorcycle(s): CB125T, Ninja250R racer, Ninja250R street bike, EX250F#1, EX250F#2, EX250E, VF500F, CBR600RR

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MOTY - 2018, MOTM - Nov '17
Yup, been hit by these things couple times and seen countless others fly off. Mostly while riding my bicycle from Santa Barbara on Hwy-101 north to Vandenberg AFB or south to Ventura. It’s worse now that they’re made from steel instead of lead. Can cause chips in windshield now due to hardness of steel. Not as damaging as rocks or other debris which tends to be larger and may actually have backwards velocity. Those can cause cracked windshields.

Wheel-weights always have forward or zero velocity when they fly off. At bottom of wheel rotation, they have zero velocity and if they fall off there, they would fall straight down to ground. At top of wheel rotation, they are moving forward at twice speed of car. Falling off there, they would fly forward at twice speed of car. This explains why I’ve always been clobbered by them from behind when riding my bicycle on side of hwy-101. Only been hit twice in 15-yrs of riding there (~37k-miles on hwy-101).

Actual impact energy is quite low due to their low weight. And they typically bounce couple of times before they hit you, slowing them down significantly. That’s from reference point of almost stationary observer on side of road.

From reference point of motorcyclist following car, it’s slightly different. A wheel-weight flying off top of wheel would be moving forward away from you at same speed of car. So 80mph car would launch wheel-weight forward at 160mph. Weight would appear to be moving away from you at 80mph and bounce off to side of road.

If fell off at bottom of wheel, it would have zero velocity and fall straight down. Now if somehow bounced up higher, it would appear to be flying towards you at your speed. So if you were following car at 80mph, it would look like it’s flying towards you at 80mph.

What kind if impact are we looking at? Let’s compare to more common household objects like... a baseball. A 1/2-oz wheel-weight is 1/10th mass of baseball. Or have momentum of baseball moving at 8-mph. Which is speed of baseball dropped into your hand from 2.3-ft above. Not that bad at all.


Relevant equations:

momentum P = mv
free fall t = v/g
free fall h = v^2 / 2g

Last futzed with by DannoXYZ; January 18th, 2020 at 02:40 PM.
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