Can I convert resistance to skidding to pounds?
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From: mm on 1 Jul 2010 02:56 Can I convert resistance to skidding to pounds? A friend's pickup was hit from behind by a hit and run driver. The rear bumper was bent 90 degrees, pointing down instead of to the rear, and also forced under the car about 8 inches. He doesn't want to pay someone to repair it. Today, with a come-along, pulling horizontally, we pulled the bumper back 8 inches and started turning it upwards, but clearly we can't finish that part because we were pulling horizontally. If we attached the come-along from above the truck, or used jacks below the bumper, we would lift the truck. He has a cap above the bed of the truck, but we could still put a bunch of heavy people in the bed to make it possible to bend the bumper without lifting the truck, I think. It would help a lot to know in advance how many people, how much weight, was needed, and it strikes me as an interesting question, whose solution is eluding me since I don't even know in what terms friction is measured. Static dry friction, I think, acc. to wikip on "friction". F = d/dt * (mv) But the formula doesn't help me at all. :-( ** We know one thing about the force needed to bend the bumper brackets. It is less than the friction provided by the tires. When the truck was in Park, with the parking brake on I think, it still moved a bit when I cranked the come-along, but when he put his foot on the brake too, it stopped moving. I can find out the weight of the truck, which is a small pickup, a Nissan 1/2 or 3/4 ton pickup about 6 years old. I think the friction of the tires wrt to the pavement is a factor of the truck's weight and the tires' design, and there must be a formula to convert these values to the force needed to bend the bumper brackets and from there to the weight needed to hold the truck down while jacking up the rear bumper. **"For calculating minimum stopping distance, a value of 0.8 is a nominal value for the coefficient of static friction between good tires and a good road ..."*** That might be the best I can find, since I wonder if specific tires are rated wrt static friction, since one article says that "sliding friction" or "dynamic friction" is what matters wrt not skidding. After reading a bit, is it as simple as taking the weight of the truck and multiplying by 0.8 (a nominal coefficient of static friction according to one page***) to get the number of "pounds of friction", just estimating for now, 2000 pounds * 0.8 = 1600 pounds, which we know is more than what it took to bend the bumper brackets? 1600 - minus the weight already on the rear bumper of the truck when it is being jacked up, at least ?? 400 ?? pounds (out of 2000 total) = 1200 = about 6 guys at 200 pounds a piece. *** http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html Thanks a lot for any help you can give.
From: nuny on 1 Jul 2010 06:12 On Jun 30, 11:56 pm, mm <NOPSAMmm2...(a)bigfoot.com> wrote: > Can I convert resistance to skidding to pounds? > > A friend's pickup was hit from behind by a hit and run driver. > > The rear bumper was bent 90 degrees, pointing down instead of to the > rear, and also forced under the car about 8 inches. > > He doesn't want to pay someone to repair it. > > Today, with a come-along, pulling horizontally, we pulled the bumper > back 8 inches and started turning it upwards, but clearly we can't > finish that part because we were pulling horizontally. > > If we attached the come-along from above the truck, or used jacks > below the bumper, we would lift the truck. > > He has a cap above the bed of the truck, but we could still put a > bunch of heavy people in the bed to make it possible to bend the > bumper without lifting the truck, I think. > > It would help a lot to know in advance how many people, how much > weight, was needed, and it strikes me as an interesting question, > whose solution is eluding me since I don't even know in what terms > friction is measured. Physics aside, if it's a steel bumper, remove and replace it. Repeated bending has weakened it to the point that it's dangerous to leave it in place. Mark L. Fergerson
From: bert on 1 Jul 2010 10:17 On Jul 1, 2:56 am, mm <NOPSAMmm2...(a)bigfoot.com> wrote: > Can I convert resistance to skidding to pounds? > > A friend's pickup was hit from behind by a hit and run driver. > > The rear bumper was bent 90 degrees, pointing down instead of to the > rear, and also forced under the car about 8 inches. > > He doesn't want to pay someone to repair it. > > Today, with a come-along, pulling horizontally, we pulled the bumper > back 8 inches and started turning it upwards, but clearly we can't > finish that part because we were pulling horizontally. > > If we attached the come-along from above the truck, or used jacks > below the bumper, we would lift the truck. > > He has a cap above the bed of the truck, but we could still put a > bunch of heavy people in the bed to make it possible to bend the > bumper without lifting the truck, I think. > > It would help a lot to know in advance how many people, how much > weight, was needed, and it strikes me as an interesting question, > whose solution is eluding me since I don't even know in what terms > friction is measured. > > Static dry friction, I think, acc. to wikip on "friction". > F = d/dt * (mv) But the formula doesn't help me at all. :-( ** > > We know one thing about the force needed to bend the bumper brackets. > It is less than the friction provided by the tires. When the > truck was in Park, with the parking brake on I think, it still moved a > bit when I cranked the come-along, but when he put his foot on the > brake too, it stopped moving. > > I can find out the weight of the truck, which is a small pickup, a > Nissan 1/2 or 3/4 ton pickup about 6 years old. > > I think the friction of the tires wrt to the pavement is a factor of > the truck's weight and the tires' design, and there must be a formula > to convert these values to the force needed to bend the bumper > brackets and from there to the weight needed to hold the truck down > while jacking up the rear bumper. > > **"For calculating minimum stopping distance, a value of 0.8 is a > nominal value for the coefficient of static friction between good > tires and a good road ..."*** That might be the best I can find, > since I wonder if specific tires are rated wrt static friction, since > one article says that "sliding friction" or "dynamic friction" is what > matters wrt not skidding. > > After reading a bit, is it as simple as taking the weight of the truck > and multiplying by 0.8 (a nominal coefficient of static friction > according to one page***) to get the number of "pounds of friction", > just estimating for now, 2000 pounds * 0.8 = 1600 pounds, which we > know is more than what it took to bend the bumper brackets? 1600 - > minus the weight already on the rear bumper of the truck when it is > being jacked up, at least ?? 400 ?? pounds (out of 2000 total) = 1200 > = about 6 guys at 200 pounds a piece. > > ***http://hyperphysics.phy-astr.gsu.edu/hbase/crstp.html > > Thanks a lot for any help you can give. Forget about math. Your problem can be solved with leverage,and hydrolic jack. That is how a body shop would handle it. Car must be a clunker if it has a bumper. Throw the bumper away and use a 2x6 oak It don't bend and is cheap TreBert 2x6 is all the math needed
From: Cwatters on 1 Jul 2010 17:39
"mm" <NOPSAMmm2005(a)bigfoot.com> wrote in message news:bvbo26lqoe1flddfiecqfge1s696grluvg(a)4ax.com... > After reading a bit, is it as simple as taking the weight of the truck > and multiplying by 0.8 (a nominal coefficient of static friction > according to one page***) to get the number of "pounds of friction", > just estimating for now, 2000 pounds * 0.8 = 1600 pounds, which we > know is more than what it took to bend the bumper brackets? 1600 - > minus the weight already on the rear bumper of the truck when it is > being jacked up, at least ?? 400 ?? pounds (out of 2000 total) = 1200 > = about 6 guys at 200 pounds a piece. Seems ok. |