Towing question

[trasher]

One of my friend wanted me to "tow" his car in his parking, because he must pull the car up a litlle hill and it is just too hard to do it by pushing the car by hand in neutral. I said it was too heavy for my car to tow his car ( he has an old Dodge colt) with my hitch. I'm I correct ? Even if it's just on a distance of 100-150 feet, this car weight about 2000 lbs!

[rajan]

i'm having a bit of trouble understanding what you're asking. are you saying your friend's car is stuck somewhere and you need to tow it just 150 feet? if this is the case, you need to know a few more pieces of information before you decide whether to tow it. the three most important things are the angle of the slope, the weight of your car, and the HP of your car.

doing the calculation to see whether or not you can actually do this will take much longer than actually doing it because you'll have to get an accurate measure of the slope, the curb weight of both your cars, the power your car has, the traction of your tires (frictional coefficient), and a couple other more minor things. i suggest just trying it and making sure both of you (especially your friend) has a foot ready on the brake. hopefully you can start in a flat area and then move to the slope.

disclaimer: i've done things that are a lot stupider than this and sometimes it takes me a long time to see how stupid they really were.

[trasher]

Sorry for the explanation... my english is not so good. His car isn't stuck.. It just won't start. I know it can be towed on the flat slowly (I can push his car myself) Anyway I said to him to pay 20-30$ to call a towing truck. But I wanted to know how to calculate, for fun, the weight to be pulled for a car of 2000 lbs on a slope of maybe 20 degrees, if we doesn't take in consideration the frictionnal coefficient. It's elemental physics calculation but I don't remenber :P

[rajan]

assuming the angle is 20 degrees and the other car is 2000 lbs, the required work needed to be done if you are to tow the car on the angle for a distance of 150 ft is somewhat simple (i think all of us would have been able to do it easily if we were still in high school). converting to metric (which i think you may be more comfortable with ): 150 ft = 46 m, 2000 lbs = 909 kg.

the calculation for work goes as: Work needed = sine(20 degrees) * 48m * 909kg * 9.8m/s^2 = 146245 Newton*meters.

the 9.8 factor is the acceleration due to gravity acting against your friend's car. this is all assuming that your tires have the proper traction (frictional coefficient) to apply such a force.

the force necessary is just 3046 N at a constant rate over the slope. i'll convert this to horsepower in about an hour when i get to work and can look back on my intro physics book (i'm just a lowly chemist now).

[rajan]

Originally Posted by rajan

the force necessary is just 3046 N at a constant rate over the slope. i'll convert this to horsepower in about an hour when i get to work and can look back on my intro physics book (i'm just a lowly chemist now).

this number is just the force along the line of the slope (the work divided by distance travelled)

anyways, this problem has been racking my brain because i really should know it much better. the final solution for the horsepower you need is, assuming you'll need to exert a force of 3046N over the distance of 48 m in a time of about 30 seconds, (3046*48)/30 = 4873 W. 1 HP = 746 W so this means that you'll need 6.5 HP to bring your friend's car up the slope under the conditions above. this seems low to me, but as far as i can see, i haven't made any mistakes. someone correct me if i'm wrong on this.

[CarNut]

If your car is an automatic you would have no problem. Manual is probably OK too if you can start on level ground. The towing recommendations in your manual are conservative and take into account highway speeds and braking. You have plenty of power for the task. The only concern is the clutch/torque converter.

There are safety considerations. Does the other car have brakes? Don't forget that without the engine running they will require considerably more force. Do you even have a tow strap? Don't try to use a rope or chain. And strap towing on public streets is not legal so don't get caught.

The real consideration for me would be is it worth it? Will the guy always be looking for tows and never return the favor?

[2004NJVue]

Originally Posted by CarNut

If your car is an automatic you would have no problem. Manual is probably OK too if you can start on level ground. The towing recommendations in your manual are conservative and take into account highway speeds and braking. You have plenty of power for the task. The only concern is the clutch/torque converter.

There are safety considerations. Does the other car have brakes? Don't forget that without the engine running they will require considerably more force. Do you even have a tow strap? Don't try to use a rope or chain. And strap towing on public streets is not legal so don't get caught.

The real consideration for me would be is it worth it? Will the guy always be looking for tows and never return the favor?

I would have done it with my old Ion (manual). I do stupid things with my cars ocasionally. I towed an F150 in a similar situation for a friend and an ocasional horse (in a trailer). That was with a 96 SL2 though (manual). I'd rather do it with a manual than an automatic...but thats just me.

[trasher]

That's exactly why I didn't tow the car... I knew it would be probably okay, but if something happened, I had felt stupid to have done it to make my friend save 40$ for his piece of crap car.

thanks rajan for the calculation!
but what the 's' is for in : 9.8m/s^2 ?

[rajan]

Originally Posted by trasher

but what the 's' is for in : 9.8m/s^2 ?

s = seconds, as in meters per second per second or m/s/s. 9.8 m/s^2 is the acceleration due to gravity that the earth exerts on an object on the surface of the earth (it gets less the further away from the earth you get, which allows objects like the moon and satellites to orbit). simply put, if you dropped a ball out of a window and there was no air resistance on the ball, each second your ball fell in free fall, it would gain 9.8 meters per second of speed. in other words [units], since 9.8 meters per second = 35.28 km/h, each second you let the ball fall without friction, it would increase it's speed downwards by 35 km/h. hope this helps. keep asking questions if you have them, then hopefully i can figure out where i made a mistake if one has been made.