Any engineers here?

Kansas Kid · November 27, 2001

Ok guys, I got a question about some forces needed to design a set of trailer brakes for a project in one of my classes. It probably isn't all that hard, but my brain is fried from working on this dang computer all day. Figured somebody on here might know what to do.

Maximum weight of the trailer is 20,000lbs, and it has 4 drum brakes. I'm also going to assume that the maximum speed of the trailer is 80mph. (I know what assuming does, but oh well. LOL) So what I need to know, is how to figure out the amount of torque that each of the 4 brakes will need to generate to stop the 20,000lbs. Once I have an idea of what amount of torque the brakes need to generate, I have all the equations to determine the sizes of the pads, etc.

I'm thinking I can calculate the momentum and figure out the frictional force to overcome the momentum instead of finding the brake torque, but don't know exactly what to do there either. ??? ???

Thanks

siume99 · November 27, 2001

I think I need more info. Such as distance to stop in. (You could always just let up on the accel. and coast to a stop or downshift.)

76BBSub · November 27, 2001

Definitely need more info...distance to stop would allow you to calculate an average deceleration rate, which would be required. Also, need to know the diameter and width of the tires and assume a coefficient of rolling friction between the tires and road, which is different depending on the road surface, but should be about the same for asphalt and concrete.

This is a tricky one, and I haven't done squat with dynamics since I graduated 6 years ago...

Jeff

Kansas Kid · November 27, 2001

Why does this not suprise me. The teacher that gave us this problem seems to like not giving all the necessary info, I think he gets a kick out of making these problems dang near impossible to get right. ???

I'm gonna have to make another assumption here, that the stopping distance is say 250ft. Tires will be 31" in diameter and 9" in width (235/85R16's), and I will try to find some value for the coefficent of friction.

If nothing else, I'm closer then I was before I asked the question.

UltraGN · November 27, 2001

I'm thinking your ASSumption of 250' to stop that thing is a bit low. I'm a Highway Engineer (roads, bridges) and according to our design stardard bible (AASHTO manual), the braking distance to stop a passenger vehicle (3-4000 lbs ave) is approx. 450'. This distance would be increased for heavier weight. This is assuming level, dry asphalt roadway.

The formula for the stopping distance is:

d=1.075[(V^2)/a], where

v=speed (mph)

a=decelleration rate, (ft/s^2)-approx 15.0 in a panic stop.

Hope this helps a little.

I should be working.....I'll call it overhead!

-Kevin

Torque Junkie · November 27, 2001

You'll also need to know the diameter of the brake drum so you can use the radius and the frictional force of the shoes to the drum to figure out the torque necessary by the drum itself.

Also, is the braking action of the vehicle pulling the trailer taken into consideration, or is this a problem for the trailer only excluding other variables like that?

-Mike

Kansas Kid · November 27, 2001

Looks like the old proffesor left alot up to assumption. (I hate when they do that)

But here is what I have so far:

20000lbs

80 mph

4 brakes

500 ft stopping distance (probably not enough, but sounded good)

approx 13.76 ft/s^2 average deceleration rate (using formula for stopping distance from above)

4 LT235/85R16 tires per axle (total of 8)

31" diameter, 9" wide

12" brake drum

I don't have a coefficient of friction for the tires on asphault yet, but will get one tonight. This is also the stopping force for the trailer only. Looks like I'm getting somewhere, shouldn't be too hard to get it figured out by next Tuesday. ;)

UltraGN · November 28, 2001

Not sure, but you may not need the CO friction for asph. You would only need that if you were skidding. You would only need the CO Friction to figure stopping distance, but you have that covered with the formula (assume dry asphalt).

Lucky for me most of my design criteria comes from charts in design manuals....I would struggle if I had to think this stuff out every day. ???

Torque Junkie · November 28, 2001

Not sure, but you may not need the CO friction for asph. You would only need that if you were skidding. You would only need the CO Friction to figure stopping distance, but you have that covered with the formula (assume dry asphalt).

You would need the coefficient of friction of the tires to the asphalt because if you exceeded the frictional force provided by the tires to the road, it would put it into a skid.

To find the frictional force of each tire, you'd have to calculate how much weight is on each tire, and then calculate the surface area of tire patch to road, based on diameter and width, and then use that to figure out the downward force on each tire and then use that with the coefficeint of friction for rubber to pavement with a formula I don't know off the top of my head

Unless of course, he is going to just throw out the idea of skid, which would make the problem all the simpler.

You will definately need the coefficient of friction between the brake shoes and the drums, though.

-Mike

UltraGN · November 28, 2001

True Mike...

I guess I was thinking of a no skid situation.

No more thinking for me!

Kansas Kid · November 28, 2001

This is where I'm at right now. It was really simple to get this far, but I was making things about 10 times harder then it needed to be.

Taking the deceleration and plugging that number, plus the weight into an equation from Newton,

F=ma

gives

F=(20000/32.2) x 13.77

F= 8553 pounds force

This force is being generated by four brakes, so the force generated by a single wheel is 2138 lb.

Since torque=force x moment arm and we know both, the answer becomes

Torque = 2138 x (31.0/12)/2

= 2761 ft-lb

Now I'm in the home stretch. I need to calculate the frictional force to see if the tires will skid, and then if my brakes I have selected will provide the necessary torque.

Thanks a ton for the help guys!!!

Jpshostr · November 28, 2001

You were having trouble deciding to use F=MA? Man, I want a class like yours with this stuff. How bout we switch, you take over my phasors and electromagnetic waves and I'll do your mass-velocity stuff.

Kansas Kid · November 28, 2001

You were having trouble deciding to use F=MA? Man, I want a class like yours with this stuff.

Ok mister smart-a$$, would you like to see the 3 pages of stuff I'll have to go through now for all brake mechanisms? I'd gladly let you go ahead and figure them up right here since its so easy.....

siume99 · November 28, 2001

Looks like you got your help. For the coefficient of friction between tire and pavement (concrete/asphalt, use which ever has the higher) If I remember right concrete was between 1 and 4 if dry and clean. Its always harder to lock up tires if you have a lot of grip. You definately want your brakes to be able stop you as fast as possible. If its possible to lock up on dry pavement, it's a sinch on the slick stuff.

BTW what is your major Kansas Kid? @ what school?