Hot Rods The Belly Button Bucket Build Thread

Trailer Hitch Design: Part One

What happens when you designed something but don't know if it's strong enough? Usually, you end up doing two things - research and make assumptions. I assumed that the most force the trailer hitch will ever see will be on deceleration.

Calculating Force due to deceleration:

• After some digging, I found a HRM track test of the McMullen clone roadster (http://hotrod.staging.enthusiastnetwork.com/articles/0402sr-performance-evaluation/). In there I discovered that car weighed 2,600 lbs and went from 60 mph to 0 in 166 ft.

• I got to thinking - I bet the weight of the T bucket + Teardrop Trailer would be close to 2,600 lbs. I don't think I could stop my bucket/trailer assembly in 166 ft, but having a guesstimate of deceleration greater than anticipated is actually good here, so I went with it.

• From here, there are at least two ways I know of to calculate force:

First, we can use the velocity equations:

• Distance = 0.5 * (initial velocity + final velocity) * time
• Convert: 60 mi/hr x hr/60min x min/60sec = 0.0167 mi/sec
• Convert: 0.0167 mi/sec x 5280 ft/mi = 88 ft/sec
• Finally, 166 ft = 0.5 * (88 ft/sec + 0 ft/sec) * time
• time = 3.77 seconds
• Now, calculate acceleration... final velocity = initial velocity + acceleration * time
• 0 ft/sec = 88 ft/sec + acceleration * 3.77 sec
• acceleration = -23.33 ft/sec^2

​

-------------> Now that we have a deceleration force, we can make more assumptions. The trailer hitch will see a maximum force of acceleration on braking, which is shown above. To calculate the Force on the hitch due to this deceleration, we need to assume a weight for the trailer (just the trailer here, not the bucket/trailer assembly). I have stated I want to be under 800 lbs, but let's say I end up at 900 lbs, and for a safety factor of 2, let's say my trailer weighs 1800 lbs. <--------------------------------

• Force = mass * acceleration
• Weight = mass * gravity
• 1800 lbs = mass * 32.2 ft/sec^2
• mass = 55.9 lbm
• Force = 55.9lbm * -23.33ft/sec^2
• Force = -1304.2 lbs

​

Second, we can use some Energy functions - because my energy unit conversions aren't too strong in English, I used the Metric system:

• Kinetic Energy = 0.5 * mass * velocity^2
• Convert: 1800 lbs x 2.2 kg/lb = 816.5 kg
• Convert: 88 ft/sec x 0.3048 m/ft = 26.8 m/s
• Convert: 166 ft x 0.3048 m/ft = 50.6 m
• KE = 0.5 * 816.5 kg * (26.8 m/s)^2
• KE = 293,659 Joules
• Now, calculate Work: Work = Force * Distance
• 293,659 Joules = Force * 50.6 m
• Force = 5,804 Newtons
• Convert: 5,804 Newtons x 9.81 kg/Newton x 2.2 lb/kg = 1304.9 lbs
• Force = 1,305 lbs

​

So as you can see in this demonstration, I guesstimated my way through two different scenarios to end up with identical Force values. That's all the time I have for today, but tomorrow or very soon I will work my way through the structural calculations of the major hitch assembly components using this calculated Force of 1,304 lbs.​

 

We used to build trailer hitches from scratch in a shop I worked at as an apprentice.
And trailers too.
Not once did we ever think along those complicated lines of math and calculations

Sounds like something an engineer might do
If you ever want to know the truth on engineers, read Dilbert comics

As a welder we just design what we think will be strong enough and then over build it.

 

One more thing, you may wonder why the force on the hitch in pounds is less than the estimated weight of the trailer. As shown above, weight = mass * gravity. Gravity is why you can weigh a different amount such as on the moon, on Mars, or even at different elevations on earth, depending on the force due to gravity. In my example, the McMullen roadster decelerates less than one unit of gravity, so the force due to this deceleration will be less than the weight of the vehicle.

 

That makes my head hurt, plus I was told there would be no math today Tim

 

Oh No is there going to be to a pop quiz?

 

*No Meth*. There will be no meth today. Or any day.

Yes; it's the type of quiz where it's best to write your name on the paper and turn it in otherwise blank in hopes for sympathy... Feel free to send cookies, car parts, and cash my way to see how I "feel" when it comes time to grade your papers... And that ladies and gentlemen, is why I am not a teacher.

 

I wish I would have studied math in High school..... I had to take Algebra. I read car magazines hidden inside my text book during class and signed my test papers for an F since there was no quarter given by the instructor that had no time to deal with someone less than inspired to study his beloved science.

I really regretted passing up this educational opportunity many times throughout my career and through my time as a hobbyist. I have always had to adhere to the if in doubt build her stout plan.

It's been fun watching Tim work through his engineering problems applying his math.

 

The thing I wonder is that because the load is attached to a vertical member it will want to twist the crossmember for & aft , thus increasing the actual force by the leverage the vertical tube offers [I hope that makes sense ]

 

Im with loudbang on this one (i just spit toothpaste all over my computer screen)

 

I saw a T shirt worn by a pretty young female student we had in for Welding.
It said:
Dear Algebra, Stop asking me to find your X.
She hates you and isn't coming back

 

Well last night at work sucked total nutsack, so I did not have time on break to translate my hieroglyphics into something we can all understand- this may have to wait until after the bachelor party but I will try.

 

The info you posted is straight forward and clear. A Goggle search can help folks get a grasp. I'm with Blue. Weld it strong and add a little more for St. Peter.

 

Tim
Do you know the meaning of a true friend?
A true friend shares pictures from the bachelor party

 

Tim, do you need a hand to get the bucket ready for the wedding? I can head into Portland for a week to lend a hand if needed. Have access to a Tig, a CNC mill and other shop equipment in your neighborhood.

 

Gary, that is VERY generous of you. If this were my wedding, I would be tempted to take you up on that. It's my brother's though, and it's in Hood River beginning of July. I had thoughts of getting the car there, but getting the body sorted out and an engine built in a little over a month seems far-fetched. I'll still keep working on it, but I'm shooting for a shake-down run towards the end of the summer, then finish upholstery and paint this fall/winter and debut at Portland Roadster Show 2018.....How many Portland Roadster Shows did I say I would have this done for at this point? Too many...I think I've been saying that since 2014 or so LOL.

 

Well Tim, I think that if the Ass monkey gang and all the rest of them TV guys can do it in 7 days then you should be able to also

 

Funny you should mention them... part one of the bachelor party is a concert at the Ass Monkey Bar and Grill... one of our favorite bands is on tour from Sweden, but the closest they came was Texas, and once I saw the venue, I knew it was too hilarious to pass up so I booked tickets!


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Bird-Poo tack welds aside, I'm pretty proud of this thing. I made a giant mess, but it's in the queue to be welded by a professional.


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Holy s**t, this thing is way too beautiful to be hidden under a body.... awesome job, dude!

 

I agree with @Baumi, frame looks great!

 

If the welder is a certified one keep all his receipts as they may come in handy when you try and get it registered.

 

Looks like it's shaping up to being a good weekend.











Hope everyone else is having an equally relaxing time!


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Only went to the Gas Monkey Bar and Grille once when I was delivering a load to Mary Kay Cosmetics which is next door, it got a little uncomfortable with the cowboys holding hands and smoking Virginia Slims.

 

Wow that is some NICE PAINT. If the body comes out that good it would be awesomwe.



ps watch out for any chaps wearing "cowboys" there

 

Yes, it looks nice. It's powder coat which technically is paint I suppose, just applied and then melted

 

Russel the powdercoater sent those pictures yesterday before lunch, and I couldn't help but let out a yell of disbelief. We were on our way to the family cabin on Cascade Lake in Idaho, so my brother and cousin thought I had lost my mind for a second. I showed them the photos, then we about ran off the road for not paying attention lol.


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Trailer Hitch Design: Part Two

If you thought Part One's math was bad, you probably won't like this.... It's still very straightforward, though. Let's get started. The page below is the hand copy of calculations from the other day. As I said, my initial calc's were some form of hieroglyphics on illegible paper that got tossed, so I re-did them. You can see the applied Force, and the Deflection of the tube with no strut support.



These next two pages show the same thing found two different ways. If you picture a side view of the trailer hitch, you end up with a basic picture as shown below. Dimensions are approximate but within reason. These calculations are what you learn in a beginner's engineering class called Statics - in other words, every problem you tackle in that class is assumed to be in static equilibrium until proven otherwise.

What I calculated is what's called reaction forces. If the trailer is pushing on the bottom of the tube, what's happening at the top of the tube and in the strut to keep the trailer hitch assembly from moving? (or in other words, to keep things in "static equilibrium")

This first method of finding the reaction forces is called the Three Force Principle. Basically you draw your known forces and angles to scale, then draw a triangle with the known forces and angles and calculate the rest. In this case, I knew some dimensions, some angles, and the 1304 lbs we calculated from the other day. I then broke the Force at point A into x and y components, which comes into play a couple pages later.



This next page shows how you can find the same thing as above using "moments". Moments are weird. They are described like so: "the sum of the moments about point A is..." Their units have force AND dimension (i.e. foot pounds, like torque)

The easiest way for a car guy to wrap his head around these next few topics, including moments, is the example of using a breaker bar to free some frozen bolts. You already tried to use your small ratchet and that didn't work... Your ratchet handle is, say 12" long. You gave it everything you had and the bolts didn't budge. You applied a "moment about the bolt", but it didn't move, so it remained in "static equilibrium". So, you get out your 36" breaker bar. You give it everything you've got again and the bolt starts to spin. In this case, your "moment about the bolt" overcame the forces keeping it in "static equilibrium", now making the bolt "dynamic" by the torque, or "moment", you've applied. Now, we're car guys, and we know that bolt isn't just gonna back out like that, so here's a more realistic example... Your 12" ratchet didn't work, so you break out the 36" breaker bar, eat a bowl of Wheaties, put on the Rocky soundtrack, lay hands on that thing and... SNAP!!! the bolt breaks off. In this case, the "moment" you applied, or torque, was greater than the shear strength of that bolt. The obvious reason being you have 3x the leverage with a 36" breaker that you have with a 12" ratchet.

Ok, I think that kinda explains moments... One thing you always want to do with moments is check your work at the end. If the sum, or total, of your forces doesn't equal zero at each point, you've got problems...



Now that we have some numbers to work with, we can break the trailer hitch into components. Each component can be analyzed for strength and suitability based on the forces calculated. First up is the 2" x 2" x 3/16" steel tube beam deflection, this time braced halfway down. The other numbers you see here are various properties of steel, which are usually in the back of your textbook. They are also online if you ever need to look something up.

After the deflection calculation, I moved to another moment calculation. In this one, I want to see what is the maximum moment that the drop tube can take and still be ok.

All materials have two values for their strength, and these two values are "yield" and "ultimate".

Anything less than or equal to the yield strength is considered to be in the "elastic region" - in other words, after the load is relaxed, the material will return to it's previous shape (like a rubber band).

Anything greater than the yield strength but less than the ultimate strength will permanently deform the material.

Anything greater than or equal to the ultimate strength will break the material.

So with that in mind, we want to know what's the most load you can apply to the steel drop tube without it deforming or breaking. To do that, you look up the yield strength for steel in a table (eventually you have all these memorized and people think you're crazy), then calculate the maximum moment, and finally solve for the maximum force. Next, you compare the maximum force to the force you calculated. I calculated this without the strut present just to show the steel can take a whole lot of force before it will deform.

Next, I took a look at the forged steel strut(s). Since the struts don't have a uniform cross-section, it is difficult to calculate their cross-sectional area, especially after I drilled holes in the webs of the brackets! In this case, the force is traveling perpendicular to the cross-section, so it's easy to calculate if you assume some things... What I did was measure the thick parts to the outside of the web, which were right at 3/8". Since there are 2 of these per strut bracket, and 2 strut brackets in total, I analyzed the tensile/compressive force of 4 3/8" rods made of forged steel.

I meant to calculate the fatigue stress of the struts, but I don't think I have a textbook anymore with the tables needed to calculate this. Fatigue stress will show you how many cycles your part can take before it fails. For example, if you had a connecting rod made of ___steel running at ___ rpm, which is in both tension and compression, you can calculate how long that connecting rod will last. Kinda neat. Usually they are in the trillions of cycles or greater before failure.



Next, I looked at the 7/8" DOM 0.156" wall cross bar. I calculated torsional shear, which is exactly what happened to the frozen bolt example above: it sheared due to twist/torsion/torque.

After that, I calculated the angle of twist on the cross bar due to the torsional force applied by the forged steel struts. This is NOT an accurate model/calculation of the angle of twist for a variety of reasons, some explained below. Basically, to calculate angle of twist, you have to assume a two-dimensional problem where you have a torque, or torsional force, going perpendicular to the cross section, which is not an accurate representation of my design. Nevertheless, the 1* of rotation is negligible, especially considering I suspect the actual angular rotation will be far less.



The last few things I calculated are below. First up is the shear strength of the 1/4" x 2.25" steel plate that bolts underneath the leaf spring. For shear strength, the only thing different you do is use the ultimate strength of the material instead of the yield strength (because you want to know when it fails due to shear).

After checking that out, I looked at the tensile and shear stresses of the fasteners, which were well within acceptable values.



So, I hope that explains the design a little bit. Using a safety factor of 2 (a trailer double the weight I intend to build), the trailer hitch assembly is in static equilibrium, and each component in the assembly is strong enough for the forces applied on the component.


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YIKES!!!!

 

Since you had to sit through that, here is a pic of the powdercoated parts I picked up Saturday morning. I also got the frame over to the welder. I have moved onto the body, figuring I could tackle the windshield/windshield posts this weekend. Well, that's gonna take awhile because the windshield posts are fairly intricate and I don't have the best tools to mitre the joints for the windshield frame. I'll figure it out, though!




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