Trailer whip for nerds

[EPGregg]

I spent some time this summer with some really great guys that happened to be engineers. When I mentioned that I was building a Tear Drop one of the many suggestions that they gave me regarded trailer whip.

I can't vouch for the accuracy, but I thought I'd share it just because it is a little different than the seat of the pants way I usually work!

This is the computer version of the diagram that was on the back of a paper plate.

Gregg

[Nitetimes]

Looks like about 60/40 for the trailer axle.

[bdosborn]

So how do you tell where the CG of the trailer is?

Bruce

[angib]

I spent some time this summer with some really great guys that happened to be engineers.

But certainly not automotive engineers who had studied trailer dynamics, or they wouldn't have come up with such a load of bulls**t.

For example, the absence of speed from the equation shows they don't understand the subject - trailer whip/sway/instability is speed-dependent, which is how come it only strikes above a certain speed (different for each trailer/tug combination).

It looks like they tried to make an equation that matched what they think they typically see driving down the road - put it in a formula and it looks like they know what they're talking about.

There are scientific papers on trailer stability but you need the Greek alphabet as well as the Roman one to cover all the terms that go in the equations. And the distance between the tow vehicle bumpers is certainly not one of the terms!

Andrew

[planovet]

So how do you tell where the CG of the trailer is?

My thought exactly...

[starleen2]

So how do you tell where the CG of the trailer is?

My thought exactly...

Spooky Magic??

[asianflava]

I've been trying to picture this formula. When you consider an 18 wheeler as an example, it doesn't work since the trailer is so much longer than the drive section. Also the CG will shift further forward as the cargo is unloaded, yet they are still stable enough to take onto the highway.

[bobhenry]

What about tongue length. There is no provision in this cock eyed equasion for tongue length. After building eggbert I hooked him to my Goldwing motorcycle and made a test run. I can closely equate to dragging an engine block on a chain. Not the weight so much as the crazy side loads it placed on a 2 wheeled tow vehicle. As a wheel hit an obstruction ( hole or hump) the tongue transfered this as a sideload. I immediatly lengthened the tongue from the stock 28" ?? to almost 4'. The tow behavior was much less violent. The center of gravity only changed a few inches ( the weight of the tongue materail) but the towing stability changed dramatically.

I am experiencing the same problem with my 5 x 10 at 65 MPH there is a side sway that is very upsetting so interstate travel is current minimalized for our safety as well as others. I have heard the term Harmonic resonance. While I do not pretend to fully understand it I know that as the speed increases above the 65 MPH the sway starts to increase in scope and frequency. I will be lengthening and reinforcing the tongue on it as soon as more reasonable weather arrives and feel confident it will eliminate the problem as I have been able to do with Eggbert my M/C Teardrop trailer.

Comments are very welcome !!! I may be overlooking something !

As to center of gravity 2 scissor jacks with a couple of bolts laid on the lift area to act as a knife edge and a bathroom scale you should be able to narrow down the CG to within a couple inches. Not real scientific but redneck engineering has its moments ask Orville and Wilber.

[Mauleskinner]

So how do you tell where the CG of the trailer is?

Bruce

Step 1: Obtain 3 scales (you can use one and move it around, but it's a lot easier with 3)

Step 2: Place the trailer on the scales...one under each wheel, the third under the hitch stand.

Step 3: Level the trailer front-to-back and side-to-side using blocking as necessary...you can put the blocking between the scales and the wheels, but you have to remember to subtract it's weight from the readings.

Step 4: Read the scales. Oh...write down the numbers, too.

Step 5: Determine a "datum" for measurement...if you use the axle as your datum, the math is a LOT easier.

Step 6: Measure the distances from the datum to the wheels in inches ...using the axle as datum, simply measure the distance from the axle to the hitch stand.

Step 7: Using the formula WEIGHT x ARM = MOMENT, make the appropriate calculations:
(Scale reading under left wheel) x 0 = 0
(Scale reading under right wheel) x 0 = 0 (I told you the math was easy this way )
(Scale reading under hitch stand) x (distance measured) = ???

Add up the three scale readings (less the weight of blocking).

Take the total MOMENT, which in our case is simply the ??? number, since we're doing things the easy way, and divide by the total weight of the trailer. The resulting number is the number of inches forward of the axle your CG is located.

(???) / (total weight) = CG

Have fun!

David[/list]

[BPFox]

Or you could just place the trailer on top of two automotive style jack stands along each frame rail. Move the stands along the rails until that trailer balances on the stands. That is the center of gravity. This can be accomplished using just the tongue jack. Start by running up the tongue jack about 3/4 of the total lift of the jack. Push the jack stands along the frame rails as far as you can to the rear of the trailer. Lower the tongue jack until the jack stands have all of the weight of the trailer. If the tires are still in the ground, then you try again jacking the trailer even higher and moving the stands back. Repeat until the trailer is in perfect balance on the stands. No special tools and no math skills required.

That's the good news, the bad news is that the center of gravity changes based on how the trailer is loaded. Pretty much make the whole exercise a waste ot time. Peace.

[stomperxj]

But certainly not automotive engineers who had studied trailer dynamics, or they wouldn't have come up with such a load of bulls**t.

For example, the absence of speed from the equation shows they don't understand the subject - trailer whip/sway/instability is speed-dependent, which is how come it only strikes above a certain speed (different for each trailer/tug combination).

It looks like they tried to make an equation that matched what they think they typically see driving down the road - put it in a formula and it looks like they know what they're talking about.

There are scientific papers on trailer stability but you need the Greek alphabet as well as the Roman one to cover all the terms that go in the equations. And the distance between the tow vehicle bumpers is certainly not one of the terms!

Andrew

I couldn't agree more Andrew. it says nothing about wheelbase of the vehicle either. Who cares what the distance is between the hitch ball and the front bumper. There are too many variables when it comes to the vehicles and trailers to make a standard calculation like that...

[planovet]

So how do you tell where the CG of the trailer is?

Bruce

Step 1: Obtain 3 scales (you can use one and move it around, but it's a lot easier with 3)

Step 2: Place the trailer on the scales...one under each wheel, the third under the hitch stand.

Step 3: Level the trailer front-to-back and side-to-side using blocking as necessary...you can put the blocking between the scales and the wheels, but you have to remember to subtract it's weight from the readings.

Step 4: Read the scales. Oh...write down the numbers, too.

Step 5: Determine a "datum" for measurement...if you use the axle as your datum, the math is a LOT easier.

Step 6: Measure the distances from the datum to the wheels in inches ...using the axle as datum, simply measure the distance from the axle to the hitch stand.

Step 7: Using the formula WEIGHT x ARM = MOMENT, make the appropriate calculations:
(Scale reading under left wheel) x 0 = 0
(Scale reading under right wheel) x 0 = 0 (I told you the math was easy this way )
(Scale reading under hitch stand) x (distance measured) = ???

Add up the three scale readings (less the weight of blocking).

Take the total MOMENT, which in our case is simply the ??? number, since we're doing things the easy way, and divide by the total weight of the trailer. The resulting number is the number of inches forward of the axle your CG is located.

(???) / (total weight) = CG

Have fun!

David[/list]

Sorry I asked!

[bdosborn]

I think I'll stick with Andrew's trailer balance spreadsheet. I'd rather design an all around stable trailer instead of picking a tow vehicle to make the trailer (hopefully) stable .
Bruce

[Mauleskinner]

So how do you tell where the CG of the trailer is?

Bruce

Step 1: Obtain 3 scales (you can use one and move it around, but it's a lot easier with 3)

Step 2: Place the trailer on the scales...one under each wheel, the third under the hitch stand.

Step 3: Level the trailer front-to-back and side-to-side using blocking as necessary...you can put the blocking between the scales and the wheels, but you have to remember to subtract it's weight from the readings.

Step 4: Read the scales. Oh...write down the numbers, too.

Step 5: Determine a "datum" for measurement...if you use the axle as your datum, the math is a LOT easier.

Step 6: Measure the distances from the datum to the wheels in inches ...using the axle as datum, simply measure the distance from the axle to the hitch stand.

Step 7: Using the formula WEIGHT x ARM = MOMENT, make the appropriate calculations:
(Scale reading under left wheel) x 0 = 0
(Scale reading under right wheel) x 0 = 0 (I told you the math was easy this way )
(Scale reading under hitch stand) x (distance measured) = ???

Add up the three scale readings (less the weight of blocking).

Take the total MOMENT, which in our case is simply the ??? number, since we're doing things the easy way, and divide by the total weight of the trailer. The resulting number is the number of inches forward of the axle your CG is located.

(???) / (total weight) = CG

Have fun!

David[/list]

Sorry I asked!

Well, the subject line DID say "for nerds"...figured I'd fit right in.

[Mary K]

ugh...my head hurts...