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Physics Lesson - CG and Tilt Effects On Trailer

J

JayTheCPA

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Am looking for an easy way to calculate how much weight shifts back-and-forth from a trailer's hitch to the trailer's axles as it relates to highway grade.

Reason for the question is that I am at a point of rehabbing a trailer's suspension and want to make sure the new axles remain able to support its GVWR with side-benefits of making sure the ball weight remains good along with some safety margin in the event that one tire blows and I need to pull the trailer for some distance (it is actually designed to do this).

From what I can tell, the answer lies in CG as it moves forward / backward, but the question is by how much? Lets take an empty flatbed trailer as the working example to avoid advance math by way of having different weight at different points on the trailer (which makes for a more complicated calculation of CG). It is as simple as when going up a 5% grade, the trailer's CG moves back by 5%?
 
I think a 60% set back of the axles from the front of the trailer should be about right. Might do a drive by some trailer sales places and do some quick measurements to see how the various manufacturers set theirs up.
I seen a pic of a trailer that a feller here has built, to Me, there was way too much trailer behind the axles. I think He is going to have a lot of problems with that unit.
 
Ok, so clarification and apologies. Lets presume this is something like a single car trailer with fixed location axles which attaches to a pickup by a ball and *not* a semi-sized trailer with adjustable axles.

And back to what is driving the question: my trailer is getting new axles, but am not so sure that the OE ones have enough carrying capacity to include a safety margin. So, am looking into slightly heavier duty axles just to make sure that they are not only able to carry GCVW on flat ground, but allow for crossing ~10% grades along with not stressing when a tire blows.
 
man there's alot of variables in that ? a top heavy load would transfer more weight than one down low to the deck. but aside from that I don't think there'd be all that much transfer, you'd have to have a pretty steep grade to make alot of difference
 
ak diesel driver said:
man there's alot of variables in that ? a top heavy load would transfer more weight than one down low to the deck. but aside from that I don't think there'd be all that much transfer, you'd have to have a pretty steep grade to make alot of difference
Click to expand...

What he said. The grade that your pulling should not have much of an effect. Make sure you position the load in the proper area, hence not too far back from the axle or axles(lifting up on the towing vehicle), or you will get the hospital wobble. What's the hospital wobble, well that's when the trailer starts violently swaying back and forth, you loose control and there is one heck of a crash, usually someone gets hurt and has to go to the hospital.

Yelp ideally it is preferred to keep loads low in order to eliminate anything being top heavy.

Big things are, have the proper rated hitch, hitch ball and high enough ratings on the axles and tires. Also trailer brakes are a big plus for sure. Load securement is very important also, you can never have too many chains or tie downs, there is no over kill. Ideally all loads should be strapped down with a tie down from each corner( minimum of 4) preventing front to back movement and side to side movement. Also on the tie downs, don't use some cheap dog chains (yelp its been tried before and people got killed) or low rated straps, make sure they have a high enough rating for what you are securing. How do I know all of this? I stayed at a Holliday Inn last night...…... ;) :smuggrin:
 
MrMarty51 said:
I think a 60% set back of the axles from the front of the trailer should be about right. Might do a drive by some trailer sales places and do some quick measurements to see how the various manufacturers set theirs up.
I seen a pic of a trailer that a feller here has built, to Me, there was way too much trailer behind the axles. I think He is going to have a lot of problems with that unit.
Click to expand...

Marty made a very good point, don't have too much trailer or load back past the axles towards the rear, remember we mentioned earlier about the hospital wobble. I assume your trailer will have 2 axles based on what you said, 2 axles are better than one for sure.
 
So, a big mountain grade is 10% which is less than 6 degree angle. If you take a level and an angle finder or protractor to hold it there, then move the level foreword even slowly - you’ll see that accelerating and stopping has far more affect on that bubble than the angle.
Do the same thing with a 2x4 where you feel the weight in your hand and no bubble to watch.
That 10% grade has so little effect compared to the rigs acceleration/ deceleration/ rapid lane changes that axle placement or loading the weight on the trailer has little to so with the hills compared to normal handling.

I used to build trailers, trucks, etc at the truck equipment shop. We built many custom trailers that would tow like a nightmare if they were empty because the load they were designed to carry was odd machinery that greatly affected the tongue and axle weight.

The generic rule of thumb for tongue weight used is 12/24. 12% of trailer weight on tongue for bumper pull and 24% of trailer weight on tongue if 5th wheel/ goose neck, but there is a lot of formulas for different situations.

Axle placement on trailer and weight per axle is all based on load. If you look at placement of the axles on two identical length camp trailers, one without bathroom, water tanks, etc vs one that has no tanks, the axle positions will be different.

So really if it is a generic flatbed trailer, copying a car trailer is the most common move.
 
ak diesel driver said:
man there's alot of variables in that . . .
Click to expand...

Completely agree which is why I wanted to start at a very simplistic example. Guess I have myself to blame for not realizing all the possible tangents this conversation could have gone and should have ID'd the trailer ;)

Trailer is a travel trailer RV. Yes, I can effect its CG by way of where I put things like food, tools, and cookware, but outside of that the manufacturer already solidified where the bulk weights lie and where the axles mount. Tongue weight is ~10%, so am good there (measured on a CAT scale when fully loaded and ~300# inside GVWR). The only curiosity item I have is that the OE axles have a combined carrying capacity which is ~6% lower than GVWR, but with 10% of weight going to the hitch, the OE axles sould remain just inside their rating.

From the commentary so far, it appears that the effect of grade is not that much which helps in the decision making of whether I want to go with a set of higher capacity axles. Again, only goal of considering higher capacity axles is for safety margin and not to carry additional load.

Another thing I am gathering so far is that while a relationship of CG and tilt exists, there is not a common formula to at least get a semi-scientific mathmatical effect.
 
I like what Will said grade alone pales in comparison to other forces. The manufactures put the cheapest axle they can that will satisfy the weight requirements statically. They don't calculate dynamic loads which would far exceed the static loads. For the most part I think its like ragu its in there.

I have cringed and stepped back watching a heavy load shift and tilt and watched tires expand to almost bursting. Watch a tandem axle trailer go over a big bump or curb while turning and take almost the entire load on what seems like 1 tire it makes me step back sometimes. I am often in amazement there are not more blow outs. I generally don't worry about axle limits but I don't like to push tires to the limit.

If it its not much more $ I would feel safer if axles could carry the entire GVWR but probably not necessary.
 
Appreciate the inputs!

Seeing as the OE axles are close to carrying their rating, am going to step-up their replacement's capacity a bit :) From what I am seeing, stepping up the capacity will not cost much, if anything, more than what I have now, but this is not about the money ;)
 
Ok.
Yes bump up the axle capacity.
The suspension could be bumped up with it, but if you do something like adjustable torsions with shock absorbers or adjustable airbags w/ shocks, it will greatly improve the ride, tire life, and safety.

I don’t think I would play with axle placement anywhere.
 
YUP, what @Will L. said about axle placement. Keep that the same.
If a higher rated spring can be had that will fit right into the existing mounts then, if it were Me, I would change out the springs too.
If the wheels are a 5 or 6 lug wheel, then, go with an 8 lug hub and axles. then the wheel and tire rating can be also improved to match the load.
 
Will L. said:
Ok.
Yes bump up the axle capacity.
The suspension could be bumped up with it, but if you do something like adjustable torsions with shock absorbers or adjustable airbags w/ shocks, it will greatly improve the ride, tire life, and safety.

I don’t think I would play with axle placement anywhere.
Click to expand...

Interesting the difference good suspension makes with trailers. Kinda funny how some people argue with little single axle offroad trailers that no suspension is needed (thought process is lacking on that idea). They all agree good suspension on the vehicle is important, but often forget about the trailer.

With springs, make sure you get something in the weight range the trailer will regularly be at. Just throwing out a number say the trailer is 9k pounds, get suspension that is rated for 10k. Going too high such as 14k will result in abuse on the trailer/cargo.

Trailer leaf springs are not really a smooth ride. Torsion axles or the Timbren 'axle less' suspension are both big improvements without the cost/time of a custom air ride design.
 
Torx axles are great, they can be found with different ride heights as well, smooth ride and much less shock than springs...
As far as grades go, it will change the cg to a extent, if you could move the axles back 6" from where they are now without getting into the wheel well it would pull better and not add much to the hitch, and a equalizer hitch would be a great addition...
 
Actually, the trailer has torx axles and I just need to work with the RV shop to figure out Dexter's current part numbers. For ride height, I will keep the OE level as it currently has a 'low-rider' stance and prefer to keep it this way as it helps minimize the sail area in addition to really good stability.

As to ability of moving the axles, there is absolutely zero wiggle room here due to wheel wells that go up into the living space (note the reference above about a low-rider) and the tires have just enough room not to scrape. Besides, with the ball at the correct height, the trailer is already very stable in its current configuration, so no need to go mucking with something that works :)

Trailer hub will keep the existing 6 lug as I plan to move the disc brakes over to the new axles. Current tow vehicle is also 6 lug, but the wheels will not fit the trailer (too wide). Both the trailer and tow vehicle have Load E tires. I would have put Load G semi tires on the trailer as I have seen great comments from people with 5'ers whom use them, but they will not fit in its wheel wells and would probably give too harsh a ride for my rig anyway :(
 
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