How far back should I move the Axle?

[donnan]

Hi everyone,

I have bought one of those kit trailers 5' x 8' from http://www.easytrailer.com.au It's probably the same kits as the Harbor Freight trailers I have seen mentioned throughout these forums.

I am building what I call is a "Deluxe Camper". It's basically a full standy with galley and shower/toilet cabinet. Here's a pic of what I plan to build.



Dan from this site has been so helpful in making the 3d Sketch Up model of this camper and he suggested that I should move the axle further back.

My problem is that I am concerned for every inch I move the axle back the more weight I will be putting over the tow ball. I am trying to keep the ball weight to about 80lbs if possible, this would mean I would have to be very careful on where I store things in the camper and how far back the axle is. Also I will have to put the water tank right at the back to try and counterbalance the weight which will naturally be at the front of the camper.

I would have thought these trailers have already been tested for where the best axle position is when they are constructed, as they are rated to carry 750kg's (1500lbs).

The trailer has been fully assembled and is awaiting to be registered, although I have to solve this axle position dilemma before I can put the floor in.

The question is: Do I need to move the axle? As most of the weight in this camper is already going to be at the front.

Please help, I am itching to get this build going.

Paul

[Roo Dog]

Paul,

Best practice is to place as much weight over the axle as possible. Too much weight in the ends creates all sorts of problems from an inertia point of view. Once that weight in the ends starts moving it needs to be brought back into line.
If your tug is a heavy weight it can control the trailer, though even this is not ideal.
There have been many studies done on trailer stability, Bristol Uni springs to mind.
I have played with trailers utilising moveable weights to assess stability at various speeds and loadings with some interesting results.
Nothing is set in concrete, standard Oz practice is to have high ball weights, this of course means a heavy vehicle. European practice is to centre the weight over the axle with low ball weights, obviously due to lighter tow vehicles.
A critical situation may never arise with an unbalanced rig but there will be a set of circumstances or an upset where things go haywire and the driver, after the event, says he had no idea what happened. Quite true. Sometimes it is impossible to replicate as those particular circumstances of wind gust, road surface, adhesion etc may never come together again for this particular rig.
I do not think you can get a simple answer due to the variables involved. I never liked the handling of my last trailer and it took a long time to solve the weight and balance to get a nice towing safe rig.
It is all in the design and this takes time, guess work is not the way to go. I towed with a Falcon or heavier for years but towing a Tear with something as light as a Getz needs a bit of thought. The Tear behind the Falcon was never an issue, depends a lot on the tug you utilise.

Take it easy.

RD

[jonw]

Remember that there are ways to somewhat fine tune uour final weight distribution by where you carry some heavy items like your spare tire, EZup, etc. when travelling.

Good luck!

[bobhenry]

Chubby is 5'7" out to out wide Just under 5' tall and 10 foot nose to tale body length without the tongue. He weighs in at 1850 Lbs. If this is where you will be be with the standie I very strongly suggest you reinforce the tongue with a center 2x2 or 2 x3 square tube ran full length under the trailer and extend it 24" beyond the stock position. With this much extra "leverage" you can measure the tongue weight at say the full 2' extention and 18" and 12" this will give you a way to fine tune by trimming after the build is complete. This extention has effectively moved the wheel rearward and will allow you to build forward of the trailers front crossmember if you wish or provide for a tongue box.

Your desired tongue weight is way too light for a trailer of this size a 1500 pound trailer needs 15% tongue weight (225 lbs). If this 80 pounds is to keep in line with your tow vehicles capabilities you need to rethink your design criteria and build smaller or perhaps a foamie.

This past weekend I towed an 8x20 dual axle flatbed trailer loaded very very tail heavy and it was continually lifting the truck. I do not like the feeling for a variety of reasons. The ride and control is greatly deminished. It puts unusual strain on the coupler and can cause it to release. The thought of a dancing trailer standing up and punching holes in my tailgate was the last thing I wanted. I made it but I had the coupler locked in with a padlock and still said a small prayer.

[donnan]

Hi guys,

Thank you for your comments. I understand that the trailer must have so much weight over the tow ball and I understand that this effect how good or bad the trailer tows. Perhaps I do need to rethink my build. Having so much of the weight at the front of the van is sure to put too much weight over the tow ball.

Paul

[bdosborn]

The consequences of an out of balance trailer are severe and difficult to correct. Rather than taking a guess at axle position or using somebody's rule of thumb from who knows where, you can calculate your tongue weight using Andrew's spreadsheet.

http://www.angib.pwp.blueyonder.co.uk/teardrop/tear81.htm

Bruce

[donnan]

Hi Bruce,

Thanks for that but it I don't think it applies to me as I don't have my build even started as yet as I was trying to find out where my axle should be. I think since the 5'x8' trailer has already had it's axle in place I will just leave it where it is. If further down the track the axle needs to be moved I will just cross that bridge when I get to it.

Paul

[danlott]

Paul,

I think you will be fine with the axle in it's original location. You can always handle final tongue weight by placement of your battery, spare tire, propane tank and other items.

Now hurry up and make some saw dust.

Dan

[Mary C]

Paul I have just finished building my HF and have been hauling stuff from my sons house. I did not move the axle. I decided that the whole discussion about weight, balance and tow weight has to do with balance and where I put the heavy stuff. I plan to just move the stuff around pull the trailer some then move the stuff again to get the best pulling, and control. I have pulled trailers before and have the understanding that you need balance and to do that, move your stuff around. Another thing if a girl can't lift the tongue and put it on the ball then it is no where near balanced. Good Luck!!

Mary C.

[mezmo]

I've always heard, as a rule-of-thumb for a TT, that
the starting point is to locate the axle on a 60-40
basis, that is 60 per cent from the front, or 40 per
cent from the back on the base of the trailer body.
Another thumb-rule I've heard of, is to have the front
of the trailer's tire radius no further forward than the
half-way point on the trailer body base. These should
give a natural forward weight bias in and of themselves,
but one always needs to pay attention to static load
balance and CG, as well as the variable load added to
the trailer which can have a big effect. And as "they"
say, @ 10 percent total of weight as the hitch weight
is considered the 'normal' or the goal here in the US.

As angib has pointed out, the European views/rules are
different and they generally have a much lighter tongue weight,
and they also tow at much slower speeds than in the US.
They must base their axle set up on the total length of
the chassis, as they seem generally to be more centered
on the trailer/caravan bodies, and I've seen some pics of
modern European caravans where the axle is ahead of the
caravan body's center point, which looks odd to our eye
here in the US.

Are there any specific Australian rules on this matter? They
do have some very good performance rules from what I
understand - one reason they have so many great off-road
trailers/caravans. It would have to be covered in those I'm sure.
I'm guessing they'd be closer to the US practice on hitch weights
and balance.

Here's a link to an article by your countryman Collyn Rivers,
as mentioned by a forum member here [I can't recall just whom
at the moment] on the topic:

http://www.noboundaries.com.au/Safety/c ... bility.htm

Cheers,
Norm/mezmo

[angib]

As a first rule of thumb, I would suggest that you put the axle 45% of the body length from the back of the body. Note - these are not measurements of trailer frame length, but of body length.

That should give you a light but acceptable tongue weight. How you load the trailer will then determine what the actual tongue weight is - if you fill up those storage areas under the beds with heavy stuff, you could end up with a dangerously low tongue weight.

It looks like most of the heavier stuff in your trailer will be near the front of the body - unlike in a teardrop - so you shouldn't need the axle as far back as a teardrop. It's quite possible that the standard axle position of the trailer may be OK.