It's been a while since I worked a statics problem in metric.

First, welcome to the forum.

Second, my answer:  Center the two axles 1.528 metres forward of the rear of the trailer.

Third, assumptions I made:

1. The horses have 60% of their weight on their front feet and 40% on their rear.

2. The load from their rear feet is at the back of the trailer.  This is conservative, but the actual placement of their feet will only give you slightly more tongue weight, which isn't a bad thing.

3. The empty weight of the trailer is 1.5 tonnes, centered on the box, not including the .75 m tongue.  (2.26 m in front of the rear of the trailer.)  This is conservative, but the tongue weight is negligible.

4. The weight in the tack room is evenly distributed.  Personally, 250 kg seems light to me, but more weight in the center of the tack room will add its weight approx 60%/40% to the tongue/axles, respectively.  No big deal.

5. The tongue carries 10% of the trailer's loaded weight.  With conservative assumptions above, it will be slightly higher.  No big deal.  320 kg = 700 lb +/- seems okay.

Fourth, I checked to make sure the front axle is not forward of the midpoint of the box, not including the tongue.  This is a check for pulling the trailer empty.  If the front axle is in front of the balance point of the trailer, the tongue weight may not be sufficient.  This is a conservative check, because we are neglecting the minor weight of the tongue itself.  Midpoint of box is 2.26 m forward of the rear; midpoint of axles is 1.528 m forward.  2.26-1.528=.732  If .732 is less than half the axle spacing, your okay.

Since, it appears you like pictures, I'll draw one and post it soon.

Nice work jdzaharia, with the 10% tongue weight when loaded equates back to 20% tongue weight when completly unloaded.  This 20% unloaded is still only 290kg (640lbs.).  This will make for a good traveling trailer whether it is loaded or unloaded.  However your tongue weights will be a little lower than calculated due to the weight of a (horse) trailer is not in the center of the trailer seeing that the rubber mats, wall lining, body divider, and axles are all in the back.  Plus at this point your axles will still be safely behind the center of gravity on your trailer.  So for what I am worth I agree with jdzaharia and believe this would make for a safe and good pulling trailer.

That's some fine drawing and deciphering.... When I looked at the first drawing the first thing come to my mind was, place the front axle directly under the horses front feet and work my way back, leaving a minimum of 6" between the tread of the front and rear tire at their closest point.

Chris... We love pictures!! So if you were to get a chance to snap a few shots while your work is in progress, we would be mighty obliged and appreciative!!

 Do you know our friend NICK, from East London, South Africa?

Chis de Beer - The total weight of 3,150kg is what was used for the calculations.  Plus, 6" between the two tires would not look right.  The average is 3" to 3.5" (7.6cm to 8.89cm)  With this size of trailer you can have 215/75R15 tires with 3" to 3.5" between them and keep it in a 68" (172cm) fender.

Those are the exact numbers I assumed and used in my calculations.  So, no difference there.  But, as haul_n_horses pointed out, the 1500 kg of empty weight may not be exactly centered on the box as I assumed.  Also, haul_n_horses checked another item that I hadn't included in my original post--the tongue weight with an empty trailer.

 haul_n_horses ... I'm no Engineer, nor do I build and or sell horse trailers,but... I got six assorted tandem axle trailer on the farm and non of the tires are any closer that 6"... What don't look right about this axle placement below?? That's probably more than 6". Just because American Spirit doesn't build them like the ones pictured below, doesn't make it wrong... Does it?

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retento - as a discaimer - I am no longer, nor have I been for a year now, any part of the American Spirit association.  But, yes as your picture shows you can do 6" plus on smaller trailers with one of the two following situations: 215/75R15 or similuar tire in a 72" fender or a smaller tire in a 68" fender.  But it is my personal opionion that it looks odd (not wrong just odd).

Try this link.  We built a trailer using this method of figuring where to put the axels and it pulls great.

http://www.trailerplans.com/index.php?option=com_content&task=view&id=40&Itemid=29

On the distance between the two axles, I would think that with rubber torsion axles I would want them as close together as possible.  That is because they work independently, unlike the shackled spring axles.  The further the rubber axles are apart, the more independent they are, and the more critical the hitch height is.

The last sentence is the important part: 

This method looks more well-suited towards cargo trailers with shackled spring axles.  In the case that the original poster brought up, the load is definitely NOT distributed evenly.