If you’re building or buying a trailer for your tiny house build, its important that the axle position is located correctly in relation to the intended load (house) on top of the trailer. Having a well-balanced trailer ensures safe and smooth towing. If the axle position results in too much weight on the towbar, hence the tow vehicle, steering and handling of the tow vehicle can be compromised. If there is too little weight on the towbar, then the trailer can become unstable when towing. In the US, typically the weight on the towbar or ‘tongue load’ is 10% of the total weight of the trailer and load. So if your tiny house setup weighed 3500kg, the tongue load would be 350kg. In Europe the tongue load is around 7%. In NZ, despite not having the large trucks that are common in the US, we still operate around the 10% mark. Anywhere in between 7-10% is ideal.
You may have heard of the 40/60 rule when it comes to positioning axles on a trailer. This is a very general rule of thumb where the axles are positioned so that 60% of the trailer bed is in front of the axle and 40% rearward of the axle. This can work well for uniformly distributed loads (such as carrying a trailer load of dirt), but is not overly accurate when it comes to tiny houses where there may be heavy point loads at different places along the trailer bed.
With a tiny house, we can assume that the load of the walls and roof are fairly uniformly distributed, so an empty shell on top of the trailer bed would have its centre of mass (balance point) in the centre of the trailer bed. But, the internal fitout of kitchens and bathrooms etc can shift the weight balance forward or rearward. Loads such as water tanks or battery banks placed on the drawbar also have a big effect. ALSIPS has put together a simplified calculator to help you determine the correct axle position for your tiny house trailer.
By calculating the moments created by different loads about the hitch we can calculate the correct axle position for a specified tongue load. Although the calculator above only takes into 3 primary loads (the trailer frame itself, the tiny house load and any point load applied to the drawbar), you can manually calculate the axle position for any number of given loads at a certain distance using the equation for D(a) below.
Manual Equations:
D(a) = (F(th) x D(th)) + (F(f) x D(f)) + (F(d) x D(d)) + …(F(l1) x D(l1)) + (F(l2) x D(l2))…)/F(a)
F(t) = (tongue load %)/100 x (F(th) + F(f) + F(d) +…F(l1) + F(l2)…)
F(a) = (100-tongue load %)/100 x (F(th) + F(f) + F(d) +…F(l1) + F(l2)...)
Definitions:
F(a) = Load on axle set
F(th) = Load of Tiny House. Although this is the hardest to calculate, if your tiny house is built with keeping weight balanced on the trailer deck, you can assume the load acts through the trailer deck center. Any heavy point loads that may upset the balance can be manually added and calculated as point loads.
F(f) = Load of the trailer frame. This will act forward of the center of the deck due to the weight of the drawbar
F(d) = Point Load on drawbar. This could be water tank, battery bank etc.
** For multiple axles sets the axle center is assumed to be in the center of the axle set.
One of many advantages of building with ALSIPS is that the weight of the structure is known from the beginning. There is no guess work when it comes to the weight of the panels or the trailer. If you are then mindful of the weight going into the fit out of your tiny house, there is no risk of exceeding the weight limit. Of course, ALSIPS is going to be one of the lightest, if not the lightest building material you can build your tiny house with. Chat to us about your project today!
