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IdBdan

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- TL;DR Summary
- Building a trailer. Beam calc's ask for (I) - section calc's give me (Ix) & (Iy)???? Which do I use?

I'm building an aluminum tandem axle (7k total capacity) trailer. Boat (18'/1300#) on the rear and a Ryker Can Am 3 wheel (750#) on the front. It's a 23'-6" x 6'-8" bed. Max live load will be approx 2250#. 900# frame weight. 230# axles.

I'm testing beam sizes with online calculators and need some help.

I'm using a "section properties" calculator to find the Moment of inertia. https://calcresource.com/cross-section-channel.html

I'm using a "cantilevered beam" span calculator for the span distance from back axle to the rear. https://calcresource.com/statics-cantilever-beam.html

And a "simple span" for the distance of front axle to trailer tongue. https://calcresource.com/statics-simple-beam.html

My beam span calculators require Length (L) - Youngs modulus (E) - and Moment of inertia (I). My section properties calculator is giving me Moment of inertia in (Ix) and (Iy) and (Iz). I'm using the (Ix) number in the beam calc box for (I).

I'm mainly looking for the deflection criteria. I'm trying to keep that under 0.15" (with load). The boats CG is over the axle center so tail bounce should not be a problem. Torsional movement (Iz) is almost non-existent with the cross bracing member size, spacing, and diagonal bracing I've used. The axle system is sliding/adjustable. So I'll be able to tune the tongue weight close to perfection by adjusting the axle location. That should greatly reduce any major lateral instability while towing.

My current choice is an Aluminum Association Channel 8x3.75 - 0.41tf/0.25tw.

Cantilever = 106 in - (69MPa) - MoI 52.04 in^4 - 40# uniform load = (0.101 deflection @ 116")

Simple Span = 144 in - (69MPa) - MoI 52.04 in^4 - 35# uniform load = (0.031 deflection @ 82")

The 32" between axles was ignored. Channel bottom flange is being reinforced with a welded 3/8"x3" sliding axle plate for the center 86" of the total bed length. Both spans are getting that extra moment support and it's not incl in calc. Adds an additional safety factor. The tongue is an A frame that goes under the side channel adding support for another 20% of the front span.

If anyone thinks I'm over designed please let me know why. Aluminum prices are crazy high right now.

Thanks

Dan

I'm testing beam sizes with online calculators and need some help.

I'm using a "section properties" calculator to find the Moment of inertia. https://calcresource.com/cross-section-channel.html

I'm using a "cantilevered beam" span calculator for the span distance from back axle to the rear. https://calcresource.com/statics-cantilever-beam.html

And a "simple span" for the distance of front axle to trailer tongue. https://calcresource.com/statics-simple-beam.html

My beam span calculators require Length (L) - Youngs modulus (E) - and Moment of inertia (I). My section properties calculator is giving me Moment of inertia in (Ix) and (Iy) and (Iz). I'm using the (Ix) number in the beam calc box for (I).

**My questions: 1.**Is using (Ix) correct? Or do I add Ix and Iy together?**2.**Do these calculators typically use member self weight in their calc's?I'm mainly looking for the deflection criteria. I'm trying to keep that under 0.15" (with load). The boats CG is over the axle center so tail bounce should not be a problem. Torsional movement (Iz) is almost non-existent with the cross bracing member size, spacing, and diagonal bracing I've used. The axle system is sliding/adjustable. So I'll be able to tune the tongue weight close to perfection by adjusting the axle location. That should greatly reduce any major lateral instability while towing.

My current choice is an Aluminum Association Channel 8x3.75 - 0.41tf/0.25tw.

Cantilever = 106 in - (69MPa) - MoI 52.04 in^4 - 40# uniform load = (0.101 deflection @ 116")

Simple Span = 144 in - (69MPa) - MoI 52.04 in^4 - 35# uniform load = (0.031 deflection @ 82")

The 32" between axles was ignored. Channel bottom flange is being reinforced with a welded 3/8"x3" sliding axle plate for the center 86" of the total bed length. Both spans are getting that extra moment support and it's not incl in calc. Adds an additional safety factor. The tongue is an A frame that goes under the side channel adding support for another 20% of the front span.

If anyone thinks I'm over designed please let me know why. Aluminum prices are crazy high right now.

Thanks

Dan

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