Morgans wrote:
"Bruce" wrote
Double axle trailers do sway, and are in general more unwieldy than single
axle trailers. A poorly designed trailer will sway at lower speed, adding
an axle seems to increase the speed at which it goes wrong

Wow, does that go against everything I know to be true about trailers.

That same tire scrubbing you talked about while going around a corner is
exactly what makes the dual axle trailer more stable, if ALL other factors
were equal. Vortex shedding can certainly be a big issue, as is percentage
of tongue weight to overall trailer weight.

Many dual axle trailers do not carry enough tongue weight. Some also do
not have both springs mounted on a common pivot, which is not the correct
way to rig 2 axles.

Another factor against many glider trailers is that too much mass is too far
behind the axles. Any weight put further out behind the axle contributes to
dynamic instability, even if the tongue weight is correct.

I know having the axles further back makes cornering a more difficult issue,
but it really does help with stability, on the whole.

If you disagree with what I just posted, you and I will have to agree to
disagree, I guess.

Sunny skies!
Hi Jim

I agree with you - no need to agree to disagree. A correctly designed
common pivot double axle will be more directionally stable than an
equally well designed single axle. But the statement that just adding
another axle will invariably improve things is false.

A poor design on a double axle often makes things worse rather than
better. Some genius added an afterthought axle to my Kestrel's trailer.
Now it has two different design axles, both suspended with cart (leaf)
springs) with different lateral stiffness and compliance. The fixed
front/pivoting rear shackle design on the axles allow - and in fact
force the two axles to move out of parallel when the wheels move over an
obstacle affecting only one side. Result is lots of steering from the
trailer. If the trailer rocks from side to side (as in vortex shedding)
there are modes when the axle steer amplifies the movement. That's a
recipe for disaster.

So again - there is no substitute for good design, and if you don't have
the resources for that, rather stick to a simpler single axle design.
Unless you can analyse the suspension geometry simpler is better.

That said - as has previously been posted - the tow vehicle and
tow/trailer compatibility is probably more important. Even the Kestrel
franken trailer is relatively easy to tow with a volvo XC70 as long as
you stay below 100km/h but wildly unstable behind a 1 ton pickup at 80.

There is one unequivocal advantage to double axle setups, in that the
ride is more consistent for the glider - the horisontal difference
between wheels tends to make the ride smoother on rough surfaces.

Bruce

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