Komet trailer tires

"Robin" wrote

I want to replace the tires on my Komet trailer. The original tire
size is P165/80 R13, which is now hard to find.

I plan to tow about 75 mph most of the time. I really want to avoid
sway.

Yes, add another axle behind the one there, then use any tire you want, and
it will never sway again.
--
Jim in NC

Morgans wrote:
"Robin" wrote

I want to replace the tires on my Komet trailer. The original tire
size is P165/80 R13, which is now hard to find.

I plan to tow about 75 mph most of the time. I really want to avoid
sway.

Yes, add another axle behind the one there, then use any tire you want, and
it will never sway again.
Not true.

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
at the expense of making the transition from stable to out of control
much sharper.

If you want demonstration try towing just about any poorly designed twin
axle trailer fast enough that you get vortex shedding. A light crosswind
is "ideal" for making this exciting.

Use a tyre rated for the job at hand. Stiffer sidewalls help. Lower
profiles help at the expense of being rougher on the glider. Correct
inflation helps.

Learn what a tyre is designed for by reading the sidewall - to
understand the sidewall code look at -

http://www.etyres.co.uk/consumer-inf...ewall-code.htm

Radials are generally better than crossplies which are generally limited
to "cheap and nasty" applications around here. If you want a durable
trailer tyre for your tractor trailer they make a good choice. At speed
on the highway I would be/am much happier with a tyre rated for the
speed, and with similar compound to the towing vehicle. That means you
should look at the traction grade indication too. It would be
embarassing to have your trailer break traction on a fast sweeping turn
that is not even making your tow vehicles tyre work...

For what it is worth - I have two trailers.

15m std class - streamlined enclosed trailer.
Single axle running on LT. (couple of extra plies in the sidewall)
LT175/70R13S
So a less compliant casing radial, with speed rating of 116. Tows stable
at 120km/h and is reasonably good at not transmitting shocks to the
trailer.

The 19m Kestrel is in a poorly streamlined twin axle trailer.
Both axles have P175/70R13R tyres.
This is unstable at any speed above 100km/h, pushes the tow vehicle
around and is generally unpleasant. Too much compliance in the sidewalls
lets the tyre develop big slip angles which then try to steer the tow
car. In tight turns one of the axles has to scrub, grabbing and releasing.
This winter's project is to convert it to a decent single axle...

Of course the bottom line is to drive a little slower - as well as
having the right tyres on the trailer.

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Of course the bottom line is to drive a little slower - as well as
having the right tyres on the trailer.


I just checked with my friend in Germany and confirmed that 80 km/hour
is their max legal glider trailer towing speed.
If we had the same law, that would be a max speed of only 50 MPH in
the USA.

So as much as the Germans like to drive fast in their cars, they
apparently tow their glider trailers at a reasonable speed.

My strategy is to maintain good tires, a tight hitch, start driving
early and try not to stop (like not stopping to thermal on a cross
country.)
60 MPH gets you there. Save gas / save your glider.

Burt
Marfa, west Texas

The 80kph in Germany is a generalisation. If the trailer meets the
requirements it can be placarded to be towed at 100kph (subject to official
DoT inspection). This is certainly true of modern Cobra trailers which (I
believe) have shock absorbers and towball clamp. The downside is that the
trailer now carries a 100kph sticker, so cannot be towed faster in France
(where the towing limit is the same as a car, 110 or 130 kph, depending on
the road)


"Burt Compton - Marfa" wrote in message
...

Of course the bottom line is to drive a little slower - as well as
having the right tyres on the trailer.


I just checked with my friend in Germany and confirmed that 80 km/hour
is their max legal glider trailer towing speed.
If we had the same law, that would be a max speed of only 50 MPH in
the USA.

So as much as the Germans like to drive fast in their cars, they
apparently tow their glider trailers at a reasonable speed.

My strategy is to maintain good tires, a tight hitch, start driving
early and try not to stop (like not stopping to thermal on a cross
country.)
60 MPH gets you there. Save gas / save your glider.

Burt
Marfa, west Texas

Burt Compton - Marfa wrote:
Of course the bottom line is to drive a little slower - as well as
having the right tyres on the trailer.



I just checked with my friend in Germany and confirmed that 80 km/hour
is their max legal glider trailer towing speed.
If we had the same law, that would be a max speed of only 50 MPH in
the USA.

So as much as the Germans like to drive fast in their cars, they
apparently tow their glider trailers at a reasonable speed.

My strategy is to maintain good tires, a tight hitch, start driving
early and try not to stop (like not stopping to thermal on a cross
country.)
60 MPH gets you there. Save gas / save your glider.

You save so much time when you don't have stop to change tires, and back
track to find the fender, then stop in town to get another tire. And if
you have find help to roll everything back up on to the tires .... well,
makes 60 seem OK!

One of my favorite trailer anecdotes is a friend that drove from Florida
to New Mexico for contest, buying 4 new tires along the way. Made good
time at 90 mph in between, though.

Lots of pilots do have tow vehicles and trailers that can tow safely at
75. The real problem is determining you have that combination without a
disaster if you are wrong.

--
Eric Greenwell - Washington State, USA (netto to net to email me)

- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm http://tinyurl.com/yb3xywl

- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what you need to know tinyurl.com/yfs7tnz

"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!
--
Jim in NC
--
Jim in NC

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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