Trailer weight distribution demonstration

At 13:57 07 October 2016, Dave Martin wrote:
Don.
You need to update your driving knowledge - the UK speed limits
for towin
a trailer is 60mph

https://www.gov.uk/speed-limits

I know that, as I should. My point was that adding 20psi to the tyre
pressure changed the handling of the trailer from unstable at
50mph to steady at 20mph faster. I suspect that a faster speed
would be possible but it was unnecessary to test as a 10mph
excess over the limit of 60mph was enough to show that just
increasing the tyre pressure was a partial answer to the problem we
had, and give a reasonable margin.
Being able to tow at 60mph means that I was able to avoid being
overtaken by large Artics governed to 56mph, avoiding the main
condition that triggers swaying, being restricted to 50mph did not
allow that.
Police generally ignore excess speed less than 10mph on
motorway/dual carriageways in the UK anyway, which is why most
traffic on those roads is doing 80mph.

At 17:13 05 October 2016, Don Johnstone wrote:
At 14:59 05 October 2016, Charlie M. UH & 002 owner/pilot
wrote:
I've wondered if adding a small (1/4" - 3/8") anti sway bar to
th
trailer
axle would help in most situations. Granted, proper weight is
best, bu
the
anti sway bar may widen the margin.
Thoughts?

Weighting is important, no doubt. Reducing the weight and
therefore th
inertia on the anchored end of the trailer does reduce the
possibility o
any swing increasing by increasing the ability of the towing
vehicle t
damp the oscillations. The trigger for these oscillations is
aerodynami
caused by the low pressure area between two vehicles as they
overtake/ar
overtaken. The situation is made worse if the vertical surface is
the sam
or more than the area in front of the axle. Cobras and Komets
are designe
to reduce the side area behind the axle.
I towed a trailer, containing a Grob 103 back from Holland. The
nos
weigh
was close to the maximum limit allowed for my car, theoretically
the bes
situation to improve stability, however it swayed readily. I have
jus
towed the same combination over 300miles. Before leaving I
increased th
tyre pressures on the trailer from 35psi to 55psi (Max 65psi).
The traile
now tows with no swaying up to the maximum speed limit in the
UK (70mph)
It does allow towing at just below 60mph which means I can
avoid bein
overtaken by large trucks, the condition which is most likely to
trigge
swaying. Tyre pressure appears to have a large influence on the
stabilit
of the trailer, greater than loading. Vertical instability is now th
majo
problem, easily fixed by a stabiliser.

Still waiting for feedback from "ME" types on this.

Tire pressures have a bit on this.

Had a large reply earlier (got lost for whatever reason) that is not here. Most recent glad to not read through it.

Basically, high trailer tire pressures may limit swaying, it also intrudes shock loads to mechanical instruments.

"Fix one thing, FUBAR another thing.....".

Charlie M. (UH & 002 owner/pilot) wrote on 10/5/2016 7:59 AM:
I've wondered if adding a small (1/4" - 3/8") anti sway bar to the
trailer axle would help in most situations. Granted, proper weight is
best, but the anti sway bar may widen the margin. Thoughts?

It's unlikely it will change the dynamics of a single axle trailer.
Ante-sway bars are used on cars and similar to change the roll stiffness
about each axle: generally, an anti-sway bar on the front axle increases
"understeer" and makes the vehicle more stable. A rear bar does the
opposite.

The place to change anti-sway bars is on the tow vehicle. I suspect a
bigger front bar would increase the towing stability, but I think it's
better to get the trailer balance and tire pressures right (ditto for
the tow vehicles tire pressures).

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

On Monday, October 10, 2016 at 9:53:59 PM UTC-7, Eric Greenwell wrote:
Charlie M. (UH & 002 owner/pilot) wrote on 10/5/2016 7:59 AM:
I've wondered if adding a small (1/4" - 3/8") anti sway bar to the
trailer axle would help in most situations. Granted, proper weight is
best, but the anti sway bar may widen the margin. Thoughts?

It's unlikely it will change the dynamics of a single axle trailer.
Ante-sway bars are used on cars and similar to change the roll stiffness
about each axle: generally, an anti-sway bar on the front axle increases
"understeer" and makes the vehicle more stable. A rear bar does the
opposite.

The place to change anti-sway bars is on the tow vehicle. I suspect a
bigger front bar would increase the towing stability, but I think it's
better to get the trailer balance and tire pressures right (ditto for
the tow vehicles tire pressures).

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

The most stable trailer configuration is a semi where about half the trailer weight is on the tongue. This is not possible for autos which can't support that kind of tongue weight. Even fifth wheels don't put the trailer wheels in that position, and they are accepted to be the most stable trailer configuration.

I have come to realize that there are two moment of inertia of interest; one referenced to the trailer axle and the other referenced to the hitch. Why is MOI important? The answer is if you have an oscillating body it will be harder to stop the larger the MOI. This is why engines have flywheels. If you want to minimize axle MOI you would put as much weight as possible near the axle, which may decrease tongue weight. If you want to minimize hitch MOI you move weight forward, which increases axle MOI. Clearly there is something contradictory about these two MOIs. All trailer towing guides I have seen emphasize maintaining a tongue weight that is 7 to 10% of the total trailer weight. Remember that MOI is calculated as the sum of the weight times the radius SQUARED, so any weight is a long distance from the reference point has a major influence on MOI. If the trailer's tires are maintaining traction with the road the axle MOI is of interest, but if the tires loose traction then the hitch MOI dominates. For certain, putting weight at the rear of the trailer aggravates both MOIs, but increases the hitch MOI four times as much as the axle MOI.

The moral of the story is to manage the hitch weight properly. Merely adding weight to the front of the trailer is not the solution: you should adjust the axle position to get the proper tongue weight. I have done this on one trailer and experimented with trailer transient response as a function of tongue weight. More tongue weight was better.

Tom

Not adding anything to the conversation, but I had purchased a Nimbus 4 that had a Pfister (spelling?) tube trailer. The tongue weight was so heavy it could only be moved by a jack. Soon after getting the bird I purchased a double axle Cobra trailer for the glider and sold the other trailer. The new cobra had a tongue weight much less than the former trailer and could be moved by hand and towed marvelously. Of course back then I was towing with a 2500 chevy Suburban and I had to put a post it on the dash to remind me that I was towing a trailer. While not an expert in any field of this particular discussion, it seems tow vehicle has much to do with the stability as the trailer. My ASw-24 towed towed great behind my little four banger Toyota pick up and horrible behind my next car a Jeep Grand Cherokee. The Suburban was my favorite tow vehicle but I had to keep reminding myself of the trailer. I am currently towing an ASG-29E with a ML 350 diesel and it tows great, but I do have a double exile trailer.

On Friday, October 14, 2016 at 7:24:24 AM UTC-7, Jonathan St. Cloud wrote:
Not adding anything to the conversation, but I had purchased a Nimbus 4 that had a Pfister (spelling?) tube trailer. The tongue weight was so heavy it could only be moved by a jack. Soon after getting the bird I purchased a double axle Cobra trailer for the glider and sold the other trailer. The new cobra had a tongue weight much less than the former trailer and could be moved by hand and towed marvelously. Of course back then I was towing with a 2500 chevy Suburban and I had to put a post it on the dash to remind me that I was towing a trailer. While not an expert in any field of this particular discussion, it seems tow vehicle has much to do with the stability as the trailer. My ASw-24 towed towed great behind my little four banger Toyota pick up and horrible behind my next car a Jeep Grand Cherokee. The Suburban was my favorite tow vehicle but I had to keep reminding myself of the trailer. I am currently towing an ASG-29E with a ML 350 diesel and it tows great, but I do have a double exile trailer.

The Jeep Grand Cherokee has a bad reputation as a tow vehicle. I personally saw one (attempting to) towing a small travel trailer. It was not stable and I witnessed severe swaying at a relatively slow speed (50 mph). The driver would slow down when it went unstable, and then creep back up to the speed that it would go unstable again. It was disturbing to watch - the driver had his family in the car and they were all at risk. I think the problem was the short wheel base in combination with the suspension and the tires. If they are too soft there isn't sufficient damping to stop swaying. This is further aggravated by the length of glider trailers. Yet they are rated to tow pretty hefty trailers. The moral of the story is just because the drive train can handle the trailer weight doesn't mean that it will be stable at highway speeds.

Tom