Dexter axle shock absorber - Page 3 - Fiberglass RV


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Old 06-01-2015, 09:51 AM   #29
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Originally Posted by tractors1 View Post
Looks like an accelerometer using a serial cable to feed Megunolink software. Curious why the Y axis units are called "Bounce" - which is a bit confusing as that indicates to me a distance rather than a force (g-force.)
Charlie,

Because in my original "discussions" with Dex I was trying to make the point (using real data) that it is the excessive "bounce" that we are trying to eliminate.
8.2 on the Y axis = 1G

Jim
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Old 06-01-2015, 09:52 AM   #30
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Dex,

We absolutely have a point of reference.
No anecdotes here - just data.
This is a "g" meter mounted on the fiberglass of a Casita.
It is showing how much energy the trailer is "seeing"

Jim
Jim,
You didn't quote the whole statement I made. It isn't if the 'g' meter measured something. It is if that measurement has any realistic meaning.

The 'g' meter measured the 3 largest shocks and from that the charts are showing they all read between 9.5 to 10 with or without shocks. That tells us that the shocks do not have a benefit for those circumstances.

After the large bump only 2 of 3 measurements show a noticeable difference.


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Originally Posted by Dextersp1 View Post
For the last two bumps we don't have a point of reference. We don't know for example if we were riding in the trailer, would we notice if there were shocks or not being used.
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Old 06-01-2015, 09:58 AM   #31
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Originally Posted by Dextersp1 View Post
Jim,
You didn't quote the whole statement I made. It isn't if the 'g' meter measured something. It is if that measurement has any realistic meaning.

The 'g' meter measured the 3 largest shocks and from that the charts are showing they all read between 9.5 to 10 with or without shocks. That tells us that the shocks do not have a benefit for those circumstances.

After the large bump only 2 of 3 measurements show a noticeable difference.
Do we really have to go through this over and over?
The initial "bang" will be the same with or without shocks.
That is the suspension doing its work.
After that, on the "down" part mostly - the shocks do their thing and dampen out the later oscillations.

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Old 06-01-2015, 10:10 AM   #32
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Do we really have to go through this over and over?
The initial "bang" will be the same with or without shocks.
That is the suspension doing its work.
After that, on the "down" part mostly - the shocks do their thing and dampen out the later oscillations.

Jim
Jim,

OK, so we agree there is no difference on the initial bump with or without shocks. So no benefit to adding shocks in that aspect.

The graphs only show dampening in 2 of the 3 after bumps - 66% of the time. What we don't know is if that dampening has any value.

If, for example, a person riding in the trailer would not notice a reading of 5 there is no value in shocks that lowers the reading to lower than 5. Another way of saying it is: If there is no problem with secondary bumps of 5 then there is no value in shocks that lower that number.
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Old 06-01-2015, 10:40 AM   #33
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Originally Posted by Dextersp1 View Post
Jim,

OK, so we agree there is no difference on the initial bump with or without shocks. So no benefit to adding shocks in that aspect.

The graphs only show dampening in 2 of the 3 after bumps - 66%. What we don't know is if that dampening has any value.

If, for example, a person riding in the trailer would not notice a reading of 5 there is no value in shocks that lowers the reading to lower than 5. Another way of saying it is: If there is no problem with secondary bumps of 5 then there is no value in shocks that lower that number.
Yes we agree on your first sentence.
The rest is back to the anecdotal problem.
We have plenty of anecdotal evidence that they make a significant improvement.
This is just data to try to get away from that.

I think we can agree that I know they work and you know they dont and just leave it at that before this thread gets shut down.

Cheers,

Jim
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Old 06-01-2015, 10:52 AM   #34
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Thanks Jim!!!!!!
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Old 06-01-2015, 11:13 AM   #35
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Jim,

How about investigating an anti-roll bar for trailers?
The side to side tipping moment could be improved. There has been very little work done on trailers since no one rides in them to complain!
However the oscillation side to side as it coincides with the frequency of the sway is a problem.
The added roll stiffness without increasing the spring rate overall would be a plus. Especially on a lifted Casita.
The key is to move the natural frequency of the system lower than the excitation to damp out unwanted oscillations. This way the damping and spring rate from the one side would be added to the other via the anti-roll bar.
Of course each bar would have to be made for the distance between trailing arms.
This would counteract some of the IRS effect of the rubber torsion axle, but only to a minor degree.
To some degree the weight distributing hitch accomplishes the same thing, but only on the tongue. This along with the added friction is a part of the effect of this type of hitch.
Here is a picture of a similar rear torsion beam car axle with anti-roll bar:
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Old 06-01-2015, 11:59 AM   #36
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Dexter axle shock absorber

Quote:
Originally Posted by widgetwizard View Post
With respect - I completely disagree with this notion -and the proof lies in that chart a few posts up. The shock is absolutely NOT transmitted to the trailer. It IS absorbed by the shock absorber and turned into heat. Please look at the waveforms again.



--Jim.

Hey Jim no problem, it is a complex thing.
The shock is a device that can be compressed or expanded easily at slow speed but the faster the rate the larger the force required. It is analogous to a capacitor. So for an impact it behaves like it is a solid object and as such any fast impact on one end is transferred to the other end. The curves above are classic. The unshocked curve shows the oscillations carrying on much longer than the shocked curve. It is the energy of those oscillations that is transferred to the shock and presents as heat and so reduce the oscillations more quickly. You are correct that the initial bump also imparts energy to the shock, but that bump is conducted better to the frame than if there were no shock. You can get adjustable shocks that allow bump and rebound to be independently adjusted and competitive racing teams use a machine to decide the adjustments that minimize oscillations without too much direct impact to the frame on bump. To further muddy the waters, most shocks now come pressurized with nitrogen. The purpose of the nitrogen is to keep the damping fluid from developing air bubbles and causing cavitation which erodes the fluid orifice, but with the nitrogen pressure the shock also acts as a spring in parallel with the shock. Springs act in an different manner, that is the more you compress them, the higher the force required. Springs are primarily not rate dependant like shocks and so do a better job of not transferring high rate bump to the chassis (although there are secondary transfer mechanisms that do). I spent more than a decade driving the national rally championship and suspension dynamics are only second to tire dynamics in terms of building a competitive rally car. So it became a bit of a passion / obsession for me and I can blather on and bore people to death given any excuse...please excuse.
What a wonderful discussion.


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Old 06-01-2015, 12:08 PM   #37
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Originally Posted by dleverton View Post
Hey Jim no problem, it is a complex thing.
The shock is a device that can be compressed or expanded easily at slow speed but the faster the rate the larger the force required. It is analogous to a capacitor. So for an impact it behaves like it is a solid object and as such any fast impact on one end is transferred to the other end. The curves above are classic. The unshocked curve shows the oscillations carrying on much longer than the shocked curve. It is the energy of those oscillations that is transferred to the shock and presents as heat and so reduce the oscillations more quickly. You are correct that the initial bump also imparts energy to the shock, but that bump is conducted better to the frame than if there were no shock. You can get adjustable shocks that allow bump and rebound to be independently adjusted and competitive racing teams use a machine to decide the adjustments that minimize oscillations without too much direct impact to the frame on bump. To further muddy the waters, most shocks now come pressurized with nitrogen. The purpose of the nitrogen is to keep the damping fluid from developing air bubbles and causing cavitation which erodes the fluid orifice, but with the nitrogen pressure the shock also acts as a spring in parallel with the shock. Springs act in an different manner, that is the more you compress them, the higher the force required. Springs are primarily not rate dependant like shocks and so do a better job of not transferring high rate bump to the chassis (although there are secondary transfer mechanisms that do). I spent more than a decade driving the national rally championship and suspension dynamics are only second to tire dynamics in terms of building a competitive rally car. So it became a bit of a passion / obsession for me and I can blather on and bore people to death given any excuse...please excuse.



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Doug L
Thanks Doug - nicely explained.
And we are using nitrogen charged shocks here. Monroe 555003

Cheers,

Jim
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Old 06-01-2015, 01:01 PM   #38
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Originally Posted by redbarron55 View Post
Jim,

How about investigating an anti-roll bar for trailers?
The side to side tipping moment could be improved. There has been very little work done on trailers since no one rides in them to complain!
However the oscillation side to side as it coincides with the frequency of the sway is a problem.
The added roll stiffness without increasing the spring rate overall would be a plus. Especially on a lifted Casita.
The key is to move the natural frequency of the system lower than the excitation to damp out unwanted oscillations. This way the damping and spring rate from the one side would be added to the other via the anti-roll bar.
Of course each bar would have to be made for the distance between trailing arms.
This would counteract some of the IRS effect of the rubber torsion axle, but only to a minor degree.
To some degree the weight distributing hitch accomplishes the same thing, but only on the tongue. This along with the added friction is a part of the effect of this type of hitch.
Here is a picture of a similar rear torsion beam car axle with anti-roll bar:

Hey RedBarron
That is a really good idea.
There is no cost with respect to ride harshness and trailers often have a lot of roll.


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Old 06-01-2015, 01:51 PM   #39
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An anti-roll bar is not going to have much effect on a trailer, since 100% of its roll stiffness is at the axle and making it stiffer won't do anything but reduce roll (and transfer some shock loads to the other wheel over bumps). Reducing roll alone won't improve handling and won't change the side-to-side weight transfer

Anti-roll bars on four-wheeled vehicles transfer roll stiffness from front to back or vice versa in order to increase/reduce oversteer or understeer, which is a whole different barrel of apples.
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Old 06-01-2015, 08:45 PM   #40
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Anti-roll bars on four-wheeled vehicles transfer roll stiffness from front to back or vice versa in order to increase/reduce oversteer or understeer, which is a whole different barrel of apples.
As far as this statementgoes it is correct, however the addition of the roll to the sway or snaking add to the amplitude of the swing.
The effect would be like adding air to tires that are underinflated.
No suspension at all would be more stable than a very soft trailer suspension. The added rate from the anti-roll bar would shift the resonant frequency in a more favorable direction.
Soft springs and no damping woule be similar to having a water tank 1/2 full across the trailer with the water sloshing back and forth. Even more like a unsecured water tank sliding across the floor in a sway event.
The anti-roll bar wold be like that tank moving to the other side against the shifting force.
The tipping from side to side or roll aggravates the swaying action making it worse.
Very little work has been done in this country on trailer dynamics and handling.
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Old 06-01-2015, 11:31 PM   #41
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Dexter axle shock absorber

The trailing arm type suspension on Bolers and Trillium's are simple but have a weak point. The tires camber angle is always equal to the trailers roll. As a result if you let the trailer roll excessively you also get excessive camber and the lateral adhesion of the tires decrease. So if you take a corner way too hot, it is easy for the trailer to slide. This is why "caravan racing" has had some popularity in the UK. There are lots of things that can promote oversteer and a sliding trailer is one of them. So an anti sway bar would both limit roll on corners and so help keep the dishes from falling, and would reduce the tendency to oversteer. Boring, but safer.
On the other hand there may not be much roll to suppress. Roll when cornering is caused by centripetal force acting on the center of gravity of the trailer. If the center of gravity is above the roll center the trailer will roll outward, but if the center of gravity is below the roll center the trailer will roll inward. If the two centres coincide there will be no roll. I think the roll center on trailing link is the center of the axil, but I am not sure. There is an algorithm to find roll center for every configuration of suspension to be found on the Internet, but I couldn't find it for trailing link. Probably there is no algorithm because on cars, straight trailing link has not been used for many decades. I found one reference that claimed that with trailing link it is easy for the suspension designer to set the roll center where he likes (no mention of how). If that is true, it explains why it is used on trailers as roll can be set as desired (minimal or zero), and the expense of an anti sway bar is not needed. When I took the dogs out tonight I rocked the Trillium and tried to eyeball the roll center. It is low, somewhere between a foot and two foot above the ground, and the center of gravity is also low. I'm kind of keen to see how much roll I get on a traffic circle next time I take it out.
OK I'm babbling, but it is an interesting topic.

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Old 06-02-2015, 01:37 AM   #42
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Name: Doug
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Dexter axle shock absorber

The trailing arm type suspension on Bolers and Trillium's are simple but have a weak point. The tires camber angle is always equal to the trailers roll. As a result if you let the trailer roll excessively you also get excessive camber and the lateral adhesion of the tires decrease. So if you take a corner way too hot, it is easy for the trailer to slide. This is why "caravan racing" has had some popularity in the UK. There are lots of things that can promote oversteer and a sliding trailer is one of them. So an anti sway bar would both limit roll on corners and so help keep the dishes from falling, and would reduce the tendency to oversteer. Boring, but safer.
On the other hand there may not be much roll to suppress. Roll when cornering is caused by centripetal force acting on the center of gravity of the trailer. If the center of gravity is above the roll center the trailer will roll outward, but if the center of gravity is below the roll center the trailer will roll inward. If the two centres coincide there will be no roll. I think the roll center on trailing link is the center of the axil, but I am not sure. There is an algorithm to find roll center for every configuration of suspension to be found on the Internet, but I couldn't find it for trailing link. Probably there is no algorithm because on cars, straight trailing link has not been used for many decades. I found one reference that claimed that with trailing link it is easy for the suspension designer to set the roll center where he likes (no mention of how). If that is true, it explains why it is used on trailers as roll can be set as desired (minimal or zero), and the expense of an anti sway bar is not needed. When I took the dogs out tonight I rocked the Trillium and tried to eyeball the roll center. It is low, somewhere between a foot and two foot above the ground, and the center of gravity is also low. I'm kind of keen to see how much roll I get on a traffic circle next time I take it out.
OK I'm babbling, but it is an interesting topic.

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