New Andersen WD hitch

eb145 wrote:

Ron Gratz wrote:

JBarca wrote:
Hi Ed, Welcome to the discussion! When you came up with 1,000# per chain load, what are the assumptions surrounding this?

John, I don't know how Ed did it, but here is my approach to defining the relationship between tension and load transfer.

First we need to define some dimensions and variables. Let:
a = tow vehicle wheelbase
b = ball overhang (longitudinal distance from TV rear axle to ball)
c = distance from ball to mid-point between the TT's axles
d = perpendicular distance from Andersen chain to center of ball (reported by Andersen owner to be 6.5")
TW = tongue weight
LTT = load transferred to TT's axles
LF1 = load removed from TV's front axle due to TW without WD applied
LF2 = load transferred to TV's front axle when WD is activated
T = Andersen chain tension per chain
M = moment (torque) generated by Andersen chain tension (total for 2 chains)

then (assuming zero pitch-axis rotational friction between ball and coupler)

M = 2*d*T
LTT = M/c = 2*d*T/c
LF2 = LTT*(b+c)/a = 2*d*T*(b+c)/(a*c)
also
LF1 = TW*b/a

If we want to restore a load equal to some percentage (call it FALR) of that which was removed from the front axle, we have:

LF2 = LF1*FALR/100, or 2*d*T*(b+c)/(a*c) = FALR*TW*b/(a*100)

solving for chain tension (per chain) gives

T = FALR*TW*b*c/{2*d*(b+c)*100}

for example, if: b=60", c=200", d=6.5", TW=600#, and you want to restore 50% of the load removed (FALR=50)

T = 50*600*60*200/(2*6.5*260*100) = 1065# per chain

Ron

Now this calculation makes me smile - it has been many years since my Statics and Dynamics classes! I especially like it because it shows my main assumption might actually be correct. But I will disclose how I came up with the 1,000# per chain estimate: several posts discussed a similar setup as mine that had various real measurements that showed about 1,000# per chain transferring the correct amount of weight to the front axle.

Ron's comment about the safety spec requiring that force for only 5 seconds is very relevant new information for me.

So a simple comparison (using rough estimate numbers - not exact) might be like this:

8 hours = 28,800 seconds. Let's round up to 30,000 seconds for a long day of driving.

And I have a 14,000# rated coupler.

Safety spec requires 3X the coupler rating for 5 seconds:
42,000# force for 5 seconds required by safety spec.

Andersen WDH exerts a constant baseline force:
2,000# force for 30,000 seconds
(Caution: THIS 2,000# force number will vary significantly for each rig)

Traditional WDH exerts a constant baseline force closer to zero.

Both couplers experience lots of dynamic forces from road use in all directions on top of the baseline (static) force.

So the obvious difference (as everyone following this thread knows and I am catching up on) is the Andersen WDH has a constant baseline force applied to the coupler in a rear direction.

And the baseline force on the coupler will vary significantly per rig depending on how much force is needed to get proper weight distribution.

And one coupler type (Atwood 88xxx series) so far has been shown to not like that constant baseline force.

Very enlightening.

And my 1,000 lbs per chain assumption is quite different from John's calculations that show almost 3,000 lbs. per chain for his 1,400# tongue weight setup. John's calculations.

I guess for now the bottom line for me is the more you have to compress the urethane bushing for proper weight distribution the more baseline force you are putting on the coupler. Know your coupler and keep an eye on it.


I just went outside, laid down on the gravel and looked "up" at the inside of my coupler for the first time. (Sunday morning in my pajamas:) )

What a great forum.

Ed

What did you see? Are you using an Andersen hitch and looking for damage? Pictures would be great if you are.

eb145 wrote:
Snip...
I just went outside, laid down on the gravel and looked "up" at the inside of my coupler for the first time. (Sunday morning in my pajamas:) )

What a great forum.

Ed

Pictures!!!! We need pictures! :B
Barney

Ron Gratz wrote:

JBarca wrote:
Hi Ed, Welcome to the discussion! When you came up with 1,000# per chain load, what are the assumptions surrounding this?

John, I don't know how Ed did it, but here is my approach to defining the relationship between tension and load transfer.

First we need to define some dimensions and variables. Let:
a = tow vehicle wheelbase
b = ball overhang (longitudinal distance from TV rear axle to ball)
c = distance from ball to mid-point between the TT's axles
d = perpendicular distance from Andersen chain to center of ball (reported by Andersen owner to be 6.5")
TW = tongue weight
LTT = load transferred to TT's axles
LF1 = load removed from TV's front axle due to TW without WD applied
LF2 = load transferred to TV's front axle when WD is activated
T = Andersen chain tension per chain
M = moment (torque) generated by Andersen chain tension (total for 2 chains)

then (assuming zero pitch-axis rotational friction between ball and coupler)

M = 2*d*T
LTT = M/c = 2*d*T/c
LF2 = LTT*(b+c)/a = 2*d*T*(b+c)/(a*c)
also
LF1 = TW*b/a

If we want to restore a load equal to some percentage (call it FALR) of that which was removed from the front axle, we have:

LF2 = LF1*FALR/100, or 2*d*T*(b+c)/(a*c) = FALR*TW*b/(a*100)

solving for chain tension (per chain) gives

T = FALR*TW*b*c/{2*d*(b+c)*100}

for example, if: b=60", c=200", d=6.5", TW=600#, and you want to restore 50% of the load removed (FALR=50)

T = 50*600*60*200/(2*6.5*260*100) = 1065# per chain

Ron

Now this calculation makes me smile - it has been many years since my Statics and Dynamics classes! I especially like it because it shows my main assumption might actually be correct. But I will disclose how I came up with the 1,000# per chain estimate: several posts discussed a similar setup as mine that had various real measurements that showed about 1,000# per chain transferring the correct amount of weight to the front axle.

Ron's comment about the safety spec requiring that force for only 5 seconds is very relevant new information for me.

So a simple comparison (using rough estimate numbers - not exact) might be like this:

8 hours = 28,800 seconds. Let's round up to 30,000 seconds for a long day of driving.

And I have a 14,000# rated coupler.

Safety spec requires 3X the coupler rating for 5 seconds:
42,000# force for 5 seconds required by safety spec.

Andersen WDH exerts a constant baseline force:
2,000# force for 30,000 seconds
(Caution: THIS 2,000# force number will vary significantly for each rig)

Traditional WDH exerts a constant baseline force closer to zero.

Both couplers experience lots of dynamic forces from road use in all directions on top of the baseline (static) force.

So the obvious difference (as everyone following this thread knows and I am catching up on) is the Andersen WDH has a constant baseline force applied to the coupler in a rear direction.

And the baseline force on the coupler will vary significantly per rig depending on how much force is needed to get proper weight distribution.

And one coupler type (Atwood 88xxx series) so far has been shown to not like that constant baseline force.

Very enlightening.

And my 1,000 lbs per chain assumption is quite different from John's calculations that show almost 3,000 lbs. per chain for his 1,400# tongue weight setup. John's calculations.

I guess for now the bottom line for me is the more you have to compress the urethane bushing for proper weight distribution the more baseline force you are putting on the coupler. Know your coupler and keep an eye on it.


I just went outside, laid down on the gravel and looked "up" at the inside of my coupler for the first time. (Sunday morning in my pajamas:) )

What a great forum.

Ed

PHS79 wrote:
Our TT has the 81911 and we have the Andersen hitch last year we put on only about 2000 miles. If we still have the TT and the couple would have to be replaced at some point down the road, I would install a Bulldog hitch. We have Bulldog couplers on numerous trailers, one of which has more than 100,000 miles on it and still has the original coupler.

I do a "pre trip inspection" everytime that we are packing for a weekend. Because of the issues that some have had with the Andersen hitch and different couplers, I make sure to check the coupler just to look for anything out of the ordinary.

Checking over the coupler is a good practice no matter the brand of hitch used. I do. Been doing it long before I got an Andersen. Some couplers are pretty flimsy.

Our TT has the 81911 and we have the Andersen hitch last year we put on only about 2000 miles. If we still have the TT and the couple would have to be replaced at some point down the road, I would install a Bulldog hitch. We have Bulldog couplers on numerous trailers, one of which has more than 100,000 miles on it and still has the original coupler.

I do a "pre trip inspection" everytime that we are packing for a weekend. Because of the issues that some have had with the Andersen hitch and different couplers, I make sure to check the coupler just to look for anything out of the ordinary.

BenK wrote:
Anyone have an image or reference link to the coupler mentioned in that Airstream link? I'd like to see it vs the others that Andersen endorses

Ben, I believe the coupler which is the subject of this post - #1772, Atwood 88010, is depicted in these Repair Kit Instructions.
A coupler cross-section is shown on page 2.

I have not seen a schematic for the Atwood 81911 which is being considered as a replacement for the damaged 88010.

Anyone who is considering using the 81911 with the Andersen WDH should be aware of this information from an etrailer.com expert:

"

I spoke with my contact at Atwood and there is not a repair kit available for the Atwood # 81911, 2-5/16 inch Ball, A-Frame Tongue coupler.
If this coupler is damaged the complete unit will need to be replaced."
expert reply by: Bob G

The date of the "expert reply" is unknown, so it would be good for potential 81911 buyers to see if a repair kit is currently available (just in case one might be needed).

Ron

JBarca wrote:
Hi Ed, Welcome to the discussion! When you came up with 1,000# per chain load, what are the assumptions surrounding this?

John, I don't know how Ed did it, but here is my approach to defining the relationship between tension and load transfer.

First we need to define some dimensions and variables. Let:
a = tow vehicle wheelbase
b = ball overhang (longitudinal distance from TV rear axle to ball)
c = distance from ball to mid-point between the TT's axles
d = perpendicular distance from Andersen chain to center of ball (reported by Andersen owner to be 6.5")
TW = tongue weight
LTT = load transferred to TT's axles
LF1 = load removed from TV's front axle due to TW without WD applied
LF2 = load transferred to TV's front axle when WD is activated
T = Andersen chain tension per chain
M = moment (torque) generated by Andersen chain tension (total for 2 chains)

then (assuming zero pitch-axis rotational friction between ball and coupler)

M = 2*d*T
LTT = M/c = 2*d*T/c
LF2 = LTT*(b+c)/a = 2*d*T*(b+c)/(a*c)
also
LF1 = TW*b/a

If we want to restore a load equal to some percentage (call it FALR) of that which was removed from the front axle, we have:

LF2 = LF1*FALR/100, or 2*d*T*(b+c)/(a*c) = FALR*TW*b/(a*100)

solving for chain tension (per chain) gives

T = FALR*TW*b*c/{2*d*(b+c)*100}

for example, if: b=60", c=200", d=6.5", TW=600#, and you want to restore 50% of the load removed (FALR=50)

T = 50*600*60*200/(2*6.5*260*100) = 1065# per chain

Ron

John, I take care of Barney by blocking tons and tons of folks...
So don't see their comments to have me comment back... :B

Even asked the admin (not moderators) to increase the allowed number
on my blocked list (yes, hit the limit several times...think mine now
has an unlimited number)

Better now, as I don't see their posts...just a line that states
'Post not shown due to blocked member xyz'

Still think the Andersen original and elegant, but their execution
lacking, IMHO

The novel use of plastic springs solves one huge problem with the traditional
spring bar and that is the rate of 'released' of energy, which contributes to
'bouncing' or 'porpoising' A few here used 'dampening', which it
does so with a slower stored energy release rate

If only they could be used in a traditional WD hitch...would that
solve that problem?

Down side is that their limited stroke married with the short moment
of the ball/shank limits the Andersen to lighter tongue weights.

Maybe an increased stack of those same plastic bushings, but just a
thought that would need more noodling

John, you posted some pictures a while back of your trailer coupler
that had thousands of miles with a traditional WD Hitch system.

It looked very typical vs the miles on it and the latch pawl was
beat up. I've seen much worse and NOTE that is with a traditional WD
Hitch system that does NOT have the constant contact that the Andersen
architecture has

Anyone have an image or reference link to the coupler mentioned in
that Airstream link? I'd like to see it vs the others that Andersen
endorses

Agree that these discussions are GREAT and am now leaning towards a
new to me, Blue Ox system. Knew of them, saw images of them, but never
looked closely till someone posted in another thread. I am now leaning
towards them...instead of PullRite (still will consider PullRite)

As for the derogatory comments/commenters...block them and if anyone
finds me offensive, please block me !!!!

Before this thread starts going negative, please lets respect each other. I do not want my thread locked by folks bantering points of view of others because they do not like someones opinion or their last post. Let's give Barney a break.

This thread was one of the 1st on the new Andersen WD hitch. It has been educational and enjoyable by many. And some, do not like the analytical nature of it. Both points of view are fine, but lets respect each other in the process.

Our forums are filled with most every brand of WD hitch on the market. And if we dig deep enough, there have been negative and positives on everyone of those brands. Many brands have failed or put the RV'er is a very negative situation and it has been discussed here on our forum. This includes the brand I own. Through education and understanding, now I know what not to do and why and so do many others.

There is no one size fits all even if some manufactures claim this to be the case. By "us" talking about them, RV net has helped many fellow RV'ers at least stop and "think" about their hitch, adjust it properly or get a better perspective to ask better questions to see if it fits "their" application. Lets keep that positive momentum going.

Agree with John that the smaller trailers will be okay, but higher weights has
both the length of the ball/shank an issue (solve by increasing either that length
and/or the amount of force via the bushings)

What isn't or hasn't been discussed is that the regular or traditional WD Hitch
systems 'do' beat up the coupler latch over time WITHOUT the constant contact
and force of the bushings.

The dynamic forces are NOT just from braking, or acceleration, but from all
directions during a common road trip over the less than pristine roads, RR crossings,
potholes, etc AT SPEED

What would you guys think or guess the forces on the latch is during a severe
whoop-do ?

Where the hitch is driven downwards to stretch the bottom plate to chain to rod to bushing?

There has been reports of the bushing shattering, or when the set screws employed
that they did not hold (allowed that bracket to move)