Dual Cam Setup

JBarca wrote:
Here I need some help understanding why TW is not a factor. I do not understand how you can discount it. The hitch was adjusted because of the TT TW. TW is one of the forces pressing down on the WD bars creating tension in them. The more down force, the more tension and less down force is less tension. So far so good? Agree?

I agree the WDH was adjusted to redistribute load which was imposed on the TV by the TT's TW.
I do not agree that TW is pressing down on the WD bars creating tension in them.

It would be entirely possible to use a WDH to transfer load even if the TW were zero. The bars transfer load because the lift chains are pulling up on the bars and the chains are pulling down on the TT's A-frame. This can happen with zero or negative TW.

When one WD bar lost tension due to hitch head roll and the tilt is relaxed, the WD on the other side of the hitch head truck can still be under tension and in most cases is. When the back of the truck drops in height due to loss of 1 entire WD bar going to zero, the angle of the hitch head is tilted back towards the TT by the truck dropping height. Similar to a back flex when the truck goes up hill just not as pronounced.

I agree 100%, and I think it might be helpful to expand on this scenario by using a numerical example.

The TV has a 130" wheelbase, 65" ball overhang, front axle spring coefficient of 400 #/inch, and rear axle coefficient of 500 #/inch.
The TT is 195" from ball to midpoint between axles.
The the WD bar length is 32" and its cantilever stiffness is 300 #/inch.

The two WD bars have been adjusted give a combined 500# restored to the front axle and 750# removed from the rear axle.
The two bars each contribute 50% of the load transfer.

The TV/TT are travelling straight ahead and one of the lift chains breaks.
The load on the front axle immediately decreases by 250# and the load on the rear increases by 375#.
The front wheelwell rises by 250/400 = 0.625" and the rear wheelwell drops by 375/500 = 0.75".

The front rise of 0.625" combined with the rear drop of 0.75" over the 130" wheelbase gives a frame slope change of 0.61 degrees
Extending that slope to the ball causes the ball to be 1.44" lower than with both bars active.

Having the nose of the TT 1.44" lower gives the TT an angle change of 0.42 degrees.
The resulting change in vertical angle between TV and TT is 0.61+0.42 = 1.03 degrees.
This means the rear end of the remaining WD bar is effectively raised by 32*sine(1.03 degrees) = 0.57".
The corresponding increase in load on the remaining WD bar is 0.57*300 = 172#.
The base load on each bar to generate the initial load transfer was 762#.

The 172# increase in bar load would cause 57# to be added back to the front axle and 71# to be removed from the rear axle.
These load transfers would cause the slope of the TV to decrease from 0.61 degrees to 0.56 degrees.

The increase in load on the remaining WD bar would end up at 159#.
The ball height would be 1.32" lower than when both bars were attached.
The primary effect of having one bar with no load is to cause a drop in ball height due to decreased load transfer, rather than causing a significant increase in bar load.

The TW of the TT is still pressing down and more TW mass is available to be resisted by the only remaining WD bar which has tension. While the head roll angles play a large role in how much weight is added to the one remaining WD bar, the head now has more rear tilt then it did before, the rear of the bar is rising up the cam and more mass available to load on the one tensioned bar then when the loads were shared by both bars.

Adding the effect of a sharp turn complicates matters. It'll take a bit more time to come up with the numbers for this scenario.

My point is, TW does play a role in this, it cannot be discounted. In order to bend the snap up, the bent snap up side of the A frame had to gain more chain tension and the WD bar had to have an increased tension in order for the chain to have more. I do not think you are disputing the fact the WD bar tension had to rise in order to increase the chain tension.---

IMO, the snap up damage is not due to an increase in vertical force produced by the WD bar. As I tried to show in the my Dual Cam free body diagram, I believe the damage is caused by a large increase in chain load resulting from the friction force between bar and cam and the increased compression in the cam bar.

I agree high friction is in the middle of this. When the load goes up, the friction goes up and as you have shown, the friction load is large multiplier, possibly 4.3 to 1 if I understood correctly. However my recollection of the laws of friction remember that to increase the friction force we have to change 1 or both of 2 things. The coefficient of friction has to change to a larger value (not likely in this case) or the force applied has to increase. (most likely in this case) If we have no force increase, how can the friction force increase? That remains the question, how did the force increase?….:h

The normal force increases because the turning hitch head is trying to pull the rear slope of the outside bar through the cam. This increases the friction force which, in turn, increases the normal force. The end result is the force magnification shown in my free body diagram example.
Without this force magnification -- the Dual Cam doesn't work.

If the rear slope of the bar were horizontal, the friction force would simply be equal to the vertical force from the WD bar times the coefficient of friction.
Since the rear slope is not horizontal, the normal force is greater than the bar force.
Imagine what would happen if the rear slope were vertical -- the force magnification would be enormous.

I will see if I can hunt up some kind of load cell for the chain and or the load pointer on the WD bar. However I do not have a way to record the constant travel under tow. Yet anyway. Also need a positive way to hold the hitch head still while measuring load applied to the end of the WD bar for the spring constant data. I'm a think'in.

Shouldn't be necessary to be under tow. You should be able to measure the effects using a slow forward turn in your driveway.
I'll see if I can come up with some ideas for measurements.

Ron