Hi Teach and others following along
See if this helps explain some of the basics on how these friction type hitches work. Like I mentioned earlier, due to patents each hitch manufacture had to come out with something new to compete against the next guy.
All of these 6 types of hitches use friction as a key element in creating a stiff connection at the tow ball. It cannot be too stiff or the TV could never make a turn or something would bend all up when you did turn. The TV plays a large role in the ability to hold against the effects of sway against the TT. Length, height and weight of the TT play a role in this too. The larger the TT, the larger the sway effect can be. None of these hitches are a fix all to sway. They are however a very important part of an anti sway program.
Since these are friction type hitches lets first talk a little bit about friction. There are 2 types of friction.
Static friction is where 2 objects are at rest and resisting forces acting upon both objects trying to move in relation to each other.
Kinetic friction is where the 2 objects still have friction between them, however the 2 objects are sliding in relation to one another.
The forces acting on a system in static friction build evenly until they reach a maximum breakaway amount of force, then we have sliding friction as the objects are now moving. The force to keep an object sliding is less then the breakaway force that broke the object free and into motion.
Next are a few things to realize in the world of friction. I’m going to quote a text book here:
Quote:
Originally Posted by Statics and Strength of Materials by Irving Levinson
From a practical point of view, three laws describe the retarding force of friction. These laws state that when one solid object slides over another the frictional force is:
1. Proportional to the pressure force between the bodies
2. Independent of the area of contact
3. Independent of the sliding velocity
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And there is a simple formula used to determine the forces which is:
Coefficient of Static friction = Force (max)/ Normal force
Or in simpler to understand terms, The coefficient of static friction between 2 objects X the force holding (Normal Force) an object at rest is = to the amount of force to breakaway and start sliding (Force (Max).
OK so what does all that means in regards to a sway control part of a WD hitch
? Simply that to prevent 2 parts from sliding against each other you either have to increase the coefficient of friction between the 2 parts OR increase the amount of force holding the 2 parts together.
By either change it will increase the breakaway force between the 2 parts before sliding friction starts to occur. By changing one or both we can change the holding power of these anti sway hitches. Also to note, each principal employed creates a different level of friction and therefore holding power against the effects of sway.
Alright enough of this star wars stuff….
I get accused of that some times…
. Lets look at some hitches
Here is the basic: A friction sway bar. It creates a stiff sliding bar on the side of the hitch to stiffen up the pivot connection between the TV and the TT. It is a brake lining type material pad being squeezed by a clamp bolt against a plated steel bar. Standard Brake material against steel is approx. 0.4 for a static coefficient of friction. That stays constant and you tighten the screw clamp to gain more force. You can add up to 2 sway bars, one on each side to make the TT,TV connection stiffer. The force is limited by how tight you can crank the screw. The friction sway bar has been around a long time and many hitch manufactures make them. Husky, EAZ lift, Reese, Drawtite to name only a few.
Here is fairly new product on the market by Reese. This came out we believe to compete against Equal-I-zer made by Progress Mfg. It is called the Reese SC hitch. Here they have installed brake pad material under the WD bar. This may be higher friction material then standard brake lining. It might be 0.4 or maybe up to 0.6 for a Coef of friction against steel. The force here comes from the hitch performing WD and can get quite high when the tongue weights are high. There are 2 pads one on each L bracket on each side of the hitch. Reese claims there are 4 more points of friction in the hitch head by the trunnion lugs pivoting. I think this is marketing hype. The tow ball also provides friction…
The next we have the Blue OX Sway Pro hitch. This hitch uses metal on metal coef of friction which is 0.8 for mild steel with no grease. Grease derates it to 0.05 to 0.1. The force comes from tightening up the trunnion pivot bolts and the WD bars pulling up on the hitch head. The more tongue weight and WD tension, the more friction can be generated in the hitch head to create a stiff connection. They also use WD bar chain guards to help hold the WD bar from swinging which reduces some effects of sway with this design.
Next we have the Equal-I-zer made by Progress Mfg. This hitch uses metal on metal coef of friction which is 0.8 for mild steel with no grease. Grease derates it to 0.05 to 0.1. The force comes from tightening up the trunnion pivot bolts and the WD bars pulling up on the hitch head like the Blue Ox. Don't know who copied who. However it has the added benefit of more friction being generated by metal on metal at the L brackets. They have true 4 points of friction generations. Here higher tongue weights and WD create higher friction forces and a much stiffer TV to TT connection.
Here we have the new Husky Centerline hitch. This hitch uses metal on metal coef of friction which is 0.8 for mild steel with no grease at the L brackets like the Equal-I-zer does. This friction force increases with tongue weight and WD adjustments. They also include a spring loaded force against a blunt surface of the WD bar trunnion. When the WD bar pivots it is met by resistance of the spring. I have no values on the spring force to help sort out how effective that is in resisting sway forces. And I can’t seem to find a different spring pack per hitch size. Just larger WD bars according to the parts list. So from a 500 to 1,400# hitch it seems to use the same spring system.
Next we have the Reese Dual cam. This hitch uses metal on metal coef of friction which is 0.8 for mild steel with no grease. It has V shaped WD bar that rests on a cam lobe where the friction is created. This one has a different twist to it. We have a constant friction holding force when the TV, cam and WD are in the straight ahead position. As sway forces act on the side of the camper and push on the hitch, the WD bar raises up on the cam. This raising up generates more force and then applies more holding force the higher up the cam it goes. It helps compensate for the loose due to sliding friction. None of the other hitches above have this feature. There friction is constant and once it turns into sliding friction the holding force starts to go down slightly where the DC force goes up. The holding force does rise with higher tongue weights and WD settings. Due to the cam feature the WD bar has to push uphill so to speak agasint the cam which adds higher degree of force until the breakaway point is reached then a flat sliding WD bar agasint a flat L bracket.
You can see the cam and WD shape in this pic as I was explaining how to adjust the cam
Also to not loose track of is dirt, grease, snow, and rain. Any of those can change the Coef of friction between the 2 mating surfaces. We get exposed to many of these and more while towing.
The key in all this is, which hitch can produce the higher friction to be the most effective? The Reese DC and the Equal-I-zer are very close to each other from my point of view. Regardless of which hitch one has, the hitch must be set up correct to work the best it can. And there is no one perfect fits all in TT applications. The are pro’s and con’s to each. Also to remember the TV and TT play a large role in this. A 1 ton long bed crew cab Dually towing a T1550 Sunline is a lot different then a short wheel base 1/2 ton SUV towing a 32 foot heavy slide TT. The hitch cannot fix it all. The hard part is sorting out which hitch is good enough for your size TV and TT. None of these hitch manufactures will rate them like that.
Hope this helps
John