Unique Tongue Weight Scale

[Frederick L. Simson]

SenZbar The Digital Drawbar
$149.99

[Benita]

What excites me most is the download link for the NHTSA brochure on safe trailer towing. A good resource for all us.

[Gina D.]

I just spent 40 on an analog drawbar.

Too late!

[Bill MacDermod]

Yeaaah, pull into a rally with that and everybody would be after you to bring it to their site so they could find out what they weighed, haaa

[Brian B-P]

I think the idea of this thing is great. What I really want is a three-axis load measurement:

• vertical, to measure hitch load;
• longitudinal, as an input to an electronically controlled "surge brake", or at least to find out how the current brake setup is working; and,
• lateral, to study the interaction between the trailer and tow vehicle.

There is one brake control product for fifth-wheel towing which apparently includes a load sensor, but I have not been able to find any real detail, and do not know if it can display a current hitch weight.

The Senzbar gets a minor mention from MotorWeek, was apparently mentioned in Trailer Life (Dec 2005 issue), is for sale from various vendors, and shows up occasionally in other web pages, but I have not found a real review or any other detail.

I am getting no response from http://senzbar.com (or http://www.senzbar.com) right now - maybe they like Benita more than me - but I did find two slightly different listings for the company...

Originally posted by http://www.thesaltydog.com/Yellowpage/marineretailers.aspx+-->

<!--QuoteBegin-http://www.thunderpress.net/PRODUCT_LINKS/1006ProdLinkbyProd.html
[b]SENZBAR
20120 New Hampshire Ave.
Brinklow, MD 20862
(410) 336-1351 EQUIPMENT
Class III digital trailer drawbar that accurately measures your trailers tongue weight.
www.senzbar.com
senzbar@hotmail.com

[Frederick L. Simson]

I think the idea of this thing is great.

I really like the idea of this thing, too. When I saw it, I had an (Aha!) moment
It is just a standard strain guage machined into the core of a hitch's ball mount! I'll bet the inventer is another weights-and-measures geek into trailering.

What I really want is a three-axis load measurement:

• vertical, to measure hitch load;
• longitudinal, as an input to an electronically controlled "surge brake", or at least to find out how the current brake setup is working; and,
• lateral, to study the interaction between the trailer and tow vehicle.

That would require 3 separate guages machined into the core of 1 draw bar! I don't think there would be much structrural integrity left at that point.

I am positive that this is only meant to be used to measure the tongue weight during a [b]weighing operation. A 1000 pound strain guage would not survive pulling a 3500 pound trailer down the street.

[b]Class III digital trailer drawbar that accurately measures your trailers tongue weight.

Which "Class III" are they referring to?
We all may be familiar with a "Class III" trailer hitch, but in the Weights-and-Measures business, a "Class III" scale is one that is deemed correct to the extent that it is permitted to be used in the conduction of commercial transactions, otherwise known as "Legal-For-Trade". Could they possibly mean both?

[Brian B-P]

...
I am getting no response from http://senzbar.com (or http://www.senzbar.com) right now...

It's back... or my network is back... or something.

That's the first on-line product purchase system I've seen which offers a "product list" with only one item.

The About page provides very little detail; for instance, there is no indication of measurement precision. Unfortunately, that's certainly not the only such site I've found. It also appears that the site has not been updated for over two years, which is also common.

...That would require 3 separate guages machined into the core of 1 draw bar! I don't think there would be much structrural integrity left at that point
....A 1000 pound strain guage would not survive pulling a 3500 pound trailer down the street.

I assumed that the strain gauge is on the top surface, not in the core, covered by the black interruption in the yellow skin. All three axes would mean gauges on three sides (all four would be better). In this configuration, the mount is just a plain bar (with minor depressions in the middle of each side), so there should be no structural concerns, but it won't have the accuracy of a linkage with a dedicated load cell element. This is why more detail would be nice...

Which "Class III" are they referring to?
We all may be familiar with a "Class III" trailer hitch, but in the Weights-and-Measures business, a "Class III" scale is one that is deemed correct to the extent that it is permitted to be used in the conduction of commercial transactions, otherwise known as "Legal-For-Trade". Could they possibly mean both?

I can only guess, but I assume that they mean only towing Class III, and even then they probably mean the informal industry definition of the class, not necessarily the SAE standard.

To meet SAE's Class III it would need to withstand 7,500 lb in the longitudinal direction (as in pulling the trailer) and 2,500 lb vertically and laterally, without breaking. The vertical spec is well beyond the 1,000 rating, but it might well structurally survive at that level but just not product a valid load reading.

[Brian B-P]

I am positive that this is only meant to be used to measure the tongue weight during a [b]weighing operation. A 1000 pound strain guage would not survive pulling a 3500 pound trailer down the street.

With the revival of discussion of this product in [b]tongue weight, dumb question, I resumed my investigation and sent an inquiry to the SenZBar people:

Originally posted by Brian B-P+-->

The response:
<!--QuoteBegin-The SenZBar Team
[b]The SenZBar is actually rated as a low Class IV drawbar to tow trailers up to 6,000 lbs.
We recommend it as Class III towing up to 5,000 lbd trailer and 500 lbs TW.

It will measure up to 1,000 lbs.

Yes you can tow with it in the Class III range.

[Brian B-P]

In another round of correspondence with the SenZBar people, I asked two more questions:

...it has been suggested that the drawbar was would not retain its accuracy if subjected to the loads of actual towing...Does the calibration change with time due to slight permanent deformation of the bar material?

Originally posted by SenZBar response
No recalibration is required as the processor Tares and zeros out each time it is turned on.

I currently do not have a 2" receiver. Do you intend to offer a 1.25" variation for Class II use?

Originally posted by SenZBar response
We do not have plans for a 1.25 at this time since the 2" size is such a majority of the market.

I'm not surprised by the 2"-only response... it's only reasonable.

[Frederick L. Simson]

[b]QUOTE(The SenZBar Team)
The SenZBar is actually rated as a low Class IV drawbar to tow trailers up to 6,000 lbs.
We recommend it as Class III towing up to 5,000 lbd trailer and 500 lbs TW.

It will measure up to 1,000 lbs.

Yes you can tow with it in the Class III range.

This response has me intrigued. Had these people discovered a lack of consequences for deflecting the bar in a direction not originally intended; (ie., pulling along the length)? I routinely make a lot of money replacing strain gauges that are (unintentionally) deflected on an axis 90* (sideways) from the intended deflection (vertically).

Compare senZbar to a common industrial strain gauge; the active component of a floor scale.

In the strain gauge, the vertical holes through the body are for mounting. The double holes are for bolts thru a spacer shim to the platform plate. The single hole is for the scale's foot.

The round plug on each side of the gauge covers the actual gauge, which is a hole through the body with 4 wires attached to the inner surface of the hole. This makes the hole's surface into a wheatstone bridge. The excitation voltage is applied at 90* and 270*, and the error signal is taken off at 0* and 180*.

[b]QUOTE(SenZBar response)
No recalibration is required as the processor Tares and zeros out each time it is turned on.

This response tells me that the they are sales persons communicating with a layman, not technical reps communicating with a technician. They missed my point...

[Brian B-P]

The illustrated device is a nice example... imagine the two-hole mounting end inserted in the hitch receiver, and a ball inserted in the single hole, and you have something just like the SenZBar might be.

I didn't really ask them if the longitudinal strain affected the vertical measurement accuracy... only if the sustained load caused a calibration issue. I think that they appropriately answered my question, but not Frederick's concern, because I didn't ask about that clearly.

For those not familiar with strain gauges and the load cells made from them, I would like to add a little to Frederick's description:

• the device in the linked document is a load cell
• a strain gauge is a piece of conductive material
  • it is bonded to a surface
    
  • when the surface strains (stretches or compresses) due to stress (applied force) the resistance of the gauge changes
    
  • since the change is small and is affected by factors such as temperature, the gauges are combined in circuits (the Wheatstone bridge) in which they are compared to get a result in which some unwanted factors are canceled out
• the wires attached to the inner surface of the hole in the load cell are the strain gauges

I assume that the SenZBar incorporates multiple strain gauges. As Frederick suggested, if force is applied to the cell in a direction other than that expected and allowed for in the design, it will not produce a correct result. So the question is whether the SenZBar appropriately considers and accounts for the longitudinal (pulling) and lateral (side-to-side control) forces, in addition to the vertical force of the tongue weight.

By the way, seeing this design I understand Frederick's earlier comment about holes weakening the bar: I was assuming that the SenZBar strain gauge wires were on the outside surfaces of the bar, where Frederick is assuming that they are on the inside surfaces of a hole in the bar.