Tires and comments back from Joe Robinson in TM

[funpilot]

I post this with all the appropriate disclaimers for TM and with their permission. This was just an email discussion in response to some questions I had and all of us need to follow manufacturer's specs and if not sure, get expert opinions. I am not taking this as an expert's response.

The email started out with this paragraph that I sent TM:

Hooked up and using the blue ox, the trailer axle weighs 3380 pounds on the CAT scale. I unhooked it and weighed the tongue weight using a tongue weight scale, and it currently weighs 650 pounds. ​ So, I think that means the total weight of the trailer is 4030. On your website you list the dry weight as 3185, the tongue weight as 490 pounds and the load capacity as 1227. I presume I add those 3 numbers up (3185+490+1227 = 4902 for max weight leaving me almost 870 pounds of weight capacity left? Is that correct?

Here is Joe's response:

Robert,


We weighed this trailer at the factory before it was shipped out and it weighed 3,785 pounds overall. The tongue weight at the factory was 595.5 pounds. (We use aircraft scales that have been calibrated to the nearest 0.5 pound.) The dry weight listed in the brochure does not include any options, batteries, filled propane tanks, or any gear.

When you weighed the unit you came up with a total of 4,030 pounds overall and 650 pounds tongue weight. This is 245 pounds heavier the weight we got at the factory. (The variance is from adding the battery & propane and scale error.)

We can use the following formula with your figures of 4,030 & 650 pounds to determine load capacity.

Axle capacity: 5,200#
Tire capacity: 2,540# each (5,080# total)
This makes the tires the lower weight rating and you should use the lowest rating when determining load capacity.

So 5,080# max capacity - weight at axle 4,030# = 1,050# of additional storage capacity. You don't include the tongue weight since that weight is not sitting on your axle. (It's supported by the tow vehicle.)

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So, for me, the last paragraph was somewhat new to me.

I then asked what would be the benefit of going to "E" rated tires. Here was that response:

The "E" rated tire would give you more capacity. The axle rating is 5200# and that does not include the axle itself (around 350#) or the wheels & tires (around 50# each.) So the overall weight at the axle can be up to 5650# if you have higher capacity tires that match or exceed that.

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More info that I did not know. One more thing on the difference of weights. I believe the Blue Ox adds about 50 more pounds of weight as well.

[davlin]

Robert,

The tongue weight on my 2007 3124KS in "normal travel configuration" is about 600 lbs. A point, often lost, on those with smaller (and therefore lighter) TM's is this argument for WDH: you are approaching or exceeding the TV or hitch manufacturer's rating for undistributed weight on the hitch.

IMHO, this argument trumps the ones for increased stability, level TV, headlight position, etc. etc. When I had to tow my 3124KS from Arkansas to Texas without a WDH (long story), I had absolutely no issues with stability, "levelness," etc. But the thought DID go through my mind about what would happen if you had catastrophic failure of the hitch. In most configurations, even the safety chains are connected to the hitch.

I know this isn't an issue in your case, but I wonder how many of our 3124 brothers and sisters are towing without a WDH?

Dave

[tentcamper]

Robert, congrats on the new camper and thank you for sharing the factory response. I had forgotten that you subtract the weight of the axle and tires for the axle load, since the axle is not caring that weight. But you need to use that weight for the tire load. It was a good refresher with the numbers that always seem to be higher than the brochure.

It's been awhile since you first started the journey of order your TM. It looks like you have it configured the right way. It's time to get out and enjoy the camper, make some mods and let the DW do some decorating.

Please keep us updated on your travel experiences and feedback on the camper. Happy traveling and camping!

PS: I agree with Davlin that a WDH is the way to go.

Have most folks found that the scales they have used are accurate? Or consistent? Just curious as I would think this is something that most states take seriously, but one wrong reading could really be misleading...? Despite ones best efforts scales can be wrong?

[Bill]

An interesting discussion here from CAT scale engineers seems to indicate that, to my surprise, CAT scales are reasonably accurate over their entire range (200,000 pounds!) Apparently they are calibrated at a number of points over their range, rather than just calibrated at the top, and then dividing the range into (assumed equal) increments. Be cautious in reading the discussion, though. The writer makes the common error of confusing accuray with resolution. They are not at all the same thing.

http://fifthwheelst.com/commercial-t...ghing-rvs.html

Bill

[scrubjaysnest]

Good comment on the resolution Bill. I think we forget that sometimes. FWIW the receipt you get a Cat scale is certified for the state you weighed in supposedly.

[Padgett]

My rule of thumb is to drop the least significant digit of a reading. e.g. if it says 3440.3 lbs then I consider that +/- 1 lb. Of course you can always ask to see the latest calibration (may be a sticker) but a commerial scale like a CAT is certified to a certain level of accuracy. The question is "what is that ?" .

My current cause celebre is tire load ratings. We know that a 225x75R15 LRD is 2540 lbs @65psi (doesn't matter whose, that is the load rating of that size).

I sent a question into Michelin a couple of days ago asking whether that load is static or dynamic ?

Scales measure static loads. If you go around a corner at 1 gee, the load on tire increases by about 50% (theory: 41.4% increase). Jounce (go over a speed bump) can do the same thing without the vectors.

Hence my question, is the tire load rating just sitting there (static) or does it include dynamic loading (e.g. the Moose Test). If static, how much dynamic can it handle ? YWTK.

At least with the low towing profile of the TM, the torque loading of the tongue under braking is minimised and engineers always add some "kentucky windage" to ratings. For land vehicles I like at least 50%.

That tongue weight of a TM is very high and does contribute to stability though I was surprised to read 470 lbs (since removed 10 lbs) uncomfortably close to the 500 lb rating of my heep (that is the system rating, hitch is a class IV).

That is where I really wonder which TMs a minivan (typically with an 1 1/4" 300/3000 lb receiver like my old TranSport) can really tow. Particularly with an add-on (mine had the factory towing package which is more than just a hitch and I insist on in a tow car.

BTW back in the dawn of time I was driving a F*rd Econoline Van with an A/SR on a tandem trailer on the back on an Interstate when the whole hitch assembly broke off the trailer. Fortunately I remembered the prime rule of racing: "first do nothing" and just took my foot off the gas. Spoiler on race car, nestled in the door of the van and I made haste to get off the concrete (ever see a driveshaft break and the car do a summersault when it dug in - why drag cars have driveshaft loops ?) and got the rig to a hault on the shoulder.

[Bill]

Originally Posted by Padgett

My rule of thumb is to drop the least significant digit of a reading. e.g. if it says 3440.3 lbs then I consider that +/- 1 lb.

Padgett, I'm surprised at you! That is the classic error of confusing resolution (in this case 0.1 pound) with accuracy (which, as you say is unstated). In the same situation as your example, if the reading shows 3340.3 lbs, then the resolution is still 0.1 pound, but the accuracy is 100 pounds. Do you still interpret that as +/- 1 lb? I hope not.

Remember, resolution is the smallest change that can be detected (in this case, 0.1 lb), but it tells you nothing about accuracy.

Resolution is useful if you are making small changes, and want to know how much change you have made. For example, if you weigh the trailer and get 3350 pounds, then use up a partial tank of propane, then weigh the trailer again and get 3340 pounds, the difference in readings (10 pounds) will tell you how much propane you used, even if the two weight readings are wrong by a hundred pounds.

Bill

[Padgett]

I agree for most things but not if the scale is certified to a defined accuracy (just spent a while trying to find the accuracy requirement for a DOT certified truck scale. Failed so sent an e-mail to CAT. Will let you know what they say.)

My RoT was for cases where the accuracy was not easily available, resolution is visible in the reading. Are many cases where just relative values are important such as comparing generator sound levels. This is not one.

[tentcamper]

In our state they use those scales undocumented trailers weight to determine your plate fees and commerce uses them. I would think the accuracy greater than +- 1%. At 1% of my trailmanor's weight (35-40 lbs) is just a rounding error for my weight and balance calculations.

But I bet it's even closer, and is somewhere +- a few lbs since commerce depends on the state to provide accurate weight information.