trailer axles and wheels calculations

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delavan

Active member
Joined
Jun 20, 2018
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30
Hi,

My old 1978 21ft trailer is causing me to learn a lot and fast! Its a 5600 lbs dry tandem axle. Both axles are rated at 2800 lbs each, from the nameplate.
I got some brand new tires for it, their rating is 1875 lbs per tire in tandem axle config and 2150 in single axle config.

https://www.tirerack.com/tires/tires.jsp?tireMake=Goodyear&tireModel=Endurance&partnum=075NR5END&tab=Survey?affiliate=FH8
They are D rated, 65PSI.

I believe the tire shop did a boo boo. I most likely contributed to that, as I just gave them the 5600 GVW rating, and I didnt know there was another nameplate with the listed axle rating. Shouldn't they have given me some 2800lbs+ tires? The 225s would have got me the required load. E rating and 80psi. I  just requested 205s as its what was on the trailer in the first place. There was an RV place that recommended me to install 225s (TOO LATE) but I didnt know width in a trailer tire was bringing extra load capacity...

How I realize something was up is when I went the bargain "cargo trailer/week-end welder" shop and eyeing a $89 (tire+rim) chinese combo for a spare. The wheel is rated for 2150 lbs max, and I did chicken out, not knowing if it was good enough...

I'm building this plane as I fly it, so to speak. Did I get the wrong tires, putting myself and my family at risk?
 
The info is conflicting, some sites would tell me that I have to add the weight rating of both tires per axles. so, with the 1860lbs of these D rated tires TIMES 2, then I would be good for 3700 lbs with these tires on a 2800lbs rated axle....

 

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If I understand, you have 2 axles, each rated at 2800#.  You need tires rated at 1400# or more.  There are two tires per axle.

Those tires have plenty of capacity for those axles.

My other question is the numbers do not sound right.  Is the camper 5600# DRY or 5600# GVWR?  If it is 5600# dry, then you have very little cargo capacity, around 600#.  If it is GVWR, it is a bit unusual for axle capacity to equal GVWR.
 
I think its written  5600 GVW only. I guess it means empty tanks, so I called this dry. Maybe a misnomer? and I agree that the 2800 lbs rating for the axles is conveniently 50% of the over all rating.
 
Federal safety standards now state that the trailer axles & tires must be equipped to handle 90% of the trailer GVWR (the max weight rating). The other 10% is carried on the trailer hitch, not the axles.  Dry weight is not relevant to that - just the maximum total load.  Trailers have been required to have a GVWR placard for decades, so that is likely what yours had displayed.  If you found a rating plate on the trailer, it is more likely to be the GVWR (maximum Gross Vehicle Weight Rating), but look closely to be sure.

In any case, if you have 2800 lb axles, the tires must be sufficient to support the max axle load, so 1400 lbs each as Grashley stated. With yours ar 2150 each, they are more than adequate.

I've never heard of derating tire load capacity for tandem axles - only for duals (two tires on each end of the axle). Tandem axles are considered to be two singles and are independent as far as weight capacity is concerned.
 
Cargo is included in the trailer?s GVWR. In the current rules, when a vehicle leaves the factory it has a GVW or dry weight assigned to it. The GVWR was a design target figure and will not change. So, deduct the GVW from the GVWR and you have the trailer?s total cargo capacity.

In the 2007 rules change the weight of installed propane systems, including the weight of propane became part of the trailer?s GVW. For your older trailer any propane bottles and the propane in them would have to be added as cargo.

As has already been mentioned, tires are and have always been fitted to the trailer manufacturer?s certified GAWRs. All tandem axles will use the ?single? tire inflation value. So your tires, inflated to provide 2150# of load capacity are providing 1500# of load capacity reserves per axle.
 
Thanks all for the valuable info.
I guess the fact that the trailer is an old 1978 and that its a Canadian trailer might have changed a little to the ratings advertized???
knowing I don't see any ratings for GVWR and I only deal with GWR on the old nameplate made me assume I was maybe towing more than I actually do.
I got 2 x 30 lbs propane tanks and and RV/Marine deep cycle battery (type 24), the trailer is full of what we need and that we don't fill the water tanks before we go.
I badly need to hit the catscale to see how is that puppy weighting. Its only 1 hour away.

As for Gary's comment about derating tires for tandem axles, it must be me lost that assumed. there is a writting on the tires that mention single /dual...i might have confused dual and tandem as being the same...

Learning a lot! I'll need to camp to decompress from getting a trailer and figuring it out. :mad:
 
In normal usage, GVWR is the max rated weight whereas GVW is the actual loaded weight.  But if a placard spells it out, it might say "Maximum GVW is xxxx"  or "Do not exceed xxxx lbs GVW", and therefore means the same as GVWR.  I've seen that sort of verbiage on various rating placards.  GVWR is just a shorthand notation for the longer phrases.
I think Canada has used the very same nomenclature for ages, but I can't testify about 1978 vintage. Here's a current Alberta province brochure on vehicle weight terminology.
https://www.transportation.alberta.ca/Content/docType41/Production/recvehtowguide.pdf
 
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