3 x 7k axle but only 20k GTW???

[hoopt01]

Looking at upgrading TT toyhauler for 5th wheel toy hauler (grand design momentum) specific floor plan still being debated. Anyhow, why... if it has three axles rated @ 7k each (not to mention pin weight) would it only be rated for 20k max? What is the weak link; pin, frame, hangers, other???

[mapguy]

JIMNLIN wrote:
Dexter axle;
The Gross Axle Weight Rating (GAWR) of your running gear is determined by the lowest rated component in the assembly. The capacity of the wheel, tire, axle, brake, springs/rubber and hub are all considered.


This from AL/KO axles says much the same:

** Weigh the trailer after it is fully loaded and ready for use.
Axles, brakes, wheels, tires, frames and suspension components are
designed to carry a specific maximum weight. Locate the VIN
(Vehicle Identification Number) plate on the trailer. It will show the
GVWR (Gross Vehicle Weight Rating). The GVWR is the total
amount your trailer (including tongue weight) can weigh when it is
completely loaded including holding tanks, propane etc. The GAWR
(Gross Axle Weight Rating) is the maximum load that the axles will
carry and the maximum load the brakes will stop.**

Jim, technically correct. Assumption is an 8 lug hub is installed when 7k axle is referenced. 6 lug hub/bearing set limits 7k axle to 6k.

[joebedford]

azdryheat wrote:
My Voltage has the same thing: 3x7000 pound axles and a 19,000 GVWR. Glad one of cares enough to pose the question.

On my Voltage 3950 with 3 x 7000lb axles, the kingpin is rated max 19000. There may be other things too.

[JIMNLIN]

Dexter axle;
The Gross Axle Weight Rating (GAWR) of your running gear is determined by the lowest rated component in the assembly. The capacity of the wheel, tire, axle, brake, springs/rubber and hub are all considered.


This from AL/KO axles says much the same:

** Weigh the trailer after it is fully loaded and ready for use.
Axles, brakes, wheels, tires, frames and suspension components are
designed to carry a specific maximum weight. Locate the VIN
(Vehicle Identification Number) plate on the trailer. It will show the
GVWR (Gross Vehicle Weight Rating). The GVWR is the total
amount your trailer (including tongue weight) can weigh when it is
completely loaded including holding tanks, propane etc. The GAWR
(Gross Axle Weight Rating) is the maximum load that the axles will
carry and the maximum load the brakes will stop.**

[mapguy]

The lowest capacity component, springs, tires, or axle, dictates axle capacity in the assembly.

This assumes (hate that word)the frame is engineered for the load of the axles at full rating.

[fj12ryder]

BurbMan wrote:
...

So if your tires are rated at 3400 lbs, then 3400 x 2 =6800 lbs. The axle is not "downrated", put on higher capacity tires if you want to carry the 7k it's rated for...

Well, yeah it is according to Keystone who put it all together. They rate the axles at 6800 lbs. according to the labels, even though they are 7,000 lb. axles. Since the axles are rated by Keystone for 6800 lbs., I would say they are downrated from 7,000 lbs.

Dexter rates the axles at 7,000 lbs. so yes I know I can put on higher rated tires, which I have done.

[BurbMan]

Some are...ST 235/85R16E are rated for 3420, but ST 225/75R15E are only rated for 2830 according to the Maxxis Site.

Check the tire sidewall for max loading info based on max inflation pressure and size. Point is that axle load is restricted by tore capacity most of the time, an axle wont carry much on 2 flat tires.

So if your tires are rated at 3400 lbs, then 3400 x 2 =6800 lbs. The axle is not "downrated", put on higher capacity tires if you want to carry the 7k it's rated for...

[fj12ryder]

BurbMan wrote:
It's the tires. A load range E tire has a rating of 3042 lbs @ 80 psi, so the axle is limited to 6084 lbs even though the axle itself is rated at 7k.

So 3 axles is a total of 18,252 plus whatever on the hitch/pin plus a safety margin for loading...side-to-side and front-to-rear, depending on how the rig is loaded will not distribute the load equally over all 6 tires, so if running at max weight it's likely that at least one tire overloaded and one is underloaded.

That would only hold true if the tires are LT tires and not ST. "E" rated ST tires would be rated to 3400 lbs. My toyhauler came with "E" rated ST tires, and the axles were down rated to 6800 lbs. with a GVWR of 18,000 lbs.

[ependydad]

BurbMan wrote:
It's the tires. A load range E tire has a rating of 3042 lbs @ 80 psi, so the axle is limited to 6084 lbs even though the axle itself is rated at 7k.

So 3 axles is a total of 18,252 plus whatever on the hitch/pin plus a safety margin for loading...side-to-side and front-to-rear, depending on how the rig is loaded will not distribute the load equally over all 6 tires, so if running at max weight it's likely that at least one tire overloaded and one is underloaded.

If I recall correctly, the Grand Design Momentums are coming with LR G tires.

Even so, my LR E tires that I originally had were good for 3,420 which brings you to 20,520 plus pin weight.

For the OP - I wondered the exact same thing. I think this would be a good question to send to Grand Design. My guess is it has to do with either the pin box (could be a 20k pin box) or the frame.

I wanted a GD but couldn't get by with the 3,500 lbs. of cargo carrying capacity in a 43' toy hauler. It didn't make sense to me.

[BurbMan]

It's the tires. A load range E tire has a rating of 3042 lbs @ 80 psi, so the axle is limited to 6084 lbs even though the axle itself is rated at 7k.

So 3 axles is a total of 18,252 plus whatever on the hitch/pin plus a safety margin for loading...side-to-side and front-to-rear, depending on how the rig is loaded will not distribute the load equally over all 6 tires, so if running at max weight it's likely that at least one tire overloaded and one is underloaded.

[Wild_Card]

Well at least it's not 20k# gvwr with 3-5000# axels

[tinner12002]

Great question as I've wondered the same thing. My RV has a GVWR of 19K, has 3-7K axles for a 21K rating and tires with a weight rating of 24.5K so go figure. My pin is at 19.5% of GVWR. Not sure where the weak link is, a factory rep told just to not exceed the axle rating if going over the 19K GVWR listed on my RV, of course he wouldn't put that in writing...lol!

[crazybanshee]

Most toy haulers run 20-25% pin weight. So a 20,000GVW trailer will have 4000-5000 K on the pin and 15,000-16,000 lbs on the axles

[fj12ryder]

The third axle is most likely to help spread the weight of whatever you put in the garage. On a non-toyhauler, the heaviest items are built into the trailer and will not be added by the owner. With the toyhauler you could end up adding an extra ton or ton and a half near the rear of the trailer. Might be too much concentrated weight for just 2 axles.

[hoopt01]

Thanks for the responses; the factory tires are g rated so a little over 4k each.

With a 3150 pin weight that leaves roughly 17k on the suspension components; spings and hangers etc. a two axle in the same model line has GvWR of 16500 pretty close to 7k an axle plus pin weight for the model... provided all things are equal just trying to figure out “why” the third axle only nets 3500lbs of added capacity to the trailer. Wondered if if it’s merely a regulatory thing? Have to classify differently over 20k? Meet other restrictions over 20k? Or if it’s actually a physical capability issue?