Replace lighter axles with heavier ones - increase GVWR?

[FlyFishn]

I suppose this is a more generic question than specific to travel trailers.

When I was studying 5th wheels a while back something that caught my attention was in a lot of cases the combined axle capacity was LESS than the GVWR. Doing the numbers the combination of 3 factors is what satisfied the GVWR:

1 - axle

2 - axle

3 - pin weight

For example - if there were a pair of 6,000lb axles under a 5th wheel with a GVWR of 14,400lbs. The combined axle rating is 12k - but the GVWR is 2400lbs higher. Where does the extra 2400lbs come from? Pin weight.

That really bothers me that the manufacturers run the numbers like that. Numerically it may "work" but there is 0 head room* on the ratings. Back in the day they didn't know how to make things cheap so they just made things good.

*Edit - if you look at the "dry" weight of trailers (and I agree with so many here and elsewhere - the manufacturers' numbers rarely are accurate - to know your weights you need to scale, point blank) the numbers allow for some room between the "dry weight" and the "GVWR", or "loaded for a trip" weight limit. I think most would agree that GVWR is usually pretty easy to hit and surpass if you aren't careful. As with the truck example further down - the axles are heavier than the GVWR - why do RV manufacturers have to be so cheap they run axles right up to the top limits with little head room?

My truck, for example, has heavier axles than the GVWR. It is 11,500lbs GVWR with a 7k rear and 5600lb front axle rating, respectively. The axles add up to 12,600lbs. So when ya think about the way the numbers roll here - the truck's GVWR is LOWER than the combined axle ratings. In the example with the "light" trailer axles - the GVWR is HIGHER than the combined axle ratings.

No matter the platform (TT, 5th) - does anyone know if there is a way you could re-register at a higher GVWR if the axles are replaced with higher rated ones? For example - on a TT with 4,000lb tandem axles and a 9k GVWR - if you replace with 6,000lb axles can you re-register with a higher GVWR?

If you go to the HDT forums this is done in reverse - "singling" class 8 trucks to drop the GVWR and the associated higher registration/insurance fees.

Either way, getting to the point of the "light" axles commonly used on RV's - they scare me from the start, and I'm known to be a heavy packer. When I was a kid the front axle of one of our boat trailers broke a leaf spring going down the road and the axle went sideways locking up the rear axle so I know things can happen with axles. Heavier ratings run lighter will have that "head room" to keep things safer. I just don't particularly like the idea of running axles right to the max.

Edited January 25, 2022 by FlyFishn

[Jaydrvr]

Low running gear ratings are common in this industry and a regular source of breakdowns and problems. However, in the instance you mentioned, the ratings are taking into account the pin weight. For example, our fifth wheel is very substantially engineered and built, but the axles are 8,000# rating times two, for 16,000#. The GVW is 20,000#, as the pin weight is close to 5000#.  Jay

[rickeieio]

In the example of the truck with a combined axle rating exceeding the total GVWR, can you load the truck to get the rated weight on each axle, or will you have more weight to the rear or front?  Same way with trailers.  The manufacturer has no control over where you put the weight.

All this is why it's important to know your limits, and use common sense.

Our fifth has three x 7k axles and about 4k pin weight, but a gross of 21k.  I like overkill.

[FlyFishn]

4 hours ago, rickeieio said:

In the example of the truck with a combined axle rating exceeding the total GVWR, can you load the truck to get the rated weight on each axle, or will you have more weight to the rear or front?  Same way with trailers.  The manufacturer has no control over where you put the weight.

"Where" the weight is wasn't the point - it was the low ratings. The "where" description/points was an illustration to get to the point of the low ratings.  

Have you scaled your rig to see what kind of payload you can get and stay "within the numbers"? I suspect that isn't going to give you a whole lot of head room, if any, on any of the numbers - trailer axle capacities, truck rear axle capacity, and either/both combined GVWR's - unless you get to a class 7 truck (classes 4, 5, and 6 are hard to get a truck configured to have any amenities with much left over payload).

That was the heart of the overall question - if you put heavier axles on the trailer so as to not risk max'ing our and overloading the OEM axles (easy to do) - can you get the registration to reflect higher GVWR than the manufacturers' rating? If not - at least the heavier axles are more head room at max gross, as opposed to riding right on the proverbial "edge".

[FlyFishn]

Somewhat in-line with my previous post - my truck's GVWR is 11,500. I've scaled around 12,200. I don't recall what my axle weights were, but likely well under on the axles. That was with gear in the bed and on top, no trailer, so I am sure my front axle was loaded up past empty by a good bit.

The opposite is true for a bumper pull trailer (even with WD - you can return some of the front axle weight over the change from empty to no WD, but not 100% of it). With a bed-mounted hitch (gooseneck/5th wheel) the loading change depends on where the loaded hitch weight is centered - if it is right over top the rear axle it shouldn't change the front axle loading much if any. If it is ahead of the rear axle it should add a bit of loading up front.

[mptjelgin]

Sure, many of us have scaled our rigs and know exactly how much is on each axle and on the pin or tongue. Our last full-time fifth-wheel had a Net Cargo Capacity of around 4000 pounds so there was considerable difference between the GVWR of the trailer and it's scaled empty weight.  And I towed it with a properly spec'd SRW F-350 that also had a comfortable cushion on each axle rating as well as the GVWR and GCWR.  It can be done pretty easily if you pay attention. 

Our current "part-time" travel trailer doesn't have 4000 pound NCC, but I don't need that.  It does have a couple of thousand pounds, and that is more than enough. I've scaled it fully loaded up and I'm well within the ratings. 

On the other hand there are certainly trailers (and motorhomes) out there with very low carrying capacity, as well as trucks with little available additional capacity. They exist, but again, if you pay attention you can easily accomplish a safe combination without having to go to a MDT or HDT.  

As to putting heavier axles on the trailer and increasing the GVWR of the trailer, legally not probably actually not either. Simply changing axles and tires, and maybe brakes will give you a safer trailer with more cushion on the running gear but what about the trailer frame, the pin box, etc. Trailers work as a system, and changing individual components without considering the overall system isn't a good idea. 

[Mark and Dale Bruss]

GVWR is more than axles.  The frame, the springs, the transmission, and the engine are all taken into account when determining the GVWR.

[justRich]

As it comes from the factory, the pickup truck GVWR is based on the tire load capacity.
Yes, there are other factors, but you will never ever see the axle rating exceed the tire load capacity.

 

[FlyFishn]

1 hour ago, Rich&Sylvia said:

As it comes from the factory, the pickup truck GVWR is based on the tire load capacity.
Yes, there are other factors, but you will never ever see the axle rating exceed the tire load capacity.

 

Right. The point is a bit off topic here, but D range tires on SRW trucks can be an issue. If someone puts D range tires on a 3/4 ton truck they might get a softer ride but the load rating on them is likely a good bit less than the axle ratings and where the GVWR would load them to (likely under the axle ratings, but also very likely under the load ratings of the tires). Knowing what the actual numerical load rating on both tires and axles is important - not just the letters. Though, letters do give you a quick reference point if you know what they mean. If you should have E range tires, for the example here, and find D range - giant red flag.

Ideally your tires should be numerically higher rated than the axle they are on. If you have a 7k axle, for example, then your tire ratings should be over that. For an SRW truck that would be, perhaps, 3650lbs or 3800lbs per tire. If the tires are, say, 3250lbs - that totals 6500lbs - 500lbs under axle rating. Considering my truck's ratings - there is over 1000lbs more capacity on axle ratings than GVWR, so a 3250lb tire rating might suffice at GVWR, depending on where the weight is, but that is just one more number to make darn sure you're not surpassing.

I'd be more scared of surpassing a tire rating than an axle rating, but scared of both for sure. I've been in a tow vehicle with a trailer axle break going down the road - don't want to be in one again, as passenger or driver.

[vlbrown86]

On 1/25/2022 at 8:29 PM, FlyFishn said:

 

That was the heart of the overall question - if you put heavier axles on the trailer so as to not risk max'ing our and overloading the OEM axles (easy to do) - can you get the registration to reflect higher GVWR than the manufacturers' rating? 

Curious if you ever got a clear answer on this specific question - i've been researching the same thing. 

[Darryl&Rita]

GVWR involves more than axle ratings. You may be safer with stouter axles, but the engineering charge wouldn't be worth it to change the registration.

[ms60ocb]

29 minutes ago, Darryl&Rita said:

GVWR involves more than axle ratings. You may be safer with stouter axles, but the engineering charge wouldn't be worth it to change the registration.

It is good that your considering the options.

One thing I haven't seem discussed here is the individual tire loading. Where I'm going the actual load of the left or right side. I don't recall the exact numbers but for example I had about 12% more weight GVWR on the ODS  than the DS of the Fifth Wheel. GVWR was 16,000 with 2,000 on the pin. Two 7,000# would seem OK but the in reality I had 8,000# on the ODS and 6,000# on the DS. Tires on the ODS didn't last long and was really stressed when in a cross-wind hitting the door-side. Under that condition a stouter axles would be a winner.

Individual tire weights are important and sometimes the way you load the RV can change the unbalance but I like a full freezer.

Clay I now a 20,000# RV with 5,000 on pin and last check had balanced weights on the 4 tires on the ground.

[Steven@146]

This has been a source of conversation many times on our specific RV manufacturers forum. Our line of 5th wheel RVs had 7000# axle ratings and G rated tires, then once all the updated options were added like better appliances, cabinets, dual pane windows, hydraulic leveling and such, the cargo capacity left for the owners stuff was really low in the 1200-1500# range. Not enough for some people especially for the price they paid for the RV. I guess they never really looked at the RV factory sticker on the left front for cargo capacity before they bought the unit.

So owners wondered if they could aftermarket replace the 7000# axles with 8000# axles to get more cargo capacity. Well yes you could but the initial factory weight ratings (GVWR, Cargo Capacity) could not be changed to reflect the higher capacity.

Owners complained enough that the factory then added the option to upgrade the axles to 8000# when building the RV  and so the factory weight ratings could reflect the higher capacity and thus giving the owners more cargo capacity. Factory supplied RV tires were also minimum G rated or upgraded to H rated.

Yeah a lot of RV manufacturers build these units right on the ragged edge.

[TXiceman]

I do not need to worry about the axles on our HitchHiker.  It has a GVWR of 19000# and has THREE 7000# Mor/Ryde IS axles and G rated tires.  NuWa was not one to worry about like the current manufacturers.

Ken

[Ray,IN]

An aside; if you install higher rated axles, tires and better than oem  brakes, it does not change the Federal Tire placard listed weight ratings that is found inside the vehicle. This is what some Canadian provinces use to determine if you are operating overloaded. If you are, it is a legal overweight violation and you are ticketed accordingly.

[fpmtngal]

I agree with others that there’s too much else that goes into the makeup of a trailer’s GVWR than just the axles, brakes and tires.  My trailer’s axle rating add up to more than the trailer’s GVWR - does that mean I can safely load my trailer to the axle weight and ignore the GVWR?  My experience with my particular trailer says no - I’ve occasionally been in that spot and have had some issues/damage that were most likely related to being overweight.