Third axle in 390RK...?

[AlphaFab]

We're in desperate need of an upgrade to the axle package on our '21 390RK. The tandem 7,000# axles are so incredibly inadequate for FT living, it's kinda ridiculous (for whatever reason, the 8,000# axles weren't available in our build). So, my question is, Instead of the tandem 8,000# upgrade, I'm contemplating just adding a third 7,000# axle, just in front of the other two. Morryde has the required additional equalizers available for triple-axle setups, so it seems that adding a third 7,000 Dexter axle would be the ticket...getting a 21,000# "load capacity" is definitely overkill, but having the extra headroom seems like a good idea to me.

I'll fabricate the brackets and any needed gussets, perform quality welds, ensure that the axle tracks correctly (I own a fabrication shop), etc... So my question comes down to: has anybody else considered, or done this type of upgrade? It seems to me that it's a very straight forward "fix", and fairly cost effective. I'm not at all worried about disc vs. drum brakes (since I would have 6 drums rather than 4 discs)...just looking for any advise or "gotcha's" that any of you may have run in to.

I would also upgrade the shackles to wet-bolt installations at the same time...and possibly add shocks (don't know until I get under the rig as to what makes sense on the shocks).

Thanks!
-joel

[TucsonJim]

Welcome to the forum. For your first post, that's a doozy. That's a big job, and as far as I know, you'd be the first to attempt it. Make sure you look into equalizers for a triple axle rig. For example, I'm posting a link to a triple axle equalizer kit for 5K to 8K axles.

Also understand that if you make a modification to the frame, Lippert may not cover it on warranty if it results in a failure.

https://amazon.com/Lippert-279689-Eq...ps%2C98&sr=8-3

Jim

[Jlawles2]

The biggest issue you may face is adding the axle in front of the current 2 will be in how it changes pin weight. You are essentially moving the rear support point forward from between the current 2 axles to the center of the current front axle. This may be enough to create sway.

If you do choose to go the 3rd axle route, is it possible to move the current setup back to help maintain pin weight? Also, if you have the ability change from the equalizer setup to a slipper setup as they are known to have better equalization capability.

[Cate&Rob]

I would also be wondering about the affect on towing stability of carrying more pin weight on an additional trailer axle. There would also be the question of adding another wheel well cutout in the body. For trailer load carrying without lightening pin weight, it might make more sense to add an axle aft of the existing ones. This might increase pin weight . . . but that would be difficult to calculate.

Rob

[AlphaFab]

Welcome to the forum. For your first post, that's a doozy. That's a big job, and as far as I know, you'd be the first to attempt it. Make sure you look into equalizers for a triple axle rig. For example, I'm posting a link to a triple axle equalizer kit for 5K to 8K axles.

Also understand that if you make a modification to the frame, Lippert may not cover it on warranty if it results in a failure.

https://amazon.com/Lippert-279689-Eq...ps%2C98&sr=8-3

Jim

Thanks, Jim...

I was looking at the triple-axle slipper setup from Roadmaster. I looked at the equalizers, and the slipper seems to be the best bet, and, it comes with shocks

I'm not terribly worried on the warrant-ability of the frame if I do this. I feel as though I'm competent enough to do a factory-like installation. I completely understand what you're saying and appreciate it though!! We've just got to get a better load capacity since even dry this thing is about at it's limit.

-joel

[AlphaFab]

The biggest issue you may face is adding the axle in front of the current 2 will be in how it changes pin weight. You are essentially moving the rear support point forward from between the current 2 axles to the center of the current front axle. This may be enough to create sway.

If you do choose to go the 3rd axle route, is it possible to move the current setup back to help maintain pin weight? Also, if you have the ability change from the equalizer setup to a slipper setup as they are known to have better equalization capability.

I was thinking about that, and calculating it. From what I can figure, we'll be shedding about 550# off of the pin with the axle forward. Currently, we're at 17,820 for the coach, with 3,880 on the pin. That's 22% of the load on the pin, which is right in line with the ideal 20%. If we move forward with the axle, we're adding the new one 34.5" toward the pin. Math shows that we'll be shedding that 550#, but then adding the 288# genset up front gives us us back some of the pin weight, to be somewhere about 3,618, which is right at 20% on the pin.

So, all that said, if I move the axle rearward, I'm going to leave the pin weight the same more-or-less (no advantage to shed that weight since the new axle isn't really helping that weight), plus the genset adding 288# gives up about 4,168, which is a bit much for the F350 that I use to pull.

I can shift everything back as well, but the numbers tell me that I shouldn't need to. Please (please!) correct my math if anything seems out of line. I'll run numbers again, because I definitely don't want to get into a light-pin situation...boy, it would be SO nice if GD fixed this little oversight before we [all] purchased. lol

The slipper setup is an absolute must for us. I did a ton of research on it last night and it's definitely the way to go... and it gives us shocks!

I really appreciate your insight - it's invaluable, and keeps me questioning my numbers. I'll definitely run them again to see where I'll be at (we're adding 2,200 pounds of batteries and solar as well, which is also figured into my calculation). The nice thing is, the batteries are nearly above the new axle if we move it forward... If it's aft, then we'll have to but a bigger tow rig! Such a fine-line.

-joel

[Jlawles2]

If you have a fab shop and the ability to "play" a bit with scales. You could use some sections of I-beam and a little creativity with some rollers to build a jig that would let you shift the center of the triple axle setup for and aft a bit to determine actual pin weight. You may find that you really need to shift the axles back slightly shedding pin weight. Without scale numbers and measurements, it's hard to verify your particular setup (until completed it's still theoretical).

In addition to adding the axle, (don't remember if it's been mentioned) you will need to verify that there are no interferences with slides or any other components in the area(s) of the axle placement.

It's not a bad idea, just may need a bit more planning and verification before diving off into the deep end of the pool.