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Flex Air drastically reduces chucking, absorbs road shock, and minimizes bounce. The internal air bag and built-in strut work with the rubberized pivoting head to achieve a dramatically smoother ride. Replaces Lippert 1621 pin boxes.
Features:
Specs:
Notes:
Getting there is half the fun, right? Not if you're being bumped, jerked, jammed, and pushed around from the chucking, road shock, and bounce that occurs as you tow your fifth-wheel trailer or RV. And imagine the costly wear and tear on your trailer as it endures that rocky ride.
Put the fun back into your trip and ease the stress on your trailer by replacing your factory pin box with the Curt Flex Air pin box. The internal air bag and built-in shock work together to cushion your ride, improve handling, and limit strain on both your truck and your trailer. But what sets the Flex Air apart from other air ride pin boxes is the addition of a pivoting head. This head, along with its built-in rubber compound, provides additional cushion, absorbs road shock, and reduces chucking for a significantly improved ride.
A heavy-duty rubber compound is built into the head assembly of the Flex Air pin box. This rubber is the first defense against road shock. It helps to absorb the bumps and vibrations experienced when towing a fifth-wheel trailer over rough roads. Standard pin boxes without any type of shock absorption transfer road shock to your trailer, resulting in potential damage to the trailer and an overall bumpy ride. The shock-absorbing rubber inside the Flex Air takes the beating so there's less wear and tear on your trailer or RV and you get a smoother ride.
Because the rubber may not be able to absorb all of the road shock that you experience, the Flex Air also implements air ride technology to virtually eliminate shock. When you encounter uneven terrain on your trip or hit a pothole on the highway, the Flex Air pin box compresses vertically, similar to how a pair of scissor blades closes in on each other. This compression transfers the road shock and vibrations to the internal air bag, saving your fifth-wheel hitch, trailer, and tow vehicle frame from enduring a rocky ride.
A built-in strut, or shock, controls the Flex Air's compression so that the internal air bag doesn't burst. The strut also causes the pin box to appropriately rebound without any bounce, and positions it to be ready for the next bump in the road.
The pivoting head on Flex Air allows for controlled back and forth movement of the king pin to reduce the uncomfortable jarring known as chucking. Unlike a rigid pin box that has little give at the point where the trailer and the hitch are connected, the Flex Air can pivot slightly to compensate for those times that your trailer suddenly pushes or pulls on your tow vehicle. Because the pin box head - not the king pin itself - is moving, the chucking that often occurs when the king pin jerks around inside the hitch is significantly reduced.
Simple Setup and Installation
First, simply remove your existing Lippert 1621 pin box from your trailer. The Flex Air uses the same bolts that held on your original pin box so no additional hardware is required for installation. Plus, the Flex Air pin box comes out of the box ready to be put on your trailer, so no assembly is needed. Because the Flex Air is quite heavy, you should put it on an elevated surface or have a friend help you support its weight during installation.
The internal air bag should be inflated after the Flex Air pin box has been mounted on your trailer. Using the air intake valve, inflate the air bag until the outer cylinder of the shock rises high enough to fully expose the yellow inflation indicator sticker.
Customer Satisfaction Score:
92% were satisfied with this product
8% of customers were not satisfied
Ratings & Reviews
Great service, very friendly staff.
I have used my Flex Air for over one year and put around 3000 miles on it. I purchased this for one reason to reduce bucking when I it bumps in the road. I can safely say that it has virtually eliminated the bucking. It has also made the ride a lot smoother. I would highly recommend the Flex air. I did my own install of this product by my self.
My rig
2018 Toyota Tundra with a Husky 16 KW Hitch
2017 KZ Durango 30 ft.
So far it's performed great. Really reduces the bucking and normal bumps. My only issue is the kingpin angles DOWN too much for me and not sure how to correct it. the plate where to pin is, is not level when hitching up which has caused gouges in my 5th wheel plate.
Really helped take away most of the jerky ride we used to get from our solid king pin. The air bag helps to smooths out rough roads. As per the advice of Lippert customer service I made 5 pound adjustments until I found the best ride. I keep a portable battery operated air pump along for adjustments. My airbag had a small air leak at the valve threads when new. After resealing the threads all is good.
Wesley
5/28/2019
Still working great. The smooth ride makes towing much more enjoyable.
Product arrived in a timely fashion in an un damage condition. Quality was better than expected. Installation for me took about two hrs, with my wife helping. I placed the pin box in the bed of my truck with the help of a frontend loader and let it down on a floor jack. From there install was fairly easy.
Pros: Good quality Heavy Duty
Cons: You will need 3 or 4 strong friends to handle it
No instructions included I did find a pdf on the bottom of the page however that listed the torque for 1/2" bolts
Paul`
8/28/2021
Works great after a year of use preforms well. I haven’t had to put air after the initial start up.
Reflection 320MKS
Dealing with etrailer was very easy. Starting with getting the correct information on which pin box would be suitable for our trailer.
The unit arrived in 10 days from ordering it. Which was less than the estimated timeframe when placing the order.
The only down side, on inspection when arrived, the air bag valve was damaged in transit. A quick email with photos, a call to etrailer & they have a replacement part ready to dispatch.
Great customer service & i would definitely recommend them.
Kevin T.
12/31/2022
Hi, I installed the Curt 5th wheel king pin unit in March 2022 on our Reflection 320MKS. What a difference it made on hauling the 5th wheel. We have traveled about 12-14,000 miles this summer towing the 5th wheel on highways & many country roads. The kin pin unit has reduced most of the stock feedback through the 5th wheelking pin & chucking has seen a big reduction as well on uneven roads. I would highly recommend this Curt king pin unit. Thanks E-trailer for recommending this unit.
This 250 lb item was shipped in a cardboard box that was just sitting on a pallet. When I opened the box I found the Flex Air was sitting upside down with the foam pad on top. I have no idea how the air filling nozzle wasn't damaged. Luck I guess. There were no "UP" markings on the box. No instructions were included.
Installation went smooth, it's just heavy so be careful. Instructions and videos can be found on-line.
The Flex Air is at least 4" taller than the OEM box that came with our Grand Design Reflection. I mounted the Flex Air as high as I could and lowered the receiver on the truck. The trailer is still nose high but seems to tow fine. My test ride was noticeably smoother. You can see a lot of motion on the Flex Air that would have otherwise been transferred to the truck.
Steve S.
11/4/2020
Fifth Wheel tows smooth. After 5000 miles from Minnesota to the south east US. Freeway and bumpy back roads. Wind gusts up to 25 mph. The trailer tows smooth. No chucking. The trailer is still higher in the front but it has no noticeable effect on towing and the leaf spring suspension can handle it. The tires seem to be equally loaded, no uneven wear.
It's still not here after 3 weeks and waiting 2 more weeks for the second shipment. Can't rate the pin box yet because i don't have it. I wish etrailer would drop TFORCE as a shipper how do you lose a 295 lbs package that is on a pallet
Install went pretty smooth. Did it from the bed of the truck, using a motorcycle jack in front and a small hydraulic in back. Make sure your torque wrench can dial to 160 ftlbs. I will update after I tow it!!!
James S.
2/5/2020
I cannot believe it has been a year....and a trip to Perdido Key, FL and Breckenrigde CO. In addition to taking care of us, I have to tell you, it will take care of your trailer too! Improved our towing experience old pin box was an upgrade Rotoflex greatly. Worth every penny! If you did not see my earlier post, my friend and I installed it ourselves. Motorcycle jack in the back of a pick up bed. :
This Curt flex air 5 th wheel pin box was probably the best investment that I’ve made in pulling my fifth wheel. It takes approx. 75% to 85% of the rebound bounce out of hitting irregularities in the roadways, of which there are many. I’ve pulled a 36’ couger and now a 41’ reflection. It makes pulling a trailer, almost, pleasant.
Wayne
2/2/2025
Fantastic! With taking our fifth wheel, a 40 foot reflection, to Arizona once and Florida once, it has taken out literally 75 to 80 of the joints when going over road transitions. One of the best purchases that we have ever made.
Kate F.
2/3/2025
I just installed it and I will take it out on a road test.And I will get back with you.
Product is great but I found out it was not compatible with my Anderson hitch so I had to return it
Awesome price! Being installed on a Grand Design 375RES. Shipment went great this is a heavy unit, arrived banded on pallets in excellent condition. Installation was simple and as instructions stated. Waiting for the next road trip to be able to test effectiveness. Always been a pleasure dealing with E-Trailer.
I will be on the third flexair pin box when it gets here on the 21st of November 2016. The problem iIm having is with the lower roto flex portion. The rubber block shifts and I have talked to others with the same problem. Lippert has been great about the replacement, But the the hassle of replacing this three times in one year is not worth it.
Top notch pin box. I received it earlier than expected, by 1 day. I received great info as to which pin box I needed. Thanks for all the good info. I followed the directions as given on the etrailer website. Yes, a guy reading directions..lol! It went on extremely easy. My wife and I installed it, with the aid of a borroed forklift, it is extremely heavy. Bolted on and filled directions to make sure it was level. It went very good, without any problems. Took fifth wheel rv for a test drive and could tell in the first block, it made the difference. Thanks etrailer!
Have not had a chance to take it for a road test yet. But it fit well, was easy to mount, the issue I ran into was getting the pin height. As my truck is 4x4, and the B&W hitch is as low as it can go, I still needed another 2 inches to get the pin box high enough, could not raise pin box any higher, and with the 5th wheel already at 13'4" did not want to lift the trailer, so ended up lowering the truck 2 inches to get everything back level again. But will find out next week how it works as we start out on a close to 12,000 mile trip to the west coast and back across through canada and back to florida.
Much was learned installing the Curt Air Pin Box. For context this was installed on a 2022 Grand Design 337RLS hitched to a B&W Slider on a 1 Ton SRW GMC.
1) Don't even think about installing this unless you have a forklift. If you are installing this yourself, rent one. United Rentals delivered one to me next day. One day rental, delivery and pickup was ~$723. Five of us using various options including truck tailgate, muscle, engine hoist, and engine stand failed until we used the forklift. The forklift method is also much safer. The Curt Flex Air plus the pallet it is delivered on weighs 290 lbs. Also consider that the Pin Box is intentionally a very tight fit into the RV frame. With a forklift and one person to guide the effort it's very doable.
2) Some of America's road are just not smooth (especially in some sections of I95 in South Carolina). This is arguably one of the best Pin Boxes. It is still not going to take all the chucking and jarring out of the ride. It will improve it.
3) On balance, the Curt Air Pin Box did reduce the harshness of the chucking and jarring. Small jarring is removed. Harsh conditions are softened--which frankly is good for both the RV and the truck. When the road is harsh the truck (even if not towing anything) is jarred, but some of that is no longer carried to the RV frame. You will still feel it in the truck however.
Likewise, road harshness under the RV axles are not passed to the truck as harshly.
5) Finding the sweet spot for air inflation will take some trial and error. I had it a little too high originally and incurred additional "bouncing".
6) Put 303 Aerospace Protectant (or similar) on the rubber ring when you apply the same to the rubber seals on your slides.
7) I used the same bolts and washers, but not the same holes in the frame or Pin Box. You will need to measure and assess which you should use based on height requirements and ability to engage the back end of the Curt Air into the frame.
I think this is a great product but gave it four stars because:
1) Cost of the Pin Box + Forklift Rental;
2) My expectations on improvement given that cost were unrealistically too high.
First, about etrailer. As always, their service was outstanding. They honored their low price guarantee without question and shipping was fast.
I upgraded from a Trailair Air Ride pin box with a fixed turret. That pin box helped with bouncing and jarring, but I still got a lot of "chucking". This FlexAir pin box solved that problem as well. Installation was a breeze, although you must make sure you have enough adjustment on your trailer to mount this pin box higher up than your existing pin box. I had to mount it at the very highest set of holes on my trailer. Lipperts web site has specs on this pin box and installation instructions, including measurements and torque specs. With the help of a friend and using the bed of my pickup truck, we unbolted the old one and bolted the new one in. Took about an hour and that was not hurrying.
I took the trailer out this past weekend with the new pin box and hit some pretty bad roads. Wow! What a difference this pin box makes. I can only describe the ride one way. Smooooooth. I am very pleased with both etrailer and this product. It's pricey, but to me, pulling a large fifth wheel trailer, it's worth every penny. Highly recommended.
4 stars as I have just recently installed this although I received it over a year ago.
I just put 750 miles on it and experienced almost every PAVED road condition imaginable and it worked great. I can't imagine what that would have been like with the stock pin box.
4 stars because I still need to see how it performs off road and I am just a few days into using it on a trip covering OR, ID, MT and WY so I still have my personal evaluation to complete and need more drive time to see if anything fails on it.
Installation was a breeze but it's a heavy hitch so caution is advised.
Arman K.
10/8/2019
I just finished a 3,000 mile trip from Oregon to South Dakota. The hitch works like a dream. Very few road issues are transferred to my truck from the trailer. This is a great hitch. Totally satisfied with this purchase.If anything I wish they made an offset pin box so that I could adjust it to make my trailer ride correctly, as it is with my 4x4 it runs a tad nose high.
This is the best money I have spent on my camper hands down. My hitch weight is a bit over 3200 lbs. I use a PullRite Super Lite hitch, photo attached. Once locked on the king pin, four bolts are set from the adapter plate against the king pin plate. The plate on the Flex Air is twice as thick as the original pin box and I have way more confidence in a solid set of these four bolts. I was blown away by how this hitch absorbed road shock. I will never have another 5th wheel without this set up.
Russ
2/11/2021
Since this original post I have gone back to a regular style 5th wheel hitch new truck with factory puck system but the Flex Air is still doing its thing and I will never pull a trailer without it unless something better is invented.
I bought the Flex Air to replace the standard Lippert pin box on our Cedar Creek fifth wheel that weighs 12K lbs. Just looking for something to smooth out the ride and the Flex Air seemed to have good reviews and recommendations.
I went to the Lippert website to download the installation instructions because I couldn't get the link on the etrailer website to work. The instructions will give you the torque values required for installation and the air pressure for the inflatable airbag on the hitch.
Installation was straight forward, but you better have a plan because this thing is heavy (300 lbs) and it will take some manpower to move around and install.
Joel
2/13/2021
Really glad we switched out pin boxes, the Flex Air was a great choice and definitely smoothed out the ride.
Joel
2/13/2021
Really glad we switched out pin boxes, the Flex Air was a great choice and definitely smoothed out the ride.
Having just installed the pinbox, it's really too early to review the functionality or performance; however, I did want to document a problem I observed.
The shock absorber was installed backwards so the "Inflate to here" markings stamped into the metal face backwards and are difficult to see. Also, your website shows a white line to highlight the location of the "Inflate to here" marking, but on mine, the white line was missing. I will turn around the shock and paint a white line on it myself.
Gene
11/26/2025
After 4000 miles of towing, I am quite happy with the pin box. Compared to my previous Trail Air Trri-Glide pin box, it dampens the updown motion just as well however, it doesnt dampen the front-to-back motion anywhere near as well. On the other hand, the Flex Air is almost maintenance-free and the Tri-Glide required a lot of maintenance and is no longer available.
I just opened the box last night on my Curt Flex Air 5th Wheel Pin Box. I could not believe the amount of steel in this setup. The quality of construction is impressive. If it preforms as well as it looks, I will be thoroughly satisfied and impressed. Another quality product that I have purchased from ETrailer.
Changed out my old pin box for a Curt Flex Air pin box. This one seems to be very heavily built and should last for years to come. Haven’t had a chance to pull it yesterday but am sure it will pull and perform great. Thank you etrailer for another great shopping experience.
Gwendolyn Y.
4/8/2025
Looks great on your camper, Greg. Do you have any plans set yet for it's maiden tow?
This item arrived much faster than etrailer said it would. It came by truck, and the freight company driver was very nice and made sure I opened the box before he left.
I installed it myself with some help from my 15 year old son. We used our forklift tractor attachment to put it in place. The Curt Flex Air totally changed the experience of pulling our 5er. It eliminates most of the chucking, and acts like shocks on a car, so ride returns to normal immediately after a bump in the road...which the Flex Air absorbs most of.
N. N.
3/27/2025
Still works great. I bought a Curt 16k hitch from a friend for $200 that still had plastic on the decals. The double jaws hold the kingpin tighter than my previous hitch and it works even better.
Kate F.
3/27/2025
Do you have a question about this 5th Wheel Pin Box?
Info for this part was:
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Katrina B.
3/24/2023
I have gotten in contact with the manufacturer and they have confirmed that the 1621 longjaw pinbox is going to have more downward angle to it because of its length. You can try to adjust the pinbox andor hitch height. The pinbox itself is not defective, you need to try and level out to get the pinbox as straight and level as possible.