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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.
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Specs:
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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
Gwendolyn Y.
4/8/2025
Looks great on your camper, Greg. Do you have any plans set yet for it's maiden tow?
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.
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.
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.
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.
Nick S.
12/1/2025
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
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. :
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.
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.
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.
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.
Wesley
5/28/2019
Still working great. The smooth ride makes towing much more enjoyable.
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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.