college of engineering white

Project 2: Adjustable Chair


Our goal is to design an adjustable chair for a young boy so that he has back and head support in his classroom. It needs to be able to ‘grow’ with him, so that he can use it in various classrooms over the next few years.

Ian Sweetin, Jacob Cotten, Caleb Harman, Peyton Pope

Problem Statement

The issue we are looking at is that this boy needs a chair that can adapt to suit his needs. He firstly needs the chair to be able to adjust to various heights, so that as he grows and moves into a bigger classroom, he is able to have the chair adjusted to the correct height for his needs. Second, he needs back and head support, ideally in the form of straps, as suggested by his teacher. Finally, the chair needs to be mobile. It should be able to move around the classroom as needed in order to help get him into the chair, and should be able to lock as desired, so that he doesn’t slide around the room while in the seat.

Design Specifications

  • Needs to be able to support his head and back
  • Boy weighs 26 lbs. and is 31.5 inches tall
  • Chair have wheels, but be able to lock
  • Be able to adjust the height of the chair as he grows (10-16 in seat clearance capability)
  • Back of chair needs a slight tilt backwards for when he eats

Background Research

For the design of the seat itself, we initially looked at modifying a car seat, as it seemingly hit two birds with one stone: the seat and the straps. The seats normally had a slightly backwards tilt, and the straps were built in order for a child to be comfortably secured in the seat. However, the rest of the car seat posed an issue, as it usually doesn’t sit level, and there would be a significant amount of difficulty in cutting away most of the outer frame of the seat, as well as securing it to an adjustable base within the desired seat height specifications, so our current idea is to design a seat to suit our needs, and to upholster it so that it is both safe and comfortable for the child.

As was already somewhat discussed, our original idea for the straps was to simply use the ones from a car seat, which would likely be one of the most comfortable options. However, the boy will need a specific type of strap in order to achieve our support goal, that being shoulder straps. A standard diagonal strap would allow to much freedom of movement while he is secured in the chair, and a waist-only strap would fail to provide the needed support.

The base of the chair is what will not only need to connect the seat and the wheels, but also to function as the primary mechanism in order to adjust the height of the chair. While the chair will not be needed to be adjusted often, we decided it would be best to keep it simplistic, so as to allow anyone that needs to adjust his chair to be able to do so. Our group initially looked at using a hydraulic base, such as one found in an office chair, but found that the height of the mechanism in combination with the other parts would normally be too tall to meet the minimum seat height requirements. It needs to be able to adjust to specific desired heights, roughly every 2 inches or so, leading us to decide on using sliding legs with a pin in each chair leg in order to secure the height of the chair.

The wheels were a fairly uncomplicated issue; the boy needs to be able to take his chair both around his classroom and to another when he needs it, and the easiest solution for mobility on a chair design is to use wheels. For the locking mechanism, however, we decided that using cheap wheels and locks would either lead to a brake failure or would lead to the wheels themselves failing. Therefore, two potential solutions arose: we could either find better wheels that come with a braking system, or we can focus on the wheels and try to add some mechanical braking system after the fact.

Concept Design 1

Our preliminary design utilizes our ideas on the locking wheels, an adjustable base with pins to lock out the legs of the chair, car seat straps, and a seat specifically designed to suit our design specifications.

The wheels and locks will allow for simplistic mobility  of the chair, as well as rigidity when desired. The pin-locking system for the base of the chair gives us both functionality and safety, as they are easy to use and quite sturdy. The straps will allow us to give the child the support he needs while also keeping him comfortable, and the seat will be designed to tilt back slightly and be tall enough for him to sit comfortably.

Concept Design 2

Concept 2 features many of the requested systems from our contact however, the cost makes it prohibitively expensive. This stroller features an adjustable back and locking wheels but has no adjustable base. An adjustable base can be purchased from the company that attaches to the stroller, but it is expensive. Modifying the base could potentially disrupt the structural stability and the company’s proprietary Balanced Safety System (BSS).

Concept Design 3

Concept 3 has many of the requested features, but the chair cannot be raised or lowered. The chair features an adjustable back, rolling wheels, and chest support. This concept is much cheaper than the stroller. Modifying the chair would be difficult because the legs are tilted inward and the frame looks very thin.

Selected Concept Design

We have chosen concept 1. Concepts 2 and 3 are too expensive and difficult to modify. We have simplified concept 1 by choosing to use an existing car seat instead of fabricating our own. The car seat has an integrated chest harness and can be rotated to aid in feeding. The legs feature an adjustable sliding lock system and are angled outward for greater stability. Our team will work on joining the car seat to the adjustable base legs.

Decision Matrix

Overview of Selected Design

Our design features a Cosco Scenera Next car seat. This seat has an adjustable chest harness, can rotate for feeding, and can hold up to 40 pounds. The base features 4 sliding lock legs used to adjust the height. The chair will be mounted in a way that allows it to be rotated, but also be stable. This chair will be able to slide under the tables in the child’s classroom so that he will be able to interact with his classmates.

Describe Design Details


The seat is a modified car seat. The seat we have chosen fits the child’s weight and age range. The seat has an adjustable harness and can support up to 40 pounds. The seat features a sloped back to aid in feeding. This option is much more cost-effective and modifiable than an accessibility chair like the ones shown in concepts 2 and 3.


The chair features PVC legs with locking caster wheels. These legs will be modified to add a telescopic sliding pin mechanism to easily adjust the height of the chair. PVC legs were chosen as a safer alternative to metal and will be easier to modify than steel.

Mounting Platform

These components will be joined together on a 14×20 inch plywood board. This board will be 3/4 inch thick. Using PVC mounting and using metal L brackets to mount the car seat.

Engineering Analysis 1

For the first analysis, we checked how much weight each leg will be holding. Based on the manufacture’s site of their product, each leg can hold around 62.5 lbs. We calculated the weight force on the chair, which is the child’s weight plus the weight of the chair pushing downward on the chair, and then how much force is needed to hold it up. We calculated that each leg needs to hold 10.1 lbs, which is plenty enough since each leg can hold 62.5 lbs.

Engineering Analysis 2

For analysis 2, we calculated the deformation distribution and the deflection of the back portion of the chair using a prototype chair. Based on the child’s weight and the chair being made out of a plastic material, we calculated that the deflection of the back would be around 1.5 mm and the distribution of deformation will be mostly on the top portion of the back. However, that is if the child’s full weight is completely pushing on the top portion of the back only, which is unlikely the case; only part of the weight will be. Therefore, the back part will most likely not be able to deform at all.

Engineering Analysis 3

For our final analysis, we calculated how much stress would be on the pins on the legs. The pins are in shear, specifically they are in double shear, but they have 2 of them in each leg. We know that each leg will experience about 10.1 lbs. We then used that to calculate the shear stress in the pin. We calculated that each pin would experience 45.71 psi. Because the pins are steel, it is definitely strong enough to hold the chair and not deform or break.

CAD Drawings

Bill of Materials

Document Fabrication Process

  1. We cut out a 3/4″ wooden board to a dimension of 14″x20″. Next we used sandpaper to smooth out all the sharp edges.
  2. Shaved down the car seat to make it flush with the ground and to lower the seat height.
  3. Cut the schedule 80 PVC to four equal lengths of 6.75″ Cut the PVC with the wheels to a length of 5 in. Milled 3/8″ holes in both the PVC pipes 2″ apart.
  4. Mounted the PVC bracket mounts on each corner using 0.5″ diameter bolts and nuts.
  5. Spray painted the wooden board and the PVC legs yellow and purple.
  6. Mounted the car seat to the wooden board using four metal L brackets with the front end flushed with the edge of the board.
  7. Put stickers of TTU and the child’s name on the side of the car seat.

Testing Results

Tests we did was to make sure the chair moved smoothly and that the wheel locks made the seat not move. Made sure the sliding of the legs was smooth and all were leveled with each other. Made sure the car seat was secured properly and would not come apart from the board.

Completed Design Photos

Instructions for Safe Use

Make sure child using the chair does not exceed the 40 lb. weight limit. Make sure you secure the child in the seat with the adjustable straps. When adjusting the height, make sure there is no one sitting in the chair. Easiest way to adjust chair is to set the chair on its side and adjust all four legs to the desire height. Make sure to lock the wheels so that the chair won’t slide around.

Project Summary/Reflection

Overall the project was a complete success. We modified an existing car seat, cut down PVC pipes to use as legs, used a pin mechanism to be able to adjust height, and all mounted it on plywood. We all had fun designing, building, and testing out our project. At times it was difficult and other times it was easy. Easy parts were cutting the board and mounting, while the hardest parts were cutting the PVC legs to the right height and adjustments. But at the end we all managed to get a better understanding and idea of how engineers use their skills to create a project. We were all proud of what we accomplished because we created something that was helpful and useful for someone in our community.


2023 Fall