Abstract

A bicycle seat is supposed to be designed for the safety and comfort of the rider. A rider may rest by sitting on the seat during longer, harder rides or simply to relax on a leisurely ride through the park. However, the height of the seat can determine how much safety and comfort are actually provided to the rider. A seat that is too high or too low can affect the ability of the rider to operate the bike safely as well as cause undue stress on the rider’s joints. There is a need for an effective and efficient way of adjusting the seat height on a bicycle. A simple push button system that can raise and lower the seat at will would allow riders to adjust their seat to meet their individual height and riding style needs. This can be accomplished by implanting a motor inside the seatpost of a bike that will raise and lower the post through a series of turning gears. A control system mounted to the handlebars would allow riders to adjust their seat on the fly, negating the need to dismount and physically struggle with tools to manually adjust the seat thus increasing the amount of enjoyment the rider gets from their outing. Through easy adjustment of the seat, more riders would adjust their seats regularly and be able to ride in the proper seat position. This in turn would increase rider safety and reduce the number of cycling related joint injuries.

1. Introduction

A bicycle seat is supposed to be designed for the safety and comfort of the rider. A rider may rest by sitting on the seat during longer, harder rides or simply to relax on a leisurely ride through the park. However, the height of the seat can determine how much safety and comfort are actually provided to the rider. A seat that is too high or too low can affect the ability of the rider to operate the bike safely as well as cause undue stress on the rider’s joints.

Cycling through Europe and most of East Asia has always been one of the main methods of transportation. Some countries like the Netherlands have 1000 bicycle per 1000 people and is by far the main method of transportation ¹. The cost of owning a bicycle is approximately $350 dollars a year where a car could be up to about $11000 per year ³. With our electrically actuated seat post it could make riding that much more popular and comfortable to those who do not cycle already. Our innovative seat post design will allow you to change the high of the bicycle seat while still moving allowing you to prevent serious injuries while riding such as pelvic instability, medial foot arch collapse and even scoliosis ⁴.

Having an electronically adjustable seat post can also prove profitable to other business and industries such as exercise equipment as well as rideshare bicycle programs and rental companies. Near downtown areas and even across major campuses today you can find more and more rental bicycles which allow you to rent a bicycle for a limited amount of time for cheap to explore the area ². An adjustable seat post would allow these companies to have a more “one size fits all” mentality and give the opportunity to have more variable rider heights and sizes. Cycling machines are becoming more widely used in the gym due to a low impact and high calorie burning exercise program ⁵. Being able to adjust the seat height without interrupting your workout to help reduce stress on lower back or knee joint areas would make this workout even lower impact on your joints and muscles keeping your workouts even more safe.

There is a need for an effective and efficient way of adjusting the seat height on a bicycle. Preferably, the system should be easy and intuitive. A simple push button system that can raise and lower the seat at will would allow riders to adjust their seat to meet their individual height and riding style needs. This can be accomplished by implanting a motor inside the seatpost of a bike that will raise and lower the post through a series of turning gears/screws. A control system mounted to the handlebars would allow riders to adjust their seat on the fly, negating the need to dismount and physically struggle with tools to manually adjust the seat thus increasing the amount of enjoyment the rider gets from their outing. Because the adjustment is easy to perform, more riders would adjust their seats regularly and be able to ride in the proper seat position. This in turn would increase rider safety and reduce the number of cycling related joint injuries.

In order to design an electrically actuated seat post for a bicycle, there are many considerations to discuss. The first consideration is how to translate the necessary mechanical energy to lift the mass of a human body, into electrical energy provided by the torque of an engine ⁸. Knowing that power is simply the quotient of work of time, and that work is a given force over a distance; we equate it to power. Power is simply the product of force and distance, divided by time. Power is then simplified to the product of force and lifting speed ⁹. The following sections will detail the calculations necessary to raise a typical average human’s weight.

After power is equated to required lifting mass, required torque can be equated to power, and necessary rotational speed. Power is simply the product of rotational speed and torque. Given one, the other may be found ¹⁰.

High torque electric motors are then to be considered, to determine the feasibility of the project. This will help to determine if the electrical seat post will be capable of lifting a human’s weight, or simply support it. Through the course of the design process, this question will be answered. Many electrical motors will be considered, such as options found in automotive and marine applications-like the Trim system found in Seadoo Personal Watercraft, and window regulators of car windows. The motors are bigger in size, and slow moving, but high in torque ⁶. Should it be mathematically unfeasible to lift a human body with a motor small enough to be fitted to a bicycle, compact high torque motors will be considered to offer the best ability to sustain the upward motion of the seat, and the mass of the person on a downward motion. Many of these motors are extremely durable, and capable of operation at various electrical voltages ⁷. Given the ability to vary voltage will give more allowance to design constraints of battery packaging, and length of charge available.

The penultimate consideration is how to drive and control the motion of the seat post. Typically, common practice is to use a straight cut gear, or to use a worm gear assembly with a heavy load of constant motion. It is imperative for the assembly to maintain the seat’s center with that of the bicycle frame, to mitigate any loss of control or comfort ability. The research will simply be trial and error, and refer to examples such as trim systems on Seadoo watercraft ²⁴. Given the high load encountered, this will be a perfect example to study, and possibly implement a solution to the seat post.

2. Background

Bicycle has an understandable varied history and been around for practically two hundred years. It was first called “velocipede” in its early invention, and is originally shrouded in mystery. One can barely attribute its invention to a single person because of its frequent use and norm in the 19 centuries. But one thing for sure Bicycle in general was first seen in the early 1800s. It was introduced back then as a form of a front big wheel with a paddle through the center of that wheel and an armrest in front of his body. Early Bicycle was then credited to Charles Baron von Drais in 1817 ²⁰.

In 1860, Boneshaker invented two-wheeled bicycle whose pedals and cranks are on the front wheel. It was back then called “bone shaker” because of its metal tires. In 1885 the first kind of bicycle that we see today was invented as it had two same sized wheels with a chain mechanism that allows the rider to go faster and travel a long distance. Since then, different bicycles have been invented in different norm and efficiency such as racing bicycle, mountain bicycles ¹⁸.

Throughout these two past centuries, one can tell that bicycles in their early age are nothing compare from today’s ones. They had drastically evolved and became more efficient in result, feasibility, and durability. Yet, their seat post has poorly evolved or one might say it didn’t have much difference compare from the early age seatposts to today’s ones. It has a dismount process that one might go through to adjust that seat to one’s satisfaction. Therefore, a suggestion to an improvement in adjusting a seat post electrically and not manual will somehow attract more customers and consumers as satisfaction and safety are involved ¹⁷.

Consisting of 3 pieces, the inner bolt is keyed to accept an electric motor input. The Bolt spins and threads into the middle shaft that is keyed to slide through the seating stem. This design is selected due to ease of use, and manufacturability. This design is intended to be implemented by bicycle manufacturers into current designs.

Classic track system integrated into the back of a seat post. The gear is powered by an electric motor, and partially shielded for safety. The design is very robust, but ultimately heavy. There are also safety concerns, such as the track pulling a shirt or accidental interaction with limbs.

This design is a lightened version of design 1, and has two keyways to guide the motion of the seat post. Electric motor power is transmitted through the inner bolt. Windows are cut into the seating stem.

This design is not optimal due to additional machining costs, and possibly integrity problems from less surface area.

In consideration of each of our designs safety was our main priority. Aesthetics was determined to be least important and therefore our most optional attribute.

3. Methods

Calculations: Most of our calculations will revolve around finding the torque in the various parts of our assembly based off of the known output torque of the motor that we choose. The formula for torque can be written as

Torque

(1)

where T is the torque, r is the radius, and F is the force. We will “walk” through our assembly with known radii and find the torque or the force depending on what is known/unknown. Once we reach the connection between the threads at the top of the worm gear and the bottom of the seat post, friction will become a factor we must consider. To evaluate this situation, we reduced the problem to a block sliding up an incline with friction and used a summation of forces set up. The summation of forces can be written as

Summation of Forces

(2)

where m is the mass of the block/seat post and a is the acceleration of the seat post. Further analysis of the block diagram reveals that

Summation of Forces Expanded

(3)

where Θ is the angle made between the horizontal and the incline of the threads of the worm gear/seat post, g is the gravitational constant, and μ is the coefficient of friction for the materials we choose to use for our parts. From this equation, depending on if the acceleration is a positive or negative value, we can know which direction the gear is turning, how fast it is turning, and if the seat is moving up or down. If the acceleration is zero, then we know the gear is not turning and our seat should be locked in position so the user can ride the bike in safety. If we do set the acceleration to zero and solve the equation for force we get

Summation of Forces Rearranged (When Acceleration = 0)

(4)

which gives us the amount of force required to move our seat. Any more force than this number will move our seat. Obviously, the greater the magnitude of the force, then the greater the acceleration of our seat position will be.

Below are a few sample calculations for various circumstances using Equation 4. Factors that will affect our calculations include the mass of the seat post, the angle at which the threads of our parts are cut, the type of materials used for the construction of our parts, and any friction reducing agents that may or may not be used in the parts. For the purpose of this project and these calculations, the addition of anything that would reduce friction between the parts has been ignored.

We have estimated the mass of a seat post to be between 3-5lbs or 1.36-2.26kg. We have also estimated that steel would be the best type of material to use in the construction of the parts. The coefficient of friction for steel on steel is between 0.5-0.8 depending on exactly what type of steel is used. We have also estimated a cut angle of 30-40° in our threads for these calculations.

We can also assume that the average diameter of a bike frame is somewhere between 3-3.5cm. For the purpose of these calculations, the radius of 1.5cm is assumed. The torque required to turn our parts is shown in the following tables.

With the highest approximate torque needed to lift or lower just the system of the seat post being only 1.65 Nm there are many available options on the market as to which electric motor to use to drive the system. Some high torque and low velocity motors on the current market can still produce somewhere around 20 Nm of torque and still stay in the size parameters that we would need to integrate into the average bicycle frame ⁷.

4. Cost Analysis

Bikes today are typically made up of 1 of 3 types of materials. Those materials are aluminum, titanium, or carbon fiber. Our parts would most likely be made from one of these materials. Current estimates on price put aluminum at $2.14/kg ²¹, titanium at $3.77/kg ²², and carbon fiber between $22.05/kg and $33.07/kg ²³. Further research would need to be done into other attributes such as strength, weight, durability, coefficient of friction, etc. Based solely on the expense, aluminum or titanium are the most cost effective choices to build our parts.

5. Constraints

Physical – The size of our motor must be small enough to fit on a standard bike frame and some of our parts will need to fit inside the bike frame. All additions we make to the bike should be light as possible yet sturdy and resilient.

Functional – The motor must be strong enough to rotate the worm but should not cause additional vibrations to the bike or overheat. The motor should be as quiet as possible, preferably not making any audible noise at all. Lastly, the motor must be weather resistant.

Economic – The design must be relatively cheap and not add a lot of extra cost to the overall price of the bike. Bikes can still operate without this feature and if people don’t see the need for it, they will not pay for it.

Legal Considerations – Because the motor we will use will be electric, there are no emissions from the motor. This means that our design will not be impacting the environment in any way. Our design must have a safety feature that locks the seat in position so that the seat post is not raising/lowering when the rider is not expecting it. An unexpected change in height could injure the rider and/or create undue strain on the motor.

Human Factor/Ergonomics – The part of this design that we chose to focus on does not have much in the way of direct human interaction. Therefore, the ergonomics of our design don’t really exist. However, there will be some kind of electronic control mounted to the handlebars to control the seat post. That system would need to be simple to understand, clearly labeled, and mounted in such a position that the rider could adjust the height of the seat by pushing a button/flicking a switch without removing their hands from the handlebars.

6. Solid Works Final Prototyping

7. Testing Analysis

The above pictures detail Finite Element Analysis of the proposed seatpost design. Using AISI 1020 steel, for the worm screw and the threaded worm shaft are substantially overbuilt. This is simply because of the threads’ overall durability. More analysis could potentially be conducted, upon testing of a prototype. This is evident, in the Stress Analysis, where stress concentrations are only at the top of the threaded shaft, and the base of the threaded screw.

The Factor of Safety approaches infinity, due to the same reasons. Solidworks does not allow threading to be included into FEA analysis.

As such, we assume that the part must be substantially stronger than needed. The ideal way to resolve and slim the design, is to use real world prototypes.

References