Analysis of bottom brackets

Mountain bikes are the focus of this assignment. Mountain biking is a sport which is performed usually over rough terrain on off-road surfaces. Mountain bikes are different from road bikes in that they are adapted to perform in conditions where the bike has to have increased durability and enhanced performance. The frame is generally smaller and tougher than other bicycle frames to cope with the extra stresses that result from the conditions the bike is put through. Some of these conditions are; landing from a height, hard turning in corners, impact with heavy objects (tree stumps, rocks, boulders, ramps etc.) and in general that the bike is put through these at high speeds.

Improvements to the design of the mountain bike in include bigger tires, smaller and stronger frame, wider handlebars, the usage of disc brakes and most importantly for us, a strong rigid bottom bracket system.

The bottom bracket requirements are related to the strength in the frame and are the base where the forces from riding are exerted. The bottom bracket holds the crank in place which in turn holds the pedals on which the rider displaces their weight. It provides a rigid base for the rider to stand in as safe and comfortable manner as possible. If the bottom bracket should fail while riding is taking place then the rider will certainly have an accident which may be fatal. In order to provide a functioning bottom bracket, some considerations need to be taken when designing the component.

Present day manufacturing has made bottom bracket axles even larger in diameter, and in an effort to increase bearing life and further increase overall stiffness of the system without changing frame dimensions, companies have moved the bearings outside of the shell. [2]

Design Considerations

The highest amount of stress when riding a bicycle is in the bottom bracket. Therefore the design has to be of the highest quality and attention. [7]

Weight – the weight in a bicycle can be directly related to the performance output of the rider on it. Weight reduction has taken an enormous role in the design of racing specific bikes but it can also have a significant impact on mountain biking, especially cross country, where there is uphill and downhill biking. The lower the weight of the bike the less force acting on the surface between the tires and the ground. The less force acting on the ground from the bike will reduce the friction and mean an overall increase in speed. Until recently weight saving was primarily focused on improving the frame, whether it by choosing lighter materials or making the frame smaller. But in today’s world weight reduction is sought in every component of the bike.

The considerations that have to be applied to the BB are the relationship between weight reduction and strength. The strength of the BB component cannot be sacrificed for less weight. This means that in order to reduce weight, lighter materials have to be used that still has the same, if not improved, mechanical properties.

Materials – materials have to be chosen that are high in strength, stiffness, are fatigue resistant, corrosion resistant and very light. The material also has to be relatively easy to manufacture as the size is small, the shape is quite complex and the accuracy is paramount. Current models of BB’s have a low range of the materials that are used. From online research the most common materials are; titanium, aluminum, stainless steel, varying steel alloys and for the high end ranges, ceramics. Ceramics provide the light weight and durable characteristics that are required for high end mountain bikes. They outperform all other materials that are commonly used in bottom bracket manufacture.

Corrosion – mountain biking is an outdoor activity; therefore it is subject to all weather conditions. Corrosion is the wearing away of metals due to a chemical reaction, contact with oxygen can make a reaction. This can very problematic in bicycles due the use of them outdoors, especially in wet weather. The contact with water will cause the components of the bicycle to rust which can lead to the frame becoming weaker, the brakes malfunctioning and the gears seizing. Corrosion in the bottom bracket will cause the turning acting of the crank (by the pedals) to become stiff and harder to operate while also adding to the danger of a malfunction. The materials used to make a bottom bracket should have some sort of corrosive protection applied to it.

This can be done by treating the surface, either by galvanizing the metal, plating it, painting or applying an enamel coating and anodizing the metal. The use of stainless steel permanently prevents against axle and bearing corrosion. In the higher range of bottom bracket choices, some are made of ceramics. Ceramics are almost entirely immune to corrosion which coupled with their mechanical properties makes them ideal for use in this component. But, they come at a price.

Cost – it is very important when designing these parts that cost is kept to fit into a specific range. For high end models, the cost can be very high, sometimes 4-5 times that of the average BB [1]. So when designing the part, the company should decide if it wants to aim its product at the higher end market, top quality product at a cost, or at the mid and lower range markets. The difference in cost of the final design should depend on what materials are used, most common is stainless steel and most expensive is ceramic, what function they have, universally fit to a range of bikes or be specifically fit to a certain range, and the complexity of the design, square tapered or splined or hollowtech or a unique design (octalink). The splined option has a standard associated with it, the ISIS BB System but remains a very strong and durable option. [4]

Roller bearings – Roller bearings are known to cope far better than ball bearings in terms of load. Unlike the ball bearing, where any weight pushing down on it is focused on one point, weight is spread out in a line along the surface of the bearing. This allows the roller bearing to handle much more weight and makes it ideal for heavy-duty applications and high load applications. The massive loads that are endured when the bike lands from a height are transferred through the frame and into the bottom bracket. The bearings must withstand this load and still be able to transfer forces into motion in a smooth and reliable manner. Unlike the ball bearing, where any weight pushing down on it is focused on one point, weight is spread out in a line along the surface of the bearing. This allows the roller bearing to handle much more weight and makes it ideal for heavy-duty applications and high load applications. [5]

Maintenance – There are quite a few signs that the bottom bracket needs maintenance. There are many ways to check if maintenance is required such as take notice of how the bike feels, you can become aware of many problems by pedaling. If you can hear or feel a periodic creak or ticking noise when the system is under high or even normal load, it could be time to grease or replace the bottom bracket. The creak, click, or rough feeling usually occurs as the crank arms pass through one particular point in their cycle; if it does, then you have a problem with either the bottom bracket or possibly the pedals themselves The creaking in the bottom bracket can be caused by the on-set of corrosion. For total confirmation of a worn bottom bracket, remove the crank arms and spin the spindle by hand judging the ease to spin will give a clear picture of whether maintenance or replacement is needed.

Manufacturing- Axles capable of use in high stress applications, such as downhill, extreme, and free-bicycle riding is described. The axle is made by a method, preferably including two forging steps. In the first forging step, a portion of solid metallic work piece is formed into an enlarged head portion that will be finally machined into an ergonomic grip portion. In the second step, another portion of solid metallic work piece is formed into an elongate tubular portion.

Factors of Safety -A factor of safety is the measure of how far a system can withstand a load beyond the actual load. There are two types of safety factors, safety factors and design safety factors. The design safety factor is the actual amount of load a part is required to withstand, while the safety factor is the amount the part will actually be able to withstand. It is important that we do not “over engineer” our bottom bracket to be withstand considerably more force than necessary as this will cause wastage in both materials and money. When setting the design safety factor it is important to consider maintenance, environment and wear as these elements will reduce the amount of load the part cans tolerate. [6]

Environment -As we are analyzing what is essentially a part designed for performance downhill mountain bike we would expect the bottom bracket to be exposed to dirt, mud and water. Water damage causes the most damage to bottom brackets. It is important for the bottom bracket to be sealed from water as it will cause corrosion, reducing the brackets lifespan and the amount of load the bracket can take before failure. The threads of the seal must be kept well greased so as to keep water and other corrosive materials out the bottom bearing housing.

Wear – In normal cases there is no appreciable wear in rolling bearings however in a mountain bike scenario wear occurs as a result of the ingress of foreign particles such as mud and grime into the bearing. Vibration in bearings which are not running also gives rise to wear and the constant abuse of forces acting on both the bearings and the bracket will result in failure. Ball bearings are preferred in high vibration applications however because the loads in our situation are so large roller bearings are the only option for long term sustainability and life span. The solution to eliminate wear as best as possible is to ensure that the bearings are incased and completely sealed in order to insure that they are free from foreign objects. They must also maintain a consistent lubrication throughout their life span if they are to reach an acceptable degree of performance. [8]

Overall Assumptions:

> 1/3 cyclists weight acts on the handlebars, leaving 2/3 of the weight to act on pedals or the bottom bracket

> Weight spread evenly across two pedals

> All bottom brackets use the same crank and pedals

> When calculating max. bending stress, the pedals in the vertical position

> The bike lands on flat solid ground, so there is no give

> The pedals are in the horizontal position when the bike lands

> No suspension

> No give in tires

> Air resistance is negligible

> Cyclist falls another 0.5m after the bike hits ground

Below are the worked out calculations for the Platinum DH bottom bracket.

Platinum DH:

P = 120kg L = 90.89mm Thickness, t = (20.51-10.51)/2 = 5mm

d = 20.51mm b1=170mm

r = 10.255mm b2 = 80mm

Second moment of inertia

Polar moment of inertia

Assumptions: During Standing

33.3% of weight is on the handle bars

170mm = length of crank

M per pedal

Shear Stress

Torsion in the axle

Average stress at crank attachment

A comes from finding the area of the Spline and adding it to the area of the inner thickness

Angle of Twist

Bending Moment

Assumptions: 2/3 weight on pedals

120kg person

Pedals in vertical position

Equal length crank arms

Stress on Bearing

Radius = 10.255mm

Width of bearing = 10mm

Taking 1/3rd the circumference because we are only going for the bottom of the bearing, as the downward force doesn’t act on the top of the bearing.

We’re taking 2/3rds of the weight on the bottom bracket, there are two pedals so dividing by 2.

Re-doing Calculations using the Impact Force

Calculating equivalent force (force when dropped from 12 feet)

t=0.12s or stopping time

Calculating Moments

Shear Stress

Torsion in the axle

Average Stress at crank attachment

Angle of Twist

Max. Bending Stress

Assumptions: 2/3 weight on pedals

120kg person

Pedals in vertical position

Equal length crank arms

Max. Stress on Bearing

Radius = 10.255mm

The same calculations as above were then done for the other two bottom brackets. All results are given in the following table. [3]

Platinum DH

Hollow-tech

Generation 1

2nd Moment of Inertia

Polar Moment of Inertia

Using Normal Force

Shear Stress

Torsion in axle

Average Stress at Crank

Angle of Twist

Bending Stress

Stress on Bearing

Using Impact Force

Shear Stress

Torsion in axle

Average Stress at Crank

Angle of Twist

Max. Bending Stress

Max. Stress on Bearing

Fatigue

As a material is loaded, and then unloaded, it goes through the hysteresis loop

For each cycle, the material is damaged slightly, inching it closer and closer to its failure point. Fatigue largely defines the durability of the bicycle. It is a measure of how many cycles a part can go through without it reaching failure. Large loads, like that of the 8- 12 foot drop, are relatively infrequent but cause are more severe than the usual loads encountered by the bicycle. If we were to engineer for the bike to be able to handle these types of loads commonly, the weight and cost of the bike would sky rocket. We need to establish a loading spectrum of the magnitude of loads and there respective frequency to determine how long the bike lasts until failure.

In steels like those used in the Platinum DH, the steel exhibits an endurance limit below which failure will occur. As the number of cycles increase, the endurance limit is reached and failure occurs. The majority of Bicycle failure is due to Low Cycle Fatigue, fatigue caused by large forces.

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