Bicycle Autopsy - Page 1 of 8


Figure 1

Figure 1 - A typical steel suspension mountain bicycle

Building custom bicycles is a great hobby that can be learned by anyone with a desire to create. The skills needed to dismantle, alter and repair bicycle components can be easily learned, and the parts and tools you will need are quite inexpensive. Discarded or worn out bicycles offer many good parts and can often be found at local scrap yards, city dumps, or yard sales for a few dollars. Even if you plan to build a custom creation using all new parts, this hobby will seem inexpensive compared to many, as you can purchase a brand new bicycle with decent components at a store for less than a hundred dollars.

The great thing about hacking and welding bicycles is that you will be working with all steel components, which are much stronger, more common, and much less expensive than high grade aluminum or carbon fiber bicycle parts. If you have never torn a bicycle apart before, then this basic introduction will show you all you need in order to complete a total bicycle autopsy in minutes, stripping an entire cycle down to the individual parts using only a few basic hand tools.

There will be some very useful tips and tricks presented that may save you a lot of frustration, especially if you are just starting out, so read through this entire section before embarking on any of the upcoming projects.

Figure 1 shows the most commonly available and inexpensive mountain bike available today, the all-steel frame suspension mountain bike from the local hardware store. This cycle cost me $120, and was used to make the StreetFox tadpole trike. The components are medium quality, and include aluminum rims, cantilever brakes, and suspension on both the front forks and rear triangle. Because the frame is made of steel, it can be easily cut and welded using any welder.

Often a bicycle like this can be found at a yard sale for a few dollars, although there may be a bit of rust on the frame, worn out tires, and the odd seized brake cable, nothing that we can't easily fix or replace. OK, now grab your toolbox, and let's tear this bicycle down to the individual parts.

Figure 2

Figure 2 - Front component details

Starting with the front of the bicycle, Figure 2 shows the parts that you should get to know by name. As per the letters, the components are:

A) Handlebars, gooseneck, brake levers and shifters. Handlebars are held in place by the clamp on the gooseneck and are available in many widths and heights. Often, a mountain bicycle will have straight or slightly curved handlebars such as these ones, whereas a road bike will have "curly" handlebars which allow the rider to hold on in two positions - a relaxed upper position, and a more aerodynamic "tuck" position. The gooseneck fits into the forks stem, and is held there by a wedge, which will be shown in greater detail later on. Goosenecks are available with two common stem diameters, so make sure you don't put the smaller sized gooseneck into the larger sized fork stem, or it will not be completely secured.

B) Head tube and fork stem. The head tube is the part of the frame that the fork stem is inserted into. The two cups on the top and bottom of the head tube carry the fork bearings, and will be shown in greater detail later. Head tubes are available in two common diameters, which means that there are also two common sizes of head tube cups and bearings. Again, always ensure that the parts are the same size, or there will be excessive friction in the steering system. There is no common standard for the length of the head tube, or the length of the fork stem, so you should keep matching parts together as a set as you collect them.

C) Front forks. Front forks come in a vast array of sizes, shapes and styles, ranging from the most basic straight leg style to the ultra heavy duty triple tree motocross style forks used for downhill mountain bikes. The front forks will fit only one size of front wheel properly, and the most common sizes for the bicycles you will be working with are; 26 inch, 24 inch, and 20 inch. Most modern front forks will also include the front brake mounting hardware such as the one shown in Figure 2.

D) Front brakes. The front brakes are the most important brakes on most bicycles, as they do the most work. Modern bicycles have cantilever brakes installed on the front forks, but you may also find some brakes that connect to the front forks using a single bolt through the crown of the fork. The type of brakes that connect to the fork using a single bolt are caliper style brakes, which are much less effective than the cantilever style shown in Figure 2 due to the fact that they do not exert as much friction on the front rim.

E) Front wheel. Bicycle rims are available in many sizes and styles, but the 26-inch rim with 36 spokes is by far the most common wheel for an adult sized bicycle. Extremely cheap rims are made of steel, do not have stainless steel spokes and should be avoided due to poor braking characteristics and strength. 20-inch diameter wheels are often used for children's bicycles and freestyle BMX bikes, and they can have as few as 26 spokes and as many as 48. BMX wheels with 48 spokes are extremely strong, which is why they are often chosen for trikes or load carrying cycles.

F) Front hub. The front hub will have spoke hole drillings to match the rim, with 26 holes being the most common number of spokes for an adult bicycle. Decent quality hubs are usually made of aluminum, but you will most likely find both steel and aluminum hubs in your scrap pile. The hubs contain a pair of ball bearings to allow the hub to spin with minimal friction around the axle.

G) Front dropouts. The front dropouts are slotted tabs on the front forks that allow the front axle to drop out of the forks once the nuts are loosened. Unlike the rear dropouts, the hole is not slotted, so it is not used to adjust the wheels position in the forks. There is usually a small hole above the axle slot where a special tabbed washer can help lock the front wheel in place in case one of the axle nuts comes loose.

H) Top tube. The top tube runs from the head tube to the seat tube and is normally under compressive load on a bicycle frame. The top tube is usually the second largest diameter tube in a bicycle frame.

I) Down tube. This tube runs from the head tube to the bottom bracket and is under tensile stress in a bicycle frame. This is normally the largest tube in a bicycle frame, and one of the most important in the strength of the frame.

J) Seat tube. This tube is normally the same diameter on all bicycle frames as it has to carry the seat post, which fits snug inside the tube. The top of this tube will also have some type of clamp which will tighten around the seat post, allowing it to lock in place at the desired height. On a suspension bike frame, this tube may or may not have the duty of carrying the seat post. On the frame shown in Figure 2, it does not.

Figure 3

Figure 3 - Rear component details

The most common components you will find at the rear of a bicycle are shown in Figure 3, and as per the letters, the components are:

A) Suspension gusset. This part may differ depending on the style of suspension, but its basic purpose is to transmit the forces from the suspension spring into the frame in a way that does not induce damage on the frame. Typically, this part will be made from two steel plates with a thickness of 3/32 inches. The top of the rear suspension spring will be held between the plates by a hollow bolt.

B) Suspension spring. The rear suspension spring includes a high tension coil spring as well as a gas filled shock absorber so bumps and vibration are not transferred from the wheel into the frame. The spring is typically rated for 500-800 pounds of compression, which is due to the mechanical advantage gained thanks to the position of the fulcrum on the rear triangle. A high quality rear suspension spring may have 3 or 4 inches of travel and cost more than a thousand dollars. The ones you will typically find on inexpensive bikes will have less than 2 inches of travel and cost only a few dollars to replace. The top ring is usually adjustable to offer a minimal amount of control over spring tension.

C) Rear triangle. The entire moving part of the rear suspension is called the rear triangle. On any bicycle frame, this assembly includes the seat tube, seat stays (M), and the chain stays (L). The three parts actually form a triangle designed to carry the rear wheel. This assembly is extremely strong.

D) Rear brakes. Much like the front brakes, rear brakes are available as cantilever style brakes as shown in Figure 3, including the mounting studs directly on the seat stays, or as bolt on caliper brakes of lesser quality.

E) Cantilever brake studs. These studs are welded directly to the seat stays and allow the brake arms to pivot, placing the pads against the rim. It is best to not remove the brake studs, as their alignment is somewhat critical to proper brake operation.

F) Front derailleur. The front derailleur forces the chain to move between the two or three front chainrings by derailing it slightly at the top as it enters the chainring. The two plates that are on each side of the chain rub directly on the chain to force it to move.

G) The chain. A bicycle chain is available in several sizes, although the pitch remains the same. A single speed bicycle chain is the widest style of bicycle chain, and is quite rigid from sided-to-side as it does not have to run through a derailleur. BMX bikes and those with coaster hubs have a single speed chain. A bicycle with a derailleur must have a thinner, more flexible chain due to the fact that the chain does not always make a perfect parallel run from the front chainring to the rear freehub. A derailleur compatible chain is quite flexible from side-to-side, and is offered in various widths depending on the number of gears on the rear free hub.

H) Front chainrings. The front chainrings have between 20 and 50 teeth, usually having two or three on a crankset for a full range of gears. The front derailleur will move the chain between the chainrings to switch gears as the rider pedals forward. The smaller gear makes you pedal faster but delivers more torque to the rear wheel (for climbing) whereas the large chainring makes you pedal slower, but propels the bicycle at faster speeds.

I) Crank arm. Normally made of aluminum, the crank arm connects the pedals to the front chainrings so the rider can pedal the bicycle. The crank arms must convert reciprocation motion to rotary motion, much like the piston rod in a petrol engine. Crank arms are available as a left and right unit that connect to an axle like the ones shown in Figure 3, as well as a single piece style crank arm, which is shaped like a large S, having both arms connected as a single unit.

J) Pedals. Offered in many varying styles and shapes, the pedals thread directly in to the crank arms and allow the rider to put force down on the crank arms. Pedals have a left and right side, with the right side (chainring side) having standard clockwise threads, and the left side having reversed threads. Pedal threads are also available in two standard sizes, the smaller size is used on three-piece crank sets and the large size is used on single piece crank sets.

K) Crank set axle. The crank set axle is only available on a three-piece crankset, and must fasten the two crank arms together. We will examine these parts in more detail later.

L) Chain stays. These two tubes run from the bottom bracket to the rear dropouts on each side of the rear wheel. These tubes are part of the rear triangle.

M) Seat stays. These two tubes run from the top of the seat tube to the rear dropouts on each side of the rear wheel. These tubes are part of the rear triangle, often including the rear brake studs.

N) Rear freewheel. The rear freewheel is a collection of small chainrings built onto a one way clutch. When the freewheel turns clockwise, the rear hub turns with it. When the freewheel turns counterclockwise, the hub does not turn with it, which is why a bicycle can coast along with the cranks not spinning constantly. The effect of gear sizes is exactly the opposite of the front chainring, with larger gears offering more torque, and small gears offering faster speeds. Most freewheels have between five and seven chainrings. To calculate the number of total gears on a bicycle, multiply the number of chainrings on the front crankset by the number of chainrings on the rear freewheel.

O) Rear dropouts. The two slotted plates at the junction of the chain stays and seat stays are designed to hold the rear axle in place, and offer a bit of adjustment for the rear wheel. Because the slot extends for an inch or more, the rear axle can be moved along the slot, allowing the rear wheel to be adjusted slightly. On a single speed bicycle, this adjustment is used to pick up any chain slack. The rear dropouts also hold the rear derailleur in place.

P) Rear axle. The rear axle is a threaded rod that contains the rear hub bearings, cones, and rear axle nuts. On some bicycles, the rear axle also clamps the rear derailleur to the frame by placing it between the right side dropout and the axle nut.

Q) Rear derailleur. Much like the front derailleur, the rear derailleur must force the chain to move across all of the rear freewheel chainrings in order to switch gears. The rear derailleur must also pick up chain slack, which is why is has a long body containing two idler gears on a spring loaded axle. The chain has a lot of slack when it is sitting on the two smallest chainrings, due to the fact that it does not have to travel as far as it does on the larger rings.

Figure 4

Figure 4 - Rear derailleur and chain details

The rear derailleur pulls the chain around the rear chainring as shown in Figure 4. Because the chain must keep tension no matter which chainring it may be on, the derailleur body must pivot back and forth to pick up the chain slack. The chain must also come in contact with at least half of the teeth on the chainring or it may skip, which is why the upper guide wheel is directly under the rear axle.

The point labeled C in Figure 4 shows the two small adjustment (limit) screws which control how far the derailleur can travel along the rear freewheel. If these screws are not set correctly, the chain may fall off the largest or smallest chainring, or fail to reach them. The chain on the top of the chainring is called the drive side chain (A), as it is always under tension when the cycle is being pedaled. The return side chain (B) is never under any tension as it simply returns back to the chainring.

Figure 5

Figure 5 - Removing the front wheel

Remove the front wheel by loosening the two axle nuts so it can fall out for the fork dropouts. Of course, you must first release the front brake pads, or the wheel will become stuck between the brake pads and the front tire. Letting out all of the air in the front tire will also work, but simply releasing the front brake pads is much easier.

As per Figure 5, press the brake arms together so the cable head can be removed from the brake arm slot (D). Also shown in Figure 5 are the cantilever studs (A), which have built in return spring so keep the brake pads (C) away from the rim when not in use. The brake pads can be adjusted for different rim style by moving them along the brake arm and then locking them in place with the brake pad bolts (B). Properly adjusted brakes should not rub on the rim when idle, but sit as close as possible to the rim.

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