Bicycle Trailer Hitch


PROBLEM STATEMENT:

Most forms of transportation in developing countries are by foot or bicycle. In some instances, people can be towing up to 400 pounds of cargo on a trailer. With braking force being applied only to the front and rear wheel of the bicycle, the trailer has a lot of momentum behind it with no braking system. By developing a low cost surge-brake hitch system; the stopping distance will decrease dramatically making it a much safer form of transporting goods and services in developing countries while still being a cost competitive form of transportation. Proper hitches, connecting the bicycle trailer to the bicycle, can be made with simple mechanical understanding and familiarity of bicycles. A problem is that hitches are usually very unique, and are designed as one-off items that connect only a specific trailer to the intended bicycle. The design of a universal hitch attachment along with a simple automatic braking system will solve this issue. The cost of materials and tools used must be kept in mind due to the nature of where the device will eventually be implemented.



external image funny_bike.jpg

Figure 1: http://myfivemen.blogspot.com

PROJECT GOALS:

1. Design and incorporate a working hitch/braking mechanism to the current bike trailer already built.

2. Test different designs and build off each design.

3. Keep manufacturing costs to a minimum and use only tools easily found in developing countries.

4. Develop a business model to help local economies.


DESIGN PARAMETERS:

1. Designed for a 150 lb rider.
2. 400 lb towing capacity.
3. Simple tools and ease of manufacturing.
4. Self-braking system.
5. Competitive pricing.


-Current Design to be Improved
The original hitch (Fig. 2 and 3) was designed with readily available materials and basic tools. A 2 inch round housing is used as a casing with holes drilled in it to attach to a trailer via nuts and bolts. Two 3/8 in fender washers are brazed onto the housing; one is at the end and the other is 2 inches from the end, inside the casing. A threaded rod is brazed onto a U-joint which was simply bent out of 1/8 inch sheetmetal. Two springs (one inside and one outside) serve as a damping system to help with braking. The outer spring (closer to the bicycle) allows movement during braking while the inner spring allows for adjust-ability of stiffness for acceleration and braking by a nut and washer on the end. A 1 inch square tube was then cut and attached to the U-joint via nut and bolt. The next piece is bent sheet metal serving as a bike attachment onto the top tube and seat post (Fig. 4). Velcro straps hold the attachment in place on the bicycle.
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Figure 2: Kevins original design

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Figure 3: Original Design drawn by Chris CarvalhoBlue: Bike attachment
Red: Pivots for free range of motion
Orange: Springs for cushioning
Green: Sliding Rod
Purple: Main housing
Yellow: Hex nut and washer

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Figure 4: Original bike attachment shown on bike

Problems With the Original Design

  • Bike attachment is not universal (Fig 4)
  • Bike attachment slides up the seat post
  • Pulling force is entirely on straps holding the hitch on the bike
  • Threaded rod catches on main housing because it is not smooth
  • Rod bends due to force applied by trailer not being along the axis of the rod
  • Spring on pulling side is too weak and spring on pushing side is too strong


Design Matrix:
CATEGORY
Cost
Tools
Aesthetics
Durability
Ease of use
Installation
Maintenance
Business potential
Safety
Materials
Total
WEIGHT
10
7
2
6
4
2
6
6
9
3

Simple brake cable
8
9
3
4
9
7
5
7
9
4
388
Hand brake
8
9
6
4
4
7
4
4
5
9
329
Brake cable with lever
5
5
3
3
9
7
1
5
8
3
276
Ball joint
3
2
7
8
5
8
7
3
7
6
283
Stiff connection
10
9
3
9
2
9
10
2
0
10
351

New Design

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Figure 5: Schematic of new design

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Figure 6: New design (left) with exploded view (right)

Changes for New Design

  • Bigger more sturdy bike attachment that eliminates sliding along bike (Fig. 7)
  • Contact point on bike attachment at front of seat post for pulling force
  • Smooth rod with threaded end to allow the rod to slide more freely
  • Stronger spring inside main housing to reduce bouncing motion while pedaling
  • Weaker spring outside of main housing to cushion the force from trailer more


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Figure 7: New bike attachment shown on bike

Materials

  • 2 in square steel tubing
  • 2 in round galvanized tubing
  • 1 in square steel tubing
  • 1/8 in steel sheet metal
  • 3/8 in steel rod
  • 1/4 in nuts and bolts
  • 3/8 in nuts and bolts
  • 3/8 in ID, 2 in OD fender washers
  • 3/8 in ID, 1.5 in OD washer
  • Springs
  • Hose clamps
  • Adhesive padding
  • Standard bike brake components

Minimum Tools Required (Other tools can be used to speed up the process)

  • Hacksaw
  • Hammer
  • Brazing Equipment
  • Ratchet set
  • Flat head Screwdriver
  • 3/8 in die
  • Vise
  • Drill

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Figure 8: Hacksaw cutting (left) and brazing (right) processes

Next Steps

  • Add a braking system
  • Testing
  • Fix problems seen in testing

Braking System:

The trailer itself requires a self-contained, self-engaging braking system to augment a bike’s braking system. This braking system will improve the braking ability of the rider, allowing it to be safer to carry up to 400 lbs in the trailer.

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Figure 9: Prototype Bike Hitch Braking Mechanism

Simple Brake Cable Design:

The simple brake cable design will make use of the force of the trailer moving towards the bike, which also moves the housing with the brake cable towards the bike. This force pushes the cable into the rod, which causes tension on the cable and engages the brakes.



Background Research:


"Avenir Discovery Bicycle Trailer Hitch." Bicycle Trailers for Toddlers. N.p., 2011. Web. 28 Nov 2011. <http://bicycletrailersfortoddlers.blogspot.com/2011/08/avenir-discovery-bicycle-trailer-hitch.html>.
The best feature in this hitch is that the spring allows for some trailer movement without affecting the rider. However, this design falls short for the design problem at hand. The mount is not universal and would not fit on the stays of many bicycle frames with complex designs. Also, at 17 US dollars, it is a bit too expensive considering it still needs a braking system to be added. There are also a few parts that may be too complicated to machine with simple tool. An example of this hitch is shown in figure 10 below.

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Figure 10: bicycletrailersfortoddlers.blogspot.com

Downs, Todd. Bicycle Maintenance &Repair. 5th ed. Rodale Inc., 2005. 19-48. Print.
There are many bike frame geometries that exist. Road racing, touring, mountain, endure, two-niner, downhill, freeride, hucking, trials, cyclocross, hybrid, and recumbent are the frame types that Downs lists. While some of these can be discarded because they probably wouldn’t be found in developing countries being used for freight purposes, there is a need to design a trailer hitch that works for all frame geometries. Also, within each frame type there are countless variations in the actual frame design making creating a universal mount even more difficult. Some mount areas such as the seat post and rear dropouts are more popular for trailers.
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Figure 11: www.burley.com/home/bur


Kou, Sindo. Welding metallurgy. 2nd ed. Hoboken: John Wiley & Sons Inc., 2003. 11-12. Web.
There are many available welding processes. The SMAW welding process would be good for its inexpensive equipment and its ability to make the welds needed for our project. It is slow and not very clean, however these problems can be worked around for this project. A person could invest in the equipment required to start a small business of making bicycle trailers.

"Travoy Hitch." Burley. Burley Design, 2011. Web. 1 Dec 2011. <http://www.burley.com/home/bur/page_164_8/travoy_hitch.html>.
This hitch which is designed to mount to your seat post, works well for a universal hitch, however it requires more parts and machining than our budget allows. This hitch sells for about 25 US dollars. A solution that uses fewer parts therefore requiring less manufacturing and money would be better. With the addition of another component it can achieve full range of motion. An example is shown in figure 11 above.

Wehrbein, William. "Wheelies and Headers, or How to Keep Both Bicycle Wheels on the Ground." Physics teacher. 42.1 (2004): 27-30. Web. 22 Jan. 2012.
Wehrbein gives some important information in his article. He claims that the coefficient of static friction on typical pavement is about 0.8. In developing countries, this would be lower due to dirt roads. This means that rear brakes alone can provide about -0.26g of acceleration or -0.56g if both brakes are safely used. With a lower coefficient of static friction in the situations our project would be used in, the acceleration that the brakes can provide would be less than the given numbers. Using these numbers as maximum stopping deceleration, we can find out the force that the trailer can exert on some mechanism that will apply the brakes.

Group Members

Matt Ramirez: Fourth year General Engineer concentrating in Mechanical Engineering. He is from Porter Ranch, California and loves playing baseball, water sports, cars and hanging out with friends.

Chris Carvalho: Fourth Year General Engineer concentrating in mechanical design and manufacturing. From Santa Clara, California and likes sports including mountain biking, frisbee, football, and disc golf.

David Hernandez: A third year Biology major concentrating in Marine Biology from San Jose, David enjoys rock climbing, cycling, and musical theater