Chain drive opportunity

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Contents

Executive Summary

Our market research began by first noting that customers interested in our sort of solution were Formula SAE teams around the world as well as entry-level racing car buyers (e.g. the Radical SR1 and the Formula Ford vehicles). From there, we ventured to do some research on the market and interview potential buyers. interviewing both technically inclined Freshmen members of FSAE and a decade-long expert in the field of chain-drive differentials, it is evident that there is room for improvement in the field of chain-drive tensioners. From our user study, it was evident that the system was not only difficult to reach and left the end user unnecessarily dirty, but it had no way to gauge improvements. An even slightly inexperienced person could leave the system in worse condition than it was before the tuning took place. The user was not confident in his or her work and did not appreciate the idea of tensioning and aligning the system at the same time. From our expert interview, we learned that the market for custom chain tighteners exists and that mass-produced solutions are either shoddy or carry a massive price tag. There is also no way of truly determining optimal chain tension, another feature that the team could bring into play.

From our research, we took some time to develop potential designs. Our baseline was two fold; it included what exists in the current CMU FSAE car (CMR 57), and a hard mount that did not allow for tensioning. Once this baseline was put in place, three potentially successful designs came out of our brainstorming methods (which included brainstorming, brainwriting and freewheeling). They included the eccentric pressure clamp design, an indexable pivot tensioner, and a idler pully spring pretensioner (please reference the Pugh Chart). From rigorous analysis, the eccentric pressure clamp design came out on top and is currently favored moving forward.

Market Research

In order to ascertain the market in which this product would exist we looked into the needs of small scale racing teams, such as Formula SAE teams and more widespread amateur racing series. This includes entry level series such as the Radical SR1 cup and other racing cars built to use motorcycle engines. In order to make informed decisions we performed a user study and interview, Interviews with professionals and survey results.

Stakeholder Needs

Because we are exploring a new product and market space we must re-define our stakeholders. The primary stakeholders in this supply chain are the manufacturers, racing shops/online retailers and the racing customer.

Consumers:
Consumers are the target audience who purchases the product, these people race dedicated track cars on the weekends. They may have built these vehicles themselves or purchased the chassis from a company like Radical.

  • Ease of use for both alignment and tensioning of the rear differential
  • Robust construction for reliable track use
  • Low weight because this is racing vehicle
  • Lowest possible cost for a quality product as these vehicles are supposed to be low cost racers
  • Ease of maintenance
  • Compatible with many frame and differential design, IE not a custom part for each vehicle
  • Easily installed vehicles not currently using it
  • Weather resistance


Retailers:
These are the people and companies that would sell the product to the consumers Most likely this would be from specialized race shops or from online retailers like Summit Racing.

  • Small size for easy storage
  • Possibility of flat packing
  • Universal compatibility use so they do not have to stock more than a couple sized/configurations
  • Ease of maintenance for customers who do not wish to do this themselves
  • Low price
  • As little hardware as possible


Manufacturers:
Manufacturers create the product for sale and effects on time and pricing impact the entire supply chain.

  • Time to manufacture is important and all custom machined parts slow down the process
  • Cost to manufacture and of materials; exotic materials would not be good here.

Competitor Products

Before deciding on a path for the rest of the market research we first looked for competitor products in the market space we defined. This meant looking both at Formula SAE and for other motorcycle engine powered low cost race cars. In the FSAE market there are several differential designs provided by companies such as Taylor Racing and Drexler. These Differentials leave the mounting up to the customer, a prospect that with Formula SAE is relatively common. However, these differentials get used in hobby cars as well as vehicles such as Radical's new SR1 which use chain drive. Mostly support and pretension for the differential is an afterthought for the company selling the differential. This is where market space exists. As we found and are documented in the pictures below these mounting plates look similar in function to what we have currently on our FSAE race car. The pivoting turnbuckle style is typical in this situation. These products also do not provide the consumer with many of their needs as a stakeholder. A lot is left to be desired in terms of universal mounting and also weight.


Taylor Racing Differential Mounting
Taylor Racing Differential Mounting
Pivot Plates for Fomula Ford Differential
Pivot Plates for Fomula Ford Differential
Radial SR1 with chain drive
Radial SR1 with chain drive
Radial SR1 with turnbuckle pretension system
Radial SR1 with turnbuckle pretension system

User Study

We designed a study to identify normal issues with the pretension and function of maintenance on the drivetrain of the Carnegeie Mellon Racing Formula SAE car. This vehicle is representative of the set up used both in the FSAE series but also in other low cost racing series where chain drive is prefered. Two mechanically inclined individuals who did not have prior knowledge of the car’s design were asked to loosen the chain completely and re-tighten the driveline. This procedure allows us to artificially create a situation of a loose chain.

The first thing observed in this study was that access to adjustment turnbuckles was very limited. This was compounded by the fact that tools were not common between the separate locking nuts and the turnbuckles. The lack of access and direct line of sight also made finding the position desired for the plates was hard. The users attempted to gauge position of the plate via number of turns on the turn buckles. This concept could be used in the future to help locate differential plate position. However, on the current set up the number of turns did not create the same movement between the two sides, making this method useless.

While the users were working it was clear that this was also a dirty project. While not a direct stakeholder need, the ability to quickly and cleanly perform maintenance is a plus for all consumers.

Towards the end of the work we noticed a lot of confusion in determining how tight the chain needed to be and whether the differential housing was indeed aligned properly. This meant a lot of time wasted trying to figure out how to determine these specifications and also how to indicate if they were within spec. “When are we done here?” was the common phrase used. This shows a key flaw in the system, there is no way to indicate alignment and tension on the plates. They ended up just pulling on the chain to determine tension (a feeler method) and then standing back a bit and closing one eye to gauge alignment. It seemed as though they got a grasp on the whole process fairly well but had no idea if they met the end criteria. There is no specification for chain tension on a project like this (custom race car) and while trying to also align the differential this made determining when they were finished hard. With an easy method to measure position or a discrete number of locations this problem would be eliminated.

Loosening and Pretension the Drivetrain
Loosening and Pretension the Drivetrain
Difficult access
Difficult access
Is it tight?
Is it tight?

User Interviews

The user interview was conducted after the user study and questions were asked of one of the two students who were told to work on the differential.

Was is obvious what tools you needed to get started?

  • "No, but it was fairly easy to figure that out. Seems like everything on the car is 7/16"

What was the best part about working with the assembly

  • "Everything was easy to see in front of us that also made it obvious to understand what was going to happen."

What was the most annoying part about working with the assembly?

  • "Tool access was very limited and the turnbuckles spun different directions from left to right side."

What was the hardest part about pretension the assembly?

  • "I would say it was trying to adjust pretension and alignment at the same time. Any adjustment you make to one affects the other and that makes it really hard to get either aspect right"

If there was one thing about this process you would change what would it be?

  • "Have specific locations for the plates so you can more easily align them"

How did you determine when you were done?

  • "We were actually fairly unsure when we were supposed to be done. It was unclear how tight the chain should be but even more confusing was knowing if the differential was aligned or not. There is really know way to know other than to just step back and look. We assumed that meant it wasn't as critical a parameter."

The user interview gave us outcomes we expected based on observing the work done by the students. Of course we knew it would be hard to align and tighten the differential all at once and that it was very hard to tell when you got it right. These answers were fairly unremarkable. What was of note is that because of the turnbuckle design and how open the area is, they were very easily able to figure out at least what the initial steps were and how the design functioned. This along with the fact that common hardware is appreciated (the 7/16 is everywhere comment) were things we hadn't thought about a head of time. It is possible that this level of intuition would make our design much easier to use, however, because this is a racing vehicle this might not be priority number one (weight/function during operation).

Expert Interview

The expert interview was conducted on the FSAE liason of Taylor Racing, who goes only by the name of "Scotty." He has just over a decade of experience with chain-drive race cars and has been fulfilling his role as the FSAE liason for over a decade as well. His design for a chain-drive differential is widely respected as the best solution available. The interview went as follows.

Scotty, what's your experience with the chain drive system as a whole and chain tightening mechanisms?

  • "Well, I've been making the best diffs [differentials] for years now, but I've never seem our chain tightening mechanisms take off - the end user usually does that themselves."

Have you ever had to tighten a chain yourself? How was your experience?

  • "I've had to do it a few times, and it's always a total pain in my *expletive removed* - you have to take off most of the body to reach anything and you always jack your fingers up on something."

So the mechanism is complicated, poorly packaged or a combination of both?

  • "I've never seen a design that was simple and cost effective. They're always either super overblown and expensive or cheap and easily prone to loosen while driving."

Do all cars need a chain tightening mechanism or is it possible to set it in place?

  • "Traditionally, cars with smaller chains need the tightening more. Though honestly there's a lot of issues with having a large chain that's sloppy as well (traction loss, unpredictable behavior of the system, etc.) It's something all cars should tune - it's always a real shame to see issues from a sloppy chain."

How do you know how tight to make a chain?

  • "Its a rule of thumb kinda thing. You can base it off similar designs with motorbikes that come with a factory specification. We tend tighten them the night before the race and just check up on them during the day."

Interesting. Some of the designs we've seen have discrete or continuous variability, is one preferential over the other from what you've seen?

  • "That sounds too fancy. I'd just focus on making something that's inexpensive and won't loosen up on you."


Scotty told us a lot of the things we already knew from the user study we conducted. One interesting thing is that again even at the more professional level the amount of pretension is still based on guess work and experience. It is possible that a more precise and objective measure of chain pretension is needed along with specifications.

Design Concepts

All differential mounting options share some common features, as can be generalized in the view below.

Diff Rotation and Chain Tension
Diff Rotation and Chain Tension

The setup contains a pair of mounting plates that allow the differential to rotate freely, dividing available torque to the wheels. In addition to locating the differential, the mounting plates must adequately react the tensile spike loads carried through the chain off the motor's drive sprocket. The tension and alignment of the chain is critical to a reliable, high efficiency vehicle. Because of this fact, four designs were developed that deal with properly aligning and tensioning the chain and are detailed below.

Eccentric Chain Tensioner

The first mechanism has a pair of symmetrical plates with an outer A frame and inner eccentric plate. The first component is the horizontal A frame which has features on the top and bottom suitable for mounting to a forward chassis tube. The A frame is also bored out for the inner eccentric mount. The final feature is a split at the tip of the 'A' that allows a bolt to pass through and apply a clamping pressure on the inner eccentric mount when tightened. Before that bolt is tightened and the clamping force applied, the inner eccentric mount is free to rotate, providing fore-aft translation of the differential and sprocket, allowing slack to be removed from the chain. Alignment is preserved throughout the rotation, once initially installed. This mechanism is detailed below.

Eccentric Clamp Pressure
Eccentric Clamp Pressure
Eccentric Tensioning Method
Eccentric Tensioning Method

Indexable Pivot Tensioner

The second mechanism, pictured below, works to tension the chain by pivoting the differential mounting plate about a pin above the axle axis. A turnbuckle is attached to the point below the axle axis and is twisted to shorten the distance between the lower pin on the differential plate and the rear chassis tube, swinging the differential and sprocket further from the engine thus tensioning the chain. In order to ensure alignment from side to side, features on the chassis and on the swinging differential plate to use as an index.

Indexable Pivot Tensioner
Indexable Pivot Tensioner

Spring Tensioner

Taken from many industry chain drives, the third mechanism utilizes a fixed differential and takes up slack in the chain with a spring loaded idler gear. The tensioning mechanism is of the drive sprocket and differential so can be attached in a variety of convenient locations within the rear box of the vehicle. The spring mechanism can be adjustable to provide a post-installation determine preload.

Spring Tensioner
Spring Tensioner

Rigid Differential Mount

The rigid differential mount has no moving components used to tension the chain. This arrangements provides a very robust and simple means to hold the differential in position relative to the drive sprocket, but relies on an accurately sized chain that can be difficult to attach.

Rigidly Mounted Differential
Rigidly Mounted Differential

Pugh Chart

The Pugh chart below compares the current design (in CMR 57) with 4 alternatives: hard mounted diff (not adjustable), eccentric clamp pressure, indexable pivot pretensioner, and spring pretensioner systems. Based on stakeholder needs, our most important criteria included minimized weight, easy manufacturability, adjustability, ease of use, robustness, adaptability, compatibility, and minimized cost. Weight must be minimized for increased speed, and the design must be robust so that the car is reliable. It should be easy to use so that even less experienced participants can have proper chain tension, and low cost for teams with tight budgets. Adjustability is important so that it can be used on various chassis with different mounting. The design also needs to be compatible with various differentials so that it can appeal to a wider range of consumers.

While the weighting was subjective, we attempted to weight the categories to best match the needs described by users and experts. Weight and robustness were most important. Most of our consumers would not bother buying a product that is heavy because it is more advantageous to risk a slightly loose chain and decrease weight of the vehicle. The design also must be robust because if it fails, the car can lose traction, which slows it down and negatively affects handling. Manufacturability was not weighted quite as important because it would be fairly low-volume. Ease of use was also important enough to get weighted as a '2' because it needs to be usable by less experienced people in order to appeal to a wide variety of consumers, and difficult adjustment is the main issue encountered by our users in the user study. Adaptability, adjustability, and compatibility got weighted the lowest, with cost, simply because users prioritized the other factors; however, they should still be considered because scoring well in these categories can make a design desirable to more consumers.

Our current design got the baseline score of '0' for all criteria. The first alternative, a hard mounted differential, gains points for weight and cost because it does not add any weight, materials, or processes. It is about as easy to use and manufacture as the current design, but it would give improvements in robustness, adaptability, and compatibility. It does, however, lose several points because it is not adjustable. The second alternative is more difficult to manufacture, and would be more expensive, but it would be much easier to use and could be compatible with a wide range of differentials on the market. Also, unlike the first alternative, it allows for some adjustability. The indexable pivot pretentioner scored the same as the current design except that it would be much easier to use. The final alternative, the spring pretentioner, is expected to be heavier and more expensive than other alternatives and difficult to manufacture. It does, however, allow for adjustments, it's easy to use, and gets high scores for compatibility.

Overall, the eccentric design got the highest score, but it was surprisingly close to the hard mounted option. All alternatives received higher scores than the current design, which shows that there is room for improvement.


Phase II Drivetrain Pugh Chart
Phase II Drivetrain Pugh Chart

User Scenario

Someone (let's name him Scotty) who has purchased a formula-style weekend racer is participating in an autocross event. Scotty is having a great time, until he accelerates through corner apexes and notices his back end going loose when it never has before. Scotty also notices a strange issue under braking where traction stops and goes without any control by him. Being a 10-year veteran of differential design, he knows his chain is loose. Since Scotty tends to be cheap on components that aren't directly related to power output, his chain tensioner is a shoddy turnbuckle design. He pulls off into the paddocks to get to work. Once he removes most of the body panels on his car, he wrestles with the system until he thinks it's ready to go. Little does he know that his misalignment is a ticking time bomb. Scotty puts the panels back on after 90 minutes of tweaking, and sets up for a quick lap to warm his tires up. While the car runs well for a while Scotty notices strange power transfer to the wheels. He re examines the pretension system and sees no issue. However he does not realize that undue stress has caused his differential to fail prematurely. With a easier to use set up he would have avoided this misstep.

Gantt Chart

The Phase III timeline is broken down into component level design, analysis, manufacture, and integration. Each team member is assigned a specific set of components to optimize and specify. The deadlines for design review I, II and the final reviews are indicated at the top of the chart. Although not explicitly indicated, prototypes for design reviews I and II will be made to demonstrate mechanisms and as proofs of concept.

Phase III Drivetrain Gantt Chart
Phase III Drivetrain Gantt Chart

Conclusions and Recommendations

Based on our Pugh chart, it is most favorable to pursue the eccentric clamp pressure design. This design is not only robust and lightweight, but allows for the highest level of adjustability. Though the manufacturing is challenging, it allows for the best possible chain tensioning and differential aligning experience. It will be a challenge to adapt it to other applications with different size differentials since the split-hub friction force is highly dependent on tolerancing, but the challenges are far outweighed by the benefits it offers. This design is very closely followed by hard mounting the differential, a solution that assumes that the chain does not lose its integrity over time, which may or may not be the case depending on how long the car is expected to last.

Appendix

Brainstorming Ideas

1. Current Design

2. No tensioning

3. Eccentric clamp pressure

4. Eccentric tensioning method

5. Indexable pivot tensioner

6. Spring tensioner

7. Shaft clamp style eccentric diff plate

8. Eccentric mounted diff (method of fixture?)

9. Bigger chassis saver (variable?)

10. Tensioning device (sprocket?)

11. Flat plane-correct

12. Continuously variable sprocket diameter

13. Magnet drive

14. Paddle slapping

15. Direct gearing

16. Buggy pushed by hired help

17. Fred Flintstone-style

18. Pedal car

19. Soapbox derby (downhill only)

20. Detroit locker (lock-selectable diff)

21. Change sprocket side

22. Easily change chain length (add / remove links)

23. Espresso Machine (caffeine for speed)

24. Duck car

25. Snowmobile (tank treads)

26. Boat car

27. Go kart style (solid rear axle)

28. Hovercar

29. Fan car (swamp boat style)

30. V-TEC

31. Reliant Robin

32. Trike w/ one rear wheel

33. Open / Brake diff

34. CVT

35. Independent chain drive

36. Dual engine

37. Electric motors

38. Dual electric motors

39. FWD

40. 5th wheel = power wheel

41. Move motor up/down to tension system

42. Move motor forward/back to tension system

43. Sliding suspension points

44. Boxless rear w/ shims for tension

45. Eccentric diff with pins

46. Centrifugal tensioning

47. Centrifugal clutch

48. Dual output shafts

49. Viscous LSD

50. Dual hydraulic (tank drive)

51. Rocket

52. Open diff

53. Belt drive

54. Turbo

55. Shaft-driven driveshaft

56. Transaxle

57. Welded diff

58. Hub motors

59. Solid axle

60. Limited Slip

61. Conventional FR layout

62. V-belt drive

63. Adjustable chassis length

64. Adjustable link lengths

65. Only drive one wheel

66. Motorcycle

67. Swappable sprockets w/ different diameters

68. Keep a few chains of different lengths hanging around

69. Automated turnbuckle adjustment from driver position with button activation

70. Propeller car

71. Jet engine powered car

72. Double chain drive

73. Compressed gas

74. Wind-up car (like mousetrap car)

75. Hamster wheel

76. Wind powered

77. Sliding cone-shaped chassis saver

78. Dual idler sprockets

79. Swappable idler sprockets with different diameters

80. Pneumatic tensioner

81. Super beefy chain that doesn’t loosen

82. Sliding chassis rails

83. Sprocket extenders

84. Liquid nitrogen cooling (chain contracts)

85. Hydraulic tensioning system that adjusts with increased oil pressure

86. Pulled by freshmen (backwards buggy)

87. Ratcheting position adjustment

88. Multiple bearing seats

89. Dual outward-pushing idler sprockets

90. Sliding diff plates (on rear box tubes)

91. Inflatable chassis saver

92. Chain that contracts when heated by engine

93. Eccentric axial clamping

94. Eccentric bolt-through

95. Telescoping chassis tubes

96. Automatic transmission torque converter

97. Heavy spring to push diff plates as chain loosens

98. Truck straps

99. Belt adjustment (like the ones for pants)

100. Pivoting rear box

Team Member Roles

  • Adam Brecher: Market Research, Gantt Chart
  • Anne Dirkes: Team Leader, Pugh Chart
  • Brian Langone: Pugh Chart
  • Mike Ornstein: Design Concepts, Gantt Chart
  • Rob Wojno: Executive Summary, Conclusion, Expert Interview
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