Progress Update 2

Introduction

Sheffield Student Downhill Design is a student-led co-curricular project at the University of Sheffield. Over the course of several years the project aims to design, manufacture, test and race a performance downhill (DH) mountain bike (named the "SUBMARINE") with the ultimate goal of the bike competing in a national level DH race.

The project was established in September 2022 and is currently in its design phase. This article covers the progress of the project over the first six months of running including initial design choices, geometry selection and beginning initial suspension linkage development. For more information on the long term project plan and the motivations behind the project please visit the Downhill Bike project page.

What we are testing?

The team decided to primarily focus the testing on the geometry of the bike, and more specifically on the ways in which the Front Centre:Rear Centre proportions affect its handling. There is currently a noticeable range in this ratio within the market therefore it was decided it could be interesting to allow for this to be changed through the design of our bike. Details on how we could integrate this adjustability into the design can be found below.

This image shows most common bike geometry measurements including front and rear centre [1].

How we are testing it?

The flip chip on a 2022 Transition Patrol. [2]

Initial ideas regarding what elements of the frame could be made adjustable

To enable us to comprehensively test the effect of altering the Front Centre:Rear Centre (FC:RC) we have decided that parts of the bike should feature in-built adjusability, these include:

Reach

We intend to adjust reach by using eccentric headset cups in an oval head tube to allow for 20mm of total adjustability, giving us a range in reach measurement between 460mm and 480mm.

Chainstay length

We aim to include 20mm of adjustability on the chainstay length, with a range of 440mm to 460mm. The exact method for this is still undecided and will form a package of work for some team members once the linkage design has been narrowed down further. Some initial ideas include a selection of rear drop-outs, adjustable rear drop-outs or flip chips in the linkage. keep an eye out for or solution in the future!

Head angle

Head angle can be altered using a selection of different headset cups. We intend to use this to give an adjustment range of +/-1 degree, between 62 and 63 degrees. This furthers our ability to alter the front centre of the bike and as such the to the FC:RC.

Other

Some less traditional ways in which we could alter the FC:RC are:

Eccentric bottom bracket (BB)
Using forks with different offsets (distance from the front axle to the centreline of the fork steerer tube)

An eccentric BB would allow the position of the BB to be moved forwards or backwards with a total range of somewhere in the region of 10-12mm, as well as up or down if need be. However this option presents its own difficulties, most notably the fact that it does not allow for total control over the chainstay length without aloso affecting bottom bracekt height, which makes it challenging compared to other methods of adjustment.

Forks with different offsets could also be used to adjust the front centre of the bike however this requires having severeal sets of fork crowns, which are a costly component to buy, and would also be very difficult to manufacture in house.

It is likely that both of these methods would only be considered if the range of adjustment available through other methods is not considered to be large enough due to their inherent challenges (be it technical or financial).

Initial suspension development

For initial linkage design the frame sub-team was split into pairs, with each of them spending time using the Linkage software to investigate a specific suspension layout for both high and low main pivots locations. The layouts that were chosen to look into were single pivot (both linkage-driven and without any linkages), four-bar/Horst-link, and twin-link (VPP, DW Link and Maestro, etc).

From this the options were then narrowed down further to just four bar and VPP designs. Considering other factors such as packaging, bearing location, and ease of kinematic adjustability the team concluded that a four bar layout would be persued further.

An Iron Horse Sunday frame, showing the DW Link layout, as well as the virtual pivot point that the design features. [3]

Sizing

Wire-frame drawing of decided geometry & table of geometry values including intended adjustment

The team decided to design the bike around someone who is approximately 5ft10 to 6ft1 in height, due to it being a common height amongst team members, and the likelihood that a rider of this height will always be available for testing throughout the future of the project. Generally speaking, it will be of a similar size to most manufacturers' size large frames.

References

[1] A. Muldoon, “Tech feature: Taking a deep dive into mountain bike geometry,” MBR, 04-Jul-2022. [Online]. Available: https://www.mbr.co.uk/news/tech-feature-mountain-bike-geometry-397826. [Accessed: 18-Apr-2023].

[2] B. K. Stancil, “Transition's mixed-wheel patrol // long-term review,” Fanatik Bike Co., 20-Oct-2021. [Online]. Available: https://www.fanatikbike.com/blogs/engage/2022-transition-mixed-wheel-patrol-mullet-bike-long-term-review. [Accessed: 22-Apr-2023].

[3]“DW-link,” Wikipedia, 13-Feb-2022. [Online]. Available: https://en.wikipedia.org/wiki/DW-link. [Accessed: 22-Apr-2023].

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