Drop Bar Bike Bell

Increasing rider safety by improving alert mechanisms for drop bar road bikes

Motivation

Current bike bells are designed to be mounted and actuated in a position near the stem of standard flat handlebars. While nothing prevents mounting these bells on bikes with drop bars, their presence can be counterproductive. Survey and observational data suggest drop bar riders primarily ride with hands in a forward position, near the hoods of their brakes, or in the drops. Existing products require the rider to reposition their hands in order to reach the bell, which can cause instability. This product aims to increase rider safety and improve current alert mechanisms for drop bar riders. 

Goal

Develop an alert system for drop-style handlebars that can be intuitively engaged from a rider's preferred hand positioning.

Skills
- human factors/
   ergonomics
- rapid prototyping
- user testing

Timeline
Sep 2023 - present

Collaborators
E. Daigle
C. Finkel
C. Pursley
B. Jacobs (Fall 2023)

Project Summary

This project was conducted as part of a year-long course in advanced product design. The fall semester was dedicated to delivering a comprehensive market and product requirements document (MRD/PRD) and developing a preliminary design with a complete drawing stack. The spring semester is dedicated to fabrication and design iteration, arriving at a fully functional and polished product by May 2024.

I will be updating this page periodically as we iterate through our design--Follow along to see our progress!

Outcomes and Competencies

- conducted interviews and surveys to identify market gaps and cutsomer needs
- devleoped comprehensive MRD/PRD to define priorotiy levels for user and product requirements
- authored project plan and created schedules and Gantt charts to set milestones, establish ownership, and define documentation structures
- designed parts for injection molding, identifying appropriate parting lines and gating surfaces
- produced assembly drawings adhering to GD&T standards

First, some vocabulary

The structure of drop bars leaves little room for bar-mounted accessories. These are typically attached near the stem, where the bar is exposed, and many riders prioritize using this space for phone mounts or GPS systems instead of bells.

From our survey of drop bar riders, 90% of respondents said they spend over 60% of their rides with hands on the hoods. Considering the variability of individual riding postures, our goal is to design a bell that is accessible from both the hood and drop positions. 

For clarity, when referencing parts of the bell assembly, we have defined the following terms:

Hood/Drop actuators: the mechanism/feature that is triggered by the user in the hood/drop position to ring the bell
Striker: the feature that directly strikes the bell
Bell Mount: the subassembly that attaches the bell/striker assembly to the handlebars 

Initial Concepts

With the intention of finding a way for the bell to be actuated in either position, we thought through different styles of ringers in both the hood and drop locations. We decided our two strongest options were a dual-actuator or rotary-style bell. 

Dual-Actuator Bell

Rotary-Style Bell

The benefit of the dual-actuation and rotary bells is the ease of access from the hood and drop positions. The dual-actuator bell achieves this with unique actuators for each position, while the rotary bell's symmetry makes it simple to turn from either direction.

While we thought through some clever internal systems for the rotary bell, the dual-actuation design allowed us to incorporate ergonomics to optimize the actuator for each hand placement independently, so we decided to move forward with that concept.

Prototyping: January 2024 - Present

Overview:

February 6, 2024

We finished the fall semester focusing on a double-action striker. To kick off our first prototype, we 3D printed the final parts from our initial concept design. 

A double-action mechanism has two primary components: the "hammer" and "trigger". In the context of bell design, the hammer and trigger would be accessible from the hoods and drops, respectively.

On the hoods, the hammer can be pulled back to directly strike the bell. In the drops, pulling the trigger cocks and releases the hammer in one motion. 

Note: We're working with 3D printed "bells" while we create tool paths for the CNC lathe and mill to fabricate our own from aluminum and brass. We're looking forward to testing the acoustic effects of size and shape soon!

The bell dome has a cutout for the striker to hit from the inside. We made the decision to align the actuators and striker behind the bell dome to reduce the assembly's overall footprint. 

Challenges:

While the double-action bell was a great start for pushing us to think outside the box, there were key risks that emerged from hands-on feedback that threatened our core promises:

Hammer: The hammer position is difficult to access in the hoods, and requires hand adjustment.

Trigger: In its current position, the trigger is close to interfering with the path of the shifters. 

Interfaces: The profiles of the trigger/hammer interface must be fine tuned to avoid jamming. Furthermore, the sliding surfaces could wear in a way that prevents the trigger from actuating the hammer, rendering it ineffective after long-term use.

The Takeaway:

While there are plenty of ways to address the above risks with some ergonomic adjustments and material selection, we believed we could eliminate these challenges more efficiently by pivoting the overall design trajectory for our next iteration. We're excited about our new ideas!

February 20, 2024

Overview

The main challenges we addressed for our next phase were adjusting the ergonomics of our striking system and fabricating the bell so we could begin testing the acoustics. 

Bell Fabrication

Our original bell tapered off at a smaller angle to reduce the thickness of the overall assembly. We decided to make a standard radius dome for our first fabricated bell to get some immediate feedback while C. Pursely looked into programming spline paths in the CNC. 

Our aluminum bell was fabricated on the CNC lathe and mill. The stock is turned to create the exterior curve before getting shelled out and adding the cutout in the mill.

Striker updates

We liked the double-action striker, but the actuators required precise alignment. We thought simplifying the connection between the hood and drop actuators would be more functionally reliable, and less prone to failures. 

C. Finkel had an interesting idea to use a spiral torsion spring to directly connect the actuators from the hood and drops. She prototyped the system by encasing a makeshift spring in a 3D printed striker subassembly.

Imagining the design from an assembly and manufacturing standpoint, I considered how we could simplify the system by combining the spring, striker, and actuators into a single part. 

Inspired by C. Finkel's prototype, I designed a double spiral spring to be fixed at its center, and tested the concept with the laser cutter. Since the shape of the printed parts reminded me of a pileated woodpecker, I decided to have some fun with the form.

Laser cut woodpecker actuator/striker and fixture to test optimal placement on bars.

Next steps

Now that we have a bell and a few options for striking the bell, we're thinking through possible mounting systems to affix the assembly to the bar. Since the laser cutter makes it easy to iterate through different versions of the woodpecker striker, we can use MDF prototypes to fine tune the ergonomics.

March 6, 2024

Overview

After getting closer to resolving the spline paths in the CNC and thinking through interfacing components, we assembled our first complete prototype. 

Striker updates

To work around the wait time for wire EDM, I decided to adjust the spacing of the spiral so that I could waterjet the part and accommodate the kerf. While the curvature of the bird was a fun place to start, I adjusted the profile to fit in line with the entire assembly so all parts would share the same origin and fixed position.

Assembly Prototype

I fabricated a custom slotted hex bolt to test proof of concept for locking the spring's orientation in the assembly. The hex head locks into a pocket on the back plate to prevent rotation. A 3D printed back plate holds the assembly to the bar with a zip tie for user testing. 

Next Steps

We're feeling good about where we stand with the high level assembly, with some adjustments in mind:

1. Adjusting the thickness of the bell dome to help with its resonance
2. Adapting part dimensions to accomodate a standard bolt size
3. Add protective plate to cover bell cavity and support spring
4. Complete official mount design to secure bell assembly to bars

Since the bell's design is versatile enough to be compatible with various placements on the bars, we're looking into creating a few styles of mounts so the user has choices for their preferred hand positioning.