Projects / Brake Retraction System

Brake Retraction System

A mechanism that pulls the toboggan's deployed brake back into its automatic latch after every run, so nobody has to reset it by hand.

Context: UW Concrete Toboggan
Role: Brake Design Core Member
Tools: SolidWorks, SolidWorks Simulation
Status: V2 in design review

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V1Hand-wheel crank and pulley

Design reviewForce path · DFM · cost

V2Spring-return cable, fewer parts

102.8 NMax retraction load

5.8 kgBrake mass

0.333 mBrake length

V1 → V2Crank to spring return

Problem
Requirements
Design
Analysis
V1
Issue
V2
Result
FEA

Problem

The toboggan's brake drops down when released and has to be returned to its automatic latch before the next run. Until now that meant someone reaching in and pulling the brake back up by hand after every run.

I started in the toboggan bay, measuring the brake area of the frame to see how much room a retraction system would have.

Steel and aluminium toboggan frame on a workbench in the team bay — Toboggan frame in the team bay

Requirements

Requirement	Why it matters
Return the deployed brake into the existing automatic latch	The latch stays as is; the new system only has to reset the brake into it
Carry the 102.8 N maximum retraction load	Sets the cable, pulley and spring sizing
Pull along the brake's reset motion	Misaligned force is wasted and loads the mounts
Fit the existing frame and route around the skis and suspension	The toboggan layout is fixed
Low cost, simple to build, off-the-shelf parts where possible	Cost and manufacturability are design review criteria

Design

Both versions use the same idea: a flexible tension member (rope or cable) routed through pulleys and guides from an input the operator can reach to the brake, so pulling the input lifts the brake back to the latch. What changed between versions was the input and how the system resets itself.

V1 bracket CAD with welded gusset plate labelled — V1Concept: crank bracket with welded gusset

Hand sketch of the V2 spring and pulley layout — V2Concept sketch: pull handle, cable path and spring

Analysis

The design is driven by the 102.8 N maximum load needed to pull the 5.8 kg, 0.333 m brake back into its latch. I used that load to evaluate:

Force path: lining the pull up with the brake's reset motion so the operator's effort isn't wasted.
Pulley routing around the frame, skis and suspension, and how each redirect changes the line of pull.

Final cable, spring and off-the-shelf component selection is still in progress.

Each version then went through the team's design review, which scored it on force transfer, cost and manufacturability.

V1: hand-wheel crank and pulley

V1 used a hand-wheel crank to wind a rope through a pulley path and rope guides, pulling the brake back toward its reset position. The crank sat on a gusseted mounting structure attached to the frame.

V1 hand wheel crank CAD — V1Crank wheel CAD

V1 mechanism circled on the toboggan frame — V1On the toboggan

Issue

Design review found three problems with V1:

Poor force transfer. The crank and the pull direction were misaligned, so much of the operator's effort didn't go into lifting the brake.
Hard to manufacture. The gusseted support would need thin waterjet-cut plates welded together, or a lot of material machined away from a solid block.
Costly and wasteful for what it does, with more parts and fabrication steps than the job needs.

V2: spring-return cable

V2 replaces the crank with a pull handle, a cable path and a compression spring. Pulling the handle compresses the spring while it resets the brake into the automatic latch; letting go, the spring returns the handle and cable on its own.

I rerouted the pull path so the applied force lines up with the brake's reset motion, and dropped the welded gusset bracket entirely.

V2 handle circled on the toboggan frame — V2On the toboggan

V2 rope path highlighted in red along the frame — V2Cable path

Result

Criterion	V1 crank	V2 spring return
Input	Hand-wheel crank	Pull handle
Reset of the mechanism	Manual	Automatic (spring)
Force alignment	Crank and pull misaligned	Pull path aligned with reset motion
Mount fabrication	Welded waterjet gusset or machined block	No gusset bracket
Part count and material	Higher	Lower

V2 addresses all three review findings with a simpler mechanism. It is currently in design review with the team.

FEA: is the V2 handle strong enough?

What FEA is: FEA (finite element analysis) splits a part into tens of thousands of tiny pieces and works out how hard each piece is being stretched or squeezed. It shows where a part would bend or break before anything gets made.

What I tested: the V2 pull handle and the line guard it sits on, with the guard held where it mounts to the frame. I pulled the T-grip straight up with the full 102.8 N retraction load, the most force the handle should ever see (about the weight of a 10 kg bag).

Why 6061-T6 aluminium: I used 6061-T6 aluminium alloy because it is the same material as the toboggan frame. That keeps the parts matched to what the team already builds with, and it starts to bend for good at about 275 MPa.

How to read the pictures: blue means the metal is barely working. Green, yellow and red mean it is working harder.

SolidWorks stress plot of the brake handle and line guard, almost entirely blue — **STRESS**Handle and line guard under the 102.8 N pull

Close-up stress plot of the T-handle, highest where the stem meets the grip — **STRESS**Close-up: highest where the stem meets the grip

Displacement plot of the brake handle, largest at the grip ends — **MOVEMENT**How far each point moves

Item	Value
Material	6061-T6 aluminium alloy (same as the toboggan frame), yield 275 MPa
Fixture	Line guard fixed where it mounts to the frame
Load	102.8 N pulling the T-grip straight up
Mesh	3 mm solid mesh, ≈ 37 500 elements
Max stress (von Mises)	2.69 MPa, where the stem meets the grip
Max movement	0.0023 mm at the grip ends
Min factor of safety	≈ 102

What it means:

Strength: the highest stress is 2.69 MPa, about 1% of what 6061-T6 can take. The factor of safety is about 102, so the handle could take roughly 100 times the real pull before it bends for good.
Stiffness: the grip moves 0.0023 mm, thinner than a human hair. The person pulling won't feel any flex.
Next step: the handle is much stronger than it needs to be, so it could be made smaller or lighter later to save material and cost.
Limit: in the model the pull is held where the handle sits on the guard. In real life the cable and spring hold it, so this is a check of the handle itself, not of the whole cable system.