Why Small Wheels Get Trapped

The Hidden Barrier Designers Don’t See

Most accessibility failures are not caused by big obstacles.

They are caused by small wheels meeting small gaps.

For wheelchair users — especially powerchair users — the small front wheels (castors) are the weak point in the system.

👉 If the small wheels fail, the whole wheelchair fails.

What Are Small Wheels (Castors)?

Wheelchairs typically have:

Large rear wheels (drive wheels)
Small front wheels (castors)

The castors:

Swivel freely to guide direction
Carry part of the user’s weight
Are the first point of contact with the ground

Unlike large wheels, they:

❌ Do not drive the chair
❌ Do not have momentum
❌ Are easily affected by small changes

👉 They are designed for manoeuvrability — not for overcoming obstacles

Why Small Wheels Get Trapped

1. Gaps and Holes

Even small gaps can trap a castor wheel.

Train platform gaps are a known hazard
Castors can drop into gaps and stop the chair instantly

Once trapped:

The wheelchair may stop suddenly
The user may be unable to move forward or back
Assistance is often required

👉 A gap that looks “small” can be a complete barrier

2. Edges and Lips

Small vertical changes cause problems:

Path edges
Playground borders
Surface joins

Castors hit the edge and:

Stop abruptly
Pivot sideways
Trap the user

👉 A castor can typically only climb an obstacle about half its wheel height

3. Uneven Surfaces

Uneven ground increases risk dramatically.

Castors can lose alignment
One wheel may drop into a depression
Direction changes increase trapping risk

👉 Turning or adjusting direction at the wrong moment increases failure

4. Swivel Movement (The Critical Problem)

Castors are always moving and adjusting.

When changing direction, they rotate (swivel)
If this happens at the wrong time:
The wheel can turn sideways
It can fall into a gap or catch on an edge

👉 This is why diagonal approaches are dangerous

5. “Ploughing Effect” (Powerchairs)

In many powerchairs:

The small wheels are pushed forward
They are forced into obstacles

This creates a:

👉 Ploughing effect — pushing into the ground or edge instead of rolling over it

6. Small Diameter = Big Problem

Smaller wheels:

Fall into gaps more easily
Struggle to roll over obstacles
Catch on cracks and debris

Larger wheels:

✔ Handle uneven surfaces better
✔ Reduce trapping risk

👉 Small wheels increase vulnerability

Real-World Example

A wheelchair user approaches:

A path with a small lip
Or a bark-to-concrete transition

What happens:

Front castor hits the edge
Wheel stops or turns sideways
Chair becomes stuck

The user must:

Reverse
Reposition
Try again

👉 Or ask for help

Why This Matters in Playgrounds

Playgrounds are full of:

Surface transitions
Edges and borders
Loose materials
Decorative features

These create:

❌ Constant trap points
❌ Loss of independence
❌ Safety risks

👉 What looks “minor” to a designer becomes a major barrier

Critical Design Mistakes

1. Ignoring Small Level Changes

Even 10–20 mm lips can stop movement

2. Mixing Surface Types

Hard surface to bark transitions
Creates instant trap zones

3. Designing Decorative Edges

Raised borders around play areas
Act as barriers to entry

4. Assuming Bigger Wheels Solve It

Powerchairs still rely on small castors
The problem remains

The Real Impact on Users

When a castor gets trapped:

Movement stops immediately
Users can feel unsafe or unstable
There is a risk of tipping forward
Independence is lost

In some cases:

👉 A trapped wheelchair is very difficult to free without assistance

Designing to Prevent Trapping

Key Principles

✔ Eliminate gaps wherever possible
✔ Keep level changes flush (0 mm preferred)
✔ Avoid lips and edges
✔ Use continuous, smooth surfaces
✔ Minimise transitions between materials

Better Design Looks Like:

Continuous hardstand surfaces
Flush joins between materials
No raised edging
Wide, forgiving pathways

👉 The goal is smooth, uninterrupted movement

The Bigger Message

Accessibility is often tested with:

“Can a wheelchair get through?”

But the real question is:

“Can the wheels keep moving?”

Because:

👉 Accessibility fails at the point of contact with the ground

Key Takeaway

Small wheels create big problems.

✔ They catch
✔ They drop
✔ They trap

👉 If your design includes gaps, lips, or uneven surfaces — it will fail

Call to Action

Designers, councils, and playground providers must:

Understand how wheelchairs actually interact with surfaces
Remove small but critical barriers
Design for continuous movement

Because:

If the wheels get trapped, the person is trapped.