A larger wheel reduces the ideal force needed to climb a given square-edged threshold, but wheel diameter alone cannot establish whether a loaded cart will cross it. On a ramp, larger wheels may reduce rolling resistance; they do not remove the force needed to move the cart’s weight uphill.
Use the tables below for screening: compare wheel sizes against measured steps, calculate the gravity component on ramps, and identify which caster dimensions and operating conditions need confirmation. The threshold values are calculated from an ideal rigid-wheel model—not manufacturer obstacle ratings or tested crossing limits.
Measure the Route Before Comparing Wheel Sizes
Separate abrupt steps, open joints, and continuous slopes. They create different problems and require different measurements.
| Route feature | Measurements to record | What the measurements establish |
|---|---|---|
| Raised threshold or plate lip | Vertical step height, edge profile, crossing direction, and surface condition | Whether the square-step model below is a useful approximation |
| Beveled threshold | Total rise, horizontal bevel run, and any abrupt lip at either end | The incline angle and any separate step that the wheel must climb |
| Open floor joint | Gap width, depth, edge condition, and elevation difference between sides | Whether the wheel may drop into the joint or encounter a raised exit edge |
| Ramp | Vertical rise, horizontal run, surface condition, and travel direction | Ramp grade and the gravity force along the slope |
| Ramp entry or crest | Change in slope, local lips, and available underbody clearance | Potential wheel, brake, frame, or deck interference |
Measure the largest relevant step along the actual wheel paths, not an average across the doorway. Check damaged edges and uneven plate seating separately.
Record the approach direction in plan view: 0° means traveling perpendicular to the threshold edge. Also record whether the swivel wheels are aligned with travel. A diagonal approach can change when individual wheels contact the edge, but it does not reduce the threshold’s vertical height.
Do not enter joint width as though it were step height. A gap requires separate consideration of wheel support, drop into the opening, and the geometry of the far edge.
Wheel Diameter Table for Square-Edged Thresholds
The table compares the horizontal axle force needed to initiate climbing over a square step. Each value is a force multiplier:
[
k=\frac{F}{Q}
]
Here, (F) is the horizontal force applied at the wheel axle, and (Q) is the downward load transmitted through that axle at the onset of climbing. Both are forces, expressed in lbf or N.
For example, a multiplier of 0.436 means an ideal axle force of approximately 43.6 lbf for every 100 lbf of downward wheel load.
Conditions Behind the Table
The calculation assumes:
- A rigid, circular, freely rotating wheel and a rigid, sharp-edged step.
- Straight-on contact, with the wheel aligned perpendicular to the edge.
- Slow application of horizontal force at axle height, without impact or a lifting component.
- A known downward axle load at the instant climbing begins.
- No bearing friction, tread deformation, edge crushing, or additional rolling resistance.
Tread width does not appear in this two-dimensional model. That does not make width irrelevant: a narrow wheel contacting a damaged edge, or a tread only partly supported by a threshold, needs a separate assessment.
| Wheel diameter, in | Converted diameter, mm | Wheel radius, in | (F/Q), 1/8-in step | (F/Q), 1/4-in step | (F/Q), 1/2-in step | (F/Q), 1-in step |
|---|---|---|---|---|---|---|
| 3 | 76.2 | 1.5 | 0.436 | 0.663 | 1.118 | 2.828 |
| 4 | 101.6 | 2.0 | 0.371 | 0.553 | 0.882 | 1.732 |
| 5 | 127.0 | 2.5 | 0.329 | 0.484 | 0.750 | 1.333 |
| 6 | 152.4 | 3.0 | 0.298 | 0.436 | 0.663 | 1.118 |
| 8 | 203.2 | 4.0 | 0.256 | 0.371 | 0.553 | 0.882 |
| 10 | 254.0 | 5.0 | 0.228 | 0.329 | 0.484 | 0.750 |
| 12 | 304.8 | 6.0 | 0.207 | 0.298 | 0.436 | 0.663 |
These are calculated values, rounded to three decimal places. Metric diameters are conversions of the listed inch sizes using 25.4 mm/in, not separate catalog sizes. The step heights are 3.175, 6.35, 12.7, and 25.4 mm, respectively.
A value below 1.000 is not a pass criterion, and a value above 1.000 is not a universal failure criterion. The multiplier only expresses the ideal axle force relative to the downward axle load.
Formula and Geometric Limit
Let:
- (D) = wheel diameter.
- (r=D/2) = wheel radius.
- (h) = vertical step height.
- (Q) = downward axle load.
- (F) = horizontal axle force at incipient climbing.
Use the same length unit for (r) and (h). At initial contact, the horizontal distance from the step corner to the axle is:
[
x=\sqrt{r^2-(r-h)^2}=\sqrt{2rh-h^2}
]
Balancing moments about the step corner gives:
[
F(r-h)=Qx
]
Therefore:
[
F=Q\frac{\sqrt{2rh-h^2}}{r-h}
\qquad \text{for }0<h<r
]
The square-root term and denominator both have units of length, so their ratio is dimensionless. Entering (Q) in lbf returns (F) in lbf; entering (Q) in N returns (F) in N.
As (h) approaches (r), the horizontal force lever arm approaches zero and the calculated force increases without bound. At or above axle height, this horizontal-axle-force model does not provide a climbing solution. A ramp, lifting action, driven-wheel mechanism, or different contact geometry requires another model.
At (h=0), the step-climbing component is zero. Actual force to start and roll the cart is not.
Example: A 6-Inch Wheel at a 1/4-Inch Threshold
Assume a 6-in rigid wheel carries a downward axle load of 200 lbf as it contacts a 0.25-in square step. The wheel approaches straight on, and force acts horizontally through the axle.
[
r=3.00\text{ in}
]
[
F=
200\frac{\sqrt{2(3.00)(0.25)-(0.25)^2}}{3.00-0.25}
=87.2\text{ lbf}
]
For an 8-in wheel under the same assumed load and step conditions:
[
F=
200\frac{\sqrt{2(4.00)(0.25)-(0.25)^2}}{4.00-0.25}
=74.2\text{ lbf}
]
The larger wheel lowers this calculated component by about 13 lbf. It does not establish the required handle force or prove that either complete caster can withstand repeated crossings.
If two wheels meet the edge simultaneously and each actually carries 200 lbf, their ideal step-climbing components sum to about 174 lbf for the 6-in wheels. Do not multiply by every caster on the cart unless those wheels contact the step together under the assumed loads.
Turn the Threshold Calculation Into a Route Decision
The missing input is often the wheel load during crossing—not the wheel diameter.
Dividing gross cart weight by the installed caster count is only an equal-load assumption. A raised edge, off-center payload, frame flexibility, or handle force can change individual wheel reactions. Use a justified load distribution or suitable wheel-load measurements; do not assume a diagonal crossing simply halves the required effort.
Interpret the calculation as follows:
- If the ideal step component already exceeds the available controlled effort, the calculation does not support that candidate under the stated conditions.
- If it falls below the available effort, the crossing remains unresolved until rolling resistance, steering, load transfer, tread behavior, and complete-caster suitability are checked.
- If crossing requires striking the threshold at speed, the slow-force model no longer represents the event. Impact loads require separate evaluation.
Blickle’s selection guidance identifies tread, bearings, wheel diameter, load, and surface condition as factors affecting starting, rolling, and swivel resistance. Diameter is therefore one comparison variable, not a complete performance specification.
For a site-specific evaluation, establish a controlled procedure before moving a loaded cart. Record gross load and payload position, approach direction, wheel alignment, and measured force. Keep threshold peaks separate from sustained rolling force; neither a single peak nor a single sustained-force value establishes ergonomic acceptability.
Ramp Grade Table: The Force Larger Wheels Cannot Remove
A continuous ramp is not a square step. Calculate its grade from vertical rise and horizontal run:
[
G=100\frac{H}{L}
]
[
\theta=\arctan\left(\frac{H}{L}\right)
]
Where:
- (G) = grade in percent.
- (H) = vertical rise.
- (L) = horizontal run, not the sloped travel length.
- (\theta) = ramp angle.
For a cart with gross weight (W), the component of gravity acting down the ramp is:
[
F_g=W\sin\theta
]
The following values assume a gross cart weight of 1,000 lbf. They are calculated gravity components only, rounded to 0.1 lbf.
| Ramp grade | Ramp angle | Gravity force along ramp per 1,000 lbf gross weight |
|---|---|---|
| 2% | 1.15° | 20.0 lbf |
| 5% | 2.86° | 49.9 lbf |
| 8% | 4.57° | 79.7 lbf |
| 10% | 5.71° | 99.5 lbf |
| 12% | 6.84° | 119.1 lbf |
These grades are comparison points, not recommended operating limits. A 5% grade is approximately 2.86°, not 5°.
At steady uphill speed, with the applied force parallel to the ramp:
[
F_{\text{uphill}}=W\sin\theta+F_{\text{rolling}}
]
This excludes acceleration, steering, and abrupt entry lips. If the applied force is not parallel to the ramp, its direction must be included in the force balance.
For a 1,000-lbf cart on a 5% ramp, approximately 49.9 lbf is needed just to balance gravity. Larger wheels cannot reduce that term. Any rolling resistance adds to the uphill effort.
If rolling resistance is represented by a dimensionless coefficient (C_{rr}), its simplified contribution is (C_{rr}W\cos\theta). Use that expression only when the coefficient is defined that way and applies to the wheel, load, and surface being evaluated. A rolling-resistance parameter expressed in inches or millimeters is not interchangeable with a dimensionless coefficient.
Downhill travel requires a separate control assessment. The table gives the downslope gravity component, not a brake rating, stopping-distance prediction, or acceptable towing limit.
Compare Wheel Diameter With the Complete Caster and Route
There is no universal overall mounting height for a given wheel diameter, nor a universal ramp grade assigned to that diameter. Use the selected caster’s dimensional drawing and ratings to complete the comparison.
| Selection item | What to compare | What diameter alone cannot establish |
|---|---|---|
| Abrupt threshold | Measured step height, calculated multiplier, axle load, edge profile, and approach direction | Actual crossing force, impact loading, or repeated-crossing durability |
| Smooth ramp | Grade, gross cart weight, rolling resistance, and available drive or manual effort | Grade-related gravity force reduction |
| Rough or damaged surface | Wheel and tread specifications, route condition, and applicable operating data | A guaranteed percentage reduction in effort |
| Overall mounting height | Mounting surface-to-floor dimension for the exact caster configuration | Installed equipment or deck height |
| Swivel and brake clearance | Offset, full swivel envelope, brake position, and frame clearance | Freedom from interference |
| Wheel-only replacement | Diameter, width, hub length, bearing bore, axle, spacers, and fork clearance | Compatibility from matching diameter alone |
| Complete-caster replacement | Mounting plate and bolt-hole pattern, or stem dimensions and engagement | Compatibility from matching plate size or load rating alone |
| Loaded ramp entry or crest | Wheelbase, overhang, underbody clearance, payload position, and slope transition | Freedom from grounding or acceptable stability |
A larger caster may raise the equipment and its center of gravity. Check the complete installed arrangement, especially on ramps and at slope transitions.
For every shortlisted assembly, retain the manufacturer, exact part number, tread and bearing configuration, drawing or catalog revision, and applicable rating conditions. A wheel rating does not automatically establish the rating of the fork, swivel bearing, mounting hardware, or complete assembly.
Use the Calculators Without Treating Their Verdicts as Ratings
The Caster Wheel Diameter Selector provides a preliminary size suggestion from obstacle, per-wheel load, and height inputs. Compare that suggestion with the threshold table, then verify the actual caster’s load rating and mounting dimensions. A nominal diameter cannot by itself prove capacity or installed-height fit.
The Caster Threshold and Curb Crossing Calculator provides threshold-related force outputs. Use measured vertical step height—not open-joint width—and check how its cart-weight and caster-count inputs represent the wheels actually contacting the obstacle. Its published limitations identify a rigid-wheel model without tread deformation or approach-speed impact, and an assumption that all casters meet the obstacle simultaneously. Do not interpret its verdict as confirmation of a diagonal crossing, a gap crossing, or complete-cart suitability.
For a threshold, the useful comparison is wheel geometry plus the load acting at contact. For a ramp, it is gravity plus rolling resistance and control capability. Confirm the selected caster against both parts of the route rather than assigning a single “maximum obstacle” or “maximum grade” to its diameter.