Vertical Stiffness Test

What Is Vertical or Radial Stiffness?

Vertical stiffness is how well a rim resists flexing and deforming vertically when a given load is applied. Typically, as the depth of a rim increases, the vertical stiffness is boosted. This rule applies to a rim made of any material.

Stiffer rims are more efficient since less power disperses as the wheels flex; however, shallower and typically less stiff rims are more comfortable to ride on, especially on rough surfaces for long hours.

Testing Goal

The rim is fixed vertically in the rig and increasing load is applied up to a preset limit. The amount of deformation is recorded automatically while the force is being applied. The aim of this vertical deformation test is to assess the behavior of carbon rims under a force pushing down from the weight of the rider and gear, and a force from the ground.

Test Standard

Deform at 50kgf

sample profile

Size Model External Width Depth Load Max Weight Rim Condition
700C WR35 32mm 35mm 50kgf 430g New rim that has passed flatness and roundness tests

Test Facility

Vertical stiffness is tested on the Universal Testing Machine (UTM). The machine has a rigid base with a clamping device to hold the rim firmly. There is a compression fixture above the rim to apply increasing force, and all the rim deformation data is recorded automatically by the sensor above the compression cylinder. The sensor data is synced to the computer and results are displayed when the vertical stiffness test is finished.

Test Conditions

Compression Item Stainless steel cylinder
Moving Speed 0.4mm/s
Moving Direction Vertical
Test Positions #1 Valve hole
#2 Perpendicular to the valve hole
Controller Remote Control

Step 1: Mark two test spots, not centrosymmetric to each other, on the rim;

Step 2: Clamp the rim rigidly on the rig;

Step 3: With the remote control, lower the compression item until it touches the edge of the rim at #1 test position;

Step 4: The compression fixture moves vertically at a controlled speed of .4mm/s to gradually increase the load to a given limit (50kgf in this test);

Step 5: Lift the compression fixture a bit and resume the touching-the-edge state;

Step 6: Rotate the rim to #2 test spot, and repeat the process;

Step 7: Export the two data charts from the computer.


test results

#1 Testing Position

carbon-rim-vertical-stiffness-test-data.jpeg MO-vertical-flex-charts.jpeg
No. Max Load Max Deflection
1 50kgf 1.2mm

#2 Testing Position

carbon-road-rim-deformation-under-vertical-load.jpeg MO-vertical-deform-test-chart.jpeg
No. Max Load Max Deflection
2 50kgf 1.41mm

These results are valid only for the tested sample.


WR35 (25mm internal, 32mm external widths, 35mm depth) flexs vertically 1.2mm to 1.41mm under a load of 50kgf.

© All photos and test data by Light Bicycle

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