TECHNICAL GUIDE / 03

How to Determine the Hub Center Height of a Dummy Wheel

What is Hub Center Height?

For regular tires and wheels, the height from the ground to the center of the hub is approximately half of the tire's outer diameter.

With dummy wheels, the vehicle is supported by the main body and legs instead of tires, so the hub center height must be determined during the product design phase.

At 813 Alignment Tools, specifications are primarily determined based on the following information:

• Tire size to be used
• Whether front and rear tire sizes differ
• Whether to change the height from the standard tire outer diameter
• Whether it will be shared across multiple vehicle models

The hub center height is not a dimension that can be easily changed after the product is completed. It is necessary to clarify the usage conditions before ordering.

Using Tire Outer Diameter as a Reference

If you want the height to be equivalent to when tires are installed, check the outer diameter of the tires to be used and set half of that as the reference for the hub center height.

The outer diameter for each tire size can be found in 813 Alignment Tools' "Tire Outer Diameter Quick Reference Chart".

If the front and rear tire sizes differ, calculate them separately for the front and rear and reflect that difference in the product specifications.

Prioritizing Calculated Values over Measured Values

It is also possible to measure the distance from the ground to the center of the hub with the vehicle on the ground.

However, actual measured values are influenced by the following conditions:

• Tire pressure
• Tire wear
• Vehicle weight
• Tire deflection
• Floor incline
• Measurement error itself

A significant difference between calculated and measured values is uncommon. Therefore, when determining specifications, it is easier to manage by using calculated values derived from tire sizes as a common standard, rather than measured values that can vary depending on conditions.

Especially for vehicles with different front and rear tire diameters, it is crucial to calculate the hub center height from each front and rear tire size and reflect that difference in the product specifications.

When measuring corner weights, even a few millimeters of change in the vehicle's support height can significantly alter the load balance across the front, rear, left, and right. If using measured values, it is essential to thoroughly check the measurement conditions and measurement error.

When Front and Rear Tire Diameters Differ

For vehicles with different front and rear tire diameters, the hub center height is set according to the respective front and rear tire diameters.

If the front and rear tires have different outer diameters, two dummy wheels for the front and two for the rear will have different hub center heights.

If all four are set to a common height, the height difference between the front and rear that exists when tires are installed cannot be replicated. For setting up an environment for alignment and corner weight measurement, specifications that reflect the front and rear tire diameter differences are appropriate.

This is because we consider different front and rear diameters not as a special feature or add-on, but as a necessary specification to accurately reproduce vehicle conditions as much as possible. Our standard approach reflects our desire to popularize tools that can measure alignment as accurately as possible.

Setting Lower than the Reference Value

It is also possible to set the hub center height lower than half of the tire's outer diameter.

Setting it lower makes the dummy wheel body shorter and lighter. This also improves ease of attachment/detachment from the vehicle, transport to the circuit, and handling during storage.

On the other hand, if it is too short, after jacking down the vehicle onto the surface plate, depending on the vehicle model or jack type, the jack might not be able to be removed from under the vehicle.

When setting it lower, it is necessary to check not only lightness and handling but also the clearance between the jack to be used and the underside of the vehicle.

Increasing Hub Center Height

Increasing the hub center height slightly improves access to the suspension after jacking down.

However, to install the dummy wheels and lower the vehicle onto the surface plate, the vehicle needs to be lifted to a higher position.

With typical garage jacks, the greater the lift height, the more the vehicle moves front-to-back during jacking. This can make it difficult to align the dummy wheels with the surface plate.

It's not a matter of whether longer or shorter is inherently better; it needs to be decided based on the equipment used and the work procedures.

Comparison of Short and Long Specifications

Adjusting Vehicle Stance with Surface Plate Height

In addition to the dummy wheel's hub center height, there is also a method to adjust the vehicle's stance by changing the height of the front and rear surface plates.

For example, if a height difference is set between the front and rear surface plates, the relationship between the front and rear hub positions can be adjusted without changing the dummy wheel body itself.

This method allows for recreating vehicle stances that assume different tire sizes or staggered tire setups by adjusting the surface plate.

The actual hub position is determined by two factors:

• Dummy wheel's hub center height
• Surface plate installation height

By combining the specifications of the dummy wheel itself with adjustments to a leveling surface plate, it becomes easier to accommodate multiple vehicle conditions.

When changing the height of the surface plate, it is necessary to measure the height and inclination of all four wheels to confirm that the intended vehicle stance has been achieved.

When Sharing Across Multiple Vehicle Models

When sharing dummy wheels across multiple vehicle models, check not only the PCD and number of holes but also the tire sizes used for each vehicle model.

Since PCD, number of holes, bolt diameter, and tire outer diameter differ for each vehicle model, individual confirmation is necessary to determine if a single specification can be shared.

If you have vehicle models you wish to share dummy wheels with, please inform us of the vehicle models and tire sizes before ordering. We will provide guidance on how to accommodate your needs, including dummy wheel body specifications and, if necessary, surface plate height adjustments.

Information to Confirm Before Ordering

To determine the hub center height, please provide the following information before ordering:

• Vehicle model
• Front and rear tire sizes
• Vehicle models to be shared (if any)
• Amount of change if you wish to lower it from the standard value