Front Suspension & Steering:Wheel & Shock Travel, Ride Height & Motio Ratio

Wheel Travel is the vertical (up/down) distance the wheel can travel.
In almost all off-road applications it is the best to maximize the wheel travel.

To get this value from an existing vehicle:
Support the vehicle weight with a lift or jack so that the vehicle is not suspended on the suspension system.

Remove shock and spring (if any).

Bring the wheel to the lowest point (also called full-droop) allowed by the suspension linkage system and measure from the floor (or any other horizontal reference plane) to the wheel center and note the distance.

Bring the wheel to the highest point (also called full-jounce or full-bump) allowed by the suspension linkage system and measure from the floor (or any other horizontal reference plane) to the wheel center and note the distance.

Make sure that no suspension link binds with wheels turned to full left or full right. Also check that the desired tire clearance is maintained check all other components (brake components, brake lines, wires, sensors, drive-axles, drive-shafts, CV joints, U-joints and etc.) to make sure they can function properly and that there are no clearance issues.

The difference between the two noted distances is the actual wheel travel allowed.

Shock Travel is the change in shock length through out the wheel travel.
It is best to maximize the shock travel; this reduces the load on the shock absorber.

To get this value from an existing vehicle:
Support the vehicle weight with a lift or a jack so that the vehicle is not suspended on the suspension system.

Remove shock, spring and swaybar (if any). If leaf sprung, do not remove all leaf springs; leave at least one to locate the suspension components.

Bring the wheel to the lowest point (also called full-droop or full extension) allowed by the suspension linkage system and measure from the lower shock mount to upper shock mount and note the distance as Extended Shock Length.

Bring the wheel to the highest point (also called full-jounce or full-bump) allowed by the suspension linkage system and measure from the lower shock mount to upper shock mount and note the distance as Compressed Shock Length.

Make sure that no suspension link binds with wheels turned to full left or full right through out the entire wheel travel. Also check that the desired tire clearance is maintained check all other components (brake components, brake lines, wires, sensors, drive-axles, drive-shafts, CV joints, U-joints and etc.) to make sure they can function properly and that there are no clearance issues.

The difference between the two noted distances (Extended Shock Length and Compressed Shock Length) is the actual shock travel needed.

Ride Height is the height of the vehicle standing still on level ground.
Ride height for high speed desert applications usually vary from 30% to 50%. Ride height for slow speed rock crawler applications usually vary from 50% to 70%.

Ride Height (RH) is expressed as percentage of total wheel travel that equals to droop travel. For example if you have 10 inches of travel and you want your ride height to be such that shock compresses 6 inches and extends 4 inches from stand still, then your Ride Height would be 40%. This also means the shock will compress 40% of its travel to support the weight of the vehicle.

Motion Ratio is the ratio of shock travel versus wheel travel. It is equal to the Shock Travel (in inches) per 1 inch of Wheel Travel.
Motion Ratio is used to account for the different suspension geometries and the resultant mechanical leverage on the spring. In almost all cases, the Motion Ratio is less than 1. In general, if there is a choice, aim for the highest ratio permitted by the wheel travel desired, shock travel available and suspension geometry. This will give you the lowest spring rate possible. Coil springs with high rates are generally not available, have less coil travel, and are heavier.

You can calculate the motion ratio by simply dividing the shock travel by the wheel travel. Example if the shock travels 0.5 inches and the wheel travels 1 inches then the motion ratio would be 0.5/1 = 0.5

For most suspension systems motion ratio varies though out the wheel travel. It is important that you use the motion ratio value at the desired ride height for the accuracy of the spring rate and ride height calculations

It is important that you measure the geometry related dimensions while the vehicle is at ride height for accuracy of the spring rate calculations.
Created by , 03-26-2013 at 09:06 PM
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