Suspension Bump Velocity

Finally computer software to tune a shim stack

Peak Suspension Velocities Over a Bump

When a wheel hits a square edge step the hub follows an arc like path over the step. The arc path results in the highest suspension velocity occuring at the start of the bump. The geometric relationships describing the hub velocity are worked out below:

The suspension velocity over a bump can be computed based on the tire diameter, bike speed and bump height.

The arc hub path creates high accelerations and high suspension velocities at the start of a bump. If cavitation is going to occur in a mid-valve it is going to occur at the start of the bump where the reservoir is at the lowest pressure and the suspension velocities are the highest.

The highest suspension velocities occur at the start of a bump.

For a one inch bump hit at 70 mph the suspension velocity is over 400 in/sec at the start of the bump. The accelerations caused by this velocity results in tire deflections which are not accounted for in the rigid wheel assumption of the above derivation. However, the plot makes the point that suspension velocities over a bump are not a simple constant velocity affair. The vertical velocity of the wheel hub is constantly changing along the arc path of the hub. It is also important to understand that when a wheel hits a bump on the order of the hub height extremely high velocities are generated.

The above plot also shows the average suspension velocities, derived below, as points on each bump height curve. The average velocities are a fair approximation of the suspension speeds over a bump and are typically used as an estimate of suspension velocities for design purposes.

Average Suspension Velocity Over A Bump

For a wheel hitting a square step-up the average suspension velocity, as a function of step height and bike speed, can be estimated from the basic geometry:

The average suspension velocity is computed from the bump height and bike speed.

A couple of examples: For a bike moving at 10 mph hitting a 3 inch bump the suspension velocity is approximately 60 inches/sec. If you have 12 inches of suspension travel this would cause your suspension to blow through its stroke in about 0.2 seconds.

Average suspension velocities for a 21" dirt knobby as a function of bike speed and bump height

At higher bike speeds, the right hand figure, hitting a 3 inch bump at 50 mph creates a suspension velocity of approximately 300 inches/sec. This would blow through a 12 inch stroke in 0.04 seconds. The suspension velocities produced by hitting a modest three inch bump at speed are substantially higher than the suspension velocities produced rolling over small bumps or the suspension motions occurring when throwing your bike down into a corner. This is the difference between high speed bump velocities and low speed suspension motions.

Whoop Velocities

Whoop spacing and heights vary dramatically. For purposes of attempting to bound the suspension velocity range occurring in the whoops the following assumptions are made: Whoop spacing of 15 feet, bike speed of 30 mph and suspension compression over 1/3 of the whoop spacing.

Those assumptions results in suspension shaft velocities of approximately 50 in/sec for a shock and 150 in/sec for a fork on a whoop height of 16 inches at 30 mph.

Higher shaft velocities would obviously occur at faster bike speeds or on larger whoops. The equations shown above allow you to estimate velocities for the conditions you are trying to tune for and the specifics of the linkage ratio of the shock you are working with.

If you need a better number high speed moves are an excellent method for estimating suspension travel and velocities for a bike in the whoops and can provide some good information on the interaction of the fork and shock in the whoops.

Rebound Stroke Suspension Velocity

Maximum velocities in the rebound stroke occur when the wheel is un-weighted. Those velocities can be determined directly from F= ma by balancing the spring force, air spring force and rebound stroke damping force against the accelerating mass of the wheel.

Peak rebound velocities are a function of spring rate, damping force and the suspension stroke depth. The calculations below start with the suspension fully bottomed. As the suspension extends through the rebound stroke spring forces are reduced while damping forces increase with the accelerating wheel velocity. At suspension velocities of 150 to 200 in/sec the damping force is higher than the spring force and the wheel decelerates as it approaches full extension.

Peak rebound velocities range from 150 to 200 in/sec for a typical MX suspension setup..

Damping rates in the rebound stroke are typically tuned to operate just short of suspension packing. This allows the spring to absorb the bump energy on the compression stroke and dissipate that energy on the extension stroke. Tuning rebound damping rates to operate just short of suspension packing allows the maximum possible energy to be dissipated without disrupting the suspension operation. Higher fork oil levels or stiffer springs store more energy and require stiffer rebound damping to dissipate the increased stored energy.

Dirt Knobbies

Average suspension bump velocities for dirt knobby tires based on diameters from the Dunlop web site.

Dirt front
- 21" Front wheel: Dunlop D952 80/100-21
- 19" Front wheel: Dunlop D739 70/100-19
Dirt rear
- 17" Rear wheel: Dunlop D606 130/90-17
- 18" Rear wheel: Dunlop D952 120/90-18
- 19" Rear wheel: Dunlop D952 120/90-19

Street Tires

Average suspension bump velocities for street tires based on diameters from the Dunlop web site.

Street front
- 16" Front wheel: Dunlop SportMax 130/70r-16
- 17" Front wheel: Dunlop SportMax 120/70r-17
- 18" Front wheel: Dunlop SportMax 120/70r-18
Street rear
- 17" Rear wheel: Dunlop SportMax 170/60r-17
- 18" Rear wheel: Dunlop Sportmax 170/60r-18

Mountainbike Tires

Suspension velocity for Mountainbike tires
- 26" wheel
- 29er