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Suspension Bump Velocity

Finally computer software to tune a shim stack

Peak Suspension Velocities Over a Bump

The hub of a wheel follows an arc like path over a square edge bump. The arc path results in  the highest suspension velocity occurring at the start of the bump. 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 path of the hub creates the highest accelerations and highest 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. These high accelerations result in tire deflections which are not accounted for in the rigid wheel assumption of the above derivation. However, the above plot makes the point that suspension velocities 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 velocities on a bump that is on the order of the hub height result in extremely high velocities. 

The points on the plot above indicate the average suspension velocity over the bump derived by the equations below. 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 suspension 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 of a wheel:

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

An example: A three inch bump hit at 10 mph produces a suspension velocity of 60 inches/sec. If you have 12 inches of suspension travel that 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 speeds, the right hand figure, a 3 inch bump at 50 mph results in a suspension velocity of 300 inches/sec. This would blow through a 12 inch stroke in about 0.04 seconds. Suspension bump velocities are substantially higher than the suspension movements produced rolling over small bumps at low speed or the motions occurring when throwing your bike down into a corner. These differences in velocity and the differences between high speed and low speed suspension motions.

Suspension velocities vs bump height for different wheel diameters are on the ReStackor web site: www.ShimReStackor.com/suspension-velocity.htm

Whoop Velocities

Whoop spacing and heights vary dramatically. For purposes of attempting to bound the range of suspension velocities the following assumptions are made: Whoop spacing of 15 feet, bike speed of 30 mph and suspension compression over the last 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 estimates above provide a ball park estimate of whoop suspension velocity. The specifics of the bike you are trying to tune for depend on the shock linkage ratio, bike speed and whoop spacing.

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

Rebound Stroke Maximum 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 damping force in the rebound stroke against the accelerating mass of the wheel. 

Peak rebound velocities are a function of stroke depth. The calculations below start with the suspension fully compressed. As the suspension extends through the rebound stroke spring forces are reduced while damping forces increase due to the accelerating wheel velocity. At suspension velocities of 150 to 200 in/sec the decrease in spring force as the suspension approaches full extension causes the wheel decelerate.

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 the suspension to dissipate that energy in the rebound stroke. Tuning rebound damping to operate just short of packing produces the maximum energy dissipation without disrupting operation of the suspension, i.e. this keeps the wheels on the ground. Higher fork oil levels or stiffer springs store more energy and require stiffer rebound damping to dissipate the higher stored energy.