Bump Velocity


Suspension Bump VelocityFinally computer software to tune a shim stack Suspension Velocity In Terms Of Bump Height And Bike SpeedWhen a wheel rolls over a square edge bump the hub follows an arc path. That arc path is defined by the wheel radius and bump height. The geometric relationships describing that arc path give the suspension velocity as a function of bump height using the simplifying assumption of a ridged wheel.
Suspension velocity is the highest at the start of a bump where the arc path is steep. Velocities asymptotically roll over as the wheel approaches the top of the bump. The figure below shows both the instantaneous and average suspension velocity as a wheel rolls over a bump.
When the bump height is on the order of the hub height suspension velocities become extremely high due to the near vertical hub path. If cavitation is going to occur in a shock, it will occur at the start of the bump where the suspension velocities are highest and the gas reservoir pressures are low. 

Average Suspension Velocity Over A BumpThe "average" suspension velocity over a bump makes things a little simpler. Average velocity is determined by the bump height and distance traveled:
The ridged wheel assumption can be partially relaxed by assuming perfect tire compliance and subtracting 1.5 inches, or whatever, from the bump height. That brackets the range of suspension velocity from perfect compliance to the no compliance ridged wheel case. 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 the suspension would blow through its stroke in about 0.2 seconds.
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. Suspension velocities vs bump height for different wheel diameters are on the ReStackor web site: www.ShimReStackor.com/suspensionvelocity.htm


Whoop VelocitiesWhoop spacing and heights vary dramatically. For purposes of bounding 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 give a fork suspension velocity of 150 in/sec for a 16 inch whoop hit at 30 mph. Assuming a 3:1 link ratio the shaft speed on the shock would be around 50 in/sec.
If you need a better estimate high speed movies are an excellent method for estimating suspension travel and wheel velocities in the whoops. Videos also provide some good information on the interaction of the fork and shock while the bike is in the whoops.


Rebound Stroke Maximum Suspension VelocityMaximum velocities in the rebound stroke occur when the wheel is unweighted. Rebound velocities can be estimated 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. Stroke depth sets rebound peak velocity. The calculations below start with the suspension fully compressed. As the suspension extends through the rebound stroke spring force reduces while damping force increases as the wheel accelerates to higher velocities. As the suspension approaches velocities of 150 to 200 in/sec the decrease in spring force as the suspension approaches full extension causes the wheel to decelerate.
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 during compression and the suspension to dissipate that energy in rebound. Tuning rebound to operate just short of packing produces the maximum energy dissipation stabilizing the bike. Higher fork oil levels or stiffer springs store more energy and require stiffer rebound damping to dissipate the higher stored energy.
