Calculate spring rate in 3 bump scenarios?

[olechampion]

Hi, Working on my vehicle and have a theoretical question I'd like a 2nd opinion on. The physics question is how much influence the force of a bump has in conjunction with a perfectly tuned spring rate can have on ride quality (as crudely defined in vertical acceleration z force m/s squared). I.e. how important is spring rate in ride quality? To look at this problem I have outlined three bump scenarios each with different heights.

The objective is to keep vertical acceleration constant at .5 m/s squared

Solving For: Spring Rate

Three scenarios:
bump height: 0.25 inch
bump height: 1 inch
bump height: 2 inchIn all scenarios the bump width is the entire road (ie like going onto a new road or bridge)Constants
vertical acceleration m/s squared: .5

vehicle speed: 60 mph
Tire Stiffness:
Tire Diameter: 32.2 inches
Tire Overall Width: 9.3 inches
Tire Loaded Radius: 15.2 inches
Coefficient of Rolling Resistance: 0.012 (10.00 -20 BIAS)
Coefficient of Rolling Resistance: 0.008 (10.00 -20 RADIAL)
Cornering Coefficient (deg -1): 0.1370
Camber stiffness: 40 lb / deg
Vertical stiffness: 4935 lb / in
Rolling Circumference: 89.33071 in

Front Axle Weight: 6900 pounds
Rear Axle Weight: 13500 pounds

Suspension Travel: 4 inches
Length of shock extended: 18 inches
Length of shock compressed: 14 inches

Wheelbase: 241 inches
Dual Tires rear axle

Side question, any decent suspension calculators to estimate problems like this? Would be great to do a sensitivity analysis.

 

[Navin A S]

Answer:The spring rate is directly related to the ride quality, so it is very important. In general, a higher spring rate will result in a stiffer ride, so you would want to choose a spring rate that provides a comfortable ride while still providing adequate support for bumps.In order to calculate the spring rate for the three scenarios, we need to first calculate the vertical load on each axle. This can be done by taking the total vehicle weight and dividing it by two (since there are two axles). The vertical load for the front axle would then be 6900 / 2 = 3450 lbs, and the vertical load for the rear axle would be 13500 / 2 = 6750 lbs. We can then use the vertical stiffness of the tires to calculate the spring rate needed for each scenario. For the 0.25 inch bump height, the spring rate should be (3450 + 6750) / 0.25 inches = 26800 lbs/in. For the 1 inch bump height, the spring rate should be (3450 + 6750) / 1 inch = 10700 lbs/in, and for the 2 inch bump height, the spring rate should be (3450 + 6750) / 2 inches = 5350 lbs/in.As far as suspension calculators, there are several available online. Some of the more popular ones include RideTech Suspension Calculator, FEA Suspension Calculator, and ChassisSim Suspension Calculator. These calculators can help you do a sensitivity analysis to determine how different spring rates will affect ride quality.