Up ]


Free Suspension Tuning Spreadsheet

Suspension Spreadsheet on Google Docs

        Or download: Suspension.xls or Suspension.ods (Open Office)

The Suspension Tuning Spreadsheet calculates:

    Front and rear suspension frequency

    Front, rear and total roll stiffness with and without bump stops

    Front Roll Couple %

    Wheel rates for springs and sway bars

    Sway bar rates from sway bar dimensions

    Wheel and sway bar motion ratios

    Active spring rate for dual springs

    Skid pad lateral g

    Dyno correction factors and corrected HP

    Automotive unit conversions

Screen Capture

Just enter values in blue. Values in black are calculated automatically.


Instructions

Simply enter the values in blue. All the numbers in black are automatically calculated.

The Suspension Tuning Spreadsheet allows you to analyze and design automobile spring and sway bar rates. After entering car and suspension values it calculates:

Front Roll Couple Percentage describes lateral load transfer distribution front to rear and subsequently the handling balance. It is the front roll stiffness divided by the total roll stiffness which is a ratio, front to rear, of the vehicle's total roll rate. Higher than 50% means the front wheels take more weight while cornering and the handling progresses toward understeer but for most cars around 55% gives balanced handling on the track while approximately 75% is best for autocross. For powerful cars a higher FRC% can help prevent power-on oversteer so a Honda S2000 will typically need a higher FRC% than a Miata.

Roll Stiffness Front, Rear and Total (given in pound-feet at the wheel per degree of chassis roll) describes the chassis resistance to roll during cornering. It is determined using track width and suspension and sway bar wheel rates but does not consider roll center and center of gravity.

Suspension Frequency is the natural oscillation frequency of the suspension. It is determined using only unsprung weight and wheel rate but damping and sway bars are not factored into the suspension frequency. The stiffer the springs the higher (faster) the suspension frequency. Street cars typically run around 1.3Hz and race cars typically come in around 2Hz. You would typically want a lower suspension frequency for a bumpier race track because large bumps require a lower suspension frequency to comply with the road surface.

Spring Angle Correction Factor compensates for the leverage applied to shocks that are not perpendicular to the lower arm (at static rest height).

Suspension Wheel Rate applies the spring rate to the leverage of the suspension motion ratio. The wheel rate is always lower than the spring rate.

Sway Bar Wheel Rate applies the sway bar rate to the leverage of the sway bar motion ratio. The sway bar wheel rate is always lower than the sway bar rate.

Bump Stop Wheel Rate applies the bump stop rate to the leverage of the suspension motion ratio. The bump stop wheel rate is always lower than the bump stop rate. The Suspension Tuning Spreadsheet assumes the bump stops are located on the shock shaft.


 

Suspension measurements used in the Suspension Tuning Spreadsheet.


My Formula for Estimating Sway Bar Rate

How to Measure a Sway Bar

My sway bar formula uses Arm Leverage, Arm Length, Bar Length, Bar Outside (OD) and Inside (ID) Diameter (or Wall Thickness) in inches to estimate the sway bar rate.

The popular Puhn sway bar rate equation consistently underestimates actual, known sway bar rates so I came up with an equation that more accurately predicts real world sway bar rates.

Definitions of the sway bar measurements used by the equation:

Outside Diameter or OD: Use a caliper to precisely measure the diameter of the bar in inches. Measure on straight sections away from bends.

Inside Diameter or ID: Hopefully you can get this information from the bar manufacturer or someone has already cut the bar and measured the inside diameter. ID = OD - 2 * wall thickness. Inside Diameter for a solid bar = 0.

Bar Length: For a bar that has bends between the bushings the Bar Length should follow those bends since the extra bar can twist. The Bar Length should be measured from the first bend outside the bushing to the other side's first bend outside the bushing (center of bend to center of bend). Use a flexible tape measure to follow the bar bends.

Arm Length would then be measured from that same first bend outside the bushing to the bar hole following any bends (center of bend to center of hole). Use a flexible tape measure to follow the bar bends. If your bar has multiple end link holes measure all of them.

Arm Leverage is the distance from the bar pivot (at the bushing) to the arm hole (perpendicular to bar, center of bar to hole center). If your bar has multiple end link holes measure all of them.

My Suspension Tuning Spreadsheet does this calculation for you.

Other Automotive math calculations: automath.xls or automath.ods


Flyin' Miata Stage II Suspension Analysis

Analyzed by Suspension Spreadsheet on Google Docs

Set: Car Wt 2465, Driver 260, 8 Gallons fuel, Wt Dist 52/48

    Suspension Frequency front 1.702Hz, rear 1.578 (Original Equipment (OE) 1.282Hz, 1.439Hz)

    Ratio of Rear to Front Frequency 0.927 (OE 1.122)

Flyin' Miata Stage II Suspension

Front Roll Couple % With Different Front & Rear Bar Settings

Front \ Rear Sway Bar Rear Stiff 525in-lb Stiff & Mid 451in-lb Mid 376in-lb Mid & Soft 329in-lb Rear Soft 282in-lb

Front Stiff 622in-lb

57.34 58.3 59.3 59.95 60.61*(2)

Stiff & Soft 560in-lb

56.49 57.45 58.47 59.12 59.79

Front Soft 498in-lb

55.62*(1) 56.58 57.6*(2) 58.25 58.93

        *(1) Most Oversteer setting and closest to stock NC Miata of 52.7%.

        *(2) FM Recommended setting.

        *(3) Most Understeer setting.

        Stock NC Miata FRC% is 52.7%. Table does not consider bump stop stiffness. By Rob Robinette

Flyin' Miata Stage II Rear Suspension

Yellow Koni Sport adjustable shocks are custom valved for the FM spring rates. FM rear adjustable sway bar is visible. Cat-back exhaust is Good-Win-Racing RoadsterSport Street Single. I love it.

Front FM bar set to Stiff, Rear set to Soft

    Roll Stiffness 1103lb-ft/deg (OE 738.5)

    Roll Stiffness + Bump Stops 1300lb-ft/deg (OE 981.5)

    Front Roll Couple* (FRC) 60.61% (OE 52.7%)

    FRC with Stock Bump Stops 60.77%

Front FM bar set to Stiff, Rear set to Mid & Soft

    Roll Stiffness 1115lb-ft/deg

    Roll Stiffness + Bump Stops 1313lb-ft/deg

    Front Roll Couple* (FRC) 59.95% (OE 52.7%)

    FRC with Stock Bump Stops 60.2%

Front FM bar set to Stiff, Rear set to Mid

    Roll Stiffness 1128lb-ft/deg

    Roll Stiffness + Bump Stops 1325lb-ft/deg

    Front Roll Couple* (FRC) 59.3% (OE 52.7%)

    FRC with Stock Bump Stops 59.65%

Front FM bar set to Stiff, Rear set to Mid & Stiff

    Roll Stiffness 1147lb-ft/deg

    Roll Stiffness + Bump Stops 1344lb-ft/deg

    Front Roll Couple* (FRC) 58.3% (OE 52.7%)

    FRC with Stock Bump Stops 57.8%

Front FM bar set to Stiff, Rear set to Stiff

    Roll Stiffness 1166lb-ft/deg

    Roll Stiffness + Bump Stops 1364lb-ft/deg

    Front Roll Couple* (FRC) 57.34% (OE 52.7%)

    FRC with Stock Bump Stops 57.96%

Front FM bar set to Soft, Rear set to Soft

    Roll Stiffness 1058lb-ft/deg (OE 814)

    Roll Stiffness + Bump Stops 1255lb-ft/deg (OE 1057)

    Front Roll Couple* (FRC) 58.93% (OE 52.7%)

    FRC with Stock Bump Stops 59.35%

*Front FM bar set to Soft, Rear set to Mid *FM recommended setting

    Roll Stiffness 1070lb-ft/deg (OE 814)

    Roll Stiffness + Bump Stops 1268lb-ft/deg (OE 1057)

    Front Roll Couple* (FRC) 58.25% (OE 52.7%)

    FRC with Stock Bump Stops 58.78%

        Notice how the FM recommended setting adds more understeer than the stock suspension.

Front FM bar set to Soft, Rear set to Stiff

    Roll Stiffness 1121lb-ft/deg (OE 814)

    Roll Stiffness + Bump Stops 1318lb-ft/deg (OE 1057)

    Front Roll Couple* (FRC) 55.62% (OE 52.7%)

    FRC with Stock Bump Stops 56.52%

*Front Roll Couple Percentage describes lateral load transfer distribution front to rear and subsequently the handling balance. It is the front roll stiffness divided by the total roll stiffness which is a ratio, front to rear, of the vehicle's total roll rate. Higher than 50% means the front wheels take more weight while cornering and the handling progresses toward understeer but for most cars around 55% gives balanced handling on the track while approximately 75% is best for autocross. For powerful cars a higher FRC% can help prevent power-on oversteer so a turbocharged Miata will typically need a higher FRC% than a normally aspirated Miata.

Sway Bars

Size and rates directly from Flyin' Miata

Front 1" 25.4mm Outised Diameter (OD), 0.688 Inside Diameter (ID), 0.156" 3.96mm wall

    498 pound-inch soft, soft & stiff 560, 622 stiff

Rear 0.75" 19.05mm OD, 0.5" ID, 0.125" 3.05mm wall

    282 lb-in soft, soft & mid 329, 376 mid, mid & stiff 451, 525 stiff

Springs

Front 300 lb/in, 5.4kg-f/mm, 52.6N/mm, ride height 13.5" (OE 165)

Rear 196 lb/in, 3.5kg-f/mm, 34.4N/mm, ride height 13" (OE 158)

 Home ] Up ]