If your spring is mounted at an angle you will need to consider that in your spring calculations. Measure the angle (A) of your spring from vertical in degrees. Use the table above to determine your Angle Correction Factor (ACF). Now divide the Spring Force (Sf) from the earlier calculation by the Angle Correction Factor (ACF) to get the Adjusted Spring Force (ASf).
Sf/ACF=ASf
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Note: This calculation determines spring FORCE not spring RATE.
The required Adjusted Spring Force (ASf) can now be used to select the proper spring rate for your application. The required spring force can be obtained several different ways. A lighter rate spring with more preload or a stiffer rate spring with less preload will generate the same spring force. The softer rate will generate a smoother ride while the stiffer spring will result in a firmer ride. You need to consider these options when you are selecting the proper spring rate for your application.
Springs should typically be compressed 25-30% of the free length when supporting the weight of the vehicle. Drag race cars will normally use a lighter rate spring (about 30%) to promote weight transfer while a street car will use a firmer rate spring (about 25%).
ASf/(spring free length x 0.25) = Firmer Spring Rate
Asf/(spring free length x 0.30) = Softer Spring Rate
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Spring rate calculations for solid axle suspension are the same as above except the Force Ratio (Fr) = 1.
kevlarcorolla wrote: If you want to use frequency to pick rates with AND your going to be doing any sporting driving with the car I suggest bumping the frequency WAY up from what Dennis suggests on his farnorthracing site.I don't think I'd hesitate to go up into the 4hz range just as a starting point. I used his info to baseline my autox scratch build,way too soft starting at the upper end for his track use suggestion.I was up in the 5.5hz range(from memory so grain of salt thing) in it and it still rode better than my street car.
The ride doesn't have to be harsh for it to handle well, and there are many reasons why the ride will be different than your ~1.0 to ~1.5 Hz "street car", so that's not a very fair comparison.
What makes you think the ride was "too soft" with your starting frequency? And what frequency did you start with?
The ideal frequency also depends on what your autocross course is like. Some lots are VERY rough and bumpy. Others might as well be kart racing tracks where you don't really need suspension at all.
NOHOME wrote: right...more information would be needed. The reason for the Miata suspension is because I like the design and handling of Miatas. The car is being built as a cruiser, and I have done a few mile plus days in the Miata, so figure it qualifies for the job. So, the goal would be to approximate the ride of a stock Miata. Coilovers will help if the vehicle needs to go up or down for a given spring rate and the extra cost of adjustables will pay off in mot having to buy new shocks to experiment. The car weight/10 rule is going to give me /10 so front springs of 280 lbs/in. Seems a bit light. Anyone know what stock NA spring rates are?
To approximate the ride of a stock Miata with your heavier vehicle you will need to know the following for both your car and the Miata:
1) Front and rear suspension motion ratios 2) Corner weights (with intended load; that means driver and fuel at minimum) 3) Unsprung weight per corner
And the front and rear spring rates for the Miata.
Or if you already know the Miata's actual suspension frequency then you just need the above info for your car.
NOHOME wrote: 350-400 would have been my gut guess. The Miata spring is not too far from the wheel,and fairly straight up and down, so would the wheel rate not be similar to the spring rate?
No. The wheel rate is the spring rate multiplied by the square of the motion ratio. The wheel rate is very sensitive to motion ratio, particularly on SLA suspensions where the motion ratio may be around 0.5 to 0.6, and is rarely much higher than 0.7.
For example, on my Fiero's front suspension (SLA) the motion ratio is 0.53 so a 800 lb/in spring gives a wheel rate of 224 lb/in. (0.53 * 0.53 * 800 = 224.72)
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