What spring rate have you all used on the rear with coil overs? It's on my 28 sedan in my aviator, no back seat and the battery and 17 gallon tank sets in the very rear. I had a used set of coil overs a friend took off his 30 model A. They are on the weak side and I don't have no idea what rate they are. I was thinking I need 200 or 250 springs but that is just a guess. I'm going with new shocks to since I don't know how old these are.
Get a bathroom scale put a spring on it compress 1 inch read the scale Sent from my SM-J737T using The H.A.M.B. mobile app
Spring weight is determined by how much weight it takes to compress the spring 1". Set spring on a scale, zero out scale, add weight to top of spring till it compresses 1". Read scale. Just how you do this, I don't know, but it will give you the actual rate. If you weigh the rear end of your car, then subtract weight of the rear axle, tires, brakes, 1/2 the shocks, and 1/2 the suspension, you will be able get a really close spring rate. Also, don't forget about ride height.
alot of things to consider to get it right. mine are 165 lbs. 15 gallon tank at the back. glass cab i have moved the gas tank from where you see it.
I just went through all of this. Have a good idea of your total weight, front / rear weight and unsprung weight. My avatar weights 2400# 1150# front 1250# rear. No people, no fuel. Ya, I was surprised also. Rear axle with tires/wheels and half the suspension components 320# Weight method, bath room scale and lever. Maybe not the most accurate method. But the net confirms it. I bought 16.5 inch coil overs with 5.5 inches of travel and 200# 12" springs. Springs installed with .75" of preload. Installed and on the ground the shocks compressed 1.5" 1250_rear weight______200_.75" preload per shock 320_unsprung weight__ 250_1.5" loaded shock compression 930 / 2 = 465________ 450_2.25" total spring compression If the supply chain accurately netted me 200# springs then my car weights are also accurate. I do plan to add bumpers, fenders and running boards so I think I'll be happy with the net.
Thanks, I'm going to try the 200's and see what happens. They're not to expensive so I'm going to stick a spring on these and this winter make some changes.
You need to know the sprung weight in the rear. Weigh the rear, then disconnect the springs and jack it up and weigh the unsprung assembly [deduct that from the 1st number] On a road car for good ride quality you want the wheel rate to be appox 20- 25% of the sprung weight [as a good starting point] This means the car should load the suspension 4-5" when the weight is upon it. Or there is up to 4-5" of compression in the springs to install them. This is all based on a motion ratio of 1:1 [meaning the springs are vertical and attached to the axle] If a car has 3000lbs weight, and 55/45 Fr/Rr bias it would have 1350lbs on the rear wheels. If we deducted an arbitrary 200lbs for unsprung weight, the rear suspension would support 1150 lbs [575 lbs per side] Using the 4:1 to 5:1 formula for cycles per second you would need a wheel rate of 143.75 lbs/in [4:1] on the high side down to 115 lbs/in [5:1] on the soft side. This incidentally is approx the spring rate of a 67-69 Camaro monoleaf rear spring [125 lbs/in] As a car gets driven faster [we all need to be honest with ourselves here] the spring frequency needs to go up [3.5:1 to 3.0:1 for road racing cars] 190 lbs/in would be quite stiff for a Road Racing Camaro. Also as the spring stiffness goes up, the spring load [free height] goes down, or the ride height will go up also. Stiffer springs are usually shorter or have less arch] This is all calculated at the wheel rate [which is 1:1 to spring rate on leafs] , but when a spring is moved inboard on an A-arm or mounted at an angle this changes the motion ratio. So the equation for Wheel Rate is the Motion Ratio squared times the Spring Rate [ The reason it is "squared" is because two factors are altered with leverage. 1: Force and 2: distance ] If a coilover is laid over 30deg from vertical, for every 1" of suspension travel requires 1.155" of shock travel. So 1.155 is the motion ratio ,and 1.15 squared = 1.334 [1.155 x 1.155 = 1.334] If we referred back to the 1st scenario of 115 lbs/in wheel rate, it would require a spring stiffness of 153.41 lbs/in if the coilovers were 30 deg from vertical. The wheel rate of 143.75 lbs/in would require a 191.76 lbs/in spring. If you aren't good at trigonometry ,here is an online calculator. http://www.csgnetwork.com/righttricalc.html Angle A or B is the coilover angle Side b is suspension travel [ make that 1"] Also change the decimal places to "3" ,then click on "calculate" Side c is shock travel [ motion ratio]
mine had 180 lbs rate , i went down to 165 lbs. much smoother ride. just drove 600 mile round trip to salina and back. have 5000 miles on truck and love the ride. mine are mounted at 28 degrees. so probably closer to 115 lbs at that angle.
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