Rancho Coil-Over RS9000X Pro Series

Coil-Over Conversion On The Jeep TJ choosing the spring rates
You get to choose the spring rates! Rancho has included different spring rates in the part numbers chart at the bottom of Ranchos coil-over page. The springs are made by Eibach. They have a strong reputation as some of the best springs made. In fact Rancho has provided a great link explaining how to choose the proper springs. Click Here for the guide. If the worksheet is too much for you, Rancho sales would be happy to help you choose the proper springs. If you want to try it, we will attempt to explain here. Caution: confusing math ahead! In order to select your spring rate you need to know how much weight is on that corner of the vehicle. So go to the local scales with the vehicle loaded as you would use it. Weigh the front and rear of the vehicle separately. Let’s say our Jeep weighs 3800lbs total. That's four people, gas, tools, oil, camping supplies, and cooler. The rear comes in at 1950lbs and the front at 1850lbs. So to figure the spring rate for the front you need to divide the front weight by two and get the weight for each side. So, 1850\2= 925lbs on each front tire (give or take a little). Now figure your wheel frequency. (huh?) That's how often your tire is going to go up and down in a second. The slower you go, the lower the frequency. For rock crawling start around .75 cycles per second. Around 30mph figure about 1.1cps to 1.375cps. Think about how many times a race truck tires bounce up and down. The faster you go the more energy the springs need to be able to absorb. Shock dampening can help your spring absorb the energy without jamming the bump stops. Valving can also hurt performance. A shock valve so stiff to compensate for soft springs at high speed will beat your kidneys at low speed. Conversely a soft valved shock on a stiff spring will do little to soften the ride. So it’s important to figure the proper spring rate first, and then figure shock valving. Fortunately the RS9000s have a variable valve, so you can go a little soft on the spring rate. I figured a little higher, as I do not have my winch on yet. With this in mind, on to the math! Take the formula of; spring frequency (SP) divided by 3.128, square the result, and multiply that by the weight, will give you the spring rate. It looks like this .75/3.128= 239769 squared =.0574702 multiply by the weight 925 = @53 lbs per inch. On our project Jeep the 80lb/250lb combination would give us around 60lbs per inch. However this would mean the tender would compress over 11.5 inches to support the 925lbs of weight. (925lbs/80 inch pounds=@11.5 inches)The tender can only collapse 9 inches before binding. This will bottom the coil at ride height. The 125lb spring would collapse 7 inches. leaving two and a half inches of down travel before binding. We will shoot for about 83lbs per inch combined rate on our project. Now you have to figure shock angle and mounting position on the axle or the arm. Check the guide for the explanation. Any angle not parallel to the motion of the axle will diminish the dampening effect. Moving the shock mount to a suspension arm away from the axle closer to the pivot point will increase the mechanical advantage of the wheel. This diminishes the dampening of the shock. They have all the formulas for this. To figure the spring rate of the dual coils, multiply the first coil rate to the second, save the result. Add the first coil rate to the second, save the result. Take the first result and divide it by the second result. This gives you the total spring rate. For this example we used 80lb tender coil and 250lb main coil. It looks like this. 80+250=330 then 80x250=20000 then 20000/330= 60.606. So we come out with 60 lbs per inch of compression. The other combination from Rancho came out to (80lb and 125lb springs) 48lbs per inch, and (125lb and 250lb springs) 83.3lbs per inch. We ordered the 125lb and 250lb springs for 83lbs total. Planning for added weight is a must, if you are going to add a winch and a bumper, you may need to order a new tender later. For more information visit www.gorancho.com
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Coil-Over Conversion On The Jeep TJ

choosing the spring rates

You get to choose the spring rates!

Rancho has included different spring rates in the part numbers chart at the bottom of Ranchos coil-over page.

The springs are made by Eibach. They have a strong reputation as some of the best springs made. In fact Rancho has provided a great link explaining how to choose the proper springs. Click Here for the guide. If the worksheet is too much for you, Rancho sales would be happy to help you choose the proper springs. If you want to try it, we will attempt to explain here.

Caution: confusing math ahead!

In order to select your spring rate you need to know how much weight is on that corner of the vehicle. So go to the local scales with the vehicle loaded as you would use it. Weigh the front and rear of the vehicle separately. Let’s say our Jeep weighs 3800lbs total. That's four people, gas, tools, oil, camping supplies, and cooler. The rear comes in at 1950lbs and the front at 1850lbs.

So to figure the spring rate for the front you need to divide the front weight by two and get the weight for each side. So, 1850\2= 925lbs on each front tire (give or take a little). Now figure your wheel frequency. (huh?) That's how often your tire is going to go up and down in a second. The slower you go, the lower the frequency. For rock crawling start around .75 cycles per second. Around 30mph figure about 1.1cps to 1.375cps. Think about how many times a race truck tires bounce up and down. The faster you go the more energy the springs need to be able to absorb. Shock dampening can help your spring absorb the energy without jamming the bump stops. Valving can also hurt performance. A shock valve so stiff to compensate for soft springs at high speed will beat your kidneys at low speed. Conversely a soft valved shock on a stiff spring will do little to soften the ride. So it’s important to figure the proper spring rate first, and then figure shock valving. Fortunately the RS9000s have a variable valve, so you can go a little soft on the spring rate. I figured a little higher, as I do not have my winch on yet. With this in mind, on to the math!

Take the formula of; spring frequency (SP) divided by 3.128, square the result, and multiply that by the weight, will give you the spring rate.

It looks like this .75/3.128= 239769 squared =.0574702 multiply by the weight 925 = @53 lbs per inch. On our project Jeep the 80lb/250lb combination would give us around 60lbs per inch.

However this would mean the tender would compress over 11.5 inches to support the 925lbs of weight. (925lbs/80 inch pounds=@11.5 inches)The tender can only collapse 9 inches before binding. This will bottom the coil at ride height. The 125lb spring would collapse 7 inches. leaving two and a half inches of down travel before binding.

We will shoot for about 83lbs per inch combined rate on our project.

Now you have to figure shock angle and mounting position on the axle or the arm. Check the guide for the explanation. Any angle not parallel to the motion of the axle will diminish the dampening effect. Moving the shock mount to a suspension arm away from the axle closer to the pivot point will increase the mechanical advantage of the wheel. This diminishes the dampening of the shock. They have all the formulas for this.

To figure the spring rate of the dual coils, multiply the first coil rate to the second, save the result. Add the first coil rate to the second, save the result. Take the first result and divide it by the second result. This gives you the total spring rate. For this example we used 80lb tender coil and 250lb main coil.

It looks like this. 80+250=330 then 80x250=20000 then 20000/330= 60.606.

So we come out with 60 lbs per inch of compression. The other combination from Rancho came out to (80lb and 125lb springs) 48lbs per inch, and (125lb and 250lb springs) 83.3lbs per inch.

We ordered the 125lb and 250lb springs for 83lbs total. Planning for added weight is a must, if you are going to add a winch and a bumper, you may need to order a new tender later.

For more information visit www.gorancho.com

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