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Originally Posted by TheONE
I believe it is about 180 pound difference.
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Ok. That may not be enough to make a big enough difference to warrant using different rates in the front. If you really want to find out for sure (that is, if you REALLY want to find out) you're going to have to corner weight the car, and do a little bit of number crunching.
Before you read on (if you're still interested), you're going to have to do a bit of reading on wheel rates. The web is littered with that sort of info (you can type "wheel rate formula" into any decent search engine and get loads of results, so I won't talk about what wheel rate is here. I'm just going to say what you're going to use it for.
After you've done the corner weighting and calculated out the spring rates of your car with the FLEX dampers installed, set those numbers aside. Later down the page I'll refer to it as "the first equation". Then do the same calculations but either add or subtract (depending on whether you haven't done the swap already or you have) 90 lbs from each corner weight in the first equation and calculate the wheel rate. This I'll call "the second equation".
Now you have the wheel rates of the front of your car as it is with the default springs, and an approximation of the wheel rate if you did/didn't do the swap. I say approximation because in calculating out that second equation, I made two assumptions:
1. All of the weight of the engine is concentrated in the front (not entirelY)
2. The weight of the engine is distributed the same was as the KA (probably not)
However, the differences wouldn't be enough to really make big enough a difference in this case.
Of the two wheel rates that you got, take the one which more closely resembles a KA powered 240 with the default spring rates (if you haven't done the swap, it's the first number you calculated. If you have, it's the second one). Take this number and plug it into the wheel rate of the formula. Now take the corner weight approximation from the other equation (the one which more closely resembles the corner weight of your RB powered 240), and plug it into the corner weight. Using this third equation, you can now calculate the spring rate that you'll need to get similar characteristics of the default Tein FLEX springs on a stock 240sx. If the spring rate you calculate out here isn't more than 0.5kg/mm (or about 28lbs/in) different than the default front spring rate (which for your car is 6kg/mm, or 335lbs/in I believe), there really isn't going to be much of a point in changing the springs. Even if the difference is more, a lot of drivers don't like to change springs with a difference of less than 10% of the total spring rate. So unless it turns out that the rate is off by over 0.75kg/mm (about 41lbs/in), you're not going to need to order a new set of springs for the front.
As you can see, this is a very time consuming and expensive method of doing it. Not only will you have to go through all the trouble of installation, corner weighting, and whatnot, you'll also have to pay for the stock Tein dampers, the corner weighting, and if it turns out you need different springs, new front springs as well. This is also not the best method either, because this will only tell you what springs you want to use to get similar to Tein's specs, which may or may not be ideal for your application. If what you want is different from what Tein specced out, the above will be useless. However, this is also one of the cheapest and decently accurate methods of calculating this all out. Whether it's worth it to you is up to you.
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