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Based on my seat of the pants dynomometer and the feedback from others at the track I need more rear spring in the car.

I understand that I must remove the springs and calculate the current rate to determine the new rate, but now how do I order a set of springs with more rate that doesn't change the ride height and from whom??



The basics on leaf springs are rate (lbs/in) and arch (in).

There's people other than Tom reading this so here's an example.

Lets say you want a 150 in/lbs spring. Then you need to figure how much arch you want with the spring installed at ride height after the weight of the rear is placed on them. That's why you need to know what the weights are at your rear tires.

So lets say each rear tire is 900 lbs of weight on it. With a 150 lbs/in spring that would need 4" of arch (theoretically) to end up with zero arch. But you probably what a little more arch at static ride height, like 1". So you would get in theroy 150 lbs/in spring with 5" arch that would end up in theroy with 1" arch left installed.

Now that's in theroy. I dont think the leaf spring will actually measure that linear. When you're done making it, you need to get the force (lbs) at each inch of compression. OR find someone that is real familiar with making/using leaf springs for race cars.

Also the amount of installed arch and total compression you get in a turn will affect rear steer. Rear steer is when the outside rear tire is moved back relative to the inside rear tire. Then the whole rear end is pointing slightly to the outside, causing the car to oversteer more when near the most rear tire travel/compression.

Is a little induced rear steer good or bad? Neither. Depends on the setup you are running. But most run as little installed arch as possible. Remember those MP circle track zero arch springs.

Also the more installed arch you have the more leverage the rear end has to move the rear end from side to side. As usual, there are all kinds of compromises taking place here.

3 and 4 links have rear steer too.

BTW...
Coil springs out of the box will not measure as advertised too. And they will vary out of the box. There are some brands/models of race coil springs that are more consistant that others.