M-5300-W FP spring rates

949Racing said:

We measured the rates of the M-5300-W kit.

You can see from the plot that the fronts are linear 250-260#. Rears are effectively 565# for travel 1 through 2 ". Increasing to 735# for 2-3".

We'll test the OEM GTG350 springs when we swap these in.

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firestarter2 said:

Can someone translate for the stupid kid in the room

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oldmachguy said:

And, oddly, Voshlag's data show the rear springs are different for the GT350 and GT350R - which is not what Ford claims. Huh.

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oldmachguy said:

Firestarter2:

Although this is kind of like the blind leading the blind:

One our our forum sponsors, 949Racing, used a device (a spring tester made by Rebco) to measure how many pounds of force it takes to compress the Ford Performance lowering springs for the GT350 and GT350R. In the first post, they listed a table of data of force required to compress the springs (first front, then rear) up to 2.5 inches, in one-quarter inch increments. Then they graphed those data. It appeared to me that 949Racing's summary of the results did not accurately reflect the data in their own table - as it looked like they were instead just "reading" from their graph. That's why I called myself either a "nerd" or "dense" (since there's even money I'm wrong). 949Racing finished by saying they would measure the factory springs when they remove them from the car.

I also posted what our owner's manual lists as the stock spring rates.

Then, while I'm writing my post, Epiphany comes along and posts a set of measurements that Vorshlag already made of the factory (not Ford Performance) springs for our cars (GT350 and GT350R).

Your take away, I would suggest, is that data doesn't lie. Ford has to pick "a" number to represent our cars' spring rates, even though those rates are not constant - i.e., they get stiffer as they are compressed. Ford says that, for the GT350, the factory spring rates are 194 lb/in in front, and 914 lb/in in back; and for the the GT350R, they are 240 lb/in front, 914 lb/in rear.

My other comment had to do with whether these rates were for just ONE spring, or a PAIR of springs. For example, road racing springs for a 1965 Shelby were 610 lbs/in - but that was for BOTH front springs, as each spring required 305 lb/in to compress it. Thus my confusion as to the convention here, whether 240 lb/in in front is just one spring, or two.

P.S. Now that I look back at 949Racing's data - it appears that the front Ford Lowering springs are softer than the factory R springs measured by Vorshlag. And, oddly, Voshlag's data show the rear springs are different for the GT350 and GT350R - which is not what Ford claims. Huh.

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BmacIL said:

It's for each spring.

The GT350 and R rear springs both are dual rate. ~1/5 of the coils are a higher pitch (more coils per inch), and the rest a much lower pitch. At ride height, there is approximately 1/4" of spring travel left of the softer rate (which means about 1/2" of wheel travel). This all shows up as some non-linearities in the graphs, and the Vorshlag data shows it best. You can see a knee point where the rate changes pretty clearly. Doing dual rate springs this way is cheaper than a separate helper spring, but also makes it a semi-progressive spring, rather than two linear springs of different rates.

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