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Not sure what you mean. Which ones are different lengths? And on what truck?
I've never actually measured my own, but since aftermarket springs don't have a "left" and "right" configuration, they're expected to fit into the same length between mounts/hangers. So should be the same length.
Or by that do you mean that the front springs are not the same length as the rear springs?
That's normal procedure for most, but I can't tell you why exactly.
Sorry for not being more clear.
I am asking about the distance from each eye to the tie bolt.
73-77.5 F250 rear springs are equal length and the fronts are unequal from each eye to the tie bolt.
That's a particular design to achieve certain characteristics. Not exactly sure what those all are anymore, but it might have to do with anti-wrap, or ride comfort/spring compliance due to certain aspects of movement during driving and braking.
Don't quote me on that, but I did know there are reasons for the differences. Some vehicles actually use offset center pins on the rear springs as well. I think Toyota is one, and GM is another only on certain vehicles.
I remember many years ago some Early Bronco folks were trying different pin locations too, just to see if it made any desirable changes. I don't remember any eye-opening results though, and haven't heard anyone talk about it in a long time. So I'm guessing it was not worth the trouble at least with the spring designs back then. Might be worth re-visiting sometime though.
Of course, we could also study the actual engineering of if and start on paper, rather than re-drilling spring packs!