What are the disadvantages of using spacers when compared with just getting longer springs?

I would think there would be less travel, but what about ride quality?

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That one really depends on the application. To a point longer springs will give you more travel and usually the longer springs wont come unseated as easily.

To counter that coil spacers do actually give you more travel as well because you can force the springs to compress further to reach the bump stop. In many cases a large spacer with not enough spring can come unseated at full droop.

A balance can be found for all vehicles, but both may do a little bit of what you are looking for.

Correct me if I am wrong anybody, but that is my understanding of it all