Bump Steer - also called orbital steer, happens when a wheel with tie-rods at unequal heights goes over a bump. The car momentarily steers the direction that the wheel turns as the toe on that one wheel changes.
For most Miata owners bump steer only becomes an issue after over-lowering. Many falsely believe that slamming the car will make it ride on rails but in reality the Miata's suspension best performs within a couple inches of the stock height. Think of your suspension and all things attached to the wheel traveling in an arc. As you can imagine, the closer that object is to the frame, the more it has to move for a certain height. Because of this, the tie rods angle increases much faster than the control arm after they go above parallel with the ground. Then your tires will toe out under load and going over bumps. This can not only lead to the car jumping over when you go over an uneven bump but it also makes the car very darty under hard braking. An overlowered car with bump steer can be much less forgiving than one on the plush stock suspension. This can also make the car's tires wear abnormally fast and hurt it's all around handling.
Even though the Miata has relatively good bump steer characteristics when lowered compared to other cars, it is still important to be aware of it and correct it if neccessary. The LE edition tie rod ends can give you a little bit of help, but most will need more than that. Bump steer correction kits for RX7's will fit if a Miata specific one can not be found. Many have found solutions and with a little searching a variety of methods can be found.
That's all for the brief explanation of bump steer. Remember not to overlower your car if you care about handling!
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