As long as you mount them up high enough, you'll be fine- of course, that's not an easy job.
I have a single 15" purple tube up under my bumper from my high school days. It's mounted to the underside of the steel bumper. Once in a while I'll turn it on- chicks still love it!
Tip- paint the metal above the tube with white or silver paint- it'll reflect the neon to the ground.
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-Tom P (Hot rodded 1986 Firebird 2.8l) from
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