If a sunset cruise and windblown hair are your idea of heaven, then a convertible top might just be your ticket there. However, metal shards, body filler dust and long nights of endless cursing await the unwary attempting to build one, and irreversible damage is a real possibility.
The Best Advice
Don't underestimate the sagacity of your local body-man when he tells your that the best advice for going topless is not to do it. Convertible cars have as much in common with fixed-roofs as four-doors do with coupes; the body, chassis and engineering are totally different. For instance, the only body components shared between a fixed-roof and drop-top Aston Martin DB9 are the bonnet and front quarter panels.
This is no bolt-on. If top-end work is a priority, then farm your chop-top out to a custom car shop unless you have the design talent of Chip Foose, the body-working skill of George Barris and the patience of Ghandi. Expect to spend at least £5,200 to £6,500 for a convertible conversion, a bit less if your model of car came as a convertible. At those prices, you might want to consider just selling the car and trading up to a factory convertible version of the same model, if available.
The biggest problem with convertible tops is that they remove a huge chunk of the chassis rigidity, especially on newer unibody (frameless) cars. The problem is especially severe on older (pre-2000) cars that are generally less stiff than newer chassis. Odds are that you've never seen a home-built convertible Honda Civic, but the few that didn't break in half have huge roll-cages. That should say something about just how weak a chassis can become when the roof is removed.
You can offset a certain amount of chassis flex by installing an X-brace beneath the car and another behind the back seat, but a proper 6- or 8-point roll-bar is going to do the most to retain rigidity. Even then, think of the chassis reinforcement as a full-length from what runs throughout the entire body. Make sure to tie the cage into your strut and suspension mounts; doing so will ensure that all chassis loads go right into the reinforcements instead of the body.
Soft Top Design
If you're crazy enough to engineer a soft-top set-up of your own, take a page from Lamborghini's notebook and think of it as an emergency umbrella for sudden showers. Make a folding tonneau cover for the rear seat, and engineer the folding mechanism for function over form with the fewest possible number of joints. For inspiration, take a look at old Model T soft tops; they're not pretty, but they keep the rain off.
Don't stress out if the top doesn't meet your windows; you'll spend the rest of your life trying to engineer a perfectly fitting system and it's still going to leak. Invest in waterproof gauges, rubber switch boots and plastic covers for your radio and electronic equipment. It's not a fatalistic approach; it's a realistic one--just get the thing built and drive it. You're going to have a lot more fun cruising the boulevard with your less-than-perfect top folded away than you'll have during the eight months it'll take to build a "perfect" top that does the exact same thing.