Hi,
I had a "street" roll bar in my TR4 until recently. Yes, with those you can raise and lower the convertible top. It wouldn't fit under the factory hardtop on my car, which is okay anyway since I'm installing a rollcage.
The reason these "street" roll bars aren't race-legal is that they aren't tall or wide enough and they don't fasten to the frame of the car. Actually, the materials, tubing diameter and tubing thickness are sufficient for even the heaviest SCCA classification (mild steel, 1-3/4" dia., .120" tube wall thicness). Thinner mild steel in smaller diameters can be used on lighter cars. Even thinner/smaller alloy steel can be used. But, personally I think mild steel is better unless weight is an overriding factor. Mild steel might bend a little in an accident, but properly designed and braced, is unlikely to break. Alloys are more likely to crack and break. Also, mild steel is easier to bend and weld and the quality of both is very important to the effectiveness of any roll bar.
Although mostly for show, even a "street" roll bar is better than nothing in terms of protection and will add a little rigidity to the car. The reason that they won't get past safety inspection at a race track is that "street" bars aren't high enough to give some clearance above the top of a driver's helmet, in the event of a rollover. Also, they aren't wide enough to fully protect shoulders. And, they mount to the body sheet metal, not directly to the frame. Finally, most only have two rearward braces, again mounted to the sheet metal and not the frame.
If you install one, I highly recommend padding a roll bar, at least in the areas adjacent to driver/passenger's head, arms and shoulders when seated in the car. Also, don't install bolts or weld on mounting tabs anywhere that a passenger or driver might strike them in an accident. And, if your seats are lowback, you might consider headrests on the roll bar.
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