Often overlooked and almost always misunderstood, a tonneau cover is probably the roadster owner's best friend. Tonneau-adorned cockpits remain dry during bad weather, thereby extending the driving season. Covered cockpits also stay safer than their open counterparts, since thieves generally don't steal what they can't see.
Above and beyond function, however, a tonneau is cool. If you're of the right generation, a covered cockpit says fast. The cover's early designation, lakes tarp, indicates its origins. As early as the 1930s, lakes racers, searching for that extra competitive edge, pulled a page directly from early aircraft construction and skinned the cockpits of their roadsters and streamliners with removable canvas. The skins covered gaping cockpits that would otherwise disturb airflow and create undue drag; as a result, tonneau-equipped cars ultimately went faster with a given amount of power. The rest was, as they say, history.
Why A Tonneau For The Street?
While the windshields on our road-driven roadsters, phaetons, and pickups pretty much offset any aerodynamic advantages afforded by a tonneau cover, a tonneau cover just may have more usable benefit to a windshield-equipped car than to a stripped-down race car. That's because air, as it courses over the windshield at speed, creates turbulence as it cascades into and summarily bounces out of the cockpit. We call this turbulent condition buffeting, and as benign as it may seem at first, that buffeting grows annoying over time. A tonneau cover reduces that buffeting drastically at all speeds.
Furthermore, any heat generated by a heater on a crisp fall evening leaves as that turbulence exchanges the cockpit's warmer air for cooler ambient air. Finally, and most importantly, an open cockpit exposes its driver more to incredible amounts of cancer-causing ultraviolet rays. A tonneau cover is capable of both retaining a vast amount of heat and shielding a driver from substantial UV exposure.
Lance Troupe whipped up this simple-yet-effective tonneau for Editor Brennan's roadster, and Brennan immediately sang its praises after his first road trip and wondered why he did without one for so many years.
Before we describe how Lance made it, we'll give you some of his insight. First, he advises at least some sewing experience before tackling a project like this. Second, he recommends a stretchy, fleece-backed vinyl like the crushed-grain automotive-bra material. "It's a lot more forgiving than, say, actual tonneau material made for pickup beds," he noted. "That stuff doesn't stretch much, if at all." Furthermore, he urges anyone contemplating trim work to go as slow as possible. "Go one stitch at a time if you have to," he said. "If you're really on the gas (sewing fast), you're bound to make mistakes."
Additionally, consider the temperature in which you construct the tonneau, and consider how the other temperatures will affect the material. For example, many materials will easily stretch and comply on the sunny day you tensioned the cover; however, if pulled tight during construction, that cover may not even span the cockpit on some chilly night as the rainstorms set in. While Brian's tonneau doesn't incorporate one, he mentioned that a longitudinal bow goes a long way to prevent the cover from flapping while bombing down the highway. Finally, balance tonneau coverage with your aggravation endurance; while a full tonneau with a tiny driver cutout looks really racy and admittedly works the best, it makes entry and egress a real ordeal.
From there, we'll leave you to your own devices to create your own, well ... devices. Oh yeah, and after you make that first long-legged trip with a tonneau, write us to ask why you went so long without one.
Auveco stocks countless design variations, including studs that incorporate their own rivets. | 
Lance recommends spacing the snaps about every 3 1/2 to 4 inches and maintaining at least one snap in the centerline of each radius so the finished tonneau won't pull away from the body. He also recommends installing the cowl top snaps within about an inch inboard of the windshield and the cowl side snaps within about an inch behind the windshield posts. | 
Cut four 4-inch-wide and six 2-inch-wide strips of vinyl from the material's longest dimension (72 inches, or two yards, in this case) and set them aside for a future process. Lance recommends cutting these slices first, as the remnants left over from cutting the tonneau usually won't yield the necessary long strips. Before proceeding, Lance removed the steering wheel since it protruded above the top of the body and would've prohibited the tonneau from laying flat over the cockpit. |
Lance then measured the cockpit and added 2 inches to the entire perimeter to give the tonneau its anticipated border and an extra bit of material for him to grasp. Lance then joined two pieces of vinyl with a zipper and cut the assembly to match the pattern. Keep in mind that he cut the individual panels oversized around the zipper for sewing allowances and a flap to cover the zipper itself. | 
Once Lance installed a snap cap in the tonneau on either side of the zipper at the cowl, he pulled the material back to the center of the tulip panel behind the seat and installed another set of snap caps at that location. He pulled the vinyl enough to remove the heavy sag, yet not tight enough to tension the tonneau material. A procedure a few steps on will explain the loose fit. | 
Once he fastened the tonneau on its centerline, Lance pulled it taut enough to the driver's front corner to make the vinyl lay naturally without wrinkling. He then installed a snap cap in the material to reference the stud immediately inboard of the windshield post. The vinyl doesn't readily lay flat on the body around the windshield post, so Lance relieved that area with a few cuts. He installed yet another snap cap in the material to reference the stud just behind the windshield. In order of progress, he repeated the stretching/fastening procedure at the passenger rear, then the passenger front, and finally the driver's rear. |