Methods of Attachment
Other Safety Considerations - Padding
Getting the top up and down
Why do you want a style/roll bar?
Chances are you either want to equip the car with additional safety features (maybe you're concerned about it being a convertible), need to meet a sports car competition/course requirement, or want to modify (improve?) the looks of the car. Herein lie the primary bases for your selection.
A word of caution at this point, roll bar manufacturers don't usually promise a certifiable finished product, primarily because they can't supervise their bar's installation. Once it leaves their factory, they will usually wash their hands of any liability. If they do make such promises they are asking for trouble. The bottom line is that with whatever additional protection you think your bar might offer (be it style or roll), it is not a justifiable rationale for taking excessive risks. Drive safely.
If you are attempting to meet rules or requirements, there some limitations you must consider. Check out Requirements to get an idea of just what you have waiting for you at the competition technical inspection garage.
Style bars (designed solely for esthetics) include a number of configurations. Those that I am familiar with include a single wide hoop (that covers both driver and passenger), a double-hooped version originally introduced by Racing Beat (the Style Bar II) I believe, and a newer single hoop version that covers just the driver (reminiscent of the old Cobra bar) referred to as Racing Beat's Style Bar III. The latest I am aware of (just stocked by Brainstorm 9/29/95) is a modified version of the Style Bar II. In Brainstorm's version they have modified the ends so they now clear the top frame hinge knuckle to keep it from hitting the bar baseplate on the way up and down. In addition, Brainstorm's design replaced the flat stock baseplate under each hoop with continuous tubing as that currently connecting the two hoops between the seats.
Solo I requirements are another story altogether and compare closely to the General Competition Rules (GCR) of road racing in the SCCA. Increased height of roll bars are required so that you won't break your neck in a rollover. This is particularly true when used with a racing harnesses (two shoulder belts, a lap belt and a submarine belt). The rationale is that in a rollover, the racing harness will not let you get your head out of the way by bending over to the inboard of the vehicle as you could with a stock street seatbelt. Again, if you are intent on racing, you need to get a copy of the current GCR and ensure that your bar meets the requirements for your intended class of competition.
The full roll cage is not addressed here only because I chose not to classify it as a "roll bar" for the purposes of this writing. Of the four listed above, the most common of these seen on the street are 1 2 and 3. If you look at the E Production race cars however you see the 4th category in common use. This last configuration is really beyond the scope of this writing, and we will concentrate on 1, 2 and 3.
The single hoop can either include two or four mounting points. The simplest types are attached only to the top of the seat belt towers and should be considered "style bars" since their integrity is severely limited due to the method of attachment. Some types however have an extended length for attachment both at the top of the seat belt towers as well as further down on the shell of the seat belt towers (and closer to the 'frame' of the vehicle). This latter configuration has been deemed by some racetracks and drivers' schools as a "four-point" attachment (which term classically identifies a much stronger configuration).
The purpose of the diagonal and the two rearward supports in configuration 2 and 3 is additional strength in the all directions. A hoop itself provides the necessary vertical strength, but is relatively weak in a fore-aft direction (consider the impact of a foot-long moment arm of the bar height acting on a two-inch attachment point around which the bar attempts to rotate in an accident). The diagonal or the two rearward support configuration provides this needed fore-aft strength and extends the attachment plane.
Materials used in most bars will be cold-rolled steel typically characteristic of 70,000 psi tensile strength. For approximations, materials engineers typically use about 1/4 of the tensile for anticipated shear strength, thus 17,500 psi. Assuming a 2.5 inch diameter tube with a 1/16th wall thickness (cross-sectional area = 0.5 sq inches), one tube of the bar will support a 35,000 pound vertical force but only a 8,750 pound force laterally. These loads sound extreme but if you consider the loads as you are decelerating from 100 mph to 0 mph in a matter of seconds (let's say 1 second for simpler calculation purposes) where a max load could be as high as 4.5 g, you've just applied over 10,000 pounds of force with your 2250 pound car!
When selecting fasteners, realize that there are various grades of quality associated with nuts and bolts. If your roll bar is for safety purposes, don't skimp on cheap grade fasteners. The cost of nuts and bolts is not high and you can get a good grade 5 or 8 bolt for just a little more. I would recommend no less than a grade 5 fastener being used on a roll bar installation (grade 5 is indicated by three equally-spaced radial lines around the top of the bolt head). Generally, markings on the head of the bolts will indicate the grade of the fastener. Those quality criteria are not within the scope of this page, but there are many good references available to explain these characteristics to you.
There are probably a variety of padding attaching devices available as well. These aren't so critical as the padding itself, but they may include a characteristic of locking mechanism that represents a hazard. A common version of these is commercially identified as "Ty-Wrap" and made of a nylon-like material. They come in a variety of colors, but most often black, blue or translucent white. You will find these in your car's wiring harness already. When attaching these you will want to avoid orienting the locking end so that they are positioned behind your's or your passenger's head.
Has anyone installed the new Hard Dog Sport Bar that appears in teh summer Miata Magazine?
1. Remove lots of parts around the seat belt towers.
2. Remove the rear carpet. A small claw hammer works great on those damn plastic tacks.
3. Remove a panel over the gas tank.
4. Spend several minutes trying to figure out how to read the pattern for cutting the sheet metal for the rear braces.
5. Spend several hours cutting the sheet metal. Send me email for some more details.
6. Struggle to get the bar in place. Call a friend to help. (Have him/her bring cold beer.) Eventually get the bar in place. Find out it must be removed to cut more sheet metal. (Give up for the night and drink the beer.)
[6.] If you have ABS, the wires to the rear sensors must be relocated. Email for details.
7. After the bar fits, install the bolts to the seat belt towers to hold it in place. Drill holes for the rest of the bolts. Have a strong friend handy for torquing the bolts to 50 ft-lbs.
8. Put the carpet into position. Realize that there is no way to cut the carpet for the bar until the carpet is in position, and no way to put the carpet in position until it is cut for the bar. Make a guess and start cutting.
9. Leave the trim off the seat belt towers until you've had time to rest. Cutting it accurately is important. Mine have been off for months.
As a side note, the driver loses no seat travel, and the passenger loses maybe an inch or two. I asked Craig at Hard Dog to make the cross brace removable. I run without it on the street so I don't have to look at it in the mirror. With the cross brace removed the passenger seat has full travel.
This may get me thrown off the list, but I leave the window zipped and just fold it with the top. There's no place to put the window if you unzip it. Just make sure it doesn't kink as it folds.
Could a neophyte install it? A determined one.
Thanks Steve for allowing us to republish this communication in the Garage. (Look for Steve's email address in the online directory if you really want to email him.)
He indicated that the installer would need tin snips, a coarse file, a 1/2 inch socket wrench and assorted sockets, and an air conditioned garage.
His experince was similar to mine in that a downward pointing bolt is difficult to thread a lockwasher and nut onto up inside the top of the seatbelt towers. Ken used a dab of locktite to hold his washer in place while threading the nut on. [I taped the washer and nut together and threading them together onto the bolt, then pulled the tape off and torqued them down (up).]
Drake adds that the current bar that works with the hardtop does not impede rearward vision any more than the first bar. The kit provides spacers to allow the installer to lower the inside rearview mirror by about 1/2". This helps, but the lower edge of the horizontal portion of the bar is still in view in the mirror.
Bar attachment points were perfectly aligned, hardware was good, fit and finish were excellent, vinyl covered padding is really nice looking.
The written directions were good, but the template for cutting the interior plastic trim panels was very poor. Drake has developed a very precise template for cutting the trim panel around the bar's attachment point at the shoulder harness tower.
Removing the plastic cover around the mirror mount without destroying it is the hardest part of the whole installation!! The bar bolts on very easily. One must remove the door sill trim and plastic quarter panel trim around the seat belt shoulder harness tower. Also remove the outside lower belt anchor bolt. Once the trim panel is out, remove the shoulder belt inertial reel and merely set it down behind the seat.
Both Drake and Ken have since installed bars for other mebers in their respective clubs and welcome any email questions that they might be able to help with.
A significant consideration in selecting a bar is its interference with the back window. Obvioulsy, if you need a specifically-configured bar to satisfy racing requirements, you don't have much choice, but keep in mind that if you do have some choice in the matter, many bars with diagonals and rear-ward supports will not allow you to lay your convertible top window down like it did when the car had no bar at all. In fact in some cases you won't even be able to reach the zipper easily. If you have one of the new glass rear windows, then you have even more restrictions to the bar type. At least one Miata owner that I know has the glass window and the Hard Dog Hard Bar. The Hard Bar has no diagonals the ability to grasp the top handle and single-handedly raise the top in one swift movement as you used to. Now you will be forced to get out of the car to put the top up, or have the passenger help you with it.
The chrome finish on some bars is not perfect. Look closely at the corners and the welds to ensure that there is good anodizing. A poor process is indicated by a variation in surface texture and/or discoloration.