Roll Over Bar Hoops.

Design and Fabrication of Roll Over Bar Hoops.

There are two types of roll over bars. The most common type is the continuous single hoop type of bar commonly referred to as the towel rail, a term left over from the surfing days of the sixties, you have probably noticed some photos from the era showing the bronzed sun loving surfie posing with his/her cool MGB complete with surf board mounted on the top of the full width roll bar and the windscreen frame, plus the oversize towel thrown over the roll bar rail to dry.

The other type is the twin roll bar Miata MX5 style hoop. This particular MX5 type might look the part but they don’t offer anything in the way of protection in the event of a roll over.

But twin roll bar hoops if built correctly are worth a closer look, a well built twin hoop roll bar can offer real solid roll over protection because of the way it is designed with integrated mountings securely mounted onto the rear of the elan chassis and lower outer body mounts, it is this type of bar that we will attempt to explain and detail for the elan owner.

Tools and skills you will need are:
Lathe: Required to machine several small parts, such as bosses, both plain and threaded.

Welding:
Oxy Acet or TIG or MIG welder. Project as shown was mostly done with Oxy Acet, with a small amount of electric Arc welding.

A well equipped workshop coupled with confidence in your own fabrication skills are a definite requirement for this project.

This is not the kind of project to be undertaken if you lack engineering and fabricating skills.

It is also the kind of project that can only be carried out when the car is undergoing a full rebuild, and by that we mean that the body and chassis will be separated, because as the project progresses there will be a need to replace the body back on the chassis to check various alignments and chassis mounting brackets, a lifting hoist or chain block is required in order to do a quick lift and replacement of the body.

In this project, the hoops are were formed with an automated CNC mandrel bending machine, the tube material chosen: 38mm OD x 3mm wall seamless  tube, the bar hoops in this instance were not meant for competition purposes, but they could easily be designed to comply with whatever governing motor body regs the competition is held under, you will probably need that little compliance sticker or piece of paper from your governing body to compete on the track.

But after you have read through and understand more about this project, you will note that the design integrity and strength of the twin hoop roll over bar/s will match or exceed existing requirements. Before this project was undertaken, several hours were spent searching the net for information pertaining to twin hoop roll over bar design for the lotus elan, the end result was that very little info was forthcoming. It appears that no one has attempted to do this in the past, or perhaps the info has not been posted on the net…. and this is what this blog is all about, we post and share projects carried out in our own workshop.

So we set out to see if it was indeed even possible, the following explanation and the photographs will help to explain how it was achieved.

As mentioned previously, the project began with a blank sheet.

Before the body is removed from the chassis, two wire templates need to be bent and shaped to try to work out where and how the twin hoops will be located behind the seats, the longest legs (Outer) of each hoop will require a bend in order to locate and mate up with a special mount at the lower outer seat belt – harness mounting point. the wire templates allow the making of the hoops so that they also will not interfere with the hardtop, (DHC) or soft top or coupe body of the elan. The height of each hoop will be governed by the roof line of the soft top, coupe or hard top of a drop head coupe, which are probably much the same.

The wire templates may not look much to begin with, but they allow the development of the ideas and provide determination of the actual final dimensions of each hoop, which of course the profiles of each side should match. Once the final arrangement has been arrived at, they are jigged with a cross wire tack welded. The wire templates are to be used to provide the dimensions for the CAD program for the automated mandrel bending machine, the wire template should indicate the centre line of the roll bar hoop, it is important to get that right, because that is what the CNC tube bending people require.

Once the hoops have been formed in the mandrel bender to match the wire templates, the fun then begins with the fitting of the hoops into the car. Be warned, it is not a matter of simply drilling a few holes to match the mounting plates and everything just slots in.  That will only happen when the project is completed, the roll bar hoops are actually quite easy to remove from the vehicle. But first there is a great deal of fabricating and fitting work to do.

The photo above shows the roll bar hoops as they arrived back from Bend Craft, the manufacturer we chose to carry out the bending job. The hoops were formed from the wire templates provided, but the wire template dimensions were converted into CNC program values.

The tubular angle cut mounting lugs are the mounting brackets for the roll bar outer mount points and the rear upper seat belt/harness points. Disregard the heavy plates with the long bolts in the photo above, the plates were an early design idea, and discarded at a later date.

To understand and form a clearer picture of the project, the photo above illustrates all of the components of the project, the completed hoops and components after the fabricating work, the trial and error fitting, fettling and welding, the job was finally complete and ready to be sand blasted before painting. Photo shows the roll bar hoops, complete with mounting points, plus the rear stays, and the fabricated brackets that are attached to the inner rear of the chassis.

Set between the hoops in the cockpit is a structure called a  triangulation frame which is securely attached with special brackets and high tensile socket head cap screws, plus machined lugs welded into the hoop tubes, the triangulation is an important integral strengthening frame because it ties the twin hoops together in the middle, and creates a very strong triangulated structure in every way possible, in other words it prevents sideways yaw in the event of a roll over, and as always you hope to never experience that, but no matter what type of roll over protection one has, it is comforting to have something instead of just an open cockpit and a frail windscreen frame in front of you which would be next to useless in a roll over accident.

Solid, secure, robust rear chassis mounting brackets. When people discuss the twin roll over bar hoop system for the elan, the general discussion is where to mount the inner part of the hoops? The only place to mount the inner leg of each hoop is to design and fabricate special mounting brackets (Shown above) that allow the mounting of the inner leg of each hoop directly onto the top of the rear short chassis “Y” sheet metal section beams.

The small bosses visible on top of the bracket are designed to protrude through the fibreglass body which covers the rear Y section. The two small bosses are welded to the top plate, located under the bracket is another plate which has been match drilled to match the top plate, the lower bracket has two threaded (3/8″UNC) captive bosses welded on. This particular section of the elan rear Y section is a deep and very strong chassis member, and would be capable of absorbing a high impact load.

The rear inboard L shaped brackets are fabricated to abut onto the top and wrap and fasten to the outside of the rear chassis member, the brackets are of sandwich plate design placed either side of the chassis sheet metal, if you are thinking that this looks like lot of work, you are correct, the mounts need to be anchored firmly and securely to the chassis.

A view of the opposite inboard mounting bracket, note the threaded captive collars welded to the inner side and top plate. Use captive nuts where ever it is difficult to put nuts onto bolts especially when working alone, make a habit of using captive nuts, they really do make the fabricator and mechanics life very easy.

The collars that are welded to the brackets are designed and machined to protrude just a small amount through the fibreglass when the body is fitted to the chassis, the holes are marked and cut with appropriate size hole saw, foam can be inserted between the fibreglass and the chassis to seal out dust and air drafts. If you work and mark out carefully, the fit and alignment of body to bracket holes should be a perfect match as shown above.

The strength of any tube weld joint rests with how good the fit of the mitre cut is, unless exact lengths are known, a lot of time is spent with trial fitting and cutting until the tube fits as required.

Much attention is also required at the other end of the rear hoop support tube, as it also must fit correctly to the support bracket which is held by the rear harness mounting point. Remember, this project is an S2 DHC elan, later model elans may have small differences. Fitting the roll over bar should be carried out before the paint has been applied to the body, if as is the case here it is being done afterwards, we had to make sure to use plenty of wet protective rags shields etc when tack welding.

The Bi Metal hole cutter works well as mitre cutter when new, but they will wear out and become blunt after several cuts, even when using the correct cutting speed and proper cutting fluid. A bench lathe can easily be set up for accurate mitre cutting of the tube, using a Bi metal hole saw, or make a DIY fly cutter using an old lathe boring bar.

An alternative method of forming accurate mitre cuts with tube is to make and set up a special holding fixture (clamp) in the lathe and use a boring bar with a fly cutter inserted, the correct cutting angle can be easily set by using the degree marks on the cross slide/tool post holder. Mitre cuts can be quickly cleaned with round/half round files as well as on a linishing belt and or a die grinder fitted with a carbide burr.

The Triangulation Of The Roll Bar Hoops.

The view with the roll bar hoops bolted in after all of the trimming and upholstery has been completed. To the casual observer, it appears that the inner hoop mounts are simply bolted to the fibreglass, but of course they would be very wrong to assume that, when in actual fact we have explained to you just how strong and secure the roll bar hoops are, having being fastened to the rear chassis members.

The mid mounted triangulation frame is constructed from 7/8″ Dia x16g ERW mild steel tube, it also features a drilled and belled mild steel sheet metal brace welded to the top of the frame, the entire frame is bolted to the hoops on each side near the base mounting brackets and at the top as shown, the drilled and threaded lugs in the hoop tubes are machined from high grade carbon steel, they are “top hat” shape and threaded for 3/8″ UNC socket head cap screws.

The triangulation frame is an essential part of the installation of the roll bar hoops. The complete structure, once fastened to all mounting points is completely triangulated, and is incredibly strong, and also forms an integrated part of the chassis, which will also help to stiffen up the rear of the elan chassis.

The requirements with metalwork should always be “best quality”, be proud of what you are building, take care, measure and cut accurately, take your time, if you are not sure about something, stop and think about what you are doing, do the very best quality workmanship that you possibly can, your workmanship will be on full display whenever you are out driving, or where ever you park your car, or heaven forbid, should the worst happen. But I will say this, having excellent roll over bar protection makes you feel more confident while driving, while taking the utmost care while driving in a spirited manner.

The Completed Project.
It is nice to sit back afterwards and look at what has been achieved and feel confident that the project has been a success, while it is several months since this project first became public, many favourable comments have been made from many other elan owners, two Aussie owners wanted roll bar hoops built for them, as did about three from the USA, but they were all declined, we are happy to outline and provide info about how the project was carried out but that’s as far as it goes.

You are right, this is no ordinary S2 elan, the numerous refinements and mods, many of which are outlined within this blog, have helped transform this S2 elan into an amazingly quick road car, there are not many cars of similar capacity (Or larger Cap) that can get near it on twisty & winding Alpine roads… could be a road going 26R but with more comfort.

Now all it needs is a a few more HP in the engine dept, (Elan now has 1700cc Twink–Very fast) S2 elan also now has Falken Azenis RT615K which are quite sticky, and just the thing for some serious Alpine road driving.

Be excited about your elan. Given the time, knowledge, and resources, you can build a great performing road elan that will give you more bang for your buck and huge grin factor, than most other sports cars on the road today.

The motoring world will absolutely never see the likes of the lotus elan, ever again, they are a remarkable sports car. Is it any wonder that prices of this awesome little sports car are climbing. Would be elan owners should grab one soon before they become really hard to get.