Cage / Space Frame

Track-proven design
In the UK Tom Walkinshaw’s TWR team developed the XJ-S into a successful Group A car for the European Touring Car Championship, Walkinshaw (also of Scottish roots) won the title himself in 1984. Jaguar eventually bankrolled a full Group C sports car program, eventually netting them world championships and Le Mans victories.

In the US, the Group 44 team of Bob Tullius ran a space-framed XJ-S.  It is this space frame car which I will be basing my chassis/cage design on now that I have received approval of the concept from the BNI tech advisers.  Here is the only image I have found floating around of an unfinished G44 space frame.

Through a lot of research, digging, questions and studying photos, I believe I have a fairly good understanding of the original design and features used.  Of course, some changes will have to be made as the PRO class we are competing in, requires that while the seats can be removed for the purpose of making runs, the seats must be capable of being in the car.  So, I’ll be playing with some of the cross bracing to allow for this.  For the most part, a relatively minor change which has little impact on the beefed up integrity of the chassis.

I’ll be using 2 inch .120 wall DOM tubing.  All the bending will be done in house here at TMR on our 15 ton Pro-Tools draw bender.  With the dies we have on hand we can bend square and round pipe and tubing from 1/2″ up to 2″ up to in excess of 180 degrees

The chassis / space frame is going to be done in a rather unorthodox manner in two stages.  The first stage will be installation of all the interior members to create the ‘cage’ aspects, with little change to the original structure of the car.  The second (optional) stage will see the installation of the 2×4 frame rails, and front structure.  Also, the tubes into the trunk area will be sandwich plated through the rear bulk head connecting the sections, rather than running a solid tube through the bulk head.  Using this method, I intend to be able to remove and repair or replace components if required, or, simply for the purpose of easing access to areas and components

Step One, the Halo

The first thing I’ll be building is, the halo, which circles the roof area, and is supported by 6 posts.  It’s a lot easier to position and fit the posts onto the halo, than to position the posts, and try to fit the halo sizing to match the posts.

Obviously, you can bend a single piece of pipe, and make it form the halo, but, it’s MUCH more practical to bend two ‘U’ shapes, which will be joined together to form the completed halo.

(web host has lost image files below.  Sorry for inconvenience.  We are attempting to locate original for replacement)


Obviously, you can’t bend a single piece of pipe, and make it form the halo.  This will require two ‘U’ shapes, which will be joined together to form the completed halo.  Interestingly enough for those not familiar, when bending components, you must bend past the angle you want for an end result.


Here is a bend I’m taking to 90 degrees, but, because of the spring back of the material, to hit the target, I must go to 94.4 degrees.  Accuracy is the key to get the results you desire.  It varies with the degree of bend, and the size and thickness of material.For instance, making the same 90 degree bend, in the same 2 inch, dimension and .120 inch thickness tubing, but in moly tubing, rather than mild steel, the tubing must be bent to 94.9


When joining or splicing, you must sleeve the joints.  In the case of this material, I’m using 1 3/4 inch .120 wall tubing. which leaves about 0.01 clearance.  Sounds great right?  Not quite so fast.  This is seamed tubing.  This means that in the manufacturing of the tube, a flat piece of metal is rolled to form the tube, and the edges are welded to complete the tube.  On the inside of the tube, is an exposed weld surface which must be ground down to allow the sleeve to fit.  Additionally, if you’ve used a pipe/tubing cutter, it has compressed the end of the tubing as well.  This too must be ground open again to accept the sleeve.


Once you get all the edges cleaned up, the sleeve should fit fairly cleanly.  If some persuasion is required, a rubber mallet, or a hammer with a block of wood to ‘set’ the sleeve might be the solution.  DO NOT beat the crap out of the sleeve, or you’ll pay the price when you go to slip the other side on because it will be bent and deformed.  Take your time, it will pay off.In the case of this joint, the sleeve is able to fit in and bottom out against the bend in the pipe.  If it did not have this ‘stop’, to make sure it doesn’t move during the fitting of the other side, you should drill a small hole through the outer tube, and spot weld it in position.


Having the two halves align perfectly would be a fortunate turn of events.  More likely, despite all efforts, you’ll have some difference in angle and width to contend with.  Once I have the two sides started, I’ve used a cargo strap to apply tension and keep a constant force pulling the halves together.  A little convincing with the rubber mallet may be required.


Ultimately, your sides will come together to form your joint.  Leaving a small gap between the ends will ensure the weld fillet penetrates the full depth, and bites into the sleeve, making it an integral part of the joint and over all structure.   Get out your preferred welding device and have at it.  I’ve used a TIG in this case, which allows great control of the weld quality, and maintains a neat appearance, without excessive heating of the material.  You might say, why worry too much about appearance, you can always grind it down, or cover it etc.  There is the rub.  Tech rules stipulate you must NOT grind or alter your welds.  This is the only way the weld quality can be evaluated to determine it is adequate.


All said and done, your halves will pull together, you will perform a beautiful weld, requiring nothing more than a wire brushing to clean up, and your halo will be complete ready to form the basis form the central point for the remainder of the interior protective cage.

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