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Strengthen The Roll Bar

One noticeable feature of Se7en style kit cars is the rear roll bar. On cars used for racing, this is functional and very useful safety item with cross bracing and additional support. On many roadgoing Se7en style cars, the Tiger included, this is more of a cosmetic item than a safety item, being bolted to the chassis by a few bolts with no additional support.

Whilst the cars are inherently very stable with a low centre of gravity, it is still potentially possible to roll the car. If the car was to turn upside down, the standard roll bar would just collapse and wouldn't help you survive the accident in any way.

To improve matters, I am modifying my standard roll bar. This will include the addition of two drop bars from the top of the existing roll bar down to plates mounted to the rear of the chassis. There will also be a diagonal cross brace from the offside top to nearside bottom edge of the existing roll bar. The drop bars will provide additional front to rear strength and the cross brace will provide additional side to side strength.

The standard Tiger roll bar is made from 45mm outer diameter tube. My rear drop bars and cross brace are made from 40mm outer diameter tube with a 3mm thick wall. I got these two drop bars bent with a 30 degree bend, the centre of which is just below the lip on the rear body tub.

To ensure I had sufficient material to cut and file, the steel tube I obtained was significantly too long. This was cut to almost the correct length, V shaped sections cut out, then profiled to fit snugly against the roll bar using the powerfile.

Drop Bars - Unmodified (Top) and Shortened (Bottom)
Drop Bars Before And After Shortening And Profiling

I completed the drop bars and the diagonal crossbrace, then tested the fit before having them welded together. I then found a big problem. The bottom edge of the drop bars wanted to go about ½" through the top corner of the fuel tank. If they were mounted higher, they would need to stick out of the rear bodywork by around 1" at the top, again not what was required. Having followed Dads instructions, making sure the centre of the bend was at the position of the rear bodywork, I telephoned him and explained the problem. I then started playing around with some 40mm diameter plastic drain pipe. I calculated that the centre of the bend should have been level with the top corner of the petrol tank, some 4-6" lower than where the corner currently was.

Upon recommendation, I took a sample drop bar, along with details of the differences in length to a local engineering firm. Two weeks later when I called to check on the progress, the person told me he didn't actually have the kit to bend the rollbar tube properly and wouldn't have chance to look at it any time soon. Suffice to say, I cancelled my order. When I went to collect my sample tube, which had the only copy of my measurement details, he didn't even say a word to me - customer service at its best!

I contacted Merlin Motorsport who are able to supply and bend steel tube. They were very helpful and I sent a fax explaining what I required which consisted of two 1 metre long 38mm CDS tubes bent to 30 degrees with the centre of the bend being 300mm from one end (and 700mm from the other). They called me back within 10 minutes of me sending the fax saying they could do it and it would be delivered at most three working days later as it takes them up to 48 hours to schedule the bending of the tube in. Within two working days, the new drop bars were delivered and were bent to my specification.

I followed the same process as above to cut the drop bars to length and profile the ends, remembering to have the centre of the bend at the corner of the fuel tank. For the bottom edge of the drop bars, I used my newly bought angle grinder to gradually grind away the material so it was at the correct angle to sit flat against the lower mounting plates. In the absense of the new tank, I put the old tank on some blocks of wood, plus ½" higher and further back to ensure adequate clearance.

Drop Bars - Correct (Top) and Incorrect (Bottom)
Drop Bars Cut To Length - Correct (Top) and Incorrect (Bottom)

End Profile of Drop Bars
End Profile Of Drop Bars

Dads next door neighbour, Pete, is a welder so the easiest way for me to get the rollbar welded was to take it to Dads, put it onto his car and get Pete to weld it. Before taking it to Dads, I borrowed his rollbar to double-check the chassis were the same. They weren't and in fact were different by between ¼" and ½" in places! I carefully measured where Dads rollbar sat in relation to my chassis. I then made up three templates from blocks of wood to get the drop bar spacing correct and to ensure they were welded at the correct angle so as not to foul the rear bodywork. All the rollbar tubes and steel were taken to Dads and arranged on his car, taking into account the differences.

A couple of days later, I picked up the finished article from Dad. Upon trial fitting it onto my car, it generally fit where it should have. The nearside drop bar had twisted round a little and thus was around 10mm from where it ideally wanted to be. However, the flexibility in the steel allowed it to be moved into position and clamped. Since taking the picture below, the feet on the drop bars have been trimmed to size and drilled.

Welded Roll Bar
Welded Roll Bar

The rollbar is now complete having been powder coated by a local company and is bolted directly to the chassis.


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