FF main hoop question

[troyt]

GCR 9.4.7 - [SIZE=1]The old main hoop may be cut off near the chassis mounting and a new main hoop of equal tube size or a section of equal tubing size may be added, and inner tube(s) must be used to mate all sections together.

My question is the definition of 'chassis mounting'. In a typical Club Ford chassis, the main hoop attaches to the top frame rail, and then there are some type of frame supports down to the lower frame rail/belly pan. So how far above top frame rail can the main hoop be cut for repair or enlargement? I am seeking to build a taller hoop to meet the 2" below horizontal test AND the broomstick test.
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[S Lathrop]

The main hoop would stop where there is a transition from the 1.375 tubing or what ever your car has to anything else. You might get real lucky and have a car with 1 1/2 roll bar and you can slip a 1 3/8 roll bar inside. But that might be too good to be true.

An example is the Zink Z10 where the main hoop welds on top of the junction of the roll bar bulkhead, the longitudinal rails that fasten to the bulkhead at that point. There are additional gussets in that area that the main hoop attaches to. A Zink Z10 is a easy car to change the roll bar. It is all TIG or MIG welded, mild steel frame. The trick with a Z10 is that the roll bar braces are difficult because the car was designed with a rear facing roll bar brace which is no longer sufficient. Forward bracing has to be added and this makes the job much more complicated.

With most English built FFs, the frames are Nickel Bronze welded. This is similar to brazing but it is not the same filler material. Brazing is not an acceptable substitute process. Unless you really know what you are doing, don't modify the roll bar mounting.

This is the reason that the rules allow you to splice a new roll bar into an existing structure. The biggest issue doing this may be the ability to properly bend the roll bar so that it mates to the existing roll bar where the splice is to be.

For the proper details of how to splice a roll bar with an inner sleeve, refer to FAA Airframe & Powerplant Mechanics, Airframe Handbook, EA-AC 65-15A. You can and should get a copy from Wicks or Aircraft Spruce. This manual is a great reference guide for how to perform many of the repairs and maintenance chores on race cars.

Using the recommendations for this type of tubing splice contained in the Airframe Handbook, the inner tube should be 6.375 inches long and of the same wall thickness as the main hoop. The splice should cut at a 30 degree angle to the center line of the tube. The there should be 1/8 in gap between the ends of the spliced tubes for welding. And there should be a provision for rosette welds 13/16 in from the ends of the inner sleeve. I would place the splice as close to the base of the roll bar as possible, which would be no closer than 3 3/16 in from the mounting of the original roll bar.

[troyt]

Thanks S Lathrop for your reply and the well thought out response. I am researching this technique and I know it will be tricky to pull off well for my Crossle. The reason I had this post in the SCCA Officials section before a moderator moved it here is because I have received different answers from different Tech Inspectors/Scrutineers and was hoping to find an "official SCCA" reference.

[mike g.]

For the proper details of how to splice a roll bar with an inner sleeve, refer to FAA Airframe & Powerplant Mechanics, Airframe Handbook, EA-AC 65-15A. You can and should get a copy from Wicks or Aircraft Spruce. This manual is a great reference guide for how to perform many of the repairs and maintenance chores on race cars.

Good reading:

http://rgl.faa.gov/Regulatory_and_Gu...hapter%206.pdf

[stephen wilson]

You have the official reference in 9.4.7, with pictures! Obviously you could replace the entire roll hoop, but as Steve mentions, that does present some additional issues. The scrutineer the gave you a different opinion needs to go back and study the GCR.

[racerdad2]

Thx for the fantastic references :thumbup: The wealth of information on this site is incredible

[Steve Demeter]

Ditto on the reference posted by Mike