Dale Amy
November 1, 2000

Step By Step

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138_16z Ford_mustang Left_front_view138_98z Ford_mustang Chassis_view
A couple of the 25.1C requirements are best observed in a pair of photos looking forward from beneath the cars. First, notice in this shot that Don Walsh’s car does not have a full-width horizontal forward crossmember, or foot brace.
138_00z Ford_mustang Chassis_view
In this shot of Billy Glidden's car, you can see the forward crossmember just behind the headers (arrow)—25.1C requires one. Also obvious in this photo are the 25.1C-required diagonal braces triangulating the inner and outer longitudinal framerails, running right from the front to rear crossmembers, as well as the mandated pair of 360-degree driveshaft hoops.
138_01z Ford_mustang Frame_rails
In this and the following shots of the area beneath the driver’s seat, the required triangulation braces between inner and outer frame-rails are apparent on Billy’s car...
138_02z Ford_mustang Frame_rails
...and notably absent on Don’s. Notice, too, the bends in Don’s framerails. These have no bearing on the car’s noncompliance with 25.1C. They are simply a result of Skinny Kid’s having originally laid out the chassis tubing to use a stock floorpan and rear subframes, as its original owner specified.
138_03z Ford_mustang Forward_cross_member
25.1C allows the forward crossmember, or foot bar (not present on Don’s), to be either one piece or three.
138_04z Ford_mustang Forward_cross_member
If in three pieces, the segments cannot be offset where they meet the inner framerails. This bar, which forms part of the perimeter cage structure, must pass within 6 inches of the fully depressed pedal position.
138_05z Ford_mustang Interior_view
The location of the foot bar is apparent in Billy Glidden’s interior. Both Don’s car here and Billy’s in the photo below have the 25.1C-mandated driver- and passenger-side secondary door diagonals (running from high up front to low in rear).
138_06z Ford_mustang Interior_view
Notice also that Billy’s cage has two small vertical bars joining the door diagonals in the vicinity of the driver’s elbow. These are to keep the driver’s arm within the cage during impact or rollover. The relative positions of the steering column and the tubing surrounding the driver (called the funny-car cage) in this shot and the one above also show how much farther back the driving position is in Billy’s car. This will be more obvious in the interior shots.
138_07z Ford_mustang Roll_cage
Don’s funny-car cage is not 25.1C compliant, since its rear vertical elements do not tie directly to the lower horizontal cage support (instead, they tie to the forward cage hoop).
138_12z Ford_mustang Roll_cage
In contrast, the rear hoop in Billy’s funny-car cage (along with the two vertical helmet-retention bars in between) are welded directly to the lower horizontal cage support. Note that Billy’s cage sits nearly a foot farther back. A padded headrest is also mandated unless a high-back seat, with head support, is fitted.
138_13z Ford_mustang Roll_cage
Compare the sections of the funny-car cage right above the driver’s head. The 25.1C specs require bars or straps to keep the helmet inside the cage area (top and otherwise). Billy’s cage has it...
138_14z Ford_mustang Roll_cage
...Don’s doesn’t. Another 25.1C spec calls for a minimum 1-inch clearance between the cage and the top of the driver’s helmet. A diagonal roof bar is required. The configurations shown in both cars here are legal. In Billy’s car, shown above, it commences at the right front corner of the cage and terminates at the front hoop of the funny-car cage.
138_15z Ford_mustang Roll_cage_gusset
At least three corners of the cage (logically, those opposite from the driver’s cage) must have corner gussets (arrow) of at least 4 inches in length. In all, the 25.1C specs are designed to provide a rigid, strong cocoon around the driver, keeping his head and limbs safely within the car.

We've been hearing a lot lately about how the Pro 5.0 field is running full-throttle into the 25.1C chassis specs. Sure, there's been a temporary reprieve from the NHRA proposal to invoke 25.1C rules for anything trapping faster than 170 mph, but the fact remains that those chassis specifications are already a requirement for any full-bodied car going 7.49 seconds or quicker.

OK. Maybe no one thought the Pro 5.0s would be anywhere near that low an e.t. for some time but, as we write this, Joe Silva has already gone quicker (7.39) and others are sure to follow in short order. The days of competitive 8-second Pro 5.0 cars are behind us. So here's a suggestion. If you're considering making the huge investment necessary to build a new Pro 5.0 missile, you'd be a tad goofy not to construct it to 25.1C standards.

So what exactly does 25.1C involve? While we lack both space and knowledge to answer that question in full detail, we can give you some ideas of the differences between a 25.1C cage and a normal Sportsman-type chassis. For help, we turned to a Michiganer who has built both and who just happened to have one of each in his shop.

Contrary to what you may have heard, Don Walsh Jr.'s year-old yellow notchback is not 25.1C compliant. For that reason, Don currently has a new car under construction. As a matter of fact, in the following photos we're using Don's sedan as the non compliant comparison to Billy Glidden's brand-new, fully compliant 2000 Pro 5.0 contender. Both were built by Keith Engling at Skinny Kid Race Cars. Keith is an accomplished chassis builder who, against his better judgment and in a weak moment, agreed to hold our uneducated editorial hands and teach us a thing or two about just what the 25.1C specifications entail.

The National Hot Rod Association [(626) 914-4761; www.nhra.com] did not invent 25.1C. It and other sanctioning bodies simply adopted the advisory specification as developed by the SFI Foundation, which has quality-assurance standards for everything from helmets to harness belts. The 25.1C specs relate to the rollcage area only--from firewall to rear crossmember. They do not set out standards for any tubing extending into the engine bay or behind the rear cage crossmember.

Some of the 25.1C standards are easily described, such as the requirement for all structural material in the cage (including tubing, flat bar, and so on) to be of SAE 4130 chrome-molybdenum steel of various specified diameters and thicknesses. Mild steel is out. Keith Engling guessed that building to 25.1C specs would typically add at least $7,000 to the cost of a Sportsman-type chassis, what with the difference in material cost and in having to craft a lot more floor sheetmetal than if simply fitting a Sportsman cage to a stock floorpan. We don't know about you, but if we were going to pilot a ton-and-a-half land-bound missile at speeds approaching 200 mph, we'd feel that to be a worthwhile investment.

In the accompanying photos we'll try to illustrate some of the basic specifics relating to cage construction (without getting into details such as tubing diameter, wall thickness, or bolt diameters). There are actually four variations in cage design depending on the fore/aft location of the driver's helmet in relation to the main roll hoop. The example we'll use--with the driver situated entirely in front of the main hoop--would likely be most common in a Pro 5.0 Mustang.

A couple of notes about Don Walsh Jr.'s chassis are in order here before we get to the photos. We mentioned that Skinny Kid Race Cars built both chassis used for this comparison and that Don's does not comply with 25.1C specs. Don actually bought the car as a complete rolling chassis from a previous owner.

Skinny Kid had built the chassis for said previous owner specifically to compete in an NMCA class requiring stock rear framerails and a stock floorpan. The point is that the chassis was not optimized for Pro 5.0. Then, at the last minute, Don bought it to use in Pro 5.0, which requires neither the rear subframe nor the stock floorpan and runs at a significantly different ride height. Had Skinny Kid built Don's chassis to run in Pro 5.0 in the first place--even without making it 25.1C compliant--some things would have been laid out much differently.