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NHRA Rule 25.1C--Chassis Specifications
Pro 5.0 Fans Gather 'round, As We Demystify The 25.1c Chassis That Protects The Fastest Racers
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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.