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What's with all the double framerail talk? What are they?
Do I need one?
by Patrick Budd, ProCar Performance
If you've been paying attention at the NSCA or OSCA races lately
you might have noticed cars in the staging lanes that look a lot like Pro Mod wanna-bes. Yards of diagonals take up
the mid sections of these cars, bracing everything from the four-link to the trans. What gives? Billy Glidden's car
never looked like that, and it has been one of the fastest 10.5 cars for years - Do you kind of feel like everyone
else gets the joke except you? Well, you're not alone. Guys like Leo Barnaby and late Steve Grebeck have pumped out
some pretty stout hardware and won championships with their designs - Why?
To answer this question, let's look at some of
the dynamics that take place when a big power, automatic, well hooked (and that is an important distinction) car lets
go of the button and sends 1500+ hp rocketing down the driveline to a waiting ring and pinion. Unlike a clutch car
that may give up some 60' time to pick up 330' utilizing wheel speed, the automatic car needs to (usually*) dead hook
the tire to take advantage of the torque converter's multiplication. The lower the first gear is (numerically) the
more critical the hook in that first 10' is- a 4.10 gear will multiply against the converter for a longer time than a
4.56 will. If the 4.56 loses grip, odds are it will spin hard enough to slow acceleration, then pull the motor down
when the tire hooks- short time be damned. The 4.10 may recover earlier, before the engine senses the tire spin thru
the converter. (*I say usually because as auto cars get faster, they do benefit from wheel speed like their clutch
cousins. Most reading this won't have the power to spin the tire, keep it spinning long enough to overcome the need
for that stall cushion -otherwise known as "getting up on the tire'- and transition to converter lock-up
gracefully. The powerglide car has to stay "up on the tire' a looong time to avoid shake. A Powerlide's first
gear is 1.80 or so and takes longer to accelerate the driveline than a stick car that has a 2.52 or so first and a .2
or so rear gear advantage. This takes a power to weight ratio unattainable by most to turn that tire that long. For
the purpose of this article, we will deal with the majority that do not.)
In other words, if you have a 1.80 first gear with a 6000-rpm
converter you'd better hook the tire or even limited wheel slip will kill your 60' and spoil your pass. A car with a
clutch will actually try to turn the tire a bit, critical to keeping the engine from bogging and flipping the hood
scoop in the air. The 
clutch car also uses much looser gearing in the trans
and rear end. The differences your chassis sees are huge; all the pushing, shoving and twisting your engine power
transmits to your four-link is bound up with nowhere to go when you dead hook a tire. The driveline is trying to
catch up to engine speed (converter stall speed at launch) with no "give' at the tires to cushion the fury.
Until the driveshaft catches up with the lock up speed of the converter, the engine is not accelerating at an
adequate rate to keep the load off the chassis. It's as if it's being held by a dyno at launch and again at the
gear change. If you have enough power, and hook the tires well enough, eventually the car will begin to flex and
lose square. It won't respond to a shock click or a flat of pre-load anymore. You can't make an accurate tuning
change because the car is unable to respond predictably at that power level, and the more you continue to flex it
the worse it's going to get.
Does this mean that some Pro Mod or Pro Street cars are actually
weaker than the latest NPS cars? In some cases, the answer is yes. The tubing in John 
Schroeder or Rob Golobo's late model cars is larger and thicker than you'll find in many
current Pro Mods. Consider this: the NPS car runs at a curb weight 900 lbs heavier than a Pro Mod, it makes
"only' 400-500 hp less (they are in the neighborhood of 1700 hp) and must cut a 1.14 60' to be in the upper half
of the field. By comparison, our elite Pro cars may run as "slow' as a 1.11 60' interval on the way to a 6.70
pass. The clutch car may have juggled ratios and lock-up to get down a bumpy track, pouring the steam on late. Neither
of these are options most automatic teams have- they compete in single stage nitrous classes and/or are locked into
their gear ratios. They need to hit it as hard as possible right away and keep it glued.
Now, it isn't all bad news for the automatic car. It has the
advantage of a hydraulic coupling between the engine and the driveline, which softens the initial hit, disguises some
tire spin, and it is very predictable to tune. Getting a car to hook up and cut low 60's is often easier with a
converter than a clutch because of the smooth way it delivers power, and once you're happy with the stall, slip and
multiplication, you're able to forget it and turn to other parts of the car. The clutch needs constant adjustment.
This is all good news for the converter manufacturers, the motor, the tires, and the time-slip, but can be tough on
your chassis.
The
answer has been to borrow a page from the Pro Mod teams and install a double framerail, typically 1.250 x .058
hovering less than a foot above the main rails. Its job is to give a positive link between the upper four link
crossmember and the "mid hoop' or reinforcing in the mid-plate area of the car. Some builders extend the rail
forward, some don't, it's a matter of preference, but they're all placed in a very deliberate and careful manner so as
to add strength and rigidity without adding significant weight. A double framerail is not a diagonal; it is not run
rearward high to forward low, terminating at the base of the mid plate or foot bar ala Jerry Bickel or Haas cars used
in Pro Stock. (Great cars, just not an example of what we're discussing.) It is a spine that protects the car from
tortional twist and flex, exactly the loads placed on a car by 632 inches, nitrous, blowers, 1.80 first gear ratios,
4.10 final drives, and 35” tires. A true double rail car is not a single rail car with an arbitrary add-on; once
in place, the upper rail sees a lot of force if installed properly. Since cars designed to run a second framerail will
often juggle crossmember heights, widths of various spans, floor material, etc., it's important to do an update with
an open mind and understand what the original intent of the car was as constructed. The update must work with the
car's strengths, but not enhance its shortcomings.
So when do you need a second frame rail? Chris Alston's catalog
suggests the 7.99 mark is a good place to start; yet contemporary Pro Stock still doesn't utilize the design.
Evidently the short stroke, low torque nature of those engines allows them to manage their torque without resorting to
the weight of a second rail. IHRA Pro Stock cars still 
don't
employ them with any regularity, but their construction is getting burlier one by one, literally as they come out. It
won't be long before they're tough to tell from the Pro Mods. We suggest it to our customers any time a car will see
more than 1700 hp or so, especially with a converter/beadlock combination. A 3500 lb 8.10 car with 33x16's will need
more support than a 2800 lb. car on 10.5's going the same times. Small tires don't seem to load the chassis as much as
a car with big tires at launch, and weight is a big factor. We don't recommend the double rail option on a 10.5”
car unless it will live most of its life in the 7's. Dan Millen's 10.5 Skinny Kid car is uncluttered compared to some
of the jungle gyms out there, but it is as fast and consistent as any in FFW or NSCA. Cars built to SFI spec 25.2 have
a more rigid floor, possibly reducing the load the upper rail will see. That's what led us to keep Nick Scavo's second
rail rather minimal in his new car, when compared to the Stanley/Weiss Pro Outlaw 25.1E piece finished mid '04.
It boils down to this; the stiffer the car, the more responsive it
is to input; be it from power, pre-load, shock clicks, spring rate or bad driving. Not everything needs two rails in
it, but using the chassis as a "fifth spring' is a less efficient way to the finish line. Today's shocks are so
much better than the ones we had just 5 years ago, springs hold their rate with much improved accuracy, wheelie bars
are built and tuned with greater precision, all contributing to a better, more gradual launch. Recent trends in
chassis construction like anti roll bars, thicker and stronger four link bars and fabricated housings have locked the
movement of the suspension in place. Why would we let everything forward of that twist around wasting energy? It only
takes a slight shift of the fourlink area to make the car react as if its preload had changed, mid run. By designing a
car with a second framerail we can end up with a faster, stiffer, safer racecar.
Copyright © Patrick Budd, ProCar Performance
