[Ron Sutton]

Part 2 of 3


Now would be a good time to discuss chassis rigidity. Like most complex things, it makes sense to peel the onion a layer at a time. So bear with me as cover several key points.

Key Note: In my explanation about chassis rigidity … I will refer to “energy loss.” Think of engine power as energy & speed as stored energy.

Rule of thumb on chassis rigidity:
The more rigid the chassis … the less energy loss the chassis experiences … the quicker & faster the car “can be” … if tuned optimally inside its handling “sweet spot.” But, the optimum handling sweet spot gets progressively smaller & smaller as chassis rigidity increases … as the car becomes more responsive to tuning changes with larger effects.

The less rigid the chassis … the more energy loss the chassis experiences … reducing how quick & fast the car can be … even when tuned optimally inside its handling “sweet spot.” But, the optimum handling sweet spot gets progressively larger & larger as chassis rigidity decreases … as the car becomes less responsive to tuning changes with smaller effects.

If the chassis is too stiff, the sweet spot gets super small & the car is too sensitive to changing track conditions. I bought a race car once like this. We used to say, “It’s the fastest car on the track for 10 laps. But as rubber gets laid down over the course of a main event, the car’s handling changed too much. That car was great for winning poles & but difficult to win races with.

On the other hand, if the chassis is too soft … often termed a “flexi-flyer” … the sweet spot is super wide … because the car is not very responsive to tuning changes. I’ve been hired to help tune on these cars and the driver doesn’t feel a 50# spring change.

Let’s talk chassis rigidity … but instead of trying to use torsional rigidity formulas & numbers, I’ll use generalizations that will make the concept easier to understand & discuss amongst us car guys. To describe levels of torsional rigidity let’s keep it simple by using these general terms, like a scale of stiffness in this range:

Extremely Stiff
Very Stiff
Moderately Stiff
Intermediate Stiffness
Moderately Flexible
Very Flexible
Extremely Flexible

And let’s embrace these terms are relative to the application. Here is an example …

In drag racing at the IHRA Pro/Stock & Pro/Mod level in the 1980’s … which raced mountain motor cars, with cubic inches ranging from 615” to 672” the “standard” Pro/Stock chassis design flexed too much, and was hurting 60’ times & therefore overall ¼ mile times. Chassis builders strengthened the torsional rigidity of the chassis by building a narrow version of a dragster chassis in the transmission & driveshaft tunnel area … connecting the 4-link to the rear motor plate. It basically added upper frame rails down the center of the cockpit along with uprights & diagonal braces connecting the two existing lower center frames rails with these new upper frame rails. It looks like the front half of a dragster chassis in the middle of the car.

There were called “Double Frame Rail” cars & measurably improved the 60’ & ¼ mile times of Mountain Motor IHRA Pro/Stock & Pro/Mod drag cars. When NHRA Pro/Stock racers bought & raced them, they didn’t go any quicker. In fact, they didn’t like them, because they became more finicky to tune the suspension & and went away from that design. (I do not know what they run today, as I haven’t been to a drag race since the end of 1987.)

So why the difference? Major differences in torque at launch. The typical NHRA Pro/Stock car achieved just under 500 cubic inches with a maximum bore of 4.625” & strokes around 3.700”. The 672” Mountain motors used the same bore … but 5.000” strokes. Even though they only made 80-100 more hp, the torque difference was HUGE … and the chassis experiences that torque at the drop of the clutch.

For the NHRA Pro/Stockers the standard Pro/Stock chassis was “Very Stiff” with minimal energy loss. The Double Frame Rail chassis was “Extremely Stiff” and too sensitive to track & tuning changes with its narrow sweet spot.

For the IHRA Pro/Stock & Pro/Mod racers running Mountain Motors, the standard Pro/Stock chassis was only “Moderately Stiff” … and while easier to tune with the wider sweet spot … suffered from some energy loss. The Double Frame Rail chassis was “Very Stiff” reducing energy loss, and making the cars quicker … but brought the sweet spot back to normal.

Just understand the car’s application … power, speed, car weight, etc, play a role in the desired chassis rigidity. There are many goals in designing a chassis, of which one key goal is designing the structure to minimize chassis flex & twist to a high degree … but not so much as to make the chassis hard to keep in the sweet spot of its suspension tuning window.

Here is an example of this …

Bob East of Beast Race Cars in Indy designs & builds some of the winningest cars in open wheel oval track racing. His Midget chassis have won a gazillion races. When I designed our Gen 2 Midget chassis for my race team, we studied Bob’s design. We initially kept his frame & cage layout, but moved most of the suspension points.

The cars were fast & had a wide sweet spot. We won races with it. After running it for a season, with our Engineers running data acquisition on our 4-6 race cars every outing, we could see where the chassis was flexing & how much. Frankly it was/is a great chassis for most racers, because the relatively wide sweet spot makes it easy to tune, & harder to “go off the range” as I call it. Because the sweet spot was wide … the chassis didn’t require a lot of tuning as track conditions changed. I suspect Bob received less customer complaints & had more happy customers due to this.

I was clear we were leaving some performance on the table through energy loss of the chassis flex. As a veteran tuner, I wasn't worried about our ability to "keep up" with the track changes. So in the off season, we redesigned it & built new chassis that stiffened the chassis in two key areas … and the car responded. It produced faster corner speeds & quicker lap times. The drivers noticed small tuning changes more. The sweet spot did narrow, but we didn’t go so far as to make the car “finicky.” But enough so, that we needed to tune the car every round to keep up with changing track conditions.

Because the chassis was more rigid, with less torsional flex & twist, the car required more tuning … but because it was more responsive, the tuning changes needed were small. We got the suspension set-ups so dialed in that tuning changes of 1/2 pound in tire pressure, 1/8” in roll center … OR … .050” in sway bar size difference was all that was needed. And we won a LOT more races.

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