In my last column, in which I discussed how reaction time is comprised of driver reaction time and vehicle reaction time, I took a closer look at the former. I also described how you, by using a practice tree, could determine your own driver reaction time. In this issue I’ll take a closer look at vehicle reaction time. Although this discussion can be applied to any drag racing application, it’s most relevant to pro tree or bottom-bulb sportsman tree racing where the vehicle does not use a delay box.
The steps a vehicle takes from the point you hit the throttle to the point the front tires break the beams is akin to the steps a series of railroad cars takes upon acceleration if they all start out bunched up as close together as possible. The engineer hits the throttle, the locomotive lurches forward a few inches before catching the first car, which then lurches forward a few inches before catching the second car, and so on. The slack has to be removed from the train before the last car can begin to move.
In our drag cars there are many potential slack points that need to be adjusted in order to have a good reaction time. The chain could look something like this: gas pedal > throttle cable > carburetor butterflies open > air and fuel mixture enters combustion chambers and ignites > converter flashes > transmission > front U-joint > driveshaft > rear U-joint > ring and pinion > axles begin to move > sidewalls begin to twist > slicks bite > energy travels forward through rear suspension to front end > front end begins to lift > launch. These steps happen seemingly instantaneously, but when we’re dissecting seconds into thousandths, there’s a lot of time wasted at various points along the way.
Although some of these slack points are beyond our control, there are three major ones that can be adjusted relatively easily. The first is launch rpm. I’m of the opinion that you should launch at the lowest possible rpm that still results in a good reaction time. If you must go high, the highest rpm at which I could ever hold a footbrake car on the starting line was about 1,000 rpm below the stall speed. The next is tire pressure, both front and rear. Of the two, I feel the rears have more effect than the fronts. Watch a drag car leave the line in slow-motion and see how much time is taken up by the rear sidewalls wrapping up before the car ever moves. If you want less wrap-up, add more air. On the rear of the Dodge I run stiff-sidewall Mickey Thompson 31 x 10.5W bias-ply slicks. I run as much pressure as the track can handle: in the summer when it’s hot and the track might be slick, no less than 12 pounds; in September–October when it’s cooler and the track has more bite, around 20–22 pounds. Last but certainly not least is front-end travel. One of the things I love about the Dodge is its torsion-bar front suspension. Armed with a ¾-inch socket, I can make instant changes in the front-end ride height. I can raise it up to take out travel for quicker reaction times, or I can lower it to add travel for slower reactions times. In lieu of having a front suspension like mine, many companies sell front end travel limiters. These can be cable-type or rubber-snubber-type. Some companies like S&W Race Cars sell both.
So what if you’ve made all of the adjustments and still come up with unsatisfactory reaction times? One possible cause could be that the car just does not have the low-end torque and horsepower to be shallow-staged; the next step is to do some deep-stage testing. Another cause could be using components that are working against you. Case in point: on my website, Dragtimenews.com (The Dragtime News), I discuss installing ATI’s 8-inch TreeMaster converter in the Dodge. You can still read the article on my home page, so I won’t go into the details here, but suffice it to say that I had been chasing reaction time with varying degrees of success for years using a 10-inch converter, and a simple converter swap was all it took to completely change the reaction of the Dodge.
I hope these last two columns have shed some light on reaction time. Feel free to let me know if you have any questions or comments.
Bob Beucler is the publisher of The Dragtime News (Dragtimenews.com), which promotes racing at 32 dragstrips from Alabama to Maine. His DRAGTIME ‘64 Dodge Polara is known at many tracks in the mid-Atlantic and Northeast. Contact Bob at Bbeucler@dragtimenews.com.
By Bob Beucler