So, what's the secret to an arrow straight pass?

DragStuff Home > Article Index >

So, what's the secret to an arrow straight pass?

by Patrick Budd, ProCar Performance

The arrow straight pass- pretty, smooth, efficient - almost looks slow. You know the one- it leaves you slack-jawed on the starting line. The one you want your car to make, the one that looks so clean, so effortless, even the gear changes didn't unsettle it. Everyone wants these runs, few have them, and fewer yet string them together. What's the secret? We'll explore some of the more common reasons for inconsistency and their fixes here. It's not light reading; Ian Rae said this reads like a dental textbook. Like all my articles, it hammers points multiple times and may cause intense drowsiness. Read at your own risk. Still, if you have a problem, this may help you. As always, formulate your own ideas and tune as you will, but consult your chassis builder or use one of the advertisers in this book to assist you before you make drastic changes. Buckle up!

The key to making clean, straight passes lies primarily with the driver. I know, that's not the sexy answer; most are reading this to learn about anti-roll bars and wheelie bars, but please don't skip this paragraph. I watched a guy, twice no less- with two different cars- prove there was nothing wrong with his suspension tune-ups by climbing in and making passes you could have gauged by a laser beam, in a car whose owner was bitching loudly about it. What did or didn't the set-up guy do? He didn't move the wheel during the run- seriously. Most drivers using Lenco-type transmissions have to fight the tower pressure to complete the shift. Unless the left side of the driver's body is wedged into the car, the effort required to move the lever with the right arm WILL affect the left, or driving shoulder and arm. The tug on the wheel and counter movement back to the center often cycles and gets worse, causing the driver to abort the pass. There are tune ups less affected by an antsy driver, but in reality it's his or her job to keep the wheel still. Keep your eyes on the sand traps- don't drive at the bumper. Your peripheral vision is much more sensitive to sudden changes or movement than looking at things straight on. Racers often drive with their left hand at ten to twelve o' clock- comfortable, but a mistake. I cannot count the number of drivers, big name guys with big reputations, that still roll their left hand down from twelve to nine at the launch, and once the tires touch down, slightly correct right again, scrubbing ET. I used to do it to until corrected otherwise - I'm not throwing stones. Lock the hand in at 8 o'clock, wedge the elbow and if you have to steer more than that position allows, you should probably be lifting anyway.

Awright. What mechanically makes for a straight pass? What can be done to the car if the driver is already doing his/her job? First and foremost, set the car up with as little mechanical preload as possible. Once you get the four link or ladder bar bound up with preload to get it to launch straight, the car often wanders around up top. Low to medium power cars or those that weigh over 2800 lbs. often have issues with the drivers front tire hiking up at the hit and the whole car arcing right in the first few feet. A few flats of preload (shortening the upper passenger bar on a four link, or whatever raises the passenger quarter on your brand of ladder bar) will often cure this, and the car will launch straighter and truer. Now, the same car 300 feet out often begins to wander to the left because pre-load binds the suspension, forcing additional weight (traction) on the passenger rear. Once acceleration slows, the car doesn't need the increased hook for the passenger tire any longer. The driver will often correct without realizing the situation, and it's usually fine. Most low 10 second back half cars are set up this way- never causes a real problem, unless the brakes are nailed hard -

This causes all hell to break loose. Seriously- how many 10.50 to 9.50 bracket cars have you seen fly into the wall, and I mean like NOW, after the driver nails the brakes? I've seen a ton, often from cars that are slower than what you'd expect to crash so hard. It's because that car has 600 hp of go, but enough brakes to equal 2500 hp of whoa. The preload screwed in to a flexible car to launch it straight was no big deal with 600 hp at the beginning of the run and moderate application of the brakes on the top end. Jam those brakes to avoid a breakout, or get into a panic, and that docile 10 second bracket car gets into the barrier in a hurry.

Remember when Pro Mods used to carry the driver tire past the eighth? They don't do that anymore, and for a reason, the flex that caused that hiked-up stance also tugged on the four-link and tweaked the load on the tires during the run. Keeping the driver's front tire in the air lowered the passenger quarter and caused the cars of the time to drift left mid-pass. Measures were taken to get the front down earlier and stiffen the cars in the areas that gave way. Seriously- for those of you that has never had a ride in a very light and stiff or very powerful (over 1500 hp and under 2700 lbs.) car, the fact the passenger quarter drops at launch can drive the car left, even though the same thing causes the 10.0 car to do the opposite. It's confusing but true. The difference lies in the amount of power and the car's ability to use and apply it. When you release your transbrake or clutch, it tries to use that power to twist the housing like a pinwheel, as the pinion drives against the ring gear. Instead, (since it's hooked to the car) it picks up the passenger's side rear tire and plants the drivers and then turns the axles, almost as a secondary motion. The body falls down over the lifting passenger rear, giving the illusion that side has the bite. Not so, at least not for the cars with average power/weight relationships. For the lightest and mightiest, the car is driven left by that same motion. If your car is really stiff and has that 1500 plus hp, it often overcomes the passenger tire's lack of bite when the body drops and lowers it's four link brackets. The big torque and chassis stiffness can harness the massive pulling forces generated by the dropping passenger side four-link and resulting change in an instant center, and drives the car left. Often, the cure for this is the direct opposite of the 10.00 car, as negative preload (lengthening the upper passenger bar) will keep the car straight at the hit. This too presents issues down track, as pulling and poor handling while on the brakes are common complaints with even one flat of negative is added in such a rigid car. It takes a lot of power and a light car to require negative preload, and most reading this won't need it. The point I'm making is that both the fastest and the slowest deal with the funky effect that spinning pinion/ring gear junction has on their racecar. The way we deal with it often dictates the efficiency or quality of the run, and the car's ability to not only track straight at the hit, but remain that way with little or no driver input through the pass.

Four-link set up and "instant center" location can play a large role in down-track stability too. Watching the various sanctioning bodies, it's obvious who is and isn't embracing the "new trends" in four-link tuning and shock selection. Without getting too specific and bogging down the article even more, the instant center that is shortest and highest (without getting into weird extremes) that still applies the tire on the starting line will keep the most weight on the front tires through the traps. Too many tuners are still setting cars up with long lazy instant centers that will go down a slop hole, but present the driver with some real handling issues as the power rises. Remember the three wheel Pro Mods? This is the other part of solving that equation - the long IC's give lots of leverage on the front end, and the more power they made, the more the front drifted. They ran the long IC's to get wheel speed at the launch, as the shocks of the day weren't up to par. Shorter IC's require different valving at the hit to grab the housing, and they weren't widely available until relatively recently. That is no longer the case, as many of the modestly priced double adjustable shocks provide reasonable valving for most cars right through the 8.0 range. When you install an unnecessarily long IC in a slower car, it often requires very low tire pressures to offset the lack of bite at launch. This too can make a car drift.

Even more racers are lucky and don't even know it. They have a flexible car that wastes energy, robbing ET but remaining consistent, day in and day out. The other guys can't use the gear they can, or the nitrous pill. The flexible car isn't any faster, but it is a whole bunch more forgiving. The guy with the flexible car doesn't have the problems with shocks or springs the new style cars do. He just keeps picking up ET bit by bit. If the track sucks, it might be his day. The problem is, the guy is usually losing pace with his (or her, ladies) class, and the racers with the newer, tricker cars. He too will need a stiffer, newer car that will waste less energy and use the power to drive forward, not up and over. The evil flipside will be the increased crankiness the new car will bring. Piss it off; it will let you know in a hurry. What you once knew about your suspension settings means squat (sucks, but true). The promise this new car brings is the lure of straight, consistent passes that require less anger to launch and sixty foot. To get there, you must finesse it in a way the loose car never needed. Getting to the "promised land" with a tight new car can be humbling, because once the flex of "old reliable" is eliminated, it forces increased attention to details like preload, spring height/rate and shock valving. The old car didn't care, so what the hell is the baseline now? That is up to your chassis builder, but we can cover some baselines that can get you close.

We've established we want as little mechanical preload in the car as we can possibly have. There are a couple of tools we have available to us to finesse corner weights without reaching for the preload wrenches. Since some new cars are dropped off semi complete- needing ignition, intercoolers, bottles or other heavy items installed by the customer, scaling the car without the hardware in there gives you a great opportunity to plan the layout of those pieces. Even if the car is together, re-mounting big items can give you a running start at the weight distribution you used pre-load to achieve once before. Spring platforms are an excellent tool to manage corner weights- within reason. Since springs don't act until acted upon, and since their rate is constant even with some platform height adjustment, the drivers front and passenger rear are often manipulated to finalize a car's corner weights. Just realize there is only so much adjustment one can make, cranking and cranking on a spring will only cause it to fight its diagonal opposite and throw the center to center dimension of the shock off. Experiment on the scales and you'll see where it stops paying dividends.

Managing the application of power is also a biggie. This, like driver technique has little to do with the chassis per say, but the way the tubing is hit by the available power has a lot to do with how the car will react. Often, the guy that upgraded from his old loose car will hit his new rigid one the same way, and experience wheel spin, wheel stands, veering toward the wall, or all of the above. Why? The old car ate up so much hp on the starting line, it didn't matter what you threw down the drive shaft. Now that the stiffer car wants to use up little to none of the available twisting forces to bend the car, it won't tolerate the same massive application of power. It needs to be kissed off the line, with a tighter driveline. Let the converter grunt against the mid mounts, the new car can handle it. It'll convert that energy to forward propulsion so much better, for so much longer, it will out accelerate a looser car to the 330'. It won't put up with getting smacked in the ass, it only needs a firm slap. What it will do is accept significantly more power down track, continuing the period of time it can accelerate at a high rate.

Once that part of the equation is settled, the next common issue on many cars is either an inadequate or improperly tuned set of wheelie bars. A wheelie bar is not only a safety device; it is a preload reducer. Properly built, properly tuned, a good set of two wheel bars will greatly reduce if not eliminate the need for preload in the four link. When a car hits its bars, there should be a slight stagger side to side, because as discussed, the housing tries to pinwheel before it allows the axle to turn. The resulting squish of the driver sidewall makes the housing crooked to the track at launch- as much as 3/8”. That's why so many cars have staggered bars in the lanes. Crooked is actually straight, once the action starts. If the car has a tire that is biting harder than the other, dropping the hooked tire's wheelie bar wheel can significantly reduce the drift at launch. The small amount of added wheel slip shouldn't affect ET if managed properly.

The most ‘in vogue' add on to anything going faster than 12.50's seems to be the anti-roll bar. It's a good item. It makes sense in most applications. In the hands of the wrong racer, it can be a source of misery. The anti-roll bar's job is simple- if the housing cocks perpendicular to the car's centerline, it uses the strength of the torsion bar (attached to the chassis) to leverage the driveline's pinwheel motion against itself, neutralizing the axle's desire to induce body roll and apply both tires equally. The problem with many anti-roll bars is that they are like computers; garbage in and garbage out. A good, high horsepower anti-roll does not have soft alloy arms. It does not have bolts that locate the arms that will start to slop over time. It is either splined or welded in place, and it should have zero tension on it when the car is on the starting line. (The one exception is low HP stock suspension cars that have little other recourse than to softly adjust the anti-roll for pre-load. They have so much travel the negatives are less severe.) This is an area where many racers aren't doing their maintenance; examining it should be part of your regular routine. The anti-roll is potentially the first place a car will show a dead spring or a twisted housing. They are absolutely sensitive to any changes in the car's hardware. After all, they are attached solidly to the chassis and housing- any change in their relationship will show up as bind. Four link changes and spring adjustments should always conclude with a cursory check of the anti-roll, on a level surface, at weight with driver. Very small amounts of bind or snugness in the heim bolts can move significant corner weight back and forth, and in an unpredictable manner. Checking to ensure it is slack will clue you into any spring/twisting issues early.

To conclude, I'd like to checklist some other items that should be inspected regularly, especially after servicing heim rods or making a four-link adjustment. First, make sure the housing is square to the body of the car. I know that sounds strange, but we don't line up the chassis on the starting line, we line up what we see, the body. You may discover that the four-link crossmember is off or other sloppy issues have made it difficult to truly center the housing. Yes, you can strike happy mediums with messed up hardware. Again, consult your builder or competent local shop for advice if you're confused or find a significant problem. Second, inspect all holes, heim ball/race relationships or any other locating item for oblonged holes or unusual wear. You may be surprised how oval the holes in your ladder bar or four link brackets are if they haven't been looked at in a while. This all adds up to potential pre load shifting, and all can kill a straight pass.

As always, use common sense and ask questions when confused. It is too easy to get in over your head and make a lap in a car that has been set up poorly, and not even know it. Best wishes, and I hope the Holidays were good for all of you.