When you drive over a bump or corner hard on the racetrack, does your car appear to have a mind of its own, with the front wheels veering one way or the other? When the front suspension responds to these bumps or body roll inputs from the track with toe-in or toe-out while moving through its bump (compression) and rebound travel, that's bumpsteer. It's undesirable because your control of the steering and traction is compromised. It's a problem that could be costing you valuable time on the track.

Ideally, bumpsteer can be avoided when a car is built. The key is to have a basic understanding of suspension and steering geometry, and to have the right parts and pieces when you start out.

"The most trouble local racers have is that when they get going, perhaps they don't have the right parts in hand," says Jeff Butcher, president of Longacre Racing Products. "Sometimes when you don't have the right parts for a specific car, there's really no way to get the bump right. It's something that has to be designed in. A local guy might have to buy something that he's not absolutely sure about bolting on. You have to be careful there."

If you're buying a rolling chassis, check with the builder to learn his normal setup for bumpsteer. This could save time and energy as you get to know the car and what it will do. If you have a new chassis, check bumpsteer early on, then check it periodically after the car has some laps on it.

Bumpsteer can make a car unpredictable and hard to control, especially on older paved tracks that may be worn and bumpy. If you're using a bumpsteer gauge and the plate moves toward the engine, you have a bump-in condition; if the plate moves away from the engine, you have bump-out.

The goal is to have zero bumpsteer or to have as little as necessary for a well-handling car. Butcher recommends this method of accomplishing zero bump: The tie rod must fall between an imaginary line that runs from the upper ball joint through the lower ball joint and an imaginary line that runs through the upper A-arm pivot and the lower control-arm pivot. In addition, the centerline of the tie rod must intersect with the "instant center" created by the upper A-arm and the lower control arm (see Figure 1).

Instant center, according to Butcher, is an imaginary point created by drawing a line from the upper A-arm ball joint through the A-arm pivot, where it is intersected by an imaginary line that extends from the lower ball joint through the inner control-arm pivot. The two imaginary lines intersect at instant center.

To achieve zero bump, Butcher says the tie rod must travel on the same arc as the suspension when the car goes through travel. It involves matching lengths and arcs to prevent any unwanted steering of the front tires.

Butcher says that some bump-out can make the car more stable on corner entry, but bump-in is, as a rule, undesirable. Regardless of how much bumpsteer is built into a car, it's a concept that must be recognized and respected.

"Every tenth of a second is going to make a difference on the track," says Kevin Smith, vice president of Irvan-Smith Inc. "If it's a tenth of a second or a half a tenth a second, in a 50-lap race or a 100-lap race-if you're making that much better time than the next guy every lap-consider how much farther ahead you're going to be at the end of the race. Setting bumpsteer is fine-tuning the race car, and it's just as important as a half-pound of air pressure, trailing arm angles, offset rearends, a sixteenth of an inch here or there, stagger, or anything else. This is just another piece of the puzzle that gets you one step closer to being complete."

"Bumpsteer is a really big thing," says Butcher. "I think it's indicative of a lot of things. Whenever you're measuring anything on a race car, if you can measure things to the next increment as compared to your competitor, it's going to give you that much more of an advantage, and it's going to help your car stay on the long run.

"For bumpsteer at different tracks, you might want to run different amounts. You might run next to none at some tracks, but for entry stability you might want to run 0.015-0.020 inch bump-out. Whatever numbers you choose to run, like all adjustments on a car, it's a cut-and-try deal, something that's specific to your driver and your location. But you want to make sure that you know what that number is so you can repeat the process the next time around. Everything's relative in racing: If you know what you did last time, you'll have a better shot at winning the next one."

Excessive bump-out, over 0.050 inch, for example, can slow your car down. You should strive to get the best bump numbers possible, Butcher adds, even if it means replacing parts.

"Quite simply, all bumpsteer is, is lining up arcs and pivot points and lengths so that the wheel stays straight unless the driver wants to turn," says Butcher. "With any kind of car, you just have to make sure that those pivot points line up with each other and that the arcs are somewhat similar to each other so that you can fine-tune with shims at the tie rod. Or maybe you have to space up the steering rack if it's a car with a rack, or move the steering box around a little bit if it's that kind of deal.

"It's one of those adjustments, again, that if you have an understanding of what's going on-it's not magic or something like what's happening inside an automatic transmission-it's really quite simple. Once you understand how to line up those arcs, then doing the adjustments becomes much easier."

Symptom: Toes-out in compression and in on rebound, all in one direction.
Cure: Increase shim on outer tie rod or lower the inner tie rod.

Symptom: Toes-in on compression and out in rebound, all in one direction.
Cure: More shim at outer tie rod or raise the inner tie rod.

Symptom: Always toes-in during both compression and rebound.
Cure: Lengthen the tie rod.

Symptom: Always toes-out on compression and rebound.
Cure: Shorten the tie rod.

Symptom: Toes-out on compression, then in on rebound, and then starts back out with more rebound travel.
Cure: Less shim at outer tie rod and shorten tie rod.

Symptom: Toes-in on compression, then moves out on rebound, and then starts back in with more rebound travel.
Cure: More shim at outer tie rod and lengthen tie rod.

* Source: Longacre Racing Products

Your front suspension must be complete and set for racetrack conditions before you can measure the bumpsteer. All components must be tight and in proper position, and you will need a quality bumpsteer gauge.

1. Set the car to ride height.
2. Use the proper size tires and air pressures.
3. Caster must be set.
4. Camber must be set.
5. Toe-in must be set.
6. Tie-rod lengths must be set.
7. Steering should be centered (tie-rod ends centered on inner pivot points lower ball joints).
8. Steering must be locked down.
9. Measure from the ground to the lower ball joint or other reliable reference point. Write it down.
10. Remove springs and disconnect sway bar.
11. Return the suspension to the proper height by using your reference number to the ground.
12. Obtain a supply of bumpsteer shims (washers).
13. Bolt on the bumpsteer plate to the hub. Level the plate and note where the dial indicator is on the bumpsteer plate so you can quickly return to the correct ride height.
14. Jack the suspension through 2 inches to 3 inches of both compression and rebound travel and write down your results.
15. Shim as needed.

* Source: Longacre Racing Products

On Street Stock suspensions, one usually must use stock-type ball joints and tie-rod ends. The location of these cannot be easily adjusted because they all have tapered shafts and fit in tapered holes. The bumpsteer on a stock automobile is normally quite accurate. Changes occur when we turn it into a race car. Chassis settings like ride height, radical caster/camber settings, and tall ball joints all affect the bumpsteer. This is the point where you should get concerned.

Bumpsteer effects will be more noticeable on paved tracks, less on dirt surfaces, and even less on small tracks.

In the absence of sophisticated measuring equipment, I have done the following to detect bumpsteer on a Street Stock. With the chassis on jackstands and the steering wheel locked, I held a carpenter's square against the brake rotor while the spindle was jacked up and down. If much bumpsteer is present, you will see the square move across the floor left or right.

On Original Equipment (OE) suspension-class cars there is little adjustment available to correct bumpsteer. Bending the steering arm of the spindle can often affect the needed correction. Another corrective action is to move the steering box. This can be a big chore. If this is done, the idler arm will need to be moved the same amount. Here I should mention many Street Stock-type cars still have the original idler arms. If the idler arm can be wiggled up and down, this will cause an even more erratic, intermittent, bumpsteer. In one extreme case, I lengthened the drag link to get rid of some bumpsteer.

Don't become too radical trying to get the last few thousandths of bumpsteer out of your Street Stock dirt track car; it doesn't make that much difference. But pay attention to it if you race on pavement. Do check it occasionally to make sure the steering arm of the spindle hasn't been bent. - Sleepy Gomez

SOURCE
Longacre Racing Products Inc.
14269 NE 200th St
Woodinville
WA  98072
Irvan-Smith Inc.
1027 Central Dr.
Concord
NC  28027
  • «
  • |
  • 1
  • |
  • 2
  • |
  • 3
  • |
  • View Full Article