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The Truth about Bumpsteer and What You Need to Know

The bump and run

Jeff Smith Sep 7, 2016 0 Comment(s)
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All the information you are about to read is true. It’s time to shine the hard light of fact on the misinterpretations surrounding bumpsteer, specifically about F-/B-body spindle swap horror stories. This story will focus on the ideas surrounding how to improve the handling of an early Chevelle but can be easily applied to any performance application. So cinch up those seatbelts, it’s going to be a bumpy ride.

Let’s start with a few basic facts. In the early 1960’s GM designed their front suspension to create understeer, which is defined as a “push” that occurs when entering a corner. The experience goes like this: enter a corner with a higher-than-normal rate of speed, turn the wheel, and the car continues to plow straight ahead. This is the opposite of what is called oversteer, where the back end of the car comes around as the car turns into the corner. All car manufacturers choose understeer because it is more controllable for the average driver. GM induced understeer by combining a rather high inner pivot point (a short spindle) with a relatively short upper control arm that creates a positive camber gain as the car negotiates the turn. There are several more factors that can contribute to understeer, but those are two important ones.

Using a stock Chevelle front suspension for example, when the right front is compressed in a left-hand turn, the camber rolls positive. Under rebound—when the suspension is lifted—this creates negative camber. Positive camber is when the top of the tire tilts outboard while negative camber is when the top of the tire tilts inboard when looking at the tire from the front. The original solution for this problem, as far as we can tell, was created by H.O. Racing owner Ken Crocie and his employee Dean Dodge back in the early ’80s. Dodge discovered that if he replaced the stock spindle on an early A-body with a 1-inch taller second-generation F-body or ’80s B-body spindle that this created a radically improved camber curve. If you are in search of better handling, a positive camber gain under compression is not desirable. The preferable move is to create a negative camber gain under compression. That’s what Dodge achieved by using the taller spindle.

This, however, created other problems. The first was that adding the taller spindle required a ton of shims to the stock upper control arm to bring the static camber back to a street alignment figure of roughly a half-degree of negative camber. Global West was the first company to create a line of tubular upper control arms that took advantage of this situation by making the arms shorter while building in positive caster.

This simple spindle/tubular upper arm solution seemed like the perfect combo. The taller spindle created more negative camber as the spring compressed—roughly one degree per degree of body roll. This was and is a major step toward improved handling since we can dial in a minimal 1/2 degree of static negative camber for street use. Yet when the suspension is loaded hard into a corner, the tire tilts in at the top—placing the tire at a slight negative angle to load the tread of the tire into the pavement. This is a classic way to improve handling.

The problem with the F-/B-body spindle conversion is that it aggravated a toe change created under load in a corner. This is what is called bumpsteer—a term used by suspension designers and tuners to describe when, under suspension movement, the steering moves the front tires into either a toe-in or toe-out condition with no other input from the steering wheel. This is caused by the steering linkage moving in a different arc than the spindle/steering arm as the suspension moves either in compression or rebound.

Let’s look at an example of bumpsteer. We enter a left-hand turn, which loads the right front corner and if we freeze the car in mid-turn we discover that while the driver has input a left turn with the steering wheel, the right front tire is actually turning slightly outward (right), creating toe-out. In certain situations, this can be useful but most of the time this is not good. But this is exactly the effect that occurs when using an F-/B-body spindle on an early Chevelle. There is also an effect known as Ackerman that plays into this complex geometry, but we’ll leave that somewhat complex discussion for a later story.

This bumpsteer issue has created a universal hue and cry about the horrible negative effects of this F-/B-body spindle conversion on a Chevelle. In a perfect world, the steering arms and spindle should both travel in the same arc, which would produce a zero bumpsteer curve. However, there are other situations at work that tend to minimize the effect of bumpsteer.

We’ve seen many statements on the Internet from well-intentioned enthusiasts who insist that you should never accept anything more than 0.010-inch of bumpsteer throughout the entire range of suspension movement. If you are Charlie Kimball driving an Indy car at 226 mph, then you should demand that of your race car because speed enhances the effects of bumpsteer. But in the everyday world of street cars, 0.010-inch bumpsteer is wholly unrealistic—unless you are Doug Norrdin at Global West—but we’ll get to that in a moment.

Rather than just blindly adding more opinions to this discussion, we decided to test a stock 1965 Chevelle front suspension, compare it to a B-body spindle conversion, and then compare those numbers to the new forged aluminum spindle from Global West that promises to minimize this issue. To be fair, there are several other companies building custom spindles, but we’ll use Global’s new spindle as our test subject.

You can see in the included bumpsteer test results (Bump Steer Curves chart) that the stock suspension toes-out under compression by 0.135-inch at 1 inch of compression and almost the same amount of toe-in under 1 inch of rebound (blue line). The B-body spindle numbers are very similar (gold line). These numbers will certainly affect the handling and there’s no denying that these are poor numbers. Detractors will point directly at 0.210-inch of toe-in at 2.0-inches of rebound to prove their point. But the reality is that the only numbers you should really be concerned with are those within 1.5 inches on either side of ride height. If your front suspension is at 2.5 inches of rebound in the middle of a turn—that means that tire is nearly lifted off the ground and isn’t contributing to the handling of the vehicle anyway. So data beyond 1.5 inches of suspension travel should be only of interest if you are a drag racer.

So let’s take the more realistic numbers of 1 inch of suspension travel on either side of ride height. These are numbers that most cars see on an autocross track or on the street. So with either the stock or F-/B-body spindle, the bumpsteer spec for our Chevelle at 1 inch of compression was 0.120-inch of toe-out. This is quite a bit, but according to Norrdin, not as bad as he has measured in the past. It is a long way from ideal, but not uncommon.

This author has over 25 years of experience with his ’65 Chevelle on track days and autocross courses and also at 168 mph in the Pony Express race experiencing that 1-inch of suspension travel perhaps thousands of times. At no time did these “horrific” bumpsteer effects ever rip the steering wheel out of my hands or even indicate that there was a problem. This is not to minimize this situation but rather to help explain what’s happening. In talking with Norrdin, I realized that these toe changes on my Chevelle were contributing to its erratic behavior at speeds in excess of 150 mph. Remember—at speeds over 150 mph. At 125 mph, my Chevelle was stable and comfortably controllable.

Norrdin says that while this bumpsteer clearly occurs, the tire compensates for the toe changes in terms of the slip angle induced into the tire under cornering. This makes sense since the tire is the elastic connection between the front suspension and the road. Again, this is not to minimize the issue but rather to point out that a narrow-minded adherence to a 0.010-inch bumpsteer maximum spec could also be viewed as unrealistic—until now.

All of this is everyday fare to Norrdin and the Global West crew. For the last three years, Norrdin has been perfecting a new, forged aluminum spindle for these cars that not only offers an ideal camber curve but incorporates a bolt-on steering arm that drastically improves the bumpsteer issue. We took our Longacre bumpsteer gauge out to Global West and stuck it on the shop’s GTO test car.

As you can see by the Bump Steer Curves chart (red line), the Global West spindle delivers on the promise of virtually eliminating the bumpsteer. No, it’s not perfect—there’s no reason to shoot for perfection. The reality is that through 2-inches of suspension travel from 1-inch of compression to 1-inch of rebound, we measured merely 0.015-inch of toe-in! This comes about as close to perfection as you will ever see in a production-based car. So at least for those seeking the ultimate front suspension for their A-body, this comes pretty close.

We’ve really only skimmed the surface of the camber curve/ bumpsteer story—which is part of a very complex independent front suspension. So don’t be afraid of things that go bump. It might help you get around that next corner!

Chevrolet Chevelle 2/11

01. Bumpsteer is a hot topic among those who like turning corners. We decided to test the effect of bumpsteer on Chevelles converted to taller F-/B-body spindles. We also tested Global West’s new forged aluminum spindle. The results are amazing.

Longacre Bumpsteer Gauge 3/11

02. We used a Longacre bumpsteer gauge to measure the amount of bumpsteer on both the author’s ’65 Chevelle track car and the pure stock left front corner of a ’65 Chevelle owned by Kris Shields. The track Chevelle was equipped with B-body spindles and Global West upper and lower tubular control arms. The numbers are listed in the accompanying Bumpsteer chart and graph. Your numbers may vary.

Baer Tracker Kit 4/11

03. This is the Baer Tracker kit for an early Chevelle equipped with a B-body spindle. The Baer kit lowers the steering arm mounting point to reduce (but not eliminate) the tall spindle’s impact on bumpsteer.

F Body Spindle Graph 5/11

04. This graph clearly illustrates how the tall F-/B-body spindle (gold line) produces toe-out in compression and toe-in under rebound. There are multiple factors that create a bumpsteer curve. Most involve the inner or outer tie rods improperly positioned relative to the arc created by the spindle. Note how the Global West spindle/steering arms (red line) approach the near perfect flat (zero) line. (A stock-suspended Chevelle is represented by the blue line.) (See also the Bumpsteer Test sidebar below.)

Global West Modular Forged Aluminum Spindle 6/11
Global West Modular Forged Aluminum Spindle 7/11

05-06. This is Global West’s modular forged aluminum spindle. The spindle combined with the steering arm only weighs 11 pounds, which is 5 pounds per side lighter than the F-/B-body unit. That’s 10 pounds off the nose of the car. Better yet, Global designed the spindle to be compatible with most popular 12-, 13-, and 14-inch front disc brake systems. This means Global’s spindle will likely fit your existing brake kit.

Spindle 1965 Chevelle 8/11

07. This is the spindle bolted to the right front of Global’s ’65 GTO test car. Note that the spindle also requires a dedicated spherical bearing and adjuster. This car also uses Global’s coilover shock conversion.

1965 Chevelle Bumpsteer 9/11

08. We checked the bumpsteer on Global’s ’65 GTO equipped with the new forged aluminum A-body spindle and tubular upper and lower control arms. By carefully engineering the steering arm, Global West has been able to very nearly match the spindle curve to minimize bumpsteer.

1965 Chevelle Undercarriage 10/11

09. We pulled this photo from a video of Global’s ’65 GTO on an autocross course to reveal how much the tire deflects in relation to the wheel. This is in the middle of a corner. So you can see how a tire could easily accommodate even a large amount of toe change without notice from the driver.

B Body Spindle Graph 11/11


Bumpsteer Test

With suspension movement expressed as compression or rebound, this chart indicates how much the tire toes-in (negative numbers) or toes-out (positive numbers). The zero position is vehicle ride height with the lower control arm roughly parallel to the ground. Compression shortens the coil spring while rebound increases its length. The critical bumpsteer numbers are those within 1.5 inches of ride height.

The stock and F-/B-body spindle bump curves were similar. But notice how the Global spindle creates a near flat or zero curve. On the Global spindle curve, add the toe change from 1 inch of compression through 1 inch of rebound and the total is 0.015-inch. Pretty damn good.

Suspension Change Stock Spindle B-body Spindle Global Spindle
Rebound 2 -0.21 -0.17 -0.018
1.5 -0.19 -0.145 -0.008
1 -0.15 -0.1 0
0.75 -0.1 -0.065 0.003
0.5 -0.065 -0.04 0.003
0.25 -0.024 -0.02 0.002
Compression 0.25 0.015 0.04 0.004
0.5 0.058 0.07 0.008
0.75 0.09 0.11 0.01
1 0.12 0.135 0.015
1.5 0.185 0.19 0.031
2 0.25 0.22 0.05

Description* PN Source
Global West forged A-body spindle Call Global West
Baer Tracker bumpsteer kit, A-body 3301001 Baer Brakes
Longacre bumpsteer gauge kit 79005 Summit Racing
* Each Global spindle kit will be individually created to fit the car and its current brake configuration. This would include the spindle, specific steering arms, brake caliper adapters, spherical bearing, and tie-rod sleeve.

Sources

Baer Brakes
Phoenix, AZ
602-233-1411
www.baer.com
Longacre Racing
Monroe, WA 98272
800-423-3110
www.longacreracing.com
Global West Suspension
San Bernardino, CA 92408
877-470-2975
GlobalWest.net

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