Technical Articles

FAQ’s.

Bump Steer

Terminology.

These terms apply to automotive suspension coil springs.

Active Coils: Coils which are free to deflect under load. Also referred to as working coils.

Approx. Lowering: The approximate amount of chassis ride height reduction. This will vary depending on optional equipment: gas shocks, age of stock springs and upper strut bearings.

Bottoming Out: The suspension reaches its maximum compressed height, not when tires rub fenders or contact the chassis.

Dead or Inactive Coils: Coils, which are in contact with adjacent coils at loaded height. Inactive coils do nothing but give the spring enough free length to stay tight in the spring perches at full rebound.

Deflection: The travel of the spring as it is compressed.

Foamy Bumpstop: Also referred to as a supplemental spring, a progressive-rate urethane jounce bumper will make the suspension progressive. Gives a smooth transition to full compression.

Free Length: The overall length of a spring in unloaded position. May also be referred to as UN-INSTALLED LENGTH.

Full Jounce, Full Compression: The suspension is completely loaded. The springs and bumpstops are totally compressed.

Full Rebound, Full Droop: The suspension is completely unloaded. Example: When the vehicle is on a lift and the tires and wheels are hanging in the air.

Linear Spring: One rate throughout the springs working range.

Loaded Height: Or INSTALLED HEIGHT, is the length of the spring compressed under the load of the vehicle weight, when the vehicle is at rest on level ground. Also may be referred to as STATIC HEIGHT.


Pre-Setting or Blocking: The spring is compressed so all coils are touching. Pre-setting gives the spring a set so they cannot sag. The springs are said to be "pre-sagged". (Super-BlokTM)

Progressive Spring: Is when there is a variable rate increase throughout the spring's working range.

Spring Clatter: When dead or inactive coils are not completely in contact with each other at loaded height, but come in contact with each other while driving.

Spring Rate: A change in load per unit deflection, generally given in pounds per inch.

Suspension: The system of springs, shock absorbers and other devices that insulates the chassis of a vehicle from shock (bumps in the road) transmitted through the wheels.

Wax Lube: Found at your local bicycle shop and is used to lubricate intricate components, including the chain. The Boeing Company has also developed a wax lube (Boeshield T-9) for long-term protection of aircraft. Their research found that none of the Teflon, silicone, or synthetic sprays held up as long as a wax lube when exposed to a corrosive environment. The wax based lube is also becoming popular due to its ability to penetrate deeply into fasteners and fixtures while displacing moisture, resisting corrosion, and at the same time detering dirt build up.

Working Spring Rate: The rate of the spring from the installed height to full compression. This is the spring rate you use when you are driving your car. This is the only spring rate which can be used when comparing Spring Performance.

Bump Steer

Micro Cellular Jounce Bumper. by Roland Graef.

Attention - calling all bumpstops.

Well, we call them bumpstops because we always have. It is what we are used to calling them. But in the age of specialized tech-talk they are "bumpstops" no more; now they are micro-cellular progressive jounce bumpers. - What? ... That was a double decafe Mocha, skinny, no whip!
"The song, "Head out on the Highway, look'in for adventure" keeps ringing in my head. Where was I, oh yeah, trying to avoid the dreaded bumpstop (dumpstop)."
Most people think of a bumpstop as a black rubber snubber hanging down from the frame above the axle to protect against metal-to-metal contact. Just like my dad's 1974 Chevrolet Caprice station wagon. You know, the one with the big block 454 and the hide - away tailgate. The Caprice was the cruising wagon of choice when highway 5 between L.A. and San Francisco first opened and gas stations were few and far between. The song, "Head out on the Highway, look'in for adventure" keeps ringing in my head. Where was I, oh yeah, trying to avoid the dreaded bumpstop (dumpstop). The thing was, no matter where you went in that wagon; you rarely came into contact with the bumpstop.

The micro-cellular progressive jounce bumper, or simply put, the foamy bumpstop, is actually a supplemental spring. It works to make the suspension progressive, allowing a smooth transition to full compression. What most people don't realize is that the foamy bumpstop is an active part of the suspension. Let's say you're sitting in your M3, 996, Accord, Golf, S4, or Civic. (Hmm I can't decide). They all use the foamy. Ok, you're in your brand new Integra, just drove it off the showroom floor and are sitting at the stoplight waiting for the green. The foamy is actively engaged even while waiting at the stoplight.

Car manufacturers use this brilliant piece of high-tech engineering for many reasons. By using an almost zero-weight supplemental spring, you can build the main (steel) spring with a reduced spring rate for a comfy ride. The softer spring uses less weight, and, saving weight is a number one priority of new car manufacturers. Cars have thousands of parts. A little weight saved here and there really adds up. When the manufactures save weight they also save costs, less material=less cost. So we now know why these foamy gems are used. But how do they really work?
As mentioned earlier, the foamy bumpstops are an active working part of the suspension. They work in harmony with the springs and the rest of the suspension components allowing the ride to be comfortable while cruising. When the car is in a handling situation, such as in a turn, they make the car feel firmer for better handling. Think of the foamy as a little progressive spring over the shock rod. All car manufacturer bumpstops have a specifically calculated spring rate and compressed height. (block height).

When a spring is designed the spring rate must be calculated with the rate of the progressive bumpstop in mind. Otherwise the spring will not work in harmony with the bumpstop and the frequency of the suspension will be out sync. The human body interprets a certain range of suspension frequency as comfortable or uncomfortable. Frequency in an automotive chassis is like chords in music. If two or more notes are combined in a pleasing combination you have harmony, otherwise you have uncomfortable dissonance. The same thing happens in suspension tuning. A balanced suspension is important for correct function. This is why it so critical to understand the true effect of the pro-active foamy and the spring. Everything must work together in harmony!

The compressed height is what limits the wheel travel and controls how far the wheel and tire move up into the fender well. Aftermarket suspension designers must consider that enthusiasts will be upgrading the tires and wheels. Therefore, it is vital that these designers take into consideration the needs of tire and wheel fitment. By trimming the bumpstop you allow the tire and wheel to move further into the fender. This can cause the tire to rub where it did not before. If your application does not require bumpstop trimming and you are concerned about tires rubbing, just load up your car with your buddies and drive over some dips and speed bumps. If your tires do not rub with a full load in the car, they should not rub with lowering springs installed.

So, what has little Johnny learned today? There is no such thing as a bumpstop in a modern suspension. The bumpstop has been replaced by a supplemental, active foam rubber spring, which is one of the essential components in the suspension.

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Bump Steer

Scrub Radius. by Roger Jackman.

This man walks into a bar... "NORM!" He proceeds to belly up to the bar and begins to talk about the daily grind and the latest in his relationship with his wife. The bartender, a former sports icon, talks to a patron at the other end of the bar about the specifics of modifying the suspension on his Corvette. He was told that after changing to an aftermarket wheel and tire combination, he needs to modify the scrub radius a bit and wants to know what in the world they were talking about. Cliff is trying to explain that scrub has been in use since early man invented the wheel. He claims, "in order to get the perfect lateral friction to rolling resistance ratio, you must periodically 'scrub' the tires to expose the unused material."

Does this scenario sound like you and your car buddies at the local watering hole after a club meet or romp through the countryside? I know I have heard it somewhere before. Now, I was able to get through school with a little intuition and a lot of B.S., but these qualities do not always work when I am dealing with inanimate objects that are not humored by my B.S. and theory. When dealing with suspension design and dynamics, little things that are easily overlooked like scrub radius can make a greater difference in your car's handling and feel than all your theory and fervent tuning can accomplish.

"When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock."
To understand scrub, you must first know about Steering Axis Inclination (SAI). The steering axis is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel, so if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the SAI.

Now back to the subject at hand. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.
You now know what elements make up scrub, now you may ask what it has to do with the price of tea in China? Well, the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

Well, now you know all about scrub, so the next time it comes up at the watering hole you have the confidence in knowing that your drunken compatriot is having more fun with creative banter than tuning and driving his car. That is not such a bad thing though. Order another beer and go with it. I will bet you can come up with something better.

Bump Steer

Bump Steer. by Roger Jackman.

My first car was a modified '72 510. After some tuning, the car was very fast and couldn't be more balanced if I had tried applying Feng Shui. My friends and I had a special road that we affectionately called the Nurburgring. It twisted relentlessly through the Northwest forest, graced with elevation changes, narrowing radius turns and off-camber corners trying to feed you to hungry ditches with deep bellies. There was a section that was similar to the Foxhole at the 'Ring, a steep downhill followed by a sharp rise, all the while making a sweeping left. It was great fun to feel the g-forces in my friend's Alfa Romeo, but piloting the old 510 through this obstacle was another story all together. The drop was exhilarating, but when the suspension compressed at the base of the dip, the steering wheel went one way and the car went another. YIKES!!!
"The drop was exhilarating, but when the suspension compressed at the base of the dip, the steering wheel went one way and the car went another. YIKES!!!"
If you have experienced excessive bumpsteer like I have, you have probably had a 'moment' or two that you will never forget, no matter how hard you try. There are others who have experienced bumpsteer and never knew what it was. It is usually felt while cornering and may seem like soft dampers, or even like a rut in the road that wants to pull the car one way or another. It is rarely checked on road going vehicles, as the inefficiencies of mass production make a more noticeable difference in handling than a little bump steer. Besides, engineers are currently doing their best to take bumpsteer out of the car during the design phase. On a modified car, or a car built specifically for racing, bump steer in one of the first elements of the suspension that is tested and tuned.
Bumpsteer is simply the change in toe as the suspension travels. If the steering rack and linkage is not in an optimal position in relation to the rest of the suspension, distances become too short or long and angles become askew causing the wheels to point in the wrong direction when in motion. When modifying a suspension by lowering the car or relocating control arms, the tie rods and control arms are not always working harmoniously and modifications need to be made. These modifications usually come in the form of spacers to relocate the steering rack or steering knuckle.

If used in moderation, a professional can add a small amount of bumpsteer to help fine-tune the attitude of a car in a corner. For example, if a properly tuned car still tends to understeer upon turn in, a few thousandths change in toe in the first couple inches of bump can alter slip angles enough to stabilize the front end of the car.

Any adjustment to the suspension is best left to a professional, as there are special instruments needed to measure and correct issues you may have. To help minimize bumpsteer without reengineering the suspension, installing polyurethane control arm and steering rack bushings will help keep critical pieces from moving in relation to each other. This will help maintain the set geometry and make the car more predictable.

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