Understanding Bump-Steer, its effects on drive-ability, and ways to
correct the problems and or symptoms

Why is Bump-Steer so confusing?

Bump-Steer can be a very complicated and confusing issue.  Grasping suspension geometry can be difficult, in and of itself, but with Bump-Steer there are two factors that increase the complication and cloud the real issues. The first factor is that, when it comes to poor handling, Bump-Steer is the "chicken" of the automotive world.  In the same way that everyone says "that tastes like chicken", automotive enthusiasts say of a poor handling car, "this car suffers from Bump-Steer."  I find it fascinating that simply by taking a test drive, many backyard mechanics, car enthusiasts, and even some automotive professionals can diagnose Bump-Steer as the problem with 100% confidence, especially when they have a limited understanding of what Bump-Steer really is and what causes it.    

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It is much like a young doctor diagnosing a serious and complicated illness in a patient simply by asking them a few questions instead of doing any real tests.  Please don’t misunderstand, I am in no way saying that if a car is handling poorly that Bump-Steer is not a possible cause.  What I am saying is that Bump-Steer may in deed be the issue, however, it is just as likely that the problem has another cause or a combination of causes.  Just as a doctor must know the actual source of the problem to successfully treat his/her patient, we, as car enthusiasts, must accurately diagnose the true issue in our suspension and handling problems to effectively correct them.  Also, we must remember that, similar to the medical profession, sometimes more relief is found in dealing with the symptom or "effect" of the problem rather than the real issue.T his brings me to the second factor that makes Bump-Steer such a confusing issue, and that is; the amount of measurable Bump-Steer is not always in a direct ratio with the amount of Bump-Steer that is actually felt when driving the vehicle.   I refer to this phenomenon as the “Bump-Steer Effect".    DazeCars Items for Sale:

What I mean to say is, if you look at a car’s suspension, there are physical quantifiable measurements that can be taken to identify the actual amount of “Measurable Bump-Steer” that is present.  However, these numeric measurements may vary from the actual handling and drivability of the car.  A car with large amounts of measurable Bump-Steer may suffer from little to no actual "effects" of Bump-Steer, or on the flip side of that coin, a car with a small amount of or even no Measurable Bump-Steer may have horrific handling and drivability due to extreme Bump-Steer Effect.  Don't get me wrong, as the "measurable" amount of Bump-Steer increases, in theory, so will the "effect", but, due to all the other suspension parts, their role in handling, and the variables they bring to the equation, the Bump-Steer effect can range from non-existent to extreme.  To make matters worse, even though Measurable Bump-Steer and the Bump-Steer Effect are two separate, yet related things, they are almost always referred to as one in the same.  This adds tremendous confusion to the issue and, in an effort to alleviate this confusion, from here on out it is paramount that we realize that Measurable Bump-Steer and the Bump-Steer Effect are separate issues and, henceforth, WILL be referred to as such.

Measurable Bump-Steer (MBS):
Whether we like it or not, almost all stock early automotive suspension systems, and even many modern suspension systems, have some degree of MBS.  The measurement of Bump-Steer is the difference of suspension and steering components moving in different arcs as the suspension compresses and decompresses.  Here is a simplified definition of MBS:  

        Measurable Bump-Steer is a situation where the suspension components travel to a point were the distance moved is not equally compensated for by the movement of the steering components and, thus, one or both wheels are pulled into an unintended turning position.  

Bump-Steer is measured by comparing the arc and the positioning of the outer tie rod and the arc of the spindle.  The spindle arc is created by a combination of movement of the  lower control arm and the A-arm.  If the spindle arc and the tie-rod arc are the same, then zero MBS exists.  The picture below illustrates this; the red arc to the left is the arc of the spindle, and the inner red arc is the travel of the tie rods.  If the two red arcs match in radius then no MBS exists, but if the arcs differ then MBS is present.  Also, in the picture, the green arcs illustrate the A-arm and LCA movement.  

When a car is aligned, all of the adjustments are made with the car in a nonmoving, resting position.  That means that regardless of the amount of MBS present, when the car is in a resting position, the wheels are pointed in the direction they were aligned to point in, and when a car is sitting motionless, the suspension should be at, or near, the middle of its travel arc. In suspension systems where MBS exists, the more extreme the suspension travel then the more severe the Bump-Steer measurement as the suspension components move away from the position they were in when they were aligned.

With all that said, tie rod length is the main factor in MBS.  If you shorten the tie-rod assembly, then its arc decreases and, in reverse, if you lengthen the tie-rod assembly, then its arc increases.  That means that the simple act of adjusting the toe on your car will either add or reduce the amount of MBS.  When the arc of the tie rod is smaller than the arc of the spindle, then the car’s suspension system is referred to as having positive MBS because the distance between the two arcs increases as the suspension moves away from its resting position.  In reverse of that, a tie-rod arc greater than the spindle arc will result in negative MBS. 

Positive Bump-Steer                            Negative Bump-Steer                            No Bump-Steer

Keep in mind that MBS is a geometric defined phenomenon that is present or it isn't!  Almost all cars have some amount of MBS and a Bump-Steer kit or modifying suspension and steering components, such as the spindle steering arm or the inner tie rod location, are the only ways to truly eliminate MBS. Also, keep in mind that to accomplish any of the above-mentioned MBS removal techniques, special measurements are required to insure that MBS has been removed and fine-tuning will be required every time the car is aligned to maintain zero MBS.  Also, there are several performance “upgrades” that increase MBS, such as some rack and pinion units, as well as Granada disc brakes on an early (64.5-66) Mustang.  Keep in mind that such upgrades may require other performance components that will help reduce the Bump-Steer Effect.

The Bump-Steer Effect (BSE):

We have established that adjusting tie rod length when doing a simple alignment will change the geometry and affect MBS.  As a whole, this is not a problem because, depending on suspension components, variable amounts of MBS can be absorbed by the suspension.  Look at it this way:  If your car is aligned properly, then both tires should be set with a small, yet equal, amount of toe in.  If, due to MBS, one tire is pulled further toe in or toe out, the other tire is still pointing in the correct direction and will help keep the car on the road.

Even with a Mustang with completely stock suspension and steering components, there will be some BSE, and there are many things that can be done to lessen or remove the BSE:  Roller Spring Perches, Adjustable Strut Rods, 1" Shelby Drop, and, most importantly, a Performance Alignment.  As these modifications reduce the BSE, there are also several performance “upgrades” that increase the BSE, such as lowered springs, stiffer springs, low profile and/or wider tires, some aftermarket A-arms and some aftermarket LCAs.  NOTE:  I am not saying that if you install the above-mentioned “negative effect” parts on your car, that you will have horrific BSE and handling.  What I am saying is that these parts have been shown to aggravate the BSE on vehicle with MBS.  The more modifications you can make that reduce MBS and or the BSE and the fewer modifications you make that increase MBS and or the BSE, then, it stands to reason, the better the car will steer and hold the road. 

Ways to reduce Measurable Bump-Steer (MBS),
the Bump-Steer Effect (BSE), and why

Bump-Steer Kit:
As we have discussed earlier Bump-Steer is a geometric defined phenomenon that is present or it isn't.  A Bump-Steer kit is the only way, short of major steering and suspension modifications, to truly eliminate the problem.  As I mentioned before, there is positive and negative Bump-Steer.  However, for all intents and purposes, positive Bump-Steer is the occurrence on most vehicles.  That means that to truly cure it, the only option is to lengthen the tie rod assembly.  The problem with this “solution” is that, without other modifications, lengthening the tie rod assembly will take the car out of alignment.  That is where the advent of the Bump-Steer kit comes into play.  A Bump-Steer kit is an adjustable mount that lowers the connecting point of the outer tie rod and the spindle steering arm.  By lowering the connecting point, you are able to lengthen the tie rod assembly and, thus, increase the tie rod arc, but still maintain the toe setting. The following (non proportional) picture illustrates how a Bump-Steer kit works.  The blue represents the kit, which typically consists of a stud that drops down below the steering arm and a special connector that allows said stud to connect to the outer tie rod. The red represents the new angle of the tie rod assembly and the green represents the amount the tie rod needs to be lengthened to accommodate the Bump-Steer kit.  Installing a Bump-Steer kit is simple.  Calibrating it, on the other hand, is not!  To calibrate a Bump-Steer kit you must take many measurement with the suspension in fully extended and fully compressed positions, which means that things like coil springs need to be removed prior to adjustment.  Also, as with other adjustments in alignment, the toe and the Bump-Steer kit need to be adjusted at the same time because they affect each other.  It is this challenge in alignment and adjustment that makes a Bump-Steer kit a non-option for many car enthusiasts, because most alignment shops will not align a car with a Bump-Steer kit and the individual car owners do not have the equipment or skills required to align their car themselves.  It is for this reason that  treating the symptom rather than the cause, for many car enthusiasts, is a more feasible option.

Shelby Drop:
The Shelby Drop is not only effective in the reduction of the BSE, but is easily the most cost effective and one of the most over all effective Mustang suspension upgrades.  The beauty of it is that, other than time and tools required to perform the modification, there is no out of pocket expense.  The Shelby Drop works by improving the camber curve which improves the contact between the tires and the road.  Changing the camber curve will slightly increase MBS however that small  increase in MBS is far outweighed by the improved handling and a  decrease in the BSE. The Shelby drop also lowers the center of gravity on the front suspension, which decreases body roll.  Keep in mind that even though the drop is 1" the actual change in ride height is only 1/4" to 5/8".   Information on performing this suspension modification, as well as ordering information for steel Shelby Drop templates, can be found at the UCA Drop page on www.DazeCars.com and ordering information for steel templates can be found on my  For Sale Page.   When performing a Shelby Drop, the UCA needs to be removed and this provides a natural opportunity to upgrade the rubberized spring perches to roller spring perches.

Roller Spring Perches:

Improving suspension response time and also freeing up desirable motion in the upper A-arm springs and shocks is very effective in the reduction of the BSE.  Many vintage Ford cars use spring perches bolted to the upper A-arm that serves as a bottom resting point for the coil springs.  The design of the spring perch, through its ability to rotate back and forth as the A-arm travels up and down, allows the coil spring to move with more consistent vertical travel through compression and decompression.  The original spring perches found on the early Falcons had brass bushings.  This allowed for a smooth rotation but had the downside of needing regular greasing due to considerable friction between the surface of the bushing and the journal it rested in.  For this reason, as well as to cut costs FoMoCo decided to replace the brass bushings with rubber ones. Although cheaper to produce,  the rubber bushing spring perches lack the smooth, easy turning radius afforded  by the brass bushing.  If you have ever tried to turn the center shaft in a rubber bushing spring perch,  you know that it takes quite a bit of force to get any movement. This excess force required for movement oftentimes causes unwanted flexing of the springs and shocks and results in a less than consistent, less-vertical travel during suspension compression and decompression.  One option in correcting this issue is to replace the rubber bushing with aftermarket brass bushings, returning the perch to the initial Ford design.  This works well, but without regular greasing, brass bushings tend to squeak.  Another option is to create the roller spring perch.  By replacing the rubber bushings with roller bearings, a smooth turning radius can be achieved and, thus, smoother suspension and drivability without the added friction and squeaking of a brass bushing.  More information on Roller Spring Perches, as well as information on building your own or purchasing a completely fabricated set can be found at www.DazeCars.com, “Opentracker" Roller Spring Perches.  Once the front suspension has been removed and reinstalled due to the replacement of the vehicle’s spring perches, it is important to have the suspension realigned.

Performance Alignment:
A performance alignment, even without all of the other suspension modifications, can be a very effective means of reducing the BSE.  The problem with a stock alignment is that it is done to Ford’s original specifications and, frankly, those specifications are not conducive to performance.  The settings that you should set your Mustang’s suspension to are:

0 to -.5 degrees of Camber but can be as much as -3 degrees Camber for the track cars

1/16 to 1/8" toe in

+1.5 to +3 degrees of Caster but as much as 5 degrees Caster for the track cars

These Camber settings allow the maximum rubber to road engagement without excessive scrubbing of the tires.  Camber setting more extreme than -.5 will increase handling, but at the expense of rapid tire wear.  The more rubber you have in contact with the pavement, the more accurately the steering will be. The toe setting allows the tires to spread as you go down the road so that when you are driving, the forward pressure brings the toe to 0.  If you started with a toe setting of 0, the forward pressure as you are going down the road will cause your tire to end up toeing out.  Caster is the big one when dealing with the BSE.  The more Caster you have, the quicker your wheels return to a centered forward position.  That means that, due to road condition, when MSB occurs, the quick returning to center of the steering will minimize the BSE. A car with significant amounts of Caster will track and hold the road much better than a car with less Caster.  Caster is one of our best weapons against the BSE.  That is the biggest reason adjustable strut rods are a natural choice when dealing with the BSE because it allows us to increase the caster further than the shims can.  The following account regarding the BSE and a performance alignment is from John Dinkel AKA Opentracker of Opentracker Racing Products:

    "A guy I know has a daughter with a 66 Coupe. They started working on it about 5 years ago. They installed Granada discs, a sway bar, shocks and other suspension mods. (Nothing too trick) The car drove like crap when they got it on the road. It would rise at highway speeds and wander all over the road. For 5 years, they could only drive the car around town. They tried all kinds of things to fix it, including having it re-aligned three times by local shops. I got a call to look at the car. It sounded like Bump-Steer to everyone else and their next plan was to put on a Bump-Steer kit. I asked who had done the alignments. The shops they had used set the cars to factory settings. I told them to have the car aligned again (they thought I was off my rocker) but it worked. They can now drive the car 100mph if they want. No wander and no rise at speed. The factory settings say the car only needs 1 deg of positive caster and 1 deg of positive camber. Positive camber!! I had them set the car to 0 to .5 neg, camber and 2.5 deg pos. caster with 1/8" toe in. The owner was so happy he couldn't hold himself together.  Before the performance alignment his daughter was ready to sell the car after all the work they had done because it was so hard to drive. Now she drives it every day and loves it. "
Also, when having your car aligned, keep in mind that a typical alignment shop will not correct for MBS, because, one, they do not check for it and, two, they do not have a way to adjust it.  Like I mentioned before, the simple act of adjusting the toe will affect MBS.  The original designers knew this and designed the steering and suspension to absorb the effect, that way, even though small amounts of MBS are present, the effect is not felt.  To put it simply, a performance alignment improves the suspension and steering ability to absorb the BSE and hold the road better.  Lastly, when it comes to alignment, factory tolerances allow small differences in the setting between the driver’s and passenger’s sides.  This is to reduce the time it takes to do the alignment.  It is extremely important that the alignment specifications be as close as possible on both sides of the car which will make the alignment take longer and probably cost more.  For this reason make sure you take your car to an alignment shop you trust!!!

Adjustable Strut Rods:
By eliminating unwanted suspension movement and maximizing desirable suspension movement the BSE can be reduced.  Adjustable strut rods are an obvious choice in this respect, due to their incredible ability to improve suspension and handling. The function of the strut rod in a classic Mustang is to stabilize the lower control arm in a fixed position that prevents its movement forward and aft, but still allows it to travel vertically with suspension compression and decompression. The stock system is designed so that, during regular driving, the rubber bushing gives, allowing vertical travel as the lower control arm moves up and down.  The give in the bushings, unfortunately, has a negative side effect of unwanted movement, as they compress and stretch during braking and acceleration. The result of the compression and stretching of the rubber bushings is an increase and decrease in the length of the strut rod. As the strut rod increases and decreases in length, the lower control arm is not held fixed and can move forward and aft a small amount, which puts the alignment in a constant state of change during driving.  The challenge is to have a joint that freely moves vertically, but is also fixed in such a way that it cannot shift forward or aft. This can be easily accomplished by using a heim joint at the bushing end of the strut rod.  An added benefit of a heim joint system is that, by having two threaded ends with opposite thread directions, one on the heim joint and one at the base of the strut rod, you can use a threaded connecting tube and make the strut rods adjustable for an easier and more fine tuned alignment. More information on Adjustable Strut rods as well as information on building you own set or purchasing a completely fabricated set can be found at www.DazeCars.com, Adjustable Strut Rods.

Granada Disc Brakes on an early Mustang:
Granada disc brakes are a popular upgrade to the classic Mustang due to affordability and availability of parts.  If the Mustang they are being installed on is a ‘67 or newer, then the swap is very effective because the geometry of the Granada and stock Mustang spindles are identical, which means there is no change in MBS or the BSE.  However, if the Granada discs are being installed on an early Mustang ‘64 & 1⁄2 to ‘66, then, due to a different location of the spindle steering arm, MBS is increased.  It makes sense that both MBS and the BSE would be an issue when installing Granada disc brakes on an early Mustang because one of the steps in this process is to shorten the adjusting sleeves, as well as the inner and outer tie rod ends. If this modification is not made then the tie rod ends bottom out in the adjusting sleeve and there is not enough adjustment to correctly set the toe when performing an alignment. As I demonstrated earlier, one of the two defining measurements of MBS is the arc of the outer tie rod, which is a direct result of tie rod length.  All this said, it stands to reason that by making the tie rod assembly shorter than it was originally designed to be, MBS would be increased.  This explains why the best option when dealing with MBS is to utilize factory components.  That means stock Mustang spindles of the same year as the vehicle are the most ideal option.  If you want to put disc brakes on your stock spindles, you have several options.  The first option is original Mustang disc brake components, which  can be purchased and installed on your original V8 drum brake spindles.  Second, there are options such as the Mustang Steve Cobra disc set up which allows you to put Cobra disc brakes on original V8 spindle. I have also seen similar kits that use "bowtie" parts. A third option, is disc brakes from CSRP There you will find kits based on the Granada disc brake configuration so replacement parts are plentiful and inexpensive. What is different about this swap and the original donor car Granada conversion is that the spindles are new and because of that there are spindles available that have been reengineered to maintain the correct 1964.5 - 1966 steering geometry.

Personal Experience with Granada Discs on a 64 & 1⁄2 Mustang:
There has been a movement as of late away from using Granada discs on an early Mustang, and some of this movement is for good reason.  However, much of this movement has been perpetuated from horror stories of individuals who claim (due to the experience with their own individual cars) that no early Mustang with Granada disc brakes will steer properly and, due to “Bump-Steer”, the car will be difficult to keep on the road at low speeds and impossible to keep on the road at high speeds.  I am here to tell you that with moderate modifications and performance up-grades this is simply not the case.  I have a 64 & 1⁄2 Mustang with Granada discs and I have yet to feel any BSE after installing my Granada discs and some performance suspension parts.  In fact, I live in MT where many of the roads are filled with bumps and potholes and, due to this fact, have put my car through some extreme suspension travel, and yet I still have not felt any BSE.  Ironically, my car’s steering was far more erratic before I installed the Granada discs and the suspension up-grades, especially at high speeds when changing lanes so as to pass.  My current front end suspension, steering, and braking combination consists of: 6 cyl steering linkage, Granada disc brakes, GT springs, KYB shocks, Roller Spring Perches, 1” Shelby Drop, Adjustable Strut Rods, a 1” sway bar and a moderate performance alignment.  My car handles like it is on rails at all speeds.   If you are interested in installing Granada disc brakes on your car, you have several options.  Some Mustangs will accept Granada tie-rod ends, making the swap simple.   However, for other Mustangs, such as 6 cyl Mustangs, this is not an option.  MustangSteve has the solution in his Granada disc brake Tie Rod Bushings.  He also has a very informative page on the ins and outs of Putting Granada disc brakes on a Mustang.

Final thoughts:
Keep in mind that every car has its own personality, that is to say what is the perfect solution on one car may not be as effective on another, even if they are the same year, make and model, it is my hope, however, that the info I have provided on MBS and BSE can be generally applied to our cars so that suspension performance and handling can be improved.  Other than dealing with actual MBS, there is no clear-cut answer.  Our only options are to use the info I have outlined, a little common sense and some trial and error to come up with the best possible handling package. 

Disclaimer on Daze Tech Tips
      I am not an expert in this field. I have performed these modifications myself with very good results. I am passing along restoration and performance tips for the purpose of education.  If you are concerned about reliability or safety issues, I do not recommend that you or any other individual perform these changes or attempt to modify your cars from stock configuration except under your own volition.  I do not assume nor accept any liability for the use of this information or how it is applied.

2013 DazeCars
The words / logos for Ford, Jaguar, Mustang, Galaxie, etc are used for descriptive and reference purposes only. DazeCars is neither affiliated with Jaguar Land Rover North America LLC, Ford Motor Company nor the manufacturers/distributors of Ford or Jaguar automobiles.