Bump Steer Explained

What is Bump Steer?

Bump steer is the tendency of wheels to steer themselves without driver input. This undesirable effect is caused by bumps in the road or track surface, as well as suspension travel due to braking and cornering, and their interaction with improper angles in the steering/suspension geometry.

From the factory, cars are designed so that bump steer effects are largely negated. However, when cars are lowered this geometry is compromised and must be corrected for. On a completely stock vehicle, bump steer compensation is not adjustable since proper geometry is engineered into the vehicle’s suspension.

For zero bump to be achieved, the tie rod’s motion must follow the arc of the suspension travel.

Simply put, bump steer is the amount of toe change throughout suspension travel.

Mustang Bump Steer Explained

Prep for Initial Bump Steer Measurement

Since the front suspension acts together as a system, you should have every parameter set before bump steer is adjusted.

  • Set Ride Height
  • Proper size wheel & tire
  • Camber is set
  • Caster is set
  • Toe is set
  • Tie rod length is set.
  • Steering is centered (wheels pointing forward)
  • Steering locked static
  • Ideally, sway bar & springs are disconnected. This is not necessary although it makes adjustment easier.

Jack the suspension on one side through ~3” of travel up and down, and record the changes in toe.

Make Bump Steer Corrections

Shimless Bumpsteer Kit
A shimless bump steer kit offers easier and more precise adjustment

Once you know your baseline level of bump steer, you are now ready to make changes to the level of bump steer compensation by changing the length of the outer tie rod.

On a classic “shim-style” bump steer kit, this adjustment is made by removing the rod end and changing the amount of shims between the rod end and the spindle.

On an FTR Shimless Bump Steer kit, this adjustment is made by simply loosening the jam nut and rotating the vertical adjuster to the desired location.

Symptom Correction (Shim-Style) Correction (Shimless)
Toe out on compression, in on rebound, in the same direction Remove shims between outer tie rod and spindle Turn adjuster clockwise
Toe in on compression, out on rebound, in the same direction Add shims between outer tie rod and spindle Turn adjuster counter-clockwise

How Much Bump Steer Do I Want?

In an ideal world, you want as little bump steer as is physically possible given your suspension geometry. This will keep your steering predictable over uneven surfaces commonly experienced in autocross events, and even some road courses.

A bit of bump out (toe out in compression) will make the car more stable upon corner entry, but bump in is almost always not wanted. Small amounts of bump steer will create an Ackerman type effect during a corner causing the inside tire to turn a bit further, increasing stability.

As a general rule of thumb, run a small amount of bump out (toe out on compression), and do not allow any bump in.

 

Further Reading:
Bumpsteer Explained – Mustang 360
The Ultimate Guide to Suspension and Handling – Bump Steer/Toe Steer – MotoIQ

S550 14″ Brakes on SN95 2001 Mustang GT

People have been asking about putting 2015 brakes on the SN95, so I thought I’d just go for it. Here’s the best part:

Total cost was under $1000!

 

 

Required hardware:
S550 Calipers – $100 each (+$50 core)
Rotors (2x): Blanks $60 ea from an auto parts store, slotted $100 ea from StopTech.
FTR Brembo Kit – $450
MM Stainless Brake Lines – $85
Pads (currently only available from Ford) ~$100
Caliper Pin Kit (2x) – ~$12

Total cost: $979!

First off, I started with our GT500 Brembo Adapter Kit (http://www.fullytorquedracing.com/sn…brake-kit.html). The S550 calipers use the same mounting pattern as S197 brakes, but there are some clearance issues so we had to machine down a section to get it to clear the bracket. The only other modification we made was to add a 1/8″ washer between the bracket and the spindle so everything would line up.

There was a fair bit (maybe took an extra 30 min) of modifying to the caliper, so I’m thinking we’ll probably come out with a bracket specifically for the S550 calipers if there’s enough interest.

The S550 Caliper is a bit larger than the GT500 Brembo, and also has larger pistons as you see here.
IMPORTANT NOTE: Use the S550 CALIPERS, PADS, AND HARDWARE, with the 2012 GT500 ROTOR

 

S550 Caliper
S550 Caliper
S550 Caliper Piston Size
S550 Caliper Piston Size
GT500 Brembo Piston Size
GT500 Brembo Piston Size
GT500 Brembo vs S550 Caliper
GT500 Brembo vs S550 Caliper
S550 14 Inch Brakes on SN95
S550 14 Inch Brakes on SN95

Developing a Quality Splitter Support Rod Design

FTR Splitter Support Rods Made in USAA brief history – back in 2013, when FTR was barely in its infancy, I need some support rods for the splitter I was making for my ’96 Mustang. Looking online, there were only a few brands out there, and frankly, they all looked like crap. I decided to machine something up myself. The first prototype was a janky manually machined clevis design with an ugly bolt going through it. After a few revisions we ended up with the low profile pressed pin design we use today, and it has barley changed since late 2014.

The big issue with the other rods on the market was the amount of rattle in the design. In our splitter support rod design, we integrate a heim joint in the end to make sure that no play or rattle is introduced

Boss 302 splitter support rod upgrade
Our Boss 302 Support rods installed on a track car

Another thing we developed was a Boss 302 support rod, which is literally just our normal support rod design with a 1/4-20 thread instead of a 5/16-18 thread. A lot of Boss 302 owners are quality obsessed, and find it obscene that such a great car comes with those pieces of junk on the nose. The stock support rods lack rigidity causing the stock Boss 302 splitter to vibrate at speed, reducing the aerodynamic benefits of the splitter. By replacing the stock part with the FTR unit, that vibration is eliminated, thereby increasing downforce and consistency.

Inside the Design of the FTR Watts Link

We recently opened up pre-orders for our new SN95 Watts Link. The purpose of this post is to go through the design step-by-step and discuss the engineering behind what we’ve created.


Part I: Conception & Goal

In 2014, we first started development on our Watts Link. There are currently a few SN95 watts link’s on the market, but they all lacked a few features that we wanted to see. Our #1 concern was the following problem: no current Watts link on the market was able to locate the car’s rear roll center lower than a Panhard rod. While the Panhard rod is good solution for rear axle lateral location, it compromises on a couple areas. First off, a Panhard rod, by nature, does not have identical cornering characteristics left and right. When compressed, the axle will be laterally offset to the left, and when decompressed, the axle will be offset to the right. When cornering, this issue becomes even more complex, as the mounting point on the chassis is moved relative to the axle and the axle deviates from its proper location.

A Watts Linkage, in contrast, provides identical cornering characteristics left and right. This concept was invented by James Watt himself (hence the name).

“Watts Linkage.gif” by Van Helsing is licensed under CC BY 3.0

 

 

Clearly, the Watts Linkage is an ideal method of laterally locating the axle, due to its consistent horizontal location throughout articulation.

Our goal is to combine the best of both worlds: Pair the superior cornering characteristics of the Watts Link with the lower roll center of the Panhard rod, previously impossible to obtain with a Watts Link.


Part II: Differential Mounting Plate

We’ll start at the center of the part. Our differential plate mounts to a Strange R5234 differential cover. This cover is part of the “Ultimate 8.8″ axle package, and has built-in mounting holes intended for an axle brace. We thought this would be the perfect cover to accommodate our mounting plate.

Prototype rear assembly, red anodized bell crank not shown

This plate is secured to the differential housing at four different places. One bolt in each corner. We also put in clearance holes for the axle girdle bolts, so that the plate is not being secured to the differential using the girdle bolts and introducing unwanted stress to the the differential itself. This way, the user does not run the risk of shearing their axle girdle bolts.

The differential backing plate has five different bell crank mounting positions, allowing a 4.5″ range of roll center adjustment. When mounted on 315/35/17 tires, this comes out to a 7.5″ roll center at the lowest setting, and a 12″ roll center at the highest setting.

This is the widest and most flexible range of adjustment of any SRA lateral-locating device on the market.


 

Part III: Bell Crank

The initial design for our bell crank was a 1/2″ threaded aluminum plate. This bell cranked function correctly, however we have since redesigned it for even more strength, rigidity, and overall sexiness.

11863258_871431339612885_5695592534727674252_n-2

The bell crank is available with three different bushing packages, depending on application:

Street: Polyurethane center bushing to combat vibration and noise
Performance: Delrin bushings and PTFE rod ends for stiffer movement
Race: Self-lubricating bronze bushings and bronze rod ends for ideal stiffness where noise is not an issue.


 

Part IV: Chassis Brackets

Our chassis brackets are laser-cut, TIG welded, and powder coated, providing maximum strength and a clean appearance. We have also added red anodized ID plates with laser-engraved part number, serial number, and QR code. The QR code allows us to keep careful track of all production data including date of fabrication, materials used, machine settings, etc.

wattsrender_final


 

Part V: The Results

In July of 2014, we sent our prototype watts link down to Brett at BTM Autosport in San Diego. Brett had previously ran a competing part, but by switching to the FTR part he found even more grip due to the increased rigidity and lighter weight of the FTR piece.

“Going from the Fays2 Watts Link to the Fully Torqued Racing Watts Link changed the car’s handling characteristics drastically. We used the same roll center height we had on the Fays and the car had so much rear end grip it produced understeer. ” – Brett Madsen, BTM Autosport

Since the installation of the FTR watts link prototype, Brett’s team has achieved the following victories:

2014 CP National Tour Champion
2015 SCCA CP National Tour Champion
2015 SCCA CP ProSolo Champion
2014 SCCA SU Regional Champion

The prototype version of the Watts Link on BTM Autosport’s car differs from the production version, which includes a more robust bell crank, laser cut brackets, and a lateral support. However, we told Brett to be as hard on the Watts Link as possible, and he has provided. After each race, the Watts Link has been inspected and has shown no signs of wear, flexing, or cracking.

Here is a video of the prototype in action:

As you can see, there is a bit of flex in the bell crank, so we have since upgraded it. However the function and strength has proven to be quite sufficient.