Suspension flex is the usable range of movement in a vehicle’s suspension system, measured by how far the tires and suspension can compress and extend while maintaining contact with the ground.
For off-road vehicles, flex is important because it helps keep the tires planted on uneven terrain. More tire contact usually means more traction, more control, and a smoother ride when traveling over rocks, ruts, ledges, holes, and other obstacles.
MetalCloak uses Corner Travel Index, or CTI, to measure and compare suspension flex. MetalCloak has put CTI to the test over 15,000 times, helping off-road enthusiasts better understand how suspension systems perform on Jeep, Bronco, Grenadier, Toyota and Ram vehicles.
Quick Summary
- Flex is the suspension system’s ability to compress and extend.
- Off-road flex helps keep tires in contact with the ground.
- More tire contact usually improves traction, stability and control.
- Flex can be limited by shocks, bump stops, fenders, tire size, brake lines, sway bars, control arms, driveshafts and coil springs.
- A suspension system works best when the components are designed to work together.
- MetalCloak uses Corner Travel Index, or CTI, to measure and compare suspension flex.
Key Suspension Flex Terms
- Flex: The usable range of suspension movement from compression to extension.
- Up Travel: The amount a tire can move upward into the wheel well.
- Down Travel: The amount a tire can move downward to stay in contact with terrain.
- Droop: Suspension extension when the tire moves downward away from the body.
- Compression: Suspension movement upward toward the vehicle body.
- CTI: Corner Travel Index, a MetalCloak measurement system used to compare suspension flex.
What Does Flex Mean in an Off-Road Vehicle?
Short answer: Flex is the movement of a vehicle’s suspension as it compresses and extends over uneven terrain.
Flex is a combination of suspension components working together as a system to produce a desired result. The result is a separation or distance of travel from an “at rest” position. Flex can be measured as the total distance that components can travel before being stopped by the limitation either of those components or an outside factor, such as a fender.
For example, flex can be measured as the distance a tire mounted on a vehicle can travel through its entire range of motion, from its most upward or “compressed” position to its fully dropped or “extended” position.
A vehicle’s suspension system is similar to muscles in our bodies. When a muscle “flexes,” it is placed under tension or stress until it reaches its maximum size and tension; the vehicle’s suspension components also have a maximum size or length before they reach their limit.
Why Does Suspension Flex Matter Off-Road?
Short answer: Suspension flex matters off-road because it helps keep the tires in contact with the ground. More tire contact usually means better traction, better control and a more comfortable ride.
In an off-road vehicle, flex is extremely important and typically, again in an off-road environment, more is better. In an on-road vehicle, such as a race car on a paved track, typically less is better. The reason is tire contact with the surface of the ground you are traversing.
On a smooth paved surface, less flex is desirable because it helps provide stability and keeps the vehicle planted to the pavement. Even then, suspension is still critical to achieve a desired result.
Off-road, the situation is different. A trail surface is uneven. The vehicle may need to travel over rocks, ruts, ledges, crevices and large obstacles. When a suspension can flex enough to avoid reaching its limitation, the vehicle can remain in better contact with the trail surface and absorb the energy from imperfections in the terrain. That helps provide a more comfortable and controlled ride while also improving traction.
What is Up Travel?
Short answer: Up travel is the distance a tire can move upward into the wheel well before something stops it.
Up travel is the distance that a tire can travel, in its wheel well, before it reaches its limitation. Its limitation may come from the fender flare contacting the tire and stopping its up travel, or from a suspension component reaching its travel limit, such as a shock fully compressing.
Think of a suspension system as being very soft so that the up travel has little to zero resistance. If you hit a large bump or rock on the road, the tire will travel from its at rest position upward until it reaches one of those limiting factors. If it is a big enough bump or rock, that upward movement will transfer its energy beyond the suspension component’s limits and into the body of the vehicle, resulting in a jolt in your seat and a rough ride.
What is Down Travel?
Short answer: Down travel is the distance a tire can move downward as the suspension extends to keep the tire in contact with the ground.
The opposite happens when suspension components experience down travel. As the old saying goes, “What goes up must come down,” and suspension is no different.
Think of slowing the above scenario down and starting from the point of the tire reaching the top of the bump or rock. Now the suspension must come down the other side to keep the tire in contact with the surface. This is called down travel. The components must now extend to reach back to the “at rest” or ride height position.
Without flex, hitting bumps or imperfections on a road would simply transfer the energy from hitting those things into the cab of the vehicle, making the ride extremely uncomfortable.
What Limits Suspension Flex?
Short answer: Suspension flex can be limited by tire clearance, fenders, shocks, bump stops, brake lines, sway bars, control arms, driveshafts, steering components and coil springs.
Limitations to flex are easily described as a balance or harmony of the individual suspension components. Suspension is a number of components that must work well together.
Think of building a computer using a number of parts from a variety of manufacturers because it was cheaper than buying one directly from a computer maker. Even if the components are great, they do not necessarily work great together. Computer manufacturers pick components that are matched and work well with each other because of the testing they have done. Aristotle, a philosopher in Ancient Greece, is often paraphrased as saying “the whole is greater than the sum of its parts.”
The same concept applies to vehicle suspension. In trying to achieve greater flex, each of the suspension components needs to be addressed. If we think of the range of suspension travel, both up and down, that would be our total amount of flex.
How Do Fenders, Shocks and Bump Stops Affect Flex?
Short answer: Anything that stops a tire or suspension component from moving through its full range of travel can reduce usable flex.
If a fender flare hangs down lower than the inner structure of a fender, that will cause the tire to hit the flare sooner on its upward travel path, limiting flex. If the flare is raised up or arched, it provides additional space for that tire to travel upward before contacting the flare.
If we hold onto that mental image of the tire traveling upward, we could also fully compress a shock, stopping that upward travel. Typically, a shock body that is too long will compress before the tire comes into contact with the fender.
Things like bump stops that are too tall, coil springs that have too many winds, and even drag links that have been flipped to the top of the steering knuckle can all limit upward travel of the tire.
On the downward travel of the tire and extension of the suspension, things like shocks that are too short, brake lines or other wiring that is not long enough, sway bars and even binding of the steering components can limit down travel.
Do Tires Affect Suspension Flex?
Short answer: Yes. Tire size and tire pressure both play a role in suspension flex.
Tire size plays a large role in flex. As an example, if a tire is 35 inches in diameter, it has more room to travel upward before reaching a contact point like a fender flare. In comparison, a 42-inch tire would have more limited up travel or less flex if the vehicle does not have enough clearance.
Air pressure can also limit flex. Think about that same tire being able to “squish” into a fender well. If the tire is flexible or has less air pressure, the suspension will have a higher amount of usable flex.
Do Control Arms Affect Flex?
Short answer: Yes. Control arms, control arm joints and their mounting brackets can all affect suspension movement and flex.
Control arms, driveshafts and coil springs are all large factors in the suspension system. When we say that items need to work together, these are critical items.
If you have ever looked at a factory or OEM control arm, they are typically thin rectangular and even stamped steel pieces that connect the axles to the frames of the vehicle. Many times they will contact the brackets that hold them in a full flex scenario. They also contain a limiting joint in the end of the arms that can limit flex.
Many control arm joints are similar, meaning they function the same, though they may differ drastically in how they perform.
Do Driveshafts and Coil Springs Affect Flex?
Short answer: Yes. Driveshaft clearance and coil spring behavior can both limit suspension travel.
Driveshafts may come in contact with other components, also limiting flex. Coil springs may bind when compressed and fall out of their places when reaching maximum downward travel, providing no real pressure or traction when extended.
This is one reason matched suspension components matter. Springs, shocks, control arms, steering geometry and bump stops all need to work together through the suspension cycle.
Are All Suspension Systems the Same?
Short answer: No. Suspension systems vary widely, and the way the components work together can make a major difference in performance.
Like in the earlier story of building a computer, the components are what make a difference as long as they work together as a system, a matched system. Suspension systems vary widely as do their components. Some manufacturers might make a kit or might make individual pieces of a suspension, but very few make up all the components in a kit.
We all have built up our vehicles by purchasing a piece at a time and sometimes they work great and other times not so good. A manufacturer cannot know how its coil springs, for example, will work with another manufacturer’s shocks. There are simply an unlimited number of combinations to try and test for optimum compatibility.
The same can be said for most of the components in the suspension system. Matching spring rates, shock valving, control arm damping, steering geometry and bump stops all add together to provide a better suspension system, but very few manufacturers can control all of these pieces and parts, and even fewer make, design and build their own.
How Does MetalCloak Measure Flex With CTI?
Short answer: MetalCloak measures flex with Corner Travel Index by lifting opposing corners of a vehicle and measuring suspension travel before a tire loses contact.
When Metalcloak designed and introduced the Corner Travel Index, it was simply a way to compare and put a number on the amount of flex a suspension system may have. It was intended to be a tool in which there could be a number assigned to a system of components working together; a rating system if you will.
By lifting up opposing corners of a vehicle until one of the drooped or extended tires lifted off the ground or platform, MetalCloak could measure flex. Since the vehicle begins from its “at rest” position, or ride height, as the opposing corners rise, the suspension components both extend and compress, doing so until a point of contact stops the cycle.
CTI allows MetalCloak to see a full cycle of suspension in both up travel and down travel in one place and measure that system.
Why Does MetalCloak Use Corner Travel Index?
Short answer: MetalCloak uses Corner Travel Index to help people understand suspension performance and build better off-road vehicles with matched components.
The idea to build a device to fully cycle the suspension system of a vehicle was solely on the minds of the engineers at MetalCloak from the early days of the company. The idea was to help people understand and help them build a better Jeep, while showing how matched components do make a difference.
Metalcloak has always believed in the system of components ideology and has built suspension systems from the ground up, making sure that the parts of the system complement each other and perform the best that they can.
In order to prove that the parts do work together, the Corner Travel Index was utilized and eventually turned into a mobile platform, the CTI Trailer, to help the off-road enthusiast build a better performing system.
This system includes everything that we have talked about, from suspension components to body armor, fenders, bumpers and steering. Everything in the system working together is the goal in order to provide the best off-road and even on-road experience.
By manufacturing all of the components, a single manufacturer can perfect each of the parts of the system without guessing what might or might not work with other components. From Metalcloak’s Dual Rate coil springs that remain seated in their mounting locations throughout the suspension cycle, to shocks that are matched for the length and valving of the spring, and the control arm bushings, called Duroflex Joints, all working together to provide the ultimate in flex as well as a superior ride both on and off road, MetalCloak has put the Corner Travel Index to the test over 15,000 times as it strives to help the enthusiast build a better Jeep, Bronco, Grenadier, Toyota or Ram.
Suspension Flex FAQ
Is more flex always better?
More flex is generally helpful off-road because it can improve traction and stability on uneven terrain. However, the suspension system still needs to be balanced for safe on-road handling, proper shock control, steering geometry and overall ride quality.
What limits suspension flex?
Suspension flex can be limited by fender clearance, tire size, shock length, bump stops, brake lines, sway bars, steering components, control arm joints, driveshaft clearance and coil spring bind.
Do larger tires reduce flex?
Larger tires can reduce usable up travel if they contact the fender, flare or body sooner during compression. Proper fender clearance, bump stop setup and suspension design help manage this.
How does air pressure affect flex?
Lower tire pressure can allow a tire to conform or “squish” more against terrain and into available clearance, which can help the vehicle use more of its suspension movement off-road.
What is CTI?
CTI stands for Corner Travel Index. MetalCloak uses CTI to measure and compare suspension flex by lifting opposing corners of a vehicle and recording suspension travel before a tire loses contact.
Why does MetalCloak test vehicles on CTI?
MetalCloak tests vehicles on CTI to compare suspension performance, show how matched components work together, and help enthusiasts better understand how their Jeep, Bronco, Grenadier, Toyota or Ram performs through a full suspension cycle.
