Articles

Understanding Your Car's Suspension

Posted:  Monday, February 06, 2006
Copyright:© 2006 Horizon Hobby, Inc.

Having so many different chassis out there from different manufacturers makes trying to get a car dialed into a specific track very frustrating. When you try to get dialed in, how do you know what specific change you should make? I recently sat down with Todd Hodge, who is not only Team Losi’s Team Manager, but is also one of, if not the most respected people in the pits when it comes to understanding a car’s suspension. Todd reveals many of the tuning secrets that he has used over the years to get his cars dialed in at many different tracks.

Q: Shock technology hasn’t changed a whole lot since the introduction of the oil-filled shock in the mid ’80s. It seems like people are changing pistons more frequently than ever, making that a major tuning piece. What will changing the piston do for you that can’t be accomplished by changing the oil?

Many times the difference is in the details. Looking at the front of these two Team Losi JRX-S’, you may think they are the same. That is until you scrutinize it further. The shock angles, spring rates, roll center, and kick up are all different, along with the addition of a sway bar to the car on the right.

[Todd Hodge] This refers to the "pack" of the shock. Increasing "pack" in your shocks is when you use smaller piston holes with lighter oil as opposed to a piston with larger holes and heavier oil. Generally for me, I will use a standard- size piston and adjust the oil depending on what the car needs. By using less pack, you will have a car that rolls more and goes over small bumps better. A shock with more pack will have more responsiveness but will not go over bumps as well.

Q: There’s been a lot of confusion about what shock changes do what. Some people believe that if you lean the top of a shock in, you will gain more grip on that end of the car; others believe the opposite is true. What’s the real deal?

[Todd Hodge] By laying the shocks in on your touring car, you will gain a softer shock, however, as the arm moves up, the shock will become progressively firm. On the rear of the car, laying the rear shocks in will result in a car that drives more fluid and has less forward traction. By standing the shock up on the rear, you will have gained more forward traction, a car that transitions left to right faster, and a car that will drive more square. By laying the front shocks in on the front, you will gain a smoother steering feel off steering center. It will get you more off power steering. By standing the shocks up, you will gain more initial steering simply because the car will be more responsive off steering wheel center. You will also gain more front traction coming off the corner, which will relate to more on power steering.

Q: How can you tell when a spring change might be more appropriate than a shock angle change?

[Todd Hodge] A spring change is more appropriate when you are lacking a lot of speed. A spring change is a bigger change than a shock angle change. A shock angle change will just give the car a different feel, while the spring change will give the car more or less corner speed. If your car is pushing on the entry of a corner (lack of steering/front bite), experiment with softer springs in the front of your car. If the car is pushing on the exit of the corner, try stiffer rear springs. If you feel like the car is loose (the rear wants to spin out) on entry, stiffer springs up front might do the trick. Or if it’s loose on exit, softer rear springs might help too. It’s all about maintaining balance.

Q: Can’t you also use your droop settings to help with this?

Adjusting the droop or down travel of your suspension arms can make a huge difference in the handling of your car. If you’re pushing on entry, you can give the rear of the car more droop (allow the rear arms to have more down travel) which will transfer more weight to the nose off-power to increase steering on entry. If the car is pushing on exit, give the front arms less droop. For a loose on entry condition, you can reduce the rear droop, which will transfer less weight to the nose on entry. For loose on exit, giving the front arms more droop can help here.

This is the familiar view of the JRX-S from the competition; from behind. Team Losi recently introduced new A-Arms for the front and rear of the car that feature a new mounting point for the lower shock eyelet. Additionally, you can see that the rear suspension mount has been flipped over to lower the suspension.

Q: People continually mix up roll center and what it does. Let’s talk first about changing the roll center of the camber links. How do you raise and lower the roll center and what effect does that have on the handling on the track?

[Todd Hodge] Using your camber links for roll center is not as drastic as using your inner suspension mounts. The hard part of using your camber links is that you are constantly changing the camber gain in the tire as it goes through its suspension travel. This almost makes a larger difference than the actual roll center change.

Q: What about at the suspension mounts?

[Todd Hodge] The suspension mounts offer a lot more in the roll center adjustability. By running a lower roll center, pins closer to the chassis, you will gain more roll in your chassis, which results in less use of the tire. By running a higher roll center, pins higher, you have a car that now drives flatter and places more of a load on the tire for traction. Generally speaking, a lower roll center will have less traction where a higher roll center has more traction. You can easily see this with foam tires, a place where we never use high roll center since it will make the car traction roll due to the increased traction capabilities of the higher roll centers.

Much like the front of the car, you can change the roll center of the rear suspension mounts by adding or removing shims from under the mounts, or as is the case on the JRX-S, flipping the blocks over. The car on the left is set up without and shims and the pins level, while the car on the right has the rear block in the “high roll center” orientation with shims under it. This configuration adds rear grip to the car.

Q: Speaking of suspension mounts, one tuning piece people have been turning to is inboard/outboard toe-in. What is the benefit of changing inboard versus outboard toe?

[Todd Hodge] Inboard toe-in will give your car more support in the rear. This will give you a car that drives flatter over the rear and will transfer more weight to the front of the car during off-power transition. With outboard toe-in, you will have less support in the rear which will result in less overall rear traction. The car will also hold more of a set over the rear of the car and less initial steering in off-power transition.

Q: Is outboard toe more favorable on some surfaces and inboard more favorable on other surfaces?

[Todd Hodge] Yes, I would say on asphalt rubber tire racing, inboard toe-in could be better since it will have more traction. Outboard toe-in is better for higher bite situations where the car needs to be freed up.

Q: Let’s shift our attention to the front end of the car for a bit. With the JRX-S there is a ton of adjustability up front, whether you’re talking about the kick-up of the suspension mounts, caster, bump steer, and more. Why would someone want to change from the standard 4-degree caster blocks to something with more or less caster?

[Todd Hodge] Less caster will give the car a more responsive feel and less steering into the turn, however, it will yield more corner exit steering. More caster will give the car more steering into the turn and make the car easier to drive.

Caster and Kick-Up are all settings that change how a car drives through a particular corner. You can see the car on the right has a few spacers under the front suspension mount to add kick-up while the car on the left has spacers under the rear mount to add anti-dive.

Q: So why would you change the angle at the C-hub instead of adding or removing kick-up at the suspension mounts?

[Todd Hodge] In all my experiences, adding kick-up simply makes the car easier to drive and adds some steering. Running flat front pins will make the car more reactive and quicker overall.

Q: The JRX-S is by far one of the most adjustable touring cars ever. One adjustment that people can make is to add spacers under the steering ball studs to alter the bump steer. What does changing the bump steer do for you and when would you change this angle?

[Todd Hodge] I generally use the bump steer adjustment to keep 0 bump steer in the car. I rarely ever run bump steer. One thing to keep in mind is that whenever you change your caster, you will need to check your bump steer. Adding caster will lower the height of the ball stud on the spindle so some adjustment may be necessary. Also when you change your roll center heights, your bump steer can change too.

By changing the ball stud location in the steering knuckle you alter the ackerman. Generally, a rear position will have a more forgiving feel while the forward position will yield more aggressive steering characteristics.

Q: What about changing the location of the ball stud in the steering knuckle?

[Todd Hodge] This just changes the ackerman. By moving the ball stud forward, you are decreasing the ackerman which makes the car have more steering.

Q: When making changes to the camber links, at what point do you look at what your car is doing and decide, “I need to move my inboard rear link” or “I should use the outer hole on the rear hub carrier”?

[Todd Hodge] It all depends. If I like the balance of my car, I will then keep that same length link but may move it around. If I want the car to initiate the turn earlier but have less steering into the corner, and carry a set over the rear where I have more on-power steering, I will move the link out on the hub. If I want the car to curl into the turn and hold a set into the turn and have more traction on corner exit, I will use the inner position on the hub.

Q: When is it more beneficial to make a camber link change than a roll center, spring, or geometry change?

[Todd Hodge] This is a hard question to answer. It really depends on what your car is doing and how you want it to handle. I would suggest a spring change, then potentially a roll center change.

One of the most common causes of an ill-handling car can be attributed to your tires. With the forces exhibited on the tires while cornering, your tire can literally be torn right from the rim. Check your tires after every run, re-gluing them where necessary.

Q: When racing on carpet or high bite surfaces with rubber tires, the grip level can be so high that a car can roll over (called traction rolling) while cornering. Why does this happen and how can it be prevented?

[Todd Hodge] This happens simply because of the large slip angle that the tire has and the tall sidewall that rubber tires have. To help this, you can use super glue on the front outer sidewalls of the tires. This will give the tire an area for it to slide rather than grip and roll your car over. While some people believe that stiffening up your suspension will help prevent your car from traction rolling, more often than not it will have the opposite effect. While it may fly in the face of what you’d expect, going to softer springs and swaybars may help solve this issue.

Q: Cars are more adjustable than ever, and one change now available is altering the wheelbase. In what situations would that be advantageous?

[Todd Hodge] This is very big. By running a longer wheelbase you will make your car more fluid on a larger track. Running a shorter wheelbase, the car will have a more aggressive feel on the track, changing directions quicker but less smoothly.

Camber can play a huge role in the handling of your car. After every few runs, check your camber with a gauge such as this one from RPM to verify that your settings are still correct.

Q: What is the best piece of advice you would give to someone who is trying to set up their car for a specific track?

[Todd Hodge] Keep it simple. Usually a setup that works at track A will work at track B. Small changes like static camber, ride height and toe in/out will make huge changes. Be sure to check out www.TeamLosi.com to get the most up-to-date setups that team drivers are using all around the world. And if you ever get to the point where you can’t get your car dialed in, revert back to the kit setup. We come up with our suggested kit setups because they will work at a majority of tracks. And of course remember this is a hobby, have fun while at the track!

Glossary of Terms

Understeer- Describes a lack of front grip. If you turn the wheels and the car still wants to go straight or doesn’t turn as sharply as desired, that condition is known as understeer. It is also known as “pushing”

Oversteer- Describes a lack of rear grip. This occurs when the rear of the car wants to come around or spin out while cornering. A car that has this trait is said to be “loose”.

Roll Center- Refers to both the camber locations and the mounting point for the suspension arms. By adding or removing shims from under the camber ball stud or raising or lowering it in a shock tower, you will change the rate of camber change. By changing the height of the suspension mounts, the weight transfer and roll rate of the car is altered.

Camber- The angle of the tire in relation to the ground. A tire that is perfectly up and down has 0-degrees of camber. A tire that has the top leaned away from the centerline of the chassis is said to have positive camber. A tire that has the top leaned in towards the centerline of the chassis has negative camber.

Caster- Refers to the angle of the front axle/steering knuckle in relation to the ground.

Toe In/Out- Looking down at your car from overhead, if the front part of your tires is closer than the rear, that is toe in. If the rear part of the tire is closer, that is toe out. By using toe-out on the front of your car, it will initiate a turn more aggressively. Toe in is not used on the front. On the rear of the car, toe in increases rear grip. Toe out is never used on the rear of the car.

Shock Oil- A viscous fluid that is used inside the shock body to vary the dampening rate of a shock. Available in a variety of viscosities, a thicker oil will resist piston movement more, while a thinner oil will allow a piston to pass more freely. Using thinner oil will also allow the car to be more compliant, handling bumps and inconsistencies in the surface better.

Traction Roll- A condition that occurs on extremely high-grip surfaces where a car will flip or roll over when cornering.

Ackerman- Describes the difference in the angle of the inside and outside tires in relation to the chassis while cornering.

Bump-Steer- Refers to the steering geometry changes that occur when the suspension is compressed.

Pack- How firm or soft a shock feels when compressed with a specific shock oil and piston

Droop- The down travel of a suspension arm once it is fully extended.


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