HPI Tuning Guide - Chapter 7 - Level Two Adjustments

Level 2 adjustments are adjustments that still have a very drastic affect on the way the car handles, but are not nearly as noticeable as the Level 1 adjustments. If you have tried some of the above adjustments and the car is closer to where you want it but still not quite there, try some of these adjustments.

Both front and rear tires have an adjustment for toe. Because they are very similar, we’ll cover both of them here.

What it does: Toe is the angle of the tires in relation to their direction of travel. 0 degrees toe is when the tires are pointing straight ahead. Negative toe angle (or toe in) is where the front of the tires are pointing in towards the chassis whereas positive toe angle (or toe out) is where the front of the tires are pointing away from the chassis. The more toe in you have the more stable the car is but the less steering it has. The more toe out you have the more steering you have but the car is less stable and feels more twitchy. In the front, toe in will give you less turn in and make the rear feel more stable. Toe out in the front will give you really good turn in but may make the car spin out. More rear toe in will give the rear of the car more stability but decrease the amount of overall steering you have while decreasing the amount of rear toe in (you never want toe out in the rear) will give you more steering but make the back spin easily.

How to Change it: For the front, most cars have adjustable steering links called turnbuckles. By turning these turnbuckles one-way, you’ll shorten the link and by turning it the other way you will lengthen the link. Shortening the link gives you more toe out (if starting at 0) while lengthening the link will give you more toe in (if starting at 0). As a general rule, you don’t want more than one degree of toe out or more than -2 degrees of toe in. For the rear, pillow ball suspension based cars will let you tune the amount of toe in to any amount you desire. Most cars do not use pillow ball suspension but, like HPI cars, use hinge pin based suspension. To change the rear toe in a hinge pin based suspension, the rear hub must be swapped out and exchanged for one with a different toe setting. Usually the manufacturer will have the rear hubs made with their toe in amounts molded into them for easy recognition. For most cars, 2 degrees toe in for the rear is a good starting point and you never want toe out in the rear.

When to change it: Generally the rear you would almost never change. If you want a little more steering, you could reduce the amount of rear toe but do it in small, ½ degree amounts. In the front, if the car doesn’t feel like it is wanting to turn in very aggressively but you would like it to, decrease the amount of toe in or use a degree of toe out. If the car turns in too aggressively and the back end wants to spin out entering a corner, add a little toe in.

Tips: If you have a car that spins out going into a turn, look here first. If your front toe is set to one degree toe out or 0 degrees toe, give it a little toe in. Many people overlook this adjustment when faced with this problem.

Cost: Free for most cars but very hard to adjust on non-turnbuckle equipped cars.

What it does: Kick up is the angle of the front arms to the ground. Zero (0) degrees would mean the arms are parallel to the ground. This is good for high bite, super smooth tracks. For tracks that are rough, you can angle the suspension arms so that the front part of the arm is higher than the rear. In off road cars, the kick up is usually not that adjustable and built into the chassis itself. Adding kick up will allow the suspension to absorb the shocks of bumps in the road better and thus allow the car to remain stable over rough sections.

How to change it: Depending on your car, kick up can be changed in one of two ways. Either you change your lower suspension arm mount (and upper arm mount angle in the case of the Pro 2, Racer 2 and Super Nitro) or you can change the front arm brace (Pro 3 only). With HPI cars, the options are restricted to 0 degrees and 2 degrees (except for the MT, Nitro MT, and Nitro Rush. All these kits have their kick up built into the chassis and it is a lot more than 2 degrees).

When to change it: If you are looking for a little more stability on rough or unprepared tracks, change from 0 degrees to 2 degrees.

Tips: When you change your kick up, you are also changing your caster. If you were running 10 degrees of caster and added 2 degrees of kick up, your caster setting would now be 12 degrees. You would need to change your caster block to 8 degrees to maintain 10 degrees of caster if you were to run 2 degrees of kick up.

Cost: Free if your kit came with the parts to make the change. If not, prices run from the cost of a plastic parts tree up to the cost of an Aluminum Lower Arm Mount in 2 degrees. Note that this aluminum arm mount is the one suggested when changing the kick up on a Racer 2 or Super Nitro. Both of these kits have upper arm mounts that are set at 0 degrees kick up. The Aluminum Lower Arm Mount (HPI Part number 72045) includes spacers that will change the angle of the upper arm mount on these kits to 2 degrees. The only other kit with an upper arm mount is the Pro 2 but it includes parts to change kick up to 2 degrees in the kit.

What it does: Camber is the angle of the tire when compared to an imaginary line perpendicular to the ground. Negative camber is where the top of the tire points in towards the chassis. Positive camber, used only on the left front wheel of an oval car, is where the top of the tire points away from the chassis. As a car turns, the chassis rolls. This chassis roll causes the tires to lean out towards the outside of the turn. If the car had 0 degrees of camber, as the tire leaned out it would begin to have a smaller contact patch because the inside of the tire would be leaning off the surface. This is bad because the bigger the contact patch the more grip the tire will have. In a perfect world, the tires would always be straight up and down in relation to the surface under all conditions. However, this cannot happen. To compensate for this, a happy compromise must be made. By setting up the car with a few degrees of negative camber and having the tire wear evenly, you are getting the most out of the tire most of the time.

How to change it: You can change camber only if you have adjustable links on your upper suspension arms/links or if you have a pillow ball based suspension. By shortening the upper arm/link, you will increase negative camber. Generally, you want camber to be set between 0 and –2 degrees.

When to change it: Usually you want your camber set up so that your tires wear evenly. To check this, clean your tires well and run the car. After running your car for one run (either one battery pack or one tank of gas) bring it in and see where the tires are dirty. If the ring of dirt around the tire is not in the very middle of the tire, adjust your camber. If it is to the inside of the tire, you have too much negative camber. If it is on the outside of the tire, you do not have enough negative camber. You can also use camber to make adjustments to the way your car handles. By giving the one end of the car a little less negative camber than it needs to wear the tires even, you will give it less traction and thus less grip. So if a car oversteers a bit, a little less negative camber in front may help solve the problem. However, most racers will agree that this is a bad way of tuning a car and shouldn’t be used but as a last resort.

Tips: Even tire wear means longer tire life and that you are getting the best performance out of your car.

Cost: Free if you have turnbuckles for upper links. Otherwise it may cost you a few dollars to change to adjustable turnbuckles.

Caster and Rear Anti-Squat are the same thing, just on different ends of the car. They work in much the same fashion but most cars have a fixed Rear Anti-Squat setting so we’ll focus on Caster.

What it does: Caster is the angle of the steering kingpin in relation to a vertical plane perpendicular to the ground. With the top of the kingpin angled back towards the car you have negative caster. Positive castor, with the top of the kingpin would be angled towards the front of the car, is never used. Negative castor makes the car more stable to drive in a straight line. Without it, the car would feel very twitchy, react very quickly to steering inputs and easily spin out. The more negative caster you have, the more steering you will have in high speed sections, during turn in, and the more stability you’ll have in the straights. With less caster, you’ll have more low speed steering with less stability in the turns and less steering during turn in.

How to change it: Most cars will let you change caster by either moving clips around on the upper arm’s inner hinge pin. Moving the clips more forward will give you more negative castor while moving them back will give you less negative caster. With other cars, such as most of HPI’s cars, exchanging front hubcarriers from stock to the optional ones that come with some of the kits changes castor. The hubcarriers will have the number of degrees molded into them for easy reference. If they don’t, be sure to keep them separated and marked for later identification.

When to change it: If you car feels really hard to drive in a straight line or wants to spin out when you enter a corner, try going to less negative caster. If the track is very bumpy and loose, try going to more negative caster.

Tips: If your car is spinning out going into a turn, try this change only after you have increased negative toe in as it may be a bit more difficult to adjust and have other adverse effects on the car’s handling.

Cost: Free if your car comes with the right parts. About $10.00 or less if it doesn’t.

What it does: Ride height is the distance between the bottom of your chassis and the ground. You measure it once in front and once in back. If either end of the car is lower than the other, the end that is lower will have more weight placed on the tires and will have more weight shift to it under braking (front lower) or Acceleration (rear lower). Tweak goes hand in hand with your ride height. Tweak is basically the left to right weight balance of your car. If one side has more weight on it than the other, then the car will turn or pull one direction more than the other. Many times when you change your ride height you may need to reset the tweak. Some beginners make the mistake of thinking that adding preload to the springs makes them “heaver” (increasing their spring rate). While this is true of the progressive springs, this is not what preload is for and will actually have no affect on linear springs. If you need a heaver spring rate, change springs. Don’t preload your springs to get a heavier spring rate even if they are progressives.

How to change it: To change ride height you can add or subtract preload from the shock springs. Adding preload will raise ride height while removing preload will lower it. When adding preload, it is important to make sure the shock is long enough to allow the shock to extend the extra distance rather than just compressing the spring. You can lengthen the shock by unscrewing the ball end that is at the bottom of the shock shaft. Just make sure not to unscrew it so far that it will not stay on. Preload is usually adjusted using clips or an adjustable collar. Add clips or lower the collar to add preload and raise ride height, remove clips or raise the collar to reduce preload and lower ride height. Always change tweak after you set your ride height. To check to see if your car is level, put it on a tweak board like MIP’s Tweak Station or get a hobby knife and place the car on a flat and level surface. With the tweak board, one side will be shown to be heavier than the other. With the MIP board, there is a small bubble like in a level to indicate this. Place your car on the board an make sure that it is centered. Compress the suspension a few times so the chassis settles to it’s proper ride height. If the bubble is off to the left side of the car, the right tire has more weight than the left. To level the car, you will need to place a little more preload on the left shock and take a little away from the right shock to balance out the car. With the hobby knife, place your car on a flat and level surface then find the center of the chassis between the left and right suspension arms. Compress the suspension a few times so the chassis settles to it’s proper ride height. Place the knife on this centerline and lift the car up. If one tire lifts up before the other, the tweak is off. Add preload to the tire that came up first and take some off the tire that came up later. The reason you add some preload to one shock and take some away from the other is so that the chassis’ ride height remains the same.

When to change it: The most obvious time to change it is when your chassis is scraping on the ground during cornering. Otherwise, you should have your ride height set to between 4mm and 7mm depending on surface conditions. The bumpier the surface, the higher it needs to be. It is important to note that if you are on a high traction track and running a very soft suspension, the chassis may scrape the ground under cornering. Try setting your ride height low but changing your roll center (see below) to decrease the amount of roll in the corners. With tweak, you may need to check it after a big crash and every time you take your car apart or change the ride height.

Tips: Set your ride height the same in front and back or, if you want a little more steering, about .5mm lower in front than in back. Also, if you drop your ride height drastically from stock, you may need to change your roll center as well (see below).

What it does: Roll Center is an imaginary point at which your car’s suspension, and the chassis for that matter, rolls around when cornering. An imaginary line connects the Roll Center of your car to the CG. The distance between the Roll Center and CG is called the Roll Moment. The greater the Roll Moment, the more the car will roll and the more weight will be transferred to the wheels. As was mentioned earlier, having a low CG will reduce your roll and thus reduce weight transfer. Generally speaking, getting your CG as low as possible is a good thing since there are many ways to get roll back if you need it but not as many to take it away as much as lowering the CG will. However, the way most people will lower their cars is with ride height as described above. The problem is when you lower your CG by decreasing preload on your shocks, you are, in effect, lowering or compressing the suspension. When you compress the suspension, you are also lowering your Roll Center thus increasing the distance of the Roll Moment, which, in turn, increases roll. Consequently, you just canceled out lowering your CG and threw off the way your suspension works. Now you may need to raise your roll center back up to get the full benefit of your lowered CG. To do this you can change your Upper Camber Link Position, which is your roll center adjustment (Another option part to change your roll center are the HPI Handling kits for the Nitro Cars, RS4 Pro2 and RS4 Sport 2. These will physically lower the chassis and CG while maintaining the stock suspension geometry).

How to Change it: On many of today’s high-end cars, the upper links have many different positions that they can be set in. However, without knowing which one to put it in you don’t know what you are changing. The two things you need to change when you adjust your roll center are the length of the camber link and its angle relative to the ground. First we’ll deal with length. A longer Upper Link will let the car roll as far as the springs will allow it. This will give that end of the car a lot of traction in the turns, especially in the middle. If you shorten the Upper Link, that end of the car will roll over less thus generating less grip. As for the Upper Link’s position, if it is parallel to the ground (or close to parallel) it will allow the roll to come very predictably and smooth. The car will feel very stable and will roll quite a bit. An Upper Link that is angled down will produce more initial grip but allow the car to roll less. The car can transition faster with the link angled. It is important to note though the there are two roll centers in your car, one front and one rear. You want to keep these two roll centers working pretty close together to keep the car balanced. If you don’t, the car will feel very unpredictable and unstable.

When to Change it: If you want a little more roll and thus more traction on one end or the other, you can change the Upper Link. A longer front will give you more mid corner steering but may make the car feel loose. A short link will give you a very stable feeling car with good turn in but less mid corner steering. If you angle the rear Upper Link, the rear will feel much more planted going into the turn and the car will feel very stable. Want to be able to accelerate out of a corner harder by softening the rear springs (more weight transferring to the rear tires giving them more traction) without loosing the turn in grip, then angle the rear arms. Want the a little more steering as you go through the corner, lengthen the front Upper Links and run them parallel. Play around with this setting some and you can see how you can fine-tune your car’s weight transfer.

Tips: Very easy change to make and when used in conjunction with springs and dampening, really has a big affect on the car’s handling. Make sure to recheck your camber and toe settings though when you change roll center as this will also affect your camber setting.

What it does: Whether your car is equipped with Ball Differentials or Gear Differentials, these can be adjusted to help fine-tune your car’s handling. You can tighten the differential to get less traction from that end of the car or loosen it to get more traction.

How to change it: To change this setting depends on the type of differential you have. For ball differentials, the change is easy. Just tighten the screw that holds the differential together to tighten the differential and loosen this screw to loosen the differential. With gear differentials, it is a little harder. You have to remove and take apart the differential then seal it using o-rings. You will then fill the gear differential with thick differential grease. The thicker the grease you use, the tighter the differential will be. These greases are rated by their weight much like shock oil but with weights being about 30,000, 50,000 and 100,000 with other weights in between.

When to change it: If your car is a little loose in the turns, you can tighten the front differential or loosen the rear. If it wants to push when turning, you can loosen the front differential or tighten the rear.

Tips: Be sure not to loosen ball differentials too much as they will start to slip and the friction created by the balls slipping against the differential rings will melt the plastic pulley the balls are in.

Cost: Free if you have ball differentials and just a few dollars for gear differential grease.

What it does: Down travel (or droop) is how far the suspension drops when the chassis is raised from ride height before the tires leaves the ground. If you set a car on the ground, push it down a few times to settle the suspension, then begin to slowly pick up one end of the car, you’ll notice that the tires will stay on the ground for a little bit before they come up. This is droop.

How to change it: You adjust down travel in one of two ways. The first method is to lengthen or shorten the shock. With most shocks, the bottom ball end screws onto the bottom of the shock shaft. The shock is shortest when this ball end is screwed all the way onto the shock shaft. The shock is much longer when the ball end is unscrewed almost completely from the shock shaft. If the ball end is threaded all the way onto the shock shaft and you need to reduce the amount of droop, you would then begin to add spacers to the shock shaft under the piston inside the shock body. Of course, this will require you to take apart the shock, take the ball end off, remove the shock shaft and piston from the shock, place the spacer on the shock shaft, and reassemble the shock. In general, you should never have more than 3mm of droop on your suspension.

When to change it: Start at your kit’s recommended shock length when setting up droop. If you need a little bit more steering in the middle of the turn or on power, add a little more droop to the front suspension. If you want a little bit more rear traction in the middle of the turns, reduce the amount of droop in the front or increase the amount of droop in the rear. While rear droop should always be more than front droop, always be careful not to put more than 3mm of droop in the rear as it will hinder the car’s handling ability.

Tips: Remember that since you are adjusting your shock length that you may also need to adjust your ride height after making a droop adjustment.

What they do: Sway Bars (or Anti-roll bars) will help reduce the amount of chassis roll during cornering. This, in turn, will reduce the amount of weight transfer to the outside tire. While you could do this with a heavier spring as well, by using a sway bar you can reduce chassis roll without sacrificing lateral traction under acceleration and braking. Sway bars do this because as the car begins to roll, the outside suspension arm pushes up against the sway bar. Because the inside suspension arm is traveling downward or not at all (if no down travel is allowed in the suspension) the outside arm must “twist” the bar to move up.The sway bar tries to fight this twist by exerting a force against this twist and it is this resistance that keeps the car from rolling more that it would without the sway bar.

How to change them: Sway bars can be added to your car using a sway bar kit. Sometimes this will require some disassembly of the car but is still something that can be changed between races. They mount to the chassis in the middle of the bar with a mount that will allow the bar free travel up and down then attach to the suspension arms with some sort of linkage that allows the suspension arms to move up and down without binding. Sway bars come in different rates from soft to firm or they can be stiffened by changing the position of the linkage on the arm of the sway bar. HPI sway bars come in three different levels of firmness, soft, medium, and hard.

When to change them: Sway bars usually are only used on very high bite tracks. If you are experiencing a problem with your car Traction Rolling through turns but if you try a stiffer spring rate the car spins out when accelerating or braking, try adding sway bars. Start by putting the softest sway bar option on both ends. If one end seems to have less traction than the other, increase the stiffness of the sway bar on the end that has more traction. For instance, if you car wants to push through turns after adding the sway bars, go to a firmer sway bar in the rear. If you go to the hardest sway bar on one end and the problem still exists, then remove the sway bar from the end of the car that has less traction and put the softest option sway bar on the end with more traction and try again. Continue to increase the hardness of the sway bar on the end with more traction until the car feels balanced.

Tips: Remember that using sway bars is, in a sense, increasing the spring rate during cornering so you may need to fine-tune the car’s handling with springs and sway bars at the same time. This is one of those adjustments that you have to experiment with.