HPI Tuning Guide - Chapter 6 - Level One Adjustments

Now that you understand about weight transfer and grip, you have probably come to realize that tires are key to any good set up. The adjustments below are in order from biggest change in handling to the smallest change. Also, for new racers, you should focus on learning these set up options first as they will not only make the most drastic changes to your car but are also the easiest to feel. Want to know what a car that pushes feels like, put some really hard tires on the front and really soft ones on the back. Switch them to see what a very loose car feels like. The more you learn what a bad car feels like, the more you’ll know what a good car should feel like.

Of all the items on your car that you can change to alter the way the car handles, tires, along with their inserts, are going to provide the biggest change. Because foam tire rules are still in flux within the major rules organizations around the world, we will leave them out of this discussion for now.

What they do: Tires are the part of your car that meet the road. Every other adjustment you can make on the car just adjusts what forces are placed on the tires and how that force is managed. The fastest way around the track is to get the most out of your tires.

Tires can only maintain traction to a point, however. Since the tires must handle linear forces (during acceleration and braking as the weight of the car pulls against the grip of the tires) plus lateral forces (cornering as the weight of the car trying to continue in a straight line fights the grip of the tires), the driver must manage both the car’s throttle and steering to get the most out of his/her tires. For example, let’s say going around a hard corner as fast as the car will travel takes 90% of the tire and track’s traction ability. Then, to fully brake or fully accelerate takes 100% of the tire and track’s traction ability. Now, from those numbers obviously you cannot go through the corner as fast as possible while under full brake or full gas. The driver would only want to brake before he turns into the turn and apply only a little bit of throttle as he exits.

How to Change: This doesn’t refer to how to take the tires off your car, you should know how to do that, but rather what are the options you have to change. First, you can decide between a slick tire that has no tread or a radial tire that does have tread. A very dirty track would call for a radial tire with a very deep tread pattern like HPI’s Super Radial tire. The treads of a radial tire will help move dust and dirt away from the tire, allowing the rubber to contact the road. A dusty track would use a less aggressive tread like HPI’s X-Pattern or Advan A038 radial tires since there isn’t as much debris. A slightly dusty or clean track would let you use slicks.

You can also change the compound of the tire. The softer compounds will have more grip yet wear faster, while the harder compounds will give greater tire life at the expense of traction. Tires have a certain temperature range that they perform best at. If they are run at above or below this range, they will not achieve their maximum grip. Softer tires will come up to their prime running temperature faster than harder tires will. Some tires are also rated to a certain temperature, such as HPI’s R compound tires, to make deciding on the right tires a little easier. These temperature-rated tires are not marked as soft or hard but rather by which temperature range they are designed to run in, although the colder weather tires are softer than the hotter weather tires.

Other than the tire itself, another option you can change is a tire’s insert. Changing an insert is like adjusting the air pressure of a full size car. The softer the insert, the more the tire will conform to bumpy surfaces. The harder it is, the less it will conform. In general, a softer insert will give more traction, however it is possible to go too soft and make the handling worse. Experimentation is necessary if you are by yourself, but as always, see if you can first get the right tire and insert combination from an experienced racer at the track. This will save you time and money!

Another tire option is width. For touring cars, there are four tire sizes: Super Narrow (21-22mm), Mid-Narrow (24mm), Narrow or Standard (26mm) and Wide (31mm). Standard is what comes with most kits. Wide tires are too heavy for racing, actually would reduce traction. Super Narrow tires have a smaller contact patch with greater pressure placed on it, making the car react quicker. They also weighed much less than 26mm tires, allowing the cars to accelerate quicker. Super Narrow tires and wheels are now hard to find because they are considered illegal for sanctioned racing. Mid-Narrow, or 24mm, tires meet the minimum width rule while reducing rotating weight compared to standard tires.

When to Change: Usually, it is best if you ask some of the faster drivers or hobby shop employees what tires and inserts most drivers are using. This is usually the best place to start. Change your tires when you feel there is a major handling problem with the car. In most cases, if you notice your car is losing rear traction in the corners and no car adjustments seem to help, you usually just need new tires.

In other cases, if the car is traction rolling constantly, or cannot turn sharp enough without slowing the car too much, then a tire compound change may be in order. If the symptoms are not very severe, such as a pushing condition or the car wanting to get sideways in corners, trying the same tire compound with a different insert may be a good first choice.

With temperature-rated tires, the track temperature will also dictate a tire change. Morning qualifiers may require a softer compound tire (since the track will be cold, a harder tire won’t be able to heat up to its optimum operating temperature range) while qualifiers or mains that are run in the heat of an afternoon may require a harder compound. It is very important with temperature-rated tires that you run them in their correct temperature range. If you run a cooler weather tire in hot temperatures, they could overheat, causing them to become slippery rather than sticky. The car’s handling will seem to go away through mid race and the tire will wear out more quickly.

If your car traction rolls you may need to change tires to a firmer compound. If you’ve tried changing springs and it hasn’t helped the problem (see below) then try going to a harder tire compound.

As for tire width, because virtually every touring car racer in R/C uses 24mm tires, the tires with the new, high-grip compounds are generally found in 24mm, making choosing which size tire to use unimportant. Tire compound and insert type make far more of a difference than tire size.

Tips: Never mix tread types such slicks and radials. You will generally you want to keep the same compound front and rear as well. If you need a little more traction on one end than the other, try a pair of tires with a harder or softer insert at the front or rear before you change tire compounds.

Cost: Perhaps the most expensive setting change on the car. Because you cannot take inserts out and change them after the tire is mounted, many racers may have several sets of one tire compound with each set having a different insert. Coupled with 2, 3 or sometimes even 4 different compounds that racers may have in their tire box, it isn’t uncommon for club racers to have several full sets of tires if they race at different tracks! While it is a good idea to have a set of tires with various inserts to be flexible, if you are only racing at one track your need for a wide variety of tires and inserts isn’t nearly as important.

What they do: Springs are a part of the shock assembly. They are the coiled metal wire (usually painted) that wraps around the main body of the shock. Based upon the length of the spring, number of coils, thickness of the wire, and how they are coiled, springs will resist more or less weight. The amount of weight that it takes to compress a spring a given distance is the spring’s “rate”. For example, the HPI # 3846 Green Pro Linear Spring has a spring rate of 247 gF/mm. This means that it takes 247 grams of force to compress the spring 1mm. The greater this spring rate, the “heavier” the spring is because it will hold more weight. There are two different types of springs as well, linear and progressive. Linear springs have the same spring rate no matter how far they are compressed. For the above linear spring, 247 gF would compress the spring 1mm. To compress the spring another mm, the total force applied to the spring would have to be doubled to a total of 494 gF. Twice the distance, twice the weight. However, Progressive springs will require more weight to compress them the more they are compressed. For instance, HPI part number 6833 White Progressive Spring has a spring rate of 298 gF/mm. The first mm of travel would take 298 gF. However, the second mm of travel will require more than 298 gF to compress it. To make things easy we’ll say it increases to 305 gF. This means that to compress that spring 2mm it would take a total of 603 gF not 596 gF. Springs control weight shift during acceleration and braking. The softer the spring rate, the more weight will be transferred to that tire. Springs are usually color coded by each manufacturer so racers can easily identify the rate of each spring relative to the other springs that manufacturer sells.

How to Change them: With springs there are only two adjustments. You can change the spring’s rate or type, either linear or progressive.

When to Change them: Because springs are one of those items that determine how much weight is shifted from one end of the car to the other, namely the front to the back, you will change them in pairs. (The only exception to this is oval cars that can also change them left to right since oval cars only turn one-way). Also, since springs only work for controlling weight transfer front to rear the way to tell if you need a spring change can be determined by what the car is doing. For instance, if the car spins out when going into a turn as it is slowing down, but is good coming out when you apply throttle, this means that you should probably try going to a harder spring in front. This will let less weight transfer to the front tires keeping more weight on the rear tires thus providing more rear traction. If a car pushes as it accelerates out of the corner, then go to a harder spring in the back. Track type is also a determining factor. A track that has a lot of traction means that you will not want the weight of the car moving around too much so you can go with harder springs. Having the weight shift too much on a high traction track (or having a tire that is much too soft) will cause a traction roll. If you car traction rolls, look to using heavier springs first. A track with little traction will require the tires to do more work. By going to a softer spring you will be allowing the weight to shift around more thus putting more weight on the tires to gain more traction. The only exception to this last rule is on bumpy tracks. If the track is very bumpy, regardless of traction, a slightly softer spring than normal will have to be used so that the car can absorb the bumps on the surface. If the car seems to “hop” a lot on a bumpy track, this is a sign that the suspension or tire insert is too firm.

Tips: It is a good idea to stay within about 2 spring rates front to back. In other words, the springs on the front of the car should be no more than 2 spring rates softer or harder than the springs at the rear of the car.

Cost: Not nearly as expensive as tires, springs run about $5.00 per pair, that’s $10.00 per set of four and they don’t wear out. For around $30.00 you can have a very good selection of springs since many times you won’t have to have four springs of the same rate on the car at one time.

What they do: Shock oil and pistons absorb the energy that compresses shock springs. A piston is a round plate with holes in it mounted on the shock shaft. It moves up and down inside the shock body as the shock is compressed. Shock fluid is then added to the shock body and the piston will move through this fluid. The holes in the piston control how much shock oil can move through the piston. The smaller the holes, the less fluid can move and the slower the shock shaft will move up and down. The larger the holes, the faster the shock oil can move through them and the faster the shaft can move up and down. Also, the same principle holds true with shock oil. The thicker the shock oil is, the slower the shock shaft moves. The thinner it is, the faster the shaft can move. A thick shock oil used with a piston that has large holes will give you the same dampening as a thin oil with a piston with very small holes. Shock springs take the energy that compresses them when the suspension is compressed (either through cornering or bumps in the road), stores it, then releases it when the suspension is no longer compressed. However, if the dampening were not there, the spring would continue to bounce thus causing the car to bounce. If you have ever had a car that didn’t have oil filled shocks, you probably already know what this is like. Since a heavier spring will store more energy than a softer one, it is important to match up dampening with spring rate.

How to Change it: You can change shock oil by selecting a different weight. Shock oils range anywhere from 5 wt to 100 wt and anywhere in between though usually only in 5 wt increments. Some companies do make odd weights that are in less than 5 wt increments though. The piston usually requires a bit more work to change. HPI does include 5 different piston sizes with any pro or racer level kit, the Super Shock set and the Threaded shock body set. They are numbered 1-5 with 1 having the largest holes (softest) and 5 having the smallest holes (hardest). Usually changing the shock piston will require you to take apart most of the shock, taking the shock shaft out of the shock body, removing an e-clip, changing the piston, then re-assembling the shock and adding new shock oil.

When to Change it: For new racers, you only need to change dampening when you change your springs. If you go to a very heavy spring, go to a very heavy dampening combination. If you go to a very soft spring, switch to a lighter dampening. Most racers will change the shock oil to change their dampening rather than the piston. They’ll build the shocks with a mid-range piston (number 3 for HPI cars) and leave that alone. Once you become more experienced, you can use shock dampening to change the way weight is transferred. Dampening will not reduce the amount of weight transferred, but rather the speed at which it is transferred. If you use a little heavier dampening than what the spring would normally take, then the weight is transferred more slowly. This would make the car more stable and less reactive to your controls. The car would react slower to weight change. If you go with a lighter dampening than what the spring would normally use, then the car will feel very responsive and twitchy. Too far either way isn’t good but making this change will tailor your car to your driving style. For example, if the car feels like it turns in very good, so much so that the car feels like it is turning in too fast causing you to hit corner dots, use a little bit heavier dampening and the car will react a little slower. Remember though that this really is a very minor change and should only be used once you are more comfortable in tuning your car.

Tips: When you are first experimenting with this adjustment, make large adjustments at a time. That way you can feel the difference between soft dampening and hard dampening fairly easily. If the big change made the car do the complete opposite of what it was doing, you’ll have a good idea that the best setting is somewhere between the two extremes. If it makes the problem worse, then you know you need to go in the other direction.

Cost: Can range from free (if your kit came with extra pistons) to about $30 for a variety of shock oils and a set of tuning pistons.

What it does: The gear ratio of your car is what determines how fast it will go and how fast it will accelerate. The faster your top speed is though, the slower you will accelerate. The faster you accelerate to top speed, the lower your top speed will be. It is always a compromise.

How to Change it: A larger pinion gear or small spur gear will increase top speed whereas a smaller pinion gear or a larger spur gear will increase acceleration.

When to Change it: You will change the gear ratio of your car based upon the size of the track and how long the race is for electric drivers. A good general rule is that your motor or engine should reach it’s peak RPMs about 10% before the braking area on the main straight for the first turn. Any more than that and you are slowing down on the infield. Some racers will even put a little smaller gear than that on their car to give them a little extra speed coming out of the turns if the infield is particularly tight. Also, for electric racers if the motor is getting too hot then you may need to decrease the size of your pinion one or too teeth. You can also ask fellow racers what their gear ratio or roll out is. If they have the same car as you do, asking the gear ratio will be enough.

Tips: Races are won on the infield, not the straightaway. It is always better to have a little bit more acceleration out of the turns than it is to have a little bit more top speed on the straight.

Cost: For $20-$40 you can have a wide range of pinions and spurs that you can use to adjust your car for different track sizes.