Because you’re probably doing it wrong.
Whether your radio is a pro pistol or an econo model, the steering system is its most adjusted feature. Basic radios only offer steering trim and servo-reversing, but many RTR radios also include dual-rate steering, and popular sport-level computer radios offer adjustable endpoints, exponential and subtrims in addition to steering dual-rate, all of which are often misunderstood by many users regardless of skill level. In this quick how-to we’ll decode steering terminology and get your radio system dialed in for dynamite direction control.
HERE’S WHERE TO START
- Subtrim: zero
- Steering trim: centered (or zero left and zero right)
- Endpoints: 100% right, 100% left.
- Dual rate: 100%
- Exponential: zero
Before you begin!
If your radio was previously in another car, be sure to reset all its adjustments before you dial it in for your new setup.
UNDERSTANDING SERVO TRAVEL
We’ll talk a lot about servo travel, which is typically 60 degrees from center in each direction, for a total of 120 degrees (60 degrees to the right, and 60 degrees to the left). And as you already know, servo travel corresponds to wheel travel. For illustration purposes, we show only 16 steps in the travel range; in reality, there are many more steps, which is why the steering servo appears to swing smoothly. In the illustration above and those that follow, note that there are always 16 steps; only the amount of movement represented by each step changes.
Subtrim will affect all your other settings, so set it first. If your steering servo is centered when you power up your radio system, consider subtrim to be set but chances are it will be a little off, and you’ll need to center it. By using the subtrim, you’ll move the left and right steering endpoints along with the center position, so you won’t lose any servo travel. A note for those of you with basic AM systems: your trim knob acts like a sub-trim and moves the entire servo travel range as shown here.
As the name implies, this setting controls where the servo’s travel ends. The left and right throws can be set independently, generally to prevent servo binding if the steering system runs out of travel before the servo does. Dial out as much travel as you need to prevent the servo from binding, but take note of the left and right settings: they should only be a few digits apart. Note the illustration, which shows full left travel and a reduced setting for right travel. When steering right, the servo moves a smaller amount for each unit of wheel travel. This is something we can exploit for greater steering precision, as you’ll see when we get to Steering Rate.
With the steering subtrim set, your car should track perfectly straight, but it probably won’t because the positions of the various steering parts changes slightly under load. A click or two of trim should be all you need. If you need more, go back to sub-trim. Avoid making large trim changes because your adjustment will add steering in one direction and take it away in the other. For example, if you add 2 degrees of right trim, you will lose 2 degrees of total right throw but gain 2 degrees of left throw.
WHAT COMES FIRST?
Some steering adjustments can override others, so the biggest adjustments should be made first. You should set the steering functions in this order:
- Steering rate (dual rate)
Once you’ve set your subtrim and endpoints, the only things that need changing to suit track conditions are steering rate and exponential. Subtrim and trim aren’t really tuning aids unless your car starts to track improperly. If the tracking error is slight, and due to parts seating themselves or wearing in, use the trimming functions to clear it up. But if your car suddenly starts to pull to one side, you probably have a mechanical issue or a bad servo. Your radio is not the fix.
DUAL RATE: IT’S AN AIRPLANE THING
Ever wonder why steering rate is usually called “dual rate?” RC airplane fliers often select a high servo rate for more elevator throw at low speeds and a lower rate for high-speed flying when smaller throws are best. To toggle between the two settings, they simply flip a two-position “dual-rate” switch. We don’t toggle between settings like the plane guys do, but the principle is the same, and the name still stuck.
Also known as dual rate, even though there’s nothing “dual” about it. Changing the steering rate alters the amount of steering throw in both directions at the same time. So how is that different from endpoints? Well, for starters, endpoints are set independently. Also, steering rate can’t override the endpoint settings; once you reach the endpoint, you can’t dial in any more travel. But the change in servo throw you see with a change in steering rate is just part of the picture. Steering rate literally changes the rate of steering, that is, the amount the steering servo’s output shaft moves relative to the amount the transmitter’s wheel is turned. Note that the units of servo movement are finer in the illustration above than they are at the transmitter’s wheel; this gives you finer control of the steering system, and that’s why you should always set the steering rate so you only have as much servo travel as you need for the track’s tightest sections. Ideally, you should use all of the transmitter’s wheel travel for maximum steering, otherwise you’ll have to make smaller movements to steer your car or truck.
When you select an exponential setting, the steering rate changes exponentially with wheel travel. Here’s the simple version: steering rate increases progressively as you steer farther from neutral. That’s if you’ve selected a negative exponential value; if you select a positive value, you’ll have a higher steering rate near neutral and less rate as you get closer to the endpoints. Negative settings are the most common and are usually used to make a car less sensitive around neutral, for more predictable handling on loose tracks without limiting total steering throw.
There you have it: everything you really need to know about steering setup. Depending on the brand of transmitter you’re packing, you may have other functions to choose from, but the adjustments explained here are the biggies that will make the most difference on race day, whether it’s at the track or in your backyard.