April 28th, 2017 by Matt Holden
We’ve gotten almost every major part of a race car covered in the articles so far. All that’s left before we dive into the meat ‘n potatoes is the adjustments in the garage menu that frequently get overlooked because they rarely need any attention. Despite this, they can often lead your setup process in a bad direction and cause you to go after a handling problem that can’t be fixed without addressing the real issue. These components on the car are far less complex than springs or shocks, so they didn’t warrant a full article to address their importance (or lack thereof). In most cases, you’ll be able to set these options before anything else and leave them alone through your entire process and finding what’s right for you is simply down to a quick test session.
Most people know what brake bias does, and most people know how to use it properly. Various driving styles will cause front and rear tires to be unloaded or loaded differently on corner entry, and the various styles can lead to wheel lockup under heavy braking. It’s important to understand that wheel lockup doesn’t necessarily require a wheel to come to a stop relative to the car. If any of the four wheels on your car slows considerably without the other three following suit, it will begin to slide and is approaching lockup. The conditions necessary for brake lockup are fairly simple: heavy braking torque that can overcome the traction from the tire itself. The brake rotor for a wheel will produce a torque to counter forward rotation, traction will produce a torque against that. If the tire wins, it stays in contact with the pavement. If the brake wins, the wheel locks up.
iRacing’s brake bias setting is a representation of the braking force going to the front braking systems in percent. If the brake bias setting is 60% in the garage, that means 60% of the force sent to the braking system as a whole finds its way to the front brakes and 40% goes to the rear brakes. On cars with equal-sized tires, having a forward brake bias (higher than 50%) is not uncommon because the rear tires are under much less strain during deceleration. For cars with large rear tires, like formula cars (especially the Lotus 79), the brake bias may be under 50% due to the tremendous amount of rotating mass relative to the front tires. There is not right or wrong bias across the board, it’s just whatever is right for the car.
Setting the bias is fairly simple: Find whatever setting prevents wheel lockup in any tire under braking. This is going to be driver-specific due to driving styles and braking techniques, so what works for your teammates might not work for you. One of the more difficult things to diagnose is when brake bias is actually causing a corner-entry handling problem and is frequently confused with a bad shock or weight distribution. A quick check on telemetry for a sudden drop in individual wheel speed is a tell-tale sign that brake bias is incorrectly set.
Above is a lap from Phoenix International Raceway, and we can see a mild lockup from the left-front tire under braking in the red trace, identified by the spikes during deceleration. A simple shift of the brake bias rearward was enough to alleviate the issue and helped the car turn into the corner better because it was no longer dragging the left-front tire along. It’s very important to understand when the brake bias is actually a problem and when the chassis is a problem, since adjusting brake bias when a chassis issue is present is not going to fix the issue and could wind up making problems worse. Just because the car is spinning on entry does not mean the brakes are being applied unevenly!
Steering Ratio, Steering Lock
Racing sims have historically flip-flopped between using steering lock and steering ratio. Some even have both! Despite the confusing, both are very simple and both are driver specific. Once you’ve found what you like, you can leave it until you retire.
Steering Lock is how far the steering will move before it hits the “lock”, or end of the steering rack. This is typically represented in degrees, but the measurement is usually arbitrary and never explicitly defined. Regardless of where it is measured, a higher steering lock angle will result in a larger amount of available steering motion. A smaller value will result in a lower amount of steering motion.
Steering Ratio is a representation of how “fast” the steering box is on a car. The simplest way to think of this is to look at the power-steering box on a car which will have an input and an output shaft. The steering wheel is connected to the input shaft, the steering rack will be connected to the output shaft. The Ratio can then be thought of as the relationship in rotation between the two shafts in and out of the steering box. For instance, if we have a 10:1 steering box, 10° of rotation on the input shaft will produce 1° of rotation on the output shaft. Similarly, 12:1 will produce 1° on the output shaft for 12° of input shaft rotation. This leads to a higher ratio producing a slower steering input when compared to a lower ratio.
Of all the options in the garage menu, this is by far the most driver-specific option. Every driver has an amount of steering they want to put into the wheel to get the car around a corner and this amount is almost a muscle-memory type of thing. If the driver turns to that angle and the car turns too much or too little, they will most likely complain until it’s fixed. Nick Ottinger, who I’ve worked with in the NASCAR Pro and WC series for almost six years, is so specific on his steering box that he can tell when it’s not right before he’s even left pit road! He prefers an 8:1 box while I prefer a 10:1, so swapping setups back and forth between the two of us has led to this issue more times than I can count.
It’s important to understand that neither of these adjustments will change how much steering is necessary at the wheels to navigate a turn. If the car needs to turn the wheels 15° to go around a corner, it will always require that much steering regardless of what the steering settings are. A 16:1 steering box will not provide any handling benefit or hindrance when compared to an 8:1 steering box since both will still need to turn the wheels 15° to get around the corner. Drivers simply see the car as being “tight” or “loose” because their expected input didn’t provide the anticipated result.
Some cars are equipped with a splined steering shaft that will allow for steering offset to be adjusted on the car. This is usually represented in degrees, and is essentially the same as removing the steering wheel, rotating it, and snapping it back onto the steering shaft. The amount rotated before re-attaching the wheel is the steering offset. This is most common in oval-track cars since they’re going to be set up to pull in one direction only, resulting in the steering wheel not being completely centered when the car is driving straight. Some drivers, myself included, prefer to have a slight amount of steering to the right when the car is going straight, some prefer the wheel to be completely centered when it’s going straight. Assuming you keep most of your caster and toe alignments the same, you can typically find a setting you like and leave it there forever. Adjusting those alignments may require a slight change in offset to get the feel you were looking for again.
Rear Steer: Truck Arm and Link Angles
Stock cars, with their solid axle rear suspension, have to have some form of locating arm to keep the housing situated longitudinally in the car. For late models and modifieds, both asphalt and dirt, this is achieved by linkages attached to the rear end housing at one end and the chassis at the other. For NASCAR’s top-three vehicles, this is achieved through the use of truck arms. These arms can be set at angles relative to the reference plane (flat plane underneath the car) to control front-to-rear movement of the rear tires when the rear housing is traveling up or down in the car.
Above is a picture of Brian Day’s NASCAR Class B car, and I’ve place a white bar where the truck arm is located in the car. (Normally I’d have a good picture of one, but we haven’t flipped a car in a while so I don’t have a good picture of it. Maybe someday!) We are allowed to adjust the front mount of the trailing arm, located within the red box. This adjustment is simply a slotted mount where the front of the arm can be raised or lowered, thus changing the angle of the arm relative to the reference plane. Here’s a picture I took of a show car way back in the day with the truck arm mounts visible, identified by white arrows. Sadly, I accidentally saved over the original with the rear roll-center image I made a few months ago, so those marks are on here as well. My apologies…
As the front of the truck arm is raised, the “longer” that arm will become as the fixed end travels upward on the rear end housing. This will push the tire the arm is associated with backwards, “steering” the rear end housing in the process. The same idea applies to late model rear end linkages, however on a much more pronounced scale due to NASCAR’s truck arm configuration being installed at an angle. This is most obvious on a dirt late model, where high angles on the left-rear linkage will pull the left-rear tire forward on acceleration and impart a tremendous amount of rear-steer. This also lifts the chassis up onto the right-front tire, producing an effect known as being “up on the bars”, the bars being the rear end linkages.
For asphalt cars, the amount of rear-steer will depend heavily on how large or small the track is, as well as how much grip is available. Low grip tracks will benefit from less rear steer to keep the rear tires stuck on acceleration, but high-grip larger tracks may benefit more from larger amounts of rear steer.
For dirt cars, it will depend heavily on the track state. Fresh tracks with large amounts of grip will benefit from less rear steer, or lower left-rear linkage angles, while a slick track will play nicely with more rear steer. In all situations, testing should show you what you need in the car, as well as your driver preferences to the amount of rear steer you like in the car. Some drivers like a lot, some don’t, it’s just a matter of personal taste.
These adjustments may seem simple and insignificant, but the small details can make or break your race. Get in a test session, experiment with everything, and find what works best for you. The days of “one setup fits all” are gone, and having the car tailored to your style and expectations can go a very long way!
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