Written by David S. Peterson
The NTM has transformed the Corvette substantially both in terms of driving technique and setup. The setup parameters still have the same effects they had before, but the sweet spots have definitely changed, and there are new nuances to understand.
Heat is a very central element now. The Corvette can overheat its tires very easily… particularly if the tires are sliding around to any degree. So the new goal is smooth and quiet.
It no longer pays to design sets around sliding the car effectively. In general, much less oversteer is called for than before. Now the main task is to plant both ends of the car as firmly as possible so that it doesn’t eek out any noises while going fast as hell.
There are a myriad of settings that can be changed. And they can all seem rather bewildering at first. Which setting gives me more oversteer, please? Well, there’s not quite an easy answer to that. Many of the settings affect the car’s balance. But the trick is that they each affect the balance under different circumstances. So in order to adjust the car properly, you need to feel out exactly where and when you want more or less oversteer, and know what settings affect that specifically.
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FUEL
Fuel is always the first setting to work with. The fuel tank in the Corvette is located toward the rear of the car. So the amount of fuel being carried changes the balance of the car.
Determine the amount of fuel you’ll need, and set that first before you change anything else. Then take the car for a spin to see what it feels like.
The fixed set starts with maximum fuel, and since you’re probably going to run less fuel than that, the rear of the car is going to become lighter. This will lift the rear of the car, change the rake, and alter the wheel cambers… all of which would tend to make the car oversteer more. However, the great loss of weight rearward generally causes the car to understeer more overall.
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BRAKE BIAS
Front brake bias
Brake bias is the first real setting to work with on the car. It has significant impact on the driving style that will be used, and thusly how the remainder of settings will come into play.
Front brake bias is the percentage of brake force that applies to the front wheels when you brake. A value of 63% is approximately neutral. 64% and higher will tend to lock up the fronts first. 62% and lower will tend to lock up the rears first.
Beginning drivers tend to favor higher values here because the car is much more stable under braking with more forward bias. As they become more experienced, they’ll tend to want to move the
bias rearward.
Brake bias below 63% tends to be used with gas on brake techniques to allow the driver greater dynamic range in their brake bias from moment to moment. That is, a bit of throttle is applied to
cancel some of the rear brake at will. The driver can intentionally lock the rears at any point by simply letting off the on the throttle while braking. Gas on brake techniques require either left foot
braking or heel and toe.
I don’t think low bias values are generally used with straight braking. Low brake bias settings are one of the chief reasons why alien setups are slow and dangerously unstable in the hands of less advanced drivers.
Fixed setups tend to have rather high brake bias settings of 64% or more. Dropping it down to 63% or 62% will reduce the braking distances, but also make the car much less stable under braking. Keep in mind that if you’re struggling to keep the car under control while braking, the potentially shorter stopping distances aren’t worth it. Don’t try for a lower brake bias than feels comfortable, or you’ll be slower, not faster.
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DIFFERENTIAL
The Corvette as a car is defined by its massive power. The differential has a dramatic effect on what’s going on with those meaty rears, and its affect on the balance and overall driving experience of the Corvette is more dramatic than many drivers might expect.
The differential tends to lock the rear wheels together the tighter you set it. The main purpose of this is to prevent the car from wasting its power spinning the inside wheel at the exit of a turn while the outside wheel remains unpowered, as would happen with an open differential.
The downside of locking the wheels together is that the car won’t want to turn properly. Imagine lifting the front of the car off the ground and then trying to rotate the car on its rear wheels while they are locked together. They wouldn’t roll. You’d have to force them to slide to turn the car. When your differential is tight, your own tires resist against your car turning.
This seems like it should promote understeer and stability, but it actually doesn’t. The forces that are preventing your car from turning properly are actually subtracting from the available traction in your rear tires. Your rear tires are resisting _each other_!
So loosening the differential promotes more understeer because then the rear tires aren’t wasting their traction on infighting. This is a good thing to do as much as possible because there are more efficient ways of tuning that oversteer back in with other settings. Loosening the differential improves the overall traction of the car.
The down side of loosening the differential is that you can start spinning that inside tire on turn exit. That will lose speed. So the general idea is to have the differential just tight enough to prevent that from happening.
The fixed sets all feature tight differentials which exhibit their locking force all the time. There’s speed to be gained by loosening them.
Clutch Plates
You can have between one and three clutch plates. More clutch plates makes the differential much tighter with the same other settings. It multiplies the effects of those settings. So two clutch plates will be twice as tight. And three clutch plates will triple the effects of the other settings.
With the OTM, I never saw a need for more than one clutch plate. But the NTM seems to respond well to settings which are roughly twice as tight as before. So two clutch plates seems to be a good standard. Though I still sometimes run singles.
Preload
The tightness of the differential is variable depending on how much engine power is being applied. This setting determines how tight the differential is when no engine power is applied at all.
I generally set this to 0. I prefer not having my rears resist each other from letting the car turn when I’m neither accelerating nor decelerating.
You may want to run with a preload if you find the balance transition of locking the rears together when you do get on the gas too jarring, but that’s the only reason I can think of to do it.
Note that reducing the preload on the car versus the fixed sets will add substantially more traction in the rear… and thus quite a lot of global understeer. Soften the front sway bar or tighten the rear to counteract this effect and restore your desired balance.
Drive ramp angle
The lower you set this value, the more tightly the rears will lock together when you get on the gas. The car will rotate more when you punch it.
You generally don’t want to set this value so low that the car wants to kick out when you get on the gas. With the OTM, this could be a desirable effect. But not with the NTM. Now what you’re looking for is the rear to stay planted under heavy acceleration and not wash out into understeer.
Coast ramp angle
When you’re completely off the gas, the engine drags, effectively applying a braking force to the rear wheels through the drive line. This setting determines how tightly the rears lock together when this happens. It’s just like the Drive ramp angle, but the opposite direction.
In general, it is no longer so effective to coast through the mid turn with the NTM as it was with the OTM. Instead, neutral throttle generally produces the best cornering speed. Therefore, the coast ramp no longer effects portions of the mid turn that it used to. The car has to coast for this setting to have any effect at all.
Now the coast ramp is primarily of value only in the braking zone prior to the turn. Whereas locking the rear wheels together tends to reduce stability in the midturn, it has the opposite effect during straight line braking. It will tend to resist any tendency of the car to want to turn while in the brake zone, promoting stability as the car approaches turn in.
The coast ramp is commonly set with lower values (tighter) than the drive ramp. This can seem counter intuitive, but remember that there’s much less force being applied through the drive line on the coast side. So the same amount of locking requires a lower angle to achieve.
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CHASSIS
Spring rate
I subscribe to the notion that the purpose of springs is to keep the car’s chassis from sitting on the ground. They are not a primary method of balancing the car. As such, we will fiddle with them early.
The things to consider with spring rates are ride height and compliance with the road. The more bumpy a track is, or the more three dimensional it is with steep bankings, crowns, crests, and other non-flat features, the softer your springs need to be in order to keep the wheels in contact with the road at all times.
The softer your springs are, the more ride height you’ll need to keep the car from bottoming out. Stiffer springs allow you to lower the car more, but they only work well on tracks which are rather
flat. And remember that curbing counts, too. The lower and stiffer you make the car, the more it will freak out over the curbs.
Contrary to common belief, stiff springs _do not_ reduce weight transfer. The same amount of weight transfers from the inside to the outside in a turn whether the car leans on soft springs or not. However, reducing the ride height does reduce weight transfer. And stiffer springs allow lower ride height. So this may be the origin of the misconception. But understand that it’s not the springs themselves.
With the OTM, spring rates near the minimum (1000lb) were generally ideal. The NTM tires seem to have considerably more bounce to them, and so the suspension needs to be relatively stiffer to compensate. Values more toward the middle of the range (1300-1600) seem to work well.
Note that if you’re making a qualifying set with low fuel, you’re removing quite a lot of weight. And
consequently the springs will lift the rear of the car up. It can be a good idea to take some spring out of the car to compensate, dropping the car back down a bit. The car is lighter, so less spring force is needed to hold it at the same level.
Spring perch offset
This is the other end of the spring rate and ride height equation. This is how to directly adjust the ride height of the car. Higher numbers drop the car down.
Keep in mind that you’re not just lowering the car, you’re also compressing the suspension which changes all of the geometry. In particular, your cambers will change when you adjust the ride height. Take note of what your cambers were prior to the change and then set the cambers back to what they were afterward.
Now the goal here is not actually to lower the car as much as you possibly can before it bottoms out. There’s a diffuser under the car, and it stalls at less than about 2 inches of clearance. The ideal ride height at the front of the car seems to be in the range of 2.2 to 2.5 inches.
The rake of the car is important primarily because of its aerodynamic effect. That is, the lower the front of the car is compared to the rear end, the greater the angle on the otherwise fixed wing and other aero elements. That’s right. We can’t adjust the angle of the wing, so we just tip the whole car instead. Clever, no?
Good values for the rake seem to be roughly 0.5-0.8 inches. The rear ride height should be in the range of 2.7-3.0 inches or so.
There’s a trade off here between getting the rear end of the car as low as possible for reduced weight transfer and raising it up higher for greater rake and thus down force. Or to put it another
way, that’s a trade off between better low speed turning (where aero is weak anyway) or better high speed turning (where down force is strong).
Camber
Negative camber tilts the tires inward so that when the car leans into a turn, the outside tire will be flat against the track.
The NTM currently has unrealistically weak side walls, and so seems to run best with rather low cambers near -1.0 front and back. The fixed sets use considerably more camber, and this does seem to have a good feel to it, but it has less traction, produces very uneven wear on the tires over time, and seems to make the tires more susceptible to overheating (presumably because the heat isn’t spread across the whole surface).
In general, the more camber you run, the harder the car will turn (up to a point… which isn’t very much right now). But the trade off is you lose straight line braking because the tires are not flat
against the track surface when moving in a straight line.
In the real world, camber is set by checking the outside, middle, and inside tire temperatures after running a few laps. These values are reported for the NTM, but they’re unusable because the inside and outside temperatures are averaged with the sidewall temperatures.
There are the tire wear percentages, too. Those do work properly and they do help, but it takes quite a few laps to get enough wear to see if the tires are wearing evenly.
So your only real tool for setting camber at the moment is driving the car and feeling for it.
You can tell whether a change in camber increased or decreased the car’s traction if a change on one end of the car changes the balance of the car in that direction. For instance, if you add negative camber to the front of the car and the car starts oversteering more, then it worked! This is increasing the overall traction of the car, not just shunting the balance around like swaybar adjustments do, so it’s definitely worth pursuing.
Note that positive camber is never used in road course sets.
Anti-roll bars
The sway bars are your primary overall balance tuning devices for the car. They pretty much adjust the balance evenly across the board.
Soften the front and/or stiffen the rear to produce more oversteer.
Stiffen the front and/or softer the rear to produce more understeer.
The more stiff the sway bars are overall, the less compliance the wheels will have with bumpy and unflat roads, just like using heavier springs.
Bump stiffness and Rebound stiffness
These are damper (shock absorber) settings. They affect the balance of the car just like swaybars do except that they only affect the stiffness of the suspension when it’s in motion.
So that means the stiffness of the suspension is momentarily increased when the car leans into a turn. Or when it levels back out again after a turn.
You can tune these settings to make the car more lively so that it has a more oversteery edge when you first start to turn. Or you can set it opposite to ease into a turn more smoothly.
One effect to watch out for in particular is when you turn the car, and the front end initially turns in very rapidly as if too spin, and then immediately washes out again and settles into the turn all on its own. This is what happens when your dampers are set for a lot of nervous oversteer while the springs/swaybars are set for much less. The balance difference is too extreme.
In general, I like to set the dampers to the same relative stiffness front and back as the springs have. This will help the car maintain steady and predictable balance as it goes over bumps. Otherwise, if you were to make the front dampers relatively stiffer, for instance, then whenever the car passed over a bump during a turn, the balance of the car would momentarily shift toward understeer. That could be desirable in some circumstances, but in my opinion usually not.
Bumpstop gap
The bumpstops limit the suspension movement in an effort to prevent the car from ever bottoming out. Essentially, they’re like very stiff springs which are reached only when the suspension
compresses beyond a certain point.
They would be important to work with on a real car, but in iRacing, there doesn’t seem to be much downside to just letting the Corvette bottom out on the track if it’s going to. So I always set these to the maximum gap possible.
I also suspect that if they are set to a smaller value where they’ll actually get hit before the car bottoms out, it becomes possible to break your suspension by just running over curbs on the track.
Caster
Caster is a wonderful thing. This is a rearward tilt of the wheel such that when the steering wheel is turned, negative camber is increased. This is great because it allows the wheels to have less camber when travelling in a straight line and more camber when turning.
Theoretically, caster can be used in combination with static camber settings to tune the car’s behavior in low speed and high speed corners seperately. However, I have found that maximum
caster values seem to work best all of the time.
Note that lower caster values also reduce steering wheel feel.
Toe-in
Theoretically, you can use positive toe-in to make the car more stable. And negative (toe-out) to make it less stable. In practice, I haven’t found a reason to change these settings on the Corvette. I don’t believe in toe settings very much because it’s another example of making wheels fight each other.
Wing setting
Unless you’re visiting the salt flats, leave this on the highest downforce setting for the Corvette. Always.
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TIRES
The tires pane shows you the condition of the last set of tires that came off the car. That is… their condition at the moment that you left the track after slowing the car to a halt. Keep in mind what effects that will have. Tires heat up and cool down rather quickly. So if the last turn you did was a hard left sweeper, your right front tire is going to show more temperature. And if you just locked up all four tires while you came to a halt, your readings will pretty much all be garbage.
Unfortunately, as I mentioned earlier, the tire temperatures being reported aren’t very useable at this time. Only the center temperature is reported correctly, but it’s not all that useful by itself other than telling you if you’re overworking a tire.
Values in the range of 220-240 degrees F are good. 250 and higher is around where the tire will lose traction due to overheating.
If you are destroying a particular tire, the most common cause isn’t setup. It’s driving technique. The usual culprit is turning the steering wheel even further when the car won’t turn any harder. Even if the tire doesn’t emit smoke (which you currently wouldn’t be able to see anyway, you’re hurting it unnecessarily when you do this. This will show up as very high overall temperatures for that tire immediately after the turn in question.
Setup can be used to try to make life easier on particular tires. You could try lowering that end of the car to reduce weight transfer to the outside tire. You could also concentrate on making that end of the car more planted so that it’s sliding less.
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CONCLUSION
Setting up the car is an iterative process. Make changes slowly and try them out for a while before giving them the yay or nay. Often times a good setup change will feel bad at first because you have to change the way you’re driving to accomodate it.
Save each iteration of your setup so that you can go back to it later if you decide that a whole direction you were going with the setup doesn’t work (and that will happen).
Also, don’t consider a setup change to be an improvement unless you can actually produce a better lap time with it. I find it can be easy to just judge a setup change to ‘feel better’. But this can be
deceptive because ‘feeling better’ may just mean easier for you to drive and can easily be slower overall.
