3 Simple & Accurate Definitions of Understeer vs Oversteer
Understeer vs oversteer, or, if you prefer, oversteer vs understeer! These two terms are used with abandon in motorsport but everyone seems to have their own definition of what they actually mean. These definitions are either overly technical or cringe-worthy – “Understeer is when you see the tree. Oversteer is when you don’t see the tree…”
When you need to cure a handling issue on your track car, you want everyone talking about the same thing. This article helps you with that by first making sure that when talking about understeer or oversteer, everyone is clear about what the terms “oversteer” or “understeer” mean.
Below you will find three basic explanations of oversteer (also known as a “loose” car) and understeer (also known as a “pushing” car.) Some charts (for the brave) and a nod to the proper explanation (for the purists).
Having a common understanding within your team is essential. Please refer anyone here whenever you feel you need more clarity.
Understeer & Oversteer Simply Explained
Here are some simple definitions for you to use and think about when discussing oversteer and understeer in a racecar.
Understeer is where the racing car will not follow the trajectory that the racing driver is requesting and results in the driver having to apply more than ideal amounts of steering lock.
Understeer in racing driver talk: “The front has no grip.“
Understeer is when your front axle looses grip before your rear axle.
Oversteer is where the racing car rotates faster than the racing driver wants and results in the rear of the car becoming unstable.
Oversteer in racing driver talk: “The rear has no grip.“
Oversteer is when your rear axle looses grip before your front axle.
And that’s it!
Now you have a common definition for when drivers are giving feedback or for when you are discussing how to cure handling problems to find more speed.
But what if you want to know a bit more about oversteer and understeer? Isn’t there a fuller definition?
Read on… ?
What Is The Proper Definition For Understeer?
The “proper” engineering explanation of understeer vs oversteer is more involved.
To do this you need to look at the difference in tyre slip angles front to rear, the resulting body side-slip angle and also your planned corner radius. This gives you a “balance” for your racecar, which will either be neutral, oversteer (unstable) or understeer (stable.)
The concepts and maths do get complicated and a bit abstract. Further the theory is only for a quasi-static vehicle state. A state that your racing car, as a highly dynamic vehicle who’s the sole aim is to maximise its available acceleration capacity at all times, should never be in. However it can still useful to know where your underlying understeer or oversteer balance is using this approach.
Interestingly, and perhaps confusingly, there are other factors that try to cloud what is going on. For example, your racecars aerodynamic, drive train dynamics (i.e. your differential settings), chassis characteristics (i.e. your damper rates), as well as the speed at which you, the driver, apply your inputs, plus the track topology (i.e. hilliness), can all contribute to vehicle behaviour that looks and feels like understeer vs oversteer but isn’t really – well not technically anyway.
If you are keen to wade into this knowledge rabbit hole then I’ve some more info below, plus you might want to pick up a few books on vehicle dynamics – the Milliken one is the place to start.
Understeer vs Oversteer with Charts!!
Think of this as a kind of introduction to a more technical explanation. I’m no professor so this is really just my understanding. Hopefully though this should help you to better understand how any car, or driving changes, are effecting your track car’s understeer vs oversteer balance.
Racing Car Rotation verses Steering Wheel Angle
When you turn the steering wheel, you want to have the racing car rotate in proportion. You might not have thought about it quite like that before but you do. Imagine if as you turned left the car first went left, then right, then straight, then left again. You’d be all over the place.
What you want is that if you turn the steering wheel left it goes left. You also want it so that when you turn the steering wheel twice as much you expect the car to rotate twice as much. Nice and consistent. Nice and “linear.”
As a result this relationship is sometimes referred to as “steering linearity.” For a well balanced, good handling car, it is what you are after.
Have a look at the chart below:
You can see three lines; one for the balanced car and one each for understeer and oversteer.
For a balanced car, you can see the line is straight. This means that as the steering wheel is turned, the racing car rotates in linear proportion.
You, as a driver, will say this car feels balanced which is good.
Understeer and oversteer are when your car is not like this. In reality this is most of the time so it is best to understand what is going on.
Understeer Requires Increasing Steering To Get Car Turning More
Look again at the chart. With the understeer scenario, as the steering is turned the line (dotted blue) curves up.
What this is saying that as you apply increasing steering wheel angle, you are getting a decreasing rotation reaction from the car.
You likely know this instinctively but take a minute now to think about that. Read that sentence again. What is really going on here?
Well, the rotation of your car is not linear with steering input. In fact, it gets to a point where you cannot get the car to rotate anymore no matter how much extra steering angle you add.
Whilst stable – and what you want in a road car – to get your understeering racecar around the corner you often have to slow down.
Excessive understeer is not good in a racing car.
OversteerRequires Decreasing Steering To Get Car Turning More.
Again looking at the chart the oversteer scenario (red dotted line) is the opposite. As steering is applied you get an increasing amount of racing car rotation.
With oversteer, the racing car becomes very sensitive to your steering inputs. This increases the chance of a spin and makes the racing car feel very unstable – even scary!
If you don’t have confidence in the car you are likely to drive slower than your car can go.
Excessive oversteer is not good in a racing car.
A Worked Example To Understand Understeer
Let us work this through a bit more. If you look at the next two charts you can see how it works more clearly:
In this first chart, you are looking for a small amount of rotation from the racing car.
With all three scenario’s there is not much difference between the amount of steering wheel angle you need to apply.
Forgive the really bad drawings – the thing on the left of the chart is a steering wheel… ?♂️
In the next chart, you want more rotation from the racing car. Now you can see differences coming in in terms of the steering wheel angle needed for different setups.
To get the same rotation in the understeering car, you need to apply nearly double the amount of steering angle compared to the oversteering car.
Whilst oversteer does have some performance benefits (less steering = less resistance = more potential speed) the trade-off you have to make is how much extra workload this adds to yourself as a driver.
All your reactions are magnified with a more oversteering car. Plus it can feel like you are wrestling a bear at times. This is why most start by aiming for the less sexy understeer car and then move towards more oversteer with increasing driver confidence.
No More Understeer vs Oversteer Confusion
Hopefully, this has got you thinking and brought clarity to your own definition of understeer vs oversteer.
Plus, next time you have an understeer vs oversteer conversation, hopefully, no-one will be confused.
Feel free to bring this explanation up (even show people the charts) if it helps you further.
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Enjoyed this? Check out these other articles:
Grab this handy setup guide to find chassis balance.
Want to take the guesswork out of setting your tyre pressures? Try this article including a free calculator: /how-to-set-your-racing-car-tyre-pressures-perfectly-every-time/
Wondering what the tyres are actually doing? This article should help your visualisation: /tyre-slip-angle-explained/