Tag: skippy

5.3: Anti-roll bar basics (Skip Barber F2000)

Posted on by Martin Krönke

Untitled-1After the tyre pressures, which we handled in 5.2, another significant setting to tweak on the Skip Barber is the anti-roll bar (ARB) in the rear. First of all, you need to understand what an ARB is and what it does to the car.

The red component in the illustration below is an anti-roll bar, which nearly every racing car has on its rear and/or front axles. The ARB connects the suspension elements of two wheels on the same axle. As a result, as soon as one wheel moves up or down, the other wheel is forced to follow that motion. However, the ARB is essentially a torsion spring which stores some of the energy when twisted, so not the entire movement of one tire is transferred to the other.

arb3

To give an example of an ARB’s importance: When a car without an ARB installed goes through a fast right-hand corner, the inertia forces the car to lean to the left side, which is on the outside of the corner. This is because the mass of the chassis is not willing to change direction, while the tyres that grip to the surface are. Relative to the chassis, the left tyres move upwards, the right tyres move downwards, causing body roll. Try to visualise this in your mind.

  1. The main purpose of the ARB is to change the roll stiffness of the axle it’s installed on, which has two important implications:
    The more horizontal a car goes through a corner, the better the chassis is at creating downforce. We’ll cover this in a later article, since here we’re covering the low downforce Skip Barber car.
  2. The ratio of roll stiffness between the front and rear axles affect the balance of the car, especially its tendency to under- or oversteer. Unlike downforce, this is highly relevant for the Skippy.

To understand how balance is affected, we need to understand that as vertical load on a tyre is increased, the coefficient of friction of that tyre decreases. You still get more grip, but proportionally less. This is known as load sensitivity.

graph

Let’s work through an example, using the chart above. At 400 kg of vertical load on a tyre, the coefficient of friction is 1.25. By multiplying the two numbers, you get the amount of friction force provided by the tyre:

400 x 1.25 = 500 kg (single tyre)

If we have a perfectly balanced axle where each wheel is loaded with 400 kg, the total available grip at that axle is:

(400 x  1.25) + (400 x 1.25) = 1000 kg (axle)

Cornering causes lateral (left/right) load transfer at each axle. Vertical load will increase on the outside tires and will decrease on the inside tires. Continuing our example, let’s assume that due to lateral load transfer the vertical load on the left tyre becomes 500 kg, while the load on the right side becomes 300 kg. The coefficient on the left tyre decreases from 1.25 to 1.11, while on the right tyre it increases from 1.25 to 1.35.

500 kg x 1.11 =550 kg (left tyre)
300 kg x 1.35 = 405 kg (right tyre)
550 kg + 405 kg = 955 kg (axle)

So while the total load on the axle remains the same (800kg), the total available grip is now only 955 kg. Just when you need grip the most!

Now we understand how an axle can lose grip under cornering, and is this precisely what causes handling issues. When the available grip of the rear tyres is exceeded first, the car goes into oversteer; when the grip of the front tyres is exceeded first, the car goes into understeer. An ARB can balance this out.

Using the ARB to balance the car
A softer ARB causes less lateral load transfer on its axle, compared to a stiffer ARB. This can improve the balance of the car, and increase overall grip on the axle as shown in the earlier example. The ARB stiffness can also determine the lateral load transfer between the front and rear axles, even if a car only has one ARB, like the Skippy, which only has one on the rear.

A stiffer rear ARB causes more of the lateral load transfer to be distributed to the rear axle. Softer rear ARB means more of the lateral load transfer is distributed to the front axle. A stiffer rear ARB thus reduce available grip at the back while increasing it at the front, hence, making the car more oversteery and less understeery. Conversely, softening the rear ARB increases available grip at the back while decreasing it at the front, hence, making the car less oversteery and more understeery.

Controlling how much lateral load is transferred on the front versus rear axle is a balancing act, to optimise how much grip is available at each axle. Tuning the rear ARB on the Skip Barber car is mostly a question of driver preference: If you find the car too unstable for your liking, you can try reducing rear ARB stiffness. If you find the car unwilling to turn, you can try stiffening the rear ARB.

It’s important to note that tuning the ARB will only make a difference if you are utilizing the traction circle, as explained in 3.1. If you ask too much of the car (overall G’s), the ARB won’t help. If you ask too little, you won’t notice any difference in handling.

Up to you

Get the Skip Barber out for a spin, and see if you can adjust the ARB to your liking! See if you feel the effect of it, and try to visualise the forces working on the car as you go through the corners.

For further explanations on the matter of ARBs, please see the more advanced chapters of this guide, which we’ll publish soon.

5.2: Tyre pressures basics (Skip Barber F2000)

Posted on by Martin Krönke

tyresOne of the few setup options available to the Skip Barber car is the pressure in the tyres. It’s also one of the most important things to get right on every single car. To find the ideal tyre pressure, it’s important to know what to look out for.

Tyre pressures influence a number of factors in the performance of the car, and both high and low tyre pressures have their drawbacks.

Factors: Deflection, contact patch, vertical and lateral stiffness
A tyre is a spring and damper unit after all, and becomes stiffer with higher tyre pressures. A higher tyre pressure also changes the curvature of the tyre, from a flat shape to like that of a bicycle.

Lower tyre pressures lower the vertical stiffness, which in turn causes a bigger contact patch (flatter tyre), and in theory sounds promising. Yet a bigger contact patch also increases the rolling resistance of the car. An even lower tyre pressure will cause the shoulders of the tyre to bare the grunt of the load, less than ideal (see illustration).

untitled-3

type2Moreover, lower tyre pressures also affect the lateral stiffness, and when running very low pressures, it’s possible that the middle of the tyre is no longer at the centre of the rim while cornering. It can cause the sidewalls of the tyres to nearly fold. This is also not ideal (see image).

This effect is most noticeable on the McLaren MP4-30 or Williams FW31, as they’re running on wheels with very tall sidewalls relative to their width. We recommend you take either of these for a spin on low tyre pressures, so you get familiar to the feeling of too-low tyre pressures.

Finding the balance
So, running low pressures has it’s drawbacks, as does having too high a pressure. As with almost every setup option that will be covered later on, some compromise is needed.

Fortunately, for the Skip Barber car and its tyres, finding the ideal pressure is fairly easy and does not depend on the track or weather conditions too much. We suggest you select a track, default weather and start a test session. Use the baseline setup, and drive a few laps to get the heat into the tyres. When the pressures stabilize, you have reached the stable operating temperature of the tyres. You have to use telemetry to find out after how many laps this takes. Let’s say that number is 4 laps. You’d need to drive 4 + 5 laps (4 laps to get the tires warmed up, then 5 clean laps within a few tenths of each other). Observe the car and check your laptimes.

Then increase the pressure a bit, and check behaviour and laptimes again. If the car is nicer to drive and/or quicker, go into the same direction with the pressure another time. If it’s worse and/or slower, go back to the original setting and then lower the pressure. Repeat this step until the improvement stops or the car becomes worse again.

The Skip Barber, as a non-downforce car, can be driven with the same pressure at practically every track. However, with higher end cars, especially those with high amounts of downforce, finding the right tyre pressure becomes a little more complex. A given tyre pressure is ideal for one specific amount of load on the tyres, yet this load varies greatly because of the different amount of downforce generated through high and low speed corners.

As a general rule of thumb, on tracks with lots of low speed corners such as Okayama, you’ll want to run relatively low pressures, because compared to high speed circuits like Spa, the downforce generated by the car is fairly low on Okayama, and therefore the loads on the tyres are lower. At a track with lots of high speed corners you may want to run higher pressures to increase grip in high-load situations.

When playing with the pressures you should be able to notice differences in traction in slow corners between higher and lower pressures. Likewise, you should notice a difference in grip in higher speed corners, although this might be more difficult to notice. Use the delta-bar to keep track of time gained or lost in individual corners.

Up to you:

The process of finding a good tyre pressure, at least for short runs, is the same iterative process as with the lower end cars. As you can already guess, for long runs it’s a little different and this will be covered later on in a more advanced section. Later on, we’ll also focus on temperature and weather differences.

For now, start your setup crafting career by finding the right tyre pressure. Continue with anti-roll bars, in 5.3.