Motorsport is all about tyres. Driving a racecar is all about getting the most out of the tyres, and racecar setup concentrates on making sure the tyres deliver maximum grip. As most racers know, the right suspension settings are vital to ensuring the racecar performs at full potential.
But for a car that doesn't come with a specification sheet outlining the optimum settings, how do we figure out what settings to use? Many racers will go with their gut feeling, or perhaps use settings that have worked on a previous car, or they may be able to copy the settings from a similar car. These are all workable methods, but there is a more scientific method to establish the ideal settings. The method involves the use of a tyre pyrometer, and a rough outline of the technique will be the subject of this article.
Spring rates, shock absorber settings and baseline suspension geometry will not be covered in this article. Baseline geometry is usually determined at the vehicle design stage, and may be limited by packaging and construction issues. Ideal spring rates will vary depending on sprung weight, vehicle corner weight, the bump frequency of circuits visited and finally the front-rear balance desired. Shock absorber settings are a specialised area beyond the scope of this article. More information on these topics is available in Allan Staniforth's excellent book: Competition Car Suspension
This article will look at how we can use a tyre pyrometer to determine the optimum wheel alignment settings (camber
) and optimum tyre pressures.
A basic probe-type pyrometer is the best kind. There are infrared pyrometers available, but they measure the surface temperature, not the core temperature that we're actually looking for. A decent probe-type pyrometer should cost around $200. There are some available here
. Make sure you carry spare batteries with you too.
You'll also need a good tyre gauge, plus a pencil and a notepad.
There is some excellent software available from Paul Van Valkenburgh
don't be put off by the basic website, the software is very good, and runs on a very basic Palm PDA (available for around $50 from Cash Converters). This software is not a requirement, but can prove to be a timesaver when using the techniques described in this article.
Tyre temperatures give a very good indication of how the tyres are working. Every tyre has an optimum working temperature, where it will deliver maximum grip for the maximum length of time. If a tyre is overworked, it will generally run hotter, it may deliver a little more grip for a short period, but will quickly deteriorate. If a tyre is underworked, it will run cooler. Comparing the temperature of the front tyres to the rears will give an indication of balance, the hotter end is working harder than the cooler end (the driver's feedback should confirm this). We can also get an idea of how well each individual tyre is working by comparing temperatures across the face of the tyre. We look at the temperature near the outside edge of the tread, the centre and near the inside edge. Comparing these temperatures will give us an idea of if our tyre is contacting the circuit in an optimal way - if part of the tyre is too cool, it may not be contacting the bitumen correctly; if part is too hot, it may be taking more than it's fair share of the load.
Measuring Tyre Temperatures
What we're after is a picture of how the tyres are working, by measuring their operating temperature. Ideally, we would take this picture at the exact moment the tyres are working their hardest - in the middle of a corner. This is not practical without a very expensive onboard infrared sensor setup (like Formula 1 teams will use during testing), so we're faced with taking the temperatures in the pits. To give ourselves the best picture, we need the tyres to work hard for at least a few laps, and for that work to be sustained for as long as possible before stopping (push the car hard right up until pit entry). Enter the pits safely, and then ensure minimum delay before taking the temperatures (a crew member or two dedicated to taking the temps as soon as the car pulls up is a good idea).
Take temperatures and pressures on each tyre. To take the temperature the probe should be inserted a millimetre or two into the tread of the tyre in three places across the tread - about 30mm from the outside edge of the working tread (tread that is working on the bitumen will have a 'rough' appearance), in the centre of the tread, and about 30mm from the inside edge of the working tread. Record all figures in a notebook (a sample recording may look something like the note on the right). Work quickly but carefully to ensure the data obtained is as accurate as possible (and so there is minimum cooling time between the readings for each tyre). You should also note on the same page the starting cold tyre pressure from before the car ran on the circuit.
Interpreting Tyre Temperatures
The ideal temperature for any tyre should be available from the tyre supplier or manufacturer (if you obtain this figure, make sure you measure your temps in the same scale - celsius or fahrenheit). If the temperature is not available, you should be able to get a good idea from other competitors, or from comparing your own recorded temperatures with how the car performed or felt. Once you have a target temperature, you can compare your recorded temps.
For our example, we'll assume an optimum of 140 degrees fahrenheit, and 21psi hot pressure.
Recorded temps were as follows:
LF OCI: 120/125/130 RF ICO: 140/135/120
LR OCI: 145/160/145 RR ICO: 145/150/145
To get an average, add up all the temps for the factor you're considering and divide by the number of temps you've added (eg; for front to rear bias, add all the front temps, then divide by 6, add all the rear temps, divide by 6, then you can compare the average front temp with the average rear temp).
From our example we can immediately see that the rear tyres are overall hotter than the front. This car is using it's rear tyres more than it's fronts. It's likely that the car has an overall oversteer balance. This may be influenced by the driver's style (he may be balancing the car on the throttle and/or driving around understeer by provoking oversteer), and could be clarified by the driver's comments. There isn't a massive front-rear differential, so it appears that the overall balance is reasonably close to optimum. (F 128 : R 148 = 20 degrees differential) More than 20 degrees differential may warrant further investigation.
Looking at the diagonal temperatures, we can see that the LEFT REAR and RIGHT FRONT are the hottest tyres at their respective ends. This could indicate a corner weight issue - generally if the corner weights are out, diagonal temps will be effected. In this case it could be that there is not enough weight on the left front and/or right rear. As for the overall balance, the differential is not very large (LF/RR average= 136, LR/RF average= 141). A differential of more than 20 degrees may be worthy of investigation of the corner weights.
Looking at the across tread temperatures for each tyre will give an idea on which part of the tyre is working hardest. To assess camber, we need to look at each tyre individually. We need to have an idea of the existing camber settings because on a tyre where there is a lot of negative camber, the outside edge can cool a little on the run into the pits.
LF: We know this tyre has about 2.5 degrees of negative camber. The temps across the tread look about right - around 5 degrees cooling between the inside and the centre, and another 5 to the outside, is about normal. Camber setting of this wheel is correct.
RF: This tyre has a 20 degree difference between the inside and the outside. The inside is working quite a bit harder than the outside. This tyre should have the negative camber reduced (tyre stood up more vertical). A rough ballpark in this case may be a 0.5 degree change.
LR: The temps on the inside and outside are the same - camber setting of this wheel is correct.
RR: Camber setting is correct.
The highest temperature across the tread will indicate the part of the tyre that is working hardest. If the outside edges are hotter, the pressure is probably too low. If the centre is hotter, the pressure is probably too high. Again, we need to look at each tyre individually.
LF: The average edge temperature (add outside and inside temp and divide by 2) is equal to the centre temp. The tyre pressure is optimum.
RF: Edge average is 5 degrees cooler than the centre. This indicates a pressure that is slightly too high. Consider reducing pressure by 0.5 or 1 psi (subsequent pyrometer readings may be used to fine tune).
LR: This tyre has a much higher centre temp than average edge temp - a fifteen degree differential. This tyre is overinflated. Reduce pressure by 1 to 2 psi.
RR: A little hot in the centre, but this tyre is not far from optimum. Consider reducing pressure by 0.5 psi.
There are other factors to consider when setting the toe. Toe out makes that end of the car less stable (this sounds bad, but can be a good thing, when you consider that being less stable can mean that end of the car will bite into, or steer more positively). Toe in makes that end more stable, but may reduce responsiveness. Any kind of toe either side of zero (wheels perfectly parallel) will generate scrub on the tyres (as they rotate against the bitumen, they are pulling a little in the direction of the toe, but can't move in that direction as the wheels remain connected to the car). This scrub causes tyre wear, but also helps to keep the tyres at operating temperature on the straights (ready to do their job at the next corner). In our example above, we may consider increasing the toe out - this will help with the steering of the car (the car will want to point into the corners) and also help to increase the average front tyre temps. We need to be very careful about this though - the added 'pointiness' may actually translate to more oversteer on corner entry.
A race car will generally run as much caster as possible, at least on the front suspension. If not, and there is scope for caster adjustment, it should be remembered that caster will contribute to dynamic camber. That is, when steering lock is applied, a car with high caster on the front will increase the amount of dynamic negative camber (the loaded outside wheel will effectively lean into the corner as steering is added). Thing start to get very complicated, but a very high caster setting may be the reason the inside edge of the tyre tread runs very hot.
Measuring the temperature of the tread of your tyres can give you a lot of information about how well the car is working. It can give you a good starting point for what changes might need to be made. Ultimately though, driver feedback is vital. An apprarent anomaly in the readings may be easily explained by the driver.
If you are going to make changes, record everything, including your reasoning for the change (and what the original settings were). This way, if you get confused, you can reset back to where you started.
And remember, you are responsible for the consequences of any changes you make, and any adverse outcomes. This article is provided for reference only. Neither iRace or the author offer any warranty or any liability for any damage or injury caused by tweaking any racecar.
Further ReadingDrive to Win - Carroll SmithTune to Win - Carroll SmithCompetition Car Suspension - Allan Staniforth