Whatever kind of racing car you drive, setting your tyre pressures perfectly is critical to maximising your racing performance (and your enjoyment).
Getting the tyres working at their optimum condition will at best give you a massive advantage, at worst, leave you equal to the best.
Get them wrong though and not only are you leaving critical performance on the table, you can easily make the racing car feel unbalanced, or even undrivable.
So racing car tyre pressures are a big deal. However, despite seeing many people who look like they mean business with tyre pressure gauges at the track, it strikes me how much guesswork still seems to go into it.
Whilst each tyre is unique, there are often published guides as to the ideal pressure ranges for your specific racing car tyres. Furthermore, if you can’t find them then generalised tables such as the one below are often quoted as an initial guide to those who know where to look:
|Vehicle Weight||Target (psi)||Target (bar)|
|Kart||14 – 18||1.0 – 1.25|
|Very Light Racing Car < 800kg||22 – 29||1.5 – 2.0|
|Light 800kg – 1000kg||24 – 32||1.6 – 2.2|
|Heavy 1000kg – 1400kg||28 – 40||2.0 – 2.75|
|Very Heavy > 1400kg||37 – 40||2.5 – 2.75|
Simple then. Just pump up your tyres to the target pressure and away you go, right?
The issue of course is that these Target Pressures are HOT targets.
Irrespective of what you pump the tyres up with (air, dry air, nitrogen etc.) the pressure will increase when the tyres are working hard and getting hot out on the track.
Unfortunately, you can’t get at the tyres when the car is on track; you can only set the pressures in the pits and when the tyres are relatively cold.
Therefore what most people do is either:
- Simple: Pump them up the same as last time and hope for the best
- Complex: Try setting the cold pressures lower than the target psi by a bit, do a short run, come into the pits, take a hot reading and then, for next time, adjust the cold setting up or down by the psi amount the hot readings are out (and THEN hope for the best…)
Hoping for the best isn’t a great strategy.
If you just take a moment to consider the time, effort and money people put into their racing car preparation, to then guess the settings on one of the most fundamental tuning devices on the car is just odd.
I’m not saying I haven’t done it in the stressful environment of a race weekend (I have) but there is nothing more disappointing in racing than performing badly because of something entirely preventable.
Setting your tyre pressures incorrectly is entirely preventable
It is more than that though. If you can get control over your tyre pressures then you are well on the way to getting more control, and more performance, out of the whole racing car.
By removing an element of guesswork, you can more easily start to refine your handling balance in a known and much more predictable way.
With tyres that are operating at the correct pressures, you can confidently start to evaluate all the other tuning tools at your disposal – springs, dampers, aero, camber angles, brake balance and everything else.
And if you get really into it, you can even start to strategically plan on whether you want more grip (or a certain balance) earlier or later in a race or stint.
Having precise control on your tyre pressures is the first step in opening up a whole range of tactical and strategic options to help you improve the balance, pace and performance of your race car.
The advantages of getting it right are huge, and yet often there we are, in the heat of battle at a race weekend, guessing which way to go. It’s crazy if you think about it. We all know the theory but carry on regardless.
6 Step Approach To Tyre Pressure Nirvana
Here I am going to walk you through a simple 6 step approach to help you reduce the guesswork out of your tyre pressure settings.
For subscribers, I’ve put together a little spreadsheet, that automatically calculates everything I’m about to describe. If you’d like it (for free) just fill in the form below to get it, or, as you’ll see it is quite easy to create yourself.
Step 1 – Decide your target hot pressure
This is the pressure you are targeting for each wheel when the racing car is out on track.
Use the data you have for your tyres or use the table above to get you going. Remember in the end you’ll have complete control over this so you’re really just choosing a sensible baseline.
In addition to getting into the right range, most of the time the challenge is simply to get even pressures across each axle.
In my example, I’m considering a 700kg rear-wheel-drive 7-style racing car so I’ve chosen 27 psi on the front and 26 psi for the rear.
Note, in the wet the tyre does not get so hot. Therefore it is best to set your pressures at or very close to your hot target pressures just before you go out in the rain.
Step 2 – Decide your scaling factor
Unless you’ve used this method before, this is where things start to get a bit different from the normal psi-delta approach people tend to use.
Instead, we are going to tune a scaling factor to ensure we get the right hot pressures.
The scaling factor is the target hot pressure, divided by the cold pressure setting.
Or, put another way, the target hot pressure divided by the scaling factor gives us our cold setting.
Scaling factors are typically between 1.2 and 1.5.
At this stage, you just need to pick four that seem to make some kind of logical sense. For example, consider the type of track, its direction and the main corners to determine how evenly (or not) the tyres will be loaded.
Typically people do this by having 1 or 2 psi stagger around the car. Here your aim is to translate that reasoning into the scaling factor instead.
In my example, I’m considering that the track is clockwise with some low-speed right-hand corners that will put a lot of energy through the rear tyres and more than the fronts.
Again, don’t panic too much at this stage, just choose something that makes sense to you.
Step 3 – Calculate your cold pressures
If you’ve got the spreadsheet you’ll see this is done for you. If not you simply need to divide your target hot tyre pressure by your initial scaling factor for each corner.
In my case, it is telling me to set my cold pressures as follows:
- Front Left: 21.6 psi
- Rear Left: 21.7 psi
- Rear Right: 20.8 psi
- Front Right: 20.8 psi
Note-1: Procedurally is it always good to take the pressures in the same order. This order (FL, RL, RR, FR) is just the one I use.
Note-2: Having a gauge that is accurate to a 1/10 of a psi reading up to 40 psi (or 3 bar) will be a worthwhile investment.
You’ll notice that the pressures are pretty similar and I’d hazard a guess not normally what you’d go with if you came about this the traditional way.
So you set the tyre pressures to these values. When you do, also note the Ambient Air temperature (if you can) as we’ll come on to that refinement in a minute.
Step 4 – Test Run and measure hot pressures
Finally, it is time to get the car out on the track. The key for this step is to get a measure of the actual hot pressures on the car.
If you are testing go out and do a representative run, I’d say of at least 5 or 6 laps. If you’re not testing, but instead going straight into qualifying (which can be common at club level) then aim to split your session in two. Go out and get the tyres warmed up. Come in and get the hot pressures measured. They are likely to be out so at this point you can also get them re-balanced for the rest of the qualifying session.
However you do it, the goal is to get a representative set of hot tyre pressures that result from your initial cold pressure settings.
In my example, you can see that the pressures have come in a bit high at the front and rear left. The rear right, however, is about right, just 0.1 psi low.
I’ve also taken the track temperature too. Again do this if you can but it’s not essential.
Step 5 – Adjusting scaling factors
If you’ve got the spreadsheet, this bit is again done for you. Otherwise, you simply need to now divide your actual hot pressures by the actual cold pressures you originally set.
Here you can see that my new adjusted scaling factors are slightly different from the original ones. What they are saying is that the fronts are actually working harder than I thought and in fact, despite being a clockwise track, the rears are seeing reasonably the same load.
Step 6 – Setting the new cold pressures
Using the new scaling factors from step 5, I can now calculate the exact cold pressures I need to get the target hot pressures in step 1.
Again in the spreadsheet, this is done for you. You simply need to set these cold pressures next time you go out and you should hit exactly the target pressures you’ve set. Bingo!
BONUS STUFF – 1 (Temperature Refinement)
Just when you thought it couldn’t get complicated enough (it really is easy in practice, just long-winded to write out!) there are temperatures to consider.
So far we’ve assumed you are pressurising your tyres cold. As in, cold, left overnight, first thing in the morning.
In practice you are going to in and out during the day. There are two temperatures to keep an eye on and which will enable you to further refine your settings model.
These are air and track temperature.
As these change then they will also effect the pressure inside the tyre.
You just want to make sure that your results are not thrown out by a large change in these temperatures. If you can’t measure them it is not so important but if you can then the rule of thumb is that you get about a 0.1 psi increase for each degree change in either air or track temperature.
For example, if you set your cold pressures at an ambient temperature of 15 degrees. Go out. Do your run. Calibrate your scaling factors but then the next morning the ambient temperature is 12 degrees you (need too / can) apply a slight offset in your pressure setting to account for this.
In this example, it is 3 degrees cooler. Therefore we’d need to reduce our cold settings by 3 x 0.1 = 0.3 psi all around.
Similarly, if you go out and set your hot run data and the track is 24 degrees C but by the time to come to the race the track temperature has increased to 29 deg C, you again need to reduce your start pressures by 5 x 0.1 = 0.5 psi immediately before the run.
This is really high levels of refinement. Not essential but in the spreadsheet anyway for you to have a play with.
BONUS STUFF – 2
So you have gone through all this and got your ideal cold tyre pressure settings. Great.
But what if you go out and you don’t like the handling of the car for some reason with those settings? Maybe it has too much understeer (push) or maybe you are getting oversteer (loose) on entry to a particular corner. Or, perhaps you felt you were losing grip all over towards the end of the stint or race.
There are lots of things you could do here to change the settings on the racing car.
These all typically require some work, expertise and take time at the track.
Perhaps you can achieve what you are after with a different set of target hot tyre pressures?
In my example say I’m getting quite a bit of understeer with 27 psi at the front. The next race is coming up in half an hour or so. Perhaps we could cure some of that understeer by aiming for 25 psi at the front?
Well we haven’t got to through the whole process again we can simply use our new scaling factors:
Running the numbers, you can see that for a target hot pressure of 25 psi across the front, you would need to drop to 19.0 psi and 17.8 psi on the Front left and right respectively.
Have a go with this approach next time you are at the track and I’m sure you’ll be pleased with the results and have much more control, and much less guessing, of what tyre pressures to set before your quali and races.
If you’d like the free tool discussed here, I’ve also scaled the sheet so it fits nicely on a phone screen so you can easily use it at the track. Just sign up below.
Wondering what the tyres are actually doing? This article should help your visualisation: https://www.yourdatadriven.com/tyre-slip-angle-explained/
Want my perspective on how to drive fast? You might like this article: https://www.yourdatadriven.com/how-to-drive-fast/