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ACC Wet Tyre Pressures – Myth Or Meta?

In this blog we outline what is happening with the wet tyres and why maximum pressures are considered superior.

What’s This All About?

In the past few days, a lot of talk has been exchanged within the ACC community surrounding wet tyre pressures and whether running the maximum configurable wet pressures, when using the wet tyres in ACC is the optimal solution. 

Rather than just put out a message to our customers suggesting to do this, we thought we would go and physically test it, share our results, explain what we saw and why and therefore give a bit of insight to people at home about what is going on. 

Testing Conditions 

We took two of our best drivers, an engineer, to go through the data, and then proceeded to test on a variety of tracks and track states to give as clear a picture as possible. 

  • 2 Drivers selected – Amedeo Castorino and Hubert Szymanski 
  • 1 Engineer – Rob Taplin 
  • 1gb of Motec data collected 
  • Multiple tracks tested 
  • 5 track states spanning different track wetness from flooded to crossover slick conditions.

Introduction

Before we dig into each track state and break it down one by one, I think it is a good time to give a quick bullet point summary as to what we saw when testing with different cars: 

  • Mid-engined cars seem to be most heavily influenced by this exploit, especially if you can run without TC in the wet. 
  • Porsche appears to be the next heavily affected car, whether with TC or without TC being turned on when driving in the wet 
  • Front engined cars are least affected, but still sizable in the benefit to it. 

What We Found Out 

With that out the way, I am sure you want to know how our testing differed for the different track states, so let’s tuck in and share all: 

Flooded Track State – 70% track wetness, 0.7 precipitation level

On the flooded track state, we have to break down the feedback into separate points, we have the lap time difference and the car behaviour difference. Both drivers experienced a moderate lap time gain, with a 0.3-0.7 second a lap difference from the normal pressure range to the 35 psi maximum. Let’s take a peak at two of those cars lap times at Silverstone for example: 

Car Lap Time GainBest Lap 30psi Best Lap 35psi 
Ferrari 296 GT3 -0.760 2:17.202:16.44
Porsche GT3R -0.4702:17.172:16.70

Now let’s look at the tyre temperatures and then wear rate differences across these same two cars: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 38 (37)36(35)40(39)42(41)35 (34)34 (33)39 (37)39(38)
Porsche 38 (37)36 (35)40 (39)42 (41)35 (34)34 (33)37 (38)38 (37)

Tyre Temperature Delta

CarFL FRRL RR
Ferrari 3c 2c1c1c
Porsche 3c2c3c4c

As you can see from the above table, temperature ranges were slightly lower on a flooded track but not drastic. The figure in brackets is the average across a 10 lap stint. Now let’s look at wear: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 2.62.62.52.52.62.62.52.5
Porsche 2.62.62.42.42.72.72.52.5

Just like above, wear is not drastically different from the normal pressures, with only a minor benefit of 0.1mm after 10 laps of Silverstone. 

Top Speed Differences: 

Car GainTop Speed 30psi Top Speed 35psi 
Ferrari 296 GT3 2 km/h242 km/h244 km/h
Porsche GT3R 1 km/h247 km/h248 km/h

As we would expect, there is a small increase in straight line speed running the higher pressures given there is a smaller contact patch on the road. What is also interesting is that running 35psi on the wet tyres in both cars, both drivers experienced a lot less aquaplaning out on track compared to the conventional pressures. Puddles became far less of a problem. 

In terms of driving, both drivers reported more rotation, more stability under braking but slightly higher jeopardy at the limit running 35 psi wets. Rear grip towards the end of the stint on the higher pressures seemed worse than the normal pressures, but it does not correlate to higher rear wear on the tyres, just a behavioural trait. 

Medium Wet Track State – 40% track wetness, 0.4 precipitation level

To keep the continuity going on a medium wet track state, let’s stick with Silverstone as a reference track so that the data can be directly compared, but now let’s add a further car to the mix, the Lamborghini.

Car Lap Time GainBest Lap 30psi Best Lap 35psi 
Ferrari 296 GT3 -0.730 2:09.202:08.29
Porsche GT3R -0.310 2:08.582:08.27
Lamborghini EVO 2-1.0102:08.912:07.90 

Two photos showing the difference in tyre temperatures on the BMW across medium wet conditions: 

Now let’s look at the tyre temperatures and then wear rate differences across these same three cars: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 63 (61)60 (58)70 (69)73 (70)55 (53)53 (51)61 (59)62 (60)
Porsche 61 (59)61 (58)71 (69)70 (68)53 (51)51 (49)59 (57)61 (58)
Lambo63 (61)62 (57)69 (64)69 (67)56 (54)54(51)60 (57)61 (59)

Tyre Temperature Delta

CarFL FRRL RR
Ferrari 8c 7c9c11c
Porsche 8c10c12c9c
Lambo 7c8c9c8c

As you can see from the above table, temperature deltas now start to really open up as the track becomes less wet. We are now seeing peaks of double figures on rear tyres for two cars. Now let’s look at wear: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 2.52.62.42.32.52.62.52.4
Porsche2.52.62.22.32.52.52.32.3
Lambo2.52.62.22.32.52.62.32.4

Interestingly, on all 3 cars, that temperature drop on the rear tyres, correlates again with a reduction in rear wear, although it is still slight and not what you’d call sizable. So again it seems the temperature is contributing to the lap time gain rather than wear saving. 

Top Speed Differences: 

Car GainTop Speed 30psi Top Speed 35psi 
Ferrari 296 GT3 2 km/h242 km/h244 km/h
Porsche GT3R 1 km/h247 km/h248 km/h
Lamborghini EVO 22.5 km/h246 km/h248.5 km/h

Overall, drivers again felt great stability under braking, better standing water behaviour and better rotation benefits under acceleration on the higher 35psi pressures. We can see, however, that the car’s lap time improvement varied. The Porsche appears less benefitting of this drop in track wetness, but the mid engined cars, especially the Lamborghini, which gained a whopping 1 second took full advantage. 

Damp Track State – 28 to 15% track wetness

Given that the talk surrounding this wet pressure exploit was mostly around a transitional phase of a race, where you are going from wet to dry tyres and being able to extend that crossover period, we thought we would focus a lot of our energy on this phase of testing and seeing how the stints developed as the track dried out and the car got progressively worse and worse lap time wise. 

This is where it gets VERY interesting…

For this test, we thought we would pick a track that has plenty of traction zones and combined braking periods so that the rear tyres are tortured as much as possible. With that in mind, the team jumped on to Hungaroring and this is the result with a full 4 cars tested: 

Car Lap Time GainBest Lap 30psi Best Lap 35psi 
Ferrari 296 GT3 -1.6001:49.031:47.43
Porsche GT3R -1.2901:48.191:46.90
Lamborghini EVO 2-1.7201:48.881:47.16
BMW M4 GT3-0.9601:48.801:47.84

Now let’s look at the tyre temperatures and then wear rate differences across these same four cars: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 120 (115)133 (125)146 (140)139 (133)101 (99)107 (103)126 (122)118 (116)
Porsche 116 (103)122 (117)138 (133)128 (117)96 (90)102 (97)122 (115)111 (103)
Lambo109 (97)120 (110)135 (125)121 (109)87 (83)102 (100)121 (116)98 (94)
BMW117 (110)136 (130)140 (135)128 (120)91 (88)112 (109)122 (118)98 (94)

BMW data showing the tyre temperatures in the bottom column, white on normal pressures vs red with maximum.

Tyre Temperature Delta

CarFL FRRL RR
Ferrari 19c25c20c21c
Porsche 20c22c16c17c
Lambo 22c18c14c23c
BMW 26c24c18c30c

It comes as no surprise that the two mid engined cars that had a big swing in rear tyre temperature delta had the biggest lap time delta from normal to max pressures. The Porsche had the smallest decrease from normal to maximum pressures which might explain why it could not keep up with the mid engined cars in terms of lap time gain when the track started to dry out. The BMW is an obscure one where it had the biggest temperature swing, however had the smallest lap time delta. Now let’s look at wear: 

Car 30psi Pressures 35psi Pressures 
FL FRLRRRFL FRLRRR
Ferrari 2.22.21.71.72.42.42.12.1
Porsche2.22.21.71.62.52.42.22.0
Lambo2.32.31.81.72.52.42.12
BMW 2.02.01.51.52.52.42.12.2

Yes, the rear wear delta really is that big, which is where the tyre temperature is really starting to take a front seat in dictating how fast the rear tyres are wearing in crossover and changeable conditions. We are seeing peaks of 30c in temperature swings on rear tyres, along with 0.5-0.7mm in wear differences. 

Let’s now reinforce these findings further by seeing how that affects the lap time delta from the 1st lap to the 10th lap in the stint on that crossover condition. 

Car Delta 30psiDelta 35psi
Ferrari 2 seconds0.9 seconds
Porsche 1.8 seconds0.9 seconds
Lambo3 seconds (Yes that’s correct)0.9 seconds
BMW 2.2 seconds0.8 seconds

As you can see, the 4 cars on the maximum pressures all have similar drop off time deltas to each other, but on the normal pressures, at this track state, things start to get wild with a big variety as the tyres start to melt and the track dries. 

To compound this evidence, we also tested wet tyres way past the window with both pressure ranges, where you were trying to run the wet tyres to an almost optimal track state where the slicks would not grain after a few laps, essentially doing an overcut on your competition There we saw:  

  • 20% less laptime loss over a 10 lap stint (A delta drop of 2.1s vs 2.6s)
  • 20% reduction in tyre wear (approx 0.3mm over 10 laps)
  • Even greater delta spreads than above 

TLDR: Run the maximum pressures and you will be faster, much faster.

What About Slicks?

As this previous exercise on the wet tyres worked so well, we also had an experiment with the slick tyres to see if this heat generation aspect to the tyre model still applied. 

So on the crossover conditions we experimented normal pressures vs running minimum pressures. The theory behind this on the slicks was to add heat into the tyre to remove graining and be in the temperature window rather than be driving around on stone cold grained up slick tyres. 

This resulted in the minimum tyre pressure run having a slower lap time by around 0.4 seconds, BUT an improvement in predictability and consistency was noted, especially when at the limit. If you went over the grip limit on normal pressures the car wouldn’t really recover the grip to stop the slide, whereas with the minimum pressures, regaining grip was much easier meaning you could push with less risk as the car could be ‘caught’ if you pushed too far.

However, in terms of lap time, it simply is not worth it and therefore the wet tyre calculations seem to be working differently to the dry tyre calculations in the model. 

Understanding the Data

Whilst analysing the data above, we can clearly see the trend. The drier the track gets, the bigger the time difference from normal to maximum pressures. What is a moderate three quarters of a second maximum on a very, very wet track, becomes one and three quarters on a drying track. 

It is clear that these gains really start to snowball when temperature starts to increase, the rear tyres especially as they start to go over 100c in operating temperature. The hotter the tyre, the greater the delta compared to normal pressures and it leads me to deduce that pressure is almost a secondary afterthought in ACC compared to temperature with the wet tyres. 

What I mean by that is, the temperature is dictating your lap speed, it just so happens that maximum tyre pressures are keeping the tyre temperatures in a window that allow you to retain that lap speed far longer. Regardless of pressure, the moment your temperatures get past the window of operation, your lap time delta plummets. The issue is, the normal pressure window isn’t the one doing this, but running balloons instead. 

There is also a disparity in standing water behaviour and the benefits become obvious as you simply do not get slowed down in puddles in the same way. 

No major differences were seen in Lat/Long G loads when looking at most of the runs across the differing conditions, although it can be noted that there were more micro-oscillations using the max tyre pressure settings. This is probably due to the stiffer sidewalls reducing the amount of damping ability the tyres have to spring/chassis movement. 

Alongside the rather insignificant top speed increase, most lap time improvement seems to have come from braking ability and entry rotation when looking through the data and speaking to the drivers. 

Both drivers stated that the stability and control under braking using max pressures in all weathers was a big improvement. In some cases being able to take almost dry track brake points onto the wet and damp surfaces. And in the lower wetness scenarios, drivers could drive as  aggressively as they would in the dry, essentially generating confidence to push, and therefore stitching together the tyre temperature decreases and tyre wear improvements to provide that big lap time improvement. 

Conclusion

Right now there is no downside to running maximum tyre pressures. The benefits happen in every track state up to flooded, it just so happens these benefits snowball the closer you get to a dry track on wet tyres. The temperature implications and how fickle the tyre is to that optimal window means you should be running 35psi at all times when driving on a wet track, and ignore the tyre widget at all costs. 

I hope the above blog was useful in providing some context as to why it’s happening and the reasoning behind it. 

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