# Visualizing Grip

It’s hard to overemphasize the importance of grip. Braking, cornering, accelerating: everything depends on grip. Understanding how rubber tires create grip is therefore really important for the racing driver. And yet, most drivers don’t know that much about it. Worse, they often have misconceptions that run against the facts.

• Amonton’s First Law: The force of friction is directly proportional to the applied load.
• Amonton’s Second Law: The force of friction is independent of the area of contact.
• Coulomb’s Law of Friction: Kinetic friction is independent of velocity.
• In addition, static friction is always greater than kinetic friction.

I don’t think many racers actually believe these laws. But should they?

The first law says that a 4000 lb car should stop in the same time as a 2000 lb car. Sure, it weighs twice as much, but it also experiences twice as much friction being twice as heavy. Theoretically, the weight of the car doesn’t matter. So why are there off ramps for trucks on long downhills?

The second law says that it doesn’t matter how wide your tires are. Skinny or fat, they have the same amount of grip. And what about grooves? The laws of friction say nothing about grooves. And yet, given a choice, racers would generally use a wider tire with no grooves. How exactly does width or grooves affect grip?

When considering how speed affects grip, most racers would point to aerodynamic downforce (or lift) rather than the rubber in their tires. Does speed actually affect grip? It turns out that it does, but not in the way you might expect.

If static friction was always greater than dynamic friction, why do race cars slide through corners? Doesn’t sliding produce less friction?

## 4 Really Important Graphs

In order to understand how tires work, you have to understand 4 graphs. In each of the graphs below, the coefficient of friction, μ, changes. Everything they teach you in introductory physics assumes that μ is constant. Maybe it is for metal blocks sliding against granite table tops at STP, but when it comes to tires, μ IS NOT A CONSTANT.

Graph A shows μ as a function of load (weight). When you double the amount of weight on a tire, it doesn’t give you double the grip. The coefficient of friction, μ, is lower at higher loadings. Is this why trucks need off ramps? Maybe a little, but actually no. It’s because their brakes overheat. However, it is why race cars are low, light, and have stiff suspensions. A low vehicle with stiff suspension doesn’t transfer much weight while cornering. As a result, the overall grip of the vehicle is high because none of the tires are getting too overloaded. Low weight also helps here, as do wider tires.

Graph B shows μ as a function of temperature. Every tire has an optimum temperature. Both cold and hot tires have less grip than one in the optimal range. If your tires are too wide, they may never get up to optimal temperature. For this reason, the optimal tire width isn’t necessarily the widest. TireRack did a great test where they tested a bunch of wheel and tire widths. The fastest tire wasn’t the widest. And when they went to a wet track, the fastest lap was an even narrower tire. One thing that contributes to heat is grooves. Squirming tread blocks are a major source of heat. As a result, grooved tires heat up more quickly than slicks. One reason for using slick tires is to spread the load better, but an even more important one is to prevent the rubber from overheating.

Graph C shows μ as a function of speed. The faster the car goes, the less time rubber has to interact with the road surface. Tires generate grip from molecular adhesion, mechanical keying, and hysteresis. At high speeds, there is less time for rubber to change shape. Under wet conditions, where adhesion no longer applies, grip is highly affected by speed.

Graph D shows μ as a function of slip angle. Every tire has an optimal slip angle. When a tire is twisted, which it always is to some degree, some parts of the contact patch are experiencing static friction while others are kinetic. This mixed state isn’t really addressed by any of the laws of friction.

## Visualizing Grip

The main point of this post is to give you a visual model of what is happening at the surface of your tire. With this model in mind, it might help you make sense of the conflicting information in the paddock or Internet.

Panel A represents a tire (squiggly line) pressing into the surface of a road (jagged line).

Panel B is what happens when you add load: the rubber goes deeper into the surface, creating more grip. But there’s only so far you can push the rubber in. This is why doubling the load on a tire doesn’t double its grip. Panel B could also be softer rubber or hotter rubber. In both cases, the rubber more easily conforms to the surface.

Panel C shows what happens at high speed. The rubber sliding across the road doesn’t have as much time to change shape, so it doesn’t deliver as much grip.

Panel D shows what happens when a tire overheats. The rubber comes apart, providing less contact with the surface. If the rubber gets hot enough, it may liquify or sublimate, creating a slippery layer between the surfaces.

This visual model isn’t perfect. For example, it doesn’t give any intuition about slip angle. However, maybe it does explain why Miatas are the best cars for rallycross. Seriously? Yes. If you look at the SCCA rallycross national championships over the last 10 years, Miatas have dominated the stock rear wheel drive class. You can literally bring any rear wheel drive car you want. There are no restrictions on power. And yet, Miatas are the dominant car. Why? Using our visual model, let’s imagine what is happening at the interface of dirt and rubber. Dirt is soft and will deform even more than the rubber. As a result, the two surfaces press into each other easily. The total amount of grip saturates very quickly, meaning heavy cars lose more grip than light cars. On dirt, you can only have so much acceleration before wheels spin, so low-powered cars aren’t really at a disadvantage. What’s the lightest RWD vehicle around? Miata. As you might imagine, light FWD cars also dominate the stock FWD class, but there’s a lot more options when it comes to buying a light FWD car.

# Teaching mid-track exit

The usual HPDE curriculum focuses on driving the racing line. The racing line is in every book on racing since the 1950s. Personally, I don’t teach the racing line to novices, or to anyone for that matter. Here are two problems with it.

1. The racing line has the highest speed. For the sake of safety, do we really want novices driving at the highest possible speed? In an educational event, do we even want to promote speed as a desirable metric?
2. While the racing line “uses the most track”, it’s also as close to the edges of the track as possible. Most self-inflicted incidents occur when drivers go off track. Do we really want novices near the edge of the track?

Instead of teaching the racing line, I think we should teach mid-track exit. Let’s see what this looks like and then discuss its merits.

The cars are going from bottom to top. The red line is the geometric line through the corner. AKA, the one with the largest radius. The blue line is also a circular arc, but it has a smaller radius. Both lines start on the outside of the track and apex around the same point. However, the blue line doesn’t go all the way to the edge of the track. It’s done turning by mid-track. Here’s why this mid-track exit is better for novices.

1. It has a slower speed. If something catastrophic happens, there will be less physical damage.
2. There is more space to recover from problems at the exit because there’s physically more track.
3. As drivers get more confident, they can add power, which naturally increases the radius of the corner after the apex, pushing the vehicle closer to the edge of the track.

Teaching a mid-track exit leads to driving the actual racing line, with the car increasing speed and radius in the 2nd half of the corner. The line develops with the driver’s skill and confidence.

In contrast, the typical HPDE coach tells their driver to “use the whole track”, which usually involves them steering out to the exit. This is a fake form of high performance driving: it looks fast but isn’t. It appears to be giving them good instruction, but tells them to follow rules (the line) rather than feel what the car is doing. It’s back-asswards. The HPDE curriculum isn’t designed to make people better drivers. You don’t have to believe me, but you should probably believe Paul F Gerrard. In Optimum Drive, he explains why the HPDE curriculum is misdirected.

If you really want to learn how to drive a car, you should go to a skid pad, possibly made from dirt, and learn how to slide a car around. An HPDE track event is not the place to experiment with oversteer recovery. However, HPDE events are a good place to have some fun with sporty cars. I think driving students will have more fun learning how to feel the grip of a car than robotically following the racing line. When I coach, I encourage students to “explore the space”. I would rather have them driving 6 tenths in the middle of the track than steering out to the exit at 4 tenths. I also teach trail-braking from day one, but that is another story.

I have some funny (at least to me) videos I made about HPDE and the racing line. They seem appropriate in the context of this post.

# Race Report

Last weekend, we entered a Lemons race and got 13th place out of ~90 cars. We were 2nd place in the C class, but not really threatening the leaders. Overall, that sounds like success, but we also had 4 black flags, which sucks. My brother did a really nice write-up, which is linked below. Check out his blog and also his articles on aerodynamics.

Beating up Miatas in a Yaris

# Homework: fastest way around 4 cones

Classes start this week and I’ll have a new cohort of students in my High Performance Driving class. In the first week we just hang out and meet each other, but in the second week we start talking about linear and circular performance. That is, how fast do cars go in a straight line (e.g. 1/4 mile or 0-100-0) and how fast do they go around circles. This will be their homework problem.

Given an autocross course with 4 cones, are you better off taking a large, circular path or a smaller square path? For the purposes of calculation, I have given them the following values:

• The straights are 280 ft long
• The small corners have 60 ft radii
• The large circle has a 260 ft radius

They are given a spreadsheet with various linear and circular formulae. It’s all very simple and doesn’t include gearing or aero. The inputs for the vehicle are these:

• Engine acceleration
• Brake deceleration
• Cornering grip

As part of the discussion, I ask the students to ponder how much each of the inputs matter. And even though we haven’t talked about aero, I ask them what they think will happen once it has been taken into account.

If you make a comment, please don’t provide any math or numbers. I want them to figure it out for themselves. You are welcome to predict which one is faster in qualitative terms!

# Final Exam

As you may know, I teach a class on high performance driving. What do we talk about? I present to you the final exam from the last time I taught the class. Feel free to suggest new exam questions!

## Question 1: Friction Circle

Given: Miata, 1.0g grip, left-hand drive, driver is highly skilled, no passenger, track is clockwise and shaped like the letter “T”.

• Draw a circle and label the axes
• Draw appropriate dots on the friction circle
• Place the letter “A” at the location where power-on understeer might happen
• Place the letter “B” at the location where over-braking understeer might happen
• Place the letter “C” at the location where lift-off oversteer might happen
• Place the letter “D” at the location where power-on oversteer might happen
• Place the letter “E” at the location where trail-braking oversteer might happen

## Question 2: 90° Corner

• Draw the geometric line (dashed)
• Draw the typical late apex racing line (solid)
• Label the entry, apex, and exit
• Place an X on the racing line where the driver should apply throttle
• Place a Y on the racing line at the position of minimum speed
• Place a Z on the racing line at the position of the highest speed

## Question 3: Suspension

• Why does lowering a car reduce body roll?
• Why does stiffening the suspension reduce body roll?
• How does an anti-roll bar reduce body roll?
• What is the advantage of reducing body roll?
• Why do racers soften suspension in the rain?

## Question 4: Grip

• State Amonton’s First Law of Friction and describe a flaw.
• State Amonton’s Second Law of Friction and describe a flaw.
• State Coulomb’s Law of Friction and describe a flaw.

## Question 5: Tire Graphs

Draw the following graphs. Label the axes.

• Coefficient of friction vs. Load
• Coefficient of friction vs. Slip Angle
• Coefficient of friction vs. Temp
• Coefficient of friction vs. Speed

## Question 6: Corners

Draw the following corners: esses, chicane, carousel, 120° decreasing radius. Label the corner as Type I, II, or III.

## Question 7: Puzzles

• You notice your friend drags his clutch during braking (i.e. uses the engine to help slow the vehicle). You ask him why and he claims it helps him slow down faster.
• Is he correct?
• How does this change braking in a FWD vehicle?
• When cars go off track at the exit, they often end up crossing the track and crashing against the inside wall. Why does this happen?
• Cars often lose control when cresting a hill. Why does this happen?
• Label each shape below with an approximate drag coefficient. Assume the object is traveling upwards on the page.

# Thrustmaster TS vs TX

I love comparison tests. Before we get to the newest test, let me look back at other wheels I’ve had the opportunity to test back-to-back.

• Logitech Momo vs. Logitech G25 – If this was a comparison of food, it would be like comparing shit to a shit sandwich. At least the sandwich has some bread in it.
• Logitech G27 vs Logitech DFGT – Shit sandwich vs. shit hogie.
• Logitech G27 vs. Thrustmaster T150 – Shit sandwich vs. shit over rice.
• Logitech G29 vs. Thrustmaster TS-PC Racer – Shit sandwich vs. a decent burger.
• Accuforce vs Thrustmaster TS – gourmet burger tastes the same as decent burger.

The two products I am comparing today are the Thrustmaster TX and TS bases. The TX can be found in the “TX Servo Base” by itself or with wheels such as the “TX RW”. It appears to be the same product as the T300. The TX/T300 base is an older design than the TS, which comes in several flavors: “TX-PC Racer”, “TS-XW Racer”, “TS-PC Racer Ferrari 488 Challenge Edition”, or by itself “TS-PC Racer Servo Base”. The TX has a built in power supply while the TS has a separate one (which looks like a turbo snail). The most important difference is that the TX has about 4 nM torque while the TS has about 6 nM. For comparison, Logitech wheels and the Thrustmaster T150 have about 2 nM. From my experience, the torque is absolutely critical and more is generally better.

Driving the two wheels back-to-back is enlightening. The TX doesn’t have the same feel. I don’t feel as connected to the vehicle dynamics. It’s definitely harder for me to drive. It’s much harder to sense and correct oversteer when the wheel has low torque. If you rely on trail-braking to sense speed and rotate the car, you will be much better served by a high torque wheel. Spinning is frustrating. You will spin a lot less with a high torque wheel. That said, my lap times were about the same with these two wheels. Should you buy the more expensive TS for the higher torque motor? Yes. The higher torque makes driving more authentic and more enjoyable. Should you buy an even more powerful direct drive wheel with an even higher price tag? Maybe. If you are using sim racing for real-world training, and have enough money to buy actual racecar stuff, then I think you have enough money to buy a direct drive wheel. If you’re mainly into sim racing for the sake of sim racing, the TS is a very good wheel base. I love mine. If you’re on the arcade side of things, then Logitech might be fine. The TX is in an odd spot: clearly better than Logitech, but also clearly worse than TS. Personally, I wouldn’t buy one. But for someone mainly interested in sim racing I think it’s fine. What about Fanatec? They make a lot of wheels and I’ve only used one. It was very good.

Over the years, I have owned a variety of sim racing wheels. Since I’m a professor, I’ll give them grades in red ink.

• Logitech Momo – The force-feedback is just vibration and the pedals are crap. Still better than a hand controller. D-
• Logitech G25 – Durable and the pedals are built well. Swapping a load cell on the brake pedal makes them top-notch. C
• Logitech G27 – Very similar to the G25 but with a less good shifter. C-
• Logitech DFGT – Cheaper than the G25/G27 but essentially the same thing. C
• Logitech G29 – Wait, are they still making the same shit? Yes, but it costs more now. C-
• Thrustmaster T150 – Thrustmaster’s attempt to make something just as bad as Logitech. With a 2 nM wheel and shitty pedals, they succeeded. C-
• Thrustmaster TX – Almost good enough to recommend. Sometimes packaged with spring-based (crap) pedals. B-
• Thrustmaster TS – Acceptable. T-LCM load cell pedals are a good match. B+

# Thanks Gary!

YSAR reader Gary emailed me and asked me if I would be interested in having my students drive his sim rig. A whole driving sim? Yes, the whole thing: Thrustmaster TX base with Sparco wheel, Fanatec ClubSport V3 inverted pedals, Fanatec H-pattern shifter, gaming computer with RTX 2080 GPU, 32″ curved monitor, Occulus Quest 2, speakers, headphones, and a custom-made cockpit with a Mustang seat complete with electronic controls. Not only did he donate this to the class, he also drove it up from Berkeley and installed it in my office. Who does things like that? Gary, apparently.

Oh yeah, my students are going to love this. Me too. This means I can get twice as many students in at a time. Also, they can race each other. I still have some minor adjustments to make on both rigs and some tidying up, but this is what my office looks like this afternoon.

The only downside to this is that my colleagues will think I’m loony. They already think I’m sus, but this is going to end up as cringe. Sorry, I don’t actually know what those words mean to young people, it’s just me trying to keep up with the times. In addition to the usual time trials and training sessions, I’m going to have to figure out how to get togue races going. Stay tuned.

Again, thanks Gary.

# Sim Coaching: Student-R drops 4 seconds

As you may know, I’m a professor at UC Davis. We are encouraged to teach First Year Seminars on whatever topic we like. These courses are supposed to be a fun way to hang out with professors and talk about mutual interests. I teach a course on “High Performance Driving”. Usually this is a lecture only course, but this year I decided to bring my sim rig into the office and have students drive it. This gives me the opportunity to collect and analyze data from a variety of experience levels, and provide some coaching to observe how they improve.

## Venue

As usual, I start with the NA Miata at Brands Hatch Indy with all settings at default values.

## Session 1

This student, who I will call Student-R (in case there is more to say in another post), had had a little sim racing experience before, but he doesn’t own a rig. He’s probably played a lot using a hand controller. In his first session, his laps were in the high 1:07s. I had him go through the 3rd-gear-no-brakes drill and he eventually got just as fast doing that as he was using brakes and gears.

## Session 2

A week later he came back to work on his driving. I say work because he clearly had that mentality. It’s much easier for me to coach someone who is serious about learning. We did the no-brakes drill again, but also worked on braking technique. Not that it matters much, but he uses his left foot to brake. As I was doing my own office work, I didn’t actually spend that much time observing or coaching. I would just comment a little about this or that and then he would go drive some more. In the end, he drove 90 laps and was in my office for almost 2 hours. Students are supposed to sign up for 30 minutes at a time, but there was nobody signed up after him, so he just kept training. At the end of the day, he improved his lap times by about 4 seconds.

## Data

The panels from top to bottom in the image below are speed, brake pressure, steering angle, throttle position, and time delta. These are my favorite channels in general because they show the driver inputs and the results of those inputs. As usual, in the answer to the question “where is he faster?” the answer is “pretty much everywhere”.

Let’s dig a little deeper into the data. There’s a big difference in the brake pressure graphs. Instead of treating the brake pedal like an on-off switch, he’s learning to modulate it. There’s still some work to do in T2, but overall the braking is earlier and softer. By braking earlier, he’s able to turn and accelerate earlier. This isn’t the advanced form of backing up the corner yet, but it’s on the way there.

## Analysis

I don’t think every student can drop 4 seconds between sessions. At least not when they start at 1:07.X. He’s now at the pace of some of the very experienced students who have their own sim rigs. How did this happen?

• Take your training seriously. I think if he came in thinking “I’m just going to have some fun”, the session wouldn’t have been nearly as productive.
• A little coaching can go a long way. I don’t think he would have made this much progress without someone looking in on him every once in a while saying “try this”.
• No bad habits. Some experienced drivers have accumulated a lot of bad habits.
• Examining data helps. He was very interested in the data as well as the driving.
• It takes time. In addition to the driving time, there needs to be some contemplative time too. I don’t think he would have improved as much if his second session was the next day. Having a week between driving sessions is useful at the beginning. Later, you need train regularly to maintain your edge, but at the start, I think you need more time to reflect than to perfect.

# Project: Assetto Corsa Endurace Server

I’ve got an idea for a new project, which is setting up an Assetto Corsa server aimed at the budget endurance racing community. That is, the cars will be older street cars, and the tracks will be those raced in Lemons and its descendants.

So why Assetto Corsa? I think a lot of people are doing their sim racing in iRacing. I find it highly unrealistic. Not the physics, which are quite good, and definitely not the tracks, which are top notch. However, the cars there look nothing like what you see in a budget endurance race. Where are the older Miatas, E30s, and FWD shit-boxes? You won’t find such cars in rFactor 2 or Automobilista 2 either. Assetto Corsa has loads of old street cars and most are free.

Here are the goals for the project:

• Free content (where possible)
• Free to race
• 2 classes of exquisitely balanced vehicles with fixed setups
• Track rotation is based on the next endurance race on the gestalt schedule (Lemons, ChampCar, WRL, Lucky Dog, AER, whatever)

## Cars

Let’s talk a bit more about the cars. I want all of the cars to be available for free. This removes some DLC from Assetto Corsa and also elsewhere (e.g. the Spec-P71 racing series). Given that the series is shadowing budget endurance racing in the US, all of the cars are left-hand drive. Thematically, the cars are mostly from the rad 80s and 90s, but there are also some cool cars from the 60s and 70s. Nothing 2000+. Also, they must be street cars rather than race cars.

To balance the cars, there are several methods. Some cars come with more than one engine or tire compound. Assetto Corsa also has restrictor settings that go from 0-100 and ballast settings that add 0-200 kg. Apparently, the restrictor equation modifies torque by the following relationship: (1 – RPM * restrictor / 400000). Adding ballast is more straightforward, but if you add too much, you can fail tech by having a car too low.

At first, I have the AI drive the vehicle to set an ballpark time, but afterwards I drive it myself to create a hand-crafted solution to balance the median lap time of each car. I do all of the balancing at a secret test track. Of course this means that the cars won’t be balanced at every track.

### A Class

Here’s the current list of A Class cars. If you know of any others that would fit here, please let me know.

• BMW 325 E30. This is one of the most popular and successful vehicles across all budget endurance racing series. This is not the M version that comes with Assetto Corsa (I could add that too with heavy restriction and ballast). I feel like the standard version is more in keeping with the theme.
• 1985 Toyota Celica. This is the first year of the FWD Celica. It doesn’t have a lot of power (150hp) but makes up for that with excellent handling and grip.
• 1969 Dodge Charger 440. Of course we need some American muscle to go along with German luxury and Japanese sport. The Charger steers like a boat but accelerates like the hot rod that it is.
• Pontiac Fiero GT. I wanted to find some kind of MR car for each class, and the Fiero is a great fit.
• VW Beetle. There aren’t many RR vehicles in budget racing aside from the Beetle. The mod I found had a stock Beetle that was too slow for the B class, but included a drift version that slots in with the A class.
• Nissan 240SX. This is an absolute staple of the drift community, but not all that common in endurance racing.
• Mitsubishi Starion. The Starion was sold in the US as the Chrysler, Dodge, or Plymouth Conquest. It’s the only turbocharged vehicle in the A class.

### B Class

• Mazda Miata NA. The model provided by Assetto Corsa is probably my favorite car to drive. There’s excellent attention to detail in the 3D model and physics. As usual, Miata is always the answer.
• Ford Mustang. There are a bunch of 1970s Mustangs in the same package. I haven’t decided which one to use. The vintage 1960s tires makes them challenging to drive.
• Volvo 240 Turbo. Volvos have been highly successful in Lemons. This one is from the 80s. Even with the turbo, it still makes only 120 hp. It’s an easy car to drive.
• Honda Civic EK9. This barely makes the 90s cutoff at 1999. In general, I’d like to find more Hondas, but most of the models are right-hand drive.
• Porsche 914. This is the MR car for the B Class. Also the only Porsche at the moment.
• 1995 VW Polo. Even though we didn’t get the Polo in the US, it fits well here. Also, I haven’t found another appropriate FF hatchback.
• Chevy Monza. I really like this boxy FF coupe. It’s very similar to the Opel Ascona, which has a little more power. Not sure I’ll use both as they drive so similarly.

## Tracks

Many of the tracks we race on are available in Assetto Corsa, and most of them are 100% free. Some have donations, like “buy me a coffee”. Others have Patreon links where you can subscribe for a couple bucks (you can subscribe for just one month, or you can be like me and keep subscribing). Finally, there are a couple tracks that are locked behind a paywall. They are still pretty inexpensive. Here’s a list of 48 North American tracks I’ve been able to find. Not all of them are high quality, but most are race-worthy.

• Area 27
• Atlanta Motorsports Park
• Autobahn Country Club
• Barber Motorsports Park
• Brainerd International Raceway
• Buttonwillow Raceway Park
• Calabogie Motorsports Park
• Carolina Motorsports Park
• Castrol Raceway
• Chuckwalla Valley Raceway
• Circuit Gilles Villeneuve
• Circuit of the Americas
• Daytona International
• Heartland Park Topeka
• High Plains Raceway
• Indianapolis Raceway
• Laguna Seca
• Mid America Motorplex
• Mid Ohio Sportscar Course
• Mission Raceway
• Monticello Raceway
• Mosport
• MSR Houston
• National Corvette Museum
• NJMP Lightning
• NOLA Motorsports Park
• Oregon Raceway Park
• Ozarks International Raceway
• Pacific Raceways
• Pittsburgh International Race Complex
• Portland International Raceway
• Putnam Park
• The Ridge Motorsports Park
• Sebring International Raceway
• Sonoma Raceway
• Summit Point
• Thompson Speedway
• Thunderhill Raceway Park
• Toronto Motorsports Park
• Utah Motorsports Campus
• Virginia International Raceway
• Watkins Glen
• Willow Springs International Raceway
• Willow Springs: Horse Thief Mile
• Willow Springs: Streets of Willow

## When?

I’m thinking this will start up in 2023. That gives you some time to get yourself a sim rig!

# Thank you Tire Rack!

Tire Rack just did one of the best tire tests I’ve ever seen. They tested the same tire (RE71RS) on the same car (2022 BRZ) with different tire and rim widths. Please go read their article because there are a lot of useful details there. I’ve copied the graphs below. But let me summarize.

• If all you have is 7″ rims, you’re faster on 225 than 215 or 245. Wider rubber isn’t always better. And in the rain, you’re better off on 215 than 225 or 245.
• If all you have is 8″ rims, you’re also faster on 225 than 215 or 245. Except in the rain, where once again, you’re better off on narrower tires.
• If all you have is 9″ rims, you’re still fastest on 225 but 245 is close. Maybe with a 10″ rim the 245 will be the top dog. And again, in the rain, narrower is better.

OK, so how does any of this make sense? There are so many unlisted variables here that all you can say is that on this day (actually 3 days), with this car, these tires, and these drivers, this is what they got. If the day was much colder or much hotter, the results may have been different. Maybe the narrower tires got to a better temperature in the rain and on a hotter day that difference would disappear? These tests are difficult and time consuming. However, they are also essential because the results are impossible to predict. Thanks Tire Rack. I’ll probably buy my next set of tires from you because of this test.

The big take-aways for me are the following:

• More rubber isn’t always faster
• You’re almost always better off with a wider wheel