FWD vs. RWD rain: part 2 (thanks Paul)

I have to thank YSAR reader Paul for sending me down this path, because it’s been really fun. I truly appreciate feedback that makes me look critically at a problem. In this part 2, I do some testing in Assetto Corsa, and come away with some surprise.

Testing scenario

To do the fwd vs. rwd and dry vs. wet experiments, I had to choose a track, two cars, and two grip levels. I like to use Brands Hatch Indy and the NA Miata as a baseline. Sometimes I use the Street 90s tire and sometimes the Street tire. The Street 90s are a couple seconds slower. When you have the AI drive the car, both tires have the same lap times. I think it uses the default (Street 90s) tire. So that’s what I did too.

For the FWD car, I chose the Chevy Monza Classic 500EF. This model is a free download. One reason I chose it is because the dry lap times are very similar to the NA Miata when both cars are on their default tires.

For the wet grip, I reduced traction from the default 0.98 to 0.75. That figure is a little bit arbitrary, but I’ve seen various tables that show a reduction of about that much.

  • Track: Brands Hatch Indy
  • RWD: NA Miata
  • FWD: Chevy Monza
  • Dry – 0.98 grip
  • Wet – 0.75 grip

How to modify Assetto Corsa grip

There are three ways to modify the grip of cars in AC that I know of: run a server, change tires, change track surface. The easiest is the last, but for completeness, I’ll describe the other two first.

If you set up your own server, you can set the grip level of the track. This requires a separate program running as the server. That’s why I’m not recommending it. But on the plus side, it’s just one line of one file.

If the cars are developed in the legacy way, they have editable text files for individual components like tyres (yes, that’s spelled with a ‘y’ because AC uses the British English spelling rather than American English). Most cars these days have binary files that aren’t easily edited. Both the Miata and Monza use binary files. This is why I’m not recommending this way.

If you look in a track folder, you will find a surfaces.ini text file that you can edit. A track may have several surfaces. For example the Brands Hatch Indy file has 11 surfaces. Before you go editing this file, first make a backup copy so that you can restore it to its original configuration later. The grip levels of the various parts of the track range from 0.98 on asphalt to 0.6 for grass. To simulate rain, I set everything to 0.75 because I was lazy and didn’t want to multiply everything by 0.75. But that would be a better way I suppose. However, I planned on driving on the track, not grass or curbs.

AI driver

The first thing I wanted to test was how much the AI driver was affected by reduced traction. Here are the values.

  • RWD -7.31% loss
  • FWD -6.95% loss

There is more loss in RWD than FWD. To put it into the perspective of a typical lap, if your dry time is 2:00 minutes, your RWD wet time will be 2:08.78 and your FWD wet time will be 2:08.34. 0.43 seconds is pretty significant in a sprint race, but we’re not talking about 10 seconds here. It’s just a little time. However, this is the AI driving. What about a human?

Human driver

Move over AI, it’s time for Ian to step into the car.

  • RWD -9.06% loss
  • FWD -6.92% loss

That looks a bit more significant. Let’s put this into perspective of my Toyota Yaris at Thunderhill last May. My fast dry time was 3:43. If we multiply these 223 seconds by 1.0906 and 1.0692 we find that the difference between RWD and FWD is nearly 5 seconds. That’s pretty significant! Given that my Yaris is heavier, higher, and less powerful, than a Miata, the Miata has all the advantages on a dry day, but given some rain, the advantage just might tip in my direction.

Here are the graphs for the simulation experiments.

However, this is a human driving a simulator, what about in real life?

More data diving

Let’s look at the actual laps from the race. On a dry track, I was averaging about 3:50 in traffic. Bring on the rain and that drops to 4:20. So about 30 seconds. I had to make a lot of passes, and when I had a clean lap, I got down to 4:03, which is a loss of just 9%. Driving around slow cars in the rain really kills your lap time.

Some of the fast RWD cars I passed included the Miata of Eyesore and the Celica of Uncle Joe’s. Eyesore’s fast lap was 3:29 but in traffic it was typically 3:35-3:40. They dropped to 4:35-4:40 in the rain, a loss of 60 seconds. Uncle Joe’s fast lap was a 3:34 and it’s traffic laps were in the 3:40-3:45 range. In the wet, they dropped to 4:25-4:30, or about 45 seconds.

Two of the fast FWD cars I passed were the Integra of Big Test Icicles and the Neon of Neon Pope. The Integra went from 3:50 dry to 4:25 wet. The Neon was 3:45 and 4:30.

The race winners, Shake and Break (E30), were typically lapping at the same speed as Eyesore in the dry (3:35) but much faster in the wet (4:10).

Let’s take a look at the relative losses of these cars.

  • Yaris -13%
  • Celica -20%
  • Miata -28%
  • Integra -15%
  • Neon -20%
  • E30 -16%

Summary

Given equal lap times on a dry track, a FWD car definitely has an advantage over a RWD on a wet track. How much? I think it depends a lot on the skill of the drivers. At the high end, maybe 0.5 sec per lap. At my level, a couple seconds. At the “you can’t drive for shit in the rain” level, I think it’s less about which wheels are connected to the engine and more about the driver lacking the skill and confidence to maximize traction in the rain. Pedal mashers who over-brake and then hammer the throttle are the ones most severely affected. A Miata doesn’t normally spin when you stomp on the throttle. But it does in the rain, and if one’s driving style isn’t very nuanced, rain will be very unkind to your lap times. However, in a FWD car, stomping on the throttle may cause a bit of understeer, which is easily mitigated by lifting. FWD cars are more noob friendly. I’m not a noob, so I don’t see that FWD and RWD are that much different. But to someone not used to sliding their car around, RWD could be a major disadvantage.

I just watched the “you suck at racing in the rain” video again asking myself “where does the Yaris have an advantage?” The expectation is under acceleration. But that’s not where I’m catching people. It’s under braking. There is no FWD braking advantage. If you’re thinking it’s because my car is newer than the others and has ABS, that’s a good idea. However, you can hear the tires sliding in some corners when they lock up because my ABS has been broken for a while.

So to sum it all up, the reason for Yaris Rain Domination (YRD) is a little bit of FWD advantage and a shit-load of “most people suck at racing in the rain”.

FWD vs. RWD in the rain?

A recent comment from an old post sent me down the path of attempting to quantify the difference between FWD and RWD in the rain. How much faster is FWD? A little? A lot? Here’s my first quasi-quantitative attempt at answering that question. I’ve taken a bunch of graphs from SpeedHive from the Lemons Thunderhill event in 2019 where it rained suddenly on Sunday. In the images below, you can see the lap times get slower as a “swell” in the graph. If there’s a big difference between FWD and RWD, the swell for FWD should be much smaller than RWD.

FWD Acura Integra

RWD Mazda Miata

RWD Lotus mongrel

FWD Hyundai Elantra

RWD Mazda Miata

RWD Porsche Boxster

RWD BMW E30

RWD BMW E30

FWD Dodge Neon

RWD BMW E30

RWD Mazda Miata

RWD Toyota Celica

AWD VW Vanagon

FWD VW GTI

FWD Hyundai Accent

RWD Mazda RX-7

I don’t see much difference between the swells of FWD, RWD, or the one case of AWD. This is just a random selection of graphs though, and maybe with a larger selection I’d see the trend. Here’s the last graph showing our Toyota Yaris. Note the Y-axis! We had a 35 minute stop, which has the effect of making our rain laps look better than they were. But of course we did rock in the rain, as evidenced by the video linked below.

Car or driver?

One of the shockers at the last Lemons race was that the winning car won by so may laps. Usually the winners are separated by 1 lap, not 9. Is the “Shake and Break” E30 vehicle that much better than the rest of the field or do they have better drivers? It’s hard to answer this question without having the same driver in both vehicles. But I’ll try anyway.

Saturday

The weather on Saturday was pretty consistent, and with ~110 cars there wasn’t that much traffic. That means that lap times should be a reasonably accurate indicator of performance. But performance is a combination of driver and vehicle. To separate the two, I segmented the laps based on the driver. I don’t know which driver was in when, but every driver change involves a long-ish pit stop, so I simply called each driver change a new driver. I’ve included the fastest lap and the median lap below (in seconds, not minutes:seconds). Laps longer than 300 seconds were removed because those represent either a pit stop or some extended full-course yellow or even red flag.

Given the similarities between the Shake and Brake #1 and #4 drivers, I think they are the same person (Anthony Zwain).

  • Shake #1: fastest 201.687, median 210.80
  • Shake #2: fastest 213.709, median 221.25
  • Shake #3: fastest 210.30, median 216.64
  • Shake #4: fastest 204.18, median 209.54

Here are Eyesore’s times. I think they ran 4 different drivers.

  • Eyesore #1: fastest 222.95, median 229.68
  • Eyesore #2: fastest 221.10, median 230.74
  • Eyesore #3: fastest 209.66, median 216.56
  • Eyesore #4: fastest 210.09, median 215.60

Rather than comparing laps between cars, let’s look at the variation within each team focusing on the median. The Shake #1/#4 driver laps in the 209-210 range. The #3 driver is typically ~6 seconds behind, and the #2 driver is another ~5 seconds behind. There’s a heck of a lot of variation between the drivers! The Eyesore times show that the #1 and #2 drivers are pretty similar to each other and ~14 seconds behind the #3 and #4 drivers. That monstrous gap represents a 6% difference in lap times, or about 2 laps per stint. The Shake #1/4 driver is around 3-5% faster than the #2 & #3 drivers, or 1-2 laps per stint. After some sketchy mathematics, I come to the following conclusion: if you remove the fastest driver from Shake and remove either of the slowest drivers from Eyesore, the result would be Eyesore winning by ~1-2 laps.

Sunday

Sunday was a little damp early, then rainy mid-day, then drying, then dry. A wet race is very different from a dry race. It’s more about the driver than the car. Let’s look at what happened in the rain. This time, we’ll look at the graphs at SpeedHive. There was a red flag in the middle of Saturday, which you can see as the big spike in lap times near lap 60. This was preceded by some full course yellow. Note that the peaks of Eyesore (orange) and Shake (white) aren’t aligned because the graph is based on laps, not time of day. By the time the red flag occurred, Shake was already well ahead of Eyesore. Look towards the right side of the graph and you can see the baseline swells. This was when it was really wet. During this period the lap time differences are really interesting. At the start, the Shake driver is lapping much faster than the Eyesore driver, maybe 15-20 seconds. But then they pit and the next drivers are doing very similar times. If there’s a big difference between these cars, it’s not apparent from the lap times.

Conclusions

I don’t think the Shake & Break E30 is anything special. If it was, you would have seen that at the last 2 Sonoma races. However, in those 2 races, they were slower than Eyesore. The reality is that Shake brought in an exceptional driver and ran a clean race. There’s a huge difference among drivers on the same team and when you add the rain, there’s more than enough variation to explain the 9 lap difference.

Lemons thoughts

Some random thoughts about the last Lemons race.

Corner workers make mistakes

Going into the race, we knew that we had to minimize black flags. Everyone needs to, of course, but our car was much slower than anything else in B class and any black flag was going to put us out of contention. Sadly, we got two in the first stint. Neither one was earned. In Lemons rules, you’re allowed to pass after a yellow flag station if the mess is cleaned up. We did that and got flagged. That’s a judgement call on both sides, and given that, we should have been more careful. The second black flag was either for contact or for going 4 off. Neither of which happened, but the corner worker at T5 couldn’t see that easily.

Tuning

A couple weeks ago, the car worked great with 225 width RS4s on the front and 205 width RT615K+ on the back. I sort of destroyed one of the tires by overheating it and couldn’t run that set in the race. So I got some stickier front rubber in the form of 225 width 595 RS-RRs. I didn’t test that combination and it turned out that the team didn’t like it. Well, actually, they hated it. The team is used to an understeering car. When you get into trouble, you lift, and the front grips again. In an oversteering car, lifting only makes matters worse. As the car owner, it’s my job to provide a car that everyone can drive. Not only would the team be faster on average, they would also be safer and have more fun driving. We did eventually change the rears to get more grip and then later switched out the fronts for even less grip. Everyone but me liked it better.

Rain

Check the last post to see a video of me working through the field on a wet Sunday. I passed a hell of a lot of cars and in return was not passed. Here’s another video from our team a little later. It takes our driver a couple laps to acclimate to the wet conditions and then he proceeds to destroy most of the field.

 

The Yaris was one of the slowest and least sporty cars in the event. Why were we so much faster in the rain? Is it because we have extensive experience in the rain? I can’t speak for Danny, but I certainly don’t. I’ve only driven in the rain a handful of times. Maybe 3 hours total, and in other cars, not this one. So what’s the secret?

On my skills page, I used to have an ABC ranking system that asks the following simple question. When the car begins to slip, what do you do?

  • C drivers slow down
  • B drivers maintain speed
  • A drivers speed up

I think rain robs people of confidence. Lack of confidence can turn an A driver into a B driver or a B driver into a C driver. How does one gain confidence? Training. Like I said, I haven’t done much rain driving. So where do I get my training and the confidence that comes with it? Simulation, of course.

Lemons is changing

The C class has dwindled to just a few teams. And there used to be lots of teams sporting ridiculous themes. Our old MR2 was one of those silly cars and was just featured in the 24 Hours of Lemons Hella Sweet Car of the Week. Back then, our MR2 was put in B class with a couple penalty laps. Today, it would go into C class. I think Lemons has become a victim of its own success. Originally, Lemons was a parade/party poking fun at high performance cars. But over the years, racers have changed its culture. Part of that comes from competing series like Lucky Dog, ChampCar, AER, and WRL, where cars don’t have to be cheap and aren’t expected to have silly themes. The teams that do endurance racing tend to race all series. Now when you look over the Lemons grid you see sleek cars with $800 airfoils instead of cars shaped like boats with stuffed animals hanging out of them. While it’s true that I didn’t dress up my car or body with humorous artwork, I did bring a Toyota Yaris. But next time we’re going all in and “bringing back stupid”.

Race Report: Lemons Thunderhill

I’ll be updating this post each day.

Thursday – arrival

In the picture below you can see how simple my race operation is. I flat tow my Yaris behind a 3.0L Ranger. It’s a very flat route so the 145 hp Ranger has no problems towing the car and gear. I arrived at the track at 4:30 the day before the test and tech day to try to get a good pit spot. I wanted something under the awning so I could shelter the pit from sun/rain. Mission accomplished.

Friday – test and tech

Tech was a breeze. The car has raced in several other series and all the safety issues are well sorted. We got into the B class with zero penalty laps. That was what we expected.

We had decided that the full test day was too expensive. $349 for 1 driver and $149 for each additional. We considered doing the half day at $249 + $100 but then decided to play a joke instead. People walked by and  puzzled: “why is the wing on the front”. We dead-panned “it’s front-wheel drive”. The look of disbelief on Daniel and Mario’s faces was worth the effort.

The weather forecast changes hourly. The latest news is that Saturday should be dry all day with a high of 78. Sunday may be wet in the morning. I told the team I get to drive the wettest stint. That may screw up driver order, but as team owner, I’m putting my foot down on that. There’s no way I can keep up with the fastest cars on a dry track, but give me puddles and let’s see who comes out on top.

Saturday – race day

The race day didn’t start the way we wanted. Our first driver got 2 black flags. One of them was for going off track to avoid a collision. I’ll take a black flag over dents any day. But 2 black flags pretty much put us out of contention. Also, there was some blisteringly fast B cars we could never catch. Our second driver didn’t like the way the car was driving. Actually, neither did the first driver. When I asked if the rear had no traction, he said neither end had traction. Puzzling. So we decided to turn the rest of the race day into a tuning day.

Mario went out and came back in after a few laps complaining that the car was oversteering badly. We were running Federal 595 RS-RR 225/45/15 15×9 on the front and Falken RT615K+ 205/50/15 15×7 rear. So we decided to switch the rears out for a stickier compound: Brigestone RE-71R 205/50/15 15×7. This time he stayed out a while and had a great race with a pickup. When he came back in, he said the car was much more neutral now and that I should get in to see what I thought.

The first thing I thought was the brakes are still mushy. The pedal starts hard but just mushes out and goes to the floor. That’s really disconcerting because it gives you very little brake feel. And without a firm pedal, it’s pretty hard to heel-toe shift. Oh well, I just did more straight-line braking and eased in the clutch. Not ideal, but I’m okay working around problems. It’s likely an aging master cylinder.

The next thing I thought was that the 225 RS-RRs 15×9 aren’t that much different from the 205 15×7 I had run in earlier races. The tires don’t actually feel very fast. Part of that is because they are miserable under braking. They slide way too easily. They aren’t a particularly loud tire, like say the NT-05, and in 225 they are definitely on the quiet side. I started to understand why driver 2 thought the car had no grip on either end. The RS-RR doesn’t feel like it stops very well, so it appears to have no front grip. But once you get into a corner, it’s side grip is really good and overwhelms the thinner and harder rear tire, leading to oversteer. Mario said it was a lot of work just keeping it on track. I didn’t get to try the 615K+ rear setup, but the RE-71R rears felt pretty well planted.

While the car felt like it had better acceleration at low speeds, surely due to the weight loss, the drag was noticeably higher. This may be because the cut down doors don’t have mirrors or the wind deflectors I added. So the inside of the car turned into a parachute. It meant that top speed on the main straight was just 90-91 mph, or about 5 mph lower than usual. That didn’t stop me from having fun though. I managed a 3:43 in my few laps on track. You can see the entire stint in the video below (quality is not good because Windows 10 Movie Maker sucks. I may re-encode this on my Mac later in the week).

Sunday – race day

The forecast was wrong. We arrived at the track to find it drying. I was expecting a lot of rain early so I could one-up some fast cars but it just wasn’t very wet. Discouraged, I decided not to drive first. Danny drove first and while he was out we got our pit crew member, Tiernan, a driving wristband. He got in the car next and despite all the warnings about the blind turn 9C that connects the East and West tracks, he did what a lot of people do, and drove straight though. When he got to the penalty box, they decided to throw the book at him. My book. I had dropped off about 15 copies of the book to be sold for the Alex’s Lemonade Stand charity. Tiernan’s penalty was to read a passage from the book while being filmed. If it doesn’t make the Lemons wrap-up video, I’ll post it here.

The rain started picking up and it seemed there was enough rain to have a bit of fun. And fun was had. I got my wish and was able to dice with the fastest cars on track… and beat them.

Mario drove next and also had a blast splashing around (in the muck and the mire). But then the track started drying and he decided it just wasn’t as much fun. We wanted to get Daniel and Tiernan back in the car one more time, so they split the time on a mostly dry track. In the end, we were 56th out of 110 entries, or something like that. After we realized we weren’t in contention, we relaxed and had a lot of fun. This weekend reminds me how much fun Lemons is. That said, Lemons is changing, and not necessarily for the better. I’ll comment on that later.

It’s raining lies: part 3

Are we finally going to end the “It’s raining lies” series? Yes, yes we are.

Screamer vs. Big Bang

Before we begin, let’s take a brief tour through a seemingly unrelated topic in the motorcycle world: big bang vs. screamer engines. A big bang engine is one where all the pistons fire at the same time (or very close together). A screamer engine spaces out the ignition pulses as much as possible. From an engineering standpoint, it shouldn’t matter much, but the screamer is a little more powerful because it vibrates less. However, from the rider’s perspective, the firing order makes a big difference. Bikes with screamer engines tend to send their riders off the high side. How the heck does piston firing order affect the rider?

In a big bang configuration, the tire gets a big kick in the ass every 720 degrees of rotation. But it also gets a long rest period before the next kick. In a screamer, the tire is getting kicked every 180 degrees (assuming a 4 cylinder motor). Apparently the downtime in the big bang configuration gives the rider more time to sense the level of grip and adjust accordingly. In a word, the big bang gives compliance.

Softer Suspension

Before getting to the objective stuff, let’s be subjective and talk about how driving in the rain makes us feel.

  • How does a car feel on a wet track? Unpredictable.
  • What are we afraid of? Crashing the car.
  • How does that make you drive? With a large margin for error.

It’s fine if you don’t want to admit it, but I will. Racing in the rain scares me a little. The tires don’t make the same sound. The steering wheel doesn’t have the same tug. The throttle pedal feels like an on/off switch. When things go wrong, it seems they go wrong suddenly and without warning. That said, I actually really like driving in the rain. The extra stress makes it extra fun.

The reason why we soften the suspension in the rain is to slow down weight transfer. A car with a stiff suspension is sort of like a bike with screamer engine. It is theoretically the faster configuration. Stiff suspension leads to less weight transfer which leads to more grip. Lap times should be lower with stiffer suspensions. This is true regardless of the wetness of the track. However, there is also the human element to consider. The weight transfer in a car with stiff suspension is much more abrupt than a car with soft suspension. A human driver needs time to make adjustments to grip, and a suspension that is too stiff does not give the driver enough time to sense and react to changes in traction. So what are the physics underlying this phenomenon?

Basics of Friction

The coefficient of friction (CoF, or µ), is a ratio of the downward force of gravity divided by the frictional force. In the old days it was thought that you couldn’t get more than 1G of frictional force, and that the CoF was limited to 1.0 (this was due to blindly following Coulomb’s Law, which doesn’t really apply to viscoelastic compounds like rubber). Racing tires can generate over 1.0G, and much more with downforce.

Tire grip comes from the interaction of the rubber with the road. These interactions occur at a variety of scales from invisible molecules to stuff the size of tires themselves.

There are two separate properties that account for tire friction: adhesion and hysteresis.

  • Adhesion – Microscopic contacts between the tire and surface. This is also called mechanical keying.
  • Hysteresis – Macroscopic contacts that deform the rubber. The energy used to deform the rubber creates grip.

Adhesion and hysteresis sometimes compete with each other. As a tire gets hotter, it increases its adhesive properties but loses hysteresis. Adhesion likes a smooth surface while hysteresis likes a rough surface. The optimal operating temperature of a tire is therefore a complex function that depends on the properties of the rubber and both the microscopic and macroscopic texture of the surface.

To simplify matters, one usually talks about the optimal friction and relates this as the CoF. The CoF of a steel plate doesn’t change, so it’s a convenient simplification to think of the CoF as a single value. But the CoF of rubber actually changes and therefore can take a variety of values depending on the situation.

Load is sub-linear

It is well known that friction increases with load. But the grip of tires with respect to load is sub-linear. That is, if you increase the load on a tire by 2-fold, it gives less than 2-fold more grip. As a result, all things being equal, a lighter car will have higher corner speeds than a heavier car. One reason for this may be that there are physical limits to hysteresis. Colloquially, once a tire has been sufficiently mashed into a surface, it can’t be mashed any further.

Optimal slip

Whenever a tire is asked to do anything other than roll freely, it will have some slip. We’re not talking about slip angle here. Imagine braking instead. There is a continuum from freely rolling to fully locked. At 0% slip, the tire has a CoF of nearly zero (there is some rolling resistance). At 100% slip the tire is locked into some amount of grip, but that grip isn’t optimal. The peak friction occurs at a relatively mild amount of slip.

Speed affects grip

A tire that is moving across a surface a high speed cannot press into the surface as well as it can at low speed. This means that tires have less grip at higher speeds.

The optimal slip ratio also changes with speed. The faster you go, the lower the optimal slip ratio. We often think of the CoF as a fixed value, but it isn’t. Given that you have less grip and a lower optimal slip ratio, it’s not just self-preservation that should make you drive more reservedly at high speed.

Water affects grip

Water affects grip by getting between the tire and both the microtexture and macrotexture. It can therefore reduce adhesion and hysteresis. Grooves or other kinds of texture in both tire and surface can help evacuate water.

The amount of water on the surface is really critical. If the water film is thin, slick tires grip better than grooved tires. But if there is too much water to be evacuated by the macrotexture, the grip of a slick tire becomes terrible.

\

Hydroplaning

Under certain conditions, a tire may hydroplane. In the figure below, the dashed line represents a constant CoF while the solid line represents a variable CoF. The actual stopping distances are given in the inset, which match the variable CoF. The take-home message here is that the grip of wet tires depends on speed. Presumably that’s because of hydroplaning.

Summary

Water interferes with microtexture and macrotexture. It can also cause hydroplaning. As a result, the coefficient of friction of a wet tire is anything but constant. A dry tire is easy to drive because it has a very broad band of traction in which the CoF doesn’t change much. You can over-drive the hell out of it and it will still perform okay. This is not true of a wet tire, whose CoF depends on the amount of water, the grooves in the tire, and the speed of the tire. Push a wet tire too far and suddenly, you’re spinning.

The reason why one softens the suspension in the rain is because the coefficient of friction of a wet tire is variable and volatile. By slowing down weight transfer, we give the driver time to adapt to an unpredictable CoF.

Let’s finish off this series of posts with a few key points about driving in the rain.

  • The reason why traction loss feels sudden in the rain is because it actually is. So be careful out there.
  • You may not notice much difference in braking in wet vs. dry but it is substantial.
  • Be extra careful at higher speeds where hysteresis and hydroplaning effects seek to rob you of traction.
  • When applying throttle, make sure you do so gradually because once a tire starts spinning, the loss of traction is catastrophic.
  • Grip in corners is pretty good as long as you don’t upset the traction with too much throttle, too much brake, or jerky inputs.
  • The more water there is, the bigger the tire grooves need to be. If you don’t have grooved tires, pump them up so they have a crowned profile. If you do have grooves, decrease tire pressure.

It’s raining lies: part 2

Where Were We Anyway?

If you recall, three weeks ago I did the following:

  • Called Ross Bentley a liar
  • Committed career suicide (see above)
  • Claimed that wet tires have 9/10 braking grip, 3/4 cornering grip, and 1/4 accelerating grip
  • Showed telemetry traces that support said claims
  • Calculated the G-forces in a Car and Driver tire test and found that braking loses much more grip than cornering
  • Lied that I would resolve the mystery the following week

In my defense, the series is called “it’s raining lies”. So let’s get back to our watery tale and see how this story resolves.

The Braking Mystery

Why do I feel like the car brakes equally well in the wet and dry when the data shows dry grip is so much better? I believe this is pretty simple. When the track is dry, we aren’t braking as hard as we could. Thinking back a couple weeks, let’s be Paul Gerrard and see if we can get to the root cause. It’s certainly not physics holding us back. Do we fear excessive Gs? Not exactly, what we fear is flat-spotting a tire. Our team races on a small budget and tires are the largest expense. Flat-spotting a tire is a huge no-no. Everyone on the team is acutely aware of that. Because we are afraid of destroying tires, we don’t brake as hard as we could.

If the car had ABS, we would probably brake harder in general because ABS prevents flat-spotting. Braking is so much easier with ABS: just mash the pedal and let the computer take over. Surely the Car and Driver tests were done with an ABS-equipped car. It makes the testing procedure much more repeatable if you minimize the human element. And why not use ABS? ABS systems probably brake better than you do. Nannies in cars are getting better and better. For most drivers, having various nannies on is faster and safer than driving fully analog.

Have you ever noticed that WWII fighter planes have wings that slope up and modern fighter jets have wings that are straight out or even slope down?

A dihedral wing, one where the wing tips are higher than roots, is inherently stable. But an anhedral wing is not. Build a paper airplane with an anhedral wing and it will flip over and fly as a dihedral. It’s very difficult to fly a plane with an anhedral wing angle. Human pilots can’t do it. There isn’t enough compliance. They need a fly by wire system that makes hundreds of tiny adjustments per second to keep the plane flying level. So why have anhedral wings? Because the inherent instability makes the plane want to turn, making it more agile. Could cars be tuned the same way, so twitchy that no human could drive one without nannies? Surely. The evolution of performance driving will someday see computers outperforming humans at every level of the sport. When that happens we’ll become even better drivers as we learn from computers whose AI can explore the parameter space more deeply than we can.

Let’s return from my crystal ball and recap: we under-brake in the dry because we are afraid of flat-spotting our tires. I’m happy to make this compromise for 3 reasons.

  1. A flat-spotted tire is a waste of money
  2. A tire that fails on track could cause a crash
  3. Time spent fixing things in the pits is time not lapping

Brake Bias

When tuning a car for driving in the rain, one parameter that is often changed is the brake bias. Since there is less overall weight transfer on a wet track, there’s less weight on the front wheels. With more weight on the rears, more braking is possible out back. In my old E30, I installed a manually operated prop valve. To adjust the bias, you lift the hood and twist a dial. Real race cars put the bias adjuster in the cockpit so the driver can make changes mid-race. Until you’ve tried an adjustable prop valve, you probably haven’t experienced how much it changes the handling of your car on corner entries. If you don’t trail-brake, you won’t notice much at all, but if you do, it’s basically an oversteer tuning dial. Want more oversteer? Add more rear brake. It’s really that simple. If you don’t have a prop valve, you can still tune your brake bias with different pad compounds, but the resolution is much lower.

The main problem with adjustable bias is forgetting to dial it back when the track dries. This can lead to disaster. The rears will lock up first, causing the back of the car to wander when braking in a straight line. This can even happen on a wet track if the brakes are horribly out of proportion. Early ABS systems were kind of crappy and just kept the rears from locking up. If you’ve got such a system and the ABS computer is defeated or the fuse is blown, the bias is dangerously out of whack. Watch below as the fast POV is destroyed by a slow BMW that loses control while braking in a straight line.

How did the BMW team not realize their brakes were so horrible? Probably because they usually brake very gently. The rain moved the lock-up G-force threshold lower and the driver found himself in unfamiliar territory. How do you mitigate this? That’s a very good question. On the one hand, you can tune the brakes appropriately with a prop valve, pad compounds, or ABS. That fixes the problem with the car. But there’s another problem, which is how to fix the driver. Should the car be good enough that the driver doesn’t matter? Or is it the responsibility of a driver to work around problems with the vehicle. Probably a little of both don’t you think?

So how do you get practice driving a car with horrible brake bias? How do you get experience with locked up rear tires? If you want to train yourself for disasters, you have to put yourself in disastrous situations… without wrecking other peoples’ property, your car, or your body. The answer, which you can guess if follow this blog, is simulation. Not every car has adjustable bias even in a sim. Find one that does and then experiment with brake bias. Once you experience how useful and fun it is to tune your corner entry oversteer, you may want to install a prop valve in your race car. It takes all of 30 minutes and costs less than $100.

There’s still more to come in the “it’s raining lies” series. We still haven’t discussed why you soften the suspension in the rain. Check back next week for the resolution (or possibly more lies).