Answer Time

I’m once again teaching a First Year Seminar class at UC Davis on High Performance Driving. I just gave them a homework problem, so I thought I bring some of that here too. Here are my answers.

Questions

  1. Two identical cars are traveling 60 mph. Car A has just the driver. Car B has 3 additional passengers. Which car stops first?
  2. Two identical cars are traveling 60 mph. Car A has 195 width tires. Car B has 205 width tires.  Which car stops first?
  3. Two identical cars are slowing down from speed by stomping on their brakes. Car A is going 60 mph. Car B is going 100 mph. Which driver feels more G-force?
  4. Why do racecars have slick tires?

Answers

  1. The laws say they stop at the same time. However, the coefficient of friction isn’t constant. The heavier car may heat up its tires quicker, leading to more friction. Up to a point when there is too much heat and it has less friction. In addition, the heavier car has  a lower coefficient of friction because tires don’t follow Amonton’s First Law: they provide less friction as load increases.
  2. Again, the laws of friction don’t care about tire width. But tires care about load and a 205 tire spreads the load more than a 195. So Car B stops sooner. It may also have more aerodynamic drag (better for stopping) and more aerodynamic lift (worse for stopping).
  3. Car B has more aerodynamic drag and probably more lift if these are street cars. Not sure which one of those wins. In addition, hysteresis is more effective at low speeds than high. So maybe Car B feels more G-force?
  4. Load and heat. Wider tires distribute load better (less loss of friction with load). Tires with grooves may heat up too much and heat kills tires.

Summary

Tires are weird. You have to do a lot of testing to find out what’s optimal.

Quiz Time

I’m once again teaching a First Year Seminar class at UC Davis on High Performance Driving. I just gave them a homework problem, so I thought I bring some of that here too. The lesson was about grip, so let’s start with the laws of friction.

Laws of Friction

  1. Amontons’ First Law: The force of friction is directly proportional to the applied load.
  2. Amontons’ Second Law: The force of friction is independent of the apparent area of contact.
  3. Coulomb’s Law of Friction: Kinetic friction is independent of the sliding velocity.

Questions

  1. Two identical cars are traveling 60 mph. Car A has just the driver. Car B has 3 additional passengers. Which car stops first?
  2. Two identical cars are traveling 60 mph. Car A has 195 width tires. Car B has 205 width tires.  Which car stops first?
  3. Two identical cars are slowing down from speed by stomping on their brakes. Car A is going 60 mph. Car B is going 100 mph. Which driver feels more G-force?
  4. Why do racecars have slick tires?

De-synchronizing

If you haven’t read “Optimum Drive” by Paul F. Gerrard, I suggest you buy a copy for yourself and then buy more copies for your racing friends. My racing/driving library has over 30 books and Optimum Drive is my favorite. Conveniently, you can get it in Kindle and Audiobook formats as well as traditional print. I suggest listening to it in audio format on the way to and from the track. I’ve listened to it 3 times so far, and I’ve enjoyed it every time. In fact, I’m looking forward to the next listen.

One of the my favorite phrases in the book is that “greatness lives in nuance, not simplicity”. We teach things in a simplified form to make the lessons easier to novices, but the subtleties end up mattering a lot than the simplification. One of the ways we simplify lessons is to synchronize 2 activities when really they should occur separately. He mentions the following instances.

  • Brake and downshift
  • Downshift blip and clutch application
  • Release brake and turn
  • Turn the wheel and turn your head
  • Accelerate immediately after brake release
  • Accelerate and unwind the wheel

Let’s go into a little more detail than Paul does at this point in the book.

Brake and downshift

Lots of drivers push both the brake and clutch pedals at the same time. I don’t think it’s a reaction to the phrase “in a spin, both feet in”, but rather, a bad habit entrenched from years of street driving. On the street, there’s no great penalty for downshifting immediately because the revs are pretty low. Let’s say you’re going 40 mph in 4th gear and downshift to 3rd. The engine will spin up from 2.5k to 3.5 (or something) as you feed the clutch out. No big deal.

Downshifting early on a race track is a completely different story because your revs are always pretty high. Downshifting from 6k will cause your revs to spike higher than 6k, and may even go past red line. I’ve seen data traces where the highest RPM was in the braking zone rather than on any straight. That’s what happens when you synchronize braking and downshifting. In addition to the extra wear on the engine, you also change your brake bias by engine braking. In most cases, this will make your braking less efficient, not more. And if you’re in a RWD vehicle, the drag on the rear wheels could cause you to spin.

What you’re supposed to do is brake first and downshift a little later. Let your speed drop before grabbing the shifter. If you’re in the habit of pushing both feet in, try to train yourself out of that. No good can come of it.

Downshift blip and clutch application

Long lost are the days when rev-matching was required on the street, so blips rarely make sense outside of a racetrack setting. When approaching a corner, one must coordinately brake, blip, and switch gears. Some people synchronize these too much by hitting all three pedals at the same time (left foot on clutch, right foot on brake and throttle). If you blip too early, you will feel obliged to shift early (see above). If you resist that urge, possibly because you don’t want to destroy your engine or your car, you’ll have to wait a bit for the speed to come down before feeding out the clutch. But the whole point of blip-shifting is to match road speed with enginespeed. If you’re going to let the engine spin down, what’s the point of blipping in the first place? It may have made you sound like a racer to people who don’t know jack about driving, but make no mistake, the people who know how to drive know the difference between heel-toe and shit-toe.

Release brake and turn

If you ever have me for a coach, you will see that the first lesson is brake release. Don’t snap off the brake pedal, release it slowly. The reason is because releasing the brake is sort of like accelerating. In both cases the weight of the vehicle shifts to the rear. When the front gets light, you get understeer. So if you snap off the brake and start turning, you will experience understeer. You may end up blaming the car, but it’s on you. The fix is to trail-brake. Keep a little brake pressure on while turning. Having weight on the front tires will make them more responsive when you turn the wheel.

Turn the wheel and turn your head

Turn your head first, of course. Novices are often focused on the hood of the car. Drivers should look where the car will go, not just where it’s currently going.

Accelerate immediately after brake release

If you can accelerate immediately after releasing the brake, you probably over-slowed the corner entry. If you’re driving at the limit, the moment you release the brake, you have maximum side loading. You don’t want to accelerate at this moment. You also don’t want to decelerate. What you want is maintenance throttle. I think this point is important enough to self-quote.

If you can get to full throttle immediately, without a period of maintenance throttle, you entered the corner too slowly.

Accelerate and unwind the wheel

You know that string analogy where there’s a string tied to your steering wheel and throttle? That’s what we’re talking about here. When the wheel is turned, no throttle. When the wheel is straight, full throttle. In between, there’s a mixture. But that’s an oversimplification. You should open the wheel in advance of opening the throttle. The advanced form of trail-braking, brake-turning, pretty much requires that timing.

Others

Gerrard mentions that these are just a few of the things we combine for simplicity. I wish he had listed more (I would list FWD control inputs). I think that in addition to the things we combine for simplicity, there are things taught separately that should be combined. Trail-braking is a great example.

Shit I don’t understand… #8: UTQG

There’s a lot I don’t understand about car culture, motorsports, and racing. Help me out.

UTQG

In the USA, the National Highway Traffic Safety Administration (NHTSA) created the Uniform Tire Quality Grading (UTQG) as a means of assessing the tread life, traction, and temperature range of tires. The tread life is supposed to be calculated by driving 7200 miles on some public roads in Texas and comparing the wear of a test tire to that of a reference standard. I say supposed to because nobody checks. UTQG ratings are provided by manufacturers, not an independent standards body. That means manufacturers can label them with whatever rating they want (subject to a fine if they abuse the system). Interestingly, the coefficient of friction of a tire is supposed to be described by the following equation.

Let’s see a few examples.

  • 40TW – 1.288G Hoosier R7
  • 100TW – 1.123G Nitto NT01
  • 200TW – 1.012G Bridgestone Potenza RE-71R
  • 300TW – 0.952G Michelin Pilot Sport 4S
  • 400TW – 0.912G BFG T/A Radial
  • 500TW – 0.882G Pirelli Cinturato P7
  • 600TW – 0.858G General Altimax RT43

Gumshoes

In the Car & Driver “Gumshoes” article, C&D test 5 tires in the 300 treadwear category. On a 200 ft diameter skid pad, they register from 0.90 to 0.95G. That’s in the expected range of 300-400 treadwear. They also show a 50-0 mph stopping distance from 79-82 ft. That can also be converted to Gs, which comes out to 1.02-1.06, which is the range of 200 treadwear. They also showed values for wet skid pad (0.75 – 0.83) and wet braking (0.77 – 0.88), in case you’re interested.

200 Treadwar

The 200 treadwear segment is particularly popular because various autocross organizations and endurance racing series have standardized on the 200 treadwear number. Due to the weird popularity of autocross, a number of tire manufactures are having a war to see who can make the stickiest 200 treadwear tire. Ask people in the know and you’ll find there’s a lot more information about each tire than a number stamped on a sidewall. Some tires are grippier than others. Some are more durable than others. Some are better in the rain. Some like to be run at high pressures and others low. Some squeal more than others. And yet somehow they are all 200s.

Among the 200s there are 3 main categories.

  1. Autocross – come up to speed quickly, don’t wear very long
  2. Endurance – consistent speed and longer life
  3. Cheap – not as grippy or as long-lived as expected

One of the most popular autocross tires is the RE-71R. Various tests show that it has the grip and lifespan very similar to an NT01, which is a 100 treadwear tire. I’ve used them and find that they are the most heat resistant tire I’ve found. Which is to say I’ve melted every other tire.

The most popular endurance racing tire is the Hankook RS-4. When the Lucky Dog Racing League made that their official tire, it wasn’t that big of a change since most teams were using them already. The grip is consistent from new to cords and they last around 3 times as long as an RE-71R.

The cheapest 200 might be the GT Radial Champiro SX2. This is the series tire for some GT86 organizations. It’s less sticky than the typical 200. The Federal 595 RS-RR is another inexpensive tire. It’s nearly as grippy as the RE-71R and even less durable. I’ve used them several times and I like them okay.

I wish

The only reason I really care about UTQG and 200TW specifically is because I race in series that cap performance at 200TW. But I’d much rather be on harder tires. Less sticky tires are cheaper and more durable. They also reduce the wear and tear on every other component and even reduce fuel usage. Aside from the economical reasons, I have more fun driving on slippery tires. The fastest teams are still going to be the fastest teams regardless of the tire, so why can’t we race on all seasons? One reason is heat. Some all season tires don’t manage heat well. The same is also true of 200TW tires. We eventually figure out which ones do and don’t work and gravitate towards the models that work well. It wouldn’t be much of a change from that perspective.

In my dream world of racing on all season tires, I wouldn’t specify a treadwear rating. Manufactures can game the system and label their tires however they like. So there would need to be a list of allowed tires and a standards body testing them to ensure they fit within some parameters.

Shit I don’t understand… #7: poke & stance

There’s a lot I don’t understand about car culture, motorsports, and racing. Help me out.

Fitment

Fenders are meant to stick out farther than the tires. There are two reasons for this.

  1. Prevents tires from hitting other tires. If two cars are traveling at high speeds and their tires touch, one vehicle or the other can get launched.
  2. Protects vehicles, people, animals, and property from stones.

In California, and probably a lot of other states, it’s actually illegal for tires to stick out beyond the body. But given how many people violate this, it’s clearly not a priority with the authorities.

Apparently, exposed tires look cool. So cool that people are willing to spend money and give up performance in order to do it. Let’s see some examples.

Poke

Car enthusiasts call it poke when the tires are exposed. A wider track width has an advantage: less weight transfer when cornering. The downside is more wear on bearings, a change in suspension geometry, and a greater tendency to oversteer. Oh, and the dangers expressed above, of course.

Stance

Excessive negative camber is called stance. Some negative camber improves grip and tire wear when cornering, but there’s a trade-off: both braking and accelerating are best with no camber. There’s really no situation where the stance of the car below is optimal. Doing that to your car has no practical application. It’s the vehicular analog of sagging. It may look cool to have your pants below your ass (to some people) but it’s a hindrance if you’re trying to outrun anything.

Shit I don’t understand… #6: not timing

There’s a lot I don’t understand about car culture, motorsports, and racing. Help me out.

Lap timers

Last week, when I was coaching at Thunderhill, I overheard a conversation where two drivers were talking about lap timers. They were mulling over the $5 phone app vs. the $500 dedicated devices. I barged in on this conversation because this is a subject I know a little about having owned 5 devices (TraqMate, RumbleStrip, AiM Solo DL, Racebox Pro, Apex PRO) and 4+ phone apps (Harry’s Laptimer Petrolhead Edition, TrackAddict, CMS Pro, Hotlap, and several that are discontinued).

There are 3 reasons to have a lap timer.

  1. To log your lap times
  2. To get immediate feedback on driving “experiments”
  3. To examine your driving after you get off track

My stance on this is that phone-based apps work great for logging lap times. Even though your phone updates its GPS only once per second, you can get accurate lap times to the tenths of a second because the software interpolates your position. I still use CMS Pro and TrackAddict from time to time.

If you want to ask questions such as “if I enter start braking at marker 3 instead of 2, how does that affect my corner?” then a phone isn’t so great. For these kinds of driving experiments, you need a delta/predictive timer that updates several times per second. 10 Hz is common, but some older devices update at 5 Hz and some newer ones are 15-20 Hz. My favorite delta timer is the RumbleStrip DLT1-GPS, which updates at 10 Hz. That’s good enough, and while I’ve never used a 5 Hz device, I’m betting that works fine too.

To examine your data when you get home, you’ll need a data logger. The AiM Solo is a really good product. You’re partly paying for the device and partly for very mature software. I’m very happy with my AiM Solo DL and I use it in the racecar all the time. It also works as a delta timer, but I prefer the giant red LEDs of the RumbleStrip.

No questions?

During the conversation, one driver said he has never timed himself. If this was a rookie driver, I could understand it. But this is a driver who had been attending HPDEs for a few years. And never timed himself? I don’t get it. How do you know if you’re improving as a driver? How do you know if your new tires are better than the old ones? How do you know if weather affects performance? How are you going to answer driving questions without data? I guess by not asking questions. I can’t imagine driving without experimenting. I never go for a drive just for fun because the street is no place to conduct experiments. I guess I’m not a very enthusiastic driving enthusiast.

Shit I don’t understand… #5: ZL1

There’s a lot I don’t understand about car culture, motorsports, and racing. Help me out.

Chevrolet Camaro ZL1

I was just coaching at Thunderhill yesterday with Hooked on Driving. The air was really terrible. I went out on the skid pad in my 2003 Hyundai Elantra GT with automatic transmission to do some drifting. Yes, you read that correctly. I like drifting in FWD econoboxes. One of the keys is to have stickier tires in front than the rear. Once you do that, the car really likes to hang the back end out. I put my helmet on and closed the windows because of the smoke. After a few minutes of training (that’s what I call my skid pad time), it got really hard to breathe. Then I started coughing uncontrollably for 20 minutes. While the air quality was bad, I probably also kicked up a lot of ash that was lying on the ground. The rest of the day I wore an N95 mask under my helmet.

My student had a 98 Camaro with some coil-overs and an LS3 swap that made something like 500 RWHP. As these are COVID times, I didn’t get a chance to drive his car. But I sure wanted to. Instead, I drove a new Camaro. I was on lap 1 of a lead-follow with my student when I noticed my fuel gauge was on empty. FUCK! So I pitted and told the HoD staff of my issue. Terri asked if I would like to use her car. Not having any other choice, I decided it would be the best option. So after a quick removal of a few items from the back seat, I was off.

I knew the car was a Camaro, and therefore had decent power, but I didn’t know which Camaro. I was very light on the throttle because I was unfamiliar with the car. But even with less than half throttle, the car leapt forward like it had actual legs. The next 20 minutes was spent with my jaw on the floor. Not only did the car have instantaneous power at any speed, the handling was better than almost anything I’ve driven. The combination was completely shocking. After the session, I got out and looked for whatever sub-model it was, and there was a ZL1 badge. I don’t follow cars very closely, but when I looked it up later, it started to make sense.

Why does a ZL1 exist?” It isn’t a car, it’s a tank (can’t see shit through the windows) with rocket boosters and ballet shoes. It’s too fast. I honestly don’t want to drive the limit in this thing thing unless I’ve got a full cage and more. And it can’t be very good on the street. Unlike an hypercar, the price is only $65K. So it’s not some priceless piece of art, it’s meant to be driven. But by whom?

Shit I don’t understand… #4: square wheels

This is the second in a series of rants about things I don’t understand about car culture, motorsports, and racing.

Staggering

Most high performance sports cars have staggered tire sizes. BMW M4, Chevrolet Corvette, Porsche 911, etc. But here’s something strange: all of the FWD cars have the same size tire in front and back. High performance FWD? Yes, they exist. The Honda Civic Type R is faster than most RWD sports cars. Its time at VIR Grand is 3:03.9. Why do RWD cars have staggered tires but FWD do not? I don’t know.

Load vs. grip

Here are some important laws of friction.

  • 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 the sliding velocity.

If we believe these laws, then it doesn’t matter how wide your tires are. They provide the same grip regardless of the area of contact. Also, it doesn’t matter how heavy your vehicle is. All vehicles with the same tires have the same grip.

It turns out that tires violate all of these rules. If you apply twice the load to a tire, it returns slightly less than twice the grip. Tires also have less grip the faster they are moving, and grip is greatly affected by temperature. That’s because tires aren’t solids, they’re viscoelastic compounds.

The reason why you put wider tires on the rear of RWD sports cars is because it looks cooler. Kidding. The reason is because they have more weight in the rear. A tire with greater width mitigates the loss of friction due to extra weight. Even RWD cars with 50/50 weight distribution put wider tires on the rear because they shift weight onto the rear tires during acceleration. If you want balanced grip while accelerating, you may need a little stagger.

FWD cars typically have a weight distribution of 68:32 or thereabouts. That’s a lot more extreme than RWD cars. With all that load up front, you want a wider tire. What happens with square wheels? The front has less grip under load and the vehicle has a tendency to understeer.

Yaris

On my no-longer-B-Spec Toyota Yaris, I run staggered setups all the time. Up front I use 15×9 and 15×8 and in the rear 15×8 and 15×7. I also run different compounds in the front and rear. This setup is actually illegal in a variety of series where they demand that front and rear tires are not only the same size, but the same brand and model. What does one do in such situations? Pump the rears up to 40 psi, align with no camber, and have a really stiff rear ARB. These things reduce rear tire grip and restore grip balance. But instead of removing grip from the rear, how about adding grip to the front in the form of a wider tire?

What’s up?

So why don’t FWD cars come with staggered tires? So you can rotate them to balance tire wear? Makes some sense, but sounds like the wrong reason for a sports car. Is it because a gorilla stance looks weird? Maybe, but look at the Polaris Slingshot. That thing is wicked looking and shaped like an inverted V.

Shit I don’t understand… #3: rain-o-phobia

This is the third in a series of rants about things I don’t understand about car culture, motorsports, and racing.

1/1000 drivers

I’m going to make a very rough estimate and say that only 1/10 car enthusiasts ever drive their cars aggressively, where I define aggressive as near the limit of traction in both the X and Y directions (drag racing will have to wait for another day).

I will further guess that 1/10 drive on track and 9/10 are street-based (autocross or illegal street racing).

You know what I’ve noticed about 9/10 track drivers? When it rains, they sit in the paddock waiting for it to dry. I’m going to call these fair weather drivers dryvers just for fun.

Taken together, maybe 1/1000 drivers intends to drive their car on a wet track.

Wetter is better

To me, the whole point of high performance driving is to balance a car at the limit of adhesion. It doesn’t matter if the weather is sunny or rainy, if the track is asphalt or dirt, if the driven wheels are in the front or rear, etc: the reason I’m there is to slide the car around under some (sub-optimal) interpretation of optimal.

Rain removes grip, making it possible to slide around at lower speeds and lower g-forces. You might feel unsafe in the rain because the traction isn’t always predictable. However, if you hit something, you are moving slower, so at least in one sense, a wet track is safer.

Are there any sports that are improved as a result of rain? I think car racing is much improved. There are downsides, however. Visibility can be bad. Tire noise is less perceptible. Paddock time is less comfortable. But on the balance I would always rather have rain. The driving is more exciting and there is far less wear on the car. On my team, I’m not very selfish about seat time or running costs. But I do look at the forecast and reserve the right to race the wettest stint.

What if you’re a dryver?

There are very good reasons to be a dryver. You are definitely putting your vehicle at more risk when driving in the rain. Traction is less predictable, and it is therefore more likely you will end up off track. Also, you can’t control other drivers, so you’re more at risk from them (assuming they aren’t waiting it out in the paddock). If you’re trying to set a personal best lap time, it’s not going to happen in the rain. Similarly, if you’re doing setup experiments, rain and surface variability are probably not your focus. I get it, rain gets in the way. But if you embrace the chaos, it’s actually more fun.

Getting over rain-o-phobia

For driving in the rain, the biggest hurdles to overcome are confidence and car control skills. You need both, and they require lots of time to develop. A skilled driver who is not confident is slow. An unskilled driver who is confident is dangerous. My brother is the former. On his home track at Pineview Run, where he has lots of confidence, you would be hard pressed to beat him. I’m not sure I could without putting in some time. But on an unfamiliar track, I will be a good deal faster because he’s making doubly sure the car drives home and I’m trying to punish the tires (in case you didn’t know, all tires are bad and need to be punished).

Both skill and confidence come with time. I think it’s easier to develop these on a sim rig, but you shouldn’t always be swayed by arguments of expense or efficiency. You’re allowed to have fun! If you learn at half the rate of the other guy it means you’re having twice as much fun along the way.

Shit I don’t understand… #2: Hypercars

This is the second in a series of rants about things I don’t understand about car culture, motorsports, and racing.

Hypercars are the worst

The most exclusive sports cars are known as Hypercars. These represent the pinnacle of technology and have a price tag to match. Hypercars include vehicles from Ferrari, McLaren, Koenigsegg, Aston Martin, Bugatti, Pagani, Lamborghini, as well as some vehicles from larger manufacturers like Porsche, Ford, Mercedes, Audi, Honda, etc. In the 2020 Road & Track Performance Car of the Year article, the fastest car around Thunderhill West was the Lamborghini Huracan Evo at 1:20.00. Not far behind was the McLaren 600LT at 1:20.42. These cars are $261K and $332K respectively. For that kind of money you might expect them to be rocket ships. Looking over the SpeedSF track records, there’s an E46 M3 at 1:19. A quick check of Craigslist shows you can get those for $10-15K. Sure, there may be differences in prep level and driver, but if it’s a 1:20 you’re after, you might as well get an M3, some track time, and a coach.

If you can afford a hypercar, here’s another idea: get a Formula Mazda, rent a garage at the local track, and drive a real race car on track days. Instead of driving a hypercar to the track, how about something more comfortable, economical, and safe? Oh, but then you wouldn’t be able to show everyone how much money you have. And ultimately, isn’t that what having a hypercar is all about? I seriously doubt there are any hypercar owners that read YSAR, but if there are, how do they make any sense? There are better racing cars. There are better commuters. What does a Ferrari do that a FM and a Tesla don’t do better?