That Smokin' Computer

Last week my nephew Amos and I upgraded his computer. We ripped out the venerable Asus A7N8X and AGP 7600 GT video card (along with CPU and memory) and slapped in a Gigabyte socket 775 mobo, Intel E7300 Core 2 Duo, 9600GT and two gigs of DDR2. Now that thing runs iRacing real well!

But that's not what I'm going to write about. I have a new computer story you might be interested in. When Amos was considering his upgrade - nervous about the necessary Windows reinstall it would entail - I bragged about how my main computer was running great since I reinstalled Windows on my main machine in December, hoping to ease his trepidation.

This computer, my main desktop, has an Asus M2N32 SLI Deluxe mobo with an Athlon 64 X2 4200+ and a GeForce 7600 GT in a gorgeous Antec LAN Boy aluminum case. It's known as the NightMonster, and it served as my racing computer until I built a new machine for iRacing last spring.

A few days ago the NightMonster moved from the top of my desk to a stand on the floor after a second 19" monitor finally arrived (two and a half weeks after I bought it off of eBay, damn the Postal Service!) I loved that computer so much that when I built it I even gave it a page on my web site, complete with a photo shoot.

Well, last night it went kaput. I was sitting there, innocently typing a PM to a fellow racer in the iRacing member site, when both monitors went completely dark and the video card fan spun up to max loud.

Tried rebooting several times. I'd get into Windows, but within a few seconds the same thing would happen. Monitors went dark, fan spun up. I tried unplugging one monitor but no joy. Eventually it quit entirely; I'd hit the power button and nothing at all would happen.

I was really bummed when I went to bed last night, as you can imagine! My main computer was dead (thank the silicon gods for Mozy backup!) and I didn't know why.

This morning after procrastinating for a while I ripped that beastly NightMonster - the most troublesome computer I've ever built - out from under the desk.

I put the old lug on the table and tore into it. I unplugged everything - hard drives, optical drives, USB card reader, even the sound card - but it was still dead. Ripped out the power supply and plugged in the one that was in my Pat Dotson G-Seat. Nothing.

As a last resort I ripped out the video card and hit the power button again. Presto!! Booted right up! I was able to copy a big file (which had taken hours to acquire via my pokey "broadband" connection) from its shared folder across the LAN to another computer. Happy, happy!

But I couldn't log into it. I tried Remote Desktop, but no joy; I'd never thought to configure the NightMonster to allow logging in through Remote Desktop since the reinstall.

But some experimenting and and a little research led to the discovery of a series of keystrokes that would allow me to turn on Remote Desktop even though I couldn't see anything! Took a number of tries but I got it.

Now it's back under my desk, nothing but air where a video card should be - and I'm logged into it across the LAN from my laptop. Heehee!

This is called a "headless" system. Well, to be strictly accurate a headless system has no keyboard and mouse, either. I plugged my spare keyboard and mouse into the NightMonster but they aren't really necessary; I can do everything I need to, including shutting it down, from the laptop.

I was going to order a replacement 9600 GT like Amos's, but after poking around a little and reading customer comments on Newegg I decided to take a look at the 9800GTX+. I thought these killer cards were too big to fit in my compact little LAN Boy.

The original 9800 cards were 10.5" long, about a quarter of an inch too much for the LAN Boy, but it turns out that the newest rev is only 9.5" long and it should fit. When it comes I'm going to put it into Wolf (my iRacing computer, which also is inside a LAN Boy) and put Wolf's 8800 GT into the NightMonster.

But meanwhile I can run the NightMonster headless and still sync my calendar to my Treo and sync podcasts to my iPod. I'm up and running. Heh.

Update: I plugged one of my 19" monitors into the laptop's VGA out port and now I've got the NightMonster displaying its desktop on the monitor (via Remote Desktop), right next to the laptop, which is displaying its desktop on its own screen. Dual monitors again. Yay!

The downsides? The laptop's monitor is a squished 1440x900 resolution rather than the nice 1280x1024 I have when using the NightMonster with its usual monitor. Also I can't drag a window from the NightMonster's display to the laptop's like I do when I am running dual monitors, although I can drag windows from the laptop's display to the external monitor.

Last but not least, iTunes - apparently in a fit of pique because it can't detect a video card, ha ha - refuses to show anything in Cover Flow and has turned its normally tasteful black, white, and gray song listing a bizarre shade of lavender.

Other than these few niggles, it's great - and now I'm essentially running four CPU's (both machines are dual core) so everything's even faster than usual!

__________________

Footnote. After the dust had settled I took a closer look at the dead video card and noticed that about half of its biggest capacitors had blown up. Seriously; their tops were split wide open. I slapped that thing into a zip lock bag real fast. Who knows what kind of nasty chemicals (PCBs? Mercury? Lead? Plutonium?) are inside those things.

Yes, I broke it. Lots of times.

I've been taking a break from sim racing for the past few weeks. I tend to burn out when I race a lot, and I raced a lot through the summer and fall. Also, I had some real-world track time, and that tends to make sim racing, even iRacing, seem a little tame.

My friend Michael Fridmann runs a shop that specializes in maintaining and modifying Lotus street and race cars. He's got a Grand Am-prepared Lotus Esprit Twin Turbo V8 that he uses for track days, and he invited me to join him for two events at Loudon.

October 26

The Lotus is a ferocious beast, with a full roll cage, race suspension, and engine beefed up to put out somewhere around 750 HP at max boost and peak RPM. Michael has pegged back the performance a bit for longevity, but it's still a mighty fast machine.

The first day was...interesting. I went out in the first session and got a black flag after two laps. Seems the car was smoking badly going up the hill out of Turn Six. I brought it back to the garage (the same garages NASCAR uses when they run at NHMS) and Michael and his friend Ray Patriacca, who did a lot of the preparation of the car for this event, looked the car over. Everything looked fine so I went back out. Two laps, black flag. Again.

This time they jacked it up and took a very hard look. Ray found evidence of a small oil leak at the rear of the left head, just above the turbo. Apparently a threaded plug was leaking and dripping tiny amounts of oil onto the turbo - which produced copious amounts of smoke.

A quarter turn of an Allen wrench and a shot of Brakleen and we had no more smoke. Michael did some laps and said the car felt good, but it lacked grip. This is because the Hoosier tires were old and had way too many heat cycles in them. Also the car still has the springs and shock settings from when it was running in Grand Am - and carrying 800 pounds of lead ballast that is now stashed somewhere back at Michael's shop. So it's a little stiffly sprung at the moment.

I went back out and started to get some laps in. Great! Yes, it could have used more traction, but it still felt terrific. It's been a year since I've been on track, and this car is worth way more money than I could afford to fix it if I smack a wall somewhere (and NHMS has lots of those). I was carefully working my way up to speed when...I'm coming out of Six, accelerating up the hill, and suddenly the cockpit fills with smoke! Augh! Not again!

I hit the master switch to kill the engine and pull off to the left, onto the grass. They shut down the course and bring everybody in so the wrecker can come and pull me back to the paddock. It's a long, slow trip on the end of the tow rope; plenty of time to contemplate that it's the end of practice and everybody's lost track time because of me and my smoky Lotus.

Back in the garage, we can't find anything. No oil anywhere, no sign of a water leak. Engine runs fine, gauges all normal. Mystery.

Then during the break before the time trials start, Michael checks the brake lights and finds they aren't working. They're required by our club, COMSCC, but weren't required by Grand Am, so Michael and Ray had added them. Turned out that a wire at the brake pedal had fallen off. The connector at the end was uninsulated, and when it touched the floor, it had created a short circuit which melted the insulation. That's where the smoke had come from.

Ray replaces the wire and Michael and I do our time trial laps uneventfully. I manage a 1:24.0, which is way slower than the car's potential, but considering the four year old tires, the ultra-stiff suspension, and my own lack of current seat time, I'm pleased.

After the time trials there's plenty of time for open practice. I get in about six laps when, going up the hill out of Six, the cockpit fills with smoke. Again. This time it's much thicker and by the time I get the car stopped it's so dense I can't breathe. I'm still coughing when the safety people arrive, and after another long tow back to the paddock the ambulance shows up and they convince me to breathe some pure oxygen for a while. To my surprise, this helps clear my head.

Turns out the new wire fell off and did the same thing as the old one. My heeling and toeing into Six is getting my feet into places Ray and Michael never anticipated, and eventually my toe snags the wire and pulls it off the connector. It falls down, shorts itself out and burns up. Since this new wire was thicker with more insulation, there was more smoke. Doh!

November 1

We had a week till the next event, so Ray and Michael had time to fix the brake wire properly. For this event Ray was my student. He's got plenty of oval racing experience, having won a local championship a couple of years ago, but this was going to be his first time on a road course.

I take him out in the car for the early morning instructor session. It's very cold, having dropped below freezing the night before, and this is a light car on big four year old race tires that are now very, very hard. I'm tiptoeing around, trying to get some warmth into them, hoping Ray wasn't too bored. (He wasn't!)

After six laps I'm feeling a little more confident, sensing some grip coming, and I start to lean on it just a little. Out of Turn Ten I squeeze on the power, being careful not to spin the rear tires but giving it a bit more than before, when the engine suddenly zings up toward redline and a big vibration hammers through the car. I shut it down and put my fist out the window to signal the cars behind that I'm headed for the pit entrance, just a couple corners ahead.

My first thought is that I've broken the transmission, but when I turn left to go around the corner the left rear of the car sits down and the vibration changes into a scraping noise. I pull off the asphalt and peer into my outside mirror. I can just see the left rear tire - lying on the grass about 20 feet behind the car. The Lotus is done for the weekend.

No tow rope this time; we (and everyone else who is hoping to be on the track about now) have to wait while the flatbed guy figures out how to get a three wheeled Lotus up onto the flatbed without ripping it to shreds.

Back at the garage, Michael directs the unloading of the Lotus from the flatbed right onto his trailer while I start hoofing it around the garage, looking for someone that will let a road course rookie drive their car so Ray's weekend won't be a total bust.

Finally a very generous Dan Baldwin allows me to take Ray out in Dan's showroom stock Honda S2000. This car is tailhappy like no other street car I've ever driven, but we get in some good laps. As we're getting out of the car in Dan's garage, right next to us Lou Milanazzo is strapping his helmet on, about to get into his ferocious black Dodge Shelby GLH. I ask him if he'll take Ray out and he says sure. Great! More track time for Ray, even if it is in the right seat.

Meanwhile Michael has gotten the Lotus squared away on the trailer, diagnosed the problem as a broken stub axle, and tracked down a drive for Ray: Marc Epstein's fire-breathing race-prepared Honda S2000.

Ray's first laps in the Honda are anything but tentative. First lap, over the hill at eight, he's flat out at maybe 80 MPH and headed straight for a wall he can't yet see until my frantic gestures get him to brake - just in time. On the second lap, going up the hill through seven, the tail steps out and Ray corrects. A wild series of tankslappers ensues. For a moment I think he's caught it, but it gets away from him and spins up the hill, coming to a stop on the right side of the track, tail off in the gravel, up against the tire wall.

In the right side mirror can see the bumper buried in the tire wall and I'm hoping desperately that any damage we've done is purely cosmetic. Marc is locked in a tight battle for the championship and the time trial this afternoon will be the title decider. I really hope we aren't responsible for him losing!

In the pits we get the all clear and Ray's subsequent laps are a little more circumspect. Back in the paddock we inspect the rear of the car. It's perfect. Not a scratch. No, wait, there are a few tiny, tiny scratches on the rear bumper where the grit on the tires in the tire wall scraped the paint. Whew! Close call!

The rest of the afternoon is relatively uneventful, except that my brother Nate lets me drive his latest project, the Bio-Diesel Special.

What a hoot! Feels almost like a shifter kart with fenders, except that you don't have to shift because the hot rodded four cylinder diesel (from a VW New Beetle) has so much torque. Good thing, too, because the shifter is so loose it feels like it's broken (but it isn't). Great brakes, steering, terrific handling, endless acceleration. Sweet machine.

Late in the afternoon Michael and Ray and I are standing around and a thought occurs to me. I've been assuming the Lotus is done for the weekend, but maybe...

Yes! Michael has been thinking the same thing. He's got a spare halfshaft (which includes the stub axle) back at the shop, plus a spare wheel bearing and spare brake lines. He's confident he and Ray can fix the car for the next day.

November 2

Sure enough, in the morning they're back at the track, car ready to go. Awesome! Ray finally is going to get a shot at driving the monster Lotus at speed.

Things go well for a few laps, but then Ray goes into Three a little hot and spins again. As he turns in I can feel it coming; the Lotus is extremely tail-happy and the ancient tires just don't have the grip Ray's asking for. Nothing I can do but wait for the spin to end; fortunately we spin harmlessly, keeping off the wall at the outside of Three, and everybody gets by us without hitting anything. I tell Ray, "I don't want to see any more spins!" And I don't.

Now another problem begins to rear its head: overheating. The Lotus had this problem all through the summer of '07. We'd get in one session with the engine running cool, temps low and stable, but in the second session the engine would start steaming and the water temp would go through the roof. We'd bring it in, wait for it to cool, and Michael would carefully top it up with coolant. We'd get another session of running cool and then the overheating would come back.

To address this, during the previous weeks he'd pressure tested the system and found and fixed a number of small leaks, mostly caused by hose clamps that had loosened up as the hoses had compressed. The fixes had worked for the first two days, but now that we were running it hard and long the problem was back.

It seemed to bite me the worst. Michael got in a nice long run; temps stayed down. I got in the car, and three laps later I was coming back in on the rope - again - after having shut it down when steam filled the cockpit. Michael and Ray both got clear runs in their time trials, but for me it overheated at the end of one lap and I had to pull off, missing out on what should have been faster timed laps.

Finally we got the procedures down and in the open practice at the end of the day I got in a nice series of laps, but I was never quite able to match my best time from the previous weekend. The colder weather combined with tires which get worse with every heat cycle were probably at least partly to blame.

In the end my only time trial lap was good enough for second in class. Micheal and Ray set good times as well. Nate won his class, Dan Baldwin won his class, and Marc Epstein won his class and his championship. Of those who helped us out, only Lou Milinazzo didn't come away with a win; instead the poor guy DNF'd. Thanks to all of you guys; good karma's coming your way - especially Lou.

Ray did a great job in his first time on a road course - and I'm pretty sure he enjoyed himself!

Despite the troubles it was a great three days and a reminder of how terrific it is to be at a race track, driving fast and having fun.

Respectfully submitted,

The Racing Addict

Pedal Project Progress

Head on over to my other blog for an update on the DIY load cell pedals that my nephew Amos and I are building.

Doh! <slaps head>

At the moment I'm a bit disillusioned with the Formula Mazda.

Grant Reeve has posted some new information on the iRacing forum which essentially renders a lot of what I said about the Mazda in my "Teasing Secrets" post moot - for now, at least.

Hopefully the Mazda's aero model will soon be enhanced enough that everything I said in this blog will be applicable.

Until then, my apologies for wasting your time (and mine) exploring and explaining ground effects on a car simulation that doesn't actually have any ground effects, and tire characteristics of tires that behave like no real-world tires I've ever experienced.

Teasing Setup Secrets out of the Formula Mazda

I spent all day on Tuesday pounding around Summit Point in iRacing's Formula Mazda. Squandered a perfectly good day doing 200 laps, tinkering with every parameter in that setup menu.

I ended up taking nearly a second off my personal best (lowered it to a 1:07.4). Also I think my latest setup is a good race setup as well as having some potential for further development.

Richard Towler's Setup, Low Tire Pressures, and Ground Effects

I started with Richard Towler's setup from an iRacing.com member forum thread called Re: Formula Mazda @ Summit Point.

It's a challenging setup - if I get a little sideways and try to recover, I wind up in a violent tank-slapper from which there is no return. But it has what I thought was fantastic balance through the final turn and also good balance through Turn 3, two of Summit's most critical corners.

Richard's setup also is very unusual in some ways, with a quarter inch of toe-out at the rear and extremely low tire pressures - 17.0 front and 17.5 rear! Tire temps show the middle temp far below the inner and outer.

In the forum, people theorize that this setup oversteers because it has a quarter of an inch of toe-out at the rear. Although this makes sense, I wasn't sure and wanted to verify this.

The first thing I did was raise the tire pressures. No other change. And bang! The Mazda's characteristic understeer came back. I've established to my own satisfaction that the oversteer - or at least lack of understeer - is related to the low tire pressures, not the rear toe-out.

I suspect what's happening is that the ultra-soft tires are letting the car squat and roll so much that the outside edge of the sidepod is touching the ground, taking away load from the outside rear tire and thereby reducing rear grip. Or, possibly, the extreme roll and squat are causing the diffuser to stall, taking away rear grip. Either way, I believe the reasons Richard's setup oversteers are related to his very low tire pressures, not toe.

Actually, I think the phenomenon of the chassis squatting under aero load explains why Richard, Wolf Woeger and others have been arriving at very soft tires on the Mazda: because when they lower the tire pressures, they feel more grip, but I believe this is because the chassis squats more at speed on the softer tires.

So as they experiment they feel that lower pressures give more grip, when actually it's the fact that the lower pressures cause lower ride height at speed, which gives more downforce, which is where the extra grip is really coming from. Make sense?

Tire Temperature Spread

But tires in the real world actually make the most grip when the temperatures across the surface are even. This is logical; if the edges are hotter than the middle, than the middle isn't being squeezed against the pavement as hard as the other areas of the contact patch, so it's not generating the grip it could.

So really, the car should generate the best grip with the tires at optimum pressure: with temperatures even across the tread, or slightly higher on the inner edge and the middle temp halfway between those of the inner and outer edge. This is the way it works in the real world.

(And yes, Grant Reeve has said that the Mazda's tires need some fine tuning, but the temperature spread relationship to grip is so elemental that I can't believe Dave K. would have made a mistake on this aspect of the tire model.)

So, I figured, I should be able to set the pressures to get even temps and drop the static ride height to get back the downforce I'd lost because of lack of squat due to the higher tire pressures.

The Formula Mazda's Underbody Design

I also believed that the aerodynamic center of pressure (see below) was too far back, and I thought that was what was causing the understeer I dislike so much. So, as I mentioned before, I thought that increasing the chassis rake should allow me to dial in better balance by moving the CP forward.

Wrong. Or, only partially right.

In fact, at one point I was so baffled by what was happening that I looked up the Formula Mazda on the web.

I realized I'd been thinking of the Mazda as a tunnel car (like the Lola/Reynard generation of Champ Cars) but really it might be flat-bottomed with a diffuser like an F1 car.

I found a lot of photos and even the aero parts list at Star Mazda. Unfortunately there are no photos of the bottom part of the sidepods in the catalog, but there are photos of the diffuser (at right). That and other photos of the entire car elsewhere (look closely at the photo of the white car below) that convinced me that this is a flat-bottomed car with diffuser, no tunnels.

So that made me think about it a little differently. Flat-bottomed cars are notoriously sensitive to ride height - and the downforce they generate isn't linear as the ride height goes up and down. Lowering the chassis does increase downforce - to a point - but then you may reach a level so low the airflow under the chassis stalls and because of this downforce begins to diminish rather than increase as you lower it further.

Real-world race engineers have information generated in a wind tunnel called ride height maps. These show drag, downforce, and aerodynamic center of pressure at various arbitrarily chosen ride heights.

I asked Dave Kaemmer in his AutoSimSport interview if he was planning to supply us iRacing members with ride height maps and he said no, these are regarded as trade secrets by the teams that allow him to scan their cars and supply him with data.

So we are going to have to rely on guesswork and experimentation to arrive at info that the real-world race engineers for these cars have at their fingertips. Dammit!

But I am assuming that Dave himself does have those ride height maps and that iRacing's aerodynamic modeling of the Mazda consists of a set of tables which mathematically represent the content of those ride height maps. I figure the iRacing physics engine, having calculated velocity, ride height, pitch, roll, and yaw at any given instant, simply looks up drag, downforce, and center of pressure in those tables, resulting in behavior that mirrors the real-world wind tunnel results.

Anyway. I found that lowering the chassis and adding some rake did get back some of the downforce, but you can't go too low or the front end just washes out completely. In Turn 3, this might be due to the front of the chassis bottoming, but it also happens in the final turn, which is nearly level. So I think possibly getting the front of the chassis too low causes it to stall, aerodynamically.

So I think maybe you can only go so far with rake and with general lowering to get downforce. I believe you have to start using the wings, too.

Fortunately the front wing should be fairly low drag, as its primary function is as a trim tab, so it shouldn't hurt too much to add a few clicks of front wing to get the aero balance into the range where you want it.

I tried going up one degree on the rear wing and several degrees on the front, and lost only about 1 MPH at the end of the straight. I think gains in balance would produce improvements in corner exit speeds that would more than offset that loss, so for now I'm sticking with 14 degrees at the rear and whatever it takes at the front to balance it.

The Impact of Caster and Camber

I continued experimenting and came to the realization, much to my surprise, that camber (and caster, because of the camber gain it causes) is critical. And none of the setups I'd tried had optimized this. This turns out to be a really big contributor to the understeer.

(This is right in the area where my friend Ricardo Nunnini began to explore a few days ago, so as usual he is way ahead of me!)

I had been operating under the assumption that at high speeds, aerodynamic downforce was so great that it pretty much overwhelmed any mechanical factors like camber. But now I think this assumption was wrong. Aero is important, but so are camber and caster.

I continued tinkering. I could dial out the steady state understeer by cranking up the front wing but this made the car unstable under braking and turn-in. For a little while I went down a blind alley by trying to tune Richard Towler's damper settings for better turn in with what I consider optimal tire pressures. Didn't work; I got slower.

After that I focused on front camber and caster. The rear temps looked good - a degree or two higher on the inner edge of the left-side rear, and the middle in between - so I figured I could assume they were reasonably close to optimum.

I started tweaking the fronts, and to my astonisment I was able to completely dial out the dreaded understeer by getting closer to optimum with the front camber and caster! I'm not sure I have optimized these yet; it's tricky because of the camber gain which occurs when you turn the wheel. But the setups I ended up with are, I think, getting close.

I posted these setups in the same thread I mentioned above. (Pesonally I was a little faster with the x1g setup, but it understeers more than I'd like. I think the x1i setup is potentially faster; I just haven't been consistent enough as a driver to access it yet.)

One of the things I like about these setups is that, unlike my earlier setups with a lot of rake and stiff rear springs, I'm not giving up rear grip, so the braking stability remains fairly good and the traction remains excellent.

The stability of the aero platform is still important, but it's not as critical as when I was trying to balance the car with rake and running the front very low. I've stuck with Richard Towler's spring settings of 600 front, 700 rear, which I think are about the only things left from his original setup. But I should also say that it still owes something to Richard's setup, particularly in the area of damper settings.

I've been experimenting with going softer on the dampers, and I think that helps calm the car down with no downside that I can see. I started with Richard's extremely stiff settings and I've backed off four clicks on all the settings except front rebound, which I think I've backed off more like eight clicks.

Richard's front dampers were extremely stiff in rebound, probably to calm down the chassis response to steering inputs. Since the car with my setup isn't as volatile in its responses to steering, it doesn't need such stiff rebound damping at the front.

Aerodynamics and Setup in a Ground Effects Car

I feel that we are still just scratching the surface of what the Formula Mazda has to teach us. It's the first downforce car in a simulation whose behavior I actually trust to reasonably closely approximate the behavior of its real-world counterpart. Therefore it's extremely interesting to me as a learning tool.

Downforce - especially downforce from the underbody - radically increases the complexity of setting up a car. You have all the usual mechanical parameters that us old GPLers are familiar with (toe, caster, camber, spring and damper rates, tire pressures, roll bars, etc.) and you also you have this enormous aerodynamic effect as well - and it varies drastically with speed. And the mechanical factors and the aerodynamic loads interact, making things even more complex.

On top of that, not only does the total downforce vary with speed, but where - relative to the wheelbase - all that downforce is being delivered also varies with speed.

This is known as aerodynamic center of pressure. If it's forward of the car's center of gravity, then at speed the car is going to tend to oversteeer, because the front tires are getting more downforce and therefore more grip relative to the rear. If the aero CP is aft of the CG at speed, you're going to get high speed understeer.

An interview with Sebastien Bourdais on autosport.com woke me up to this factor. In it, Sebastien pointed out that the Toro Rosso/Red Bull chassis has extreme migration of its center of pressure throughout the speed range. At low speed, the CP is very far forward, so the car oversteers in slow corners. But as the speed rises, the CP migrates toward the rear. At very high speeds, it's very far rearward, so the car understeers in very fast corners.

Bourdais says he doesn't do well with either of these characteristics. Mark Weber and Sebastian Vettel have been able to adapt their driving styles to drive around these problems, but Bourdais hasn't (and neither has Coulthard, apparently).

The reason "Sea Bass" was so quick at Spa is that almost all the corners are medium speed, so the car's CP was in the middle, giving it good balance in almost every corner. And so he flew.

Other Factors Affecting the CP

Speed, of course, is not the only thing which impacts the center of pressure. Ride height, chassis rake, pitch, roll, and yaw all have their impact on the airflow over and under the car. The underbody of a flat-bottomed diffuser car like the Mazda is particularly sensitive to these things. The CP might be migrating all over the place, for all we know, and it might not always be doing this in a way that is intuitive. It makes sense, for example, that increasing the chassis rake would move the CP forward, but does it always?

And, of course, spring rates, damper settings, and tire pressures all have an impact on dynamic ride height and rake, as does the track surface. A steeply banked corner is going to squat the chassis down more than a flat corner of the same radius; stiffer springs are going to reduce the squat compared to softer springs, and stiffer damper settings will impact both overall ride height changes and rake changes during transients.

This is why a downforce car is so much more complicated to set up than a non-downforce car like the Skippy. Almost anything you do to anything affects other things. And it's often very difficult to pin down exactly what is causing a particular reaction to a change. Is it the thing you changed, or is there a ripple effect, with the greater impact coming from a secondary or even tertiary factor?

The Mazda's Center of Pressure

My issue with the Formula Mazda is what I perceive as relentless understeer, particularly at high speeds. I figured this pointed to an aerodynamic center of pressure that was fairly far aft and/or migrated further aft at high speeds.

Therefore, I've been trying to balance the car by moving its CP forward, both by increasing the chassis rake (raising the rear/lowering the front, which should move the CP forward) and by running more front wing.

My experimenting the other day suggested that - lo and behold - tire alignment, both camber and caster (via its dynamic camber change) are also very big factors in the balance of the Mazda, even at high speeds.

It turns out that those big wide bias ply slicks need to have their surface pretty much flat on the pavement. With the front camber and caster in the ball park, the balance of the car is transformed - and radical tricks like very soft tires or extreme chassis rake aren't necessary to get its handling into the neutral zone.

But, as I say, I feel we are still just scratching the surface.

DIY Load Cell Pedal Project

My nephew Amos and I have been working on an interesting sim racing project this summer. We're building a set of pedals patterned after Todd Cannon's CST pedals. The key factor in these pedals is that the brake pedal operates a load cell rather than a potentiometer.

This is turning out to be quite a substantial project. We decided to build a hybrid of Todd's original DIY pedals and his current production pedals, but without any welding or painting. This has required a significant amount of design work.

The load cell and its nearly complete actuation mechanism is shown in the photo at upper right. At lower right the not-quite-complete clutch and brake pedal units sit on a scrap wood mockup of the planned floor stand, built for the purpose of verifying pedal height and angle relative to the floor.

You can read more about the project and see lots more photos at my blog, Amos and Alison's DIY CST Pedals. Actually you might want to start at the first post, Building a Set of Load Cell Pedals.

Mazda Magic, Part II

Laguna is not my favorite track. I don't like all that steep banking; the rolling in and rolling out really increases the complexity of the corners, and not in a way that's fun for me.

But last Tuesday I raced the Mazda with aslightly modified version of Ricardo's Mazda setup and had a fantastic race. (All I'd done to Ricardo's wonderful setup was to add a few notches of front wing to help the car turn in better.)

These days my iRating seems to be high enough that iRacing's automated race control keeps sticking me in races full of aliens, with whom I haven't a hope of actually racing. I wasn't gridded anywhere near the front, but after the first lap shuffle I wound up with a fellow named Dennis Griffen right behind me. Dennis and I were about the same speed; I had an advantage in some corners, he in others, but he was quick enough to put pressure on me the entire race.

I was pushing myself quite hard but trying to stay clean, and although he got alongside a couple of times, Dennis was never quite able to make a pass stick. He was driving very cleanly, which made it fantastic. The only contact we had was when he bumped me from behind once entering the corkscrew after he got a good run out of Turn 6 and I, being a little inconsistent as usual, braked a little earlier than usual at the top of the hill. But the contact was clearly unintentional and he apologized right then and also after the race. What a pleasure to race with people like this!

We had a terrific fight right through until the penultimate lap. The whole race (except for that one blooper) I'd had an advantage through Turn 6, that nasty banked corner before the hill going up to the corkscrew, so most of the time Dennis was never close enough to threaten me at the corkscrew.

But this time he got another good run through Turn 6 and I messed up and went wide on the exit, so I had to lift a little. Going into the corkscrew he was right on my tail and, under pressure, I left my braking too late. I skittered off the outside, shortcutting the corkscrew. Not by much, but just enough to get a furled black flag for a second or two. I had to slow down to clear it, and Dennis was through.

Damn and blast!

But it was a great battle, and we both had a terrific time.

I'm really getting to like this Mazda!