Marshall Pruett’s Racing Tech Mailbag for March 13

Marshall Pruett’s Racing Tech Mailbag for March 13

Insights & Analysis

Marshall Pruett’s Racing Tech Mailbag for March 13


Welcome to Marshall Pruett’s new Racing Tech Mailbag on If you have questions about the technical side of the sport for Pruett, who spent most of his life working as a mechanic and engineer in open-wheel and sports car racing, send them to We can’t guarantee your letter will be published, but Marshall will always reply.


When might IMSA release the technical study of the GAINSCO DP crash from Daytona? While I don’t like DPs, many of the violent crashes involving LMPs over the past 15 years didn’t seem all that abrupt.

What changes are taking place with roll cages as many recent GT machines now have aluminum chassis?
Rick Kappler

MP: I spoke with a friend at IMSA and he said while they are still doing some further studies on a few items specific to the crashed Riley chassis, once they are done, the report will not be public.

On the roll cages, steel has been the standard material for quite some time, making its installation with cars formed from aluminum frames a newish exercise to incorporate. The FIA’s standard on roll cage construction, materials and attachment is used almost everywhere, including IMSA. Corvette Racing went to great lengths to weld a steel cage to the Aluminum C6 chassis with their first ALMS GT2 car, but the FIA outlines something much easier – bolting the cage inside the car.

How many DW-12 chassis (tubs?) have been built? How many are still “raceable” (i.e., not damaged to the point where they are show cars only)?

MP: My man Stefano DePonti from Dallara came through on this one for us: 77 tubs built, 75 complete cars sold, and the best we could come up with was five destroyed/yet-to-be-repaired DW12 tubs.

Why are standing starts on street courses so difficult? Seems to me that every time a car leaves its pit stall, it’s the same as a standing start? What is the problem/difference?
Bruce Ingberg, Sparks, Nev.

MP: I’m not sure standing starts are as difficult as you might think, Bruce. Launching from the pits is a different situation altogether that involves dumping the clutch and leaving with little else to master. Standing starts call for finding the bite point on the clutch, raising and holding the revs, and having to coordinate the lights turning out with the perfect clutch release. Between the two, one is fairly basic and primal, the other is somewhat procedural and with more steps, there’s more to get wrong.

We did see at least one stall during most races where they were used by IndyCar, and you could put that down to it being a new procedure for everyone. Also, by the numbers, having 24 out of 25 drivers get it right is a ratio I’d be happy with in any scenario.

Formula 1 drivers tend to come into the series with a lot more standing start experience due to it being the normal starting procedure in so many European open-wheel series. It used to be a staple for the various Formula Atlantic series here but, with its demise, that area of driver training has been lost. I’d expect IndyCar’s mandate for anti-stall to be activated this year to help, and with more practice, IndyCar drivers should be able to nail their starts. I hope.

I’m very excited about this new mailbag. With Derrick Walker stating that IndyCar intends to bring innovation back and opening up the cars for development, what do you see as the first area to be opened up for development other than the aero kits that we know about?
Tyler, Azusa, Calif.

MP: Great question, Tyler. With aero becoming the responsibility of the aero kit manufacturers, there isn’t much left on the car for teams to monkey with. The new bodywork will conform to the size and shape Chevy and Honda desire, so at least for what goes on beneath the body, any changes would need to fit within the car’s aero needs. If you ask the teams, they’d love to go in their own direction on brakes; the current Brembo system at each corner has never been popular, or consistent, so if that door can be opened, look for that area to receive a lot of attention.

I could see some suspension development being permitted, but the argument from the series will be whether opening up those areas does anything to improve the show. I’m sure a cost-benefit analysis will also be required; some teams can put a hurting on the others with their financial resources, and with so much of the car being spec, I fear the series will shy away from letting the DW12 become individualized.

I feel really old when I say things like, “I grew up at a time when people wanted to see the best team with the best ideas win.” Now, the mantra seems to be “All that matters is the show.” I’d argue that if the show is all that matters, and it comes at the expense of stifling ingenuity and creativity, we’re better off racing shopping carts and saving everyone an awful lot of money.

Can you quantify the amount of downforce lost when following a car? The motivation behind my question is to help better understand the necessity of the DRS system. 30 years ago in F1 when they weren’t producing so much downforce the racing was better because the cars didn’t suffer as much in dirty air. I’d like to better understand that dynamic.
Ryan, West Michigan

MP: Yes, you can, provided the cars are instrumented and you can measure the changes in things like downforce data generated from strain gauges, potentiometers, etc. The DRS system simply bleeds off downforce (and drag), allowing the car with DRS activated to increase its speed. Less turbulent air comes off the rear wing elements when DRs is engaged, so it’s more about the leader car accelerating than the trailing car being slowed.

As for F1 downforce 30 years ago, yes, once ground effects and sliding skirts were banned, downforce figures were definitely reduced, but don’t discount what those big wings and large flat-bottom floors were able to generate. As for the racing being better, that’s debatable; I know I enjoyed F1 more back then, but that was because the cars were very dissimilar in design, construction, had engines that made different amounts of power, etc. If we’re talking about the racing itself, the quality is much higher today due to the teams being better and the rules steering everyone toward designs that are more common than creative. 30 years ago, you tended to have a few cars battling hard up front, while everything else going on behind them was rather forgettable.

Do you think the sonic flow restrictors used on the DPs at the Rolex 24 this year affected the turbocharged V6s differently than the normally aspirated V8s? If so, could you explain (simply if possible) the physics behind the possible difference?
George Brucks

MP: Yes, George, there is a difference in how the sonic inlet restrictors affect turbos and NA V8s. The intake stroke on a NA V8, for example, is incredibly powerful, and despite having an air restrictor choking down what’s fed to the intakes, it will still pull the air in with great force. On a turbocharged engine, the turbo itself is does not allow the full force of the intake stroke to pull in air the same way a NA V8 does. The turbine side of the turbo is doing its best to draw air in through the restrictor, but the force – and volume – is slightly below what the NA V8 can achieve. As RACER wrote about last month, IMSA spent time on the dyno after Daytona to match restrictor sizes between both types of DP engines. In essence, they spent the time to learn that if they want to apply an equal power reduction to turbos and NAs, the turbos need a little bit more air.

I’ve noticed a difference in the fire suits that IndyCar drivers wear vs. those that NASCAR drivers wear, with the biggest difference appearing toward the bottom of the fire suit. IndyCar drivers appear to wear a type of fire suit that tucks into the driver’s shoes, while NASCAR fire suits appear to go over the top of the driver’s shoes. Is this a matter of personal preference, or is there a technical reason behind this difference in design? Additionally, what other differences exist between these two different types of fire suits, i.e. materials, fire ratings, etc.?

I’ve always wondered about these differences. 
Jay Matheny, Mayfield, KY

MP: It’s pretty much a preference thing, Jay. Some drivers like cuffs, some prefer them straight. Every racing series has its own requirements for the suits their drivers must wear – how many layers, in particular. Fire resistance requirements are pretty much universal. Sparco’s newest Hocotex suit is all the rage this year and, as my man Townsend Bell said, “It feels like a Snuggie! It weighs about a pound, and anytime you can save weight, even with a suit, it’s a big deal. I love the suit.”

Back in the day you used to hear about tire stagger in IndyCar all the time. Seems I remember reading once that teams literally took their tires and measured the circumference with a tape measure in order to decide which corner of the car to use them. Is stagger still an issue or are modern tires so consistent that it’s a non-issue? And is stagger only an issue on ovals or is it important on road/street circuits as well? Does stagger change after a tire is heat cycled? And do/did other series besides IndyCar use stagger as a tuning tool?
Tim Elder

MP: Yes, stagger is still a big deal in IndyCar on ovals. The move from bias-ply to radial tires did away with tricks like overinflating tires to stretch them and increase stagger. That doesn’t stop teams from measuring the diameter of the rears to confirm the stagger, but they simply work with what they are given. Firestone sends out a nice media alert before every race with stagger info, BTW. Their last, at Fontana, included this nugget on what they brought for the MAVTV 500: “Tire stagger of approximately 0.45 of an inch for all sets of tires.”

When the current IndyCar formula started manufacturers could use either a four-cylinder engine, or a six-cylinder engine. I recall at the end of the first year, that rule was still in place, though no one built a four-cylinder. Here we are a few years down the road and there have been a few changes to the rules, such as only twin turbos for 2014, but I was wondering if it’s still in the books that a four-cylinder is still permitted?
Mark Holden, Cutler Bay, Fla.

MP: It was until 2014, Mark. Like F1, IndyCar has mandated the V6 as the only engine layout permitted. Granted, no one showed up with anything other than a V6, but I’m not sure closing the rules to force new manufacturers to build a V6 if they’d prefer something with six cylinders or less (the rule from 2012-13) is wise. From the rule book:

Constraints – The engine will be a four-cycle, four poppet valve per cylinder (two inlet, two exhaust), V6, turbo charged engine subject to the constraints below:

a) Capacity –2.200 liter maximum

b) Cylinders – Six, all of equal capacity; each round to +/-0.13mm

c) Bank Angle – between 60° and 90°

In December 2013, it was announced that Chevy and Honda will provide aero kits for the entire 2015 IndyCar season and that Dallara kits will still be allowed. Do you think any teams will elect to continue to use the Dallara aero kit? The current Dallara punches a pretty big hole in the air and allows for the trailing car to sling shot past the leading car, as shown by the record number of passes for the lead in the 2013 Indy 500. How will IndyCar be able to keep a high rate of passing with three aero kits available? What would prevent one of the three from designing an aero package that creates so much turbulence in it’s wake, that it is nearly impossible to pass the leading car?
Mark Z, Discovery Bay, Calif.

MP: Mark – this one is easy: No! Dallara’s said they will not built an aero kit to compete with whatever else appears in 2015, and with Chevy and Honda having aero kits manufactured, look for all of their respective teams to use them. Can’t say on the rate of passing – personally, I don’t care about the volume of passes. I watched CART’s Hanford Device era and yawned each time. It’s hard to create a lot of turbulence without producing drag at the same time, which is why you don’t see that concept making it into aerodynamic designs.

LMP1/F1’s KERS works by recovering waste energy created under braking and transforming it into electrical energy to help the regular engine, right? Is there any way to have another system like that but working on transforming into electrical energy the efforts made by the driver by turning the cars around the corners of the circuit (steering wheel movements)? Everything produces energy, right?
Giu Canbera, Sao Paulo

MP: I’m sure there’s energy to be harvested there, but the costs to create that system and the weight of that system, for the amount of actual energy produced, would be a waste. It would be like spending $1000 on a system to generate enough energy to power a light bulb.

Is this aerodynamics possible? Or even Plausible? It looks revolutionary… IMO it looks better than CART and it should work way better according to Hot Wheel’s imagination
Grid RX

MP: It’s a really cool concept car, but those really aren’t aerodynamic devices in the conventional sense. The flat, angled surfaces will push down on the car to some degree, but these are for show, and nothing more. The “Downforce” graphic is nothing more than something an artist came up with.

F1 has decided to implement a new rule this year that seems absurd. With the new cars and new 8-speed transmissions this year, cars have to run the same eight gears for the whole year, unlike previous years when they could select various ratios at each track. So we will now see a team running the same gears for Monaco and Singapore that will be running at Spa and Monza. How does the top tier of motor racing allow such a rule? For IndyCar fans it would be like a car running the same gears at Indy that they run at Barber. It diminishes the race and takes away from the thrill of speed I believe. Your opinions or beliefs?
Dan, Springfield, Ill.

MP: It’s weird, isn’t it? I didn’t understand the move to a spec gear stack from a performance standpoint (although one change is permitted during the year if teams want to use different ratios), but as I’m told, it was done to save on costs. The craziest part is it asks teams to, as you rightly mentioned, use the same stack at all types of circuit, and due to testing regulations, they’ll have to use simulation to predict the best ratios for most tracks without having run the new 1.6-liter turbo engines or 2014-spec tires at those facilities. Sim technology is frighteningly advanced, but it’s still a tall order to get things right by selecting eight perfect ratios before the season starts.

A lot of closed-wheel racecars have vents or gaps behind the front wheels. What is this for? Is it to reduce drag by pulling in the air flow coming out of the front wheels with vacuum or route air on to some surface for downforce?
Larry Lee

MP: That’s exactly why the venting is done, Larry. Left closed, fenders act like parachutes. Opening those parachutes reduce drag and increase speed.

During the CART era, Indy cars were turbocharged but they didn’t have an air intake above the drivers head like they do now. Why the difference?
Tim Falkiewicz

MP: Tim, it’s all about the rules. The Dallara DW12, per IndyCar, must have the overhead intake where it then feeds the turbos.


When the iconic committee first introduced the concept of aero kits, they showed a example of which aero pieces could be modified. Basically the engine cowling and side pods. Since Dallara announced they will no longer produce a aero package starting next year, does this open up the whole body for OEMs to develop? Have you seen any renderings or actual bodywork yet? Do you know if either Honda or Chevy has wind tunnel tested their aero kits yet? Does the implementation of competitive aero dynamics make the IndyCar series more attractive for another OEM to partner with Cosworth? When a team leases a engine, who are they leasing it from? The engine builder or the OEM? Thank you for taking my questions. You have a great way of explaining things to everyday Joes like me.

MP: Darren, the rules on what’s allowed has changed a few times since the aero kit concept was unveiled. Here’s a look from one of the recent versions of the rulebook. No, I haven’t seen any renderings – those would be kept hidden from the public. Can’t say in the wind tunnel part, but you can bet they’ve spent plenty of time using CFD. The manufacturers say it gives them a chance to have stronger brand identity, but in the same breath, they’re also worried about costs. Bottom line, I don’t know of any auto manufacturers that want to build an engine for IndyCar because they can also do their own aero kit. Most manufacturers have no idea what an “aero kit” is, frankly. Leases are supposed to be facilitated through the series.

The TUDOR SportsCar series has brought together the Grand-Am and ALMS together. I was thinking that they finally got a match. But in the Le Mans entry list for the 24 Hours, what is the problem that no Prototype (formerly known as the DP) is not on the list. Does this have to do with the engine specifications required by WEC and FIA?
JLS, Chicago, Ill.

MP: The DPs are built to a set of rules that are unique to America. They do not comply with the ACO’s regulations, therefore DPs, Indy cars, rally cars and all other types of racing vehicles that aren’t built to ACO regulations, are not invited to participate at Le Mans.

Do you see any direct “IndyCar only” technology that can be used in today’s automotive industry? If so, you would think Honda and Chevy would want to promote this.
TJ Spitzmiller

MP: TJ, if anything, they use their IndyCar programs to advance the state-of-the-art on things like direct injection, but at the same time, they also draw from the expertise their road car DI experts have to offer. I can’t think of anything that’s truly “IndyCar only” that’s come from the recent engine formulas and made it onto the production line. It’s not an issue reserved for IndyCar, BTW.

Could you please document for me exactly what data IndyCar mechanics and crew can see on their telemetry systems and how they use said data during a race?
Bob Sobie, Glen Ellyn, Ill.

MP: I don’t know about the “exactly” part, Bob, because every team sets up their own telemetry page to suit their needs. There isn’t a single standard, basically. Teams will monitor a number of performance-related items, a number of safety items, temperatures, pressures, lap times, etc. You’ll also have engine technicians, usually standing behind the timing stand, who watch every imaginable parameter. Thresholds will be set for many of the items monitored via telemetry to act as an alarm – when something is either too low or too high. My eyes used to go numb trying to keep track of everything on the ovals – the laps are so short, it’s a test of one’s ability to focus (or maybe that was just me.)

I asked my man Mark Feldhake from Cosworth Electronics to take a deeper dive into the topic for you – Mark is the primary data rep in the IndyCar Series:

The new-for-2012 Live on Air telemetry system in IndyCar allows for teams to approach logging rates that would be used to store data on the box for telemetry. This has allowed during practice sessions for teams to have high quality data to begin analysis and actually know what the car did on track and be able to make true setup changes that are accurate. In the past, you were using telemetry to get generalizations and mainly used as a safety device (I.E. tire pressures) and as an engine monitor for temperatures and pressures. This change has allowed the normal items for suspension geometry and suspension spring/damper changes to be done before the car comes in. The sensors used to assist in changes would be the damper position sensors which monitor the displacement of the dampers. This can be related with math to the wheel displacement, damper velocity, and much more.

Strain gauges are used on all four corners to assist with setup changes as well. This includes aero and/or suspension. Sometimes during an accident, a team has enough data to diagnose the fault of the crash down to a component just by looking at the logged telemetry data. The team is able to log the telemetry data as it is broadcast wirelessly to the team’s pit stand and then be able to review that data immediately.

Teams are able to use the Infrared contactless tire temperature sensors (which now show the temperatures across the tire with up to 8 different points) to determine suspension setups as well. Tire Pressure Monitoring Sensors allow for the tire pressures to be measured. This is typically used to determine flats and pressure lost, but pressure information is also used by teams for setup changes.

Team’s strain gauge the steering columns, drop links, bump rubbers, and anything else you can imagine. Teams use lasers for ride height but also will fit sensors to check for contact around the track.

Normal driver inputs are also recorded and these are used to help the driver correct or improve their performance. These would include the throttle sensor, steering angle, gear position sensor, and brake pressures.

Synchronized video and data is very valuable as it allows drivers to visually compare driver’s lines with full data as well. So not only does the data trace show a difference with the other driver on the team, but the driver can see exactly where the car is being placed and how they are hitting the curbing or using the full track, or how close they are getting to the wall on the straight at Indy.

The sky is the limit as to how many and what sensors you can put on a car. The challenge for the team is what is going to help them the most and then prioritize their purchasing and time on those that make the biggest differences.

Why don’t they repave Sebring? It seems like the potholes are getting worse.

MP: The short answer is: Because then it would be like any other circuit, and who wants that? Here’s what my man Ken Breslauer from Sebring said when I posed your question to him:

The asphalt portion of the circuit is paved regularly, and will likely get resurfacing in the near future (maybe this summer). The concrete portion (T17, front straight and T1) is original Hendricks Field concrete poured in 1942 and will hopefully NEVER be resurfaced. That is sacred ground and its bumps are part of what makes Sebring the ultimate endurance test.

Your answer to the question in your first Tech Mailbag regarding Memo Gidley’s Rolex 24 crash has led me to pose this question: under a different rule paradigm, is there a place in IndyCar for steel tubeframe chassis? I ask because I would imagine that chassis construction costs would be significantly less expensive if such an approach was allowed. This could have many benefits, including reducing costs and barriers to entry for teams and drivers, increasing field sizes, and allowing for a move away from a single, spec chassis. Granted, cost is only one side of the triangle, the others being performance and safety. I’m curious to hear your thoughts on whether or not such chassis, which are good enough for top-level sports car racing, would be feasible in IndyCar.
Larry in Springfield, MA

MP: Weight is a big concern with smallish open-wheel cars, as is steel’s energy dissipation properties compared to carbon fiber/aluminum honeycomb tubs. Plus, steel hasn’t been used for many, many decades in open-wheel at the top levels. It would be cheaper, but it’s akin to building houses from mud. Yes, it can be done, but there’s no reason to abandon modern construction methods when the tools and materials are available. Now, for prototype sports cars, I don’t see it being as much of an issue due to the larger chassis size. Daytona Prototypes currently use steel frames, for what it’s worth.

When reading about suspension setup I always see toe in, camber, and caster. I get toe in and camber, but not caster eludes me. I understand what it is, but not really what it does to affect handling.

I remember when TK was struggling to make the 500 after 2 crashes in 2010 or 2011, he mentioned prior to a qualifying attempt that the engineer had made some caster changes. What effect would caster have at a place like Indy?
Matt McGowan, Collegeville, Pa.

MP: It has been a long time since I’ve engineered a car on an oval, Matt. I can only give you generalisms as the use of caster has become a bigger tuning tool than when I had a clue as to what I was doing. Also, if that’s what TK said in public, go with it, but there was more to it than that. I’ve asked a friend who engineers a Dallara DW12 (and quite ably, I must say) to give you an up-to-date answer on how he uses it to tune a car and give his driver what’s needed:

Castor will affect a car at Indy in a couple ways. First is steering weight, while remembering these cars do not have power steering and you want to be as smooth as possible, as any movement causes a weight transfer in the car. If the weight in the steering wheel is too heavy he will have trouble turning the car into the corner. In fact, high amounts of castor may induce an oversteer moment as he fundamentally has to jerk the wheel to overcome the load. On the flip side it could induce understeer if he is physically not strong enough to get the steering angle he needs when he wants.

Also, if it’s too heavy and he does have a “moment,” he might not be able to correct in time to save the car as he is incapable of getting the steering rate needed to counteract the moment from the weight in the wheel. You also have to be careful not to have the steering feel too light. Therefore he would not be able to feel the front end of the car while cornering. This could be bad as it might cause him to wiggle the wheel around trying to find where the grip level of the tire is located. The wiggling from the driver could upset the car and cause him to think the rear is never underneath him when in reality the car might have understeer, but he simply cannot feel the front tires gripping to the track.

The other effect of castor at Indy is running different castor settings side to side. Castor does affect diagonal weight jacking of a car. This would have the car rolling into the corner on its own or the driver has to turn the car into the corner. In very simplistic terms, a split in one direction and the driver could relax his grip and the car will turn ever so slightly on its own, a split in the other direction and the driver has to turn the wheel to get the rotation started in the corner. This, from my experience, is driver preference and something you test with each particular one, and this also does change sometimes year to year with drivers.

What do you think about Cosworth and Audi getting together to build an engine for IndyCar, since they are both interested?
Zach, Plainfield, Ind.

MP: I’ve not heard a word from Audi about wanting to build an IndyCar engine, Zach. Their chief racing engine designer has expressed his desire, but that’s a personal statement, not one from Audi.

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