Race Car Stuff

The Mark Ortiz Automotive
CHASSIS NEWSLETTER
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March 2006
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WELCOME

Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: markortiz@vnet.net. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.

DIFF DIFFERENCES
Which differential would be best for road racing, in my 300hp Porsche? The question arises because a supplier suggests that a plate-type limited-slip is better suited to road racing than a worm gear style torque bias diff. I have used both and found Iliked the torque bias diff. I thought it was a better design, from what I read. The supplier states that the lsd will be better in corner entry and exit. What is your opinion?

Clearly, both worm gear and clutch pack differentials have their adherents, and both are used successfully in racing. You say you have experience with both types, and have already formed a preference. The most obvious answer would be that you’ve already answered your own question, and don’t need advice.

However, the situation is actually a bit murkier, because the behavior of both types of differential can vary according to design and tuning details. Both types are similar in that they generate a locking torque in response to the total torque being transmitted. In both types, the locking torque depends on pressure angles. In a ZF-style clutch pack design, the angles are those of the ramps on the spider shaft and the housing halves. In a worm gear design, it’s primarily the helix angle on the gear teeth, and secondarily the pressure angle of the tooth profile. Lubricant choice also influences behavior.

Consequently, all clutch pack diffs don’t act alike and neither do all worm gear diffs. A lot depends on how a specific example is tuned.

That said, the clutch pack design probably offers a greater range of tuning options, and probably greater wear resistance. With the worm gear designs, we are trying to make gear teeth act as a friction device. Clutch discs are designed to be a friction device; gear teeth can be made to act as a friction device, but they are less comfortable in that role.

This affects the ability of the differential to maintain consistent properties over time, and its longevity.

The pressure angles determine how rapidly locking torque builds as transmitted torque increases. The preload in the diff determines how much locking torque there is when no torque is being transmitted. A clutch pack is easily preloaded, and it maintains its preload relatively well, especially if the preload is applied by springs or some other compliant system such as dished clutch plates. Worm gears can also be preloaded, but because they are not very compliant, the preload rapidly goes away as the teeth wear.

One limitation in worm gears is that the pressure angle is generally the same for forward torque and rearward torque (as when engine braking, or when transmitting brake torque from a single rear brake, as seen in FSAE cars). In a clutch pack diff, we can use different ramp angles for power and decel.

Another peculiarity of worm gear designs is that because power and decel apply force to opposite sides of the gear teeth, preload doesn’t have identical effects in both directions. If we preload the gears in the direction they’re loaded under power, what happens under decel is that we have diminishing friction with increasing reverse torque, until the preload is overcome, at which point locking torque is zero. As reverse torque increases beyond that point, locking torque builds again. With a clutch pack, preload has similar effect in both drive and decel modes.

This does mean that we can make a worm gear diff act different in drive and decel, but not in a manner that’s independent of preload.

One interesting, though uncommon, trick we can use in a worm gear diff is to use plain thrust washers to absorb the thrust of the worm gears in one direction, and needle thrust bearings to absorb the forces in the other direction. This can afford us some limited measure of difference in friction depending on torque direction. Last year’s North Carolina State University FSAE car had a diff like this.

It will be clear, however, that using these tricks is not nearly as straightforward as varying the ramp angles in a clutch pack diff.

Finally, neither option is ideal, because neither is speed-sensitive. Both clutch pack and worm gear diffs rely on Coulomb friction, which is largely dependent on normal force and not speed. We would rather have the locking torque vary with the speed difference between the wheels, either entirely or at least in part. This argues for either a pure viscous limited-slip, or a design that uses a pump, driven by relative ouput shaft rotation, to load a clutch pack, or a design that combines viscous effects with a clutch pack.

It was nice to see Jeff Burton do well and win Dover. Seeing him and Matt Kenseth fight for the lead reminded me of Dale Earnhardt and Mark Martin doing the same thing somewhere around 1992 at the Dover fall race. It was a real pleasure to see. It was also nice to see the two of them express thanks for the good race after it was all over. They were the only two who enjoyed it more than the fans.

Tony Stewart apparently commented about how the Chase has become a contest of who can do the best job of avoiding problems during those ten races. I think the results of the chases so far have pretty much proven him right. For what it is worth, those were my comments when I first heard about the Chase for the Cup. No matter how it is structured, it will still be the same contest of avoiding accidents, mechanical gremlins, and pit lane mistakes. Tony knows that he won his championship without a win during the Chase. Kurt Busch only won one race during the Chase in 2004 when he won the championship. So thus far there is only one Chase win by the eventual champion.

I find it very interesting that none of the “experts” are favoring Burton to win the Cup, even though he is the point leader now. Most are still picking Harvick; even though his history as a driver is that he is very inconsistent. RCR has gotten things together this year, but I still have a hard time thinking of Harvick as the favorite. Jeff Gordon is somewhat silently right there and I know he would like to add another Cup to his trophy shelf. My personal favorite would be Matt Kenseth, but I surely would not bet a mortgage payment on it. I think that Kyle Busch is the only driver that should be considered out of it. Even Kasey Kahne could win the thing.

Anyone remember that Jimmy Johnson lead the points for the first 25 races of the season?

I doubt anyone is looking forward to the Car Of Tomorrow being thrown into the mix of the chase races next year.

Most viewers of Speed Channel have probably caught what looks like a tacky low budget show called “Back In The Day” as it comes on or goes off. Few have probably taken the 30 minutes to watch it. The show looks very low budget, and it probably is. It has Dale Earnhardt Jr. introducing the show and leading in and out of the commercials.

In spite of the poor quality of the show, it is something that every race fan NEEDS to watch. Back in the late 60’s and early 70’s we did not have cable TV. There was only ABC’s Wide World of Sports that occasionally showed races in tape delay format spliced in with an afternoon of figure skating, or Olympic weight lifting. There were very rare occasions that we would see some live coverage.

During this time there was a Saturday afternoon show called Car and Track (wonder where they got that name???). They mixed in some test drives of current performance cars and film coverage of various races. There was coverage of NASCAR Grand National as well as other forms of racing. The coverage by todays standards was poor, but it was all we had at the time. There was a guy named Bud Lindeman that went with a friend, a microphone, a tape recorder and two handheld cameras. If you didn’t live near a race track, this was your only way to see many of these old vintage cars in any form other than still photos in Competition Press magazine.

Today Dale Jr. seems to have a real respect for the racers from this era. It seems to be more than lip service in front of cameras. He has put the old NASCAR footage from Bud Lindeman with some info about the era done in “Pop Up Video” format along with some introduction. It is not great TV, but it is fantastic to see these old races, cars, crews, and drivers in action.

You get to see pit crews in short sleeve shirts, drivers in open face helmets, gasoline spilling everywhere as the gas man twists off the gas cap, and Bud marveling at 25 second 2 tire pit stops. It is truly a window into the past. This show just can’t stay on Speed for very long, so catch it before it is too late.

I have been thinking quite a lot about how the face of NASCAR racing is going to change next year. There have always been such nice people who fill the grid at a NASCAR race. Back in the 70’s there were many unheard of racers like Bill Champion, Blackie Wangerin, J. D. McDuffie and literally hundreds of others. I often had the chance to meet many of these people over the years, and I always pulled for them…. not to win the race, but to run well, finish well, and hopefully make enough to be able to continue their racing efforts.

I realize times have changed and there are huge opportunities in NASCAR these days. The success of NASCAR has really spread around to where there are three top level touring series that are funding many teams, many full time mechanics, technicians, publicists, etc. This is all good, great actually. But something is missing from being able to see J. D. McDuffie standing in the infield at Darlington after qualifying 41st, and being thrilled about making the race.

Now NASCAR has a new rule that gives a sure starting position to the top 35 teams in points. Being in the top 35 all season is really worth somewhere around $3-4 Million in prize money for the season. That is a serious chunk of change no matter how you look at it. That can make a huge difference in how a team approaches a race.

For 2007 there are several new upstart teams that will be fighting for starting spots. I forget the number of new Toyotas that will be hitting the track. A couple will be in the top 35 for the start, but there will be quite a few fighting with a few other new cars, and enough existing cars that, there are going to be several well funded racing teams going home from every race. I unfortunately don’t see any room in any race for drivers like Morgan Shepard, Carl Long, or Kirk Shelmerdine. There is going to be no room for a new driver on a new team to race their way into a cup race.

The part of this that upsets me the most this week is realizing that a blown engine with 10 laps left at NHIS last weekend dropped Kyle Petty to 36th in the points. At 46, Kyle is in the later years of his driving career. He has had a good career with many wins, but he will always be overshadowed by being the son of the winningest driver of all time in Grand National/Winston Cup/Nextel Cup/Grand racing.

Kyle’s on track performance has definitely suffered somewhat by all the things he has done for other people the past 6 years since the death of his son Adam. He is one of the most liked drivers in the garage area. He works very hard running Petty Enterprises, Victory Junction Camp, and the #45 car around the racetracks. He has benefited from running soundly in most of the races all season long. Unfortunately with the mediocre performance of the #45 car, the blown engine dropping him to 36th in the points, and the influx of new teams making qualifying for the eight available positions outside the top 35, I see a possible end to Kyle Petty’s driving career. It really bothers me that he may be forced out, rather than being able to end his career as it should, by his own chosing.

2007 might sadly be the first NASCAR season ever (since its inception in 1949) without someone from the Petty family competing. Maybe this is our last link back to the days of finding one of the drivers out in the infield leaning against the fence, happy about their 41st starting postion.

It was a somewhat exciting last couple of races leading up to the Chase For The Nextel Cup. I am not a fan of the concept of the chase, but it is with us now and will probably be revised next year. So much for consistency.

There are seven Chevrolets, two Fords and a Dodge in the Chase. In the past whenever Fords or Dodges have shown this sort of dominance, NASCAR has adjusted the rules to either slow them down, or give advantages to the Chevys. Anyone wonder if there will be adjustments this year? I seriously doubt it.

After week one it looks like Kevin Harvick is the car to beat. But there are 9 more races. The first two races are where the Chase has been lost in the past. It can also be fairly said that it was the place the Kurt Bush won the Chase in its first year, and from there he held on. It is safe to say that in the past two Chases that the strategy was different for each of them. Most of the drivers in the chase will probably have one bad race. All it takes is form someone to finish all of the races in the top five or ten positions and all of the strategy is out the window. But in the past nobody finished without any problems.

Every investment advertisement has fine print at the bottom that says that past performance is not guarantee of future performance. So what has happened before might be a good guideline, but anything can happen. After week one, any of the ten drivers is still in the hunt.

I still think that Matt Kenseth is my choice of someone to watch in the Chase. He has problems with the first two races and each of the past two seasons. He got out of NHIS with most of his points. If he can do the same in Dover he could be very strong through the remainder of the Chase. He is very strong on the 1-1/2 mile cookie cutter tracks, and there are several remaining.

As far as Dover. This could be the race where one of the “Grizzly old veterans” could be dominant. Watch for Mark Martin or Jeff Gordon to be strong. Joe Gibbs Racing seems to have the setup for Dover and Tony Steward knows how to muscle the car around the track. I would list any of these guys as favorites, but I must keep open the option to change my mind after practice.

Don’t forget that Harvick is still on a hot streak. He should run strong at Dover, but it is a track where mistakes can really bite.

More of an attempt to get caught up on things, here is what I did a couple of weekends ago.

The challenging Shenandoah Circuit at Summit Point Motorsports Park was the setting for the MDA driving school. I was scheduled to be an instructor for this event driving my tired old first gen GSL-SE RX7. As usual something in my life altered my initial plans for getting myself to the track. This time it was the workload at my real life place of employment that was putting my schedule in a crunch. I knew that I would not make it to the track to drop off the car and trailer on Friday night like I had planned, so it was a drive straight to the hotel and get to bed early. There is something about the drive to Summit Point that simply relaxes me. I can feel my blood pressure lowering as I get in the Winchester/Berryville area. I can tell this is the place I belong.

The next morning at the track I had to rush a little getting the car unloaded and the site set up. Then off to the instructors meeting. There is nothing unusual, with the usual emphasis on safety, attitude, and enjoyment. MDA makes a big deal of student to instructor introductions. Then there is a meet and greet time where the plan is for the instructor to learn how worried to be in the passenger seat.

I had the usual two students, Jonathan and Steve. Steve has been to a few driving schools and is preparing an RX7 to race in the Spec RX7 class in SCCA racing. Jonathan tells me that he is a little bit of a daredevil and that this is his first time on a racetrack. For future reference this is a good series of sentences that will make an instructor nervous. But then he said the important thing. He said that his wife wanted to drive the Miata he was driving this weekend to work Monday morning. Whew….. It is probably going to be OK.

It has been a year since I have been on this racetrack, so I have one session to get my lines back, remember all the braking points, etc. to teach my students. The session was uneventful for me, but it is cut short because of another instructor’s car going into the tires at turn 14. I think I have things together well enough that I can teach the track.

During the next session (which I don’t have a student) another car goes into the tires. What a way to start the weekend off. I take an extra few minutes with Steve and Jonathan talking about “things”. I want to make sure they have the right attitude about the first session.

Steve and I go out first and he has some things to work on during the event. All students are intimidated by the Karussel (a replica of the Karussel at Nuerburgring) but they learn that after the first session on the track, there is nothing to it and they pretty much forget about it the rest of the weekend. Steve is no exception, but I am bothered by his RX7’s rear setup. It is bottoming on the banking, the one thing that could lead to control issues as he goes faster. I identify the areas Steve is going to have problems with during the weekend and plan how to work with him.

Jonathan tries to run his first lap faster than I am comfortable with, but on his own he realizes that it is probably a little too quick for him to learn anything. He quickly settles into a driving pace that is fine for his experience, without me suggesting it. The rest of the session is uneventful but he seems to listen to advice and try what is suggested. After the session he asks many questions. This makes me realize that he definitely has the right attitude for this event.

During the rest of Saturday there are a couple of other incidents and several spins. This is a very tough track to learn and it appears that both students and instructors are having problems with it. Shenandoah is the most challenging track I have ever driven. The overall average speed is fairly low for this track since it is so tight and twisty. I think it is the ideal track for this type of high performance driving seminar.

I gave each of my students a ride in my car and rode with them the rest of the day. Both made huge improvements. Their cars were both so much faster than mine that it was embarrassing. That is something that is going to have to change soon in the future. By the end of Saturday I was not ready to sign either of them off to solo, but they were making progress.

Back at the hotel I decided to watch the Richmond Nextel Cup race on television, but I lasted about 10 laps before I fell asleep. Did I mention that the previous couple of weeks had been very stressful and I was now able to relax? Problem was that I had not set an alarm or left a wakeup call. So I am late getting up, but get out to the track in time to make the meetings and warm-up. The first thing I find out is that the schedule had been changed to start everything later. So with extra time, I spent it talking to my students some more about attitude. But this time I am stressing how to get quicker by being more efficient on the track. Steve had video taped both in-car and from his friend Mike’s shoulder outside the track. He had spent Saturday evening watching all of the videos and learning from his mistakes.

My morning session is my best one. I finally am able to run consistent laps and work towards being quicker. This was my first time trying out Falken Azenis RT615 tires. If you are not familiar with the tires, they are one of the 2 or 3 tires that everyone is using in SCCA Solo II as tires for classes requiring true street tires, as opposed to DOT race tires. I have been using the DOT race tires for a while now, and it took me a while to get used to the step backwards. But in this session I was finally getting things together between the car, the track and the tires. Jonathan had gone out with the chief instructor (who also was driving a Miata) during this session.

Both of my students drove like new drivers during the first session on Sunday. They were both driving more efficiently, letting the car and the physics do more of the work. They were understanding, using more of the track and not fighting the car.

One of the interesting things MDA does is they use the skid pad at this track. It is a large circle that gets watered down with sprinklers. It is used to teach how the car responds to various control inputs. Steve and Jonathan each spent time on the skid pad learning the basics of car control. They had different instructors on the pad but seemed to have learned quite a bit. This was a very beneficial exercise for them both.

My car started suffering from fuel starvation during the next session. It upset my concentration enough that I decided to call it a weekend. I signed Steve and Jonathan to solo but gave them the option to keep in the car if they wanted. They each wanted me in the car until the last session. They were now each one of the fastest cars in their respective sessions.

For the trip home, Steve and I caravanned most of the way together. This turned out to be a good thing as I developed trailer-wiring problems. Fortunately between Steve, his friend Mike and myself we figured out the problem and were able to continue. I owe thanks to Mike and Steve for helping me get home. I am sorry I made you guys later getting home.

MDA and Shenandoah make for a great combination for an enjoyable and safe weekend.

Much has gone on in the world of racing since I last wrote an update. Work has kept me busy in other ways. I have barely had the opportunity to catch up on the racing that has gone on. I did have the opportunity to take in the Mazda Driving Associates High Performance Driving Seminar on the Shenandoah Circuit at Summit Point Raceway. Here is what as happened the past couple of weeks.

MDA Track School

I attended the MDA High Performance Driving Seminar last weekend. It was nice to get back on a racetrack. The event was the usual well run event put on by Dave Nugent and the rest of the MDA staff. The very first sessions had a couple of incidents of sheet metal into the tire barriers but after everyone got their nerves settled back down the rest of the weekend was incident free. I had two great students, Jonathan in a Miata and Steve in an 83 RX7.
Jonathan initially gave me a little concern by claiming that he was a bit of a daredevil and this was his first time on a racetrack. But one thing made me realize that he had his priorities straight. He told me that his wife wanted to drive the car to work on Monday morning. Steve on the other hand had a few track events under his belt and was building a Spec RX7. They both turned out to be great students, both making great progress over the weekend.
I had some good sessions on the track. I was using a new set of Falken RT-615 tires. These tires have been touted as being one of the best true street tires for solo and track events. It was difficult to return to a true street tire after spending years on various types of racing rubber. But overall it was a great weekend, thanks MDA staff.

Michael Schumacher Announces Retirement

It had to come sooner or later. It is difficult to make the argument that Michael is not the best Formula One driver of all time. Some claim that Michael did not have the strong competitor that Senna and Prost faced in each other. Others claim that Fangio and Stewart raced in totally different eras, but the argument is still tough to make.
He may have been the reason that many Americans have lost interest in F1 over the years. But he holds every major record in the sport including two more world championships than anyone else.
It will be a sad day when the first race is run without Michael on the starting grid.

NASCAR Sets the Chase

The format of the Chase for the Cup in NASCAR Nextel Cup racing it not my personal favorite. I am in the camp the prefers the old scoring method. But I admit that it generates much excitement allowing NASCAR to compete fairly against NFL Football for the sports fan’s attention.
Going into the last race of the “regular season” there were 11 drivers hoping to get one of the 10 spots in the chase. Tony Stewart was the driver that had a bad enough night at Richmond to be eliminated. He had gone from the 2005 champion to missing the playoffs in 2006. Much is being made of his faltering, but it is no big deal really. He had a bad string of races and barely missed. He will be back next year.
Seven of the ten cars in the chase are Chevrolets. When Ford teams showed this sort of dominance, there were performance adjustments made that slowed the Fords down and made the Chevrolets faster. I doubt there will be any adjustments now.
The one surprise was that Matt Kenseth is the leader going into the Chase. He seems like an odds on favorite right now, but it should be pointed out that these races have proven to be very tough for Kenseth the past three years.

More to come later….

This is the big debate that all of us racers and racing fans have to debate. Ford is being brutally honest in the previous episodes of this series about the “remaking of Ford”.

It seems like they are truly committed to racing. Of course to see this and and the other episodes check out their site:

FordBoldMoves.com