The Mark Ortiz Automotive

CHASSIS NEWSLETTER

Presented free of charge as a service

to the Motorsports Community

August 2007

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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: mortiz49@earthlink.net.  Readers are invited to subscribe to this newsletter by e-mail.  Just e-mail me and request to be added to the list.

 

 

IS IT BAD TO LIFT A WHEEL?

 

After watching the new NASCAR Car Of Tomorrow (COT) run on some short tracks and then at the Sonoma road race it certainly appears that the NASCAR teams have some real development to do.  The Sonoma road race certainly illustrated what I would consider a real problem with the COT and that is the very radical lifting of the inside front wheel during heavy cornering.

 

The three wheeling was not incurred by going over curbs, it was the classic three wheeling very similar to a dirt car with lots of cross weight and plenty of traction and torque.  The lifting of the inside wheel was especially evident in the 8-9 turn complex where the car was carrying good speed and especially coming out of turn nine, which is a flat sweeping left hander, many cars would carry the inside front, left wheel, to the point that they had to brake for turn 10.  Some cars would carry the wheel 4-6 inches off the ground.  Of course once they braked for 10 the left wheel, which is now going to be the outside wheel in 10, would regain contact with the track and of course it is not turning so there would be a pretty good "unsettling" of the car before it was set into turn 10.  I think that the drivers are using third gear for this series of turns so there is pretty good torque available as they exit turn 9.  Having run sports cars at Sonoma back in the '80s, I remember that we did not do anything related to side to side weight distribution and we ran our cars with pretty much 50/50 side to side weight.  Although Sonoma is a clockwise track and is predominantly right hand turns we found that trying to bias weight to the right side of the car would adversely affect the car in the left corners and we ran the "carousel" at that time which was a very long, high speed, down hill sweeping left turn onto a good straight so the right side weight was not the thing to have in this corner.  NASCAR does not run the carousel so it is highly possible that right hand weight would be a advantage in the track configuration that NASCAR runs.  I am not sure if it is legal to run lots of inside weight on the COT.

 

At the short tracks that they have been running these cars on, you can also see the cars lift the inside front wheel as the power is rolled on exiting a turn, which I am sure is very much assisted by cross weight jacking trying to keep the inside rear planted  for a good drive off the corner, but at Sonoma the cars could and would carry the inside wheel in both right and left hand corners.

 

I have always believed that poor chassis stiffness in torsion is a major contributor to "three wheeling" and looking at the COT chassis it certainly doesn't appear to be well braced for torsional loads.  What are your thoughts on the COT as to its attribute of going onto three wheels while corning.  Causes, and possible fixes???

 

It is not necessarily bad for a car to lift the inside undriven wheel.

 

Ideally, we would like to use all four tires equally at all times.  This would involve not only having all four wheels driven, but also having the c.g. at ground level.  That's impossible, of course.

 

With only two wheels driven, and the inevitability of lateral load transfer, we have to make some compromises.  The nature of these compromises varies depending on the rules, the track, and our overall design and setup strategy.

 

If the rules impose the same tire size limit on all four wheels, cornering speed is moderate, and aerodynamic lift or downforce is negligible, we get best steady-state cornering with around 50% static rear weight, and similar overall roll resistance at both ends of the car.  In this situation, we should not lift a wheel.  To win a moderate-speed skidpad competition, we might want such a setup.

 

However, a road race is not a skidpad test.  On most road courses, most of the turns are of modest duration, and are separated by significant straightaways.  In this situation, when cars are nearly equal in power, the race is won on the straights, but the straights are won in the turns.  That is, it becomes very important to have good turn entry and exit speed, and late braking points.  The brakes also have to last through the race, which is a big factor in Cup cars on a road course.  To make the car brake as well as possible, and put power down as well as possible, we need as much static rear percentage as possible.  This costs us some steady-state lateral acceleration, but it gains us longitudinal acceleration, both forward and rearward.  It also saves the front brakes, which are normally the ones that give out first, because optimum rearward acceleration will be achieved with less front brake bias than we would otherwise use.

 

To make a tail-heavy car corner neutrally with equal-size tires at both ends, the front end has to absorb the greater part of the lateral load transfer.  When such a car is accelerating hard both laterally and forward, it my very well lift the inside front wheel.  This does not necessarily mean the setup is bad.  It means that some lateral acceleration has been sacrificed to increase forward acceleration.

 

The COT has front and rear clips that are designed to be more deformable in a crash than was the case in the old cars.  That probably does cost some torsional rigidity as well.  However, if anything, low torsional rigidity makes a car less prone to wheel lifting, at least for a given suspension setup.  This is largely academic, because with a less rigid frame we will normally run more front roll resistance; we will have to, to get the same wheel loads.  If we are comparing flexible versus stiff frames, with equal dynamic wheel loads, there will be little or no difference in tendency to lift a wheel.

 

One thing that does affect the tendency to lift a wheel is the c.g. height, and here there is a substantial difference between the old car and the COT.  I hear that the COT has a center of gravity fully two inches higher than the old car.  I have a hard time understanding where that much difference could come from, but certainly the roof is higher by that much, and I understand the frame rails are at least somewhat higher.

 

Once the front end reaches 100% lateral load transfer, any further increase in roll moment can only be reacted at the rear.  Consequently, the car has much less angular roll resistance beyond the point of wheel lift, and any further increase in lateral acceleration produces a relatively large increase in roll angle, with a correspondingly greater amount of daylight visible under the inside front tire.  It often becomes quite difficult to carry the wheel just a little.

 

I understand that at Sonoma, a car spends about three times as many seconds accelerating rightward as it does accelerating leftward.  That would mean a right-heavy weight bias would be advantageous.  However, I doubt that it's legal.

 

It's not easy for people not on a team to get NASCAR rule books.  They are normally only sent out to people getting a NASCAR license, and the rules are subject to revision and interpretation in mid-season.  I do know a person who works on a Busch team that started out as a Cup team, and he has the Cup rules as they existed at the beginning of the season.  At that time, there was a minimum left side weight for the old cars on clockwise road courses, but none for the COT's.  As of the start of the season, the old cars had the same minimum weight for the left side on road courses as for the right side on ovals: 1625 lb., without driver, out of a minimum total of 3400, without driver.  COT's have to have 1650 right side out of 3400, and there is no rule specified for road courses.

 

Evidently, the plan at that point was to have the old cars run the road races, and that got changed.  With 200 lb. of driver weight, distributed 50 lb. right/150 left, an old car would have 49.3% left for a road course.  If the COT is required to have 1650 left for a road course, it would be exactly 50% left with driver, based on the same assumption for driver weight.  Actually, the driver sits slightly closer to center in the COT, but there wouldn't be any possibility to make the car markedly right-heavy.

 

One thing that would make the car carry the inside front wheel more readily, and higher, in left turns is torque wedge: the effect of driveshaft torque on the chassis.  This tries to roll the car to the right, unload the left front and right rear, and load the right front and left rear.

 

 

HOLLOW VS. SOLID ANTI-ROLL BARS

 

I have a question about sway bars.  Im looking to upgrade the OEM bar set on my 2001 Pontiac Trans Am, and with all the different manufacturers out there that produce many different diameters/grades, there also is the issue of a solid vs. hollow sway bar.  Do you have any recommendations on this?  I'm sure weight is definitely a reason for hollow but my question is, is the performance of a hollow bar close to that of a solid one?

 

For the same outside diameter, a hollow bar is softer than a solid one.  A hollow bar can provide the same stiffness as a solid one, with less weight, but the outside diameter has to be bigger.

 

Other things being equal, a hollow bar has higher stresses than a solid bar.  If the bar is short, or the arms are short, in some cases the bar needs to be solid to avoid stress levels that would cause the bar to take a permanent set or fatigue prematurely.  Short of this point, there is some reduction of longevity with a hollow bar.  The bars on a Trans Am go all the way across the car, and have long arms, so hollow bars should work fine.

 

To give you some idea of what diameter you'd need with a hollow bar to equal a solid bar, if you had a factory rear bar 5/8" in diameter, a " O.D., .060" wall hollow bar would be about the same stiffness.  A " O.D., .090" wall bar would be about 30% stiffer.

 

If you have a 1 1/8" solid front bar, then a 1 " O.D., .156" wall hollow bar, or a 1 5/16" O.D., .120" wall hollow bar, would be about the same stiffness.  A 1 3/8" O.D., .120" wall bar would be about 20% stiffer.

 

If you buy bars by an advertised rate in lb/in at the arm end, be aware that there are two ways of expressing this rate, and not all manufacturers use the same convention.  The more common method is to rate the bar like a ride torsion bar.  That is, one end is moved a known linear amount and the force per inch is computed.  This gives you rate in pounds per inch per end pair: the force when each end moves half an inch, meaning there's an inch difference between the two ends.  Some manufacturers prefer to double this figure to get the cataloged value.  This method gives you the rate in pounds per inch per end: the force when each end moves an inch, meaning there is 2" difference.  This method has the advantage of being easier to equate to a change in ride spring rate.  Neither method is more correct than the other, but you do need to know which method a manufacturer uses, if you want to make comparisons based on rate.