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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: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
SPRINT CAR ON A ROAD COURSE?
I'm a pavement sprint car guy that went road racing this season for the first time (American Sedan) and have completely fallen in love with road racing. I want to marry the two disciplines so to speak. I want to take a sprint car and make it a road racer, at least as a decent track day car. At our track here in S.E. Michigan, Waterford Hills which is a short road course at approx. 1.45 miles, one can get away without having a gearbox. One of the guys I race with is geared to never shift out of third once we go through the rolling start. So I’m not worried about putting a trans in the car for this. I think putting a quick change late model rear end with a diff in the car would solve the right turn dilemma at the rear, along with taking any offsets out of the car (make a new centered up motor plate, etc...) My question is when making a new front axle is there anything I can do to make the car turn better with the beam axle other than making sure it’s "straight up" as well? The car will be an older pavement car, 90 inch wheel base with a 45 inch measurement from the rear axle to the back of the engine (front of the motor plate), which is a Beast chassis measurement for good pavement handling.
Like I say, this is mostly for fun and to see a sprint car out on a road course, but it would be nice if the beam axle can work decent like it does on the pavement, so the car can be really fast! Of course, here we won’t have the rear stagger and offset to help us in turning.
Any input would be appreciated....
Also Mark, would you use two right rears for the rear tires or two left rears? Would using LR's let the car turn in better because of less rear grip (smaller overall contact patches)?
There is some history of open-wheeled oval track cars running successfully on road courses against cars intended for road racing. One famous instance was Rodger Ward’s 1959 Formula Libre win at Lime Rock in a midget. Here’s a link to an article by Chris Economaki about that race:
This really was a remarkable feat. The midget was 11 years old at the time and had run about 1000 races. It not only had just 1500cc and one gear, but also had no front brakes. It was up against sports cars with much bigger engines and even a Maserati 250F Grand Prix car. Most of the sports cars were front-engined, but there was also a Cooper Monaco, which is rear-mid-engined and has a 2.5 or 2.7 liter Climax.
Much of the credit has to go to Ward’s driving, but also the track was conducive. Lime Rock (https://www.google.com/maps/place/Lime+Rock+Park+(Main+Track),+Salisbury,+CTemail@example.com,-73.3877439,1125m/data=!3m1!1e3!4m5!3m4!1s0x89e7819ab026bccd:0x361ae2b0b3cccca4!8m2!3d41.9283497!4d-73.3813924?hl=en) is relatively small but is a momentum track with mostly short straights and not a huge variation in car speed compared to many road courses, and all turns but one are the same direction. They’re right turns, not left, but it would be possible to use tire stagger on a locked rear axle (bigger tire on the left, for right turns) and only have locked axle push on the one left turn. I have no idea whether Ward’s car had any tire stagger, but such a strategy would work better at Lime Rock than most tracks.
According to the Economaki article, Ward had also entered a midget earlier that year in the first US Grand Prix, which was held at Sebring. Sebring (https://www.google.com/maps/place/Sebring+International+Racewayfirstname.lastname@example.org,-81.3571794,1342m/data=!3m1!1e3!4m5!3m4!1s0x88dc5ab1c2e40a09:0xe9faf90eadde9e62!8m2!3d27.4539057!4d-81.3523943?hl=en) is much more a stop-and-go track. It has long straights and some tight turns, and plenty of turns in both directions. The midget did not do well there against F1 cars.
I’ve been to Waterford Hills (https://www.google.com/maps/place/Waterford+Hills+Road+Racingemail@example.com,-83.3935987,556m/data=!3m1!1e3!4m5!3m4!1s0x882497a38fc105a1:0xdca3c426a3d836e!8m2!3d42.706563!4d-83.3963667?hl=en). In 1991 I lived in Warren, Michigan and attended some events there. It looks small when you’re there. There is a back straight down the east side, with a fairly fast turn leading onto it, but it’s only about a sixth of a mile long and has a fairly slow turn at the end. The rest of the track is mostly turns, and none of them are really sharp. The track runs clockwise, so right-handers predominate, but there are enough left-handers so the car has to turn left well.
So the questioner is probably right that the car should be able to do well with just an in-and-out quick change, as long as this is combined with a diff. This would be an unusual rear axle, but I don’t see any reason you couldn’t put one together. It would have the usual sprint car torque tube and center section, with probably a Winters Track or equivalent worm gear diff and closed tubes. Birdcages could be specially made to fit the closed tubes and pick up the existing trailing links and torsion bar arms.
The car will need two rear brakes. I would mount the calipers to the axle tubes, not the birdcages. If they are on the birdcages, anti-lift in braking will vary a great deal as the suspension moves.
Beam axle front ends work just fine on road courses. I’d consider giving both the front and the rear wheels a little static negative camber – probably no more than a degree.
It isn’t really necessary to have everything centered in the car. It’s just necessary to avoid having the car left-heavy. For a clockwise track, the car might even be fastest if it’s a bit right-heavy. If we’re replacing both front and rear axles, we can get any right or left percentage we want by moving the wheels to the desired location with respect to the rest of the car. There should be no need to move the engine with respect to the frame.
The car probably should have dual master cylinders, with a balance bar. Finding room for that might entail moving the engine over, but hopefully not. I also recommend running a proportioning valve in the rear brake line in addition to the balance bar. This allows the car to have more rear brake on gentle application than on hard application. This is particularly helpful when it rains – which happens a lot in Michigan.
Ideally, the torsion bar arms and the shocks should be the same distance inboard from the wheels on both sides of the car. This might call for cranked or offset torsion bar arms. However, a little asymmetry here probably won’t be disastrous.
The spring rate at the end of the torsion bar arms should probably be the same, or very nearly so, right and left. This calls for unequal size torsion bars, because the bars are transverse, one behind the other, making the right and left arms about two inches different in length.
Sprint car torsion bars are stamped with a “rate” number that doesn’t actually indicate their rate; it indicates the diameter of the active part of the bar in thousandths of an inch, if they’re solid, or the diameter of an equivalent solid bar if they’re gundrilled. The rate in pounds per inch at the arm end varies inversely with the square of the arm length and directly with the fourth power of the bar diameter. This means that to get equal rates from two bars with different arm lengths, the effective bar diameter needs to vary with the square root of the arm length. The numbers on the bars need to be in a ratio equal to the square root of the arm length ratio. For example, if the arms are 12 inches and 14 inches, the number on the bar with the 14 inch arm needs to be bigger by a factor of the square root of 14/12, or about 1.08. If we have a bar stamped 1000 on the 12 inch arm, we’d want a bar stamped about 1080 on the 14 inch arm.
Regarding tire sizes, for this application we are not constrained by any rules. We don’t have to run tires intended for sprint cars. We probably want the biggest tires we can fit on the rear. The front tires should be whatever size gives an acceptable understeer gradient when the car is set up so it just barely lifts the inside front wheel exiting a right turn. Unless we plan on staying off the track when it’s wet, we’d like tires we can get with a rain tread, although the rains don’t necessarily have to be exactly the same size as the drys. For enjoyment on track days, we probably want a compound that is not highly temperature sensitive. However, the highest coefficients of friction come from tires that need to be hot (but only when they are hot).
If the object is just to have fun, we might want to run without wings, but if the idea is to go fast, we want wings. Probably it’s no news to anybody anymore that wings really pay off, and on a slow, short track they need to be as big and aggressive as possible because drag is not much of an issue.
If you doubt how much wings can do for a sprint car, watch this: https://www.youtube.com/watch?v=JHukIQ7-V20. This is a Super Modified feature at Madera, California, this year. There are two winged sprint cars mixing it up with the Supers. The faster one finishes second. Really, it’s the third fastest car; one of the Supers blows an engine while leading. Still, it’s a really impressive performance for a car that doesn’t have the low, radically offset construction of a Super. The sprint car is not only competitive on lap time but isn’t losing any ground in the turns. If anything, it’s losing ground at the end of the straights.
The sprint car is probably lighter than the Supers but both types of car use similar materials and construction methods. I think one big reason the sprint car is competitive is that it has a bigger main wing. The wings on the Supers are multi-element but are constrained by the rules to a smaller size than those allowed on sprint cars.
For a track day toy, we can run any wings we want, so we’d want to put as much wing on the car as possible. There’s no reason the wings couldn’t be passively moveable, as on east coast Supers – mounted with air struts so the wings flatten out as air speed increases. Both front and rear wings could be like that. However, for Waterford the wings could probably be fixed, with little penalty. We’d definitely want the main wing to be adjustable fore and aft as on most sprint cars.
The side plates on the wings should be symmetrical rather than staggered as on a left turn car. They should be big.
The front wing doesn’t need to be small and on top of the nose. For inspiration, check out what wins in A Modified autocross competition: http://www.gotcone.com/pgallery/images/2010_scca_tirerack_solo_nationals/am/img_0209.jpg
Note that this car not only has the biggest multi-element front wing the driver can see over but also has skirts on the sides of the car that keep air out from the underside of the car. There’s no reason a sprint car can’t have this stuff. We do need to get air to the radiator. It might be desirable to relocate the radiator. It might work to have one on each side, fed by sugar scoop shaped side pods.
And if there are no rules, powered downforce is legal too.
Of course, the more we pursue downforce, the less the car visually resembles a sprint car. But if the idea is to go fast on a short track, there are no rules, and the car has lots of power, downforce offers the biggest available bang for the buck.