A lot of people think that there is a vast difference between dirt and asphalt race cars. And for the most part, they’d be correct. That is except for the evolution of the setups for both genres. If we study how each has evolved over the past twenty years, we can see a similar pattern.
Way back when I got seriously involved in racing technology, both dirt and asphalt cars ran tight setups to where a lot of load went to the right front tire through the turns. I was fortunate enough to get into the inner circle of both dirt Late Model racing as well as asphalt Late Models and Cup cars as well believe it or not. That’s a long story, but I was privy to some amazing insight.
At places like Daytona, the Cup and ARCA cars used the right front suspension way too much, I know because I was able to look at the shock travels of one of the top teams, a previous recent winner of the Daytona 500.
And the dirt Late Models were carrying the left front tire off the ground more times than not. If that tire is not on the ground, then all of the front weight of the car must be supported by the right front. And the dirt cars were sliding the corners hanging way out. Not so much the Cup cars and asphalt late models.
Some thought went into aero back then for both dirt and asphalt, but not like what has come to be. Both of these types came to know a lot more about how to use the air they were driving through to better advantage.
So, there you have it, things were not very efficient back in the day. But then in the late 1990’s and early 2000’s, things started to change for both dirt and asphalt. If I could explain it in as few words as possible and sum it all up it would be this. Dirt and asphalt teams started increasing the spring rate of the right rear springs, period.
Neither forms of circle track racing use the left front tire as much as was needed in the old days, so to speak. Once they started to balance the setups, the left front tire gained loading and traction. It then started to help turn the car and everything got better.
We have all come to know that if you can get a car to turn, dirt or asphalt, you can always find ways to gain rear grip. And that’s what happened. When the dirt cars stiffened the right rear spring, the cars started to turn much better, they kept the left front tire on the ground, and now they were searching for ways to gain rear traction where before they were tight/loose.
If I could explain it in as few words as possible and sum it all up it would be this. Dirt and asphalt teams started increasing the spring rate of the right rear springs, period.
The asphalt cars started running stiffer right rear springs too and gained use of the left front tire with the car turning more efficiently. They not only went faster but stayed fast longer. Teams noticed that the tires had less wear over the course of a race, they stayed cooler and the lap times dropped off less. Life was good, for those who got on the balance train early.
The coming together really hit a high note, and this might have been a beginning for the dirt teams, when Billy Moyer won both the Dream and the World 100 at Eldora in 1998 using basically an asphalt late model setup. It’s true and although I’ve talked about this in the past, it bears repeating. In that setup, the right rear spring was equal in rate to the left rear spring when most all of the time the dirt teams ran a softer right rear spring.
On asphalt too, especially in the upper mid-west, the asphalt teams ran softer right rear springs until Brian Hoppe kicked some serious butt in 1999 running equal rear, or even stiffer right rear spring rates. The evolution was getting started in a big way.
Now it is common to see much stiffer right rear spring rates in both dirt and asphalt race cars. This forces more load onto the left front tire which provides more front grip and a better turning car. Some teams overdo this aspect of setup and force too much load onto the left front, but that’s food for another story.
And, both dirt and asphalt teams learned that if they could get the front of the cars lower to the track, they could create more aero downforce and more free traction. Sure, aero drag went up some, but was easily overcome by horsepower, especially with the high HP dirt late models. The gain was in faster turn speeds.
If you look at modern day dirt and asphalt Late Models, the nose is on the ground, the cars are turning well and the dirt cars are driving more straight ahead. But there is more to the story. The dirt Late Models, asphalt Late Models and Cup cars are all utilizing bump setups.
Now the technology has finally come together in a way never imagined back in 1997, twenty years ago. Stiffer RR springs, lower noses, cars that turn well, and now everyone is running on bumps. What has the world come to?
So, to summarize my lead-in statements, this perceived vast difference in dirt and asphalt cars setups is no longer vastly different. In fact, they are more similar than different. Sure, the dirt cars still run with the rear hanging out somewhat, but that is mostly for aero flat plate affect that helps push the car towards the infield. And truth be known, some asphalt cars are utilizing the same effect, but you didn’t hear that from me.
I guess the final statement from me about all of this is, what works to help a car turn and what provides more speed is common among all race cars, be they dirt or asphalt late models, Cup cars or even Formula One. Making all four tires work just makes a car faster. And that is why when we present tech on race car suspension and setups, in many cases that tech relates to both dirt and asphalt. And that’s all I have to say.
If you have comments or questions about this or anything racing related, send them to my email address: chassisrd@aol.com or mail can be sent to Circle Track, Senior Tech Editor, 1733 Alton Parkway, Suite 100, Irvine, CA.
Dirt Modified Build
Bob,
I am a 70 year-old former Sprint Car driver who has not raced in over 35 years. As a devout reader of your tech articles, I think your thoughts and ideas could be a great help to me as I design and build a dirt Modified. My single desire is to campaign a totally “balanced” car, preferably a three link, four equal 5″ coil springs of maybe 500# each with 60/40 shocks, same size tires with 15# nitrogen in each, 302 SBF at 650HP on alcohol.
My goal is to work all four wheels WITHOUT lifting the left rear like a four-bar car. A friend who is an aero engineer at Boeing developed a computer model with the above design, using a 5.10 gear ratio on a 3/8 mile track that showed the car to be more than a little competitive. At my age I am more interested in being different than doing like everyone else. Do you think such a car could work? Springs in front, on top, or rear of the quick-change?
Your thoughts and comments would be greatly appreciated. Is it possible to buy a Shaw 4-bar chassis and set it up so the left side does not lift way up? front to rear scale weight 54% rear. Left weight 52%. Before wings I drove my sprint in the corners without sliding sideways. Even with wings I was still able to drive into the corners deeper before scrubbing off speed.
I am a strong believer in balance and gear ratio. Torque not HP is what makes the car go forward. Please do not laugh, this is a very serious desire to race on the National USMTS tour next year. Your help would be greatly appreciated.
Thank you, Dave Reidt
I would never laugh at anyone for their dreams. Many of the teams running the dirt mods don’t really understand the concept of balance. But, on dirt, we don’t always strive for complete balance due to the slick conditions later in the event. But, we do advocate making changes to bring the car from balanced, when the track has grip, to un-balanced somewhat to compensate for the slick track conditions. Running a whole event with the car perfectly balanced could be a problem, but who knows, you can certainly try that.
You stated that you wanted to run four equal rate springs on the four corners? That would not work very well to balance the dynamics unless your rear suspension is a swing arm design where the car feels about half the spring rate. But you were asking where to place the rear springs, so I don’t think that is the case.
So, that said, I’m not sure what your friend the aero engineer did to work out the balance, but running 500lb springs on the back of a three link won’t work to balance the chassis. The front would out-roll the rear and that car would be very loose.
I wrote a computer program 20 years ago that helped racers and myself develop a balanced setup and it has sold thousands of copies to dirt and asphalt racers without any complaints in all those years. I seriously doubt your guy could make that work knowing what I know then and now.
And, most teams have been getting away from high rear percent numbers and going with 52 – 52.5 percent max. The higher rear percent tends to make the car loose from unbalanced polar moment effect.
You should probably mount the rear springs behind the rear axle on both sides. Mounting on top has its problems when the rear end rotates, which it will always do to some extent and then the springs are not straight if mounted on top of the axle tube. And, you can setup a dirt modified, even a four-bar car, to have zero rear steer and not hike the left rear of the car.
In dirt modified racing, mainly the more successful drivers, are driving more straight ahead. When I ran karts on dirt years ago, that’s the way I drove and I could drive off and leave those who wanted to sling the cars sideways into the turns. I agree with that idea. And overall, I agree with the whole concept of your car.
Anti-Squat Questions
Bob,
Thanks for the thought provoking tech article on Anti-Squat. I have gained a better understanding of three-point suspension. Please explain how one figures out the percentage of A/S.
And I know you warned us no to go here but curiosity begs me to ask, if suspension angles were configured to go beyond 100% A/S, would the rear of the chassis rise up under acceleration, then drop under braking?
John
Yes, it would. Here is a good way to look at percent of anti-squat. If you produced a lifting force from the third link angle and rear end rotational force, that would displace all of the rear load, both the static load and that which transfers due to acceleration, then you would have 100% anti-squat.
This is very hard to calculate due to the variables of calculating the rear end rotational forces from engine torque and the loading at the rear from dynamic loading caused by load transfer under acceleration. You cannot do it with just link angles alone. Anyone who says they can is just barking at the moon.
The rear end would lift if you exceeded 100% anti-squat. There is no real advantage to that because there is only so much load to use. You cannot create added load that is not there to begin with by going beyond 100% a/s. If you lift the front wheels off the ground under acceleration, like some sprint cars do, then all of the static front load is transferred to the rear axle and you have 100% load transfer of the front load to the rear.
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