Leaf-spring rear suspensions...
Leaf-spring rear suspensions adapt well to various conditions and are very common in dirt-track racing.
Leaf Spring Science
I have a couple of questions I am hoping you can help me with. I know that rear-spring shackle location in reference to its height above or below the leaf spring's solid front mount can make a difference in roll steer, under both acceleration and braking. I've also seen a sliding shackle used instead of a swinging one. Can someone enlighten me more on shackle placement and design in relation to handling characteristics?
Another question that relates to leaf springs is what are the benefits and/or drawbacks of a locating pin offset from the center of the spring length-that is, the front sections of the spring being shorter than the back half. Chrysler products seemed to use this quite successfully on drag-race cars in the '60s to control rear spring wrap-up. These cars seemed to rise up in the rear during initial takeoff, instead of squatting and transmitting more weight to the tires. Does it make any difference whether the front or rear section is the shorter length?
Jeanne Worthy - Evaro, MT
I am going to assume you are talking about multileaf springs and not mono-leaf springs. With that as a base, here is my answer.
Think of the front half of the leaf spring as being a trailing arm and a torque arm. That is the portion of the spring from the locating pin to the front frame mount. Most of the travel that occurs in this portion of the spring is the result of torque through the rearend housing, because the leaf spring is the only thing that prevents the rear housing from rotating.
The angle of the leaf spring from the rear housing to its frame attachment point at the front is what causes the rear of the car to rise or squat on acceleration. The more the front portion of the leaf runs uphill to the frame, the more lift or anti-squat you have on acceleration. When you raise the front mounts, you do induce roll steer. The amount of steer depends on the length and angle of this portion of the spring.
By stiffening the front portion of the leaf spring, more lift or anti-squat will be induced on acceleration. This stiffer front rate transfers engine torque to the frame faster than a softer rate does.
Think of the rear portion of the leaf spring as being the spring. This rear portion of the spring has no effect on roll steer or the roll center of the car. Notice how the shackles or floaters attach to the frame. They can move front to rear, so they do not induce steer-only the fixed front mount does.
The rear shackles should be mounted exactly parallel to the angle of the leaf spring (when viewed from overhead) to prevent bind. Personally, I prefer the sliders. One of the benefits to this type of mount is that as the body rolls over, the shackles pivot, and this affects the diagonal weight or wedge in the car. The rear mount in this application is used to set ride heights and the diagonal weight.
The point at which the leaf spring contacts the rearend housing determines the roll centers in any leaf-spring car. You measure this point to the ground on each side, then add them together and divide by two. The sum is the rear roll-center height. Measure the distance between the two locator pins on the leaf springs (side to side) and divide by two. This will give you the lateral roll-center location.
The point at which these two intersect is the true rear roll center. When lowering blocks are used, the rear roll-center height is calculated by measuring halfway between the rearend housing and the leaf spring to the ground. In other words, if you add a 1-inch lowering block to the rear, you lower the rear roll center by 11/42 inch.
The front of the leaf spring should be shorter than the rear so it can function as described above. The Chrysler products were among the first to incorporate this design. This design was very effective on dirt late-models in the '70s and early '80s. However, when the front of the leaf spring was as long as the rear, the resistance to torque from acceleration and braking was too violent and caused considerable amounts of wheelhop.
If the questions you asked were to be applied to a mono-leaf-spring car, the answers would be basically the same with a few minor exceptions. The mono-leaf does not hold up the car. That is accomplished by coil springs, usually coilovers. The torque arm usually controls the torque from the rearend housing. Everything else is pretty similar.
Any type of leaf spring used in racing should have some sort of a damper shock-mounted horizontally or both horizontally and vertically (on a torque arm) to the rearend housing to help control wheelhop under acceleration or braking.
The problems inherent to the leaf-spring design, such as lack of roll-center adjustability or rear-steer adjustability, have led to their demise in most forms of racing. I still believe they have a place on limited classes (such as Street Stock) on dirt. They do afford a certain measure of adjustability over four-link passenger-car suspension.
As a side note, I would recommend you contact Landrum Spring Services, (404) 622-9348, a good source for both mono-leaf and multileaf applications.
Mark Tutor General Manager
America Online Racing, Mooresville, NC
Mark Tutor, general manager,...
Mark Tutor, general manager, America Online Racing
Second Gear...Faster, Faster
We race a '79 Ford Courier pickup with a 2.3L Ford four-cylinder. It has a four-speed standard transmission with a rearend gear ratio of 3.76:1.
The problem is that Second gear is high enough to run, but we are at max rpm with the engine about three quarters of the way down the straight. We currently run Second gear, but Third gear is too low to keep our revs up to where we need them.
We run 14-inch tires and have thought about going to 15-inch tires to get more height, but we are not sure if this would help us enough.
Our other obvious option seems to be a gear change. We are unsure whether it would be better for us to run Second or Third gear. What is your opinion, and what ratio is our best option for that gear? We run on a 31/48-mile dirt track with approximately 5-degree banking.
We also need to know what would be a good spring rate for each wheel of the truck. Our truck weighs about 2,000 pounds.
Some further information would help us answer your questions more accurately. So I will give you a broad-brush response that may cover what you are doing.
If at all possible, it would be better to run a higher gear. If this is not possible, your choices should be backward through the transmission gear selection. The reasoning for this is simple. When you are in a higher gear, you are delivering the engine torque straight through the transmission. The fewer number of times you have to reduce the gear ratio, the more torque you have made available to rear wheels.
I do not know the tire circumference you are currently using or the tire circumference you are contemplating using. I made a guess that your current final gear ratio is in the mid-6.00 range, probably 6.56:1. If you increase your rear tire size by 1 inch in circumference, you are making a gear ratio change of about 0.10. This change at this gear ratio range should yield an rpm decrease of about 100 rpm.
If you go from 14-inch tires to 15-inch tires, you are talking about a circumference change that, most likely, will be in the 3.25-inch range. This should give you about a 325-rpm decrease. I think this will be pretty close for a starting point.
Remember that when you make a final gear ratio change of 0.10 (6.56 to 6.45) you are going to see an rpm decrease of about 100 rpm. When you make a tire-circumference change of 1 inch, you will see a change of about 100 rpm. Now as you move through the gear ratio range, the amount of change will increase with higher ratios and decrease with lower ratios. In the range you are running, these rule-of-thumb tips should be pretty accurate.
There are some things to remember. When you change the ratio of the transmission, you must multiply the ratio change in the transmission by the rearend gear. For example, if you have a 3.50 rearend gear and a 1.23 Third gear in your transmission, your final drive is 4.305 (1.23 x 3.50). You change Third gear to 1.35, and you have a final drive of 4.725. The change in transmission ratios is very dramatic on the final-drive ratio due to the multiplication factor.
Now please understand that I made some very broad assumptions. There are many other variables that come into play. These variables include the rpm range at which the engine makes power.
If you have a gear that is so high or so low that you are out of the rpm range of the engine, a very small gear change can yield little or no results. Conversely, if you are close to the gear ratio, a very small change can produce dramatic results. Even the air pressure in the tire affects the final drive. If you've ever seen a Top Fuel or Funny Car leave the starting line, you can understand what I am talking about.
I hope I have helped you.
Mark Tutor General Manager
America Online Racing, Mooresville, NC
327 needs more torque
I race at a local 11/43-mile asphalt track in the Street Stock division with a '78 Camaro. I currently run a 327ci that is my backup engine. I had trouble with the 350. I like the way the small engine responds in traffic and coming out of the corners, but the 350s are killing me during the last half of the straights. Many engine builders have told me the 327 is a very reliable engine, because the shorter stroke is easier on the pistons, rods, and bearings. With my budget, reliability is a priority. Can you offer some suggestions on how to overcome this problem? The engine rules allow only cast-iron heads, intake, exhaust, flat-tappet cams, Holley 4412 two-barrel carburetors, and 11-1 max. Also, the rules give the small engine a 100-pound weight break.
Marc Utheim - via e-mail
The question you ask is interesting, and I would suggest several things. Let's start with your first problem. Basically, you have a lack of torque in a 327 compared to a 350. This can be explained because of the difference in the stroke characteristics of each engine. A 327 has a stroke of 3.250 inches, while a 350 has a stroke of 3.480 inches. This means the 327 will have the ability to accelerate more quickly and obtain a higher rpm than the 350 engine-but with less torque. So this obviously means the 350 will win out when it comes to torque.
Torque is a very important component in making horsepower. Here is a formula you can use to prove this to yourself:
Torque x rpm = Horsepower
As you can see, the 327 engine with less torque will require much higher rpm to equal the horsepower output of the 350 Chevrolet engine.
For the 327 to achieve any similarity in horsepower to a 350 it would, by definition, have to develop higher rpm levels. Doing that, of course, would have some negative ramifications for a 327. For example, the reciprocating weights in the engine (which include the weight of the piston, pin, rings, and pin end of the connecting rod) are increased on the square of the rpm. This produces thousands of pounds of force on the lower end of the connecting rod, rod bolts, rod bearing, and beam of the connecting rod, which are holding everything together. So, by trying to match the performance of the 350 through increased rpm, the stress on a 327 engine will be greater than on the 350.
Now let's move to the next issue. Should you decide to continue to use the 327 and raise the rpm level, you will have a significant problem trying to supply sufficient air to the engine through a two-barrel 4412 carburetor (500 cfm). When you add that to the fact that you are only allowed a stock cast-iron intake and exhaust manifold, the possibility of matching the output of a 350 seems even more remote.
The only advantage you have going for you in this situation is the 100-pound weight reduction, but this is not likely to be enough to compensate for the superior power of the 350. The fact is that on an equal basis, it's usually very difficult to give up cubic inches and at the same time remain competitive.
So far I have given you reasons not to continue using the 327 in your race program, but there may be a scenario in which it would make sense to continue using this engine. For example, if you have a rule limiting the size and compound of the tires you can run, a smaller engine might run better. If a hard compound is required, the 350 engine with greater torque could spin the tire, making it difficult to hook the power to the track. As we have established, the 327 has less torque, and it's less likely that the tires would spin in this situation. Add in the fact that the car is 100 pounds lighter, and this could be a successful environment for a 327 engine. In a case like this, it isn't how much total horsepower you make, it is how much torque is produced over a given rpm band and how well this power band fits your racing requirements. Too much power can hurt a mishandling car a lot quicker than too little power.
I hope this gives you some insight into the potential solutions to your problem. While I believe there are some situations in which a 327 may work well for you against the output of a 350, as a general rule, I think it's safe to say that for best results there is no replacement for displacement.
Crane Cams, Daytona Beach, FL
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