In Dirt Late Model racing the name of the game is all about planting the rear wheels to ge
To get maximum traction on a four-link suspension, mounting the shock and spring on the bi
...and in front on the right takes advantage of the bars to load the springs and press th
The shock mounts to the birdcage on the left side behind the axle. The birdcage allows the
The key to nailing your shock/spring setup is understanding what a four-bar suspension sys
The opposite happens on the right side of the car. Now as the car rolls over its mov
Dirt racing is increasingly becoming a buy-it-and-bolt-it-on sport. One of the few ways to get an advantage over the next guy when it comes to the race car is good suspension tuning. To get the lowdown on getting a Dirt Late Model hooked up with proper shock and spring selection, we went to Jeff Smith of J&J Racecars. In addition to being a darn good racer, Smith sells GRT and Warrior race cars from his shop in Gastonia, N.C. and spends a great deal of time helping racers find the perfect combination for their driving styles.
Circle Track : When tuning for a specific track, what areas of the car are the biggest factors.
Jeff Smith: It depends on how close we are on the setup when we get there. We generally have three areas we go to to tune. We go to the right-front spring, the Panhard bar or the top bar on the left rear (Smith runs a four-link rear suspension). Well change the length of the bar or its mounting height to change the way the birdcage indexes. After that well get into shock valves. Well run the same on the left front and the two rears. Well run the same springs there probably 90 percent of the time. What we change a lot is the right-front spring to affect the car getting in and off the corner. Thats basically how we tune the GRT cars. I think most other cars are probably the same way.
CT: Like most Dirt Late Model racers these days, you run a four-link rear suspension. Please explain the importance of running the shock behind the axle versus in front on the left rear.
Smith: Putting the shock behind the axle on the left rear gives the car more drive off the corner. It also makes it tighter. It does it because of the way the birdcage indexes, or moves, as the car rolls into the turn. As the car turns, it rolls over on its right side, picking up the left side. As the top bar on the left side moves down, it spins the birdcage around the axle. If the axle is mounted on the birdcage behind the axle it actually loads the spring, helping it push the left-side tire down to keep it in good contact with the track. The more pressure you put on that spring the harder it pushes the tire into the ground.
Its the opposite if you have the shock and spring in front of the axle. As the left side rolls up and the top bar pulls the birdcage, its actually unloading the spring. So if we are at a racetrack thats taking rubber and really getting hooked up, and you are able to be on the throttle all the time, the car will start rearing up with the shock behind and get too hard to drive. Then well go to the shock and spring in front of the rear end to calm the car back down.
The rule of thumb is you go in front of the axle tube on the left rear when the traction is there already. Also, putting the shock on the front of the axle will free the car up and keep it from rearing up so much. Its just reducing the traction available to the rear wheels.
CT: Do you switch types of shocks depending on track conditions?
Smith: I run both monotube and twin-tube shocks. For consistency, I stick with just one brand of shocks and springs, Afco in my case.
CT: What factors determine what type of shock you will run and where?
Smith: If Im going to a racetrack that Ive been to several times and I know what I need setup-wise, Ill put my regular shocks on, which are Afco twin-tube units. I like the standard shock the best because I feel they react a little quicker than a gas-charged shock (monotube). But thats just my seat-of-the-pants feel.
If Im going to a racetrack Im not familiar with and Im not really sure what Im going to need, Ill put on a set of double-adjustable shocks. That way I can adjust both compression and rebound individually and not have to be slinging a bunch of shocks and springs around. Its a big advantage to be able to tune your shocks without having to chuck a bunch of units out of the rack. These shocks are also twin-tube, and they are neat because they allow you to adjust in any increment you want, not just whole numbers.
Monotube shocks are more consistent on rough tracks, so Ill switch to the gas-charged shocks if I think the track is going to get rough. With the constant up and down of the wheel, you dont run the risk of foaming the oil with a gas-charged shock.
CT: As a driver, can you tell the difference between a standard twin-tube shock and one that offers adjustability?
Smith: Not really. Lots of times on my double adjustables, Ill set the compression and rebound a half-pound softer than I will run my regular shocks. If I normally go on 75s and 94s, on my double adjustables Ill put the settings at 74.5 and 93.5. Im not saying thats the right thing for everybody, but it seems to work for me in terms of getting the same feel from the drivers seat.
CT: Ever mix shock types?
Smith: Ive played with some monotube stuff on the right rear, then loaded up on the gas pressure just to try to get the car back on the left rear quicker. Ive mostly just experimented with things like that. Its got some potential in some situations.
CT: Some chassis builders now allow you to change rear linkages quickly. Does switching from, say, a four-link to a Z-link require a change in valving?
Smith: Usually, no. Switching the linkage configuration is mainly for controlling rear steer, especially when going from a four-link to the Z-link. You are just flipping the top bar from the front to the back. Now when the car rolls over its keeping the rear end in the same place up and down. The only reason you would change your shock valving in this situation is driver preference.
CT: Give us a guideline of some situations where shock valving is a good tuning option.
Smith: Every situation is different, so its difficult to give hard and fast rules. But a good example is say your car is a little lose getting in and across the middle of the corner. You think its because the car isnt planting its right side hard enough. What you can do is go to your left-side shocks and soften up the rebound a little so that side can rise up faster. Then soften up the compression on the right so it will drop faster. You can go the opposite direction if the car is too tightstiffen the compression on the right and the rebound on the left. You may not have to adjust both sides. Sometimes you can go to just one corner of the car. For example, the left rear has a much greater influence on the car coming off the corner than it does coming in.
CT: What is the one piece of advice you most often find yourself giving to inexperienced drivers and chassis tuners?
Smith: I guess the thing I get most often with people who are still learning is they see what somebody else is running and they hear about things like split valves and soft compressions, and automatically they think theyve got to have the same stuff. They may be way off somewhere else on the race car, but suddenly its the shock thats supposed to fix all the problems. Its tempting to think you can fix about any handling problem with shocks and springs, but you are only covering up a problem, not fixing it. If somebody tells me they need a certain shock combo, my first question is always, Why do you think you need it? The right answer is never because thats what somebody else is running. Youve got to be looking to solve the problems on your car, not somebody elses.
CT: Now for the fun one. Whats the next evolutionary step in Dirt Late Model suspension tuning?
Smith: Some people have begun playing with double shocks on the right rear, one in front of the axle and one behind. The idea is to have zero compression with rebound valving on one and zero rebound with compression valving on the other. Generally, with the double springs you will run half on each spring to total what you would with a single shock-and-spring setup. But then, some people do it a little differently, say if you normally run a 200-pound spring thereinstead of going to a pair of 100s some people will use something like a 110 and a 125. So it isnt a hard rule that you put even weight springs on this configuration.
We havent done enough yet with the double shock/spring setup yet to know exactly what we need with it. Ive tried it a little at some test sessions and found things that I thought were really good. But then when I got to a race and tried to apply what I had learned, the car didnt respond the way I had expected, and it wasnt what I needed.
The double-spring setup seems to be a good idea for people running with a spec tire rule. But then when theyve gone to an open-tire race, theyve always struggled. Ive even struggled trying to run the thing in an open-tire race, because when you can use any tire compound you can get the car so hooked up you are going to run tight, and the double-spring setup makes the car tighter. On a spec tire rule you are not as tight and the setup will help you out. But again, as far as exactly what spring rates and valving work best, we havent quite figured that out yet. It will just take some time to figure it out unless something else comes down the pipe before then.