J-Bar talk with Billy Moyer Jr.
We are going to talk about the J-bar on a race car today. It is sometimes called the pan-hard bar and is located on the rear of the racecar.
It hooks to the left side of the frame and goes across in the shape of a “J,” which is what gives the part its name. The other side of the J-bar hooks connects to the car, adjacent to the drive shaft (pinion side).
We have different options when it comes to a pan-hard bar such as the straight bar, bud bar (also known as the Rayburn bar), and the most popular is the J-bar. We use the J-bar to keep the rear-end of the car in the desired position. The placement of the J-bar controls several settings on the rear of the car.
With it we can set the side-to-side measurement, which allows us to define the placement of the rear-end from one side or the other. The placement of the rear-end directly affects how and where your shocks and 4-bar rods are attached. As you can already see the J-bar can have a major impact on how well or how poorly the car handles.
The J-bar can vary in length from roughly 18 – 22 inches from center to center. We have numerous adjustments on our race car, and moving the J-bar left to right can really change the handling characteristics.
Moving the J-bar to the left side of the car tends to make the car tighter, which creates more traction. Conversely if you pushed the bar to the right it would result in or freer or looser handling car.
A longer – 22 inch – bar tends to stick the car harder in the corners. You have to be careful because this can also create a “push” in certain conditions. A shorter J-bar makes the car react a little quicker but tends to make the car looser. I usually run a long BSB Manufacturing J-bar on my Billy Moyer Victory Race Car. I like the feel of the car sticking harder on entry and try to adjust the car elsewhere if I am stuck too hard or if the car is too tight.
You can adjust the J-bar with the measurement on the frame side or the pinion side. However, it’s just important to be consistent on how you are documenting it. Otherwise your notes might not always match, and you could be in for a big surprise the next time you hit the track.
The higher the measurement from the frame side tends to stick the car harder and make it tighter in the corners. The lower the measurement would make the car turn in easier and have less stick.
You can lower the J-bar on the pinion side in order to tighten the car on corner exit, which gives the car more traction
Examples:
Too tight (The RR won't step out to free up the rear of the car)
If I am at a track like Tri City Speedway (Pontoon Beach, Illinois) – which is known for being a “tight in” type of racetrack – I would take a look at my J-bar placement on the frame if I was too tight getting in the corner. I would lower the J-bar on the frame with hopes of it making my corner entry freer, which would allow me to carry more speed into the corner. Update: 11/2/22 this is a rear tire adjustment not a front tire, if the car don't turn in the corner you may still need to make a front end adjustment to the setup. What you are wanting is the rear tire to step out some before taking a set on the RR or scotching up.
Too loose ( The car is not going fast enough to load RR on entry) If I am at a track like Belle-Clair Speedway (Belleville, Illinois) – which is known for being a “looser” type of racetrack – I would think about dropping my J-bar on the pinion. It is a very low speed track, and you need all the scotch you can get. If I drop it on the pinion, it would tend to give me more scotch and allow more traction since I am stuck harder in the center of the corner and on exit.
A standard setting for most J-bars would be 8-9 inches from the bottom of the frame to the center of the J-bar heim. The standard setting on the J-bar would be the center of the pinion. Update: Lets start at center of pinion with a 6" to 7" split on mounting points and tune the split to the speed of the track.
Here is a quick rule of thumb we have used over time when it comes to moving the J-Bar. Frame side for entry, up to tighten and down to loosen the car on gas, and on the pinion side on exit.
Hopefully this article gives you some insight on exactly what a J-bar is and its function on a race car. Thanks again to people in racing like Billy Jr and his dad for all they have done, we are going to add to this as a on going blog about the j-bar and how it works. Many ideas into its function and some that are opinions and not fact but still make sense because of the unknown about the rear roll center and its migration in the car.
Update: 5-4-20
With the growing amount of unknown info about the function of a j-bar lets look at a couple of basic ideas in how it works. The reason I say we are learning less is because we have more input the subject due to technologies which is creating theories not facts. So, with out adding to them lets agree on these few facts.
· If we move the whole bar up, we will free up the car.
· If we lower the whole bar down, we will stick the right rear tire harder.
· Pinion swing will move the rear end to the left making the car tighter on gas.
· Big split in the j-bar will move the rear end more to the left under gas.
I would like to see use work on a basic plan to develop a set split amount in the J-bar and move it up and down to tighten the car, instead of just changing one end. I know it is more work, but I feel the outcome will be better for the car. Each chassis manufacture should have a basic set split for their car based on rear roll and weight movement. Than we could move the bar up or down to determine how hard we want it to load the right rear tire. This is a very deep subject with a lot of engineering going into it and we can't get to the bottom of it any time soon, but let make it as simple as we can and learn a little at a time. I will come back to this and update it over time so we can have a better data base on this subject.
Updated 7/20/21
Some of the J-bar movement is going to come from weight placement in the rear of the car, the higher the weight the easier it will move. When you bolt weight on a car in the rear it tends to be higher than a car with weight in it, so that will move easier and may take a different J-bar placement. The one size fits all will not apply here for sure. I do think a chassis manufacture could tell you where they need to be with there cars because they are more alike, so this would be a good starting point. Again, here are some basic starting points for a dirt car with more of a crate type motor or a car that turns in on gas. A modified with a 604 motor and 54 % left side weight would be 1” to 1.5 inch above pinion on a quick-change rear end and center to a 3/4” above on a 9” ford. On the frame side start with a 6” split from pinion as shown below.
Checking height of the J-Bar
Note: the shape of your J-bar will not change how it loads the RR tire, it is a point A and point B device. The design will determine how much load is on the rod ends and how fast they wear out or bind.