J-Bar Theory

[Twenty-Five]

Was hoping someone could give me some insight on j-bar adjustments. Theres a few different attributes of the j-bar we can change so im wondering, when do you move the chassis side vs. rear-end side, when do you make an angle change and when do you move the entire bar up or down?Thanks

[RaceEngineer]

Was hoping someone could give me some insight on j-bar adjustments. Theres a few different attributes of the j-bar we can change so im wondering, when do you move the chassis side vs. rear-end side, when do you make an angle change and when do you move the entire bar up or down?Thanks

Overall height change is roll resistance change. Lower means more roll. Angle change means more downward load on the rear tires when the bar is carrying lateral load.Not really a good adjustment, per se. The front and rear of the car need to work together. Much adjustment, and you disrupt that.

[Matt49]

Moving the entire bar down, decreases roll resistance just like raceEngineer said.
In my opinion, more angle increases dynamic load on the RR tire at the expense of load on the LR tire.
For this reason, more angle tends to make the car tighter on entry and looser on exit. And flatter tends to make the car freer on entry but tighter on exit.
I think it is more common to sort of set and forget the chassis side and tune with the pinion side.
Keep in mind that if you don't have close to perfect radius'd climbers for your application, a j-bar adjustment could mean a side-to-side rear end movement which could be compounding or cancelling what you are trying to do depending on which way it moved. And then if you adjust the j-bar length to compensate for the side-to-site movement, well now you've made another change. The length of the bar is a pretty substantial thing.

[RaceEngineer]

Moving the entire bar down, decreases roll resistance just like raceEngineer said.In my opinion, more angle increases dynamic load on the RR tire at the expense of load on the LR tire.For this reason, more angle tends to make the car tighter on entry and looser on exit. And flatter tends to make the car freer on entry but tighter on exit. I think it is more common to sort of set and forget the chassis side and tune with the pinion side.Keep in mind that if you don't have close to perfect radius'd climbers for your application, a j-bar adjustment could mean a side-to-side rear end movement which could be compounding or cancelling what you are trying to do depending on which way it moved. And then if you adjust the j-bar length to compensate for the side-to-site movement, well now you've made another change. The length of the bar is a pretty substantial thing.

Your opinion makes sense. You are taking weight from the left side, and supporting it with an infinite spring that is connected to the rear end near the center of the track. That load had been supported by the LR coilover.

[Twenty-Five]

So what is the difference between moving the bar on the chassis side vs the pinion side?

[RaceEngineer]

So what is the difference between moving the bar on the chassis side vs the pinion side?

The easiest way to think about it is what is the average height after these two adjustments.

[Austin34471]

It’s important to understand that, negating the CoG height change when the left side jacks, and negating the leftward movement of the rear track (neither of which should be negated, this is just for understanding’s sake), moving the jbar will *not* change the amount of weight transfer. It’s a 0 sum game, the weight transferred through the jbar is at the expense of the weight transferred through the springs and vice versa. The jbar is increasing or decreasing jacking/anti-roll. I think the amount of lateral movement of the rearend to the left caused by the jbar angle and height is very substantial and what causes most of what the driver “feels” when moving the jbar. How the front and rear track is aligned dynamically is one of the most important things that needs to be measured and kept track of IMO.
Example: if you had the front and rear track dynamically aligned at [arbitrary measurement], then moved both the frame side and rearend side of the jbar an inch, the driver would feel that change LESS, than if you: left the jbar height and angle the exact same, but shortened or lengthened the jbar, changing the [arbitrary measurement].
Also important to note that moving the rearend left and right dynamically changes your rear spring/shock angles, 4 link angles, along with a few other unintended changes. Lots to consider
Sorry for being long winded.

[Confused?]

These points have my curious about one other change. When would someone move the j-bar right or left without changing angles or lengths?

[Kromulous]

I am glad you started this thread, because some stuff i held as true in my head didnt add up on the track, and often led to confusion. Now, after reading a better understanding of J Bar Theory its starting to ring the bell, so Thanks !

[billetbirdcage]

I am glad you started this thread, because some stuff i held as true in my head didnt add up on the track, and often led to confusion. Now, after reading a better understanding of J Bar Theory its starting to ring the bell, so Thanks !

J Bar at times can be extremely confusing:

I will try to explain this but really not sure I can as it is more an experience thing and knowing the driver and car well.

Typically the general thoughts are if you increase angle, the car will be tighter most of the way around the corner. However there are times when I will increase angle to loosen the car.

Two times this really sticks out in my head:

One was at lakeside in Kansas city with Purkey as track was hammer down and everyone was fighting tight in hot laps. We was OK but tight across the middle pretty bad and was starting in the back of heat due to pill draw. He came in and we discussed it off what he felt and what I saw and came to the conclusion it needed more J-bar (I know seems all wrong). So I increase the J-bar on the frame 3/4" and the guys pitted next to us says (They was watching because we where generally one of the faster cars in MLRA so others tend to watch what we did to learn something or just out of curiosity):

Other team (OT): Wasn't you guys tight?
Me: Yes
OT: then why are you raising the J-bar?
Me: To loosen Al thur the middle
OT: Didn't say anything, just a confused look on his face

We went on to win the heat from the back by a large margin and after race they ask me how that loosened the car? I have really tried to explain this several times and can't really put it to words as it's an experience deal and that is what determines when it goes the other way like that time.

The next one is way easier to explain:

Crate car at magnolia, track has a nice slightly ledged cushion but track is good enough that if you going to be fast you can't lift. In hotlaps he was tight across the middle and had to lift to keep it turning, so after watching the car I came to the conclusion that it needed more J-bar. Driver is confused and not really sure that is the right call, but trusts me and lets me do whatever I want. I raise the J-bar 1/2" on frame. He can now run thru the corners and never lift and killed them by about half a track. Needless to say, he thought or assumed I did more then just the J-bar but is really impressed the J-bar alone fixed the car so well.

The reasoning I did the J-bar in that situation was in the middle of the corner I could see the car set down about 3/4 to 1" on the LR (crates on fast tracks tend to set down in straights because they don't have enough power to keep them solid on the chain). So when the car settled down it was loosing rear steer and making the car tight thur the middle. After the J-bar change the car even if it settled down on the straight as soon as he turned in to the corner it was now back solid against the chain and steering. Now again, there are several reasons this could have tightened the car and several things that could have loosened it (refer to Austins post above). It's more about knowing (or guessing for that matter) when which one of those is going to overpower the others or be the more dominate adjustment. Attitude of the car is crucial in all of this and is what I base most of it on, I have to see the car run

Like I said it's really hard to explain and give you definitive scenarios as to when and why this happens. I still get confused at times over certain things and it's just part of it and that's why you get so many different opinions on stuff. It's not that they are wrong per say they just tested or had things happen that gave them get different reactions.

[billetbirdcage]

So what is the difference between moving the bar on the chassis side vs the pinion side?

Again hard to be a 100% rule here but I think the best one that fits is this:

Frame height controls more how much the left side lifts
Pinion side controls more how much the car rolls down on the RR

A rough generalization^

2nd general rule would be: Pinion is roughly twice as sensitive as the frame, meaning down on pinion 1/4" is similar in adjustment size as up on frame 1/2'

[billetbirdcage]

Another generalization on J-bars would also be: (Granted I don't think this applies as much as it used to with the lower RF spring rates and/or with bump stops or some sort of hard stop)

More rake in the J-bar can/will have a profound effect on RF travel. Meaning I could have a 350 RF spring with 7" of rake and travel 3.5" on the RF, but with 8" of rake I could travel the 350 way more or travel a 400 that same 3.5"

Just something to be aware of, but like I said with hard stops on RF it's not as critical now a days.

[GRT62]

some good info here

[Matt49]

Billet I have a theory on your Lakeside hammer down track experience.
In following my theory that more j-bar angle tightens "entry" and frees "exit" we should really stop using those terms in quotes because the car doesn't know where it is on the race track. It only knows if it is decelerationg or accelerating or turning and which direction the weight is transferring based on that.
So I'm wondering if (in those hammer down conditions) you were dealing with a "tight on entry while on throttle" condition which leads us to the same logic of stiffening the LF instead of softening to free entry. Am I making sense?

[over4T]

This makes my head hurt.