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?ThanksOverall 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.

[billetbirdcage]

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?

"Entry, middle, or Exit: To me these has 2 meanings/locations depending on who or when someone is describing them. I'm guilty as anyone using them both ways, but I typically use them in context.

Entry: can be the physical location on the track like turn 1 to the middle of both turns or the place which the car is decelerating/out of throttle.

Middle physical middle of turn 1 and 2, or the place the driver picks up throttle

Exit: physical out of turn 2 or the place past after which he's been in the throttle a while.

Obviously if never lifting there really is only the physical places and not 2 meanings.

Yes, I understand what you mean by reverse adjustment effecting entry because your not lifting and there really isn't an entry. However with the Lakeside case, while I said hammer down you was still lifting for brief second to rotate the car (sorry that wasn't probably clear).

However I'm NOT a believer in the "that more j-bar angle tightens entry and frees exit. Not saying I'm right and your wrong or vice versa, but as a general statement not something I agree with 100% in my experience. The crate was tight in the physical middle of the corner (not lifting) so your theory could explain that, but with the lake side it turned better upon brief off throttle and when he picked up the throttle. So, that doesn't really follow that theory and one could say it should have make the brief off throttle worse.

A person could make a ton of legitimate arguments as to why it did that or to why it's misinterpreted and I'm not here to argue but just state that sometimes things don't always follow a rule for whatever reason.

[Matt49]

I'm not here to argue but just state that sometimes things don't always follow a rule for whatever reason.

Ain't that the truth. Good stuff though.
For such a simple thing (a single piece of suspension connecting a beam axle to a chassis), it sure does things that don't make a lot of sense. It's so simple that it's complicated.

[Austin34471]

Ain't that the truth. Good stuff though.
For such a simple thing (a single piece of suspension connecting a beam axle to a chassis), it sure does things that don't make a lot of sense. It's so simple that it's complicated.

It’s original purpose is very simple, but us dirt racers like to use suspension parts to do things they were never intended to do... we use the jbar to jack the left side a mile in the air, move the rearend a half of a foot back to the left to negate the rear steer added by the 4 bar setup, which changes the CoG, track alignment, dynamic spring rates, anti roll and squat values for the back, etc etc. I’m usually pretty good at explaining what 15 things happen when you make one adjustment, but Billet is better at explaining what actually happens to the car.. cause it’s hard to know how many of those 15 things are contributing to making the car handle one way or another. Only way to really know is test it and find out, something Billet has done more than everyone on this thread combined. Lol

[fastford]

more than any one on this thread combined ? thats saying a lot billet , what say you?????

[Burke1118]

good info guys, thanks those taking the time to give you're opinions

[Jim11h]

Billet, have you thought that maybe you was already at max adjustment for desired effect and went more which can (usually does) give opposite effect? Seems to me where you was at and effectively learned how to use as tuning device where other folks it throws them and they start chasing their tail

[fastford]

i had a driver that used the rear brakes to get in the turn , raised the j bar 2 inches on frame , he could still get in pretty good , but he could drive off the corner on the right rear liker a bullet , this fit his driving style better than a driver that likes to keep the car straighter in and through the turn , point is , sometimes you have to go against the norm to find a balance between driver , car and track.....i also use to think there was nothing better than a short strait bar , that was before all this soft rt frt stuff ,

[billetbirdcage]

more than any one on this thread combined ? thats saying a lot billet , what say you?????

While there is no way to say for sure, I'd be willing to bet yes by a large margin. I've done tons of private tests for the last 12-13 years with several racers and at times that can get upwards of 20 times a year. This isn't including open practices nights at a track as I don't generally accept them as gathering good data.

Not a 100% if Austin meant total hours testing or all the different types of stuff I've tested over the years.

[Matt49]

A little off topic but along the same thread of thinking about j-bar...
Has anybody thought about or experimented with some type of tether system for the rear end as a safety net in the event the j-bar (or something holding it on) breaks. Not sure if any of you have ever broken a j-bar but suffice to say that the j-bar isn't the only thing you'll be replacing if you ever do. The shocks and birdcages are usually going to end up destroyed. It seems like some sort of tether system could be easily developed to "save" some of this additional damage and potentially increase safety. I was sitting still the time mine broke (got hit square in the LR while at a complete stop sideways) but breaking one at speed in the corner could get ugly.
I think it's something to consider. I've thought of putting tabs on the rear frame rails some sort of mounting mechanism above the center of the rear end and then just using some old harness belts. The belts being adjustable would make it sort of a universal design. Thoughts?

[billetbirdcage]

Billet, have you thought that maybe you was already at max adjustment for desired effect and went more which can (usually does) give opposite effect? Seems to me where you was at and effectively learned how to use as tuning device where other folks it throws them and they start chasing their tail

Sure it can happen, but in these instances typically doing that adjustment would not loosen them up. It's on of those deals where it's situational and a lot of it is based on experience with it and knowing the drivers/cars well from being with them for a long time. Now if every time I got over that certain point it did that, then I would agree but that wasn't the case here.

The experience I'm talking about isn't all that different then knowing/guessing from experience when to do 4 soft tires, 3 soft and 1 hard, or left soft and right hard. While not exactly the same, sometimes you just know or base what is gonna work the best off experience. Like knowing that the soft tires tonight isn't gonna work as well as it typically does and they are gonna give up.

[billetbirdcage]

A little off topic but along the same thread of thinking about j-bar...
Has anybody thought about or experimented with some type of tether system for the rear end as a safety net in the event the j-bar (or something holding it on) breaks. Not sure if any of you have ever broken a j-bar but suffice to say that the j-bar isn't the only thing you'll be replacing if you ever do. The shocks and birdcages are usually going to end up destroyed. It seems like some sort of tether system could be easily developed to "save" some of this additional damage and potentially increase safety. I was sitting still the time mine broke (got hit square in the LR while at a complete stop sideways) but breaking one at speed in the corner could get ugly.
I think it's something to consider. I've thought of putting tabs on the rear frame rails some sort of mounting mechanism above the center of the rear end and then just using some old harness belts. The belts being adjustable would make it sort of a universal design. Thoughts?

The drive shaft as it gets into the tunnel/(not a nice word)(not a nice word)(not a nice word)(not a nice word)pit or when the RR tire hits the 4 link bracket or frame, is our safety stops, lol

Not saying it isn't a good idea but what about back ups for other things? I've torn up way more when the torque arm or 6th coil chain breaks, so where do you stop on back up's?

[Matt49]

The drive shaft as it gets into the tunnel/(not a nice word)(not a nice word)(not a nice word)(not a nice word)pit or when the RR tire hits the 4 link bracket or frame, is our safety stops, lol

Not saying it isn't a good idea but what about back ups for other things? I've torn up way more when the torque arm or 6th coil chain breaks, so where do you stop on back up's?

Billet, I realize you're kind of playing devil's advocate (maybe not) but if NOT destroying the drive shaft is the outcome of a tether system, it kind of supports my point. I just look at single points of failure and try to think of ways I can limit the damage if/when those failures occur. Obviously routine maintenance is our best friend but things do fail outside of what a small team can reasonably quality control. With a failure like I'm describing, you're obviously out of the race, but if you can salvage some form of financial losses and/or increase safety, it has to be worth evaluating, right?

Love this discussion by the way. 4m tech section has been lacking these in the past few months.

[95shaw]

Billet, I realize you're kind of playing devil's advocate (maybe not) but if NOT destroying the drive shaft is the outcome of a tether system, it kind of supports my point. I just look at single points of failure and try to think of ways I can limit the damage if/when those failures occur. Obviously routine maintenance is our best friend but things do fail outside of what a small team can reasonably quality control. With a failure like I'm describing, you're obviously out of the race, but if you can salvage some form of financial losses and/or increase safety, it has to be worth evaluating, right?

Love this discussion by the way. 4m tech section has been lacking these in the past few months.

Are we thinking of creating another tether which can be morphed by someone into a tuning device?
My other concern would be that we are simply moving the stress points when the failure occurs.


Thoughts?

[billetbirdcage]

Are we thinking of creating another tether which can be morphed by someone into a tuning device?
My other concern would be that we are simply moving the stress points when the failure occurs.


Thoughts?

You mean like the bungee cord type chains/limiters guys are using now? Joking. . .

















or am I. Shrugs

[billetbirdcage]

Billet, I realize you're kind of playing devil's advocate (maybe not)

Yes, I’m playing devils advocate. My point was if you had a back up/safety for every item on the car, it might weigh 100# over weight or have so much stuff in the way it’s hard to work on or something else to get tangled and cause issues while racing. If you can do it without causing additional problems or things I listed for such a rare occurrence, absolutely nothing wrong with doing it.

[fastford]

While there is no way to say for sure, I'd be willing to bet yes by a large margin. I've done tons of private tests for the last 12-13 years with several racers and at times that can get upwards of 20 times a year. This isn't including open practices nights at a track as I don't generally accept them as gathering good data.

Not a 100% if Austin meant total hours testing or all the different types of stuff I've tested over the years.

and i would be willing to take that bet if , like you said , if there was a way to say for sure , ive been involved with building and racing dirt cars going on 25 years and drag cars 15 years before that , i have a heap of hrs testing my self , but one thing i will say , you never stop testing and learning , or you will go backwards , I have defiantly gained knowledge from your post and i do appreciate it..........

[Austin34471]

I would stay away from the tethers because you would obviously have to attach them to the rearend in a few places... which would, you know, increase the unsprung weight.....

Heh.. heheh.... heh......

[Matt49]

I would stay away from the tethers because you would obviously have to attach them to the rearend in a few places... which would, you know, increase the unsprung weight.....

Heh.. heheh.... heh......

Now you've really got me thinking! Maybe my left side tether needs to weigh WAY more than my right side tether. Ha ha

[Jking24]

I have seen what billet it's describing at lakeside quite a few times and in my opinion it is a very experience related change. Example if the driver had not lifted when he initialy turns the car in but then burps the throttle to let the car rotate. The weight transfer has already happened so the effect of going up on jbar (on frame) tightening entry ie. Decel has already happened. The right sides are loaded their for minimal effect from added loading. More jbar would keep the car up during the off throttle and late straight away periods then when the driver did get back to the fuel which was probably still pre apex the added angle adds rr traction and general car attitude witch rotate the car more across the center. I don't know if that came out right but their is alot of what where and how that goes into what effect the j bar will or won't have

[95shaw]

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


Generally speaking, moving the j-bar right or left, moves the point of weight application on the axle.
Meaning weight that is applied by the j bar is moved closer to one tire.

Of course, moving the both ends to keep bar length the same, moves the frame end in relation to the center of gravity, changing which spring the weight is coming from. Dynamically, could net a zero change in wheel weights.

[let-r-eat]

Years back when I started smashing tires together with springs I learned some things about the J Bar that I never realized before. The placement of that JBar is more critical than I ever expected. Leaving the J Bar in a box stock configuration could be a big compromise.

[Kromulous]

Reviving this thread, i wanted to ask more questions. Ran this past Saturday night, when the track has some water in it the car is pretty good overall decent, but not great. Florence Speedway BTW.

Now when it gets slick, we struggle.

What i see is, the car has a lot of rear lift, LR and the RR it seems, the RR it is not compressing at all, and probably against the chain on the RR, and the LR for that matter. I run a chain on both sides. Now that i think about it, its probably maxed out on both, against the chain.

Does the amount of bar angle, on the LR, and the RR, play into how much angle is in the J bar? I mean if you have a lot of angle on the LR bars, and angle in the J-bar it should make it easier to lift the LR, correct?

I am thinking i need to lower the J bar on both sides, maybe 1/2" pinion, and 1" on the frame. and try to get it to roll on the RR better on entry, the car is lose on entry, but he says its not horrible. Its just terrible off, no grip.

RF is pinned, 2300 lbs at 4" about 160 lbs extended load on the LR. Droop travel on the LR is about 5.5" Stagger was 1.5" RR spring is a 250 lb.

I will see if i can find a video to help, this car has me cornfused, it doesn't behave properly LOL. We're almost ready to dig out a Reveloution we have in the back of the shop in the weeds!

[rjs21]

I appreciate the conversation that you guys have had on J-bars. I have a question and the modified section doesn't seem to have much discussion.

Do any of these theories relate to an IMCA Sportmod with a straight panhard bar behind the axle housing?

[Lizardracing]

Yes it does with some small differences.
Mounted behind. The panhard bar is longer so the side to side change is less, the axle mount moves up and down less, so it's less of a reaction than pinion mounted but the roll center theory still applies if you into that sorta thing.

[rjs21]

My car has a slider on the frame side so it is adjustable in small increments. The housing side has 4 holes on it 1" apart, not a small increment adjustment.

My question is; what is more important if I was to adjust the frame side? The height, which would affect roll center? Or the angle?

Suppose I wanted to snug up entry with a 1/2" panhard adjustment. Would I raise it on the frame to get more angle but a higher roll center? Or lower it with less angle but a lower roll center?

Which aspect of the adjustment is more critical?

I was wanting to avoid the housing side adjustment because it seems to be a big change.

[Lizardracing]

Generally speaking the height will be felt more by the driver on a long PH bar.

[billetbirdcage]

My car has a slider on the frame side so it is adjustable in small increments. The housing side has 4 holes on it 1" apart, not a small increment adjustment.

My question is; what is more important if I was to adjust the frame side? The height, which would affect roll center? Or the angle?

Suppose I wanted to snug up entry with a 1/2" panhard adjustment. Would I raise it on the frame to get more angle but a higher roll center? Or lower it with less angle but a lower roll center?

Which aspect of the adjustment is more critical?

I was wanting to avoid the housing side adjustment because it seems to be a big change.

Just to be clear, how is the panhard mounted? Left side frame and right side axle/rear end or left side axle/rear end and right side frame? That will make a huge difference on adjustments!

[rjs21]

Panhard mounted on left frame rail to axle housing about 4" right of the pinion.

[hipower17]

wasn't it moyer that tried the jbar on the right side won a lot of races then quit using it? why is this setup used more often looks like this would work on certain tracks?

[MasterSbilt_Racer]

wasn't it moyer that tried the jbar on the right side won a lot of races then quit using it? why is this setup used more often looks like this would work on certain tracks?

I have first hand experience making a Rayburn bad fast with the panhard on the right. The front and rear need to roll/work together. It can certainly be made to work.

[ZERO25]

I have first hand experience making a Rayburn bad fast with the panhard on the right. The front and rear need to roll/work together. It can certainly be made to work.

Hasn't Mr. Bolles from Circle Track been advocating this for years, as well as Mr. Bush, too!