J-bar lengths

[merc123]

I have a two hole frame mount. If I get a shorter j-bar and move it from the hole closest to the left side to the right hole, moving it about 2 inches to the right, abut keep the bar angle the same will there be any difference?

I'll be going from a 23" c-to-c to about a 20" c-to-c j-bar.

It's on a 3-link car with a long, solid upper link if it matters.

[Egoracing]

A shorter bar will react faster than a longer bar. You are also moving the roll center of the bar to the right.

[dualdj1]

You are also moving the roll center of the bar to the right.

That's a tricky wording, that could be confusing. The rear roll center, side to side, is set by spring/shock mount location. The length of the bar effects the angle of force applied to the axle, and the dynamic increase in angle (as you said acts faster, gains more angle). It doesn't really have a roll center, or effect roll center side to side; it does however effect side to side movement of the axle (shorter bar = more movement left for same angle and same hike).

[MasterSbilt_Racer]

That's a tricky wording, that could be confusing. The rear roll center, side to side, is set by spring/shock mount location. The length of the bar effects the angle of force applied to the axle, and the dynamic increase in angle (as you said acts faster, gains more angle). It doesn't really have a roll center, or effect roll center side to side; it does however effect side to side movement of the axle (shorter bar = more movement left for same angle and same hike).

There are different theories, but I believe the truth lies in the middle. The panhard bar does effect the point in space about which the chassis rotates. It is not the sole influence as spring locations and rates come into play as you pointed out.

[dualdj1]

There are different theories, but I believe the truth lies in the middle. The panhard bar does effect the point in space about which the chassis rotates. It is not the sole influence as spring locations and rates come into play as you pointed out.

Yeah I don't disagree there, it just (both from theory and experience) seems to have more effect height wise than side to side. the side to side effect seems to be from the movement it creates on the axle, not by changing the actual roll center. moving the RR tire more under the car and changes the car's footprint, letting it get over on that side easier. But roll center is really just a theoretical tool to let you have a baseline frame of reference on how your changes effect roll, and doesn't really apply well to dynamic changes with a lot of movement on the axle. I think the term "roll center" doesn't always apply well. Would you agree? I respect your technical expertise here :)

[MasterSbilt_Racer]

Yeah I don't disagree there, it just (both from theory and experience) seems to have more effect height wise than side to side. the side to side effect seems to be from the movement it creates on the axle, not by changing the actual roll center. moving the RR tire more under the car and changes the car's footprint, letting it get over on that side easier. But roll center is really just a theoretical tool to let you have a baseline frame of reference on how your changes effect roll, and doesn't really apply well to dynamic changes with a lot of movement on the axle. I think the term "roll center" doesn't always apply well. Would you agree? I respect your technical expertise here :)

Yes. We are thinking the same.

[dualdj1]

Yes. We are thinking the same.

Cool, just making sure, sometimes I get my logic mixed up, lol



So back to original question, with shorter bar you will roll over easier, on top of the faster acting effect. Keep in mind that if your springs angle in, that you get more dynamic spring rate increase as well, as your axle travels side to side, possibly negating some of the roll improvement.

[Egoracing]

That is why I said "bar roll center" and not rear roll center. If you look at the bar and the weight of the chassis, going to a bar that moves the left side in and not the right side will move the weights center on that bar farther to the right. Originally the chassis rear roll center was directly referd to as a point on the bar. I Also believe that the springs come into play as to where it will roll BUT if the springs were NOT in the equation there is still a roll center in the rear of the car, where would it be located?

[Matt49]

Wheel rates and roll rates have a great deal to do with it. Roll rate is a product of the angle that the springs are at.
Imagine two different extreme hypothetical scenarios:
1) The RR compresses and the LR doesn't move up or down
2) The LR rebounds and the RR doesn't move up or down
The roll center given these two scenarios would be in two very different places.
In the first example, the roll center would be pretty far to the left. In the second example, the roll center is pretty far to the right. So I disagree with the theory that the lateral roll center is between the tops of the two springs. That may be true if the RR was always compressing the same about the the LR was rebounding but we know that is never really the case.
I think if you do enough real-world modeling you will find that the roll center in most setups is very low (below both j-bar mounting points) and pretty far to the right (at least farther right than the pinion mount of the j-bar) and then moves to the left as the car rolls over as a result of the rear end shifting to the left.

[merc123]

Well I was able to get the same length j-bar as my old one. I might try to play with the short one and see if I like it later on in the season or at a practice though.

[dualdj1]

Wheel rates and roll rates have a great deal to do with it. Roll rate is a product of the angle that the springs are at.
Imagine two different extreme hypothetical scenarios:
1) The RR compresses and the LR doesn't move up or down
2) The LR rebounds and the RR doesn't move up or down
The roll center given these two scenarios would be in two very different places.
In the first example, the roll center would be pretty far to the left. In the second example, the roll center is pretty far to the right. So I disagree with the theory that the lateral roll center is between the tops of the two springs. That may be true if the RR was always compressing the same about the the LR was rebounding but we know that is never really the case.
I think if you do enough real-world modeling you will find that the roll center in most setups is very low (below both j-bar mounting points) and pretty far to the right (at least farther right than the pinion mount of the j-bar) and then moves to the left as the car rolls over as a result of the rear end shifting to the left.

Your roll center doesn't change. That's the problem with using that term. Your Center of Gravity can change, because that's relative to gravity/ground, and is effected by movement, and you have other means of increasing or decreasing roll, but those don't change the roll center. Your roll center is relative to your sprung weight (ie chassis). it's determined by points of resistance to that weight rolling, which is why changing the roll center changes the effectiveness of the suspension - it either increases or decreases that resistance. On the rear, those resistance points horizontally are the spring mounts on the chassis (where the resistance is applied). Vertically the resistance points are the ends of your panhard bar. The only time roll center changes is when you change those resistance points. Again Roll Center is just a theoretical tool, that tells you what the weight "wants" to do, by where the resistance points are located.

Now, arguably, with todays 4-bar cars, you have other tools that can be used to reduce that resistance dynamically (LR hike, bird cage indexing, etc). But that doesn't mean your roll center changes, just that you're able to get more roll through other means. Which is why the term Roll Center (for rear) just doesn't mean as much as it used to, other than giving you a baseline to work with.

[dualdj1]

That is why I said "bar roll center" and not rear roll center. If you look at the bar and the weight of the chassis, going to a bar that moves the left side in and not the right side will move the weights center on that bar farther to the right. Originally the chassis rear roll center was directly referd to as a point on the bar. I Also believe that the springs come into play as to where it will roll BUT if the springs were NOT in the equation there is still a roll center in the rear of the car, where would it be located?

Wasn't disagreeing with you, just wanted to clarify for those that haven't researched much and might get confused.

If there were no springs, your roll center would still be the same, if your attachment points were the same. Take a ruler, and prop it up on both ends, at equal distances. Now, angle one of your props in towards the middle. The ruler has more weight hanging over 1 mount point, meaning it wants to roll over that end; so you moved the roll center towards the other end. Doesn't matter if it's sprung or not at that point.

[Egoracing]

Wasn't disagreeing with you, just wanted to clarify for those that haven't researched much and might get confused.

If there were no springs, your roll center would still be the same, if your attachment points were the same. Take a ruler, and prop it up on both ends, at equal distances. Now, angle one of your props in towards the middle. The ruler has more weight hanging over 1 mount point, meaning it wants to roll over that end; so you moved the roll center towards the other end. Doesn't matter if it's sprung or not at that point.

I agree with you, just opening up the thinking a little. Rear roll center, weight centerline and chassis centerline all need to be addressed differently. I have seen to many people think that the chassis center is the weight center, roll center OR that the center of the rear is actually centered in the chassis.

[dualdj1]

I agree with you, just opening up the thinking a little. Rear roll center, weight centerline and chassis centerline all need to be addressed differently. I have seen to many people think that the chassis center is the weight center, roll center OR that the center of the rear is actually centered in the chassis.

Yep that's the big problem, that the one term gets applied different ways. Really need a better term for rear dynamic pivot point, that takes into account your axle movement, roll center, etc, and gives you a better sort of idea how changes effect your overall roll, since there are so many more factors now.