J-Bar explanation

[racefastnow]

Can someone explain how the points on the jbar should effect handling. High to low on frame. Than moving it on pinion. Also length???

[MasterSbilt_Racer]

Lower j bar = chassis rolls more
Higher j bar = chassis rolls less
More angle = more of the load applied to the j bar pushes the tires into the track suface instead of trying to slide them across it. Too much angle will stop chassis from rolling. The downward force is applied to the rear where the j bar attaches to it.

Now you have to look at the attachment point. Is it in the middle of the track width? Closer to the lr? Closer to the rr? If closer to the rr, then what happens if you increase rr tire load on entry? exit?

This should give you some idea about how to look at it.

[Egoracing]

Also shape does nothing to the function of it. The chassis only knows it by the attachment points as a straight line.

[dualdj1]

And to clarify a little what Mastersbuilt said...

Lower j bar, means both frame and pinion low, higher means frame and pinion high.

Think of a straight line between your mounting points, extending to the axle. where that line hits the axle is where the force is effectively applied. going lower on pinion is going to bring that force more to center. going up takes it more right. Going up on frame brings it more center, going down more to right.

Going up on either point raises roll center (less roll), going down on either lowers (more roll).

[lovin dirt]

I was under the impression that you get more roll if you either lowered @ pinion or raised @ frame. I this incorrect?

[MasterSbilt_Racer]

I was under the impression that you get more roll if you either lowered @ pinion or raised @ frame. I this incorrect?

Neither of those are always correct. Angle changes are complex.

[Matt49]

Just like with the front roll center, the rear roll center is dynamic/moving all the time.

[dualdj1]

Just like with the front roll center, the rear roll center is dynamic/moving all the time.

Your amount of roll can change, and your center of gravity can change. Your roll center does not, unless you change mounting points. It is static.

[Matt49]

Your amount of roll can change, and your center of gravity can change. Your roll center does not, unless you change mounting points. It is static.

I disagree that the roll center is static but I agree that the roll center does not change unless the mounting points change. But the mounting points do move and thereby so does the roll center. As the car rolls over, the pinion mounting point moves down and to the left relative to the car's center of gravity.
As throttle is applied and the liftbar moves up, the pinion mounting point of the j-bar moves up with it.
The only thing that doesn't change is the length of that j-bar. The mounting points (at least the pinion mounting point) is moving and therefore the roll center is moving.

[dualdj1]

Sorry I should have phrased it differently. You are correct in your description; the problem (as is being discussed elsewhere) is that roll center term is used as a baseline (static) measurement, that doesn't accurately represent dynamic changes, since it's calculated in one way statically, but when you go into motion or as the car rolls, there are many other factors that come into play which can increase or decrease the resistance to roll. Which is why it can be so confusing. The changes you talk about are correct. It just feels like we need a better term for it and better ways to calculate that increased or decreased roll from the other factors (such as axle movement, etc), which go beyond the traditional definition of roll center. dynamic roll center, dynamic center of gravity, you get the idea (or maybe I'm just being overly complicated about it, lol)

[Matt49]

Sorry I should have phrased it differently. You are correct in your description; the problem (as is being discussed elsewhere) is that roll center term is used as a baseline (static) measurement, that doesn't accurately represent dynamic changes, since it's calculated in one way statically, but when you go into motion or as the car rolls, there are many other factors that come into play which can increase or decrease the resistance to roll. Which is why it can be so confusing. The changes you talk about are correct. It just feels like we need a better term for it and better ways to calculate that increased or decreased roll from the other factors (such as axle movement, etc), which go beyond the traditional definition of roll center. dynamic roll center, dynamic center of gravity, you get the idea (or maybe I'm just being overly complicated about it, lol)

I think we're on the same page. For something that appears so simple, there are many things that make it complex.
I was watching the Nationwide race at Road of America a week or so ago and they had a camera under the rear of one of the cars and the "analysts" were going on an on about how much movement there was and change in pinion angle. I couldn't help but just laugh.

[dualdj1]

I think we're on the same page. For something that appears so simple, there are many things that make it complex.
I was watching the Nationwide race at Road of America a week or so ago and they had a camera under the rear of one of the cars and the "analysts" were going on an on about how much movement there was and change in pinion angle. I couldn't help but just laugh.

It's easy to get caught in the terminology (obviously i fall victim to that), but understanding the concept is the important part. I just know it took me a long time to understand it, because of how the terms got thrown around, which is why I try to clarify things for the new people trying to learn.

yeah, put em under a dirt car sometime, lol.

[Egoracing]

It's easy to get caught in the terminology (obviously i fall victim to that), but understanding the concept is the important part. I just know it took me a long time to understand it, because of how the terms got thrown around, which is why I try to clarify things for the new people trying to learn.

yeah, put em under a dirt car sometime, lol.

I was at the PRI show several years ago talking to our rep form Hyperco and he had some Indy car engineers there and we all got into a conversation about suspension movement. The Rep told them that I was one of "the dirt guys" and worked on a dirt late model. One of the guys turned to me and asked soemthing to the effect "How do you plan on the amount of change the track runs thru with a car that has a weight transfer and traction amount that is directly linked to the track surface and the amount of chassis movement the car generates. We went to where the TNT Chassis car was and went over a few things and they look at me and said they were glad it was not them and said that they make changes with a micrometer, they would have NO IDEA where to start on the rear of that (pointing to the late model). The other guy pulls out his phone and asks about what spring rates and shock rates do we run in, he said there were WAY over a million different possabilities on the rearend alone." I invited them to the Nesmith races at Eastbay that weekend but they could not make it.

[dualdj1]

When we got big into dirt racing, we had some connections in Nascar (knew a guy who was Unocal fuel rep for Nascar), which for him it was just business. But in and around, let us talk to and meet various different Nascar people, and the most of what we heard was basically the same thing you just said. The other common things were amazement at the short amount of time dirt trackers had to prepare, when the Nascar guys are used to a few practice days, whereas we get 5 laps if we're lucky, and then the track's probably changed by the next time we're back out.

There's definitely an art to it all, but I like learning the theory behind it all, to get that extra leg up, and know (at least probably) why a change works, and have a little better baseline for adjustments.

Appreciate all the good input and discussion here. Been very productive I think.

[racefastnow]

Can someone giv a few scenarios on what to do and when generally.. Specifically in tacky conditions. Loading the right side will help loosen and turn the car in the cornercorrect?? And keeping low angle also. Maybe what are some baseline numbers as far as the difference in height at points. We dont move jbar alot and just started and wanna loosen car up. Too tight in tacky situations. Tried alot of other things. So working on jbar stuff plus other things now. I understand not alot of info. But in general. Car is literally 2 seconds slower in the mudd than black ice dry slick. Haha

[dualdj1]

go up on pinion, and frame if possible. if you like the way the car reacts when you step on the gas, then leave the angle alone if you can and raise both sides. at the least, go up on the pinion.

if you can't go up on pinion at all, then bout have to flatten the bar by going down on the frame.

[let-r-eat]

If its tight when you pick up the gas what would you do?

[MasterSbilt_Racer]

If its tight when you pick up the gas what would you do?

Nothing with the j bar.

[let-r-eat]

I was just reacting to what he stated "like the way the car reacts when stepping on the gas"So tight in center on the gas you wouldn't change jbar? Just asking

[dualdj1]

I was just reacting to what he stated "like the way the car reacts when stepping on the gas"So tight in center on the gas you wouldn't change jbar? Just asking

that was in regards to the angle, and how it effects roll. more angle means car reacts quicker and can be more erratic as you're getting back into gas, or as you're transitioning through the corner. I guess not just on throttle, it's more as you transition into the corner and start rolling. throttle can just increase roll depending on how it's set.

So, as I said before...

If you want to try j-bar change, go up on pinion at least, and frame if possible, and try to keep your rake the same. if it feels like the car is flopping over to the right, you can take angle out, first by going up on pinion, and then by going down on frame only if you can't go up on pinion.

However, as Mastersbuilt said, if you are only tight when you pick up throttle, the j-bar probably isn't the problem. J-bar effects multiple areas. if you're tight on throttle you need to look at things like bite, springs, bar settings, roll steer, etc.

MIDDLE CORNER

(on throttle)
SHOCK ADJUSTMENTS
• Increase rebound front shocks
CHASSIS ADJUSTMENTS
• Decrease wedge
• Drop left top 4-link rod
on chassis
• Raise right top 4-link rod
on chassis
• Stiffen RR spring (can also tighten off
throttle handling)