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)

[racefastnow]

Never had these problems with other cars. Just this one. But its great on dry. Competes for win almost every time. Gets last in mudd. Weve gotten it better this year by changing the lowers to steer better and takin roll steer out and a few other things. But last time its was still tight. Than it was loose and spinning on straight aways. Weve had top left bar at like 30 degrees when were at 47 on dry Does nothing. Car is not a balanced car. Are old car. Drives perfect in heat. Add gas drives good enuff to win in features. This car is really sensative. Generally its tight everywhere and will not even go around a wide corner track. Needs straight in and out. I was leaning toward the rear roll center must be rolling over too much. Thinking of stiffening rr spring Maybe messing with j bar. So if u lower jbar on frame will it actually make it roll over more? Just changes the angle of force. And some small adjustments to lower links. Its crazy the difference from mudd to dry. Faster lap times in dry slick than fast heavy tack. It has gotten better tho. Weve tried alot of stuff. Went to schools to fix mudd setup. Ha. Bet most people wish they had that problem. Any ideas are always good. I was thinking jbar adjustment so it didnt hav so much force and that would free it up??

[dualdj1]

No, ONLY lowering j-bar on the frame should decrease roll, because even though you're lowering the roll center, you're taking angle out, which seems to have greater effect on j-bar. Decreasing angle takes away lifting force, and tends to push the RR across the track instead of into it. A heavier RR can be beneficial there as well. Pinion side, however, is still preferential to raise, because you don't just take angle out, but raise roll center as well, giving you more overall roll decrease.

[let-r-eat]

Never had these problems with other cars. Just this one. But its great on dry. Competes for win almost every time. Gets last in mudd. Weve gotten it better this year by changing the lowers to steer better and takin roll steer out and a few other things. But last time its was still tight. Than it was loose and spinning on straight aways. Weve had top left bar at like 30 degrees when were at 47 on dry Does nothing. Car is not a balanced car. Are old car. Drives perfect in heat. Add gas drives good enuff to win in features. This car is really sensative. Generally its tight everywhere and will not even go around a wide corner track. Needs straight in and out. I was leaning toward the rear roll center must be rolling over too much. Thinking of stiffening rr spring Maybe messing with j bar. So if u lower jbar on frame will it actually make it roll over more? Just changes the angle of force. And some small adjustments to lower links. Its crazy the difference from mudd to dry. Faster lap times in dry slick than fast heavy tack. It has gotten better tho. Weve tried alot of stuff. Went to schools to fix mudd setup. Ha. Bet most people wish they had that problem. Any ideas are always good. I was thinking jbar adjustment so it didnt hav so much force and that would free it up??I see several cars tight *especially off the throttle* going in. Have you taken the spring/shocks off left rear and checked total droop to make sure you aren't topping out? I see several doing this with their limit chains. They set the limit and while entering on the heavier tracks and the fact of having more drive and hike due to the track surface when they enter the left rear goes light /pickups up not off the ground or anything but enough to severely reduce left rear grip/causing the right rear to grip hard on decel/trail braking increases hike/ and we all know what that means, goes drastically and violently tight just like braking the right rear only. You then have to wait on the car to slow up or violently brake the car for the car to turn. Some apply the gas and when the thing sets back down it violently turns in an out of control style fashion.You can generally tell if your coming close to this situation by driving one when the car starts the set and feels as though the front wheels aren't driving a thing. Just goes where it wants to. Accelerating will make the car turn but when the left rear starts to get back in the ballgame the right rear is loaded for bear and turns you faster than you want to turn.I see many cars doing this.

[racefastnow]

Its mostly tight on throttle middle and exit. I raised the jbar this weekend. That helped. I was doing some thinkin on the 5th spring. If generally a stiffer spring is for mudd and softer for dry. If i hav too much drive would i actually want a softer spring for mudd or should i try stiffening?? Also, how soft is to soft. Were at 325 right now

[dualdj1]

Its mostly tight on throttle middle and exit. I raised the jbar this weekend. That helped. I was doing some thinkin on the 5th spring. If generally a stiffer spring is for mudd and softer for dry. If i hav too much drive would i actually want a softer spring for mudd or should i try stiffening?? Also, how soft is to soft. Were at 325 right now

you could also just run less fuel (if possible, not going under weight), or just add more stagger.