Just a thought on jbar

[Dirtmod13]

Was talking with mark bush about the jbar and adjusting the roll center. Seems like the little difference in roll center location couldn't make that much of a difference but maybe, it's more of the fact that the left rear moves further out from the body/ frame that it has the same effect as putting a spacer on it/ moving right rear in. Just seems like those adjustments made more difference to me than moving jbar 1/2" Looking for thoughts / theories

[Dirtmod13]

This is in reference of when the jbar is mounted at a higher split, then it gives more left rear out when in roll in the corner. Just because if the jbar was pushing down harder on the right rear when in the gas, it should be looser due to higher load placed in that wheel. But it has the opposite effect.

[RCJ]

I think you are on the right track.The jbar is effecting more than just the roll center.I don't move it a lot because the results aren't as predictable as I would like.

[Kromulous]

Couldn't you look at it another direction, the shearing force on the RR tire actually lifts the chassis via the Jbar. The more angle in the J bar its easier for this force to drive the Left side of the car up in the air?

I know it controls the pivot point, or RC of the rear end, but i notice it when i groove tires for the RR and i will put more circumferential grooves towards the outside of the tire. I can then see if have an effect on the car.

All i think its a constant trade off. Seems like when you find a sweet spot for the car, i kinda stay right there.

[Austin34471]

Think about it this way: when cornering, the body of your car wants to keep going straight (Newtons first law of motion) and the only thing making it go left are the those 4 black rubbery things called tires. So the relative motion of the tires is leftwards, and the body is still trying to go right (as illustrated in my little drawing below). The jbar is what keeps the rear of the car from continuing to go straight! Because the rearend stays planted on the race track (in most cases), the body will be forced to rotate about the mounting point of the jbar to the pinion plate (or whatever you have it mounted to on the rearend). As you can imagine, there's a great deal of force going through this point and you can change the direction and location of the where the forces are being fed to the rearend! Imagine if that white line in my picture was completely flat. The forces would be perfectly parallel with the race track and all of the centripetal Forces would be trying to "shear" the tires from the track. Now imagine that white line being at a 45 degree angle.. As the tires tried to move left and the body tried to move right relative to each other, the body would be forced in an arc around the pivot point on the rear end (with the radius of the arc being the length of the j bar.) So the direction of the force would now be down into the rearend at 45 degree angle, with the weight of the car helping add some force downward, as well as the centripetal force adding some shear forces! And also, as you've pointed out, the rear track will be pulled to the left relative to the body, so more weight should be sitting over the top of the RR, and the front to rear track alignment will have changed. Also remember that the body can rotate at the point that the jbar connects *to the frame* and what determined how it rotates is the rear springs. That's why the rear spring's rate, angle, and mounting points have just as big of an influence on the "roll center" as the Jbar (all of which effectively change when you make jbar adjustments lol). There's a lot going on in the rear of these cars and the jbar is just one tiny piece of the puzzle. Understanding how all of these pieces work and understanding how they work together is what will make you be able to make the right adjustments and not just be a "direction follower" who looks in your chassis cheat sheet for what adjustments to make when the car does X Y or Z.

[Pilgrim 28]

I bent a J-bar and bought a longer J-bar ,I struggled with a tight entry problem. I never put the 2 together, that the J-bar length could be the problem. I changed back to the shorter J-bar and now can turn in Bottom , middle or top. just something to keep in mind. frame and pinion location stayed the same height, just different mounting hole on frame bracket..FYI

[JustAddDirt]

I bent a J-bar and bought a longer J-bar ,I struggled with a tight entry problem. I never put the 2 together, that the J-bar length could be the problem. I changed back to the shorter J-bar and now can turn in Bottom , middle or top. just something to keep in mind. frame and pinion location stayed the same height, just different mounting hole on frame bracket..FYI

same thing I noticed as well. just turn wheel and car rotates in corner very easily, and sticks harder.
(I noticed this on a modified)

[powerslide]

same thing I noticed as well. just turn wheel and car rotates in corner very easily, and sticks harder.
(I noticed this on a modified)

Have you played with moving the j-bar from the right side of the pinion to the left and moving the frame mount equally? I think I would be able to and keep it the same length.

[JustAddDirt]

Have you played with moving the j-bar from the right side of the pinion to the left and moving the frame mount equally? I think I would be able to and keep it the same length.

no. haven't done that. will loosen entry tremendously. but it can help traction off a bit I suppose.
better be smooth track too. All my experiences with straight bar and a 4 link car, they can be a bit wild on rough or rutty track. I suppose the length would help that a bit.

[a25rjr]

no. haven't done that. will loosen entry tremendously. but it can help traction off a bit I suppose.
better be smooth track too. All my experiences with straight bar and a 4 link car, they can be a bit wild on rough or rutty track. I suppose the length would help that a bit.

That's been my experience, too. I think our bar was about 13". I am actually in the middle of building a new straight bar in the 16" range. That is what Mark Bush recommends and very little angle to not make it so erratic. I don't believe it will work with a conventional setup so we will be experimenting with springs and wedge.