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Author Topic:   Lets talk J-bar tech
JT54
Dirt Maniac

Total posts: 170
posted June 22, 2004 05:34 PM  
First off I have studied the archives on this subject thoroughly and would like to hear some fresh discusion, what I understand is raising on both ends raises roll center and produces a loose entry,lowering doese the opposit ,what I dont understand is the affect of moving it at one end or the other.......right now my 3 link car is bad fast on a tacky track with good entry and exit the J bar is 4 1/2 higher on the frame side and even with the pinion on the other end .....when the track dries out I still have good forward bite but I think side bite is lacking....I moved the pinion side down to the lowest hole in a dryslick heat race and car realy rolled over hard .....not too sure I liked it that way.....would just moving the bar down 1 1/2 on the frame side cause the car to roll a little less than moving it on the pinion end and still increas side bite?.......I am new to J bars .......I know it is going to come down to trial and error ....but a few hot laps on a greasy track before the races isnt teaching me what I need to know .....Good fast top 5 car but I am missing something to get me to the front......I know there are lots of smart people out there on this subject so lets here some track condition senarios and adjustments as the night goes on....thanks in advance Jonnie "T"54

JT54
Dirt Maniac

Total posts: 170
posted June 23, 2004 01:01 AM  
anyone ?

rocket36
Dirt Freak

Total posts: 209
posted June 23, 2004 02:31 AM  
if the track is rough, lowering both ends to lower the roll centre usually works, this way you get a little more side bite but not too much as you don't need alot when the car is running through bumps as the bumps will help to promote side bite anyway. you will also not get as much rear end movement (rear-steer) as the j-bar is not working in the lower part of its arc.

with more angle (either lower at the pinion or higher on the frame) you really plant the right rear hard (good for smooth slick tracks). you get alot more rear steer as the bar is really starting to move sideways in its arc of travel. when excessive, the chassis may even start to climb onto the left rear bars (4-bar car mainly) which will really give you some drive off the left and start to gain even more rear steer as the left bars drop into there lower part of the arc they move. when we do this with our late model, we have found it is really easy to go too far (too much angle) and the car will get a push very easily. we mainly just move the j-bar at the pinion (1 1/2" increments).
if the track is real hooked up or rubbered up, we usually take some angle out of the j-bar.
it really is just trial and error as to what you and your car like and how you want to drive it.
don't forget to re-adjust your rear end lateral location (j-bar length) every time you move one end and not the other as just a 1/4" difference in rear end location will make a difference. we purchased an aluminium j-bar from "weirs machine" which has a turnbuckle at the heim joint end so that you can adjust the length without removing the bar (one of the best "trick" parts i've ever bought).

dluna
Dirt Freak

Total posts: 334
posted June 23, 2004 10:37 AM  
rocket, can you explain this upper and lower part of the arc on a j-bar that you are talking about? Are you talking about the bend that is in the j-bar, or another characteristic of a panhard bar as it moves under chassis roll? Why is the j-bar not working in the lower part of its arc when it has less angle?

>THANKS<

JT54
Dirt Maniac

Total posts: 170
posted June 23, 2004 11:19 AM  
Thanks for the reply ,this is the type of explaination I was looking for ....anyone else want to add to this ?

rocket36
Dirt Freak

Total posts: 209
posted June 24, 2004 05:34 AM  
the best way to see this is to actually do it.
unbolt your j-bar from the pinion.
hold the j-bar (pinion end) at your normal hieght and let it swing down in its arc 2 inches and notice how far the end of the j-bar moves sideways.
now hold the end level with your lowest mounting point (maximum angle) and let it swing down 2". notice that the end moves sideways (to the left) further. this is because the bar (any bar for that matter) moves in an arc. this is why if you lift your left top 4-bar rod as high as it will go on the frame, you will increase dynamic roll steer.
hopefully i explained that well enough.

JT54
Dirt Maniac

Total posts: 170
posted June 24, 2004 11:34 AM  
I forgot to consider that the right rear moves left wich tightens the car.

rocket36
Dirt Freak

Total posts: 209
posted June 24, 2004 02:05 PM  
with more angle, the car will tend to try and jump or pole vault up on the j-bar, that means the weight of the car is being held and pushed down onto the track by the bar and not the springs. this really plants the right rear hard (side bite) and is good for slick/flat tracks. i'm not sure that i would run alot of angle in the j-bar if the track is, or is going to, rubber up. when the track takes rubber, treat it like a tacky track.

JT54
Dirt Maniac

Total posts: 170
posted June 24, 2004 02:20 PM  
The track I run on is slick like ice and has one groove on the bottom ,It seems when the car gets enough bite in the rear to get off the turn without rosting the tires then the front tires loose too much grip....I know ....Its a matter of ballance ....its just frusterating getting from being in the ball park to being the first one to cross the checkers......alittle more trial and error and we should be able to Git -er-Done.

rocket36
Dirt Freak

Total posts: 209
posted June 24, 2004 02:47 PM  
if you run a lift arm, maybe try shortening it up a little. we did this on our latemodel and it made a significant difference. if you run a pull bar i have no idea..LOL

JT54
Dirt Maniac

Total posts: 170
posted June 25, 2004 01:21 AM  
I have a lift arm and a pull bar ...the lift arm is just used for braking and the pull bar is pre loaded against the spring but has no affect on braking......I am thinking of running a 125 spring on the lift arm with the pull bar if I still have exsesive travel on my pull bar.

rocket36
Dirt Freak

Total posts: 209
posted June 25, 2004 04:38 AM  
i have absolutely zero experience with a pull bar so i can't comment. i've only ever seen one.

JT54
Dirt Maniac

Total posts: 170
posted June 25, 2004 11:23 AM  
Thanks Rocket.

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