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Author Topic:   help with 3link handling....PLEASE
dluna
Dirt Freak

Total posts: 334
posted August 18, 2004 03:52 PM  
Looking for some assistance on our DW8 3-link setup. Car is good on the tacky (like most) but missing something on the dry. I'm still new to driving, so I could be the root of all evil . When running low, I can back off the gas WAY early and turn through the corner just fine, however, I loose a ton of time. If I go in as hard as the rest, the car just wants to push around mid turn. If I tap the brake to make the rear come around (adjusted to the rear), I end up drifting right into the black stuff. Throttle push is probably the best way to describe it. Here is the setup:

Spring: LF 650 RF 750
LR 200 (front of axle) RR 175 (on top of axle)

Percentages: LS: 52.5%
CROSS: 51.5%
REAR: 58%
BITE: 100 lbs.

Trailing Arms: Short LR (16”) @ 20* Long RR (22”) @ 10*

Pullbar: 600-1200 Progressive Spring 39” @ 15*

Panhard: J-Bar, Bottom hole of pinion, 6” of rake

Wheelbase: ½” static trailing on the RR

On a typical setup for dry, it seems like the right thing to do would be decrease the RR arm angle to decrease the rear-steer, however, in my case that would make me tighter in the center I would think. It seems like the trend is to put more angle in the panhard as the track slicks off to get some sidebite, but 6” of rake in a j-bar seems plenty. I’m actually wondering if this is my problem having too much rake (but I would think it would be more evident on the tacky where bite is built into the track).

I should mention our car is running a little heavy it seems. Without me in it (180lbs), it is 2458 on a full tank of fuel. I was looking to put a 225 on the LR before this started happening…but I’m thinking that might be the wrong direction.

Please help!


ctfarm
Dirt Freak

Total posts: 206
posted August 18, 2004 04:39 PM  
What are you doing with your tire pressures from tacky to dry,wheel offsets,and shocks?Are you using a 90/10 shock?Your setup looks good,supply the above info and we'll try to fine tune it.


autoshop
Dirt Freak

Total posts: 298
posted August 18, 2004 05:17 PM  
Dluna , where is your pull bar side to side in your car, You might think on moving it to the right on the rearend about 1" or moving the front over too. I fought mine for three weeks moving spring , percents and shocks before moving the pull bar to the right on the rearend and that fixed my same problem.

mod70
unregistered Total posts: 298
posted August 18, 2004 05:38 PM           send a private message to autoshop   Edit/Delete Message   Reply w/QuoteBBBBBB
First off, when you say that you have a throttle push after going in easy, that is your problem. (at least part) I had the same problem when I first started driving. I would take it in too easy which caused a push mid, and forced me to over use the throttle to get the car to come around. What I would suggest is going up on the rear springs 25# each, and take a little rear bite out of it down to 75#. You may also want to try pulling the RF offset in or moving the RR out. You didn't say what offs you are running now. Then work on going in good and hard and letting the rear rotate more to set you up with a better angle for comming off.
On a dry/slick track you do want more rear steer. A tacky track will require less. The reason for that is that a tacky track will hold more throttle input and can hold the car around the corner. When it is dry you want to use less throttle to get the car to come around, so you use the rear steer to get it around the corner.

PEDDLER
Dirt Freak

Total posts: 446
posted August 18, 2004 07:25 PM  
Something I don't see here...TIRE STAGGER.
Tire stagger is your best friend when you want a car to turn. Look at a styrofoam coffe cup...Something I always remember from the old wing mod days
Stagger gets you in,,,wedge gets you out.

155fan
Dirt Maniac

Total posts: 163
posted August 18, 2004 10:11 PM  
He said his panhard bar is 4th hole down with 6" rake .......looks like if he would raise it up some that would be another fixer .

boas51
Dirt Maniac

Total posts: 190
posted August 19, 2004 06:59 AM  
j bar is a good start, up one hole on pinion.
he could also try moving the rr spring to the front or lr to the top.

dluna
Dirt Freak

Total posts: 334
posted September 07, 2004 08:16 AM  
I’ve been really busy at work lately so haven’t been able to get on here much and supply some more info. After running at 3 different tracks this weekend, I’m REALLY ready to figure this thing out. I do believe the throttle push that I am experiencing is from going in a little too easy; however, I think I’m going in too easy for lack of side bite. Sunday night in the B, I was trying to run low and the car would drift almost all the way to the top. I started running the high line and the car was a ton better, but still lacking some. Slider broke in the A after a few laps, so we didn’t get much time there.

Offsets:
LF: 4 RF: 3
LR: 3 RR: 4

Stagger:
Front: ½”
Rear: 1” (all I have been able to get with these tires…I bake the RR in the sun on Saturday)

Pullbar:
Running inline with the driveshaft

Spring change:
LR up to 225


Saturday night, had 6” of rake in j-bar for heat and still lacking side bite. Raised the frame side ¾” (lengthened the bar at the same time) and it didn’t seem to help at all. Sunday night, same problem in the heat so I raised it another 1”. So now we are bottom hole on pinion with 7 ¾” of rake. As I mentioned above, could only run the high line or it would drift like crazy.

I’m not going the wrong way am I? Putting more angle in the panhard is going to move the roll center down and to the left…forcing the car to roll over more and plant the RR…I could go on and on but I better stop for now so I can get some ideas.

THANKS!!!

mod70
unregistered Total posts: 334
posted September 07, 2004 10:43 AM           send a private message to dluna   Edit/Delete Message   Reply w/QuoteBBBBBB
What are your ride heights? You might try raising them by a 1/2". Try running a 93/5 on the RR to get it to roll quicker. By going up on the LR spring only you were tightening up the car on exit without really loosening on entry that much. I think you were trying to use that stiffer spring to catapult the car over, but that would be done better by using a higher rebound shock on that side. If you have the room, run a 5" off on the RR, or move the rear end left 1/2" to 1" to get that roll center left.
Also, have you ever tried running a shorty bar on the left side of the pinion? It will react much quicker than the long J-bar. Hope some of this helps dude.

doghouse racing4
Dirt Forum Racer

Total posts: 91
posted September 07, 2004 10:55 AM  
check you ride heights. I would be willing to bet that they are to low. Just a thought and it is quick to check.

dirtywrench13
Dirt Freak

Total posts: 324
posted September 07, 2004 11:33 AM  
WE RUN OUR D-W 8 ALL CORNERS FROM 5&7/8 TO 6&1/4,700,900,200,200. AND STILL WANT MORE SPRING SO WE CAN SET IT BACK DOWN TO GET THE ANGLE ON THE UPPER A'S THAT DIRT WORKS SAYS TO RUN. J-BAR LEVEL AT PINION AND 6-7 INCHES UP THE FRAME SIDE. I'D TRY LESS BITE ON YOUR CAR, SINCE THE LEFT REAR IS PUSHING THE FRONT UP THE TRACK. ALSO TRY MOVING YOUR PULL BAR TO THE RIGHT AT LEAST AN INCH TO HELP LOAD THE RR TIRE MORE.

droper23
Dirt Freak

Total posts: 233
posted September 07, 2004 12:30 PM  
The lower you panhard or jbar is mounted the lower the rear roll center. As you move it up on the frame, you are moving the roll center higher and to the left. Lower roll center creates more body roll. If you want it to plant the left rear more, lower the j bar down some on the frame. I had a short bar and ran it in the second from the bottom hole with about 4" of rake. If I went more than 4", it would barely turn. I also had both springs on top of rear end with a 150 rr and 200lr

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 07, 2004 01:53 PM  
where is the car at? what tracks are you running?

ctfarm
Dirt Freak

Total posts: 206
posted September 07, 2004 02:48 PM  
Adding rake or raising the chassis side raises the rollcenter.Trailing the rt rear will make the car loose on entry.I would suggest lowering the jbar to 4 1/2" and squaring the rearend.To measure the rollcenter on a jbar,measure in a straight line from the center of the mounting bolts.The rollcenter height is the middle of this measurement.Hope this helps


dluna
Dirt Freak

Total posts: 334
posted September 07, 2004 04:12 PM  
Ride Heights:

LF: 5 ½”
RF: 5 ¾”
LR: 5 ½” (when I jack the frame all the way up in the rear, the slider is adjusted all the way up snug against the spring right now…turning it more to raise the ride height on the LR would start compressing the spring at all times even when jacked up)

RR: 6”

We tried the short panhard two years ago. My Dad was driving then and didn’t like it. The track we are running at this weekend is smooth, so maybe I’ll give it a try.

As far as the roll center location, I keep going back to this site:
http://www.rpmnet.com/techart/panhard.shtml

In illustration 2, it shows the logic that I was going by. The first pic shows that if you raise the frame side, you move the RC down and to the right. If the roll center on a j-bar is just the middle of the two mounting points, I can totally understand CTFARM is saying. I know raising the frame side did not help out with the side bite issue this past weekend. What gives guys???

Zero – We ran Midway Speedway (new track in Lebanon) on Friday, I-44 (Lebanon) on Saturday, and Monett on Sunday. I went over the bank at Monett and the rod pulled out of the slider on the LR a couple laps into the A. The drop-off there is pretty wild. We normally run at I-44 during the year. I’m just trying to get as much seat time as possible before the end of the year now that we have a motor that seems to want to stay together. I think I’ll try the Modified Shootout at Lakeside just to experience the faster speeds. I’ll need another motor if I want to be competitive there. The car is setup pretty good right now for I-44, although I could definitely use more side bite. I know I could drive it in harder if I didn’t feel like it was going to come on around.

dluna
Dirt Freak

Total posts: 334
posted September 07, 2004 04:17 PM  
Also, I failed to mention that the RF weight jack maybe has 2.5 - 3 full turns left in it before I run out of threads. The LF still has a ton of thread showing. Sound like I need a stiffer RF?

zeroracing
Dirt Forum Champ
Total posts: 1875
posted September 07, 2004 04:57 PM  
increase the rf to plant it, and decrease lf if you really want. loosen up entry and plant the lr for hard exit.

ctfarm
Dirt Freak

Total posts: 206
posted September 07, 2004 05:33 PM  
What exactly does the car do when you lift off the gas and turn into the corner?It should start to setup on it's on.Also what are your caster settings on the front?


dpfranklin
Dirt Full Roller

Total posts: 35
posted September 09, 2004 10:50 AM  
dluna what car number are you? I often go to Monett to watch.. just curious

dluna
Dirt Freak

Total posts: 334
posted September 09, 2004 03:23 PM  
ctfarm - we are running basically the same caster camber that DW calls for…

CASTER:
LF: +1 ½
RF: +4

CAMBER:
LF: +1 ½
RF: -2

The track we typically run, I can make the car enter the turn real easy. The problem is that I can’t sling it in hard enough to get the car turned how I want it and start heading down the straight. That is where they are beating me. Sometimes it will just want to push. If it would plant the RR going in, I think I could feel a TON more confident in really laying it into the corner ( I can do this when the track still has bite). Last Sunday, if I went in as hard as the guy in front of me, the car would just skate all the way up the track.

If raising the frame side of the J-Bar is raising the roll center, I understand why the car keeps getting worse every time I make a change. Does anybody know the formula to calculate the RC? It has to include spring rates, etc. I just keep going by that “Panhard Tech” article and the location of the RC in their drawing.

I’m really trying to figure this out before this weekend. I’ll be running on a pretty flat track that will take all the side bite that I can through at it!

dpfranklin – Sunday was my first time at Monett. I was just taking advantage of the holiday weekend to get some more seat time. Car number is 11w.

cd2
Dirt Maniac

Total posts: 161
posted September 09, 2004 09:22 PM  
dluna,if i understand right your problem is the car is sliding sideways an out from under you correct? if so try raising some weight up heigher in the chassis & this will help on side bite.Also on the left side you can use shocks that are soft on rebound & on the right soft on comp.This will let the car roll over & get side bite quicker. Low left side weight is good for tacky tracks but on the dry you have to raise some weight up so it will roll over & down on the right side tires for side bite. Moveing the R.R.in or the R.F.out will help on dry.A stronger R.R. spring will help on entry or a stronger L.F.will help. cd2

droper23
Dirt Freak

Total posts: 233
posted September 10, 2004 06:56 AM  
Your rear roll center is in the middle of your two mounting points for your jbar/panhard bar. Take a measuring tape and measure straight from hole to hole, and that is where your rear roll center is located. You do not need to know anything about springs, shocks or anything else. Just take that measurement. The lower your roll center, the more roll your car will have therefore planting the RR more. The higher your roll center, the less roll you will have.

mod70
unregistered Total posts: 233
posted September 10, 2004 09:23 AM           send a private message to droper23   Edit/Delete Message   Reply w/QuoteBBBBBB
ctfarm & droper23 -
I need to correct you on the roll center. I made the same mistake at one time, and a little more research in my Circle Track mags clarified it for me. The point halfway between your mounting points is actually called the "moment center".
Volume 22, number 6 June 2003- article titled "Stock Car Dynamics, part II"
The rear moment center is used to determine the true overall rear roll center. It is a part of the equasion, but not the same thing. See point #2.

Quote from the article:

"There are seven primary things that affect the magnitude of the rear roll angle:
1. The sprung mass of the portion of the car supported by the rear suspention (scale weights at the left-rear and right-rear tires minus unsprung components).
2. The height of the rear moment center.
3. The magmitude of the lateral force.
4. The overall spring stiffness as well as side-to-side spring split (i.e., left-rear stiffer than the right-rear spring).
5. The width of the spring base, which is the distance between the centers of the top of the springs.
6. The rear sway bar's (if used) anti-roll effect must be taken into account. The larger the diameter of the bar in this system, the more resistance there is to roll.
7. The track-banking angle."

The acutal roll center is a complex equation that I'm not even going to try to figure out. The article that dluna posted is the better way of describing the rear roll center. Hope this clears that up for everybody. (yeah, clear as Missouri River mud!)

droper23
Dirt Freak

Total posts: 233
posted September 10, 2004 10:22 AM  
I have to disagree. I have read multiple IMCA Modifieds technology books. They all say that the rear roll CENTER is the point in the middle of the panhard bar bolts. What it looks like you are explaining is the rear roll ANGLE not center.

ctfarm
Dirt Freak

Total posts: 206
posted September 10, 2004 10:55 AM  
For a quick fix, try this.Swap your front springs,lower j-bar to 4" from top of frame to bottom of bar.Square the rearend,the bar angles will control rear steer and try some of the things cd2 suggested.Weight placement and shocks are very good tuning tools.Raise panhard bars to loosen,lower to tighten.Good luck with what ever you choose.

MikeN
Dirt Full Roller

Total posts: 27
posted September 10, 2004 12:36 PM  
Those IMCA technology books are telling you that the roll center HEIGHT, not lateral location is the mid-point of the panhard. Spring rates, and all kinds of **** do come into it. Also, I beleive it's called the moment center because the car doesn't always rotate around that point. It will change. It's only the center for that moment. A moment later, the center is elsewhere.
For most dirt track racers, the guideline in the above post is rock-solid advise. Move the bar up to raise roll center, down to lower it. That's more than likely all you'll ever need to know about roll center.

droper23
Dirt Freak

Total posts: 233
posted September 10, 2004 01:50 PM  
That is exactly what I was saying. Lower the bar to lower roll center, raise it to raise roll center. Lower roll center creates more body roll. Higher roll center creates less body roll. That is what I said from the start. Lower roll center creates more body roll, in doing so, it loads the right rear more in the corner. You say the the "moment center" is always changing. That has nothing to do with the spring rates and other stuff. That has to do with the fact that as the car moves, the suspension moves, therefore, moving the roll center. You can get deeper with it but I would say that would be useless to many people, myself included. All I said from the start is lower the bar to lower the roll center and load the RR during cornering.

dirtbuster
Dirt Forum Champ
Total posts: 2007
posted September 10, 2004 02:38 PM  
Remember that body roll and weight transfer(or sidebite) are different things. You can put really soft springs under the rear and get a ton of body roll with a high roll center but yet not transfer a lot of load to the RR for side bite.
Vice versa: stick some stiffer springs in the car, lower the roll center and even though the body may not physically roll to the right you will get more weight transferred to the wheel and stick it to the ground.


MikeN
Dirt Full Roller

Total posts: 27
posted September 10, 2004 03:47 PM  
Droper,
I wasn't trying to say that you were wrong, just that there is more to it. Also, you perhaps didn't notice that I said moving the bar is all you need to know, essentially agreeing with you.
Spring rates effect how a car rolls, therefore effecting roll center. Not that I'll ever take that into consideration, I follow the KISS principle myself, but some may wish to, and should not be mislead.

Still Learning
Dirt Roller

Total posts: 21
posted September 14, 2004 07:35 PM  
I am going to jump in here a little late... on one posting it says to put the j-bar on 4" from top of frame to bottom of bar. I am confused... what does that mean? On my DW8 I measure my frame side from bottom of frame to center of bolt and it is around 7". Do I need to go lower than that by the way I measure? And will this give me more side bite if I do go lower?

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