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Author Topic:   4 bar ?
mod2_x
Dirt Full Roller

Total posts: 38
posted October 26, 2005 07:49 PM  
I am having a new rear clip put on my car. It was a swing arm on both sides and will now be a 4 bar on both sides. Its on a metric front and I am looking for a good starting point for a set up. I will be able to move the springs ahead or behind and would like to know where to start. If you could please explain your choices. Thanks

FlyNLoIMCA17
Dirt Forum Champ
Total posts: 753
posted October 26, 2005 09:32 PM  
LR spring behind axle and RR in front. Both mounted on the birdcage.

LR spring- 225 RR spring- 200

Bar angles
LR:
Top- 15-20 deg up
Bottom- 5 deg up

RR:
Top- 12-15 deg up
Bottom- level

Now for the explanations. The spring location is due to birdcage indexing. Basically it works just the opposite of a standard 2 or 3 link. When the RR of the chassis compresses, the bottom of the birdcage rotates forward therefore you put the RR spring in front of the housing so that the cage compresses the spring as the car rolls.
The LR goes behind the housing. It works just like the RR but remember when you accel the LR of the chassis doesn't compress, it raises. So then the bottom of the birdcage rotates backwards. By putting the spring behind the housing, the cage rotates towards the spring on accel. (thats the easiest explanation I could come up with LOL!)

Hope that helps!!

zeroracing
Dirt Forum Champ
Total posts: 1875
posted October 26, 2005 09:44 PM  
Here is my advice starting out with each the Left Side.
Left Rear-- Many modifieds right now are running "clamped up" which means they clamp the spring in front of the rear end housing, so as you get onto the gas the spring will wrap up and help to hike up and plant the LR tire. This is a very simple theory behind it, stomp gas, car hike up.
Good points--This set up is ok for rough and tacky tracks, it can work good in those aspects, sometimes works good in slick. More of a stop and go tacky track set up.
Bad Points-- This is VERY throttle dependant, ever watch a 4 bar car with this in traffic, sometimes they will stomp gas hike up let offslam down, stomp gas hike up, very hard to maintain speed in traffic for an aeverage driver. Inconsistant, the car gets the inital hike up from kicking the LR spring up by axle rotation. Well one slick especially you can stomp the gas but not everytime the car will "bite and load the spring" Some of the time, the driver will hit the gas, the car will get power to the tires, and not actually grab and load the spring, instead kinda spin the tire some and eventually load the spring. This setup on slicker tracks can lead to lots of inconnsistancy and overall tires beginning to fall off toward the end of the race.

LR behind floated- THis set up is where you mount the LR spring behind the axle and on the birdcage so it rotates with the movement of the bird cage not the actual rear end wrapping up. This set up is very versitile.
Good points-- car is very connsistant, loads the spring off body roll not rear end rotation, you canforce a car into starting it by jerking the wheel to get the car to roll over. the car is very tuneable with this set up. The car is easier on tires, and they are not as prone to dropping off at theend of the race.
Bad points--harder to tune, more adjustment so you can get the car out of the park if you go hog wild on changes without knowing what to do. If not properly set up then can be a handfull on rough.

My take on it, I run lr behind All the time. There are so many ways to alter the way the car reacts with this so you can make it work in any condition.

Right Rear- Place it in front and on the Bird cage, this is what pretty much everybody does. IT loads the spring more as the car rolls over which plants the rr tire more as the car rolls over. Pretty much set it there, and forget it other than bar angles and spring rates.

baseline
LR clamped-250 LR spring SHock clamped behind.
LR floated-225 LR spring(on coil over for slick and smooth, and on slider with the real shock on front of birdcage for rough or tacky).
LR angles- starting out about 15-18 deg uphill upper and level lower

RR Floated spring 200-225 rr in front on birdcage on coil over.
angle 15 upper level lower.


When you have the new clip put on, make sure they DONT USE SHORT 4 LINK BARS they must be 16 at least upper to be controllable. many mods came out iwth 14 upper the cars are like pogo sticks and hard to drive and tune.



mod2_x
Dirt Full Roller

Total posts: 38
posted October 26, 2005 11:40 PM  
Thanks for the info hope they keep it comming. I had them set up the rear clip for 16 inch top and 14 inch bottom bars. I designed the mounting brackets myself on these rod lengths. I had planned on doing the work myself but the more I looked at what was going to be involved it seemed easier to just reclip it. So I let the local race shop do it. I dont fully understand the 4 bar stuff and I have always just made small adjustments on the swing arm. I try to keep things simple and try not to go overboard on adjusting things . I think it helps the learning curve. Also I need info an percentages and where to start on lr bite and will there need to be any front end changes from what I ran on the swing arm deal. and what about rear steer. Should I square up the rear end or add static to it. I really dont know what to expect from the switch. I believe the 4 bars have a lot more rear steer to them and I would hate to enter the corner and have the thing pointed to the infield mid corner.

[This message has been edited by mod2_x (edited October 26, 2005).]

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