Four Link adjustment and set-up

How a four-link works and general set-up and adjustments for modifed four bar suspension.

The Four Bar Set-up is a highly adjustable suspension that promotes traction and rear steer. When adjusted properly to track conditions and driving styles, the four link set-up can be a very effective weapon.

General Guidelines

  • Do not overtighten a racecar. To much traction (Left Rear) will not win races.
  • Adjust your chassis so you can keep the most momentum. Momentum requires less traction.
  • As a general rule- Upper 4 Link bars mostly affect corner exit, while the bottom 4-Link bars mostly affect the entry.
  • More angle in any bar adds more weight to that bar (traction) but also creates more rear steer.
  • Too much upward angle will create to much roll steer and/or can top shock out.
  • The trick is to find a happy median between traction and roll steer.
  • Bigger tracks require less upper four-link angle.
  • Smaller stop-and-go tracks require more upper four-link bar angle.
  • Upper bars are generally longer than lower bars to induce traction but limit rear steer.
  • Raising Left Rear Bottom 4 Link bar creates more "hike up" inducing roll steer and loosening car throttle
  • Lower Left Rear Bottom 4 Link bar holds chassis on spring more inducing less roll steer therefor loosening car on throttle

4 Link Adjustment Dirt Modified

TO TIGHTEN CORNER EXIT

  • Left Rear Top 4 Link Bar: Raise on chassis
  • Left Rear Bottom 4 Link Bar:Lower on chassis (Reduces roll steer and holds more spring load to tighten)
  • Right Rear Top 4 Link Bar: Lower on chassis
  • Right Rear Bottom 4 Link Bar: Lower on chassis.

TO TIGHTEN CORNER ENTRY

  • Left Rear Bottom 4 Link Bar: Lower on Chassis (More noticeable on throttle than deceleration)
  • Right Rear Top 4 Link Bar: Raise on chassis
  • Right Rear Bottom 4 Link Bar: Lower on chassis

TO TIGHTEN CORNER MIDDLE

  • Left Rear Bottom 4 Link Bar: Lower on chassis (On throttle)
  • Left Rear Top 4 Link Bar: Raise on chassis (On throttle)
  • Right Rear Top 4 Link Bar: Lower on chassis

TO TIGHTEN CORNER MIDDLE/EXIT

  • Right Rear Bottom 4 Link Bar: Lower on chassis

TO LOOSEN CORNER ENTRY

  • Left Rear Bottom 4 Link Bar: Raise on chassis
  • Right Rear Top 4 Link Bar: Lower on chassis
  • Right Rear Bottom 4 Link Bar: Raise on chassis

TO LOOSEN CORNER MIDDLE

  • Left Rear Top 4 Link Bar: Lower on chassis
  • Right Rear Bottom 4 Link Bar: Raise on chassis

TO LOOSEN CORNER EXIT

  • Left Rear Top 4 Link Bar: Lower on chassis
  • Left Rear Bottom 4 Link Bar: Raise on chassis
  • Right Rear Top 4 Link Bar: Raise on chassis
  • Right Rear Bottom 4 Link Bar: Raise on chassis

 

11 Responses to Four Link adjustment and set-up

  1. Pingback: 3 Excellent Resources for Setting up your Dirt Modified | Racer Setup Sheets

  2. Jacob says:

    In your “General Guidelines portion”:

    Raising Left Rear Bottom 4 Link bar creates more “hike up” inducing roll steer and loosening car throttle
    Lower Left Rear Bottom 4 Link bar holds chassis on spring more inducing less roll steer therefor loosening car on throttle

    It appears there is a missing word… since you say both raising and lowering LR bottom 4 bar loosens the car on throttle. Can you please clarify? Thanks!!!!!!

  3. Jeremiah says:

    Has anyone switched to a straight bar and got rid of their j-bar? If so did it work better for you or did you go back to the J-bar.

  4. nick messner says:

    I have a straight bar and it seems to be working for me. Does anyone have problems with a straight bar as the j bar

  5. Chad says:

    Jacob,

    There is no missing word. Pleaser remember that all i explain below is under throttle. It is key to remember that this applies only under power, throttle down.

    Raising the lower left rear bar on the chassis ( more angle ) hikes the chassis faster on the left rear and also because of additional bar angle will pull the left rear tire further forward in the car. This makes the left rear bite harder but will rotate the car faster, remember this is the shorter bar, it changes roll steer quick. This condition can and will cause a loosing of the car to the feel of the car due to the increase in rear roll steer.

    Lowering the left rear lower bar on the chassis will not hike the car nearly as fast, which will not load the left rear tire as fast. You will not get the roll steer as explained above, but you also do not get the forward drive as i explained above. Staying on the left rear spring longer and not hiking as fast will also cause a loose on throttle condition.

    I hope this helps you.

  6. Flip says:

    I found your technique of factors surpassed by the need to know of where things are now! Set-up to set-up it would surely help in any move made by those trying to improve to know better chassis to chassis design! I found no info in indexing to achieve desire in movements of bars by length or is this just understood by all in there builders notes and specs per set-up?

  7. TJ Smith says:

    Absolutely awesome guideline.

  8. Lock & load says:

    As far as bigger track less bar angle vs small track more bar angle. Much of that has to do with gearing and momentum also. and keep in mind that for each gear and track condition you still need to squash that rf spring the same amount each time. Sometimes a softer spring on the rf may help hike the bars the prescribed Amount. For each corner radius and banking and track width (c to c of rear tires )there is an optimum amt of stagger.
    Axel lead. Not many posts on this issue. Keep in mind that pre rear steer suspensions tracks had x amt of stagger necessary to turn the car. Now when the car gets up on the bars the front centerline is moved to the left. You can take some of that out by starting with the lr behind square whilst in the static state. Please reread that sentence again. Notice I DID NOT say you move the lr back. The lr axel does not move when you lengthen or shorten the bar lengths——- you actually move the front of the chassis l or r of center. The angle of the rear axel is determined by the rear stagger whilst on the throttle, provided you aren’t blowing the tires off. Putting in 5″ of stagger is just a gigantic crutch. Adj the drive angle ( bar lengths) to the correct setting. Even better, have several sets of bars pre adjusted so when you swap them at the track your static scale weights and bird cage adjustments are where you want them and you not guessing.
    J bar vs straight bar. Flat large radius tracks transfer more weight l to r. Lower the weight or increase left percent. Longer j bar
    High bank short radius track, raise the weight or decrease left percent- quicker short straight bar. Somethings to digest, eh? Everything affects everything. Anytime any of your adjustments gets way out in left field, you are missing something somewhere else. Go back to the notebook ( you do have one?) for each track you run, don’t you?
    If you want to impress the front runners, come to the track with an open trailer 4 tires in the rack and a few guys that know what’s going on and run in the top 3/4/5 all weekend. I will be impressed.

  9. Aaron B says:

    Thank yall for the input it’s hard to find reading material on the 4 bar setup as I’m moving into a mod and eager to learn

  10. Anonymous says:

    A clearer explainion of what reaction the bars cause. if one had to set from scratch! Would you set the bottom bars first, how would you start. Sometimes the bottom bars first?

  11. Roy says:

    Dirt late model to tight go in
    What is best thing to do

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