Weight Jacking

Discussion in 'The BS Topic' started by aramirez80z28, May 29, 2008.

  1. aramirez80z28

    aramirez80z28 Member

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    Jun 29, 2004
    East Biggs CA
    I just got 85 olds cutlass race car it has weight jacks on it. This car is setup for dirt track racing. Hows do these things work? The weight jack are in the rear. Will have pics later
     
  2. muscl car

    muscl car Veteran Member Lifetime Gold Member


    weight jacks are used to adjust suspension pre-load or to add/subtract the race cars cross weight to obtain faster lap times .the old way was to use a screw jack on ea wheel but like on our dirt track car we use afco race shock with a coil spring kit . to adjust our suspension we just loosen the lock collar and put in or remove a round or two to adjust for the track

    this explains it better :

    Cross Weight:

    Weight distribution and cross weight are helpful tools for tuning your suspension. Unfortunately, many racers misunderstand these properties, setting up a potentially winning car to lose. The following example of this science will demonstrates how chassis weights and preload work. This is how it was explained to me in simple terms, so I will explain it the same way. Mind you this dates back to my Racing education in the 80's, so bear with it.
    WEIGHT DISTRIBUTION
    Imagine that you take your kitchen table and place a bathroom scale under each leg. Stand exactly in the center of the kitchen table. If you are exactly in the center of the table, then all four scales will register the same weight.
    Now if you take half a step to the left. The left side scales will now show more weight than the right side scales. In this example the weight is distributed to the left. This is obvious because of where you are standing on the table. But it is also evident from the reading of the scales.
    From this position, move forward one half step. The left front scale will now register the most weight. This is logical due to your position on the table. Again, by reading the scales it's also possible to tell where you are standing on the table.
    Just like positioning your weight on the table, your race car's weight will respond exactly the same. However, reading the scales to determine the weight location within the car is a little more complex than this kitchen table we used as an example. Refer to our section on roll centers, this section will help you in your quest to find absolute center of your race car.
    PRELOAD
    Up to this point, all four table legs are rigid. Now place a spring under each table leg, And, just to make it a little more fun, suppose the bottom of each leg had a threaded collar resting on the top of the spring. This simulates your race cars suspension, if your race car doesn't have jacking bolts, omit the jacking springs.
    Now the table now has weight jacks just like your race car. Place the scales under the springs. If all four springs were exactly identical and the threaded collars were adjusted to be the same height, and you stand in the middle of the table, then the scales would read no differently than prior to adding the springs (except for the newly added weight of the springs and jacking collars).
    However, if you were to adjust the right front spring collar to slightly compress (preload) this spring, then the right front scale will register more weight. At the same time, the left rear scale will register slightly more weight. In this condition, t he right rear and left front scales will register less weight, too.
    In this example, if you were to add the weights registered by the left side scales you would find they were 50% of the total. Even though the weight at each corner changed in this example, the weight distribution remained unchanged because you are still standing in the middle of the table.
    About this time, many racers give up on trying to understand weight distribution and cross weight. Dont give up, with the use of this section you can and I stress understand how cross weights work.
    IN THE REAL WORLD
    The kitchen table up to this point has responded exactly the same way your race car would respond for these same changes. In reality, your race car has suspension parts (A frames, tie rod ends, ball joints, link arms and so on.) making it slightly more complicated, but, the physics is the same for both cases.
    With no preload, the weight distribution is clearly understood just by reading the scales. You must take in account the difference in spring rates when you look at your weight distribution. The stiffer the spring the more weight it will move to the opposite corner of the car. You can with some playing around figure that for every 50 pounds of spring change you make you will change the weight distribution about 2-5%. With uniform weight distribution the prelaod can also be understood by simply reading the scales. But, throw in a mix of preload (cross weight) and non uniform weight distribution and you have scale readings that are no longer clear indications of these kitchen table examples.
    CROSS WEIGHT; Your worste nightmare...or is it?
    Simply adding the scaled weights from diagonal corners of the car tells you little about the race car. By the time you factor in different spring rates, sway bar rates and motion ratio, different spring locations, different wheel offsets and rear axle offset, the cross weight is not much more than a number. Preload, however, is a real and scientific aspect of your race car. You must understand what your combination is going to do. You should try setting up your car with different springs, swaybars, and wheel offsets to see what is affected to get an understanding of how things are affected. Major changes can occur just by changing your wheels offset. Start off with a base spring rate and change one thing at a time to see what the effects are. NEVER, EVER change more than one component at one time..This is also true at the track. I have seen teams change their air pressure in one tire and change the wedge (spring weight) of one or more corners of the race car and have it handle worste than when they had started. Always remember if you change your cross weights you must turn the opposite corners to reflect in respect to your ride heights. If you want more cross weight and compress your right front spring and left rear spring you must turn them the same amount of turns while taking the same amount off the opposite springs. Like I said, this effects your ride heights. If you don't follow this to a tee, you will end up having a car that wont handle on the track. BE WARNED.......

    Preload is also a good diagnostic tool for selecting springs on short track cars.
    INTERPRETING PRELOAD
    When reviewing your Chassis Weights, if you find a preload greater than 25 to 50 pounds, then you could benefit from a spring change. Some people have found preload in excess of 200 pounds. The best way to check for preload is to use a spring tester and measure your compressed distance as the same as is set on the race car. The number you end up with when checking the compressed number will be your preload weight. You can purchase a tester for around 250 dollars, it is a worth while investment.
    If the scales shows excessive preload in the right front and left rear, then you should interpret this as the car needing a larger front sway bar. If your car does not have a sway bar, you must change the spring rate on that wheel. Use of sway bar is only recommended on asphalt tracks. Another benefit from using Chassis scales, and spring testers, is if you find excessive preload in the right rear and left front corners. In this situation your race car could be improved by using stiffer rear springs
     
    Last edited: May 29, 2008
  3. Twisted_Metal

    Twisted_Metal Moderator Staff Member Lifetime Gold Member

    25,386
    328
    Feb 26, 2004
    Bloomington, MN
    I think Chuck covered it. Great info! :D

    I got my race car suspension lesson today too! ;)
     

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