|To support our commitment to help you reach your racing goals, we've
included this informational section to our site. It includes articles written by the
Race Depot owner, Bob Seymour.
Squaring the Rear Axle
By Bob Seymour
Racing today, especially at the higher level is becoming so
sophisticated I sometimes feel that without an engineering degree or an aerospace
background I might as well get into a different sport.
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Teams equip themselves with chassis specialist, shock dynos, engine tuners and more. Seems
like the days are gone when guys like Junior Hanley in stock cars, or Lee Osborne in
sprints could design and weld up their chassis, come up with their own shock package,
assemble and tune their motors and compete on a national level. Unfortunately today that
type of racer is as hard to find as a baseball player that plays every position and bats
But what made guys like Ozzie and Junior so successful is they did everything with
precision, paid attention to detail, kept things simple and used good common sense. So
many times today we get caught up with getting too trick. We tend to overlook the simple,
basic things about set-up that can mean more than shock valving or dyno tuning.
That brings me to one basic open wheelers frequently overlook and means a great deal
SQUARING UP THE AXLES!!!
If you were to ask a group of open wheel racers if they set their rear ends square to the
motor plate, you probably would get several different answers. One may say dead
nuts. Another may lead the left side or right. Some may not want to tell you and
others may not even know.
In my opinion for most cars and tracks starting from square is theplace to
begin. The bigger the race track the more critical this becomes. On a small, round track
you may try rolling the right front ahead and the right rear back. This may help the car
to roll freer through the center of the corner or even on a round straightaway. On a tight
corner and long straightaway track you may try to lead the right rear just a little to
tighten the car up off the corner. (A word of caution there would be to make sure your
drive line is still free to move up and down without any binds!) These are set ups you may
want to try during testing not for the first time during race conditions.
Regardless which direction you go the first thing you need to establish is that the rear
axle is square to the motor plate and the rear end in the center of the chassis (or offset
as much as the motor plate). There are a number of ways to accomplish this. Here is the
procedure we use.
1. Check the motor plate for flatness with a good straight edge, install the motor plate
with light bolt tension. Be sure to check that the plate sits flat on the frame tubes.
File off any high spots in the paint. Do not flex the plate while tightening the bolts.
2. With the bare chassis on stands, pull a length of waxed dental floss from the middle of
the front chassis tube, through the hole in the motor plate, to the middle of the rear
3. Using a nice readable tape measure, at the front, measure from the line to the side
frame, left and right, and center the line within a sixteenth.
4. Perform the same procedure at the rear. Now double check both ends. When satisfied ,
mark the line location on the tubes with the edge of a triangular file.
5. Use a long sheet rock t-square, check the squareness of the motor plate with the
centerline of the chassis. The edge of the square should be within a sixteenth of the line
at the end of the square. Project the square both forward and rearward. Be careful when
setting the square on the motor plate as not to disturb the line to achieve the most
accuracy. Patience counts. Shim the motor plate carefully to correct any out of
6. I prefer to have the chassis on the floor on ride height blocks, but you can perform
this procedure on stands. Install the rear end, torque tube, and ball housing (cover not
required). Assemble the rear suspension completely (less shocks and springs), connect the
jake ladder or panhard bar (rear) and all the links so the rear is completely located and
has only one degree of freedom (up and down). Do not take any shortcuts here by omitting
any link, bolt etc as it will severely affect the accuracy of the setup. Set the rear at
the correct distance from the motor plate. Set the rear at ride height.
7. With a very straight bar clamped to the motor plate, measure back to the
rear axle at the widest point, left and right. Be sure to measure to similar diameters on
the rear axle. Adjust the links to achieve parallelism between the motor plate and the
rear axle (to zero). Now check to see if the ball pops in and out of the housing with
absolutely no friction. Adjust the rear left to right with the panhard bar rod end to
center the ball. Unfortunately this will change the parallelism of the rear considerably,
so perform the entire operation several times until the rear is parallel and the ball is
centered. Close enough is not good enough. Spend the time required to get the rear perfect
and the ball perfect. There will most likely be a small error if you check the centerline
of the rear between the frame rails. A little clearance here and there between the
driveline boltholes accounts for this. The most important measurements are rear axle
parallelism and ball location.
Now for grins, bump the rear up one inch or so and check the measurements back to the axle
again. This simulates the car under acceleration. Ideally the rear will remain perfectly
square, but unfortunately the geometry of the single link rear is not that good. Now move
the right side up and the left side down an inch or so. This simulates chassis roll in the
corners. There definitely will be some parallelism change but hopefully the right side
will be a bit shorter than the left with the ball still centered.
8. With the rear back at ride height now install the front end completely and check the
location distance to the motor plate and square the front axle.
I hope that these guide lines will help a few of you to get to victory lane or at least
have a better handling chassis. And who knows
.maybe beat up on a few of the high
tech teams along the way.
Sometimes its the little things that make a big difference. Good Luck!!
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