The Mittler Pull Down Rig...
The Mittler Pull Down Rig is more than just that, it's an all-around chassis evaluation apparatus used to prepare your car at the shop. Let's begin the evaluation.
There is a new setup tool that is designed to be used for the evaluation of the chassis components that is unique and interesting. I visited the race shop for Xpress motorsports in Mooresville, NC, to look over its pull down rig designed and manufactured by Mittler Brothers Machine and Tool company. Mike Mittler and company develop, build, and sell some of the most useful tools in our industry. And this rig is no exception.
My personal fascination with these types of test rigs comes from the understanding that there is much we don't know about what happens with all of the various chassis components on the racetrack. Any chance we have to look more closely at the workings of all of the parts and pieces of our race cars, the better. And every one of these contraptions has its pluses and minuses. As far as this one is concerned, I saw a lot of usefulness and potential.
The Mittler rig is designed to literally push/pull the car down to simulate both the attitude and forces that the car experiences on the racetrack in the turns. Once we have done that, we can do a series of evaluations for compliance (bending of the chassis and/or individual components), clearances, alignment, stress and tire cross sectional loading.
The unit is fairly compact and somewhat portable as compared to other machines of similar design that must have a facility built around them. The cost to prepare a site for a seven-post rig runs in the hundreds of thousands of dollars in and of itself, aside from the cost of the actual rig. Mittler's rig is bolted to the floor, any floor basically, and can be moved if necessary. It can even be made portable although no one has actually asked to have a portable unit built, yet.
How It Works The race car, or truck in the case of the Xpress Motorsports team, is rolled onto the unit and hydraulic pistons are attached to the chassis with specially designed attachments to three points, usually two up front and one in either the left rear or right rear (most common) corners.
The tires sit on specially designed scales that have three long pads, under the outsides and middle of the tire. These measure loads at three points on the tire contact patch to judge the evenness of tire loading. More on that later.
The race car is pulled down to preset loads or ride heights based on design criteria, on-track data accumulated, or predicted loading and chassis travel depending on the particular track you will be racing on.
The mounting brackets used...
The mounting brackets used to connect the chassis to the pull down cylinders are attached to the main frame channels with heavy-duty steel plates. The unit slides into the open end of the framerail.
The push up portion of the...
The push up portion of the Push/Pull Rig is located under the tires. Note the three-part specially developed scale that reads pressures on the inner, middle, and outer portions of the tire.
Changing springs is very easy...
Changing springs is very easy with the PDR because you can lift the chassis to provide plenty of space to remove the springs. During an evaluation process, the team can make spring and sway bar changes and re-evaluate the load distribution as well as clearances and alignment.
Once the loads are applied, various observations can take place that teach us a lot about our chassis that we would otherwise never see. If you think about the possibilities, there are many.
Different Approaches You can approach the loading and pull-down distance and magnitude from different angles, so to speak. If you have your shock travels from on-track testing, you can pull down to those exact dimensions and check the tire loading, clearances including the spring coil gaps, and other criteria.
In some cases, you can get engineering data such as the predicted tire loading that includes the weight of the vehicle, load transfer due to lateral forces, aero downforce loading corrected for front to rear percent, and mechanical, or dynamic loading that is the result of the combination of lateral loading and gravity.
The predicted loading method is a bit complicated, but not impossible to calculate. I just wrote an Excel calculator that does just that for a Grand Am team and a DP car. Sorry, it's proprietary, so don't ask for a copy. The team can do both the shock travel method and the calculated method and then compare the results. They should match up, but if not, the reason why is sought out.