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.
The reason why the shock travel method might not match the predicted loading is because of the compliance of the components during high loading due to stress. Many of the components including control arms, bushings, spindles, crossmembers, axle tubes, brackets, and more, bend under high load. This affects the shock travel and the way we load the chassis affects those changes.
Real World To Test Rig In the real world, the loads affecting the chassis are spread out throughout the entire chassis and include the effects of parts bolted or welded onto that chassis such as the engine. It's virtually impossible to replicate that on a machine, but we can come close enough to gather some important data.
On a test rig, we have to attach to the chassis at specific points that are easily accessable. So, we introduce loads to those points in excess of what they would see as the car negotiates a turn. Nonetheless, we accept this deficiency and work with the simulation to help us understand what goes on with the chassis. The same is true of any current model of dynamic analysis apparatus.
Both the simulation of loads and the measurement of compo-nent movement are valid tools for replication of the event of cornering. I might be inclined to use a combination of both, utilizing the exact science of dynamics along with the somewhat imprecise science of translating a measurement although the measurement method helps to eliminate errors in the application of the loading from the track to the rig.
Uses For The Pull Down Rig Once we understand the way the rig works, we can think of many uses for it. Here are just some of the ways a team can utilize the rig to better understand how their chassis is working and to allow better design of components and setups for specific tracks. From the Mittler information sheet, "Typical studies, actions, or items of interest include:
Matt Teeple is a consultant...
Matt Teeple is a consultant to the team and handles all of the computer work during test sessions. The PDR motions are controlled at the computer, meaning he moves the cylinders individually using the mouse and cursor placed over specific buttons on the screen.
The team has fabricated an...
The team has fabricated an extension off the front of the unit so that exact measurements can be taken for the height of the splitter, which is critical to aero control. The measurements are made at the spots where the orange tape is placed.
A quick look tells us that...
A quick look tells us that the spring is almost completely compressed into coil bind. The team can measure the exact spacing between the coils at any pre-determined ride height. Observations are made at this point to determine the amount of compliance for various parts. Compliance is movement and/or bending of components. The angle of the top and bottom of the spring relative to the upper mount and lower control arm are just some of the considerations.
• Stroking the chassis through complete travel to check for binding or tight clearances
• Study toe changes over the full suspension motion range
• Study camber changes over the full suspension motion range
• Evaluate corner weight effects of sway bar changes
•Study clearances to shock bumpstops
• Evaluate load distribution across tire
• Prepare for rapid testing/practice spring changes
• Accurately align chassis spring ends in mid corner positions
• Evaluate feasibility of unusual bumpstop setups
• Review mid-corner dynamic wedge percentage
• Evaluate accurate front splitter height and/or valance setting
• Study rear steer and camber changes
• Good substitute where sanctioning body testing bans are imposed"
Here are my thoughts on some of these evaluations and how they apply.
Load Distribution One of the most critical uses for the PDR is to measure and evaluate load distribution. As the chassis is compressed and rolled, the sway bar influences the load distribution just as the camber changes and coil binding and/or compression of the shock bump rubbers do. It is of great importance that we know what these changes are.
There is a complicated rearrangement of the loads that takes place as the attitude of the chassis changes. We could never predict or calculate the result of all of the influences. But the PDR shows us exactly where the loads go and that is translated into actual wheel loadings and crossweight percents.
If our crossweight does change through various increments of dive and roll, then the handling balance will necessarily change. It is possible with the PDR to make component changes and reach a point where the load distribution change is minimal and the balance stays consistent. Handling is all about how the loads are distributed and re-distributed and this use of the PDR seems to me to be of utmost importance.
Clearances Once we have determined the correct loading and/or suspension travel, we can measure the crossmember clearance, ride heights, front valance clearance, spring coil spacing and degree of binding, spring angle to top and bottom supports, sway bar angle and alignment, shock piston position, spring-to-shock clearance (for a coilover car), tire-to-fender clearance, component deflection, and more.
Now, those are a lot of areas to look at and you might be able to think of more. Most team goals these days involve attaining an attitude of the chassis and body that will be low and level to the racing surface to achieve maximum aero efficiency. With the correct input and pull-down travel, you can know exactly what the clearances are for many areas of the chassis.
When we calculate ride height changes due to the dynamic and aero forces, we often forget to include the tire deflection. The tire has a spring rate and thus a compression amount when a load is applied to it. So, we have a ride height change associated with tire compression that must be added to the ride height change due to spring compression. On the Pull Down Rig, all of that is taken into account and visualized.
Alignment One of the functions of the PDR is a system for measuring the wheels, front and rear, for camber change and steering angle changes. As the chassis is loaded, we need to take precise measurements at the wheels and the PDR does that, and the information is transferred to the computer for evaluation.
As the chassis compresses under loading, the suspension parts move and changes occur to the geometry. Cambers and casters change, bumpsteer-associated angles change with the control arms as well as the tie rods and even the drag link in that system. We can do a bumpsteer evaluation any time and at any chassis attitude or steering angle in our shop, but that doesn't take into account the compliance of the parts and pieces due to loading.
On the rig, we can do dynamic measuring of front end toe changes, Ackermann, camber and caster change, rear steer, and rear toe all under real track loading conditions. One of the least understood aspects of alignment involves the rear solid axle system. Under high loading, we often see with high lateral and vertical loading, there is compliance of the axle tubes and control links.
The tool used to measure bumpsteer...
The tool used to measure bumpsteer and camber change is shown here. There is a potentiometer, or LVDT as the engineers like to call it (Linear Variable Differential Transformer) that records steer movements of the wheel and an inclinometer to measure wheel angle for camber readings.
Here we see a close-up of...
Here we see a close-up of the LVDT and at the center of the plate, the inclinometer for camber readings. The measurements are very accurate and are able to be recorded for each setup combination.
This screen shows the truck...
This screen shows the truck at normal ride height and static weight of 3,202 pounds total. Note that the red portion of the weight columns represents the loading. The RF and LF tires are more heavily loaded at the outside and middle when at ride height.
The Mittler PDR has the capability to measure cambers and toe changes as the chassis goes through its loading phase. With the late-model cars, the rearend will rotate as the chassis is compressed and that changes pinion angle, rear-wheel toe, and cambers. Imagine being able to physically measure all of that with the chassis loaded as it is on the racetrack. That is exactly what you can do with this rig.
Measuring The Effect Of Changes Each change we make to spring rates, sway bars, and so on comes with peripheral affects. One change can affect other settings and load distribution. Making changes and then evaluating those with the PDR can alert us to the consequences of our actions so that we are not surprised when we go to the racetrack.
Let's say we want to make a sway bar change to a larger bar. We can see the physical effect that change has on ride heights with the same tire loading as before the change. We can tune certain component selections to affect the chassis's attitude on the track. Our overall loading may be the same, but the attitude of the chassis and body to the racing surface can be fine-tuned with different combinations of springs and sways.
We may find that we need stronger components in certain areas of our suspension to reduce compliance and we may be able to reduce the thickness or strength of certain components that show to be less stressed to save weight. The beauty of the PDR is that you basically take a picture of the components and the overall chassis while it is fully loaded similar to the way it is on the track, but not going 165mph. There may be some things that will surprise you when you make your observations.
Advantages Of The Mittler Pull Down Rig The advantages of the PDR are as stated before, ability to measure toe, cambers, clearances, and load distribution while under dynamic loading. You are able to quickly make component changes and read the effects of those changes.
The team that uses the PDR told me that its use of the machine saved a lot of time when the team gets to the track. It has more beneficial practice time because it doesn't have to guess if the fenders rub or the crossmember will hit the track. It doesn't have any surprises such as binding or detrimental toe changes to affect the truck. I guess you could say the PDR provides a certain piece of mind.
The Future Of The Mittler PDR Mike Mittler never sits still for very long. He is constantly looking to improve all of his products and invent more. The PDR is no exception. The company is looking to expand and create more models and configurations for the PDR.
This screen shows the truck...
This screen shows the truck loadings after it has been pulled down to the mid-turn attitude and loading. Note the tire loadings now. The RF and LF are much different than at static ride height. The total load is now 6,369 pounds, or 3,167 more pounds than at static height. The added load represents a combination of aero downforce and mechanical downforce.
This screen shows the graphs...
This screen shows the graphs generated for Bump Steer as the truck is pulled down. It represents the toe angle for each inch of movement. A straight vertical line would represent perfect or zero bumpsteer. This system needs work.
Here we see the camber curves...
Here we see the camber curves which will always be excessive when a double A-arm system has a lot of movement. The key is to plan out what cambers will be ideal after full compression and then setting the static cambers so that you will have the best results in the mid-turn attitude.
One idea is to provide a scaled down version that might be more affordable to the average racer. It would have less sophistication in the way of computer controls and machinery, but afford many of the benefits of the PDR. There would be no wheel measuring apparatus, but you could attach your own lasers to accomplish that.
Another idea is to introduce the effects of lateral loading. There is a way to do this with the PDR with add-ons that would necessarily increase the cost, but also increase the capability of the machine for the high end user.
Conclusion In every machine created for the evaluation of a race car chassis, there are benefits. As of today, there is no machine that can do it all. But the Mittler PDR, in my opinion, provides the most benefit for the buck of any system I have seen. The proof is in the comments and accolades I get from the user, and it is clear that this is one very useful setup tool.