Track Height vs. Static Height
You unload the car at the track, anxiously waiting for the first practice to see how the changes you made this week help – or hurt. You go out and the car looks good, but it’s in the ground. You add a 1/8” packer and go back out, only to find that you’re still in the ground. You come in and add another packer and then repeat the same once more before the chassis is off the ground. Problem is, practice is now over, and you haven’t got to tune the car at all.
This scenario is a common occurrence. You get to the track with big plans only to waste all your practice time fixing a basic problem. This can be completely avoided.
Let’s start from the beginning:
There are two primary positions of the racecar. Static height and Track height. These are the positions that we use to measure the chassis for set-up.
Static height is the natural relaxed position of the chassis. We often refer to this position as “Tech Height”, because it’s the position the car is at during tech inspection.
Static or Tech height is also important because that is the reference position for your chassis’ set-up. Wheel weights are taken at tech height. Toe is often set at tech height. The rear end is squared at tech height.
Track height is the average position the chassis is on the race track. On the track, your chassis operates within a travel “window”. Track height is the average position between max travel and minimum travel on the track. The front of your chassis typically never returns to tech height once it leaves the pits. So, measuring the chassis at track height is very important.
Knowing the average chassis position on the track requires data acquisition. Since we typically don’t have data for every car, we traditionally use max travel as the track height.
It is our goal to keep the chassis (and body and everything else except the 4 round rubber things) off the ground and keep all of the car’s force (or load) in the tires. Whatever you have hitting the ground with whatever amount of force it’s hitting with is that much load out of the tire.
How we fix it
The first step is to determine the wheel loads for the track you are going to be at. If you have one of our pull down fixtures, we give you a good starting point for most tracks. Here’s how you determine the wheel loads on your own:
1. Go to the track with a known conventional set-up* and measure the travel. (RF, LF & RR)
2. Come back to the shop and pull the chassis down to the max travels and measure the front wheel weights. This is the amount of load that track is generating with that tire and track conditions.
3. Now, you can change from your conventional springs to a bump stop set-up, pulling the chassis back down to the loads and checking the chassis height.
4. Adjust your spring/bump stop/packer combination until the car reaches the wheel load just before it gets to max chassis travel.
5. Max chassis travel is the point just before the chassis contacts the ground. We supply a chart and instructions to know how to measure this using our ground plane laser kit.
6. Once your wheel loads are determined, you can test and combination of set-up changes in the shop and know how far the car will travel at the track, eliminating wasted time getting the car off the ground.
*We use a conventional, non bump stop set-up to determine track loads because bump stops can ramp up wheel loads very quickly (hundreds of pounds in thousandths of an inch) making accurate measurements with travel indicators not accurate enough. Lighter weight bump springs (<1600lbs) can be used.
Difference Between Spring Loads and Wheel Loads
You can use your spring load machine to determine the max travel too. The difference is the spring load is not the wheel load. Spring loads are higher than wheel loads and do not take into account the motion ratio or spring angle. More importantly spring loads do not measure suspension binds, tire rate and sway bar loading/unloading. For example, your spring loads will typically show your left front to be the heaviest or dominant tire. On the track, however, the right front is always the dominant tire.