The reasons why and the methods we use to change the rear trailing arm links are many. Those lie somewhere in among the various ways we try to improve our performance. But there are ways to do this and ways not to do it. And, we need to be aware of other settings we might be changing when we make changes to the trailing arm angles.
Rear Square – Of primary importance in our overall setup routine is making sure we know which way the rear end is pointed. By pointed, we mean, where is a line that is at ninety degrees off of the axles pointed. This is also the direction of the wheels and tires. In most cases, we want them pointed straight ahead at normal ride height.
We can check this by employing a simple routine using strings and marks on the floor. Or, we can use lasers in todays advanced setup tool environment. Either way, we want to make sure we know where the rear is pointed.
Which Way To Point On Asphalt– The next step is determining which way we want the rear end to point in the three basic phases of the turns: 1) Entry, being the initial turn-in during braking, 2) Mid-turn, meaning the part where there is no deceleration and no acceleration called steady state because all of the motions are steady now, and 3) the Acceleration phase where we need the most bite off the corners we can get.
On the entry phase, it would be nice if the car turned in easy so we can brake more straight ahead, at least having the front wheel more straight ahead. If the rear turned the car during that initial phase, we won’t need to steer nearly as much. But this phase doesn’t last long and we don’t want the rear to steer very long at all or we will go loose as we enter the middle phase.
As we enter and travel through the middle phase, we want the rear to be steered straight ahead, or even slightly to the left which will provide the tires with more angle of attack and therefore, more grip.
We have to be careful here because if we give the rear tires too much grip, the front won’t be able to turn with the rear and we’ll be tight through the middle phase. A little rear steer goes a long way.
Then as we begin to accelerate, we need maybe more rear steer to the left to give the tires even more angle of attack and counter the torque of the motor making the rear tires slip. This “slip” angle can be produced by causing more rear steer. In any case, we’ll need for the rear to either be steered straight ahead coming off the corners, or steered slightly to the left.
We can position our trailing links so that as the car enters the turns and rolls to the right, the right trailing arm pushes the right rear tire back, steering the car into the turn. Then as the car rolls more to its final roll angle, the right trailing arm travels past level to now going downhill and pulling the right rear tire back to where it was originally. The trailing arm angle we talk about here is when the front mount higher than the rear axle mount.
Now the RR tire and the rear end is pointed straight ahead and ready for the mid-turn phase. We can even set the RR trailing arm angle so that it travels a little more and then pulls the RR tire forward a slight amount to give the rear tires more angle of attack and more grip.
But what about the left rear link? This link does not usually move much during initial entry, and even through the mid-turn phase on most low and medium banked tracks. It’s only when we drive through very high banked tracks that we see any significant LR chassis travel to affect the LR trailing arm angle.
What we do see is LR travel when the car accelerates off the corners. It is here where we can make a difference and produce rear steer, to the left. If we put angle (front mount higher than the rear axle mount) in the LR trailing link, then as the car squats coming off the corner, it will move towards level and then push the LR tire back causing rear steer to the left, like we need. We can vary the amount of steer by varying the angle of the link.
Which Way To Point On Dirt – Dirt control arm angles are a bit more complicated. Depending on the system, you can create many different scenarios for dirt cars. The most common systems are the three-link, four-link and Z-link. We’ll concentrate on the four and Z-link systems because the three link is talked about in the asphalt section.
Both the four-link and the Z-link systems have four links. The four-link has them all going forward from the axle, whereas the Z-link has the bottom links going forward and the top links going back from the axle.
These two systems can be setup to do either of three things, 1) produce zero rear steer, or not move the wheel/tire either forward or back, 2) produce forward movement of the tire, or 3) produce rear movement of the tire. In most cases, its only one of those per wheel.
And, we need to think about which way the chassis moves, up or down. With some link angles, we get jacking forces in the left rear links that push the left side of the chassis up on acceleration. It is almost inevitable that the LR tire will be pulled forward when this happens. The amount of travel forward is dependent on the link angles.
The right rear chassis might move down, up or stay at the same height as the car enters, goes through mid-turn, and then accelerates off the corner. No matter what the movement is, we can keep the RR tire in the same position fore and aft if we set the right link angles and do all of the steering with the left link angles.
There are various thoughts and opinions on why we steer the dirt cars so much through the turns. It may look like the car is loose and the rear tires are sliding, but they are not. They are just pointed to the right and must travel out way past the path of the front tires in order to gain their angle of attack to get around the corner.
The most popular thought, and one that is most true practically, is that having this extreme rear steer gets the car turned in better and pointed to come off the corner. But mostly it provides a flat plate aero effect that puts the large flat side of the car at an angle to the direction of travel causing a side force that helps to counter the lateral force trying to push the car off the turns. And on dirt, especially dry slick dirt, we need all the help we can get.
How To Make Changes – We need to make sure when we change our link angles that we don’t inadvertently change the rear alignment too. If we take a measurement from the wheel rim forward to a point on the side of the car, we can then recheck that distance after the change to bring the tire back to its original position fore and aft.
Some cars have alignment tabs that can be used too, but make sure your alignment tabs are correct before you rely on them. And if you have a laser system, you can easily check the rear alignment after a link angle change. Remember, make only one change at a time.
The post How to Change Trailing Arm Angles Properly appeared first on Hot Rod Network.