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Where you mount the left rear front shock looks like a small detail. It is not. The same braking event moves that shock in opposite directions depending on which one you bolt it to — and that difference shows up on corner entry, every lap.
This is one of the most common questions I get asked, and I think it is one of the most important things to understand if you want to optimize the handling of your racecar.
Let's start by defining the two setups. A birdcage mount places the shock on the front of the floating birdcage, typically somewhere around 7 or 8 o'clock when you look at the cage from the left side of the car. An axle tube mount fixes the shock to the axle tube itself, anywhere from about 9 o'clock to 12 o'clock.
Figure 1 shows what we are actually looking at. The birdcage floats on the axle tube. The top and bottom four link rods attach to the birdcage at top and bottom, and the LRF shock hangs off a forward mount below the axle centerline.
Although the mounting location seems subtle, the impact on the dynamics of your left rear suspension is significant.
In my opinion, the left rear front shock has the most impact on corner entry. I also believe that most handling problems in circle track racing begin at corner entry. Put those together and it makes it critically important to get your left rear mounting setup right.
Corner entry begins with one of two actions. Either the driver begins to turn left, or the driver decelerates and brakes. Which one comes first depends on track configuration — size, speed, banking — and on track conditions.
At a high speed racetrack like Pittsburgh Motor Speedway or Eldora Speedway, the driver will often enter the corner on the gas. He is beginning to turn left before he ever lifts or touches the brake. Compare that to Florence Speedway at the end of a 100 lap feature, where the racetrack is slow and slick and the driver lifts well before corner entry.
What happens in your left rear suspension in those two events is vastly different.
As the car approaches the corner, the left rear suspension is at or approaching full droop. The droop limiter is tight and preventing the suspension from extending any further. As the driver begins to steer left, weight transfers from the left side tires to the right side tires. Because the droop limiter is already tight, it immediately begins to unload the left rear tire. We have all seen pictures of late models with the left rear tire pulled clean off the ground — that is the extreme version of what I am describing.
Now the opposite. There is so little traction that the driver lifts and begins braking well before corner entry. As soon as he lifts, the left rear suspension begins to compress. When it compresses, the angle of the left side rods decreases, and two things happen. First, dynamic rear steer is reduced, which creates an instantaneous tight condition. Second, the J-bar angle decreases, which reduces the side bite of the racecar.
I want to be clear about what that combination feels like, because those two things sound like they contradict each other. They don't. The car isn't simply tight or simply loose — it is inconsistent. The driver gets an initial push as the rear steer comes out, and then the car lets go underneath him as the side bite disappears. That is a very difficult car to drive with confidence.
In each example there are two events working against the handling of your racecar. And problems that start at corner entry almost always follow you to the apex and off the corner.
The good news is that we can use the shock mounting location to help the chassis in each of these conditions. But before we get there, I want to detail exactly what happens in the left rear during deceleration.
When the driver steps on the brakes, the torque about the axle tube changes from clockwise to counter clockwise. The easiest way to think about this is what happens on the fifth arm. Under acceleration it is wrapping up. Under deceleration it is pulling down on the sixth coil.
Every part of your rear end follows this motion — your 5th arm, your rear end housing, and both axle tubes.
Your left rear birdcage does not. When the car decelerates, the left side rods no longer have the thrust of the left rear tire driving them forward and up, so the rods begin to decrease in angle. That drives the birdcage clockwise — the exact opposite of the axle tube.
Same braking event. The axle tube rotates counter clockwise. The birdcage rotates clockwise. Everything else in this article follows from that one fact.
Let's go back to Florence, where the track is slow and slippery and the driver is braking well before turn in.
Mount the shock to the axle tube in that condition. The braking torque drives the tube counter clockwise, which pulls the shock mount away from the chassis and pulls the shock down. If there is any rebound in that shock, it is pulling the chassis down with it. That compounds the problem I already described — the chassis drops, the rod angles drop, rear steer comes out, and the J-bar angle flattens. All of it makes the car harder to drive and slower in slick conditions.
Now mount that same shock to the birdcage. Under deceleration the birdcage rotates clockwise, indexing the shock mount upward and driving the shock into the chassis. That helps keep the chassis up, keeps the rod angles up, and maintains the attitude of the car despite the conditions. In my opinion the birdcage mount is the superior option here.
Now back to Pittsburgh, where the driver enters the corner at high speed on the gas. Remember — when he turns left before decelerating, the left rear tire begins to unload.
What is holding that corner of the chassis up at that moment is not the shock. It is the forward thrust of the left rear tire driving the rods forward and jacking the chassis up, with the droop limiter simply defining how far that travel is allowed to go. The suspension is at the end of its travel and the shock has nowhere left to work. Whether it is bolted to the birdcage or the tube, the chassis is staying up. Neither mount is doing much yet.
The mount starts to matter the instant the driver lifts. The thrust that was holding that corner up disappears, and now the mounting location decides what happens next.
With the axle tube mount, the braking torque rotates the tube counter clockwise and pulls the shock mount away from the chassis. The shock extends, and rebound pulls the chassis down with it. At Florence that was the last thing we wanted. At Pittsburgh it is exactly what we want — bringing that corner down puts load back into the left rear tire instead of letting it keep unloading, and it gives the driver something underneath him at the point in the corner where the car is most free.
With the birdcage mount, the cage rotates clockwise, indexes the mount upward, and drives the shock into the chassis. The chassis stays up. The left rear stays light. On a slick track that is a gift. On a fast, high grip racetrack it exaggerates the problem you already have.
It is the same mechanism in both cases. Neither mount is right or wrong. What changes is whether the racetrack is asking you to hold the left rear up or put it back on the ground.
| Slow and slick, driver braking well before turn in | Birdcage |
| Fast with grip, driver entering on the gas | Axle Tube |
| Rough racetrack, lots of character | Axle Tube |
Everything above depends on something I have not mentioned yet, and I don't think you can have this conversation honestly without it. How the shock is valved.
Here is the key. Rebound damping is what couples the shock to the chassis on the extension stroke. If the mount indexes away from the chassis and there is rebound in that shock, the rebound drags the chassis down with it. If there is no rebound in that shock, the mount can index away all it wants — the shock extends freely and the chassis never feels a thing.
We build our birdcage mounted left rear front shocks with no rebound damping at all.
That turns the birdcage mounted shock into a pure compression device. It can push the chassis up under deceleration, and it can never pull the chassis down. That is not an oversight. That is the entire point of running it on the birdcage.
Think about what happens under acceleration. The birdcage rotates counter clockwise coming off the corner and indexes that shock mount away from the chassis. If you had rebound damping in there, it would be fighting the chassis coming up on exit — working directly against the attitude you spent all that effort building. With no rebound in it, the shock simply gets out of the way and lets the car do what it wants to do.
The axle tube mount is a different animal. It needs rebound. And that is where this gets complicated.
I have made the entire case above on corner entry. There is a third condition I have not accounted for, and it happens to be where the axle tube mount makes its strongest argument. A rough racetrack.
When the shock is mounted to the axle tube, the input from the racetrack goes almost directly into the shock. Hit a rut and travel one inch, and your shock sees just about one inch. If you were designing a suspension purely to control a rough racetrack, that is how you would do it.
Mount that same shock to the birdcage and that one inch input at the tire first has to translate into rotation of the cage, and that rotation into shock travel. Your shock sees some ratio of that input — never all of it. It is a less direct and less effective way to damp the axle.
So on a rough racetrack you want the axle tube mount, and you want rebound damping in it to control that axle.
And here is where it gets difficult. That same rebound is what pulls the chassis down under braking. You cannot add rebound for the ruts without also making the chassis fall harder on corner entry. They are the same adjustment.
On a fast and rough racetrack that works out in your favor. You wanted the chassis coming down on entry anyway, so both effects push the same direction and the setup gets easier.
On a rough and slick racetrack they fight each other. You need the rebound to control the axle over the ruts, and you need the chassis to stay up to hold your rear steer and J-bar angle. The same adjustment cannot do both.
I am not going to pretend I have a clean answer for that one. What I can tell you is which direction each adjustment pushes — and in my experience, knowing that is usually enough to find a compromise you can live with, and to know which way to move when the car isn't right.
If you run the axle tube mount, where you put it on the tube is not a minor detail. It changes how much of the tube's rotation your shock actually feels.
Picture the tube rotating counter clockwise under braking, and watch where the mount travels as it does.
At 9 o'clock, straight forward on the tube, the mount travels straight down. All of that motion goes into extending the shock. This is the most aggressive position.
At 12 o'clock, straight up, the mount travels straight forward. Almost none of that motion extends a shock that runs close to vertical. The shock is nearly decoupled from the rotation.
Between them, the mount travels down and forward on a diagonal, and only part of that motion reaches the shock. Halfway between the two, roughly seventy percent of it does.
So the clock position is a coarse adjustment on how strongly the braking torque couples into your shock. Move toward 9 o'clock for more of this effect. Move toward 12 o'clock for less.
Most of you reading this cannot swap mounting locations between hot laps and the feature. If you are stuck with the mount you have, you are not out of options. You are just working with a coarser tool.
With a birdcage mount, compression is your adjustment. That is it, and that is by design — there is no rebound in that shock to work with. If the chassis is not dropping enough on corner entry, reduce compression and let it fall more freely. If it is dropping too much and too fast, add compression to slow it down.
With an axle tube mount, you have both compression and rebound available, which is one of the real advantages of that configuration. Get compression in the right range first, then use rebound as your secondary adjustment — remembering that rebound is doing double duty, controlling the axle over the ruts and setting how hard the chassis falls on entry.
Neither one gets you all the way there. But understanding which direction your mount is pushing the car tells you which direction to adjust. And that is most of the battle.
Every shock we build gets set up for your car, your track, and your driving style. Reach out at racemartin.com — I read every message.
— Bruce Hordusky, Martin Dynamics
