Rainy day suspension basics... Arm yourselves!

[exwestracer]

I've had a couple of questions on figuring out control arm length. Rather than bury the answer in another thread, I figured i'd put it up for discussion... This is only one method, and your project may have different priorities that will determine control arm length (like you have a set laying around...?)

We'll concentrate on the upper arm and get back to the lower one later...

1. Start with the available space between the inner lips of the front fenders and subtract 1-1 1/2" for tire clearance on each side. That will be the outside width of the tires. Divide that number in half. We are now working on ONE side only.
2. From there we need to subtract the width of the tire and ADD back the backspacing. If you only know the wheel offset, subtract 1/2 the width of the tire and add or subtract wheel offset as needed. That gives us the wheel bolt face width. (here's a pic showing the difference between backspacing and offset if this is confusing. Racers tend to use backspacing for measurements, but street wheel manufacturers often supply an offset number instead)
   
3. Next we need the dimension from the bolt face to the center of the ball joints. You will have to have the spindle, hub and rotor assembled to get this measurement.
   
4. From the upper ball joint centerline, measure to the center of the top of the frame rail to determine upper control arm length. If you don't know how wide to make the frame, that's a whole different issue.

The lower arm should be 125-150% of the length of the upper arm for a typical modern camber curve.

[exwestracer]

Sorry for the abrupt ending.... SHHH, still at work!

[exwestracer]

Ok, so now we have an idea how to set up the length of the arms...but what does that get us? There are WAY too many variables to factor in to finding the "best" control arm length. Most of those are concerned with getting the optimum camber gain for our choice of tire, or the available suspension travel. It does us NO good to set up the suspension camber curve for a 60 series tire when we are running a 35 or 40. Also, if the suspension has a great camber curve in 5" of bump travel, but we only have 2" available, it's pretty useless. Getting the idea?

Here are some general guidelines:

1. The "length" of a control arm is the straight line distance from the ball joint center to the centerline of the mounting bolts or cross shaft. (see red arrow in photo below)
   
2. The ratio of the lengths of the upper and lower arms plays a big role in how fast a suspension changes camber. The closer the lengths are to each other, the slower the camber gain. The longer the arms are in general, the more stable the car tends to feel to the driver.
3. The more angle between upper and lower arms (in front view), the faster the camber change. This adjustment is usually made with the upper arm. (remember, we are assuming the lower arm starts out level at static ride height)
4. The taller the spindle assembly (ball joint to ball joint), the slower the camber change is for a given set of arms.
   And finally, probably most important... The shorter the tire sidewall, the less camber change that tire needs to keep the tread surface flat to the road. Now, that doesn't necessarily mean we want to eliminate the camber gain from the suspension. Remember, we only have camber gain with suspension travel, so if we are using VERY little travel (super low car, etc.) we may still need an aggressive camber curve to make best use of a short sidewall tire.

[avewhtboy]

thanks for starting this thread ray

[CTX-SLPR]

Ray,

A general set of questions:

1. Does a taller spindle need a larger ratio between upper and lower arms to keep a given camber curve?

2. Is there a general "rule of thumb" for a car that sees a good amount of street mileage to the relationship between sidewall profile and camber curve?
i.e. Does a 45 series tire want to see around 0.5º of camber gain per inch of compression while a 35 would want ~0.3º per inch?

3. Is there such a thing as too long an arm if it will fit in the chassis and you can keep a proper ratio?

Thanks,

[exwestracer]

Ray,

A general set of questions:

1. Does a taller spindle need a larger ratio between upper and lower arms to keep a given camber curve?

2. Is there a general "rule of thumb" for a car that sees a good amount of street mileage to the relationship between sidewall profile and camber curve?
i.e. Does a 45 series tire want to see around 0.5º of camber gain per inch of compression while a 35 would want ~0.3º per inch?

3. Is there such a thing as too long an arm if it will fit in the chassis and you can keep a proper ratio?

Thanks,

1) Yes.
2) You are on the right track with your thinking. The actual amount needs to be tailored to the tire AND the travel. If the car has more roll (bump travel in cornering), it will need less gain per inch.
3) I don't see that you can go "too" long. At some point structural integrity becomes an issue, but that's all part of a properly designed package...

[L_O_O_M]

Your the best Ray.

[L_O_O_M]

2) You are on the right track with your thinking. The actual amount needs to be tailored to the tire AND the travel. If the car has more roll (bump travel in cornering), it will need less gain per inch.

Hi

How do I determine the right amount of camber for my car with different tire sidewalls and travel?

[TheBandit]

[*]The taller the spindle assembly (ball joint to ball joint), the slower the camber change is for a given set of arms.

For existing muscle car chassis, isn't it typically just the opposite? I guess it depends on your starting point. Assuming a SLA suspension and a ride height corresponding to a flat lower arm, the greater the discrepancy between the spindle joint vertical separation and the frame joint vertical separation (either shorter or taller), the greater the rate of camber change.

[exwestracer]

I see where you are coming from, Bandit. My comment referred strictly to a taller spindle with NO change in upper arm angle like we'd get by going to a tall ball joint, etc.

[exwestracer]

Hi

How do I determine the right amount of camber for my car with different tire sidewalls and travel?

Sorry, no way to answer that question without a LOT more info. I can tell you that tire temps are the best way to tell if the camber curve is correct...ONCE the car is built and you're driving it. I'm sure that if you have a 69 Camaro, someone will tell you the "perfect" combo of tire, spring, geometry, etc. They might even be right...

Somewhere around 1/2-3/4 per inch is a good place to start for a modern tire combo.

[TLWiltman]

Hi

How do I determine the right amount of camber for my car with different tire sidewalls and travel?

When you start getting into racing tires, they'll usually tell you about what the optimum camber is. It's generally assumed that you do want some dynamic negative camber during cornering. One way to show this would be to take an eraser, and hold it vertically on a tabletop. You would then drag the eraser across the tabletop. It's fairly easy. Then you would lean the top away from the direction of travel and repeat... It grips a lot more. When it comes to a street car, then tire wear becomes more of an issue. and so on and so on...

[exwestracer]

When you start getting into racing tires, they'll usually tell you about what the optimum camber is. It's generally assumed that you do want some dynamic negative camber during cornering. One way to show this would be to take an eraser, and hold it vertically on a tabletop. You would then drag the eraser across the tabletop. It's fairly easy. Then you would lean the top away from the direction of travel and repeat... It grips a lot more. When it comes to a street car, then tire wear becomes more of an issue. and so on and so on...

Tad, in essence that's what this thread is all about. Figuring out how to measure up and lay out arm lengths that will provide good "dynamic" camber change (camber gain) for a particular tire. As I mentioned earlier, there are a LOT of variables in that simple statement. We haven't even delved into caster effect, as camber change in roll is pretty much always accompanied by some steering angle...

[L_O_O_M]

There is some awesome information here guy's please keep I up I am learning allot.