100% Camber Recovery

[bjhines]

The main complaint people have pertaining to their new track suspension set ups is that the car is squirrely/trammels/darts under braking. These are all trade-offs when setting a street car up for high-grip track-duty.

 

The next compaint comes when they wear their tires funny on the street.

[S30 SPL]

I know in many other cars, more caster is an alternative to more static camber in the front. Is this the same for a S30 chassis Z?

[blueovalz]

This is where the caster really makes the argument interesting. Again, it's a compromise, but less of one. Lots of caster helps increase the camber curve on tight runs (solo for example), but may be too much on a road coarse where the same g-force (read: sway) is had with less steering angle. Like John said, compromise... compromise... compromise. This is why one track setting is a different setting than the next track for fastest times.

[S30 SPL]

I think John might have come up with another discussion while sitting with a brew that can go on until closing time.

[johnc]

I forgot to post the spark for this discussion, yet another Mark Ortiz chassis article:

 

WHY NOT GO FOR 100% CAMBER RECOVERY?

 

I enjoyed this newsletter on a car dear to my heart [referring to the issue about Corvair rear suspension, April 2008]. I had a question though. I am about to begin installation of a C5 Corvette rear end into my V8 car, using an adapter I designed to couple to a big car Saginaw 4 speed. I will need to center the engine in the car, and then design/build all new rear suspension. The question is how much camber gain to design in. You mentioned 50% as a typical gain. Why would you not design for more, even 100%?

 

Camber gain usually refers to the change of camber in degrees, per inch of wheel travel. Its units then are degrees per inch. It is commonly measured with the car stationary in the shop, the springs removed and the anti-roll bars disconnected. When you see a single number for this, it’s most commonly the camber change in the first inch of compression from static. It increases as the suspension compresses and decreases as the suspension extends, if the upper arm is shorter than the lower. Conventionally, change toward negative in compression and toward positive in extension is taken as positive camber gain.

 

Camber recovery is the percentage of the roll angle that is recovered at the wheel when the car rolls. If the wheel leans the same amount as the body, that’s zero camber recovery. If the wheel leans half as much as the body, that’s 50% camber recovery. If it doesn’t lean at all, that’s 100% recovery.

 

For a car with a 57.3” track width (57.3 being approximately 180/π, the number of degrees in a radian), 1 deg/in of camber gain gives 50% camber recovery. To get 100% camber recovery, we have to have 2 deg/in of camber gain. The problem with that much camber gain is that the camber changes too much in situations other than roll: in longitudinal acceleration, over humps and dips, over bumps, and with fuel burn-off and other load variations.

 

With a passive independent suspension, it is impossible to have zero camber change in both ride and roll. The best we can do is to strike a compromise between the two, so that we don’t have any big camber changes in any situation. With a 57.3” track, one degree of roll is one inch of displacement difference at the wheels, or ½” per wheel. Two degrees of roll is twice that: 2” per wheel pair or 1” per wheel. So if the camber gain is 1 deg/in and camber recovery is 50%, we have one degree of camber change per inch of wheel movement in both ride and roll. The only way to get more camber recovery without more camber change in ride is to increase the track. This also reduces lateral load transfer and thereby increases cornering power, but unfortunately it makes the car bulkier.

 

The formula for instantaneous rate of camber gain is:

dφ/dz = tan-1(1/Lvsa)

where:

φ = camber angle, degrees

z = suspension displacement at the contact patch center, inches

dφ/dz = first derivative of camber with respect to suspension displacement (instantaneous camber gain), deg/in

Lvsa = length of virtual swing arm

 

This formula also works with units of length other than inches, provided you use the same units for z and Lvsa. Note, however, that 1deg/mm of camber gain is nowhere near the same as 1deg/in.

 

A simplified version is:

dφ/dz = 57.3/Lvsa

 

One question that sometimes arises is whether to measure Lvsa horizontally, or along the force line. Unless the instant center is at ground level, the true or vector length will be greater than the horizontal distance to the instant center. The radius on which the contact patch moves is actually the true or vector length, right?

 

Well, yes. But also, if the instant center is anywhere other than ground level, an inch of contact patch movement about the instant center is not a vertical inch; it's some lesser amount. And it turns out that the ratio of vector to vertical motion at the contact patch is the same as the ratio of horizontal to vector swing arm length. This ratio is also the cosine of the force line's angle of elevation.

 

So if you use vector distance for the swing arm length, and apply the formulas, you get the rate of camber change per vector inch of wheel movement, and if you use the horizontal distance, you get the rate of camber change per vertical inch. Convert a vertical inch to vector inches, and use the

 

vector swing arm length to find the camber change rate per that unit of distance, and you get the same number as if you hadn't converted, and used the horizontal distance.

 

Formula for instantaneous rate of camber recovery is:

Rφ = (1 – dφ/dθ) * 100% = (t/(2Lvsa)) * 100%

where:

Rφ = camber recovery, in percent

φ = camber angle, degrees (measured with respect to the road, not the car)

θ = roll angle of sprung mass, degrees

dφ/dθ = first derivative of camber with respect to roll, i.e. the instantaneous rate at which the tire leans per degree of body roll

t = track width

Lvsa = virtual swing arm length, taken horizontally

[tube80z]

This is where the caster really makes the argument interesting. Again, it's a compromise, but less of one. Lots of caster helps increase the camber curve on tight runs (solo for example), but may be too much on a road coarse where the same g-force (read: sway) is had with less steering angle. Like John said, compromise... compromise... compromise. This is why one track setting is a different setting than the next track for fastest times.

 

There's also the cross weight change that happens because of caster. Put a car on scales and turn pads and you can really see it all working.

 

Cary

[johnc]

That's the nice thing about caster, it helps plant the inside rear wheel in a corner. It does other, not so nice things with steering effort and rolling resistance, but if your car is making good power it helps get the power down earlier in a corner.

[Flexicoker]

That's the nice thing about caster, it helps plant the inside rear wheel in a corner. It does other, not so nice things with steering effort and rolling resistance, but if your car is making good power it helps get the power down earlier in a corner.

 

I disagree with this statement, John. More caster (in conjunction with some scrub radius) is going to lighten the outside front/inside rear crossweight, and tend to lift the inside rear with increased steering angle. The advantage of this is to give the car more oversteer/less understeer with increased steering angle, reducing all cars tendency to push in tighter corners. The disadvantage is that you're going to tend to lift the rear tire, and have trouble putting the power down.

[johnc]

In the case of a front engined RWD sedan, positive castor has a jacking effect on the suspension of the outside wheel that is loosely related to the kingpin angle. Granted this effect is small but it does work to increase the corner weight diagonal by some amount - greater in low speed, high steering angle corners. It can be measured on corner weight scales but I think those measurements are irrelevant due to the dynamics involved out on course. The suspension jacking also helps to reduce lateral load transfer by some very small amount.

 

In the case of a formula car with a more optimized chassis layout, the effect is greatly reduced to the point of insignificance.