Two Guys Rally

The Volvo Chronicles: New Parts! [ September 11th, 2008 ] By:Mark Ozimek

As Charles mentioned the other day, the rallycross was pretty tough on my suspension. There were a few dips in which my suspension bottomed out going over it, and I’m sure the OEM dampers were having a tough time dissipating all the heat that the roughness was generating. Somewhere in all the chaos, the dampers broke. The rear dampers are not damping suspension travel at all, one of the front struts is making an awesome grinding noise, and the other is also not damping.

Tough, but it’s a reality we all face in rally; off-road surfaces are hard on cars, and you need durable parts engineered to take the abuse if you want to keep stuff around for more than a few races. In my case, it’s not really a big deal. I was aware that the dampers were degrading, and I have suspected them to be the same parts that were on the car when it rolled out of the factory in Sweden. Both rear dampers were leaking oil, and I’m sure the fronts were close to being in similar condition.

So I ordered some replacement parts, and decided to go more for better road handling than off-road handling, seeing as it’s a Volvo that really belongs on the highway at speed, not pitching around slow hairpins in the wet grass. I’m sure I’m not the only one, but I get really excited when big packages arrive in the mail! Looks like my kitty also gets really excited about packages too…

(click for larger picture)

H&R springs, should stiffen up the car a bit around turns and over bumps. There are also some Koni Sport dampers still in the mail that will hopefully be here tomorrow. I’m really looking forward to those, since the damping rate is adjustable, I can play around with it to see what firmness level gives the best handling. With 205-55-16r tires on the car, I’m suspecting I can get away with going pretty stiff before it becomes unreasonable.

However, over rough surfaces, stiffer is not better, since it reduces the amount of time the tires are in contact with the road when the surface suddenly changes height. The stiffer dampers will slow down the speed of the wheel significantly, but on the flip side, the stiffer springs will push harder against that damper to make the tire move faster.

There are a lot of things to consider when getting springs and dampers. Without delving too much into the mathematics behind it, a car can be modeled as a mass+spring+damper system mathematicaly. Solve a few second order equations and you can calculate things like the oscillation period, transient response, and all sorts of other neat things to try to match a spring rate and damping rate to your particular vehicle and preferred handling.

Really, there are three scenarios that occur, overdamping, underdamping, or critically damped. The latter is really hard to achieve, but it’s not difficult to get close. Here’s a relatively complicated picture for those uninitiated with system dynamics, stare at it for a while and try to make sense of it all, I’ll do my best to explain each one.

Underdamping is when the spring rate is too high in comparison to the damping rate. Most cars are underdamped, as this provides a more comfortable ride, and good traction in most conditions, despite the poor response time However, when the car is too underdamped, it will become uncomfortable and uncontrollable, as the car will be bouncing for a long time after hitting a bump. Think of your Grandfather’s old old Caddilac here.

Overdamping is the opposite, the damping rate is too high in comparison to the spring rate. This is bad because it puts a lot more strain on the suspension mounting hardware, as the bumps are barely absorbed by the suspension and is translated into chassis movement instead. This means that a moderate bump can cause your tires to be airborne for a moment. Obviously not good unless you’re racing on a really really smooth surface.

Critical damping is when the movement stops in the shortest time possible, technically the ideal balance between the spring and damping rates.

So which one is best? It really depends on a lot of things. Smoother surfaces can use higher damping rates to trade a little bit of traction for better response, while rough surfaces can do the opposite. the amount of suspension travel you want, the geometry of the suspension, the weight balance of the car, and even the driver’s preference all matter towards making the optimal setup for performance, or in some people’s case, comfort.

Personally, I am based towards critical damping, but only from a theoretical standpoint. More experience with suspension setup may change my opinion. Until then, I’ll just have to play with what I’ve got and make the most of it.

Suspension Setup Basics [ August 15th, 2008 ] By:Mark Ozimek

I’ve heard that a few of our readers would like to know a little more about things like camber and toe, and the effects the basic suspension settings have on vehicle stability and control. Before reading this, keep in mind that the optimal setup for any combination of car and road can vary a lot. This is just a guide to help understand what three settings do:

1. Camber
2. Toe
3. Caster

There are many more variables in the suspension setup, but these three seem to be the most easily changed, and have the largest effect when tuning the car.

Camber is the angle of the wheels from vertical when viewed from the front. Negative camber means the top of the wheels is closer to the center of the car than the bottom. Positive is the opposite, with the top of the wheel further away than the bottom. The measurement is degrees off from vertical.

Usually the suspension in a car is designed to decrease camber as the suspension compresses. This way, when the body rolls as it goes through a hard corner, the outside suspension compresses and pulls the top of the wheel in, the inside decompresses and pushes the top of the wheel out, counteracting the roll from the body, keeping the tire closer to perpendicular with the road.

The main idea behind changing the camber angle is to maximize the tire’s contact patch for when you need it most. Typically it is set slightly negative to maximize traction during hard cornering. The downside is less traction when traveling in a straight line.

Positive camber causes more wear on the outside edge of the tire, while negative camber causes more wear on the inside edge of the tire.

Toe is the angle between the wheels and the car’s centerline when viewed from above or below. Toe-in means the tires point inwards, ie front of the tires are closer to the car’s centerline than the rear of the tires. Toe-out is opposite, with the front of the tires out and the rear in. The measurement is degrees off from parallel with the car’s centerline.

Toe mostly affects straight line stability and turn-in response. Toe-in improves straight line stability, negating the effects of things like surface irregularity, bumps, crosswind, and generally makes the car want to travel in a straight line.

The downside of this is that the turn-in response is reduced. Consider that the inside tires must travel through a smaller radius when turning than the outer tires. When turning with toe-in, the inside front tire will have a smaller angle of turn than the outside tire, meaning that it wants to go through a larger radius, and is fighting against the outside tire during a turn. As the weight is transferred to the outside tire, the effects of the inside is reduced.

Conversely, with toe-out, the car will be unstable at high speeds, anything that transfers weight to one side of the car will make the car want to turn in that direction because the tire is pointed outward. Keeping this in mind, it seems a contradiction that toe-out improves steering response. Remember what I mentioned before about the inner and outer tire’s turning radii. With toe-out, the inside tire tries to turn a tighter turn than the outside tire, which is exactly what we want. This way, the tires aren’t fighting against each other until the weight transfers to one side.

However, just like camber, any toe away from 0º increases wear on the tires; Toe-in causes more wear on the outside edge of the tire and toe-out causes more wear on the inside edge of the tire.

Caster is slightly more difficult conceptually, and it only applies to the steering wheels. The angle between the axis upon which the wheel turns and vertical is caster. The best example I can think of is a bicycle. The front wheel rotates about an axis that is not vertical, but is angled so that the axis of rotation is in front of the contact patch. When viewed from the side, positive caster means this axis of rotation is tilted backwards, the top is towards the rear of the car and the bottom is forward. Negative camber is when this axis is tilted forward.

What does this do? Well, when the contact patch is behind the steering axis (Positive caster), the wheels want to travel in a straight line, and will have a tendency to center when turning. As you would expect, the opposite is true with a negative caster, the wheels want to turn away from going straight and more in the direction that they are currently turning.

Negative caster was used a lot back in the 70’s and earlier to make the feel of the steering lighter, since less force is needed to turn if the wheels want to go in that direction. The problem there is that negative caster gives some instability when going in a straight line.

Almost all modern cars have positive caster to improve stability and ease of driving at speed. Although the steering wheel will be more difficult to turn, power steering helps that.