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.