Tell me about 'Spring Rates'

I always thought that "spring rates" referred to the higher prices commanded by open-topped cars (convertibles) because of the coming of spring. No?

 

There is no simple answer to your question, that is way you see people talking about what they prefer, not what the right spring is for a car. Basically, the smoother you want a car to ride, the more suspension travel you need to use and that means soft springs and shocks. A softer suspension also tends to have more grip because the tires don't get bounce off the ground. But more travel means the car must sit higher off the ground, which causes more body roll. Body roll makes the prevents the tires from sitting flat on the ground and you lose grip. Body roll and pitch also unsettle the car going into and out of turns, or at least sets the rate that you can smoothly transition form a right to a left turn, so here stiffer is better. Antisway bars also aqre important. They let the wheels move up and down together when you hit a bump, but act to prevent one going up while the other is going down, reducing body roll. The problem with them is when you hit a bump with only one wheel, in that case the antisway bar tries to move both wheels, making the car feel as though it has very stiff spings. It's a big balancing act and what is best for you depends completely on how and where you drive.

 

By removing adding weight to the car the spring rate will be alterd,
The strenth of a spring in imperial measurement is lbs per in'' or weight needed to compress it one inch,say 200lbs spring will need 200lbs to compress it one inch,well i think thats the theory behind it,

cheers
Dale.

 

You lost me on that one. The car will sit at a different height because the springs will compress less or more respectively, but the spring will still have the same rate it had before. The only way to change the spring rate is to change the spring. When they make the springs, they use different wire diameters and number or coils to set the rate.

 

Yes sorry Mark i messed that up,think the rest was there though,

cheers
Dale.

 

a few more items on the subject...

- spring rates can be linear or progressive. linear means they provide the same resistance through the compressed travel. progressive means they will resist even greater the more they are compressed. progressive rated springs will be found on daily driven street cars.

- shocks only modulate the amount of travel that the springs will be floating the rest of the car. without shocks, the car would bounce up and down. this is why you have compression, rebound and high speed compression (and now rebound) adjustments as well as gas pressure that you can adjust in some high performance shocks. of course, even a single adjustable rated shock, you are only adjusting the rebound.

 

Engineering a system is always a compromise.

For production car (street legal) ride and handling, this is a huge area of compromise, and $Billions are spent by manufacturers to get this area right. It shows in the impressive variety of capable and comfortable cars and trucks for sale now. Huge improvements have been made just in the last three years.

Spring rates as related to your cars suspension is determined by a number of factors. The process starts with suspension frequency and what the human body will tolerate and what will be perceived as comfortable. Too high or too low, and you'll literally vomit due to the motion. There is a limited range of acceptable suspension frequencies, rated in Hz (hertz). One also needs a higher frequency in the rear suspension vs. the front, so the vehicle's body doesn't pitch like a ship at sea after a bump.

This is ride frequency, important during straight line driving. There is also roll frequency, which determines how a car goes around a corner. A road car spends more time driving straight than going around a corner, so the bias is toward comfortable ride frequencies, or ride rates.

Using the term frequency connotes vibration. The vibration occurs when the vehicle's platform is excited either by a bump in the pavement, or by a control input from the driver. Without shock absorbers (dampers), the suspension and platform would vibrate until the energy was dissipated by friction/heat.

Look at the suspension closer now. When you add in the geometry of the suspension, we get closer to spring rate. Suspension geometry determines how the position of the wheel/spindle moves in relation to the body or platform. For modern independent suspensions, this relationship is not linear. The spindle prescribes an arc of sorts, and this is what Mark's talking about when he speaks of soft vs. firm suspension: a soft suspension will allow the tire to move away from optimal contact with the pavement, a.k.a. camber gain. Modern sophisticated suspension design yields the best of both words in regards to camber gain and steering feedback.

Okay, back to spring rate...

Add in a suspension motion damper (shock absorber). Due to packaging issues, this is mounted in a manner in which the motion of the shock relative to the spindle (or wheel) is also non-linear.

The relationship of spindle to coil-over shock absorber mounted spring is called motion ratio.

Starting with suspension frequency, adding in the motion ratio determined by suspension linkage design, we come out with a spring rate. Whewwww.

This complex motion, this adventure in mechanics of moving bodies (physics), four-bar links, trigonometry, etc. is why everyone talks about "spring rates" and not suspension frequency. It's too ethereal. Gives me a headache and I understand this stuff.

Therefore, in conclusion, comparing spring rates from two different cars is a total waste of time, as the suspension geometry is different.

Yes, typically spring rates in the US are expressed in units of lb/in. You'll also see N/mm, kg/mm.

As for shocks (dampers), of course they dampen the suspension excitation. They also are a timing device for tuning. Whole nuther story....

More here Chassis Tuning Ferrari's for the Racetrack. And here Setup F355 Challenge for Racing.


Best regards,

Rob Schermerhorn

 

Many aftermarket single-adjustable shocks adjust both bump and rebound simultaneously. Koni, KYB, Tokico come immediately to mind. Koni builds many different adjusters.

Rob