ideal spring rates for coil-overs on Yaris

[Loren]

Cali, you're right. Shock damping has more to do with ride quality than spring rate. You can get pretty darned stiff with the springs, and as long as you use mild damping, the street ride will be okay. But, as you go stiffer, you'll find that it takes a certain amount of damping to keep the springs from oscillating (bouncing) over a bump... and when you have them set stiff enough to do that, they'll ride harsh. So, there is a point where you have to stop with the spring rate to avoid having to underdamp them for street use.

IllusionX, as someone mentioned (first post, I think), you need to "know what you're doing" when you start monkeying with the spring rate bias. You don't just want to go and make extreme changes like changing from 2:1 to 4:1 without knowing exactly what you're doing.

But, a subtle change in the right direction never hurts. It is well known that the Yaris understeers because it's FWD. The stock 133/117 rates give a ratio of 1.14:1. Changing that ratio to 1:1 is not too far of a stretch, and that gives us even rates front and rear.

Don't get too confident in the simple "front/rear ratio" thing, though. There's a lot more to it than that. The motion ratio of the suspension comes into play, too. In the front, the MR is 1:1, the motion of the wheel is the same as the motion of the spring because it's a MacStrut. The rear beam axle is not 1:1. The wheel moves a greater amount than the spring does because the spring is closer to the fulcrum point of the beam axle assembly. That motion ratio could be .6 or .7, which means that the effective spring rate, or the "wheel rate" (which is what really matters) is actually less than the actual spring rate.

Is your head spinning yet?

[ddongbap]

Originally Posted by Loren

Cali, you're right. Shock damping has more to do with ride quality than spring rate. You can get pretty darned stiff with the springs, and as long as you use mild damping, the street ride will be okay. But, as you go stiffer, you'll find that it takes a certain amount of damping to keep the springs from oscillating (bouncing) over a bump... and when you have them set stiff enough to do that, they'll ride harsh. So, there is a point where you have to stop with the spring rate to avoid having to underdamp them for street use.

IllusionX, as someone mentioned (first post, I think), you need to "know what you're doing" when you start monkeying with the spring rate bias. You don't just want to go and make extreme changes like changing from 2:1 to 4:1 without knowing exactly what you're doing.

But, a subtle change in the right direction never hurts. It is well known that the Yaris understeers because it's FWD. The stock 133/117 rates give a ratio of 1.14:1. Changing that ratio to 1:1 is not too far of a stretch, and that gives us even rates front and rear.

Don't get too confident in the simple "front/rear ratio" thing, though. There's a lot more to it than that. The motion ratio of the suspension comes into play, too. In the front, the MR is 1:1, the motion of the wheel is the same as the motion of the spring because it's a MacStrut. The rear beam axle is not 1:1. The wheel moves a greater amount than the spring does because the spring is closer to the fulcrum point of the beam axle assembly. That motion ratio could be .6 or .7, which means that the effective spring rate, or the "wheel rate" (which is what really matters) is actually less than the actual spring rate.

Is your head spinning yet?

Never thought about the fulcrum point. Damn you're smart.

[Loren]

Originally Posted by ddongbap

Never thought about the fulcrum point. Damn you're smart.

Nah, I'm not smart, I've just studied suspension tuning a little bit. Motion ratio is a pretty fundamental concept to calculating proper spring rates. The "wheel rate" is what matters, the effect of the spring "where the rubber meets the road". The rate of the actual spring is irrelevant.