ideal spring rates for coil-overs on Yaris

[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.

[cali yaris]

^ Yes, but thank you

[botchilah]

Ooh I hate that bobbing / thumping / bouncing sensation!!!

[Loren]

Originally Posted by botchilah

Ooh I hate that bobbing / thumping / bouncing sensation!!!

Not necessarily all the same problem, though.

Bobbing = underdamped
Thumping = bottoming due to insufficient suspension travel OR really stiff damping
Bouncing = underdamped

Typical "lowering springs" on stock shocks will often cause all of the above. The spring rate is enough that they're underdamped on stock shocks, and they typically reduce ride height too much for the spring rate allowing the suspension to bottom out.

[thebarber]

i was talking with my friend who races his mr2 competitively....we worked it out to about 370lb front and 550lb for an ideal-ish racing setup with street tires....but thats assuming 60/40 weight distribution....

the calculations went as follows:
take half the weight of the car and multiply it by the opposite distribution expressed as a percentange.

so front is 1150 x 0.4 = 460

then multiply that by the generalized gripping ability of your tires....r comps = 1, summer tires maybe 0.8....but its all a generalization

so 460 x 0.8 =~ 370lbf/in

rears are 1150 x 0.6 x 0.8 =~ 550lbf/in

you want to have the rears in a fwd about 150% of the fronts....

also in discussion was that of a rear sway bar....we surmised that if you have a rear sway bar you can go with a more livable rear rate (say 450 or 500) as the rsb stiffens the rear under cornering load so the rear doesnt need to be as firm

there are lots of items to consider when tuning a suspension...more than just spring rates....but i guess that calc can give you a pretty good idea of whats req'd

but ill be on megan coils in spring with are 5kg fron and 6kg rear (or 280lb front and 335lb rear)

[Loren]

His generalized calculation is most likely going to give you a general WHEEL rate. Don't forget to include the motion ration in the equation, which means the rear rates need to even higher.

[thebarber]

Originally Posted by Loren

His generalized calculation is most likely going to give you a general WHEEL rate. Don't forget to include the motion ration in the equation, which means the rear rates need to even higher.

ah yes, thats what i forgot to add....so thatd maybe bump up the rears 50lb?

im not honestly GREAT w/ suspension items, but im learning slowly...

[Loren]

Originally Posted by thebarber

ah yes, thats what i forgot to add....so thatd maybe bump up the rears 50lb?

im not honestly GREAT w/ suspension items, but im learning slowly...

To answer that question reliably, someone would have to measure the motion ratio. Easy enough to do with the spring removed and the car on jackstands. Put a jack under the hub and lower it as far as it will go. That's your zero point, take a reference measurement from the ground for both the hub and the lower spring perch. Then raise the hub as high as you can and take both measurements again. Subtract the before and after measurements and compare the result, that's your ratio.

So, for instance, if you had 6" of travel at the hub, and 5.25" of travel at the spring, 5.25/6.00 = .875

Now, if you're looking for a wheel rate of 500#, you'd DIVIDE that by the motion ratio, 500/0.875 = 571

ALL OF THE ABOVE NUMBERS ARE JUST FOR DEMONSTRATION PURPOSES, THEY ARE NOT ACCURATE... THEY'RE NOT EVEN GUESSES. (I know how stuff posted on forums can take on a life of it's own, just thought I'd add that)

[Loren]

Engineering is not for the timid.

[Loren]

I know you guys are just goofing around, but a serious question was asked and serious answers were given... this kind of response does not inspire people to want to take the time to post serious answers.

[eTiMaGo]

yeah, you're right, trying to keep the mood light but this is a very educational thread

[yarstar]

Wow! This thread is a awesome. I'm thinking about getting coilovers, and this has taught me alot about what I should be looking for. Lots of good info here. As usual....Loren brings it. THANKS LOREN!!