ideal spring rates for coil-overs on Yaris

[Loren]

For track driving, you most certainly DO want to change the front/rear ratio.

Most of the suspension companies leave some understeer on the table for liability purposes, they also typically avoid over-springing the rear because it makes for a less-comfortable ride. Remember that 95% of their market wants their product for "bling" and will never drive on a race track! They know that, and they design their products accordingly.

Botchilah, you've stated that typical Tein coilovers are too stiff for your liking. Understand that there is a lot of setup involved that can affect how harsh the ride is on a rough road. Set the suspension too low and you don't have enough travel to absorb a big bump without bottoming out hard on the bump stops. Set the shock damping too stiff and the suspension simply won't move over most bumps! You need to find an adequate ride height for the surfaces you drive on, and then tune the shocks so that they're soft enough to not beat you up, but firm enough to not wallow around.

Then, when you get to the track, if it's a smooth track, you can dial the shocks stiffer for better response.

As for spring rates, the stock rates are about 133 lb/in front and 117 rear. (I don't remember where I got those numbers, but I have them in a spring rate reference sheet)

To combat the understeer of the FWD platform, you want the rear rates to be close to the same as the front rates, sometimes even MORE. I'm running 4kg/mm front and rear on mine, and it could still use a touch more in the rear. I tried 7kg in the rear, and it felt REALLY good on a short test drive, but I was afraid that it might be too much on the track and lead to problems. So, maybe something like 4kg/5kg would be ideal for a "trackable street" setup. For a more track-biased suspension, you'd want to go a lot stiffer, maybe 6k/8k.

The problem with the Yaris suspension is the spring perches on the rear beam axle. I'm pretty sure this is why many of the aftermarket kits stick with a near stock rear spring rate. If you fit a stiff spring in the rear that's short enough to give the ride height that you want, it's too short to stay seated under suspension droop. The solution to that problem is a tender spring, a softer rate spring that stacks on top of the main spring to hold the spring stack in place at full droop. BUT, that doesn't work all that great on the Yaris, either, because the lower spring perch doesn't rise straight up and down, it rotates up and down as the axle swings on it's pivot point at the front. Makes for an awkward situation. I stuck with a tender spring stacked on a standard spring, but I think ideally, I should be using a heavier tender spring (more of a "helper" spring) and the springs should be welded together. That might work better.

Alternately, I think I've seen a Yaris coilover kit that uses a "true coilover" setup in the rear. The stock spring perches aren't used at all, instead, 2.25" ID race spring is used around the shock. Much better solution, I think.

Buuut... if all of this talk of the hassles of tuning the rear suspension with a proper spring rate is scary to you, just fit the Tein kit with its default rates, fit a big rear swaybar and be happy. It's close enough to goodness that you won't know the difference.