ideal spring rates for coil-overs on Yaris

[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.

[largeorangefont]

Originally Posted by Loren

...

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....

That scares and intrigues me at the same time. That is a small single shear bolt at the bottom of that shock.

Undoubtedly a conventional coilover would work better at the limit, but it is just a beam axle in the back so I'm sure you could get 95% there with a good shock/spring setup.

Loren, do your rear K sport shocks have less droop travel than the factory units?

[Loren]

Originally Posted by largeorangefont

Loren, do your rear K sport shocks have less droop travel than the factory units?

I didn't do a lot of checking with the stock suspension, but I'm pretty sure they do have less. I've got them set to droop as far as possible (maximum suspension travel) and they still don't seem to droop very far. Lots and lots of "impressive" rear wheel lift when cornering.

[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!!

[rob323]

Originally Posted by Loren

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.

I was contemplating the same thing on my echo yesterday while installing the new shocks on the rear. I still haven't decided what I am going to do spring-wise yet and was thinking about those kits that put the rear springs on the shocks (which would be easy to do on my shocks). It worries me though that if using a spring on the shockie mounts, it could stress the mounts more than they are designed for.
Although, the standard shocks have the bump stops on the shock so any bottoming out would be transmitted to these points anyway.
I might have a bit of a closer look at this method tonight to see if it has any merit, and check for any clearance issues.

[rob323]

Had a good squiz at my echo last night with tape measure in hand. Maybe a Yaris has more clearance than an echo, but it would a very tight squeeze to fit 2.5" ID springs in there. They would fit with the standard 175 wide tyres and 5.5" rims (about 10mm clearance to tyre) but any other offset or width could cause problems and may require spacers. There is also the rear brake line to the drums that would possibly get in the way. Not really worth it for my application but might be worth investigating further for smooth tarmac type work where limited suspension travel is needed (and therefore shorter springs can be used).