twohoos

SakeBomb,
Thanks so much for doing these measurements and sharing with the community! (I'd almost given up on anyone actually doing this!)

I hate to ask, but would you be willing to post (or just send me) the numerical data from your plots? The reason I ask is that, just eyeballing the curves and trying to read off the numbers, I actually calculate different MR's than you posted. For example, for the front bar, your data seem to show about 80mm of deflection over the course of about 150mm of wheel movement. That's an MR of 0.533, vs. your estimate of 0.492. The other plots also seem to result in higher MR's than you indicated.

Thanks again and best regards,
John

robrob

Can you cite a reference for your wheel rate MR^2 formula. I've never seen that before.

SakeBomb Garage

Hey Rob , miss you in the RX-7 world

That's a pretty common formula for wheel rate, you will often find some extra terms like an angle correction factor but that is do to the lever style MR measurement and isn't needed in this case.

Wikipedia Wheel Rate
Eibach Wheel Rate

You will sometimes find the equation written as WR = SR / MR^2 but that is with the MR being defined as wheel travel / spring travel instead of spring travel / wheel travel

Optimum G Upside Down Motion Ratio

robrob

Thanks SakeBomb for the links. My suspension spreadsheet does indeed have the motion ratios squared, I just forgot about it.

I have always applied the s2K's 20 degree shock angle to the wheel rate formula, what is your reasoning for excluding it? The more the shock lays over the lower its effective spring rate.

SakeBomb Garage

If you look at the Eibach link, their motion ratio term is really vertical damper bolt movement per wheel travel. The real definition of motion ratio is damper travel along it's axis / wheel travel. We measured the damper travel along its axis directly so the angle correction factor isn't needed.

The Eibach page defines the MR as d1/d2 but it's should really be d1/d2*cos(damper angle).

robrob

So to make sure I understand, you measured shock travel and wheel movement so your motion ratio isn't really d1/d2, so it won't be accurate when used with a shock angle factor like in my spreadsheet--unless you know to set the shock angle factor to 1.

Edited to remove "nonstandard."

twohoos

SBG's measurements are along the correct dimensions to translate spring movement to wheel movement.

They've been kind enough to share more details with me via PM, and in turn I'll be sharing my thoughts here sometime soon.

robrob

I really appreciate SakeBomb sharing their data, I just want to make sure I and others understand them. If someone uses their motion ratio numbers (which are different from previous s2k measurements) then puts in a 20 degree shock angle correction their results will be incorrect.

SakeBomb Garage

That is correct, this data should not be used with a shock correction factor, I will edit the top post to make it more clear. The angle correction factor is used to transpose the damper bolt movement to the axis of the damper shaft, as we measured the damper travel directly the angle correction isn't needed.

Depends on your definition of "standard" . The industry standard definition of motion ratio is Spring Travel / Wheel Travel. The standard in motor-sports is to simply measure the damper travel vs wheel travel which is exactly what we did. This is often difficult for enthusiasts to do so people have come up with formulas to estimate the motion ratio using the lever arm formula d1/d2*angle correction factor which approximates the motion ratio. Even then, the motion ratio number should include the angle correction factor. In layman's terms d1/d2 is not the motion ratio, it is the lever ratio of the lower control arm.

-SBG

robrob

Yea, "nonstandard" was a little harsh so I edited my earlier post.