Motion ratios

[122 Vega]

Has anyone taken the time to measure these out or have access to this info? Jeff at Tripoint? Let me know asap please, or I'll measure it out tonight. Sway bar rate would good as well as swaybar motion ratio.

Thanks, making some changes to the suspension here soon.

Britt

 

[Rogue]

Can you explain what a motion ratio is?

 

[low_psi]

From my measurements:
Front: 0.98:1
Rear: 0.91:1

I gave these and a spreadsheet I've been tweaking to Jeff. He said he'd ask the Tripoint guys how my measurements compared, but from what he remembered, they were pretty close. If you can find out the wall thickness of the Racing Beat sway bars, I can calculate the sway bar rates. I would be nice to have that info to compare the RB rates to the theoretical rates I've calculated. I can measure the swaybar MR later next week. I was planning on starting a thread soon about suspension tuning. In my talking with Jeff, the Tripoint cars surprised me in their approach.

Rogue- The suspension's motion ratio tells the the relationship between spring and wheel movement. In the case of the front of a Protege, for every 1" the wheel moves vertically, the spring moves 0.98". Strut based cars typically have MRs in the 0.9x:1's.

 

[Rogue]

thanx for the explanation. How can knowing motion ratios help with suspension setup?

 

[aMaff]

Knowing how much wheel travel you have vs how much shock travel you need? Just making a somewhat educated guess. That'd probably be a good tidbit to have for the rear of the miata. I'm sure some1's calculated all that before...

Does that mean that for every 1" the wheel travels, the spring compresses .9"? (ie, a 7" spring, after 1" of wheel travel, would be ~6.1" long?)

 

[low_psi]

Yes, but obviously different spring rates will limit both spring compression and wheel movement. Think of the spring and wheel being on two different length levers. Knowing the MR allows you to calculate your wheel rates, which is more important than spring rate. I will start a more indepth thread later, hopefully Britt, Justin, Jeff and others can share their input.

 

[dmitrik4]

in other words, think of the suspension arms as levers. the vehicle weight is a force compressing the suspension. it acts on the end of that lever. the spring is mounted somewhere along that lever arm; where it's located (i.e., where it is between the pivot and the end) determines what spring rate is going to be needed to generate a desired force at the end of the pivot arm (where the vehicle weight is acting). the farther away from the end the spring is, the stronger the spring needs to be.

another factor is the spring angle; if the spring isn't oriented exactly vertically, only a portion of its force will act on the vehicle weight. a car like the '89 civic has a motion ratio of around .7:1, IIRC. that's part of the reason they run such high spring rates; if the spring is 600 lb/in, there might only be 400 or so of that acting on the wheel.

here is some motion ratio reading:
http://sccaforums.com/forums/thread/82118.aspx (start around post 82113)
http://eibach.com/cgi-bin/htmlos.exe/07130.7.6752809533900016946

and here is some setup reading:
http://farnorthracing.com/autocross_secrets.html

 

[xelderx]

You guys are thinking too much. That's my thought.

 

[122 Vega]

It's 34 degrees outside and raining. What the hell else are we supposed to do?

Britt

 

[xelderx]

It's 34 degrees outside and raining. What the hell else are we supposed to do?

Britt

Good point.