Effective Motion Ratio and Wheel Rates

[240sxTTC]

Has anyone calculated their effective front motion ratio and wheel rates? I'm still getting more body roll than I want even with the bars on full stiff. It seems I need to increase spring rates so I thought I would calculate the effective motion ratio and wheel rate of my current front set up. Keep in mind I've pushed the wheels out pretty far in the front to fit 245 Hoosiers. Just curious what others have calculated.

Wheel: 17x9
Effective Wheel Offset: 11mm
Spring rate (C): 650lbs/in
Sprung Corner Weight: 615lbs
Motion ratio (MR): 0.71
Angle Correction Factor (ACF): 0.98 (for 10 degs)

Wheel Rate (WR) = (MR)^2(C)(ACF) = 324lbs/in

My wheel rate is 50% of my spring rate and 53% of my sprung corner weight, which is pretty soft. The motion ratio is definitely lower than I thought. I'm going to increase the WR 25%, which will require a 800 lbs/in spring.

What do you think? Does the math make sense

[Def]

Where did you get the motion ratio? Is that for the front?

Seems a little low, so it seems like it's influencing your calculations a bit.

[240sxTTC]

Where did you get the motion ratio? Is that for the front?

Seems a little low, so it seems like it's influencing your calculations a bit.

Yep for the front. I agree it seems low but the dimensions are what they are. I think it's lower than usual because of the offset of the wheels.

MR - see diagram at link

http://www.worksevo.com/Spring_Rates_1.pdf

[josh18_2k]

Call me crazy, but shouldn't the outer measuring point for MR be the ball joint? Taking camber curve out of the equation, the bj and wheel move the same distance vertically. I think the method in the link only really works I there's no camber curve at all. Good assumption for some cars I suppose.

[Matt93SE]

For a McStrut design, I would expect the motion ratio to be much closer to 1..
the best way to do it is to:
1. measure hub to fender distance at ride height.
2. put car on jack stands, remove wheel.
3. pull the spring off the shock and reinstall in car.
4. jack up from the wheel hub, rotor, ball joint, whatever until 1" below (droop) of static ride height.
5. measure hub-fender distance. measure strut shaft length.
6. jack up wheel hub 1/2". measure both spots again.
7. repeat until you're out of travel.

plot measurements and check whether linear or curved. if it's linear (most are pretty close), then you can use that for your motion ratio. if it's a curve, check the closest 1" or so around your static ride height.

Edit.. additional info that you'll find helpful: http://www.shock-shop.com/Critical%2...asurements.pdf

[Jason M]

Ok, this thread has my full attention... I always assumed the MR to be 1:1, or so close that it does not matter.

I just ripped apart the front of my car and took some ruff measurements. Moving a reference point on my front wheel bearing cap 1", the piston of the strut moved about .925".

I'll have to bring home some dial indicators from work to get us more accurate numbers. The wheel bearing cap mark was scribed with a razer blade & is 1.25" away from the hub mounting face of the rotor. The rotor was at a relative -3 deg's & the spring/strut was about -5 deg's.

What other measurements should I take??? I was thinking the length of the LCA pivot point center to center, t/c rod length, rotor angle, strut angle, roll center adjustment, & SAI. Also think I should throw in the camber curve, which is no problem to measure. As far as measuring a refernce point on the rotor/hub assembly. Any preference???

[Jason M]

I like this method, it consist's of using an installation ratio to determine the wheel rate. Also recommends measuring the wheel travel at the center of the contact patch. Not sure how to do that accuratley. It may be easier to just do some math based off of the hub face.

http://eaglewoman.rscsites.org/cobra/tweaks/motionratio2.htm

[Matt93SE]

Think of it this way.. if any one piece of your hub, rotor, tire contact patch, or ball joint are moving as anything other than a single unit, then calculating perfect motion ratios are the least of your worries.

If you jack the wheel hub up 1", then the tire will move up 1". so will the brake rotor and lower ball joint.

sure, there's negligible effects of camber change in there, but you want to be concentrating on the center of your suspension travel, not the ~3deg of camber change present from full droop to full compression. even with that, that camber change will affect your calculated motion ratio in such a small number that it just doesn't matter.
a 0.5% miscalculation motion ratio isn't going to mean jack when spring rates come in 25lb increments (those are 5% steps if you're looking at springs in the 500lb/in range.)

check your significant digits. this is hand-grenade math, not bearing clearancing. Don't over fresh-outta-college-engineer this stuff.

[djsilver]

Yep for the front. I agree it seems low but the dimensions are what they are. I think it's lower than usual because of the offset of the wheels.

MR - see diagram at link

http://www.worksevo.com/Spring_Rates_1.pdf

The attachment matches my understanding of the subject. I'm running 550lb springs in the front but I have extended my strut ears to minimize scrub when going to wider wheels, so my wheel rate will be lower than a stock setup with the same springs. Conversely, I haven't modified the mounting points for the sway bar links so it's still the same rate as it would be on a stock setup. The offset/backspacing of the wheels do affect the WR as well, since they affect the center of the contact patch.

[SoSideways]

That would make sense djsilver, as I am running 9k springs up front and it feels kinda like I'm running a softer spring due to the low offset wheels and extended LCAs up front.

The rear feels softer than 6k springs too, again, from the lower offset wheels.

[Def]

Yep for the front. I agree it seems low but the dimensions are what they are. I think it's lower than usual because of the offset of the wheels.

MR - see diagram at link

http://www.worksevo.com/Spring_Rates_1.pdf

The note about MR mentions a strut suspension doesn't necessarily have a 1:1 MR, but it's usually .98 to 1.02. .71 just seems really low with everything out that far.

Have you jacked up the wheel by a few inches and measured shock travel?

[Jason M]

Using A dial indicator on the hub & a dial indicator on the strut housing gland nut, I measured .944" of strut movement vs 1.000" hub movement. Verified that number three seperate times. This was taken from a starting point of -2" below normal ride height, ending @ -1" below normal ride height. @ the .5" hub travel mark the strut moved .475", also verified multiple times. The motion ratio is not linear. A small amount can add up since that number is squared.

Squaring the measured number is supposed to make up for the mechanical advantage as well as the less distance traveled. Does the wheel offset give us a mechanical advantage & or disadvantage? If so how should it be calculated?

.944^2 = .8911136 = motion ratio .89 or 89% of the spring's rate.


.

[240sxTTC]

The note about MR mentions a strut suspension doesn't necessarily have a 1:1 MR, but it's usually .98 to 1.02. .71 just seems really low with everything out that far.

Have you jacked up the wheel by a few inches and measured shock travel?

I agree the MR w/o the dynamic loading at the tire for the S13 should be ~.95-.98. This would be using the hub movement (instead of the tire) compared to the strut. My point is the offset of the wheel affects the dynamic loading (lever arm) at the hub. The farther out the tire contact patch is the longer the lever arm and higher the load applied to the hub. Again this makes sense based on the performance of my car. I have a full cage, large bars on full stiff and 650lbs/in front springs and my car still rolls a fair bit. My sprung weight on a front corner is 615#. If the MR was .98 than the WR would be ~615lbs/in, which is the same as the sprung corner weight and considered stiff.

Anyways, there are a lot of different ideas of MR and how to calculate MR. The reason for this thread is to openly discuss our knowledge. Keep it coming!

[240sxTTC]

Using A dial indicator on the hub & a dial indicator on the strut housing gland nut, I measured .944" of strut movement vs 1.000" hub movement. Verified that number three seperate times. This was taken from a starting point of -2" below normal ride height, ending @ -1" below normal ride height. @ the .5" hub travel mark the strut moved .475", also verified multiple times. The motion ratio is not linear. A small amount can add up since that number is squared.

Squaring the measured number is supposed to make up for the mechanical advantage as well as the less distance traveled. Does the wheel offset give us a mechanical advantage & or disadvantage? If so how should it be calculated?

.944^2 = .8911136 = motion ratio .89 or 89% of the spring's rate.


.

If you get a chance, measure the movement at the center of the tire contact patch (instead of the hub) and the strut gland nut.

[Jason M]

If you get a chance, measure the movement at the center of the tire contact patch (instead of the hub) and the strut gland nut.

Sure.

What do you think about placing a floor jack on a scale & moving the tire .5" or 1" & measuring the force it takes to moves the tire a certain amount of distance?