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Suspension Tech / Motion Ratio / Unsprung Weight

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Has anyone calculated the motion ratio of the front and rear suspension on a 280Z and obtained an answer that you are have confidence in the accuracy?

 

Using a tape measure in the rear I have calculated a .82 MR. Measuring from the pivot to the base of the strut and from the pivot to the hub face (assuming zero offset wheels).

 

Using suspension analysis software (Mitchell's WinGeo), it shows a ratio of 1.142, which I assume is the inverse, so that would be .876

 

Does anyone have a good answer for comparison? How about a good sketch showing proper measurement and calculation? I need to go buy a book or two, but I want answers now! :-P

 

I am planning to do some shock dyno testing and trying to get the suspensions natural frequency...

 

Thanks in advance.

 

Tom

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Erik Messley has all that information in Mitchel's software. I'll see if I can get the file for you and send it over. Right now, using my memory, I had calculated a .98 front and a .97 rear motion ratio (to the center of the contact patch) on my 240Z running:

 

1. 275/45-16 tires on 16 x 10 rims with 6" of backspace.

2. No wheel spacers.

3. 8 degrees of postive caster.

 

Again, those numbers are from memory so they are suspect. We did this a while ago and we used the centerline of the contact patch for a reason I don't remember right now.

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It changes the motion ratio.

 

Yeah, I know that. But I can't remember why we went with the contact patch over the ball joint pivot.

 

BTW... I just ordered the full Mitchell ProLine/Advanced software package. I've been wanting to for a while and this post got me off my ass. I have two 240Zs in the shop for complete suspension builds so I'll take the time and carefully measure everything.

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HI am planning to do some shock dyno testing and trying to get the suspensions natural frequency...

 

I used to do this all from calculation but I found a really cool method to do a direct measurement on the car. You need to remove the shocks (and what I've done is use a set of old modified stock struts with no oil). You put these in and then then lock the opposite end of the car. Then you bounce the end your interested in.

 

Here's a video of the process in action (bounce test) http://www.racing-car-technology.com.au. What's cool about this process is you get friction effects included and can see how this changes with friction.

 

For a direct measurement of motion ratio all you need to do is measure the change in strut shaft length as compress or extend the suspension. I have a bunch of quarter in AL chunks I use under the suspension to make the change. The ratio is not linear and if you're doing shock work that may make a difference.

 

In general I've found a number of ways to measure the car directly and found this gives more realistic results than calculation. Billy Shope has some procedures based on Miliken's book using scales and a floor jack. You can read about them here, http://home.earthlink.net/~whshope/index.html

 

Cary

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Yeah, I know that. But I can't remember why we went with the contact patch over the ball joint pivot.

 

Because you were interested in the spring rate at the contact patch. My guess is Erik was doing calculations for wheel or axle percentages when setting the car up.

 

BTW... I just ordered the full Mitchell ProLine/Advanced software package. I've been wanting to for a while and this post got me off my ass. I have two 240Zs in the shop for complete suspension builds so I'll take the time and carefully measure everything.

 

Cool, I can't wait to see what you find.

 

Cary

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Wouldn't this number simply be the COS of the kingpin inclination, which is 13º. This comes to .974 for the rear. The front though has to also incorporate the caster angle which means a smaller ratio of (just a guess on this one) .965.

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I just came in from the garage. Since I had the car up and the springs removed, I decided to measure the motion ratios of the front and rear suspension. I did this as follows:

 

1. Jack and level the car.

2. Remove springs and reinstall strut.

3. Let strut go to full droop and perform following measurements.

---a. Measure from ground to center of hub.

---b. Measure length of shock tube to line on bump stop.

4. Jack wheel up 1/2 and perform following measurements:

---a. Measure from ground to center of hub.

---b. Measure length of shock tube to line on bump stop.

5. Repeat step 4 seven times to get measurements through the range of travel.

6. Graph shock measurements versus hub measurements.

7. Fit regression line and get slope (inches of shock travel/inch of hub travel).

 

 

For the rear, I came up with a motion ratio of 0.882.

For the front, I came up with a motion ratio of 0.904.

 

The suspension ratios at the center of the tire contact patch are related to the ratio at the center of the hub by a constant that is equal to one for wheels that have a zero offset.

 

My suspension is modified as follows:

Front:

Stock length lower control arms (fitted with inboard spherical bearings)

Shortened strut to fit Koni 8610-1149 insert.

Lower Control arm pivot point moved up 3/4" and out 1/4".

Top of strut tower moved back ~3/4"

1" bump steer spacer

Ground Control camber plates.

 

Rear:

Custom rear control arm adjusted 1/4" longer than stock

Eccentric camber bushings adjusted equally to maximum negative camber.

Shortened strut to fit Koni 8610-1437race insert.

Ground Control camber plates.

 

Though my suspension is fairly heavily modified, none of the modifications will have a huge affect on the motion ratios.

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I don't see how it could be non-linear unless the angle between the spring/strut travel and the control arm was really far off from 90 deg. Even then it would only have a slight curve, especially in such a short swing range. No?

 

So I feel like I have contributed something, here is a spring calculator that might come in handy.

 

http://www.efunda.com/DesignStandards/springs/calc_comp_designer.cfm#calc

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Today, I spent the entire day in the garage weighing and measuring suspension components. This is all in an effort to quantify my unspung weight and also to get a good handle on my instant centers. I'll publish my results for the unspung weight here:

 

Front:

Group A:

Strut housing with hub, lug nuts, and brake disc_________31.0 lbs

Ball joint and steering arm ___________________________2.6 lbs

Wheel and tire (Hoosier A3S04 245/45/16)______________39.6 lbs

Caliper and pads (Outlaw 2800 caliper w/Hawk 237 HP+) __4.5 lbs

 

Group B:

Lower Control Arm __________________________________2.4 lbs

T/C Rod Assembly __________________________________2.8 lbs

Strut Insert (Koni 8610-1149) ________________________4.3 lbs

Tie rod ___________________________________________1.4 lbs

Spring (Eibach 10" x 2.5" x 200 lb) ____________________2.9 lbs

 

To get the total front unsprung weight for one corner you sum the items in Group A and add half of the sum of the items in Group B. The total for the front is 82.5 lbs.

 

Rear:

Group A:

Strut Housing with brake rotor, stub axle, spindle pin, CV adapter,caliper adapter, and lug nuts.__________________________________________________42 lbs

wheel and tire (Hoosier A3S04 245/45/16) ___________________39.6 lbs

Caliper and pads (Outlaw 2800 caliper w/Hawk 237 HP+) ________4.5 lbs

 

 

Group B:

Strut insert (Koni 8610-1437race) __________________________4 lbs

Rear Half Shaft (300 ZX turbo) ____________________________14.7 lbs

Rear Control arm (Custom tubular) _________________________ 7.5 lbs

Spring (Eibach 10" x 2.5" x 250 lb) __________________________3.1 lbs

 

To get the total rear unsprung weight for one corner you sum the items in Group A and add half of the sum of the items in Group B. The total for the rear is 100.75 lbs.:shock:

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nice. i think the unsprung weight might be a little closer if you weigh the swing arms ect. by fixing the hinge area and weighing them. some objects may be heavier at one end than the other making the unsprung weight more or less depending on where the majority of the weight is at. just my .02. good going though.

 

jimbo

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The weight of the Group B items is divided by two because one end is fixed to the sprung mass, and the other is moving with the unsprung mass. The approximation of 1/2 applied for the Group B items (control arms, half shafts, tie rod) assumes symmetry and introduces a small error for items whose mass is not evenly distributed. Fortunately,(or unfortunately) the weights all of the Group B items are small compared to the Group A items. All of the Group A items are 100% unsprung, and no approximation was necessary. The error from approximation of the Group B items is very small compared to the overall unsprung mass.

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Dan, what's your next move? If it's going to be another really important measure or weigh in of some part of the suspension, can you do it here? I think I'm going to merge this thread with the motion ratio thread eventually and give it a title like motion ratio / unsprung weight / etc. These are the types of questions that one has a very hard time finding answers to and they become increasingly necessary when you're ready to get really serious. Thanks for sharing this really valuable info with the rest of us.

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Where am I going with all of this?

 

I'm working through chapter 16 of Race Car Vehicle dynamics because I'm an engineering geek, and because I want to have a better understanding of how my existing suspension works before I change it.

 

Chapter 16 deals with ride and roll rates and requires that you have knowledge of the following before you can do any meaningful calculations:

 

Corner weights (been there done that)

http://forums.hybridz.org/showthread.php?t=92703&highlight=corner+weights

Unsprung weight (check)

Motion ratios (check) http://forums.hybridz.org/showthread.php?t=129623&highlight=motion+ratios

Front and rear Roll center heights (That's next)

Center of gravity height (I would like to do this but I may have to estimate)

Tire Spring rate (This would be nice, but I'm not holding my breathe). Does anyone have a good ballpark figure?

 

In parallel I plan to work through some of the Chapter 6 stability analysis.

 

As I said, "I'm a geek".

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Again, thanks for posting your findings, this is really valuable info. I'll merge this thread with the motion ratio thread. If you could then keep going on that one thread that would be great. The B/W/S/C forum is getting plugged up with too many stickies, and all of this info is certainly sticky worthy!

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Wow! You've been as busy as I have!

 

I went through a similar unsprung weight excercise, but took a different approach based on laziness.

 

In the front, since I had the upright removed and left the rest of the control arm intact, I weighed the control arm on the car, resting the end of the arm on the scale. Since I have a heimed front end, friction was minimal so I figured it would be close. so my numbers were:

 

Control arm with brake caliper - 16 lbs

Wheel+ Hoosier slick - 36 lbs

Upright + rotor - 30

Shock + spring - 8 /2 = 4lbs

So - 86 lbs on each front corner

 

In the rear I used a similar approach but there was a lot more friction in the control arm so there is probably a larger margin of error. I tried to bias the friction each way and took the average. So:

 

Control arm w/upright/half shaft and caliper - 40lbs

Wheel/slick - 36lbs

Rotor - 10lbs

spring/shock 8/2=4lbs

90 lbs per rear corner

 

We were pretty close in the front and I suspect the differnce in the rear is attributable to the halfshafts. Not sure where the CG of your CV joints are but my car using stock halfshafts the bulk of the mass is towards the diff so less outboard weight...

 

Getting back to motion ratios... Thanks for measuring that! I was planning to do the same excercise this weekend. I probably still will, but for the time being I had been using the inverse of the motion ratio as calculated by the Mitchell suspension software. That being .942 in the front and .876 in the rear. I am impressed with the presumed accuracy in the rear... our differences easily within the margin of error of my measurments or our probable camber differences. Not sure whats up in the front...

 

Tom

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Another busy weekend:

 

I did the following:

1. Determined my roll centers.

2. Re-measured the motion ratios

3. Played around with Smithee's Weight Transfer Worksheet using the data that I know about my car.

 

First, I'll detail the procedure that I used to determine my roll centers. The process required measuring the following:

- Front and rear track (center to center)

-lengths of front and rear control arms (center to center)

---14.5" rear and 11.25" front.

-spacing between the left and right LCA pivot points

-height of front and rear LCA pivot points from ground.

-spacing between left and right pivots for the tops of the struts (center of spherical bearing).

-heights of front and rear strut top pivot points above ground.

 

I performed the whole process twice on the front suspension (once with no bump steer spacer and once with a 1" spacer). My results were that my rear roll center is 3" above ground and my front roll center is 2 " below ground (with no spacer). The front roll center rises to ground level (0.014") when I installed the 1" spacer. Here are screen shots showing the excel graph of the front and rear data:

 

Front_Roll_Center.JPG

 

Rear_Roll_Center.JPG

 

I reperformed the motion ratio measurement (just to be sure). This time I did the front at the hub center like last time and I also redid it at the tire centerline. The hub centerline and tire centerline data agreed with each other, but both were slightly different than last time (I think I was a little more careful this time). Additionally, I decided to install the bump steer spacer and see if there was any change in the motion ratio. The results from all of this is as follows:

 

Front:

--Hub center and tire patch center (no spacer) Installation ratio = 0.915

--Hub center with 1" bump steer spacer installed Installation ratio = 0.8911.

 

Rear:

--Hub center installation ratio = 0.901

 

What can probably be discerned from the variation in my results is that the error in my measurements doesn't justify my reporting more than two significant figures. So we'll just say that the rear motion ratio is 0.90 and the front motion ratio is 0.91.

 

So, where does this information get me? Hopefully, it gets me closer to picking some new spring rates. Toward that end I plugged all of my data into Smithee's Weight Transfer Worksheet and found some possible spring combinations. I'll write up the details of that later, but I believe that I am going to end up with 350 lb/in rear springs and 375 lb/in front.

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