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post #451 of 641 (permalink) Old 01-19-2014, 10:07 PM
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Quote:
Originally Posted by CanyonRider View Post
Thanks very interesting. I'm at a complete loss for why they would use a bearing there. The strut and spring do not rotate - in fact the fork that connects the strut to the LCA prevents any rotation.

For comparison here is my strut mount assembled:

And disassembled:
From what I've read in the past, and after talking to some of the spring manufacturers, they say the spring twists as it compresses. Without allowing one end to rotate, it supposedly binds the spring a little bit, which changes its rate. BUT, those same spring manufacturers (and the folks at rogue engineering, and probably one or two others) said that with short springs that don't have a lot of travel, it's pretty much a negligible amount. The stock MS6 springs are awfully long, maybe the mazda engineers saw it worthwhile. Since I have them available, and after seeing how well they hold up, I'm going to keep mine in. At worst, they don't help anything and I'll probably never be able to tell one way or another.

Quote:
Originally Posted by CanyonRider View Post
From the Formula MR=(a/b)^2 * (c/d)^2
where
a = LCA inner pivot point to the point where the spring attaches to the LCA
b = Length of the LCA from the inner pivot point to the ball joint
c = LCA ball joint to the instant center
d = Center of the tire contact patch to the instant center

The instant center is the intersection of two imaginary lines: one formed by the angle of the UCA and the other formed by the angle of the LCA. This is where you can have some variability. Also, the instant center changes with ride height.

On my car I found
a=9.25"
b=12.625"
c=163"
d=167"
The angle between the UCA and LCA was about 7deg.

The dominant term is a/b. Being off a couple of degrees and even a couple of inches in the c/d measurements makes little difference, but a 1/4" error in the a/b measurements makes a big difference.

I look forward to your build thread.

CR
My goal here is to figure out what spring rates I really need. I was using the simpler formula of MR=(a/b)*sin(θ), where θ is the angle between the LCA and spring. [Which is looking at it like a force vector, using only the force perpendicular to the LCA.] Using this, we would have a rising MR, because θ is moving towards perpendicular through bump.



I will double check my measurements, and refine it with the actual θ. I also need to compare this LCA to what's on the car right now, because it's an aftermarket replacement. [Hopefully it isn't different than OEM.] I made those measurements with the aftermarket piece off of the car. I'm pretty confident of those measurements, but I'll double check.

Using your numbers and formula, I get MR = .51. Way back here, you said you got a motion ratio of .73 for the front, which is much closer to what I'm finding. Can you clear that up for me?

This is my suspension thread - MS6 Suspension Data - Mazdaspeed Forums Mostly just gathering information right now, but I have a couple things in the works (like the perch adapters). In the Mazdaspeed world, the MS6 is an unknown and enigmatic beast. Most people think it's just a bigger, heavier version of the speed3 that is only good for 60' times...with mcphailson strut and all. Because of that, though, no one (besides you and me) has gathered this information in an attempt to improve it.

06 MS6

Last edited by phate; 01-19-2014 at 10:16 PM.
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post #452 of 641 (permalink) Old 01-20-2014, 10:07 AM Thread Starter
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Ok this may get a little confusing:

Quote:
Originally Posted by phate View Post
Using your numbers and formula, I get MR = .51. Way back here, you said you got a motion ratio of .73 for the front, which is much closer to what I'm finding. Can you clear that up for me?
Sorry, my bad- I omitted the SQRT function in the formula I gave above. It should be:

MR=SQRT[(a/b)^2 * (c/d)^2]

The function basically normalizes the two terms. Now if you plug & chug, you get 0.72 not 0.51.

Strictly speaking: MR=SQRT[(a/b)^2 * (c/d)^2]
or more simply: MR=a/b (as in your example)
The answers should be very similar.

Contrary to the name, MR is not WR/SR. You do not multiply MR by SR to get WR. If you look way, way back at this post I said WR=0.5SR.

What I left out is:

WR=(MR)^2*SR*ACR
where
WR = Wheel rate
SR = Spring rate
ACR = Angle correction factor or sin(θ)

Remember, the angle changes slightly with ride height. I found mine to be 75deg.

Alot of folks (me included - and apparently you) call (MR)^2*ACR the Motion Ratio because it is the value that the SR is multiplied by to get WR, but it is not technically accurate - it is just a constant C.

In this case C=(0.72)^2*sin(75)=0.5.

Thatís where I got WR=0.5SR in the earlier post.


So lets do a real world example of of selecting the spring rate for the front of the car:

Letís say weíre going to lower the car to the point where we have 3Ē of suspension travel before hitting the bumpstops. When last on the scales we had LF=844lbs, RF= 848lbs. For simplicity, letís say 850lbs each. I havenít weighed the wheels, tires, knuckles, control arms, strut, spring and CV shaft, but I guessing thereís about 150lbs of unsprung weight at each corner - leaving 700lbs of sprung. Letís assume we are going to be driving on a fairly smooth track (AutoX tends to be much bumpier) so we design for a maximum 1.5g bump. Based on these assumptions, we would be absorbing a 700*1.5=1050lb force over 3Ē, therefore requiring a minimum wheelrate of 1050/3=350lb/in.

Of course WR and SR are not the same, but they a related by a constant that is a function of the control arm linkage ratios, the instant center of the front suspension and angle of the spring.

From above we found WR=0.5SR or SR=WR/0.5

Therefore we, need a 700lb/in spring to handle a 1.5g bump in 3 inches.

The new springs I just installed on my bilsteins are 700lb/in.

Hope this clarifies and I apologize for the confusion. The key is getting the LCA dimensions correct. It is best to remove it from the car.

CR

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post #453 of 641 (permalink) Old 01-20-2014, 10:41 AM
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My brain just made a funny noise, sounded something like a civic axle back..


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post #454 of 641 (permalink) Old 01-20-2014, 04:10 PM Thread Starter
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Suspension Engineering Continues

So we have our new 700lb/in, 2.5"dia race springs on our newly adjustable perches. As @phate has pointed out in another post, the 6 suffers from a lack of stroke. The front struts have a max of just under 5.5" with no weight on them; add the weight of the car and you have, say 4"; add a bumpstop on the strut piston and you may have around 3" of usable stroke. Remember that because WR=0.5*SR, this means 3" of spring travel equals 6" of wheel travel so all is good.

The problem comes when you start to lower the car. If we lower the car to the point where we only have 3" of wheel travel, this will mean about 1.5" of travel available at the strut. We need to increase the available travel for two reasons:
1- So we aren't riding on the bumpstops.
2- So we don't damage the strut by bottoming it out.

So here is the stock strut mount:




We disassemble it and throw away the top (the plate on the right) and rubber mount in the middle.




Then we cut the bumpstop cup off the bottom plate which is actually the upper spring perch. This will allow the entire strut body to pass thru the spring perch.



Here is the underside of the strut tower where the strut mount would normally be.



Here is our modified mount installed in the car. It will no longer function as a strut mount - only a spring perch.



Here is the strut installed with the spring and bump stop removed and the suspension in full compression (no more available travel). Keep in mind, you would never set your ride height this low, but under the worst bump condition this is the limit of suspension travel.



Close up


Notice that the top of the strut tube is basically where the bumpstop cup used to be. To increase the available stroke and improve our safety margin we fabricate a new strut mount plate and install it above the factory mount. Here we have raised the strut mount 2.25" above the stock location.



It is a little too high and just barely hits the hood. I will remove the lower nuts to drop it down 1/4". I think you get the idea. Additional note: remember my hood has no insulation pad or reinforcing structure:



so I'm guessing a stock hood would not allow the strut tower to be 2" higher. YMMV.

CR

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post #455 of 641 (permalink) Old 01-20-2014, 07:06 PM
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Are you concerned at all about the torque that will be put on the new plate and/or shock rod and seals? You have removed the bushing in the stock mount that allows a bit of flex as the angle of the shock changes through its travel.

I double checked my LCA measurements today as well, and came up with very similar results to my originals. Makes me wonder if the MS6, or later years in general, were changed slightly.

06 MS6
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post #456 of 641 (permalink) Old 01-20-2014, 07:43 PM Thread Starter
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Quote:
Originally Posted by phate View Post
Are you concerned at all about the torque that will be put on the new plate and/or shock rod and seals? You have removed the bushing in the stock mount that allows a bit of flex as the angle of the shock changes through its travel.
.
Yeah I was thinking the same thing. I ordered some poly bushing to go above and below the plate.
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post #457 of 641 (permalink) Old 01-20-2014, 09:10 PM Thread Starter
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Quote:
Originally Posted by CanyonRider View Post
Yeah I was thinking the same thing. I ordered some poly bushing to go above and below the plate.
CR
This is what I ordered.




When they come in I'll enlarge the hole in the plate to accept the small nipple and put a washer on top and bottom. Should work don't you think?

CR

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post #458 of 641 (permalink) Old 01-20-2014, 09:26 PM Thread Starter
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Here's another thing that occurred to me while under the car (I do some of my best thinking under there). When the car is lowered the upper balljoint is raised causing the UCA to be angled upward from the pivot point. It would be nice to raise the pivot point also to correct the geometry. Recall that we have installed eccentric bushings in the UCA. Here we have the eccentric set toward the balljoint to give us more negative camber.



Our adjustable balljoints give us ample negative camber, so we really don't need more. But if we rotate the eccentric toward the bottom, it will raise the back of the UCA a little and help offset the raising of the balljoint.



Every bit helps. What do you think?

CR

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post #459 of 641 (permalink) Old 01-21-2014, 02:18 PM
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I think, as long as it doesn't decrease the amount of dynamic camber we gain through bump, go for it. We don't have a great camber curve (it's good, but just doesn't seem to be enough), so I would avoid anything that decreases it. I would like to model this suspension at some point, but don't have the capability right now.

Those shock tower bushings should work great. I've been trying to come up with a way to increase shock travel, and this might do just fine. On the MS6, the front camber curve from full droop looks like this. Full droop, of course, is limited by the shock extension. Raising it a little bit shouldn't hurt a thing.

I might be a little concerned about removing the frame stop from the UCA. There have been many cases on other cars where the control arm will dent, or even punch through, the shock tower. I was toying around with the idea of putting a normal type of bump stop out there, but haven't done any measuring of the UCA or the math to see what would happen where.

I'm a little confused again about your motion ratio and shock/wheel travel ratios and shock travel calculations. The shock motion ratio is a direct relationship between the ball joint and strut mount, along with the angle of the shock. I come up with .73, meaning the shock/spring move .73" for every 1" of wheel travel (it's dynamic, but probably close enough for anything we're doing). I'm not sure why you're using the .5 figure, I guess.

06 MS6
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post #460 of 641 (permalink) Old 01-21-2014, 08:24 PM Thread Starter
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Quote:
Originally Posted by phate View Post
I might be a little concerned about removing the frame stop from the UCA. There have been many cases on other cars where the control arm will dent, or even punch through, the shock tower. I was toying around with the idea of putting a normal type of bump stop out there, but haven't done any measuring of the UCA or the math to see what would happen where.
We're thinking the same thing. I've measured it up and ordered a bumpstop that will line up with the upper ball joint as it reaches the top of its travel. I don't want to rely on my modified strut mount as a bumpstop because if it fails I'll have a hole in my hood. Also the force at the ball joint will be less (by the factor of the MR) than the force on the strut mount. I'll post pic as I get it fab-ed up.

Quote:
Originally Posted by phate View Post
I'm a little confused again about your motion ratio and shock/wheel travel ratios and shock travel calculations. The shock motion ratio is a direct relationship between the ball joint and strut mount, along with the angle of the shock. I come up with .73, meaning the shock/spring move .73" for every 1" of wheel travel (it's dynamic, but probably close enough for anything we're doing). I'm not sure why you're using the .5 figure, I guess.
I recall from engineering school, whenever they gave us a new formula they would send us to lab to collect experimental data and prove the formula accurately predicted the behavior we expected. So here we go:

Starting with the LCA horizontal (approximating normal ride height) We have the center of a wheel stud at exactly 5".



And at the same time the top of strut sleeve is 8" from the strut mount.



We then raise the wheel exactly 4".



And we find the strut has risen 2-9/16"



Based on this empirical data it appears WR=0.64SR. Hmmm - very interesting. Looks like I'm getting a bad lab grade on this one. Thoughts?

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