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Miata IRS (now with dimensions)
http://www.locostusa.com/forums/viewtopic.php?f=5&t=3181		 Page 1 of 1
Author:  Stein [ January 19, 2008, 2:00 am ]
Post subject:  Miata IRS (now with dimensions)
I think that I have built my frame to the rear as much as I dare until I come up with some dimensions for control arm pivots and heights. Does anyone have any control arm pivot dimensions to build to? I really like the Westfield design and have PMed Keith to see if he has made any measurements yet. Is there anything out there? The more I look at this thing, the lighter the Miata subframe looks. :lol: I really don't want to go there, though.

I'd like to finish the rear before going to the front. I think the car would be easier built around the rear, as there is some leeway on placement of the front to match.
Author:  Miatav8,MstrASE,A&P,F [ January 19, 2008, 9:47 am ]
Post subject: 
You could jig off of the stock subframe and arms. If you feel the miata design is inadequate, you can still use the jig and arms for everthing but the upper control arm pivot position.
Author:  Tralfaz [ January 19, 2008, 10:24 am ]
Post subject: 
A little info here:

http://www.locostusa.com/forums/viewtop ... t=0&t=2308
Author:  JackMcCornack [ January 19, 2008, 10:38 am ]
Post subject: 
From that link Tralfaz directed us to in the above post)
chetcpo wrote:
I have a 1990 subframe out back, what dimensions do you need?
Hi Chet, I know it was a lot warmer in July, and by now the subframe elves may have left with that subframe, but...do you recall what it weighed?
Author:  Stein [ January 19, 2008, 9:16 pm ]
Post subject: 
OK, I spent a lot of time in the shop this afternoon, but didn't cut a single tube. After reviewing the measurements on the aforementioned thread, also, going over the dimensions on the attached website, I also measured a subframe that has both uprights and control arms but no diff. I knew I needed good overall lengths to make the halfshafts work well. So, after levelling up the lower control arms so that all four points were in one line, I came up with this. I am fairly confident in these numbers for the existing geometry. That being the case, I suck at Paint. Actually, this is the first time I ever tried to draw something in paint. I usually just sketch it up and give it to the CAD guys to model. :lol:

Image

I then spent a bunch of time trying to scale the pictures on the FM site to come up with the Westfield geometry. I know I am close, but there were two good pictures and they yielded different dimensions, probably due to camera angle. I was able to scale off of the outer top upright bolts, so that gave me a large dimension to scale. I backed it up measuring a known size on the picture as best I could. The two pictures yield slightly different dimensions, but either could be right.

The first one here is probably the right one, as the vertical height is the same as the stock Miata (7 1/4") It also yielded exact even numbers for upper and lower spans between pivot points ( 23" and 11") so it makes sense.

Image

The other picture yielded slightly different results (7 3/8" vertical, 23 1/2 and 11 3/8 horizontal)

Image

I guess my question is, would someone be willing to run the existing chassis dimensions through some suspension modelling software, as well as the second two to see how they come out. I have no clue on such things. I am planning on a 6" ride height on street tires. Pan will be 1 3/4 below and I live on a gravel road so think I need the height.

Thanks in advance.
Author:  Auto-X Fil [ January 20, 2008, 12:04 am ]
Post subject: 
Frankly, it is worth you learning it. Get the Wishbone program in the suspension thread and punch in the numbers. It's really easy. At that point, run it through combinations of roll, bump, and steering angle and see what happens.

If you REALLY want to just bolt up a suspension you don't understand (c'mon a Locost is a learning experience, and suspension design is the coolest part!) I can hook you up with some numbers that will work just fine for whatever you want to do. I'd need to know track width F/R, ride height, intended purpose, and what steering rack you're using.
Author:  Stein [ January 20, 2008, 1:24 am ]
Post subject: 
I got the Wishbone and started to play with it. As this is for the rear IRS, are toe control arms both supposed to be set at zero?

One other thing on the original Miata measurements. If the axle is made to be straight, the lowers droop about 1/2-5/8". That puts the uppers up about 1/2" as well. Just a FYI.
Author:  chetcpo [ January 20, 2008, 1:55 am ]
Post subject: 
Jack, IIRC the miata rear subframe wieghed less than 45 lbs. I can get a definitive measurement if needed.

I spent a lot of time playing with the rear geometry. It wouldn't be hard to get a good stable roll center if it wasn't for that big ol wide diff carrier. In order to get the roll center down around 2" where I wanted it and stable in roll and bump etc. it needs to share space with the outer arm of the diff carrier. This isn't impossible, since we aren't limited to one dimension, but it does make it necessary to straddle the diff carrier arm mount with the inboard upper control arm pivots.

I couldn't come up with and elegant way to do this and that is why my rear suspension kit looks like steel orgami on crack.
Author:  Tralfaz [ January 20, 2008, 10:27 am ]
Post subject: 
If memory serves correctly.....In the Miata, at static ride height the Upper control arm is roughly level, angling upward only slightly.

My 'Guess' would be 1/4"-3/8"

T
Author:  Stein [ January 24, 2008, 10:10 am ]
Post subject: 
Well, I took everyone's advice. A buddy that has built a lot of stock cars came by this weekend and we talked a bit. He brought me a couple of chassis design books and I read them both twice and then went back to Wishbone with a clean slate.

Got something together that looked decent on paper and then found that I had fitment issues with the diff arms.

Scrapped that plan and went out and measured everything again and limited my placement of pickup points to "known safe" areas. I knew I had limited areas to work in. After reading the books, I knew what changes were influencing various parameters, instead of just guessing, seeing that it was worse and then going the other way. So, I am glad I did some reading. Knowledge is a good thing. Thanks for the push guys.

I was a bit suprised, but I came up with pickup points that look good, fit everything, show .7, .8 and .9 degrees of camber gain for 1, 2, 3" of bump. Roll center drops exactly 1" per inch of bump. Starts at 3.5 and ends up at .5" above ground. Lateral roll center is completely stable. Worst case is 2" bump, 2* roll to moves to the unloaded side by .007" Jacking is .004" at that position. That is my one question. Is that an issue?

My biggest problem getting things to work was that I kept trying to start with a 100-150" IC arm, per Adams' book. I could never get it to work that way. I also was trying to start with lower control arms that were level or lower arm pickups that were lower than the lower BJ's to get both ball joints on the "right" side of the camber curve. Both of these things kept putting me at an unstable RC.

I did end up with the lower control arm "down" and the upper is "up". Technically, I thought this would lower my camber gain too much as the lower arm had to travel past horizontal, but the gain is exactly what I was looking for.

I have gone back and drawn a scale drawing of the back half of the car and everything fits perfectly. I'll be cutting steel for the IRS tonight! My design is a very close approximation of the Westfield design, including removable diff mounts that tie into the upper shock mounts for easy installation and removal of the diff. If it works as good as it looks on paper, I will provide scale drawings to someone that would be willing to update the McSorely iges files. My design is book all the way through, with the exception of the IRS mounts so it should be fairly simple to update the solid model.
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