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Jon those numbers I gave you were just meant to be used as a starting point. Don't be discouraged, when designing a double A arm suspension you can acheive whatever roll center, swingarm length, etc. you choose all by simply moving around a single suspension pivot. Once you've settled on a lower control arm length, the upper inboard pivot's location is really the only one you should be moving around. The lower inboard pivot should be the same height as the lower pivot of the upright, so the only question with it is how far to place it inboard which will determine your control arm length. From there, you focus soley on the upper control arm's chassis pivot. Expiriment with it in different locations until you achieve the characteristics you want, it really is the only point you need to move around when messaging a design. Good luck!

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He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has.

I have finally had some time to revisit my suspension design. I’m starting with the geometry once again. I will start by explaining my method of evaluation, to make sure that my method is correct.
I’m using SolidWorks sketches to find all my lengths and angles. Below is a picture, which shows one example of the suspension at ride height, and another at full compression. Full compression was done in this test by adjusting the length of the “shock” from 14” to 12” to represent the 2” of shock travel from ride to full compression. The readings that I recorded are shown as grey dimensions. The roll center is measured in relation to the frame, to see how much it moves in relation to the CG. The Swing arm length is measured and the camber gain is measured.



I set my lower arm parallel with the ground, and made it as long as I could, without comprise. (This is about 6” shorter than my previous design) I tried various positions for the upper A-arm to connect to the frame. I then recorded the data that was needed, and put it a spreadsheet, as seen below:

Height is the vertical distance from the lower inner joint to the upper inner joint
Distance from the frame is the horizontal distance from the center of the car to the upper inner joint
Initial is before compression
Final is after compression



Now, I need to evaluate which is best. I’m finding it hard to narrow down which one is best. The ones that I thought were good are highlighted. In my opinion, the 6.5/6.75” height with 12” distance from the frame are the best. Does anyone agree or disagree with this?

Any other thoughts/ suggestions?

Thanks!

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"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054

If you can post some coordinates for the arms, track width, and toe link coordinates, I can toss them into WinGeo and post some graphs.

It might be easier to visualize and decide between them.

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-Andrew
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Hope this is what you are looking for A.Moore:




Thanks so much for the help!

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"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054

Jon,
I know this is pretty late in the game but I saw you had blown a PS hose and had to deal with it. Have you ever considered "de-powering" your rack? I have a mustang rack that I shortened and took the seals out of (although I am not sure it was absolutely necessary). The steering effort is completely reasonable even stopped. You would have to get a shorter belt but seems like you could drop ~20-30lbs and eliminate some crap in your engine bay. Just a though, Craig

Here you go Max; give these a shot.

Each graph represent the suspension being moved from 2" of bump to 2" of droop. For each graph, the chassis is at a different roll angle. The first graph represents 0 degrees of chassis roll and the last graph is 2 degrees of chassis roll.

The roll center movement does not look bad in droop but in bump it seems to shoot off into space once you get beyond 1" of bump.

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That's due to the "1 / zero" pole that's part of the kinematic role-center method. The best you can do is try and keep it out of your operating range, just realize it isn't real. That is, the car won't go crazy when you pass through that point - it's an artifact of the math, not what's really going on.

If you want to discuss differences between the force-based vs. kinematic roll-center calculation method, that's for a separate thread.

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Yes, I have considered it, but have chosen not to. I'm more attracted to a stock steering rack, than the potential weight savings by modifying it. I don't enter my car into any competitions, so saving weight isn't too big of a deal for me. Maybe some day, if I get bored....




Thanks Andrew for making the graphs for me, I really appreciate it. But, I'm not too sure how to interpret that data? I understand that the kinematic roll-center method has an "error" in it, as Kurt has said, but this leaves me even more confused. I am unaware of the forced-based analysis, and don't really want to take the time to study it at this point. Should I then ignore the left side of the graph, and assume that this suspension has good control over the roll center? With my limited expertise/ understanding, I think that this geometry will be good. Will others agree or disagree?


I have a 9 day break from school starting this Saturday, so if I can get my designs wrapped up this week, I can start to make some A-arms this week, and maybe cut off my rear end next week (scary thought). I bought some steel today for the A-arms, and may buy some aluminum sheet this Saturday. I can't believe the price of it! It was $150 per sheet for a 4x8 0.040"THK sheet 2 years ago, now its $68! Wow!

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"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054

Well, the bottom line is to keep the wheels upright in roll - assuming you care most about cornering. You can probably figure it out backwards. That is, figure out how much your car will lean in a hard turn and figure out how much camber you need to cancel it out. You'll also have to guess or calculate how much the shocks compress during that hard turn to know the suspension travel. Fix the lower arm at whatever length and width you want - with an eye on steering rack length. The upper arm is the only issue, specifically the inboard ends.

OTOH, if you're into dragracing, then you don't care much about camber compensation and want the tires upright all the time.

If it's just for crusing down the street, it's somewhere in between. Without reading the entire thread, what's the car going to be for?

While this doesn't answer all your questions, it at least cuts down on the number of variables.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Kurt, this is a retrofit of an IRS (using T bird diff axles and uprights) into an already completed solid rear axle car.
This is him driving it a while back: http://www.youtube.com/watch?v=iUQPsi8lg_g

Jon, the big question I have is with that V8 torque are you gonna want that much camber gain? All of the selections you have highlighted seem to have quite a bit of it. You mentioned before that your car had very little body roll, so with that in mind you may be better served with a setup with less camber gain, especially considering the task of hooking up on launches could be compromised if you get much weight shift to the rear and introduce some negative camber.

What did you say your front RC height was?

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He is a wise man who does not grieve for the things which he has not, but rejoices for those which he has.

I have no interest in drag racing, and this car may never see the strip. The only time I have concerns about traction is in the corners, when I'm attending a track day. I have less concerns about traction during acceleration.



Front RC is 1.33in off the ground.

Yes, my car currently rolls very little. However, I know it will have a greater tendency to roll after this is done due to the 4 wheel independent suspension, in addition to the fact that I will be getting softer springs throughout the car. The springs I have are way too stiff, and make it very uncomfortable as a daily driver. This is why I have tried to get a decent amount of camber gain in the suspension. This being said, I don't know what amount of camber gain will be ideal. I know it depends on a lot of different things, especially body roll, but I have no way of predicting this.

_________________
"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054

Yes you do, if you can estimate CG height. You already know everything else you need, track, spring rate, and probably have a good handle on what the installation ratio is. With all those, and how fast you feel you can go around a given corner, will tell how much the car will lean. RC height determines how the cornering force is shared between springs and A-arms. That is, if it's at ground level, all cornering goes through the springs, while if it's at the CG height, all cornering goes through the arms, and the car won't lean at all.

With what you'll be doing, shounds like you want something around 0.4 deg/inch, but without real numbers it's only a guess.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

An easier way to visualize it is the distance between the roll center and the CG is the roll moment arm. If this value is larger, the roll moment will be greater and more bar/spring will be required to minimize chassis roll. If this value is small, the moment is smaller and less resistance is required to control chassis movement.

Regardless of your belief on kinematic or force based roll centers, I would still attempt to move the asymptote to a region of the curve the suspension will never see. It should not take much effort and it will ensure no weird handling characteristics are introduced into the final suspension.

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-Andrew
Build Log
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I did some calculations tonight, and found that the car should roll 2 Deg, cornering at 1.0G. I don't know how accurate my calculations are, but this sounds reasonable to me. I used references from a Dynamics course I took in school, and a couple of books on chassis engineering to come to this conclusion (A lot of assumptions were made). So, I wont need all that camber gain. (Was looking at 2.32Deg before) I have chosen a new point, where I will get 1.5Deg of camber gain, which should be much better for my car. I will be re-doing my 3D CAD models this weekend, and will post some pictures as progress is made.


Thanks again for everyone who has pitched in to help me with this rear suspension geometry. I have learned a lot, and it looks like the geometry is going to perform well.

_________________
"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054

I'm on "study week" at school now, which means I should have about 5 hours a day this week to work on my car. Wahoo! Finally!

Earlier this week, I made some threaded inserts for my suspension arms. This will be lightly pressed in, and welded in. I did this at my school's machine shop.


I'm working on detailed Solidworks drawings now. Before finishing them, I needed to take a couple more measurements of my car, and verify many others. This required me to disassemble everything in the rear of the car. I started the day yesterday with this:


A couple hours later:


Doing some mock-up with the new differential:


I also had the chance to verify that the old drive shaft had the same bolt pattern the new differential. And it does! This is exciting to me, because I don't want to do another drive shaft. The only downside to this is that I need my differential to be 1.5" closer to the front of the car. This little change means that I basically need to re-do all of my SolidWorks drawing, move the shock locations, re-doing the a-arms etc. I got started on it last night, and will hopefully finish it by the end of Monday. This means that on Tues, I can buy more steel, make a mini build table, and cut off all the steel I don't need in the back of the car.

_________________
"The decision to build a car is not one reached through a rational mindset. It is a decision that is made because we have to do it! It makes no sense, but neither does love, children and taxes" -Sam Buchanan

My Build Log: http://www.locostusa.com/forums/viewtopic.php?f=35&t=3054