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Front Suspension Design (was: North American Standards)

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JackMcCornack

Post subject:

Posted: December 5, 2007, 6:40 pm

Joined: July 29, 2006, 9:10 pm
Posts: 3169
Location: Oregon, usually

Gosh, was it something I said? This thread went from a flurry of activity to five days of silence...

Anyway, I put another drawing up at http://www.kineticvehicles.com/Drawings.html
showing the front upper control arm tube parts--now all that's left to draw for those control arms are the lower tubes and bungs. I'd like comments and criticism if available.

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Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

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dhempy

Post subject: Bash my design

Posted: December 27, 2007, 4:03 pm

Man of Constant Hazard

Joined: February 20, 2006, 11:18 am
Posts: 3186
Location: Lexington, KY

OK...either no one else is having better luck than me, or they're keeping their designs close to their chest.

I'll start this ball rolling by posting my current design and let you all shoot it down. (Save steel...shoot now!)

This is my fifth iteration of a design for a McSorley +442, Miata spindles, shortened rack, and 20" race tires. It seems to be the best iteration I've come up with, based on tinkering with it in Wishbone. I do not know yet if it can actually be fabricated in the real world.

One problem I see is a lot of camber gain in bump. Other than that, camber gain in roll and steer are probably okay, and bump steer looks dead-on. (At least I think so...have I mentioned I'm a newbie?) I don't know enough about any of the other numbers (Ackerman, KPI, etc.) to comment on those numbers

Here is a screenshot and the .WSH file you can load into Wishbone.

[oops...Looks like .WSH files are not allowed attachments. Let me talk to our man Chet and see if he can fix that...in the meantime, you can get it here: http://www.davidandjanine.com/david/car ... COST05.WSH ]

As this design evolves, I'll post updates, and I encourage others to freely update and post their own updates or completely fresh designs.

(BTW, This kind of collaborative effort might be a good use for a Wiki...does LocostUSA have a wiki feature?)

You do not have the required permissions to view the files attached to this post.

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...nowadays people are so intellectually lazy and lethargic that they can't build ANYTHING with their hands. They'll spend hours watching whiny people marooned on an island, but won't spend a second adding anything to the world. -weconway
Visit my [Locost 7 build log]

Last edited by dhempy on December 27, 2007, 8:12 pm, edited 1 time in total.

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chetcpo

Post subject:

Posted: December 27, 2007, 4:37 pm

Joined: August 15, 2005, 10:13 pm
Posts: 7043
Location: Charleston, WV

I haven't had time to play with those numbers in Wishbone, but based on some of the discussions about the tight fit of a +442 chassis and the Miata rearI wonder if a "book" or +2 wouldn't be better than the +442.

Just throwing that out there.

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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.

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dhempy

Post subject:

Posted: December 27, 2007, 6:58 pm

Man of Constant Hazard

Joined: February 20, 2006, 11:18 am
Posts: 3186
Location: Lexington, KY

Oops...Just spotted an error in my numbers above... :oops:

Got some of my numbers mixed up between wheel sizes.

Here is what I've got now... This is for a McSorley +442, Miata spindles, shortened rack, and 23" stock Miata tires. I do not know yet if it can actually be fabricated in the real world. After I get this nailed down, I may explore options for 20" tires for autocrossing, but at this point I'm eager to get some rubber on the road, and autocross will be a slim minority of the miles I'll put on this car.

You can download this version from http://www.davidandjanine.com/david/car ... COST10.WSH if you want to play with it in Wishbone.exe

Code:

Co-ordinates  X(fore/aft) Y(height) Z(width) F1 Help F2 Info F5 Dimens F6 Diag
LOWER REARWARD      -6.60      6.40      7.50     22.75    TYRE DIAMETER
LOWER FORWARD        4.65      6.40      7.50     55.5     TRACK WIDTH
TOE CONTROL ARM      4.00      6.85      7.90     90.0     WHEELBASE
UPPER REARWARD      -5.35     13.10     11.90     13.00    CG HEIGHT
UPPER FORWARD        2.70     13.10     11.90    -55.0     CG LONG
LOWER BALL JOINT     0.00      7.75     26.00
TOE CONTROL ARM      4.00      8.50     26.35     -0.50    STATIC CAMBER
UPPER BALL JOINT    -0.75     16.25     24.40      0.13    STATIC TOE-IN
BUMP, ROLL, STEER   -0.90      0.00      0.00       Home,Enter = Zero BUMP etc
↑↓→←=Select: (Ctrl+)PgUp/Dn=(Fine)Adjust: Esc=Exit: Enter(x2) = Recalculate
               UNLOADED / LOADED                         UNLOADED / LOADED
RC HGHT/JACK     3.028   0.000               IC HEIGHT      6.4    6.4
RC LATERAL       0.000   0.000               IC LATERAL   -30.5  -30.5
CASTER (deg)     5.047   5.047               SQUAT HT      16.9   16.9
CAMBER (deg)     0.466   0.466               SQUAT LG   %-2734.9D+%9%-2734.9D+%9
TOE-IN (deg)     0.099   0.099               %ANTI I/B      0.0    0.0
TOE AT RIM       0.011   0.011               %ANTI O/B      0.0    0.0
KPI (deg)        9.839   9.839               SCRUB RAD     0.400   0.400
LIFT            -0.021  -0.021               ACK % STAT    42.8    per cent
TRAIL            0.683   0.683               TURNRAD FT      0.0     0.0
Data was read from file: LOCOST10  F10 to save data. Last save was to file:
COSINE Suspension Geometry Analysis.. (Wishbone)...(C) M J McDermott 2000



LINK LENGTHS                      ANGLES (STATIC)

Lower rear leg    19.688          Steering arm/track rod angle     83.000
Lower front leg   19.123          Steering arm ('Ackermann') angle   7.004
Upper rear leg    13.344
Upper front leg   13.687          Ackermann percentage (nominal)   42.9
Track rod length  18.524
Steering arm effective length   4.072

JOINT SPACINGS

LF to LR chassis pickups   11.250
LF to UF chassis pickups    8.112
LR to UR chassis pickups    8.249
UF to UR chassis pickups    8.050
King pin length             8.682

PERPENDICULAR LENGTHS

Lower wishbone    18.505
Upper wishbone    12.701

-dave

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dhempy

Post subject:

Posted: December 27, 2007, 8:30 pm

Man of Constant Hazard

Joined: February 20, 2006, 11:18 am
Posts: 3186
Location: Lexington, KY

chetcpo wrote:

I'm not convinced about the +442 being too wide. I've had my rear wheels on the car, and while it was close, they cleared the frame, and I could have cheated anohter 2" out of it without narrowing the cockpit. I'll have to go back to my pictures and see how close it was. (My subframe is out of the car at the moment.)

Anyway, I'll follow up on that thread and leave this one about suspension design.

-dave

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horizenjob

Post subject:

Posted: January 12, 2008, 2:57 pm

The voice of reason

Joined: January 10, 2008, 4:47 pm
Posts: 7652
Location: Massachusetts

DHempy writes:

Quote:

I originally planned on running 22" - 23" tires on the street and 20" race tires for autocross. But that was before I knew *anything* about suspension design (not like I'm a pro now, mind you) and didn't realize that you can't just change one parameter and have everything else stay the same. I hadn't bought the Kimini book yet, obviously! I still have some hope that I'll be able to do both, but I'm more interested in finishing the car with a single tire size and escaping this paralysis by analysis. ( I'll worry about perfection on my next car! )

I used to do time trials and hillclimbs, so I'm interested in what you see as the issues
here. I guess I have trivialized here as well. So I can think that the scrub radius will
be wrong, where else do you get in trouble dropping the car an inch with tires.

My desire would be to get good times at the track, and live with more compromise
on the street ( no one times you there and not inclined to race on street ). But any
evil handling would not be acceptable...

Seems like you could choose some values that would tolerate this change. I am
leaning to fabricating or buying race type uprights, so that may or may not help
compared to trying to use Miata parts...

Thanks for any opinions offered...

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horizenjob

Post subject:

Posted: January 12, 2008, 3:19 pm

The voice of reason

Joined: January 10, 2008, 4:47 pm
Posts: 7652
Location: Massachusetts

Jack McCornack writes:

Quote:

Please forgive the pompous disclamer on the drawings; I've been sued too many times to set these drawings free without qualifiers.

Jack your license seems generous and sensible. I was wondering if there
should be something similar for the sticky thread being proposed. I like
the GNU free software license which allows free use, even commercial use,
but requires that modifications be availabe. So community development
is encouraged...

That seems suitable for the mix of amateur and also commercial people
contributing here.

[/quote]

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chetcpo

Post subject:

Posted: May 6, 2008, 9:14 am

Joined: August 15, 2005, 10:13 pm
Posts: 7043
Location: Charleston, WV

Randy's request for some workable numbers has renewed my interest in this project. I'll post the numbers I come up with and let the gang critique them. :wink:

So far here is the objective. Come up with a "good all around", not too difficult to build suspension design for a "book" chassis. Based on the Miata as a donor it will have the Miata's track and be built around a ride height of 5.5". It will also be built around a Stock Miata wheel offset and 23" tall tires.

How does that sound to everyone? Any suggestions before I get too far down this path?

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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.

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RandyBMC

Post subject:

Posted: May 6, 2008, 10:21 am

Joined: February 29, 2008, 7:15 pm
Posts: 346
Location: Denver, Colorado

Sounds good - I'm working on it too, so we should keep our notes together. What is the stock Miata wheel offset?

I am thinking that the issues to tackle will be having a roll center low enough but not so low that the lateral movement under bump is crazy, which will determine the control arm (and in turn swing arm) lengths as well as attachments.

The shorter the swing arm length (the distance from the ball joint line to where the upper and lower control arms intersect) the better the roll center stability, but has radical camber changes in both droop and bump.

The longer the swing arm length, the lower the roll center, but you have less stability - the roll center wants to move laterally. It also has very small camber change in bump and droop but bad control over the outer wheel in droop.

Obviously, the balance must be struck to provide good camber change control and good roll center control while keeping the roll center as low as possible.

I am planning on trying to get a good offset wheel, like 40-45mm, on the wheel to get the best scrub radius possible too. The closer the ball joints to the wheel centerline, the less scrub radius you will have (which is a good thing). In bump, the twisting force on the steering is proportional to the scrub radius length. If you take a line through the upper and lower ball joints to the ground, the difference between that line and the wheel cL is the scrub radius, so if you use a higher offset wheel, you reduce that length.

How's that for a start! How do you copy the page from Wishbone into a thread? I plan to draw out the initial design and then use wishbone to test it.

YeeHaw!
Randy

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RandyBMC

Post subject: Re: Bash my design

Posted: May 6, 2008, 10:41 am

Joined: February 29, 2008, 7:15 pm
Posts: 346
Location: Denver, Colorado

dhempy wrote:

I don't know enough about any of the other numbers (Ackerman, KPI, etc.) to comment on those numbers

Dave,

The KPI is king pin inclination, and is really already set by the mounting points on the upright. It is the line between the upper and lower bal joint mounts on the upright compared vertical - it is related to the scrub radius (and in some discussions, people use king pin offset instead of scrub radius) in the fact that it uses the same two reference points, but the scrub radius can be moved due to its relation to the centerline of the wheel instead of just vertical to get the angle. Scrub radius is a length - cL of wheel to line between UBJ and LBJ to the ground, and king pin inclination is an angle of that line through the ball joints to the vertical plane. It is usually helps center the steering, but if you have too much, the tires flop during steer. You want about 5-9 degrees of KPI.

Ackerman turns the inner wheel further than the outer in a turn to minimize scrub. This is usually done with rear mounted steering arm mounts on the upright. You can also make some Ackerman available with an upward tilt to the tie rods. I really don't think it is that important, because of the load on the outer wheel making the inner wheel less and less useful, but every little bit helps.

Hope that helps!
Randy

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davew

Post subject: Ackermann

Posted: May 6, 2008, 12:08 pm

Joined: September 22, 2005, 8:12 am
Posts: 1880
Location: 4AGE in S.E. Michigan

Ackermann can be accomplished by either front or rear mounted steering arms. On front mounted steering arms, the arms are angled outboard in vehicle position and for the rear mounted steering arms they are angled inboard. You do not want to add additional upward tilt to the tie rod because that will effect the bump steer. You can slightly increase the Ackermann effect by moving the steering rack closurer to the center line of the spindle. Unless you can machine new steering arms or alter them, you will probably not have any choice in the ackermann angle without spending big bucks for custom arms.
If you plan to race, then it could be worth the effort.

@ 55% ackermann, Dave

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chetcpo

Post subject:

Posted: May 6, 2008, 12:35 pm

Joined: August 15, 2005, 10:13 pm
Posts: 7043
Location: Charleston, WV

The Miata's arms are angled out like Dave mentions to provide some ackerman geometry. On the longer wheelbased (longer than Miata) Locost they seem to fit the bill nicely. I haven't studied it carefully, but FWIW low speed manuvers at full lock on my Locost are smooth. There is not scuffing/sliding/scooting stiction between the tires and the ground.. I'd be willing to bet the ackerman is really close to ideal.

Here's some numbers for the front for folks to play with based on the specs given earlier:

Roll center is 1.7" above ground.

You do not have the required permissions to view the files attached to this post.

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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.

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JackMcCornack

Post subject:

Posted: May 6, 2008, 12:53 pm

Joined: July 29, 2006, 9:10 pm
Posts: 3169
Location: Oregon, usually

Does increasing scrub increase the car's self-centering...probably using the wrong phrase here--what I mean is, the tendency of a car to straighten itself out if you let go of the steering wheel. I've driven/ridden a few Locosts that, if one put one's hands in the air, pretty much went whichever direction the steering wheel was pointed when the driver last pointed it, and that's not altogether a good thing.

Then again, having heavy steering isn'a altogether a good thing either. How does one tune for a happy medium?

_________________
Locost builder and adventurer, and founder (but no longer owner) of Kinetic Vehicles

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RandyBMC

Post subject:

Posted: May 6, 2008, 4:49 pm

Joined: February 29, 2008, 7:15 pm
Posts: 346
Location: Denver, Colorado

That's more likely a combination of not enough caster with zero or even worse, toe out in the front. If you design with some usable caster - 5 to 9 degrees or so - then you should still be able to use a neutral toe or even a little toe out for very quick turn in.

Hope that helps!
Randy

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RandyBMC

Post subject:

Posted: May 6, 2008, 4:55 pm

Joined: February 29, 2008, 7:15 pm
Posts: 346
Location: Denver, Colorado

chetcpo wrote:

Very cool Chet - how do you reference those numbers? Is that from frame centerline in the X, and what from the Y - ground?

I wanted to plug them into wishbone.

Thanks!
Randy

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