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"sony1492" (sony1492) 01/13/2020 at 08:33 • Filed to: None

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Ive got a suspension design program demo that lasts another 9 days, so in that time I’ve got to get the front end ironed out.

Sould be a 72" width with a 29" wide chassis, no idea where the center of gravity will end up, but the roll center still looks too low(maybe?)

Basically we are working with this, but a square frame and significantly wider in all regards

C urrently the arms are not drastically different lengths which means not much camber change under dive bet I’m not sure how much should be traded off for camber increases in roll (to maintain contact patch). In it’s current form it will need a few degrees of static camber to be level at 3" of roll but has less than a degree of camber change under d ive

If anybody has any input or ideas please share.

PartyPooper2012 > sony1492 01/13/2020 at 09:54

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OK GOOGLE: H ow much should be traded off for camber increases in roll (to maintain contact patch)  

CobraJoe > sony1492 01/13/2020 at 10:25

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I took a vehicle design class back in college, but that was 12 years ago now... (Geez, that makes me feel old).

I know I’m forgetting a lot, but the thing I remember the most is that you would tend to want a roll center at the front to be a little lower than the roll center at the back. That would put the chassis at a slight inclination to understeer, rather than oversteer.

And, if I’m recalling correctly, having a lower roll center than COG will cause a little less responsiveness in the handling, but also makes the car less twitchy.

As for the camber gain, I think that partially depends on what kind of tires you’re planning on using. Taller sidewalls might require a bit more static camber or camber gain to compensate for more deflection during cornering.

Though this is all coming from a guy who once studied the concepts, designed a suspension, but is still in the process of building it.  (In other words, I don’t know exactly how my own suspension is going to react in the real world).  

sony1492 > CobraJoe 01/13/2020 at 11:03

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This is more insight then the guy who watched a few youtube videos.

Itll be on 245/40/17's or anything in that ballpark so reflection wont be huge. With this new design the upper arms got pushed out a tad and the lowers pulled in, camber now stays flat on roll. Not sure how much ill worry about dive with a longish wheelbase 96-98".(maybe fiddling with the pitch center and upper A arm inclination?)

The center of gravity is raised, though the rear end will be a solid axle so I imagine its roll center is fairly high.

Snuze: Needs another Swede > sony1492 01/13/2020 at 11:16

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I’m an engine guy, not a suspension guy. I have a friend for that. I linked him to this, if he has any input I’ll add it. But I think CobraJoe is pretty on point (I also took a vehicle dynamics course and suspension is still black magic to me). 

sony1492 > Snuze: Needs another Swede 01/13/2020 at 11:22

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Engines arnt black magic!!?  Also, thanks.

Snuze: Needs another Swede > sony1492 01/13/2020 at 11:32

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Nope, air and fuel goes in, torque and exhaust gases come out. Simple. Just gotta make sure it all happens at the right time and in the right quantities.

But suspension... suspensions require complex math. Geometry, differential equations, etc. I once sat through a lecture on understanding suspensions through “bifurcation analysis. ” I’m still not entirely sure what that even means! I think that was the moment I realized suspension stuff wasn’t for me. 

CobraJoe > sony1492 01/13/2020 at 11:54

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Not sure how much ill worry about dive with a longish wheelbase 96-98".(maybe fiddling with the pitch center and upper A arm inclination?)

Back when I was considering anti-pitch or anti-squat geometry, I had one guy on a forum point out that anti-pitch or squat is also anti-suspension. Adding in those geometries might also reduce the reaction to bumps or weight transfer.

Pitch shouldn’t be much of a worry, your COG will be much lower than a sedan, and your spring rate and shock stiffness should be much higher too.

My solution was to lay out a basic design with some ability to change mounting points in the future, if I decide it’s worth the hassle. (It’d require cutting off existing tabs and welding on new ones, but I have a vertical tube to allow for a wide range of mounting locations) .

Otherwise, I’m trusting in the adjustability of the shocks and spring rates to get me to my desired handling (I’m not even making sway bars until I test drive it some) . But, it’s worth noting that I’m going to set mine up for street duty first, with maybe some autocross or track days later on. I t’s also a FWD drivetrain in a mid engine setup, not a standard FR locost .

A solid axle does likely mean a high rear roll center (depending on exact setup), so that combined with your current higher front roll center should put the roll axis closer to the COG, which means less body roll. I’m not certain of the exact consequences of that design, but I’d guess it means less worry about camber gain during cornering, and less waiting on weight transfer before the tires take a set in the corner .

The only other thing I’d mention while looking at the front suspension design:  Bump steer.  Where is your steering rack going to mount?  Are the tie rods an appropriate length compared to the the A-arms with the rack at it’s mounting point?  Adjusting the A-arms slightly now would be much easier than cutting a steering rack later on.  

sony1492 > CobraJoe 01/13/2020 at 12:23

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The front end width is being dictated by the rack width of 29", otherwise the chassis would be a bit more slender. I figure getting the car built even if its sub optimal is what matters most on the first one.

Ive got no plans on running sway bars since weight will be 2000lbs or less, not sure if 7.7" roll center is particularly high or low? The center of gravity will still be fairly high for a locost since the v8 is pretty much a 28" cube with a bunch of weight up top (twin cams ).

You raise an interesting point about anti squat, sounds like I should limit that to however much ca ster I want, since the handling goal is autoX and street use. Softly sprung enough to stay settled over mid corner bumps, with more then a little of droop.

With your chassis do you think sway bars will be necessary with your weight goals? 

CobraJoe > sony1492 01/13/2020 at 13:12

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A DOHC V8 in a sub 2000lb car should make for one hell of a ride. Hopefully you can put all that power down in an autocross course.

My build is only using a B18B1 from an Integra, so I’m expecting a weight closer to 1500lbs on a 94 in wheelbase. I’m also planning on keeping the engine stock for a while, 140hp should be more than enough to keep it fun on the street, at least to start with.

I’m also going with some droop and soft springs as well, paired with some smaller wheels (15s or 16s). O ur streets are not always the smoothest, and I don’t want to stress the frame too much until I know what it can do . I’m really not sure if sway bars will be necessary, it is a lightweight design and the center of gravity should be rather low, but I also kept the roll centers low to try and make it forgiving on the street (it is going to be rather rear heavy) .  So, I guess I’ll install some sway bars if I need to tune the handling beyond what the shocks and alignment can accomplish.  

DasWauto > sony1492 01/13/2020 at 13:27

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If you have time, get a copy of Race Car Vehicle Dynamics by Milliken to read. I’d offer more advice but I barely started reading it myself. :P

sony1492 > DasWauto 01/13/2020 at 16:35

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$$$

Bu t itll be my first choice for books regarding  handling and chassis

DasWauto > sony1492 01/13/2020 at 17:44

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There’s a digital copy out there on the bay of pirates, iirc.

Also, when I build a bike engined car, I’m going to do it a la Matt Brown and just use Miata suspension front and rear, though I likely won’t bother narrowing it down  to fit whatever body I end up using.  Not that Miata suspension is perfect but the geometry being done for me and plentiful parts availability make it an easy choice. 

sony1492 > DasWauto 01/13/2020 at 18:51

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Any rough ideas on the chassis thus far?

DasWauto > sony1492 01/13/2020 at 21:09

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For myself? I’m thinking I’ll get an old roadster or some sort, remove the chassis (if not unibody) and cut out basically everything that isn’t the body shell. Put a pair of healthy sized square tubes (maybe 2x2 or 2x3) on the inside of the sills, and build off that with lateral pieces to frame out the floor, firewall and rear bulkhead. Make a trans tunnel out of smaller square tube (1x1 or 1.5 x 1.5) and build some torsional rigidity into it with triangulation to that and possibly some door bars. I’ve thoug ht about welding the doors shut and just stepping over them to get in, likely using the roll bar I intend the car to have. Suspension mountings will come off the firewall and rear bulkhead likely with a pair of frame rails each and torsional rigidity coming courtesy of triangulation with the fender supports and firewall/bulkhead.

It’s all conceptual right now but I’ll draw up a plan once I have a body, bike and Miata to start with.