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Had an old thread that wasn't very well named, or focused. Trying to do a new one that's better named and more focused! We'll see. (https://forums.somethingawful.com/showthread.php?threadid=3762097) OK, so this whole thing started out with wanting to do a sustainable roadrace car - IE I'm tired of fixing unibodies. Also wanted competitive class and ... frankly I love 1st gen RX-7s and rotaries and stuff. This all led me to SCCA GT-3, where you can run a full tube chassis, with a given wheel size (15x7 but Hoosier has 10 inch wide tires for that) limited downforce and in the 240-260hp range. Various engine spec lines but relevant ones are streetport 13B with unrestricted intake, bridgeport with 42mm per-cylinder venturis, or peripheral port with a single 38mm venturi before a plenum. This went through various iterations which I'll talk about in the next post. Basically this is going to focus on this thing which is still in the design stage; I'm hoping to come up with a bunch of improvements over conventional cars but at the very least it's fun to build and design.
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Nov 5, 2020 21:41
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OK, where did all this start?    So square tube, based on Wide 5, and so on. Very chonky though and not packing optimized at all. I think it was over 400 pounds which ... isn't great. I was also thinking GT-2 at that point; it's not a great idea because the cost goes up significantly and it would be a major quixote thing trying to chase down what's currently competitive in that class.    Converted everything over to round tube and lost a bunch of weight. This is also when I'm starting to do FEA on revisions.    Working through details including checking a cockpit view  Got a commercially available 3d model of the body - my halfassed one was pretty close! About now is when I'm moving to GT-3 specs.    Narrowed a bunch of stuff down, more revisions... a lot of FEA checking going on in the background.   And that's the last major revision. Another one's in progress though to fit stuff around the revised rear suspension. Next post, custom designed parts!
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Nov 5, 2020 21:54
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OK, so, all the aftermarket stuff for live axles and quick changes and the like are based off of 4x4.5 which was fine in the 90s but sucks now. Basically the only good/light/cheap 15x7 wheels you can get are in 4x100 for Miatas and Civics and stuff. So clearly custom hubs are needed.    First iteration... so basically Wide 5 bearings and cambered snout because rear camber on a live axle is :chefkiss: Sphere on the end of the axle in the cutaway is actually a crown spline to allow angular misalignment.   This is looking better, but the bearings are basically unavailable which is a PITA. "Basically unavailable" for a loving wheel bearing isn't acceptable.  Final version, with brake and wheel stuff thrown in. Back to Wide 5 bearings but everything moved around to fit well!
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Nov 5, 2020 22:01
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Engine stuff... well, the engine bay is tight. And god knows broadening powerband would be nice. So first try at a custom short-runner-for-13B-with-12A-irons intake manifold. And then I got an idea.  So this is copied from a bunch of different things, more directly Yamaha's YCCI intake setup. The velocity stacks are on 4-bar linkages driven by a servo that kicks them up at high RPM. Basically either add 75mm to the intake tract or don't. A few revisions and...   So it all works in CAD; I was printing out a test version but my printer took a crap. Will fix shortly. Now the angle on the manifold was wrong to fit in the chassis so let's fix that.   Now while I was dicking with stuff I went and designed a bellhousing to mate a 13B to a Jerico pattern trans with a Tilton SBC small diameter flywheel. Like so.  And playing with geometry went and banged out a Mumford Link because it lets me do things with the roll center I want.  Next post will be parts coming in.
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Nov 5, 2020 22:07
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So parts coming in, in vaguely chronological order.   Not directly a race car part, but Stuska water brake engine dyno I got here with the help of NitroSpazzz! Still WIP on that but it'll get there.     Trans Am spec centerlock spindle and BBS magnesium wheel center. Intended to reverse engineer but ... it's a loooooot of extra time and effort for no real performance advantage. Cool oh hell yes, but not necessary.  Dry sump tank! I think 3 gallon, but nice and tall and skinny.  Chassis table!     Tilton small diameter SBC flywheel fit to modified RX-8 counterweight.    Quick change rear end  Nice pimpy master cylinders.
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Nov 5, 2020 22:16
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That was a fun trip, was even more fun carefully lowing that thing from a loading dock. Look forward to things starting to come together. If COVID is under control next season I'll come up for a race weekend at BIR, RA, Gingerman or wherever. NitroSpazzz fucked around with this message at 23:52 on Nov 5, 2020 |
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Nov 5, 2020 22:34
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I don't have anything interesting to say but this is very cool.
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Nov 5, 2020 23:19
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Ummm, what.... insanely awesome. I'm glad you pulled this out into a new thread, I'd have completed missed it otherwise.
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Nov 6, 2020 00:05
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Yeah, it was pointed out to me that the old thread title was... uh... underwhelming. People who followed it followed it but I figured there were probably people who'd be interested who weren't following. So. Text post for a bit. Let's talk about suspension design and philosophy. I've read a _lot_ about suspension design over the last 20 years. The latest version I'm following a lot of guidelines Neil Roberts laid out in Think Fast. His basic treatise is that the main job of everything is not to do anything unexpected and have any weird handling interactions. Some consequences of this are the front roll center at ground level and rear roll center about 2 inches above, long side view swing arm lengths in the front, and most importantly consistent roll center motion with the chassis. See, roll center is an idealization of where force is applied to things, but it works well enough... the moment between roll center and CG determines moment generated and relative moment/stiffness between front and rear determines, for example, handling balance. So if you have the roll center radically move you can have handling balance radically change under acceleration or braking. NFG. Furthermore I figure that a lot of things should be adjustable without affecting other things. I'm in the process of designing front uprights right now, but things like... lower pivot movable vertically by shims to change roll center if desired. Upper pivot connected to the upright via shims (maybe copying Porsche LCA shims for this) so that camber can be changed repeatably without affecting toe or even kingpin inclination. And upper link on the rear suspension (going 3 link) is adjustable vertically to affect anti-squat.  This was a lot of iterations to get here. If everything is made to print, roll center moves with the chassis within a couple thou for the whole suspension travel.  Surprisingly this was only a couple iterations. Roll center moves with chassis within a couple hundredths for most of the suspension travel... a chassis mounted Watts link would do better there, but virtual roll center trumps that. And no, the rear chassis hasn't been altered with Mumford mount points yet. The more basic stuff... I got some guidance from people who've been running the class about what kind of wheel rates the tires in the class like, and back calculated that into ride frequencies so there's targets. Shocks will probably be basic Penskes to start or something - I have weird ideas about trying to make a homebrew controller for off the shelf (Camaro application or something) magneride shocks but there's enough to build first.
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Nov 6, 2020 01:03
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This is super cool, I wish I had noticed the new thread before. One thing I've always wondered about suspension design for a new unibody is how do you figure out where to bolt a swaybar to the body? Does it even matter as long as it's bolted somewhere?
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Nov 6, 2020 01:13
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I mean the new thread is all of "today" old. Sway bars you can get away with all sorts of weird locations, and for illustration look up Subarus. What is ideal is the same sort of stuff as is ideal for shocks, having it so that the ratio of wheel travel to sway bar end travel is fairly constant over wheel travel... so links to suspension arms not angled too much, stuff like that... but there's all sorts of ways you can do things where they still work fine and in practice, OEMs seem to mostly put it wherever there's room.
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Nov 6, 2020 01:25
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mekilljoydammit posted:Furthermore I figure that a lot of things should be adjustable without affecting other things. I'm in the process of designing front uprights right now, but things like... lower pivot movable vertically by shims to change roll center if desired. Upper pivot connected to the upright via shims (maybe copying Porsche LCA shims for this) so that camber can be changed repeatably without affecting toe or even kingpin inclination. And upper link on the rear suspension (going 3 link) is adjustable vertically to affect anti-squat. This would be so nice. Change one thing and not have to go fix all the other things that changed. I had to add a full degree of camber going from slicks to treaded and it of course messed everything else up. I'm still very new at fully adjustable suspension so it took me most of a day to make the adjustments needed on all four corners then re-corner balance the car.
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Nov 6, 2020 12:08
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Well, this is why I check stuff in CAD. Long story short the lovely Mumford link... doesn't fit around the QC axle. Will try to figure it out this weekend, it's far from the first time I have to modify designs to fit.
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Nov 7, 2020 15:30
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Oh hooooooooly poo poo, I came up with a new revision for rear suspension location that uses a linkage that, as far as I know, has never been used for this before, does everything the Mumford does, and clears everything. Pictures to follow after I rebuild it in a "real" version instead of a mocked up version.
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Nov 9, 2020 00:14
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 So it works! It's the same idea as the Mumford link with a virtual pivot point, but with the linkage underslung and pivoting in different planes. This means that I have all the advantages but it's actually possible to package.
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Nov 9, 2020 21:28
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More clarity on how it moves. Single wheel bump:  3 inches up or down from rest position  
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Nov 9, 2020 21:43
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So, if I understand right, it's fixed to the chassis at the two bronze bushings that are otherwise unoccupied in the model?
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Nov 9, 2020 23:20
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Darchangel posted:So, if I understand right, it's fixed to the chassis at the two bronze bushings that are otherwise unoccupied in the model? Correct - it's constrained in the CAD model as if there were pivots there because while I was figuring stuff out I didn't want to go modify the chassis for it. The Mumford link a few posts back was similarly constrained. The difference is the Mumford had parts going through the QC axle with everything at static ride height, whereas the new thing can handle about 3 inches of compression before something hits, which is more than a purpose built roadrace car will ever see. Fundamentally it works the same way though.
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Nov 10, 2020 00:11
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This is already incredible. Can't wait to see more of your work.
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Nov 10, 2020 03:32
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That is a really clever way to get around packaging issues.
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Nov 10, 2020 13:07
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I mean, it's basically just formula car bellcranks but upside-down and with a linkage instead of springs. Or the linkage is exactly the "roll" element of the Mercedes Project One suspension - if you flipped it upside-down and used a link instead of a coilover. What's novel is just using it for lateral location of a live axle. The thing I like that's not obvious is that it was actually a lot easier to analyze than the Mumford link. The axle-to-bellcrank pivot is 4 inches from the bellcrank-to-chassis pivot because it's a number I pulled out of thin air, and it worked the first time I tried it, and the bellcrank-to-bellcrank pivot location doesn't matter as long as they're mirrored across the chassis pivot. But the cool part of how all this moves is that roll center migration can be tuned by changing the chassis pivot angle - like, if the bellcranks were horizontal the chassis going down (axle going up relative to chassis) would make the roll center move farther away from CG... I could tune it at some other angles to make it move closer to CG under bump. Instead it's at an angle that keeps it almost perfectly the same distance away from CG under bump at any axle location. None of those force vectors matter for using this sort of linkage for springs the way formula cars do, but they totally do for axle location.
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Nov 10, 2020 14:02
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 Maybe a little more clear now what the end goal is.
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Nov 12, 2020 06:49
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Maybe I missed this but I'm curious why you're using a live axle in a full custom chassis like this for track use. I thought most track cars had IRS? I mean as a hillbilly truck builder I love SRA builds, don't get me wrong, I just thought they were passe on pavement builds.
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Nov 13, 2020 02:05
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kastein posted:Maybe I missed this but I'm curious why you're using a live axle in a full custom chassis like this for track use. I thought most track cars had IRS? I mean as a hillbilly truck builder I love SRA builds, don't get me wrong, I just thought they were passe on pavement builds. Boils down to a couple things. First and importantly, per class, you're not allowed to put IRS in something that didn't come with it and I like 1st gen RX-7s. Secondly, there's a weight penalty for IRS. So basically nobody in the class goes IRS, even things where the body looks like 3rd gen RX-7s or Nissan stuff. But the rules side aside... with as smooth as race tracks are single wheel bumps aren't really an issue, unsprung weight isn't too bad since it's a vey light axle, the ends will be cambered enough for radials to work properly, and live axles have advantages with putting power to the ground efficiently. Normally it's a pain in the rear end to get the roll center down as far as you might like but the Mumford link solves that too.
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Nov 13, 2020 02:14
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Various supporting stuff going on; also holidays and jobhunting and stuff. Whatever. Decided to start playing with the AEM dash designer software.  Standard "race" page - obviously no tach, that's handled by shift lights. Intent is that really, anything on the LCD either lights up a color that can be seen in peripheral vision, or isn't that important. I personally like numbers for a "I'm glancing at this every so often or under pace car conditions"; input appreciated. Most of the layout concepts are cribbed from various IMSA GTLM class cars; probably closest to the Corvette. Oh yeah, the TPMS box with temperatures is an "I'd like this eventually"; the sensors aren't hugely expensive but aren't necessary to get on track.  Alarm page for fun - this would be switched to at "shut the car down" levels of badness, though the text at the bottom is just the generic example warning for setup.
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Nov 30, 2020 19:05
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Agreed the numbers are useless while pushing it but can be handy. Does aem support a graphical predictive lap indicator? Something like iRacing where you're doing better off that corner makes a green bar and worse red? Been years since I setup any digital dash stuff so I'm sure it's improved a lot since then. Otherwise looks solid to me.
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Nov 30, 2020 19:44
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honda whisperer posted:Agreed the numbers are useless while pushing it but can be handy. I'm trying to figure out a good way, within the context of the software, to do that. The delta in the lower left is PLT; thinking green/yellow background on that because I don't really want to use red for anything other than immediate action (on this screen: "you locked the tires" on the grip bars, or "you way overcooked these things" on the tire temp squares) Also thinking of something useful to do with the graph in the lower right; I stuck an XvY acceleration graph there for now, but that's not really necessary.
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Nov 30, 2020 19:49
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Not important in the grand scheme of things but... startup screen idea!
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Dec 19, 2020 00:26
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Right-click, save as.
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Dec 19, 2020 01:27
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 So, the for-real this time final revision. But boy that's a lot of tubes to notch. Looking around I remembered this guy's build: https://imgur.com/a/l7HpO Which... having now dicked with CNC router stuff seems honestly pretty quick and easy to bang out. And would save me so much time. Going to have to get stuff together for that. Other things are taking some time; toy box for the kid for example. I'll post pics of that when it's done too even if it's not really related.
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Jan 2, 2021 17:01
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That tubing notcher is a lot simpler than I pictured, since you already have the drawing with tube lengths and cuts it seems well worth it. I remember finding some shop years ago that would take a drawing and do all the cuts and bends then send you everything on a pallet. Even if you need to clean up all the cuts you'll be saving a lot of time. Heck being near Road America and a lot of racing you might be able to do it as a side gig to help offset your build cost, I'd think there's enough people interested in building cages that don't have the bending and notching equipment available.
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Jan 2, 2021 17:56
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Cages is a whole bunch of trouble though, with fitting within unibody cars. On the other hand, I've been meaning to do a suspension-and-FEA-optimized Locost, and I could always just make single donor 7 clones for far less than Westfield...
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Jan 2, 2021 18:03
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mekilljoydammit posted:Cages is a whole bunch of trouble though, with fitting within unibody cars. On the other hand, I've been meaning to do a suspension-and-FEA-optimized Locost, and I could always just make single donor 7 clones for far less than Westfield... As someone who's been dreaming of a turbo rotary locost for close to twenty years, I fully support this plan.
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Jan 2, 2021 18:15
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McTinkerson posted:As someone who's been dreaming of a turbo rotary locost for close to twenty years, I fully support this plan. I mean, it was going to be Miata donor but I mean, I'm me so.
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Jan 2, 2021 18:22
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Just leave enough room for a k20 and I'm in.
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Jan 3, 2021 13:19
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Well in advance of needing it but I wanted something that spat out CAN streams for testing and it's just too cool... ordered https://trailbrake.com/tire-tpms/ the other day. Not as high of frequency read out as some of the motorsport ones, but something like 15x less expensive than the next least expensive easily-loggable one I was able to find...
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Feb 12, 2021 18:05
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So you'll have live tire pressure readings, are you going to do IR temp across the tread as well? I've always thought a live tire temp would be neat to watch and really help you dial in alignment.
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Feb 12, 2021 19:18
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It's my intent to get to there as soon as practically affordable.
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Feb 12, 2021 19:59
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 Proof of concept... so had an FC BBS wheel kicking around, so been working on a work flow to get 3d scans into solidworks. And, well, here we are! No I was not very careful about the spokes. I'm not going to dick with it too far but if you look at the profile stuff, I got it to spit out something I can convert to a 3d sketch and blah blah blah, dimensionally accurate wheel inside barrel to design brake fitment around.
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Feb 27, 2021 22:04
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This stuff is so cool.
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Feb 27, 2021 23:17
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