13 Dec 2008

Worked on CAD most of the day cleaning up the chassis, still not done. After getting tired of that went out and layed out the bottom chassis tubes on the table; if you look closely you can just see the lines. With it actually taking shape it’s another vote for a large table. The difuser behind the engine and the radiator support at the front will both make the frame about two feet longer. Looks like cutting steel will start tomorrow…

Earlier Cooper came in to check why I spend so much time staring at the dumb computer. He’s perfectly happy walking around the house with a towel like this, looking like the Emperor in Star Wars. With his short hair he likes the warmth on cold days.

11 Dec 2008

Phil pointed out a couple things about tube chassis construction that I was aware of but hadn’t really thought through. He stressed trying not to have a bunch of tubes coming together into one node. Technically it’s the right way to do it but causes a number of fabrication problems. The cuts on each tube can end up being very hard to fabricate, including being impossible to install without cutting off one fishmouthed end. Many tubes in one spot results in such a large mass of metal that a lot of heat must be pumped in to weld all the tubes which leads to heat warpage. Also, it can be near impossible to reach in between all the tubes and weld every inch of every joint. It’s easy to skip the hard-to-reach ones, which start corroding from the inside out since it’s open to oxygen and moisture (never mind rain water wicking inside.) Seems like good advice so I’ll see what I can do, space the tubes slightly to allow getting the torch in between and to simplify the tubing cuts. In many cases it doesn’t take much to greatly simplify the end-cut, sometimes just moving the end of the tube 1/4″ is enough to change a 3D cut to a single-plane cut.

Ordered some Christmas presents for myself. Google Sketchup works pretty well for creating the chassis but Google knows what they’re doing. Their free version only exports JPEG drawings which is very limiting. No .EPS format for the book, no .DXF for sending to metal fabricators, or .PDFs, or .TIFFs. Fine, they win; Sketchup Pro was purchased which has all the export formats.

Back when building Kimini, there were times it was nearly impossible to use the TIG welder foot pedal when welding in awkward positions. A finger-operated current controller fixes that. Since moving a finger to change current will likely move the torch head slightly it’s not a perfect solution, but better than the antics of struggling with the foot pedal.

Replaced the leather cover for the TIG torch hoses; it was worn through and contrary to my cheapskate nature it was replaced rather than waiting for a hose to spring a leak. Also ordered bandsaw blades for the weenie-but-faithful Harbor Freight bandsaw; we’ll see if it lasts another build. They have a larger $600 saw but as long as this one keeps running there’s no reason to consider it, plus it would consume sacred floor space.

10 Dec 2008

Added a tubing rack under the build table. Keeps everything clean and visible, off the floor, easy to check stock, and saves floor space. After that, dismantled the wood mockup(!) and cleaned off the table in anticipation of starting for real. To be honest, things are moving a lot faster than expected. With the Mini it was nearly five years before the steel chassis was started.

Coming up in three weeks is the appointment with the DMV, to see if a coveted SB100 exemption number is in my future. It’s kind of a big deal; if I get one it’s a huge relief as it means there’s no concern about getting the car registered, smogged, and provides incentive to have it rolling in 2009. If it doesn’t happen it’s not terrible, but it does drag out the fear of the unknown another year and there would be no chance of driving it in 2009. It would give time to concentrate on the construction chapters of the book though it would be nice to know how it handles before that. If there’s anything dire that needs changing it would be nice to have the right stuff in the book the first time rather than reworking chapters.

I’m just holding my breath until then. The real fear is that this law is too good to last; at any time they might pull the plug which would make getting the car on the road much more difficult. Of course, with California’s $15 billion deficit, maybe the DMV will continue taking our money for this service. Have to keep a positive mental attitude.

9 Dec 2008

The tubing is here, bent tubes, too! A big thanks goes out to Phil Burke and everyone at BurkeBuilt Fabricators for helping me get the tubes bent. Lots of places can bend 0.125″ wall tubing but not many can do 1.5″ x 0.095″ wall. Unfortunately I had the wrong bend radius on my drawing but they fixed it, which means some tube drawings have to change… which is another reason the tubing drawing issue bugs me. Anyhow, different businesses use different bend radii in their benders, making the main hoop and windscreen slightly different shapes and affecting tubes mating to them.

Received some good comments on what information to provide for tube fabrication. Of course the suggestions range from, “they don’t need anything”, to, “give them everything and let them pick what they need.” The answer’s somewhere in the middle; the trick being to not spend a lot of time providing info builders don’t need or can’t use.

Since Burke deals with this stuff all the time I asked his opinion on how much information to include, noting that a builder is typically one guy in a garage using a grinder or tubing notcher to do the ends. He had a good example: a simple angle cut on the end of a round tube. At shallow angles, the tip of the tube on the drawing will be quite long and sharp. However, in the real world it won’t be nearly that long because it’s not possible to weld such thin metal; it’ll either melt back or get ground off during deburring. His point is that measuring to the tip is only useful as a guide of how long to cut the tube before notching it. Another example for this same tube: the open end of the tube is an ellipse, so the centerline length of the tube is measured to the center of the ellipse. But how is a builder going to do that? It’s very difficult if not impossible before it’s cut and dang hard to do even after.

I know I keep beating this topic to death but when people who know this stuff say don’t bother with detailed drawings it makes an impression. Maybe it’s my excuse why tubing detail will be minimal on unimportant diagonal tubes but more detailed on important corner tubes. As was pointed out, probably the most important drawing is the one showing where the suspension pickup points go; everything else is just a big bracket to keep them in place.

A shelf will be added under the build table to store the tubing up off the floor. Learned that lesson the hard way last time; the tubing rusted quickly from the moisture coming up through the concrete.

7 Dec 2008

The final chassis is progressing though the round tube drawings bother me. As mentioned before, it’s not clear the best way to convey how to cut the ends – or even how much to try and make the drawings precise. The overall length is good, and the angles of the end cuts, but I’m not so sure about index angles, which I think will be rather useless to a typical garage builder. If a drawing specifies that the miters are offset at each end by, say, 27.3 degrees, how is a typical builder going to make use of that information, really?

Curious about build accuracy, I asked FSAE teams how they make use of laser-cut tubes – just what does someone do once they have perfectly cut tubes? The theory is that with them accurately cut, the chassis becomes “self-jigging.” Teams confirmed that while having the tubes pre-cut is a tremendous time-saver, that’s all it saves. They said a builder cannot just weld laser-cut tubes together and expect the chassis to be square; it’s just not going to happen without a big heavy fixture to combat weld distortion. The reason I mention this is that just because tubes are accurately cut has little to do with them fitting due to tolerances. What good are tube drawings with 0.01″ resolution when the tubes are cut and installed into a chassis with a realistic accuracy of >0.125″?

If someone hands me accurate chassis drawings, what would I do with them? I’d make sure a few major tubes are accurately placed and not sweat the rest. About 90% of chassis tube placement just don’t matter much; the various minor tubes just keep the big ones in place and the inboard suspension points where they’re supposed to be. All the tubes in between, eh. That’s how I intend on building the chassis myself, using the drawings to double-check overall dimensions, but other than gross lengths and angles, they’re a guide only. That’s all they can ever be due to weld distortion making accurate drawings fairly pointless.

This isn’t anything new. The Gibb’s book (“Build your own sports car”) supplies complete drawings of the chassis accurate to 1mm (0.040″). Yet right up front he warns: ” Precise dimensions are provided for all components, but in practice allowance must be made for welded joints and distortion. It is advisable to cut tubes – and check and trim as necessary – as the chassis is built up, rather than cutting all tubes before work starts.” So there it is from another source, having drawings with high resolution aren’t of much use other than a reference.

6 Dec 2008

The final chassis is taking shape, right side. It’s accurate as opposed to the “pretty good” original, at left, ensuring the final drawings are right – and the chassis symmetrical. The bottom tubes and bent tubes each have a dimensioned drawing which is why tubing is being purchased. The drawings will be kept ahead of fabrication to make sure each tube has a drawing to keep the chassis honest. This way when the chassis is done there is no doubt that it’s a faithful copy and nothing will have to be remeasured, edited, or rebuilt. Of course, only after the first trackday event will it be known whether the chassis needs any final tweaks. I’m trying hard to do all the work up front so that it “just works.” Kimini was pretty good right out of the box so we’ll see if it can be done again.

A lot of progress is expected over the next four weeks ūüôā

Speaking of Kimini, at the last second the buyer asked that a color copy of the Kimini book and spare windshields be included. Since he’d already paid for the car I wasn’t concerned. However, after a couple months and a few polite reminders, well, so be it. Someday he’s going to need to know something about the car and I’ll be happy to explain, right after he pays the bill.

2 Dec 2008

With tube drawings starting an incentive seems in order. All the tube lengths are known – though not yet formally drawn up – so the numbers were totaled and tubing has been ordered! Since the main hoop and windscreen tube drawings are done they’ll get bent at the same time the tubing is picked up. All this happens next week, so between continuing with tube drawings and having the tubing on-hand, there’ll be plenty to do over the holiday break. First thing will be cleaning off the build table! Happy now?

28 Nov 2008

I’ve received comments questioning several aspects of the car and thought I’d give an explanation of my mind set:

Roll cage lacking tubes:
I anticipate builders configuring their car for what they want it to be. I have no interest in SCCA or NASA events and want the ease of getting in and out, so, I’m not adding any additional tubes to the rollcage. For hardcore racers, they can install longitudinal tubes to brace the two hoops together to make sure the cage meets the rules but most builders won’t want them. Builders building a car for cruising or going to weekend lunches probably aren’t interested in a fully-triangulated SCCA-approved roll cage. The way I look at it is that the car will be much safer than a Locost, and besides, competition rules vary around the world and are always changing so there’s no way to present a be-all-end-all solution. It’s up to each builder to double-check their local competition rules to make sure there isn’t a problem.

(Example: It was pointed out that the diagonal in the main roll hoop does not meet NASA’s rules because it’s installed to the opposite corner (though it does meet SCCA regs.) The reason it’s that way is it gets the diagonal away from the driver’s seat for better clearance. If a builder is going to participate in NASA events they’ll have to change things around which is fine. For all other builders it’s that way to keep the tubes away from the driver’s head. This is just one example of how everything’s interconnected and how each builder will be responsible for their own car – as it should be.)

The car is too tall, lay the seats back further to lower vehicle height:
This is a double-edged sword by possibly making it too low for a street car, becoming invisible within 50′ of SUVs. Right now it’s 45″ to the top of the main hoop. How low can it go before it’s unsafe in traffic? Laying the seats back means lengthening the wheelbase, slowing slew response, something important to autocrossers. Lean the windscreen back too far and the driver’s line of sight will become distorted by the glass or Lexan windscreen, or the headlights may start blocking the field of vision, never mind being too low to meet local laws. Being too low can actually be a detriment at an autocross event due to the “forest of cones” issue but it does lower the CG. Builders will have to decide what’s most important to them.

How about side-pods?
Other than the yet-to-be-rendered side inlets, I don’t think Midlana will have them. Not because they’re a bad idea; they provide several benefits such as under-chassis diffusers, space for radiators and better side-impact protection. However they also make the car heavier, more complicated, and expensive. Not by a lot, it’s just more “stuff.”

I appreciate all the input, really, but there’s a growing sense of needing to move on. The car’s a compromise and at some point I have to draw the line somewhere – figuratively and literally – but draw it I must or I’ll be forever stuck in the design phase of trying to make everyone happy. The book will note areas where builders must make decisions – which I encourage – based upon their own goals and preferences. It’s okay to be different. I’ll try to include renderings of side-pod ideas so that individual builders can pick and choose what they like, or they can do their own unique modifications.

I’m far from any sort of design and styling expert; I want people to customize their car. I’m 6′ tall, so if you’re taller or shorter, the roll cage can be changed to suit. Or, maybe you’re heavier or more slim. Widen the car, narrow it, move the seat forward or back, or tilt it and bring the cage down. Want to add tubes to the cage, great. Want to change the engine cover look? Great. Variations are a good thing, really. It’s like the book is offering a recipe for, say, a cake. If builders/chefs want to add a little more or less sugar or butter, great! The point is, it’s a starting point, a known-good solution (or, it will be) to provide a fun safe competitive car that works, but individual variation is a great thing and can result in some pretty cool cars. As long as the suspension geometry isn’t changed, where the tubes go isn’t a big deal, really. But as some point… like about now, I have to stop experimenting with different chassis designs or the car will never happen. I want to move on, keep things simple, and let builders handle the individual variations. I hope this doesn’t come across as harsh; being a one-man show means doing everything myself but has the advantage of being able to decide when to push forward in spite of not achieving perfection on all fronts.

23 Nov 2008

This explains the thinking behind how the tubing drawings are evolving.

Since the basic chassis is complete it’s time to start a redraw to: eliminate inaccurate nodes, remove overlapping tubes, double-check that nodes are symmetrical, create individual tube assemblies, and start dimensioning everything. The overall drawing of the bottom tubes is complete though not the individual tube drawings, which brings up an issue that’s been nagging me for a while – how to draw the tube ends.

It would be great to have an exact drawing of each tube showing the precise shape of each end; cut them per the drawing and it all “just fits.” However, cut tolerances and welding distortion change the shape of the chassis, unless of course the chassis is assembled on a 10,000 lb welding jig. Since we don’t have that we tack-weld the chassis, clamp it together the best we can, and weld it up. This often results in hearing a loud “bong!” as the tube that’s being welded causes a tack-welded tube elsewhere in the chassis to let go due to the tremendous heat distortion. Once that happens it means the chassis has changed shape and exact-sized tubes aren’t going to fit. Kimini was welded on a fairly heavy jig which included 4″ I-beams (H-beams to you Brits!) Even then the chassis accumulated about 1/4″ of heat distortion, bent up at the ends like a banana. Part of the trouble is that after the lower chassis tubes are welded on the top sides, the chassis has to be unclamped from the table and turned over to weld the bottom. Having precise drawings does little good if a tube that’s¬† exactly 10.000″ long in reality having to fit into a space that might be 9.912″ long. This is one problem.

Then there’s how to convey to builders the shape of each tube ends; square tubes are easy to show on drawings but round tubes are much harder. Some chassis plans deal with this by showing tubes with the ends “unwrapped”. The idea is that a full-size paper template is cut out, wrapped around the tube, marked, then the tube cut along the line, and presto, perfection. However, I’m skeptical; there’s always some cutting error, which if the chassis is started at one end and built toward the other, can accumulate into a surprising amount, and then there’s material thickness. Wrapping a paper guide around a tube gives the correct line to cut, if the wall thickness is infinitely thin, which it isn’t, or if the tube junction will be 90 degrees, which it seldom is. What happens is that facing inside surfaces of tube junctions don’t fit up because the ID of the tube prevents the edges from properly mating up, and the problem becomes worse the more acute the angle.

Because of the above, tubes will be dimensioned but not unwrapped. Square tubes will have basic dimensions while round tubes, thought they’re very hard to show numerically, are trivial to fabricate if a tubing notcher is used. Since inexpensive¬†tubing notchers are now on the market it’s reasonable to require one. With a notcher, only the length of the tube and angle of the cuts are needed; the exact path of the cut on the ends of the tube becomes moot <em>and</em> the notcher deals with the wall thickness issue.

Somewhat related is the issue of customized tubes; no two Midlana cars are going to be the same due to different drivetrains, seat choice, pedal placement, engine cover design, etc. This will be dealt with on the drawings by colored tubes where placement depends on the parts or style the builder uses. For example, on the drawing of the bottom tubes, there are two diagonals stiffening the bottom of the engine bay. While they will be dimensioned they will only correctly fit a Honda K24 drivetrain. For other drivetrains they’ll have to be different in order to miss the oil pan and transmission bell-housing.

Brackets are missing from the drawings for now since it’s too early to know where exactly they go. They’ll be added in due time though for the most part, it’s pretty obvious when a bracket is needed when the time comes.

22 Nov 2008

My brother took his Super Stalker to its first trackday event at California Speedway in Fontana, CA. He had a great time (other than being sore the next day.) The car did well other than a miss in 5th gear that may be hard to diagnose, and the right front tire seems to have too much camber which is odd. He did just under a 2-minute lap and was barely outpaced by a 300 hp Arial Atom. Given that the Atom is supposed to have 25% more power (versus the Stalker’s 240 hp) and had stickier tires it should have walked away from him but didn’t. Than again much of driving is mental. My brother – and other drivers – admit that while their cars could go faster, driving on a superspeedway is mighty intimidating, having a wall to the right that’ll stop you like a bug on a windshield. Just how far out do you hang the back end at 135 mph when there’s no national championship on the line, never mind being older and more aware of one’s mortality? Anyhow, he had a good time and the car did well.

21 Nov 2008

Turns out that a set of good brand-name gauges approaches the price¬†of some flat dashes. One big perk of some flat dashes is that they connect to the ECU, displaying the same values that the ECU is actually using versus only reading their own sensors. The advantage is that what the dash displays is what’s being used by the ECU rather than a copy. If there’s¬†wiring or sensor problems it’s readily apparent instead of reading fine¬†from the gauge’s sensor while the ECU sensor has a loose connection. Then¬†there are the configurable alarms which is real nice because if any parameter has a problem it flags it Right Now rather than driving a couple more laps before noticing.

I’m not sold on this one way or the other but am surprised that flat dashes are getting close to being cost effective. On the other hand I read about unhappy flat dash users discussing flaky software (with some dash brands.) There is something to be said for the simplicity of traditional gauges though there’s plenty of time to think about which way to go.

20 Nov 2008

Here is Midlana with a traditional Seven rear end which looks pretty good. It has a convenient area on top to vent engine compartment heat yet not be visually cluttered when viewed from behind, and gives room to fully hide the muffler. However, the engine cover that builders chose will be entirely up to them due to strong and very differing opinions of what looks best. The book will provide suggestions but not dictate a solution since it’s clear everyone wants to do their own thing, which is great! The only question is if hot engine compartment air might swirl forward due to the low pressure behind the windscreen. Or, will air pressure be lower behind the car so it isn’t an issue? Guess we’ll find out.

What’s not yet decided is how much of the bodywork will be removable around the engine. It would be nice if the entire rear area could be removed: fenders, rear panel, and top cover, to ease access, something I didn’t do so well on Kimini.

19 Nov 2008

Odds and ends.

I’ve been thinking ahead about what gauges to use. I’d decided upon the combination flat dash and datalogger from¬†www.race-technology.com. With Kimini sold and cash in hand it’s all too easy to go nuts and buy really cool stuff, and cool it is. A flat dash, all instruments nicely integrated into one package. Alarms on all the important parameters, a datalogger and GPS, what’s not to like – and then a funny thing happened…

First off, Midlana is an open car. The thought of having $1600 of dash and logger sitting there begging to be stolen bothers me. Then there’s thinking of it getting beat on by sun and no doubt someday rain. Then there’s something else – the economy. I’m surprised to admit it but what’s going on is affecting me, like millions of other consumers. “Do I really need this?” No. I’m becoming turtle-like, slowly pulling arms and legs into a shell in anticipation of an economic storm of unknown size. I guess I’m getting older and a little wiser, no longer as quick to spend money. Who knows what’s coming next year, the year after, or even the year after that. I think we’re watching history in the making – what’s that Chinese proverb/curse, “May you live in interesting times”? I digress.

So the current thinking is that there’s nothing wrong with old fashioned gauges. Looking through all the various tachometers it’s a bit surprising to end up right back with the Spa Techniques tach/speedometer that was used in Kimini. It has an odometer which is nice for road use (and just for curiosity.) A nice tach, shift lights, 0 – 60 timer, electronically calibrated, it works well. Stack has a similar “all in one tach” that also has two sensor inputs. However, at around $750 versus $350 for the Spa unit, it’s a no-go. For the $400 difference a couple extra gauges will work just fine.

Regarding the engine cover, there are a lot of very differing opinions about what looks best. This part of the build will definitely be left open for each builder to do as they please. I’ll provide suggestions but there isn’t much point in saying “do it this way” when everyone wants something different. I think it’s great that each builder will create a different car; why build a car with plans that dictate ideas you don’t like; I think variations are great.

The side inlets are proving difficult to model but there’s no much pressure to dump a lot of time into them. At this point it’s not so important to produce nice renderings; the ones already posted give a decent idea of where it’s headed. Seeing an actual car is more important that a CAD rendering. Time is best spent pushing forward with finalizing the chassis, dimensioning it, and moving toward cutting steel.

Headlights, wipers, taillights. Headlights, tradition dictates the typical chrome bolt-on units but there are some interesting alternatives now such as 2″ projector units, though they don’t include housings as far as I know. Today I saw some 4″ LED(!) “work lights” that might work as headlights but are probably not bright enough. Wipers, ugh, haven’t given them any thought; how much does someone drive an open car in the rain? However I realize most areas require them by law and it’s tempting to just specify hand-operated ones sold by hot-rod shops. We’ll see. Taillights will wait until later, too. While they aren’t a big deal they certainly change the appearance of the car. If they go on the fenders it pretty much dictates an external-mount style like the original Seven to stay clear of the tires. OTOH if they go on the body then just about anything can be used. However, they seem so far off I haven’t given them much thought.

And finally, I just got another e-mail requesting to be added to the pre-order list and thought I’d yet you know there is one… now. If you want to be on the list let me know. No, there’s no free gift for doing so, just the knowledge knowing that you’ll be the first to know the moment the book and plans are available.

17 Nov 2008

Using Sketchup components of a Caterham 7 posted to the internet by “terradog”, the nose and fenders were borrowed and rescaled to quickly create (very) rough renders of what Midlana will look like. And yes, I am partial to the burnt-orange color that’s been appearing on new cars.

There is some discussion about how to style the curved back panel. Thoughts are: nothing (no cover), painted aluminum mesh, aero shaped louvers, an aluminum panel with many punched louvers, or a more traditional Locost back end (thanks to Dean again for bouncing these ideas around.) Regardless what is used, good air flow is key. If there’s no clear styling winner then all will be presented in the book and builders pick their favorite – variations are encouraged!

The CAD chassis has been sent to a generous reader to run it through an FEA analysis. The results will only be approximate since stiffening by the attached panels isn’t being taken into account, but it should give a good idea if additional triangulation’s needed. It also has the benefit of not counting on the panels to be structural. One feature not dealt with yet are the side air inlets just forward of the rear fenders.

9 Nov 2008

Slow work churning through the rear suspension redesign, coupled with long hectic weeks at work.

Adding colors makes an enormous difference for clarity, so much easier to see things. With a rough model of the drivetrain in place it’s apparent why tubes are being run the way they are. Note how the transaxle differential housing extends rearward, preventing simpler triangulation. Hopefully – for the last time – the rear suspension is settling down; current total bumpsteer is less than 0.020 degrees over full suspension travel. A few tubes will change and a few are missing but the difficult ones are placed.¬† The diffuser has been enlarged, using otherwise wasted space, currently at 12 degrees. The turbo is approximately where it’ll go, though it’s unclear if it’ll be top or bottom mount.

The large radius rear curvature of the chassis is slighly larger than the rear fenders, visually integrating the rear curved deck and fenders, or that’s the plan. The rear wheels and fenders will be rendered to see if it looks dumb or brilliant.

3 Nov 2008

Due to drivetrain placement and the overall shape of the car, there’s really only one place the shocks “want” to go, just ahead of the axles. The bottom of the shocks attach to the lower trailing link, but the proposed upper trailing links run right through where the shock body wants to be. While the shocks <em>can</em> go there they would have to lean inboard to miss the upper arms, getting too close to the drivetrain. Since it’s unknown what drivetrains will be used it’s best to provide as much room as possible. So, the upper links are returning to A-arms…

Mitchell’s uber-cool BumpSteer program made short work of determining the toe-links locations, resulting in 0.016 degrees of bumpsteer from 1″ of droop to 3″ of compression, or about 0.008″ toe-change measured across the face of the tire. (A big thanks to my suspension advisor buddy – it’s great to have someone to bounce numbers off of who’s been through this before!)

I hope this stuff is helpful, though perhaps a bit dull and boring; weeks of going round and round to get everything to work together. Sometimes this means violating engineering practices, such as the lower rear suspension arm having the trailing link intersect the lateral link part way along its span. The shocks will attach near the intersection, feeding a large bending load into the assembly. As mentioned, it’ll be dealt with by using large OD tubing and overlapping plates, not a perfect solution but a reasonable compromise. The realities of using existing suspension uprights and clearances with the drivetrain and body sometimes dictate a less than perfect solution. It’s all about compromise, which are all over cars, one-offs, and even production vehicles. As long as the reasons are understood and a safe solution arrived at, it’s okay, and allows moving on with the rest of the design. This reminds me of what someone said during the Kimini build, about “walking a fine line between doing it right and getting it done.”

FWIW, the long lateral links provide near-zero vertical roll-center migration in roll – within 0.001″ over +/-2 degrees.

30 Oct 2008

Things are busy this week and next so it’s a bit slow on updates. It’s one of those deals where by the time I get home, take a shower and eat, it’s late and I don’t feel like doing anything!

However, the parallel trailing arms have been modeled in WinGeo and nothing bad has appeared. I’ve had a couple readers express concern that moving away from A-arms is a bad idea. What’s driving the suspension change is packaging; one reason is that there isn’t enough room behind the drivetrain to have A-arms with a base wider than 10″ which I feel isn’t strong enough. I understand the concern but feel trailing links aren’t much different than A-arms when it comes to toe control. No one bats an eye when they have to dial out bumpsteer at the front yet everyone freaks when it’s the same situation at the back – it’s the same thing.

In the 1960’s, F1 cars commonly attached the rear toe-link to the lateral link, so this was tried first – it was terrible, the wheels toing in 0.440″ over the full travel! The reason is because as the suspension moves into bump the trailing links pull the upright forward. Since the toe link is attached to the inboard pivot point, the upright and lateral link rotate forward as one about the inboard pivot, leading to really bad toe-in. The trick is to have a dedicated toe-link not attached to the lateral link, resulting in the much better control.

I was reminded that I got in trouble using trailing links once before, with Kimini. However, in that case I purposely designed in rear toe-in in bump. Older and wiser, I know better now and will design in a very small amount of toe-out.

26 Oct 2008

Things are changing – again. After a buddy reviewed the chassis he asked, “Why didn’t you use simi-trailing arms for the rear suspension like on Kimini? It’s a nice compact layout and allows long links.”¬† Um…

It’s a combination of things: the forward links can’t go too far outboard because they hit the wheels; they can’t go too far inboard because they hit the drivetrain; even in-between they hit the intercooler on one side and the alternator on the other; I wasn’t sure, due to the simi-trailing nature of the arms, if the camber and toe curves would be polite. Perhaps it was the workload of getting the chassis into the computer that kept me from spending the time on it. Or, perhaps I just got tired of dumping so much time into such a small part of the chassis with little to show for it. Well now that the chassis is (more or less) done it’s being redone – again.

The trailing links can be made parallel to chassis centerline if they’re attached part way along the lateral links. It’s not the best engineering decision though because of the bending loads imparted into the lateral links – unless they’re made really strong. This might be an acceptable place for engineering compromise because it solves a number of packaging problems all at one time: it greatly decreases the amount of “stuff” behind the drivetrain; the lateral links can be made as long as I want; because it’s then true trailing arms (parallel to chassis CL), there’s no toe-steer going on; it also frees up more space for the muffler, an necessary component that easy to forget. The shocks/springs can package fairly straightfoward, but again, only if they’re mounted slightly inboard of the lower pickups. Since the trailing links pick up from this area of the lateral links already, it’s probably okay – if it’s strong enough. I’ll run the numbers, but since Kimini had to use 1.25″ tubing for the lower tube – without the spring load – it may go as large as 1.5″ OD square tubing, due to the separate large vertical and longitudinal loads. Overkill? We’ll see what the numbers say.

So if it sounds like I’m talking myself into redoing it again, well, yes. Better this way than to build it from steel and then become unhappy with it. I wasn’t real happy about it before, and my buddy’s comments pushed me over the brink of redoing it.

In other news, I think I found a bug in Sketchup. Recall I’d been bitching about how sometimes tubes don’t “snap” to where they’re expected to. I just caught it red-handed; it was easy to see this time because it was in a clear area of the drawing. Clicking on a node I wanted to connect to, it looked like it connected fine, but it was very slightly off. Zooming way in showed that it was snapping to a point in thin air, ignoring the valid node right near it. Huh? Selecting the area confirmed that there’s nothing there and yet it wanted to connect. Sigh, going to have to keep a close eye on that.

20 Oct 2008

A major chapter has closed – Kimini was shipped to her new owner last night.

As this is my first full-blown CAD project, I get to learn all the quirks like everyone else, like CAD’s wonderful ability to see everything with such exact precision. The snap-to feature is something I’ve been using all along and taking – too much – for granted. Move a tube to a junction and it “snaps” to the node, as it should. However, had I zoomed in close, I would have seen that it does indeed snap to a node… just not necessarily the one I intended! So where several tubes come together; the situation’s ripe to have an added line be “slightly off” due to snapping to something nearby. The real damage (timewise) is that later tubes carry along the error, building errors the further along I went. My payment for being lax is to have to go back through the entire chassis and confirm each node is truly a singular point. I also noticed that the fuel tank frame doesn’t quite fit… huh? Sigh, oh well, another thing that I thought was finished isn’t. Add it to the list… but it’s coming along, quirks and all.

That aside, the last of the tubes are in place, minus the rack and radiator mount; now it’s a matter of figuring out how to optimize all the tubes. After that each tube will become its own component and get the end profiles cut; it’s not hard, just very tedious. After that, everything gets dimensioned – that’s the fun part – then it’s time to start cutting steel! The idea is to build the car from these plans, adding notes and corrections as I go along, so that by the time the car is built there’s a known-accurate set of plans. Unlike with Kimini, it means that if Midlana ever gets bent, there will be accurate drawings to make new parts.

17 Oct 2008

Since no one likes the footwell reinforcement (I admit I wasn’t sure about it myself), I took it out for now. All this stuff is subject to constant change as it progresses; even after it goes to steel there will no doubt be further refinements as the chassis solidifies. Per a suggestion from Dean, and remembering how I handled Kimini’s floor, part of the floor is going to be double-skinned. This prevents the driver’s legs from dropping out if the floor panel gets ripped off in an accident. (This is what happened to race car driver Bob Bondurant, so, as with Kimini, it’s called a Bondurant panel.) This may change slightly, maybe using smaller diagonals, but you get the idea.

Reader Brian reminded me that there are many race organizations, not just US-based SCCA and NASA. While it’s impossible to meet all the rules everywhere, he brings up a good point. It doesn’t hurt to review rules from elsewhere and see what’s easy to implement. I realize many people won’t race their cars in officially organized events – I’m not. But because these organizations learned the hard way about what survives crashes and what doesn’t, it pays to follow their guidelines. An accident at 40 mph with an SUV on the street can be just as bad – or worse – than a single-car crash at 100 mph on a racetrack.

I discussed the future fabrication of the turbocharger exhaust manifold with my race car fabricator buddy, Alan. For fun I mentioned possibly using Inconel and was surprised when he didn’t laugh. He said it really is the best material to use, build it and forget it, if it’s in the budget. It is not, at $200 per U-bend, I’d be spending about $1200 just for tubing! No, can’t go there, but I will use 321 stainless. At $40-50 per U-bend it’s still expensive but much less so.