Tag Archives: technical stuff about building cars

All those words and I forgot to say what I came to say

Since this is a follow-on post I won’t use much space this time. What I originally came to say was that while I was trying to figure out how to build the Mini Sprint-T I figured out how to build a better frame for the 1:1 Sprint-T. Anyway, what I had decided for the Mini was front and rear hoops that went all the way down to the belly pan and had the frame rails glued to the front and back of the verticals of the hoops, instead of continuous frame rails and the hoops on top of the rails. What evolved from that was a lower rail with 0.120″ wall inside the roll cage area and 0.060″ wall in front and back of the cage so that in front or rear collisions the lower rails would collapse first and send large objects attached to the frame under the driver compartment and not into the driver or passenger.

So, when I build the frame I bend up the hoops and leave the legs 1.5″ longer (scale) than I would with the legs on top of the frame, and glue the lower rail between the legs. On the real 1:1 car that part would be the 0.120″ wall. Also the top rail would be one continuous piece from the front bulkhead to the rear bulkhead, which means I will need to figure out how far to bend it where it comes past the front and rear hoops. On the 1:1 car it will be 0.120″ wall and the biggest length of tubing on the car and run the full length of the frame and require careful gusseting where it passes the front and rear hoops. That’s also true on the Mini Sprint-T, but the mechanics are completely different because of the scale.

And since this is just what I should have posted yesterday instead of rambling, this is a good place to end.

Can’t stop thinking

And one of my favorite subjects is how to make the Sprint-T lighter and safer, which was not caused by seeing Ryan Newman’s Daytona crash. Actually it was brought on by wanting to make the frame fail in such a way that the engine didn’t try to join me in the driver’s compartment in a frontal collision. Second consideration was using as much 0.060″ wall tubing as possible. I say “as possible” because while some of the 0.120″ wall tubing is required by safety rules, some of it is required because of the stress risers created by the 0.120″ wall tubing.

And once again I wish I could show you what I see in my mind, when I’m thinking about the Sprint-T. The rest of the time I don’t want you to see what’s in my mind, that’s like being on the wrong side of the eyewall of a hurricane full of garbage. But seriously, I wish I could show and not tell about the frame for the Sprint-T.

OK basically The Rules require the hoops and diagonals and upper parts of the roll cage to be 0.120″ wall, but to balance things out so the cage holds together that means the bottom frame rail has to be 0.120″ as well at least between the rear and the front hoops. The fun(?) part is deciding how far back the 0.120″ wall has to go, at least to the rear hoop but behind that do I want 0.120″ all the way to the rear bulkhead/bumper, or do I want that to be a crumple zone up to the rear hoop? Going back to The Rules at least one of the diagonals must be a single length of 0.120″ wall tubing the same diameter as the hoops (1.5″), but do I want/need more than one diagonal, and if I want/need to have two diagonals do both of them need to be 0.120″ or can the lower stressed one be 0.060″? Add into the mix that I can also have a rear hoop and a left and right hoop, and run a diagonal from the front crossbar to the rear corners of the left and right hoops and get a much stiffer but slightly heavier frame.

But we were approaching the point of diminishing returns for frame stiffness given the suspension design of “stick” axles front and rear. I mean the main point of torsionally stiff frames is to keep the front and rear wheels at the best camber angles and also to balance the roll rates so the weight transfer between the front and rear outside tires can be tuned for desired handling behaviour. Well stick axles don’t change camber angles unless the inside tire is lifted out of the plane parallel to the ground which likewise limits the weight transfer ratio between the outside tires. And that one didn’t come out completely right, as the inside wheels can also go over a bump and remain in contact with the ground and not be lifted by trying to transfer more weight than exists on the inside tire by either excessive roll angles or by roll centers that are too high and transferring weight without compressing or extending the springs. This is the mechanism that allows changing the handling by raising and lowering the roll center on one end. The closer the roll center is to a line running through the center of gravity of the car the less control the springs and anti-roll bar exert over the weight transfer and also the less the car will roll over on the suspension in a turn. Get the roll center higher than the center of gravity and the car will try to roll opposite the cornering force and pick up the inside tire. This is why standard kit T-buckets are no good for autocross and Solo Racing because they have such high rear roll centers to compensate for the “normal” size difference between the front and rear tires on the street.

Anyway, back to the frame. Running left, right, and rear hoops will give me four uprights on the rear so lots of crush resistance where the majority of the driver sits, which is good. But also less tie-in at the top of the rear hoop which is ungood (not actually bad becauuuse there are other ways to tie-in and brace the top of the rear hoop). And running left, rear, and right hoops means an extra hoop of 0.120″ wall plus the crossover bar over the front of the cockpit and the verticals under the crossover and the diagonals from the verticals to the rear hoop have to be 0.120″ where front and rear is just the two hoops plus the crossover bars between the two hoops and whatever gussets they get attached with and the diagonals that are a single piece of tubing between the hoops. Like I said, compromises and where weight can be saved.

Also, while I’m trying to do this for the 1:1 car I’m also trying to figure out how to build the 1:25 Mini Sprint-T, which is where this mess got started because of the upper rail running from the front bulkhead spring mount to the front hoop.

Still here, still nothing much to report, except a shocking picture of a sick cat (CW)

Basically the most newsworthy thing this entire week was my son is taking a long weekend in San Antonio so no grocery run Saturday, and my feet hurt too much to take the bus to the store and backpack the groceries home. Yay, news!

On other fronts one of the people I follow on Twitter turned out to be a scam artist from Ghana, asking me for money after a long conversation by DM. Fortunately my BS detector started going off well before the request for money for her mother and babies. And no it isn’t because I want her mother and babies to starve, it’s because I don’t believe those are her mother and babies in the pictures she sent and put in her account. I might be cynical (YA THINK?!?) but I’m also experienced in getting scammed, having been scammed many times in the 61 years I’ve been crawling around this planet. Actually most of the time when people try to scam me I didn’t have any money to get scammed out of.

On the Sprint-T front, currently the best option is the junkyard 5.3L iron block LS engine and a 4l80e transmission which will give me a 1800 lb. car with 300 HP at the wheels if I just leave everything stock, or 400 HP with a cam swap and headers. Since I have to build headers to get the exhaust out of the firewall anyway, spending the extra $200 for the extra 100 HP seems like a decent trade off, since I’ll have to tune the computer setup either way.

Now what would be cool is if the engine comes with the VVT cam drive which would let me tune the cam timing for best low end and high end power, if I also get the ECU and harness that originally came with the engine, which kinda supposes I get the engine from the vehicle it came in. But I have been reading articles about tuning the VVT and compared to fixed cam timing it can be worth as much as 50 lb-ft of torque below 3000 RPM compared to running a fixed timing setup for high RPM power. This means I can get more dig out of tight corners when racing and still have power at high RPM. It also means I can squeeze the last MPG out of the engine for the transit from one race to another, as getting more power at low speed means I don’t need to burn as much fuel to get the same speed as power required doesn’t change.

Other things dancing around my brain included seeing cars at the Chili Bowl Midget Nationals that had either the right or left halves of Chevy or Mopar V8s in front of the driver and thinking “Throw an LS3 engine and a clutch in that and you would have a killer A-Mod autocross car.” Sure I would need to run a larger radiator to cool 376 cubic inches of gas powered V8 than 166 cubic inches of alcohol powered 4 cylinder, but it would be a practical race car with a high power to weight ratio. The cars running half V8 engines were rules-limited to iron block and heads, so there wouldn’t even be that much increase in weight to go to the all-aluminum LS engine, and the radiator could go to the rear of the car right behind the driver to keep the balance the same front to rear balance and a 3 gallon fuel cell would be enough to run all day if I stayed with pump gas, so no heavy fuel tank hanging off the back. I could even take a wrecked sprint car and chop most of the front end off, keep the cage and rear suspension, and rebuild the front to the 72″ minimum wheelbase for A-Mod and get pretty much the same thing as there is very little difference between the two types of racer behind the firewall. There are wrecked sprint cars locally but they tend to be scrapped out pretty quick, so I would have to have $$ in reserve to buy the wreck practically at the track. Mind is still trying to solve the problem of not having a race car even though I have no way of getting the race car to the races. I don’t know why, besides I want a race car that I can race.

In other news I finally got Clyde to sit still for a minute so I could take a picture of how he looks now. Image not suggested for sensitive viewers.
Clyde has a mess and we can't put the medicine he have on it.
Yes that is blood oozing from those cracks in his skin, and that’s also the reason why we can’t use the medicine we have to make the itching stop. It has a big black box warning in the instructions to not apply to open injuries or sores, and will make him very sick if we do. So we have medicine to stop his itching we can’t use because he scratches until it’s raw and bleeding, and he still has itches he scratches until they are raw and bleeding 😿. Poor kitty! We might have to take him to the vet again.

A little late, but here it is

Sorry to take so long but I finally got a decent picture saved someplace I can use to post it to the blog.
See that teardrop opening?
There’s a wrinkle that doesn’t show well in the picture because of the angle, but is glaringly obvious IRL. So this part is Just Practice while I perfect my technique. Also I have been looking at pictures in catalogs and online of turnouts that used this method and they have egg-shaped openings, not teardrops. That means I have to work on my technique even more to get the opening right every time.

Since I spent so much time trying to get a good picture today after trying to get a good turnout yesterday I’m up past my bedtime and I really need to be awake when the stormdoor installation team shows up in the morning, I’m putting this to bed and then myself. Hasta la Friday. Which I know is ungrammatical Spanglish, but anyway, Mañana

I spent hours practicing

Last night I did the bender again and started trying to do the “sliced tangent to the inner radius” exhaust tip. Now while I was building my tool and practicing bending the raw stock I was listening to YT Music on my phone, which ran the battery down so I couldn’t take pictures after I was done. But I promise pictures for tomorrow after the phone gets charged.

The tricky part is the heating of the raw stock to make it soft enough to bend without causing the tube to collapse. I ruined many bends because I overheated the tube and collapsed it. I also discovered that heating the stainless steel bolt I’m using as a bending post doesn’t work for tubing at all, unlike solid rod. If the post is warm enough to soften the plastic, the wall of the tube next to the post collapses before the outer radius starts to bend. In retrospect I should have expected that, but I was thinking in metalworker mode not plastic model builder mode. The way to go for the plastic tube is to spin the tube on its long axis with the intended bend area in the heat source (butane torch flame) until it gets soft but not floppy, then put it in the bender and make the bend. This has to be done extremely fast because the temperature spread between floppy and rigid is very small and soft enough to bend and too cold to bend smoothly is even smaller.

Anyway I discovered another reason for using the sliced bend exhaust tip, it leaves a delightful teardrop shaped opening besides directing exhaust away from the car, similar, shorter, but not exactly like when the reverse technique is used for side-exit exhaust. But I wasn’t expecting this shape from the inside bend, I was expecting a more oval shape opening. Maybe when I get a better quality bend the shape of the opening will be more oval than this attempt. But better quality requires more practice and there is a limit on how much raw stock I can waste on practice. I really need to reserve one stick of tailpipe material for the final build so I have enough length. But I will get a picture of the pretty practice tailpipe for tomorrow’s post

Here’s the link to that lean-burn Corvette

Lean burn Corvette testing video.

You can see what it does IRL on real roads. This video was done last week and uploaded Saturday 12/7 so cold weather which is very bad for gas mileage because of increased aero drag and rolling resistance. Now my car is going to have horrible aero especially compared to a 1999 Corvette. If I’m lucky I’ll get about 30 MPG highway because of that, but that is still about as good as I got with my 1987 Hyundai with the non-overdrive 4-speed transmission. My best Dallas-Nashville mileage was about 31-32 back in the early ’90s partially because I was driving at night and missed everybody’s rush hours for all the cities I hit. Again, 1980’s technology and no overdrive with a carburetor but I did have an external oil cooler running as low viscosity oil as was available at the time, I think 10W-30 non-synthetic because synthetics were like 3 to 4 times as much as petroleum-based oils and as always we were Not Rich even then.

So, now that we know what’s possible, let’s see what we can do with what we have.

I have been practicing on raw stock

This is a technique I haven’t used since the last time I was building models for/with my son when he was a child, who will be 42 next birthday. So to say I was out of practice would be a gross understatement. So I decided some practice on some of the raw stock I’m using for the exhaust would be in order. I decided to start on the hardest exhaust end first because if I could a decent job on that the others would be a piece of cake.

Well the pictures didn’t turn out so good because the new phone doesn’t do macro very good, the old phone used to do macro real good, but this one stinks. Also it’s all white plastic so while I can see it, the camera just doesn’t pick it up well at all. But given the limitations of the technology here’s a before pic. Actually the other end of the tube that hasn’t been worked yet, but you get the idea.
that's a thick-wall tube if I'm trying to emulate exhaust tubing
And after, notice the thin edge looks like the whole tube is really thin.can you see how much thinner the tubewall looks now?
On a different background and lighting to make the difference more noticeable.That's really thin now
And before really needs a contrasting color on the edge but the tube wall is still discernible.needs better lighting and a better photographer

So this was the straight-cut tip, the tip I’m going to be emulating in plastic is the turn-out tip, where the tube is bent around a radius and then cut tangent to the inside of the bend right where the bend begins. This is a classic hot rod design motif that forces the exhaust in a different direction without enlarging the width of the exhaust in tight confines. The normal use is to turn the exhaust down in front of the rear axle when a combination of no ground clearance and not enough floorpan clearance when the axle is at full bump prevents routing the pipe over or under the rear axle. Or like I’m doing putting a side exit exhaust where there isn’t a lot of room to the side and you don’t want to hang a sharp edge out on the side of the car.

And if a picture is worth a thousand words then I have posted enough for today. Next step is to fire up the bender and make that turn-out tip.

Waiting on the door installation people

OK I have selected storm doors to replace the ones installed when the house was originally built in 1985, now I’m waiting for the installers to arrive and measure the door to get the right size storm frame. As this is one of the few standard size things on the house, I don’t expect too much hassle getting it installed.

One thing I have to say, it’s easy to get through the door once it’s open, the frame is 3′ x 6′. Looking through the Lowe’s listings this is the largest standard door available. There are taller doors, but they are only available by special order. There are lots of doors that come narrower, but I didn’t see anything wider. So 3 by 6 is as big as you can get without spending buttloads of money. But the installation people have to verify that in person so they get the right thing.

I have been practicing with the bender for the plastic stock of the frame and exhaust for the Mini Sprint-T, as the exhaust in particular requires lots of work to get right and there is a bend for the muffler/tailpipe in particular that really needs to be done right. To simulate the thin wall exhaust tubing of the 1:1 scale vehicle the end of the exhaust tip needs to be filed to a knife edge from the inside and the inside painted matte black or matte silver. That’s something I’m going to have to research, does E85 leave a carbon deposit when tuned for best power? I already know that the highway mileage tune for 87 octane won’t, as it is about as lean as the engine will run right with, emulating a tune that let a ZO6 Corvette get just over 38 MPG in a real-world highway test. For airplane drivers this is the car equivalent of tuning 150° lean-of-peak on the EGT. And for people not aware this is over 40 MPG steady-state from an engine that displaces 427 in3 (7 liters) and put out ~500 HP at the rear wheels back in the day. But anyway the highway tune will not remove carbon deposits at the end of the exhaust, so if the tune for E85 carbons up the exhaust then I need to emulate that in the model. And I just checked the E85 forum and there are 3 posts that say they don’t get any carbon from running E85, and one that said he still has carbon in his tailpipe from the 87 octane the dealer put in when he first bought it, but nothing more than that in 5000+miles since he bought the car, so I’m going with the matte silver for the inside of the tube.

The fuel map is going to look really strange when I get this tune done, because most of the map will be slightly rich except for the RPM block and throttle setting for steady-state freeway cruise at 60 MPH which will have a “hole” that leans the engine as far as it will go and still run right. On 87 octane this will be leaner than the max NOx point so low CO, CO2 and middling NOx.

And I have been sitting here writing for about 3 hours waiting to hear from these people, and it’s about 1630 CST so I’m going to assume they aren’t going to answer today. And this is a good word count to stop at, because I don’t want to bore anyone today. So, I have to go out again tomorrow to deposit a check, which means going on the bus again. But not as bad as Black Friday in the rain.

Still stuck on imagi-building that A-MOD car

I have a mind that must chew on problems to solve, not social problems, physically solvable problems like bicycles or race cars. And the one that’s occupying my mind is the A-MOD car. Like all good engineering problems it has a simple premise: Make the quickest car possible on an autocross course within these constraints; 72″ minimum wheelbase, 42″ minimum tread, 900 pounds minimum weight with driver and full fluids, and 10″ minimum wheel diameter. There are safety rules common to every car in the series, and because having a really stiff suspension is a huge advantage you have to have sprung suspension, not that actual springs are required, there are cars that are riding around on actual hockey pucks between the suspension and the frame, but something that isn’t a solid piece of metal against another non-moving piece of metal.

Anywho, working through those minor strictures I have evolved literally thousands of cars with uncountable variations of “suspension” over the last few weeks. But what I keep coming back to is a single-pivot rear suspension with a solid axle and a single disk brake halting the axle. It’s simple, strong, and fairly lightweight. And relatively easy to adapt to a chain drive. Another thing is it works extremely well with a common trope in cars this size, mounting the engine on the rear suspension. Basically this is done to simplify the drivetrain and shorten the chain on chain drives. It’s really bad for unsprung weight so I’m only doing the designs as due diligence to make sure I’m covering all the bases, and because researching this adds to the knowledge base for other designers, and what I learned to pass on to other designers is the unsprung weight is pretty close to zero if the Center of Mass of the engine is mounted right at the pivot point of the suspension which is physically impossible unless the rear swing arm is mounted in such a way as to create a virtual pivot point in or near the CG of the engine, which kinda negates the purpose of mounting the engine on the swingarm in the first place, simplifying the rear suspension and drivetrain.

Another suspension I have been looking at is the classic 4 link and panhard rod with the forward pivot of the 4 link even with the front drive sprocket of the chain drive and the rear pivot right on the centerline of the rear axle so the chain and the suspension swing through very close to the same arc and making having the same chain tension through the suspension travel simple.

Another thing I have been looking at is mounting the rear part of the chain drive solidly to the rest of the frame and using CV axles to run the power to the wheels as shown in a book I bought, How to Build Motorcycle-Engined Race Cars by Tony Pashley. and in this video by Vasily Builds where he modifies his swingarm buggy into IRS. Way more complex to build but less unsprung weight than almost anything else, but the 4 link is lighter overall, allowing weight to be added where it will do the most good, making the final decision a tossup or a painstaking virtual build of both to see which would be better. I have the time but not the software to do the virtual build, and the 4 link is both cheaper and easier. And since I have more time than money, cheaper is the way to go on this one.

But I would still rather be building the Sprint-T. This is just something to keep my mind occupied so I don’t build a doomsday device and just destroy everything. Speaking of which, I can’t find my yellowcake, I have the lemon and the cinnamon, but not the yellowcake. (Mad scientist in-joke, you don’t have to laugh if you don’t get it, but if you don’t get it and want to get it google “Nigerian yellowcake” for the source of the joke.)

Nothing much to write about since putting my head through the door

Basically my mind won’t stop building A-MOD cars and it’s been keeping me awake at night. How much of that is related to last week’s adventure in DIY home-wrecking and how much is just frustration at not being able to build anything (or even able to type anything as I will explain later)?

I have been thinking about taking the driveline out of one of those AWD quadbikes and stretching it out to the required wheelbase and track for an A-MOD car. Basically that would require making one new drive shaft and 2 new axles to put everything on one side of the car so the driver could sit beside the engine instead of over it, and get everything as close to the center as possible. Quadbikes are literally available all over the place out here because nobody can find parts to fix them so when they break they get thrown away, and because nobody does routine maintenance on them they break with appalling frequency. So they are literally available for towing fees or “How much you wanna pay me to haul that off?” If you get one that has good parts support they are somewhat fixable but those are not the kind that get thrown away, otherwise you try to find a bunch so you fix one and use the rest for parts. So the build process is basically find a quad, cut off the front end and build new pickup points for the front suspension that moves the wheelbase out to 72″ and the track to 42″ and widen the rear to match the front. Then move the controls to the side and down so that the CG is close to centered on the new layout with the driver and because the driver is on the same plane as the rest of the vehicle the vertical CG is lowered a bunch. Swap out race slicks for the all-terrain tires of the quad and semi-instant A-MOD car!

The other idea that has been prancing through my mind is a variation of the sidewinder build trailing A-arm rear suspension that puts the pivot point of the suspension next to the output shaft of the transmission so that chain tension wouldn’t change much as the rear suspension traveled through its range. Basically the design change was a single structural member from the pivot point of the A-arm to the rear axle bearing support next to the chainline, to prevent flex from the chain pulling the axle forward, and one diagonal that runs from the pivot behind the engine to the far support bearing while another runs from the chainline support to the support on the driver’s side of the axle, with another running in front of the rear axle sprocket to the engine side bearing support on the axle to prevent flex when cornering. The best way I can describe this is it will look like a flattened A with one leg missing and a line up the middle and another leg to the center on the side away from the driver from overhead. Because I plan on running the same size tire on both ends it will have only one spring and shock absorber on that center bearing support, as close to the axle as possible to get what is technically a “zero roll stiffness” suspension so that all the roll stiffness is in the front and the car will corner on the outside front and both rear tires to prevent massive oversteer under power. Having the full length and width support only on the side away from the driver allows the driver to sit further back in the car while still allowing for suspension travel.

Now, about that typing issue mentioned earlier. Basically what it comes down to is stuff from the cuts on my hands got all stiff and I couldn’t hit the keys with my fingers. Long story is some of the cuts opened up and started bleeding so I had to put bigger bandaids on them with more antiseptic, leaving me with two fingers that couldn’t bend, the index and third fingers, and typing became a single-handed operation of hunt and peck. And slowly because sympathetic pain every time I moved the fingers of the other hand because I use both hands to type and I kept trying to use the injured hand. So no new posts until the hand healed enough that I could type. But now I’m better (or as the saying goes “good enough”) I’m doing another post.