Tag Archives: technical stuff about building cars

Perpetually thinking about “things”

Yep, I’m still thinking because I can’t “do” because my money is getting spent on other things, like buying windows I can actually see through, and health care. Yes, we are replacing the opaque thermal windows that have degraded so bad that one corner is almost like the insides were painted over. Also paying for seeing a doctor who is not the Lab Rat Keeper.

But that’s not what I was thinking about today. I was thinking about the frame, about how I can make it easier if I make it in bolt-together sections with bolt-in braces that keep the sections rigid, and allow for different suspensions for real A-B-A testing. Like I could build one front clip with the straight axle, and another front clip with independent front suspension, and swap back and forth between the two to get a true comparison. Like swap in a few hours from straight to wobbly.

But the first part will be building the roll cage with the two hoops that are exactly the same size as the body and barely clear it as the body passes through the halo opening in the top of the cage with the bolt-in brace removed. The thing is to route the bracing so that as little needs to be removed as possible to get the body inside after everything gets painted but still braces everything against torsional flexing. What I have is a full cross brace all the way from the left to the right but split to unbolt and allow the body to get through. Then the diagonals run from the corners to the transverse brace in the middle to where the brace is removed for the body to get through. There is not that much vertical thickness to the body so most of the opening can be blocked by the diagonal bracing and still allow the body to get fed through sideways from the top. Just spin the body through the roll axis and let it fall through the halo by overlapping the diagonal bracing.

Don’t forget I have to get through that hole to get in and out of the car. I have been thinking about that and the motions required, where I have to grab and brace my body. From inside getting out is easier than from the outside getting in. Getting out is a matter of bracing my feet against the transmission crossmember on the floor, grabbing the cross brace of the cage and using my legs to push me up against my arms and sitting on the top of the cage and swinging my legs over the side and sliding down until I’m standing outside the car. Getting in requires climbing until I can sit on the side of the cage and swinging my legs over the side and into the car and using the bracing to lower my body into the seat. Anyway I have to get two 12 foot sticks of roll cage material (the 1.5″ diameter 0.120″ wall) and get two legs bent at right angles 46.5″ apart, centered on the stick so I have lots of material to work with to set the height of the cage. The top part of the upper frame rail and diagonal braces are also part of the cage, so they have to be made from the same material. But the front clip can be made from 0.060″ wall tubing which saves a few more pounds.

And I didn’t get good sleep last night, so I’m nodding off at the keyboard. I’ve basically spelled out the changes in the new design, so I’ll put this post and me to bed.

I want to stop thinking about stuff and start DOING stuff

The biggest problem I have now is I lack funds to buy tools and raw stock, or to buy the parts I can’t make. And this frustrates me no end. On the plus side I have time to think about how to do it better, like I recently did by going from a fabricated steel panhard rod to a straight piece of aluminum that weighs less than half as much by moving the frame out of the way.

“Think before doing” is a good maxim to live by, but I have reached the point of “Do something even if it’s wrong!” Or more accurately, “Do something before you can’t do anything!” as my infirmities multiply and grow worse.

At the rate things are going I won’t even be able to get in and out of the Sprint-T after I finish it because I won’t be able to bend at the hip far enough to get past the roll cage. And then I would have a true Greek Tragedy, all that planning and work but unable to harvest the fruits of my labors.

Moving on from self-pity, I’m still trying to find a transmission that will work for the mission of racing around in first gear and hitting redline at 40 MPH but getting good freeway mileage, but still not weighing anything or having to change gearing between racing and highway, and not coming up with any good answers. So far the best (lightest) solution was the 2.43 1st gear Super T-10 and swapping the gears in the quickchange rear to get stupid tall final drive in 4th from the 40 MPH in first ratio. The next best I have found is the Super Street 5 speed with OD that comes in at 35 pounds heavier but has a 3.33 first and a 0.77 5th. No need to swap spur gears to go from redline @40MPH in 1st to cruising at 2068 in 5th @70MPH.

[squirrel!]Excuse me a moment my music app just played The Song That Must Be Played As Loud As Possible (Won’t Get Fooled Again by The Who) and I had to crank my headphones up to 11. Now I have to stuff cotton in my ears to keep the blood from running out.[/squirrel!]

Getting back to the post, one has simplicity in its favor, and the other has lightness on its side. Neither has anything for cost, the cost of buying an extra spur set for the quickchange and the ongoing costs of buying rear end lube for changing the spurgears easily offsets the higher cost of the 5speed. And as I don’t have funds for either one at the moment it’s mox nix. (Or macht nichts in the original German). Also the torque limits on the Super T-10 are so low that even a stock tune L33 will stress the transmission to its limits, except the 2.43 ratio 1st gear model’s 375 foot-pound torque rating is comfortably inside the L33’s 335 foot-pound rated output.

Everything else is marginal to “ain’t no way”. 2.64 1st is 325 foot-pounds (iffy) and the 2.88’s 300 rating and the 3.42’s 266 rating are both “grenade city” except when you pull the pin on a grenade you have a pretty good idea of when it’s going to blow, about 2.5 to 3.5 seconds after you throw it. The 3.42 ratio might last until you floor it or a few seconds after, but the 2.64 is the real question mark. It might last for a season or 2 or blow up while you’re hundreds of miles from the shop and spares with no way to get there. No, if I go with the T-10 the only way to go is the 2.43 1st gear. But that 35 pound weight savings looks sooooo goooood. It’s the difference between 1490 pounds empty and 1525 which is 2.3% and all I have to do is crawl around under the back of the car a few minutes to half an hour before a race and the same amount of time after. It’s tempting as hell.

Still thinking alert, take cover immediately

J/K there’s no imminent danger. I was thinking of ways to improve the front of the Sprint-T. Also still thinking about the drivetrain with a possible manual transmission.

Starting with the front suspension and related things I decided it might work better if the panhard rod (AKA track bar) was straight. That requires redesigning to the front part of the frame so there is nothing that the panhard rod has to go around to get from the driver’s side of the frame to the passenger side of the front axle. I did this by moving the front diaphragm that carries the loads from the coilovers to the rest of the frame from in front of the axle to behind the axle. This requires moving the steering behind the axle as well as the steering box back to the kit location. This also means the radiator has to be moved back and up to clear the steering shaft from the steering box to the steering wheel. This is how designing a car goes, you can’t just change one thing, it cascades across most of the car. And I forgot to mention the complete redesign of the bumper support structure because the top and bottom frame rails stop at the front axle.

The other thing I was doing was trying to find a lightweight transmission with overdrive, and basically what I found was the Super T-10 couldn’t be made to have an OD without basically redesigning the transmission because the cluster was a solid hunk of steel. Now if it had followed the path of the sister/progeny transmission that used to be called the Nash 4+1 which uses a cluster with replaceable gears, then I could just have a replacement 3rd gearset machined and installed instead of a full replacement cluster. Anywho, the mentioned 4+1 is now the Super Street Five Speed with much higher torque capacity compared with the Super T-10, and extra weight from the structure to resist loads from the increased input.

The 5 speed handles 600 ft-lbs, compared to the 300 to 375 rating for the T-10, and also has a much lower 1st gear (actually several options that are as low or lower than the lowest 1st gear on the T-10). The price for gearing this low and higher torque handling is weight. The 5 speed comes in at 105 pounds compared with 70 for the T-10. Still lighter than any of the self-shifting transmissions with overdrive, by 60-80 pounds, plus it takes up less room inside the car, especially since on the Sprint-T the inside of the floor is also the top of the bellypan and nothing hangs lower than the bellypan, that’s another design paradigm for the Sprint-T.

The top of the bellypan is attached to the bottom of the bottom frame rail, and if there were no bellypan the bottom of the bottom frame rail would be the lowest part of the car except the wheels, because that lets the car be as low as possible without dragging the road if a tire goes flat. Theoretically all 4 tires could go down and the only thing touching the road would be the tires. This requires the road to be pool table flat, but even on normal pavement I can still get both tires on an axle going flat without anything touching anything except tires. I picked this one up from off-road trucks that need to be able to keep going with flat tires.

Anyway, going from a bent panhard rod to a straight one allows a smaller size rod and also allows changing from steel to aluminum, and the combination allows for the drastic reduction in weight of about 2/3, and as half of the panhard rod is unsprung a reduction in unsprung weight as well. This results in a slight increase in grip on bumpy roads and courses as less unsprung weight improves the ability of the tires to follow bumps without leaving the road. This is why I try to reduce unsprung weight every chance I get.

Couldn’t stop thinking last night

And it kept me awake until nearly dawn thinking about getting an overdrive for the T-10 by swapping in an OD for 3rd and changing the shifter to allow it to be the new 4th.

In doing my research for this I discovered that the OE 3rd gear is already a slight OD as the ratio between the input and cluster shafts is 1.28:1, while the output from 3rd is 1.23:1, which means the ratio of the gears on the 3rd position of the transmission is already an OD just not very much of an OD, 0.96, probably just a tooth larger on the cluster than the main shaft. It’s looking like I will have to order a custom cluster with a custom gear for the main shaft that is as small as will fit. This is not an inexpensive alternative.

And it’s not like I could amortize the costs by selling duplicates at a profit until the tooling costs are covered. This application is very niche, not many people are in the market for this combination of light weight and OD top gear, and can live with only 4 speeds. Also I have been looking at the ratios in use on the transmission and most of them are not actually useful in making 3rd OD. Swapping the gear for 2nd gives an OD that is not even enough to make the output OD, the gears are 1.25:1 as they are before swapping which is less than the 1.28:1 of the input gears. So basically I’m looking at finding the smallest gear that will fit the output shaft and designing a gear to machine into the 3rd position on the cluster and hoping the resulting ratio is enough to be more overdrive than the underdrive ratio of the input shaft to the cluster. Also most of the different ratios available are just from changing the ratio of the input to the cluster, the 2.43 S cluster, and the 2.64 W cluster are the same ratios except for the input/cluster gear. In fact checking the spares list shows that most of the ratios are shared with common 2nd ratios for the S, W, CC, and Z clusters, common 3rd for S and W, and common 3rd for X, CC, and Y. So yeah, I will have to order a special cluster and gear for the main shaft if I want to hang an OD 3rd on my transmission.

And I have been going back and forth between the PDF of the spares list, the calculator app, and this post for like 4 hours, it’s time to put this one to bed.

Been thinking again about suspensions

Basically what I was thinking about was tall front uprights for camber control. Basically the taller the upright the less the suspension arms change the camber of the wheel because the side to side motion induced by the arms rotates the upright in the camber plane in inverse proportion to the distance between the ball joints.

GLOSSARY

Camber – The angle between the tire and the road surface. Positive is the top of the tire tilted away from the center of the car, negative tilts toward the center of the car. This is backwards because positive camber is almost universally bad for tire grip and is therefore a negative, but the terms were defined prior to the invention of the pneumatic tire and now we are stuck with them.

Kingpin inclination – Another term that is archaic because kingpins are from solid axles except for a very tiny number of early independent front suspensions. It refers to the angle of the axis of steering rotation for front wheels between the upper and lower ball joints.

Scrub radius – The distance between the intersection of the steering axis with the ground and the center of the wheel. Basically this is the moment arm of the force generated by the brakes through the steering system. The greater the absolute value of this number the larger the force felt at the steering wheel under braking. The direction of this force depends on the positive or negative value of the radius. If positive the steering wheel will turn in the direction of the greatest braking force, and opposite if negative. Now there are schools of thought that if the system is designed to turn away from the stronger braking force it will be self-correcting in the case of brake failure. There are other schools of thought that drivers will instinctively turn away from forces trying to steer the car so positive scrub radius is better, with the debate coming down to do you want your driver to be an active part of the car.

Upright height – The distance between the upper and lower ball joints of an independent front suspension

This is a hypothetical discussion because the front suspension for the Sprint-T has been frozen as a straight axle, and because my fabrication tools are not up to building suspension uprights. So theory only. No labs for this lesson (shoutout to my teacher friends who “get” this joke). Theory for this starts with the statement from the opening paragraph camber change is inversely proportional to the distance between the ball joints. Second is that the position of the bottom joint is constrained by the dimensions of the wheel used, because kingpin inclination and reducing scrub radius dictate the location somewhere inside the wheel. Excessive positive scrub radius has been proven bad experimentally by excessively increasing the force required to steer under braking.

And it just dawned on me this post needs a glossary to keep track of all the technical terms. We already have camber, kingpin inclination, and scrub radius besides upright height. Basically I’m assuming my reader has a working knowledge of terms used in suspension design, which is probably not the case. So I’m inserting one following the first paragraph. It’s there now but wasn’t before I started this paragraph.

Anywho, the design parameters basically dictate the lower ball joint be as far inside the wheel as possible constricted by the brake disk because of scrub radius minimization. It doesn’t matter if you want positive or negative scrub radius, you still want it to be small or variations in braking force caused by changes in road surface can tear the steering wheel out of the driver’s hands which is bad no matter how you look at it.

Also, the amount of camber change caused by body roll is proportional to the ratio of the distance between pivot points on the frame and the distance between the pivot points on the upright. Desirable geometry of defining the instant center dictate this is going to be close to unity and usually less than 1:1. There are some old GM cars that had this greater than 1:1 and they understeered excessively with the outside front wheel riding on the sidewall of the tire. Look for images of the late ’60s Chevelles making hard turns and you can see this in action, but not many of those pictures made it to the internet and the only ones I have are in paper books on suspension and my cell phone camera did not get a good picture of the picture. One of the things about having the ratio between the distance of the suspension pivots greater than unity is the front roll center is underground, which causes the body to roll excessively and further causes the car to ride on the sidewall of the outside front tire. This meant that racers using this car as a base for stock cars had to crank in ridiculous amounts of static camber and excessive amounts of roll stiffness to keep the car driving on the tire treads instead of the sidewalls. They made it work but it would have been easier if the front suspension was designed right in the first place.

Getting back on track for this essay without any more digression (squirrel!), we want to basically define a height range for the front upright. Too tall and either we get “funny” geometry in the suspension or we get the pivots for the upper suspension arm somewhere in the boonies, which technically is “funny” geometry. Too short and we get poorly controlled camber as the car goes through bump and/or roll. My personal take is I design about the same as the inside diameter of the front wheel as my max, and about 8″ as my minimum except for suspension karts. No matter what you do those are “funny” because no room.

And I seem to have lost the point of this post except that I woke up in the middle of the night thinking about front suspension uprights. So since we hit over 1000 words this seems like a good place to stop and hope I get my train of thought back on the rails for a future post.

Still thinking, remain in cover until the all clear has been given

I’m still reading my new book about suspension geometry and contemplating the information, but it hasn’t resulted in any changes to the Sprint-T. The front is still a tube with fore and aft location by parallel 4-bar and laterally by a bent Panhard rod to clear frame members, rear is still fore and aft by dual trailing links and laterally by a Watt’s link and a torque arm for control of reaction torque from the engine. And torque reaction links for the brakes. All rotational reaction forces are isolated from the suspension to prevent locking up the suspension from either engine torque or brake torque, allowing the suspension to move freely at all times. The front springs are mounted as close to the ends of the axle as will clear moving parts, and the rear springs are mounted to the trailing links originally designed for swingarm suspensions of much heavier vehicles on rough tracks. This allows easier tuning of suspension stiffness by changing the motion ratio for the springs as there is quite a bit of motion ratio to play with.

To clarify that, the bottom of the spring mount moves less than the wheel in both single wheel and double wheel bumps. This means I can get smaller changes in wheel rate than steps in spring rate which gives me finer control over wheel rate. Also the total travel of the shock is less than the travel of the wheel which reduces the amount of force per inch of wheel travel. There is a formula that gives us the effective wheel rate when the spring rate is known (mr2*Spring Rate) or the square of the motion ratio times the spring rate. On the rear suspension for the Sprint-T there are two motion ratios, the motion ratio for a single-wheel bump or body roll and the motion ratio for a two-wheel bump which is just the motion ratio of the swingarm. The single wheel bump is the swingarm ratio times (the distance from the far wheel to where the swing arm attaches to the axle housing divided by the track).

OK engineering problem here

I have been thinking about the front suspension of the Sprint-T, specifically how to get anti-dive without getting bump steer. Since this is a solid axle technically I could just change the angles on the links of the 4-bar same as if it was a rear axle trying to get anti-squat, except that moves the axle differently back and forth and changes how the wheels point depending on how hard the brakes are pushed. And since the wheels don’t change camber with bump there’s really no reason to have anti-dive in the front suspension, except to not have the front suspension travel under braking.

And really I don’t have any reason to do that, there’s nothing close to the ground up front that might hit under heavy braking. The only thing even close is the front lip of the nose, and it is about 6″ off the ground at ride height, and not very far from the front tires. The actual position is even with the fairing in the front fenders or about 5″ from the front edge of the tire, or about 18½” from the center of the front axle. To get the lip to hit would take both tires being about half flat and extremely heavy braking enough to use all the travel available in the front suspension. So really there’s no reason beyond only having a little bit of travel left in the front suspension. So, it is an intellectual exercise that won’t be used on the Sprint -T and has only served to keep my mind occupied so it doesn’t wander off and leave me without a mind. Because that would be bad.

Got a new book

I bought Advanced Race Car Chassis Technology HP1562 for the Kindle a few weeks ago and I’ve been absorbing it a little at a time. It’s a complex subject, but from what I’ve been able to read so far, if I get it right I don’t need to make the frame all that torsionally rigid. If I’m reading this right there’s no twisting load on the frame if I have the suspension geometry right and tuned the spring rates. If my sums are close there will be about 300 pounds ± on the springs of the front corners, and about 350 ± on each rear. That’s not a good sprung/unsprung ratio, but that’s what I’m stuck with.

The front axle weighs about 65 pounds plus the hubs and brakes, because it’s a honkin’ huge chunk of iron steel with tons of thick brackets welded to it, and the spindles are likewise big chunks of forged steel. The rear axle is right at 100 with the diff and and axles (drive) but without the hubs and brakes, which are almost the same weight for both ends. The rears are a touch lighter than the fronts because the rear disks are 11.75″ and the fronts are 12.19″. All of that is unsprung (bad) weight, axles, drive axles, hubs, brakes, the whole shootin’ match all bad weight. Well none of it is good but unsprung weight is super bad compared to sprung weight. Like excess unsprung weight causes tires to lose contact with the ground over a bumpy road even if the bumps aren’t that big, bad. And a lightweight car makes unsprung weight even worse because sprung weight pushes the unsprung into the road and not having enough sprung weight means the unsprung weight is just kinda floating out in space. It only contacts the ground intermittently because the two weights, sprung and unsprung, are just oscillating independently. I’m getting close to that condition with the front of the Sprint-T. If I understand the theory right the only thing you can do with a car that has sprung:unsprung ratio approaching unity is make the dampers stiffer to prevent oscillation from setting in, assuming there’s nothing to do to make the unsprung weight less. Right now the only option I have is to use “drillium” in the front axle, that is to remove weight by drilling the front axle full of holes. Since this axle was designed for a car almost twice as heavy as the Sprint-T this is actually an option.

As far as the geometries of both ends are concerned, the front end will not have any adjustability beyond caster and toe, the roll center height will be fixed to prevent the front axle moving under load and changing the steering angle. But the rear axle will be located by a Watt’s link with the pivot located by a nut on a threaded bolt that adjusts up and down to change the balance of the car for different conditions, like understeering for highway travel, neutral for road courses, and slightly oversteering for autocross. That can be done by turning the bolt to move the rear roll center up and down.

Oh, here’s the link to the build video for the GenIII Hemi™ giveaway. Lotsa power and torque from a nearly 600 pound engine.

I’m annoyed, again

My parts arrived yesterday after I posted my previous blog post about them, and after a quick test fit, they don’t fit.

I mean seriously don’t come even close to fitting, like several inches from fitting. I put one fan on the shroud in the upper right corner and traced the outline, then moved it to the lower left corner and traced again, and the result looked like the Venn diagram of racists and Republicans there was so much overlap. OK that was an exaggeration, but seriously there was way more overlap than I expected, which was none to nearly none. This had more than 2 inches of overlap at the widest point. The answer is the advertised size of the fans is the functional diameter of the fan, not the installed width. There is a slight radius at the base of the fan, plus a generous mounting flange that adds up to the mounted diameter of 14¾”, not the advertised 14″. Add in that the shroud was built to only handle a single, engine-driven fan and you can see how this all went to heck in a cargo bike.

So I have to return the dual fans and just get the biggest single electric fan and hope the ¾” mounting flange is a constant, or get two much smaller fans and live with it. That’s because the next size down from the 14″ is the 12″ which I’m still not convinced will fit after the fiasco with the 14″ so I’m probably going to go with two 10″ fans or the single 16″ fan that is the largest in the catalog and has a similar flow rating to 2 10″ fans and costs less. The other alternative is to trim the mounting flanges on the 14″ fans so they fit, but that voids the warranty and also makes them not returnable, ignoring the fact that they still might not fit, because of the taper of the shroud to make it work with engine-driven fans.

And as I was composing this post I got notice that there was yet another mass shooting in the USA, the only country that has mass shootings on the regular and isn’t a conflict zone. This time it was 8 dead in a San Jose CA light rail staging yard with an as yet unknown number of wounded, and also the shooter is dead but not included in the number of the dead. So as I write this that makes 9 total dead and an undisclosed number of injured, in the only major country where this happens on a regular basis.

And on that point, I’m outta here.

Trying to think straight through the fog

My new meds are starting to kick in, I didn’t have any neuropathy kicks or twitches last night and my tinnitus is getting quieter. But my sleeping is seriously borked because of the meds. I overslept and missed taking my laptop to get the bloatware removed because this med just knocks me out. Normally when I’m not getting sleep I’m not thinking too good, but for some reason when I get my coffee on this med I’m rarin’ to go, even though I’m still walking like an extra in a zombie movie. It’s the craziest thing, I’m thinking clear as a bell, but my legs and feet and fingers are acting like they never got out of the bed. I have been spending almost as much time fixing typos as writing because the fingers are only about 90% accurate in hitting the keys. And if you’re thinking “well that’s not too bad” just remember it takes a minimum of 3 keystrokes to fix a typo I catch immediately, and I’m averaging one mistake in 10 keystrokes. So, average of every 7 letters I have to fix something.

On a similar but not entirely related note, I love the verb “bork” when used in it’s non-sexual meaning of seriously SNAFUed. This is one of the few contributions to the English language from a SCOTUS confirmation that has survived longer than a couple of years.

On a brighter note, the cooling fans are going to fit the fan shroud nicely. One will fit in the upper right side next to the inlet for maximum heat transfer from the water coming from the engine, and the other will fit on the lower left to get as much as physics will allow from the second pass across the radiator. The idea here is to keep the coolant going into the engine as cool as possible to keep the combustion chambers cool for either best power or best fuel economy depending on which mode the engine is in. Don’t you just love thermodynamics working for you for once? And in case you don’t understand how this works, the formula for heat transfer from a radiator has the fluid temperature in the top half of the equation, meaning the hotter it is the more heat it transfers to the airflow. Which means most of the heat comes from the first pass across the double pass radiator, and by careful fan placement I can maximize that transfer for the lowest possible outlet temperature.

Going off on a tangent, current writing jam is “Dear Mr. Fantasy” by Traffic. There is an interesting dynamic that is peculiar to depressed people in the lyrics, how we use comedy to hide our depression. “You are the one that can make us all laugh, but doing it you break down in tears.” This was me for decades, “Laugh, clown, laugh!” while hiding my depression with comedy. “I love how your poetry makes me laugh,” for years.

Anywho, while I’m waiting on the parts to arrive I’m figuring out how they will be installed in the car, because what else can I do except write about it?

So, how was your day?