Tag Archives: fussy design work

Designing gas tanks instead of sleeping

Just because I can’t get words out because brain not working right doesn’t mean I stop thinking. The exact opposite happens.

I was trying to come up with a frame that would put me on the driver’s side of the car and still be stiff and that I could fit inside the body without chopping the body into little pieces and hanging a couple of hundred dzus fittings on the frame to attach the body. Physically can’t be done. So I’m back to sitting in the center of the car and the gas tank is now sitting on either side of the frame instead of the passenger side of the interior. The fun part is now I have to design a pair of fuel tanks that literally have no parallel sides and figure out how much gas (E85) I can carry between them.

First things first I measured the body to find out what space I had to work with. The major design constraint is the sides of the tank are going to be flat pieces of steel for ease of construction, so where the curves in the body make that impossible defines the volume of the tank. First constraint is where the back of the body starts to curve up from the floor, which defines the length, 41 inches from the inside of the firewall. Second constraint is being able to slide the tanks in from the top without removing anything, so the tanks have to clear the dash and the top flanges of the body and the bottom of the dash is 10.5″ from the inside of the firewall, leaving 30.5″ as the length of the tanks. Measuring the inside of the body at the floor gave me a width of 33″ at the front and 34″ at the back. Measuring the same locations at the body top flange gave me 37″ at the front and 44″ at the back, and a depth of 19″ inside the body flanges. Now not all of this volume will be available for fuel storage, I’m going to occupy a large chunk of it, 27″ down the center front to back.

This leaves us with two methods to determine the available volume, find the total volume and subtract the volume taken up by the frame around the driver, or remove the space taken up by the frame around the driver from the measurements and calculate the space left over. The easy part is the volume occupied by the frame around the driver 27″ wide by 30.5″ long by 19″ deep, or 15646.5 in3. The volume around that is a bit trickier to calculate because the space is a trapezoidal polyhedron, and the volume is the average area of the top and bottom times the depth. The quick way to calculate the average area of the top and bottom was to average the top and bottom widths on both ends, by adding them up and dividing by the number of measurements (4) and multiplying by the length (30.5) and then the depth (19) to determine the total volume. Subtracting the volume occupied by the driver and frame leaves 25.4 gallons for fuel. Which is not enough to get across the E85 barrens of west Texas and NM at the projected fuel economy for the vehicle. That means I need to make the tanks bigger or find room for another tank. Now one way I can make the tanks bigger is taking less space for the frame (and me) but it is late and I’m getting tired and unmedicated brain is not thinking good and wants to sleep.

More on that T-bucket

I know my old readers from back in the day 8 years ago today (yay!) might be getting a little tired of my building a car here. but I have to write and I can’t write about bicycles for a while. So building a car from scratch and scraps it is for now. Are any of you reading now still reading from back then? Leave a comment please if you are.

I have pondered the plusses and minuses of the three suspension designs I mentioned a couple of days back and made a decision based on what will work best and still not break the budget. Before I tell the winner I have to explain what went into the decision.

The overall design of the original car called for a tube front axle with reproduction early Ford spindles running adapted Mopar disks and GM Metric calipers held up by coilovers and connected to the frame with a parallel 4 bar and a long panhard bar for lateral location much like a sprint car. Since I had already bought much of this setup economics demanded that I make the rear suspension compatible with this. The biggest feature with this suspension is zero camber change in roll compared to the road as long as both tires remain in contact with the road, and a mostly fixed roll center height. This makes for a predictable and controllable front end which is worth way more than extracting that last 0.01 lateral g from the front tires. But to make the front suspension work the rear suspension has to be equally predictable and controllable.

So we start with the minivan strut suspension. Plusses are mainly low cost as all the parts are right there. Most of the minuses were spelled out in that other post, the biggest being lack of tunability , and a secondary one being that ride height would have to be set during the frame building process without knowing the spring rate or how much the back end will weigh. Well in addition to the construction issues there are the geometry issues. The biggest of those is the camber changes pretty much in tandem with the body roll, causing the outside tire to roll over on the outside shoulder and pretty much killing mechanical grip making the back end much less predictable and also drastically reducing overall grip. So as much as it pains my Scots ancestry this cheap solution ends up not the correct solution.

The second possibility was using a heavy wall aluminum tube like the axle tubes on a quick change rear end for a front engine car to make a de Dion rear suspension. This has pretty much the same geometry as the front end making it a much better choice for the midengine Bucket than the salvaged struts from the minivan. It is very easy to make the camber and toe adjustable at the plate that mounts the minivan knuckle to the tube which aids in setting tracking and adjusting the rear to match the front. Another advantage is the coilovers can be located pretty much anywhere on the top of the tube making setting the initial roll stiffness very easy. The major disadvantage is I don’t have the equipment to weld aluminum except in very thin material so the welding would have to be farmed out. That is an additional expense.

That brings us to the third possibility, the steel truss de Dion rear suspension. Again we have a very compatible geometry for the front end, that is also highly adjustable for everything and has a similar weight to the aluminum tube version, but one that I can build using the equipment I have on hand. One slight disadvantage is the mounts for the coilovers have to be over or very close to a vertical member of the truss (I decided on a modified Pratt truss) which means either the truss will have to be iterated somehow (probably with a shear plate that would be removed after the mounting point was determined) or the initial mounting point will have to be SWAGed (Scientific Wild Ass Guess) and then tuned with an anti-roll bar. For simplicity I would prefer to not have anything extra on the car for street use, so SWAG for the initial mounting point on a shear plated truss then do a limited iteration on both sides for the final mounting point, then complete the truss. Or just leave the extra mounts and one bay of the truss shear plated for adjusting the rear roll stiffness at the track. I like this because instead of bringing a rack of anti-roll bars to the track I just move the coilover from side to side on the axle to adjust the balance of the car.

So the Final Solution to this problem is the steel tube Pratt truss with one bay shear plated and several mounts for the coilover along that bay and matching arrays of upper mounts on the frame crossmember on the rear, with the tube axle and early Ford spindles and the coilover mount as close to the spindle as possible without getting tire rub, parallel four bar longitudinal location on both ends with a long panhard rod on the front to eliminate bump steer from the cross steer steering and a Watts link mounted low in the center of the frame running out to the knuckle mounting plate for lateral location. And I literally came to this conclusion while composing this post, because that’s how I think. You just got to watch my mind at work iterating a design specification. I had already decided on the truss but the details got finalized while doing the post.

Billed @€0.02, Opus the Unkillable Badass

My Warlock died yesterday and more on a Wreck-Free Sunday

Yep, my poor Warlock was severely killed (along with my apprentice NPC Bard) in the kinky D&D game yesterday. I was down to 6 HP when I got hit by a 34 HP attack centered on me and my apprentice because we were not in the room in an earlier attack that made the entire rest of the party magically pregnant with the half-siblings of a half-dragon daughter of a Green Dragon. She was pissed at us for not being there to get pregnant during that attack. I managed to get off a few good shots but a 5th level Warlock going up against even a half-dragon just ain’t gonna work. And so, I died.

Fortunately in the party loot was a clear bottle with a lead stopper filled with orange smoke and flashing lights, a “bottle of djinn”. This djinn was fairly powerful in that he had enough moxie to do two resurrections and bring both me and my apprentice back to full life with no loss of levels or XP. The senior member of the party then ordered the djinn to be free, much like the climactic scene in Disney’s Aladdin. We did this because we only had control over the djinn for 24 hours and if he was powerful enough to bring two characters through a full resurrection he was more than powerful enough to do a TPK in a single attack. So, “bring my friends back to life, and be free” seemed like a good choice.

On the Sprint Cup race today Martin Truax finally got a win this season after practically pounding the door down all season long with the Furniture Row Racing team. They won simply by virtue of having the stronger car pretty much all race long, and were able to outrun Happy Harvick at the end. This puts him in for the Championship Chase at the end of the season. I have liked Truax as a driver since his days with Michael Waltrip Racing, and was highly disappointed when MWR let him go at sponsor request, even after winning races. Well now he won driving for a single-car team with a tiny fraction of the resources of the big name teams. I mean until now had you ever heard of Furniture Row Racing?

With the parts that arrived last week detail design work on the Sprint-T progressed a little further as now I know how big the things holding the rear axle from moving relative to the frame are. Knowing this I know how much leeway I have to provide to keep them out of the bodywork and frame, and where they have to connect to the rear axle so that the coilovers don’t hit that. Having the actual items in my hands to measure makes things that much easier to draw. The issue I’m now facing is that I need the instant center for the rear suspension to be above the rear control arms slightly so that the rear axle doesn’t chatter under heavy braking, and when you throw that in with the “don’t hit the body” during suspension travel, or violate the “nothing lower than the main frame rail or the rims” parameter that is one of my basic rules for building a street vehicle it kinda boxes you in.You might have noticed that there was a bit of a bow in the control arm in the picture last week to accommodate the mount for the coilover unit.
This means I can’t just run the part between the rear end and the frame or it will run below the frame rail as this is a dual-use part. Not only does it locate the axle fore and aft, it also controls the up-and-down motion by mounting the rear springs, and the anti-roll bar bolts to the bottom to control roll (obviously, part of the name of the part). Why let a part do only one thing when you can use it to do three jobs at no increase in weight? I think it was Colin Chapman (founder and first designer for Lotus Cars) who said that, but he might have been quoting an earlier engineer.

I just returned from evening service where Mrs. the Poet was installed to the board of the church as a member-at-large for 2 years. It was quite moving. It was also rather frightening on one level, as the oath of office was basically in invitation for your gods to hunt you down and kill you in the most painful way possible if you screw up. I don’t know if that was the intent of the oath of office, but that’s surely the way it came out. I guess that is an effective way to ensure that screw-ups will be unintentional.

And I should let this end here as I have already gone a bit over my planned word count. And I’m having trouble putting the abstract concepts into concrete words.

PSA, Opus the Poet