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
Witch on a Bicycle 