Designing and Fabricating the Steering Rack Mount

Rack Mount

Steering rack mount, ready to weld gussets.

Next step in getting the car rolling is to mount the steering rack. These parts were not laser cut but were cut with angle grinders and my bandsaw, because they’re, um, a design improvement. Yeah, a design improvement, that’ll work. It took me months to find a blade for my bandsaw, because Thailand. Anyway, now it’s useful for these kinds of tasks.

Once again we have something that looks simple, but took a great deal of thinking to arrive at. As Steve Jobs said, “Simple can be harder than complex: You have to work hard to get your thinking clean to make it simple.” I used to do almost exactly what Steve Jobs did; in fact, he once called my boss to persuade him to cancel the design we were working on, because Jobs was going to do it better (he didn’t). So I understand precisely what Jobs meant about simplicity. That might help explain why this was my fifth complete design for this subassembly. One downside of mechanical design: when you’ve sweated out an elegant design, anybody can take a casual glance at it and say “OBviously”. To which I reply, “OK, there’s probably an even better design out there somewhere. Let’s see you find it.”

Also attached are a few of the finite element analysis (FEA) tests that I performed to verify and improve this design.

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Shiny Happy CNC Parts

I’m getting some interest in the car from here in Thailand, but they want it SOON. So I’ve had to step up the rate of progress, even though it hasn’t showed here on the blog. Instead, I’ve been designing and designing and designing… It’s a big step from an assembly design that seems pretty much correct, to a set of drawings and IGES or DXF files that you’re willing to pay real money to fabricate. Everything has to be checked, from the load cases used in the finite element analysis to the hole clearances for every bolt.

For example, did you know that a 1/4″ bolt doesn’t go into a 1/4″ hole? The proper size of the hole is 0.257″ for a close fit or 0.266″ for a free fit. Of course, then you have to take into account the width, or kerf, of the laser beam used to cut the metal, which can be 0.01″ or 0.25mm but varies with the type of material and thickness being cut, and the laser beam is actually a cone that can be focused on the top, middle, or bottom surface of the piece. Many parts had to be redesigned for the materials and processes I’ve been able to locate in Thailand. Here it’s not a simple matter of looking up all the local suppliers on Google and giving one a call. Thailand has a great number of very small companies that rarely have websites, and even if they exist they’re mostly in Thai, which as a special favor to web search engines uses no spaces between words. Yes, you read that right. Using no spaces would be OK if there were only one way to parse a stream of Thai characters, but haha, you make joke, eh? And then if I can actually get someone on the phone, I have to communicate in Thai. Like that’ll work.

Last year I was looking for a foundry to cast aluminum uprights, and I found one (using Startpage, not Google) less than an hour from here. Their website had a Google map and everything! So I drive there, and I’m within 100 feet or so, asking motorcycle taxi drivers where the company is. No idea; there’s never been a foundry around there. I show them the address and they say oh, that’s way over on the other side of town. The person answering the phone number has no idea what I’m talking about. I give up.

On the other hand, recently one of those Google ads that look like the first links on your search actually showed me something that I needed and couldn’t find with a search: a small local company that fabricates custom radiators. So I managed to find their shop this week and got a quote for the radiator. Quote comes by email entirely in Thai. Google translate does a pitiful job of translating Thai, but I caught on the email wasn’t spam. Fantastic price, by the way. Cheaper than what I paid for a used race car radiator on EBay. I decided I didn’t really like how that one would fit, so I designed one that’s ideal for my application and figured I’d worry about fabrication later. Problem solved, yay!

The Thai racers who are interested in the project have emphasized that the price is crucial, which means more redesign. Surprisingly for me, this is the same thing I’ve heard from the people in Singapore who’ve contacted me. Sometimes it’s easy to dash off a quick machined aluminum design, but it takes a lot more thinking to do it with laser-cut steel pieces. The equivalent in steel typically ends up being a little heavier, but that’s an example of why Formula 1000 has a minimum weight rule. Laser-cut steel hand-welded into a complex component is just about free in Thailand.

Where before I was planning on making just about everything in my machine shop, now I’m looking at fabricating as much as possible at subcontractors. Luckily a friend found a large CNC machine shop not too far from here, and the guy running it speaks English and understands drawings and computer files and tolerances and clearances and everything. I’ve now received my first batch of CNC machined parts from them and they look great. Combined with the first batch of laser-cut parts and a few parts I’ll fabricate and modify myself, I now have everything to make the car a roller.

Anyway, on to today’s gallery:

Hot Off the (3D) Press: Suspension Upright 3-D Print

The suspension uprights have gone through a long evolution, but I’m zeroing in on the goal.

First Design: Machined Billet Aluminum

This design uses radial-style brake caliper mounting. Needed to be redesigned when someone ordered the lug-mount calipers and had them delivered all the way from the USA. Probably a Freudian slip as they’re less expensive.

Machined Upright

Second Design: Fabricated Steel

Fabricated Upright

Second design was fabricated from steel. Unfortunately, welding will eliminate the temper in the heat-affected zones. The weakening due to this is hard to predict, and can only be eliminated by heat treatment. That would mean days or weeks finding and learning to deal with a heat-treatment supplier.

Mesh Quality

Mesh used for finite-element analysis of fabricated steel upright.

Upright FEA

Finite-element analysis stress plot for fabricated upright. Strong enough, but where are those heat-affected zones?

 

 

Third Design: Cast Aluminum

Here’s the final result of literally hundreds of design revisions, ensuring that the upright is strong enough and as light as possible. This design is made possible by the new technology of 3d printing, which will be used to make the master “plug”, from which molds will be made to cast the actual part. Note that the steering arm is not an integral part of the upright, but is modeled together with the upright because the FEA runs much faster this way.

FEA mesh plot for cast upright

FEA stress plot inner

Cast Upright FEA

View from outside

Finally, the Master Copy

The upright had to be split into four pieces for 3D printing; split vertically so I can make two mold halves and remove the masters from the molds, and split horizontally to fit the 3D printer. The 3D printer extrudes hot ABS plastic in X-Y layers onto a heated Z-axis stage with 0.3mm resolution. The print is slightly rough and the parting plane is slightly warped, which will have to be corrected with auto body putty, primer, sanding, and paint. The cast aluminum blank will still require several machining steps to cut off the gate and sprue, drill mounting holes, bore the bearing hole and retaining ring slot, and mill the brake caliper mounts. Still far better than trying to machine individual parts (or even a mold pattern) this complex, which would be approximately impossible and semi-infinitely expensive.

Beautiful, huh?

One half printed in two colors to highlight the split required to fit it into the 3d printer

Set of 4

Full set of four 3d prints. The cylinder protruding in the upper left is the gate, where molten aluminum will be poured into the finished mold.

Top View

Some people get excited by shoe sales. I get excited by this!

 

 

Calculating the Loads on the Front Suspension

Suspension FEA

Simplified left front suspension finite element analysis result

It’s time to finalize my parts order, which means I have to pick all the rod ends and spherical bearings for the suspension. Up to now I’ve relied on rules of thumb, which isn’t good enough for a state-of-the-art race car. So I simply opened up one of my higher-level chassis models in Solidworks, one with most of the suspension attached, and created a 3D sketch of the upper and lower left front suspension A-arms, constraining the endpoints of the lines to the centers of the rod ends and spherical bearings. Then another line to simulate the upright and tire, and I’m done with the sketch. Then I created a structural member along each line and opened a static simulation study. In the study I constrained the inboard ends of the A-arms to zero translation and applied a 1400 pound load toward the rear of the car to simulate 4-G braking. Run the study, and voila! axial loads on each member.

Upper Front: 785 pounds

Upper Rear: 1211 pounds

Lower Front: 2639 pounds

Lower Rear: 3096 pounds

This allows me settle on 1/4″ rod ends for the inner upper attachments, 3/8″ for the inner lowers, a 7/16″ spherical bearing for the upper ball joint, and a 5/8″ one for the lower ball joint. All chrome moly alloy. Even these are far stronger than indicated necessary by this analysis, but I don’t want to deviate too far from standard practice on this design. Maybe transitory loads can be far higher?

Calculating the Loads, Round II

At the urging of a poster on ApexSpeed.com who insisted my calculations were inaccurate due to the non-zero joint stiffnesses, I re-ran the analysis with the suspension a-arms modeled as trusses. A truss model allows only axial loads on a member, using hinges at both ends to zero the moments. This gave me results that were the same +/- 2%, which makes no difference given the uncertainty in estimating the operating loads.

However, I also refined my model to include the suspension pushrod and that did indeed give me a different result. With the pushrod in place I could model the vertical load due to the weight of the car and the aerodynamic downforce. I’m now working with a braking force of 1600 pounds and a vertical load of 1230 pounds, which assumes a coefficient of friction of 1.3 for the tire on the surface of the track. This gives me the following axial forces:

Suspension

Left front suspension modeled as a truss with pushrod

Upper Front: 1103 pounds

Upper Rear: 1258

Lower Front: 4432

Lower Rear: 2366

Pushrod: 2357

Note that including the vertical load and increasing the braking load shifted the most highly-loaded member from the lower rear to the lower front, and increased the maximum load from 3096 to 4432 pounds, or 43%. The chrome-moly 3/8″ rod ends I’ve specified for the lower a-arm inboard joints still have a safety factor of 2.14 under these conditions. The posters on ApexSpeed have strongly warned me against using 1/4″ rod ends in the suspension, so existing inventory will be used in a different application and the upper rod ends will be upgraded to 8mm. It appears that only metric rod ends are available locally. Under these assumptions the pushrod rod ends will have to be further upgraded to 10mm, although I hate to have a whole separate part for just the pushrods, so I’ll have to re-examine the assumptions.