Load test--Pedals

The equipment

Don't worry, the guy who welded this fixture (that's me) is not the person who welds our chassis and suspension components. As with sausage making and legislation, not everybody wants to see how structural testing is done, but when us citizens spend billions on, for example, developing a B-2 bomber, some of that gets spent load testing the pedals--and those test fixtures aren't much more sophisticated than this one.

Load test fixtures for individual parts have to meet two needs: they need to support and stress the part in a realistic simulation of the part's actual use, and they need to be strong enough to test the part, rather than test the fixture. When designing a test fixture, weight doesn't matter and pretty doesn't matter, but time is money so one gets points for expedient, quick and functional, and this one fills the bill.

The pedal is mounted in our pedal bracket, which is welded to the fixture frame, and pivots on the lowest quality hardware we think it's likely to encounter—a Grade 2 coarse thread bolt with no nut. The brake pushrod, with pushrod clevis from a first generation Miata, fits into a rounded pocket on the fixture, which allowed us to test pedals drilled for a pressure ratio of 4/1 to 6/1 (need #1: realistic simulation) without mounting it to a master cylinder (need #2: test the part, not the fixture—using a master cylinder would have tested if its seals could handle over seven tons psi* but might not have tested the pedal). The load (sandbags on a wooden platform--not shown in this photo) hangs from a length of seat belt webbing (not placed on the pedal in this photo) and the fixture is lifted by an engine hoist until the load leaves the ground. The sandbags were weighed on a Pelouze Model 4010 digital platform scale.

The test standards

In aviation the standard for any metal structural component is a demonstrated ultimate strength 1.5 times the design requirements, times 2 again for castings. The X2 casting factor is based on centuries of experience with structural castings (though only a century of that includes airplanes), and while there is some variance in strength between parts cast from the same mold, the worst is at least half as strong as the best.

I don't know what the SAE calls for in brake pedal design strength (any card-carrying automotive engineers out there that can help me out?), for Light Sport Aircraft it's 125 pounds for pedals (rudder or brake)--times 1.5 times 2 is 375, so a 375 pound ultimate static test would qualify these pedals for an LSA, suitable for FAA certification to carry passengers and fly a mile above your house. But since automotive standards for strength of critical controls may be greater than the FAA's requirements for equally critical controls, let's double that to 750 pounds.

We tested four samples. I'll admit I breathed a sigh of relief when the first one reached 750 pounds, but I sure wasn't surprised. At 1000 pounds I wasn't surprised either—it was designed to be good for 1000 pounds, plus a tiny fudge factor—but I was keeping my faceshield down and my padding in place and it wouldn't have been a shock if it had failed at that point and half a pedal had been hurled about the room at Civil War artillery speeds**.

Anyway, it didn't. Here's a photo of a pedal holding up 1072 pounds of sandbags and support structure.

We stopped at 1072, and all four pedals survived without yeilding (bending), much less failing (breaking). The pushrod clevis did yeild, but never failed, so I'm not going to complain to Mazda.

Here's a closer look at the fixture, still with 1072 pounds on board. I set it up with the pedal horizontal (at right angles to the load) because that is worst case, and used the seat belt webbing to distribute the load over the footpad. The seat belt webbing is about 1/2" off center because I didn't want it to slip off, but oh well, close enough.

We stopped loading at 1072 pounds. Our next available sandbag was an 81 pounder and the hoist is rated at 1000 pounds in this setting; I was willing to bet that the manufacturer has a 10% fudge factor built in to that figure, but that's as high as I'll go.

Also, at 1153 pounds (the next step up) I'd expect a pedal to fail, and I didn't care to replace a window just so I could say "I told you so." 750 pounds was all I needed to be happy, going over 1000 was just plain showing off.

...or back to ...or back to Pedals

*Using a 6/1 pressure ratio at the pushrod, a 1072 pound load, and a master cylinder with a 3/8" radius (3/4" diameter) piston, we get (6 x 1072) / (pi x ((3/8)squared)) = 6432 / .4418 = 14,559, which is a bunch. It's about 1000 times atmospheric pressure, it's the pressure at the sea floor about six miles down...I don't think most brake seals could take it.

**Not a joke, by the way. Doing tests of this sort, I wear a full coverage helmet with face shield <and> safety goggles, gloves, a leather jacket over a Mr. Rodgers comfy sweater, and other strategic padding. The energy stored in the stretch of the seat belt webbing and rope, the flex of the engine hoist beam, the bow of the 4 by 4s and plywood under the sandbags...at 1000 pounds load they're capable of tossing a quarter pound of aluminum faster and harder than any crossbow bolt of the Middle Ages, and armor didn't stop those.