Tuesday, November 8, 2016

Struggles of Design: Alfa Harness Bar

We developed our 4Corsa Harness Bar for the Alfa 4C knowing that people would like to track these cars.  This blog entry discusses our harnessbar development process, a mistake we discovered AFTER we had began production and the solution that we came up to fix it.  We are happy with the final product but it took some extra effort to finally get it 100% right.

Brainstorming A Solution
Getting a bar into the Alfa presented some challenges as the chassis and seats did not give us much latitude.  We've designed harnessbars for over 10yrs for Lotus Elise/Evora and Mini Coopers.  Some of our design requirements for the 4C included:  be economical, 100% reversible, require minimal modification, and most importantly, be safe. Safe means it must be strong AND correctly position the shoulder belts.  Solutions we've seen in various cars, including the 4C, position shoulder belts too high.  After MUCH head scratching and going through a number of concepts, we landed on the one you see for sale today.  We hit our targets for safety and reversibility. 

Joe (our engineer) earning his paycheck.

Measurements were taken inside the car, a rough 3D model was made, and a prototype was fabricated. Making a one off piece is actually relatively easy. However, we needed to take this part and make an accurate 3D model for our fabrication vendor so they could make a batch of bars. We took measurements from this bar and had some tubes laser notched, they were off a bit, so we had some more tubes laser notched, these were correct. We basically remade our prototype with these laser notched tubes (the originals were notched by hand) to verify the design. It fit so we kicked off production.

Prototype bar in place. It fit our car!
Fitment Problem
Great! Right? Well, when dealing with these low volume cars, sometimes it is difficult to get your hands on a single car for testing, much less multiple cars. Before production, we identified a potential issue. We assumed there would be some variation in the distance between the seat belt mounting points that the bar bolts to. We did some estimation that the variation would be 1/8" or less which the bar was designed to deal with.  We were wrong.  After we started selling our bar, we found out we were wrong. It turns out the variation from car to car is more. This may be due to the introduction of a new model called the 4C Spider - though we are not sure.  Regardless, small volume cars generally have much more car to car variability and that must be considered in the design of any hard parts.

Problem Resolution
 We needed to get creative again and come up with a solution to take up the car to car variation. Our current bar fit some Coupes but not Spiders (2 so far). Most cars have 'slip joints' to take up build tolerances.  This gave us an idea.  We decided that the best way to allow the bar to accommodate the width variation is to allow it to telescope.  We brainstormed several different approaches to make this happen and finally landed on a solution.  Obviously safety was critical and we are happy that it is probably now even stronger than before.

TSSJ Technology
Our solution is a slug and sleeve arrangement to give the bar some telescope ability in the width direction. We cut the bar in half, insert and bond a slug inside the tube and slid a sleeve over the top. This sleeve is also bonded to one side of the 4Corsa.  The other side of the 4Corsa bar slides into this joint.  This arrangement is total overkill. The slug in the center is a piece solid steel round bar and the sleeve, being bigger than the tube it is covering, is even stronger. The both of these parts combine make sure that the bar is not any weaker than the original design.  We came up with the acronym, TSSJ (Telescopic Slug Sleeve Joint) because necessity is the mother of all invention and why shouldn't we have some fun at the same time?!
TSSJ Technology:
Section view of slug and sleeve retrofit.
Slug is red, sleeve is blue, original tube is grey.
Though we had considered adding additional adjust ability, earlier in our development, we ended up tabling it.  In hindsight this proved to be a mistake.  Ultimately our TSSJ solution makes our bar a product that will work on both models and offer a degree of adjustment that a small volume car truly needs.

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