MKII Rear Brake Scoops, my summer project

[Adam C.]

Well I call it my summer project because it began sometime in June I think. Like many things however, it went over timeline and over budget, but what a learning experience!

I thought I would share my first real experience in fiberglass. Although I have helped out on making parts, and have done a little carbon work, this is the first thing I have done start to finish. I am certainly no Chris Melia or Bill Hough, just an engineer with too much free time.

My college buddy, Stacy Henson and I decided that we would like to build MKII’s from scratch, and so over the past year or so we have been gathering info, parts, etc. To me, two of the most defining things on the MKII are the T-44 (one recently sold for $42,000), and the rear brake scoops (sell for around $400). Since we aren’t exactly loaded, we started with the two-order of magnitude cheaper rear scoops. We figured, rather than buy two pairs of scoops, we’d just copy a set and make our own. With some initial figuring we thought that we could surely make the mold for under $800 in no time, and then the parts that came out of the mold would be practically free. We also figured that we could do it all in a couple of months, just working on it during weekends. Ha!

We began with our borrowed scoop which we spent a lot of time on sanding, priming, etc. Once we were happy with the finish, we constructed a jig to hold the snorkel in place, make the various parting lines for the mold pieces, and provide flanges for the mold. Below is a shot of the fixture and original part. The mold was split into three pieces, left, right, and front.

The little rubber bouncy balls are to provide locating features to all the mold pieces fit back together nice. We used Kleen Klay to fill the voids between the part and the board. Wax, wax, and then wax the whole thing some more.

[Adam C.]

With the buck ready to go we brushed on the epoxy surface coat. This is a two part tooling epoxy coat that is pretty tough and sands real nice. It is more expensive than gel coat, but we wanted to do the best job we could.

We then followed with the first of many layers of chopped strand mat. The rule of thumb that I hear is the mold should be at least three times the thickness of the part you intend to pull from it. You want the mold very stiff so it doesn't crack with repeated use.

[Adam C.]

After around 4 layers of chopped mat, we piled on about another 8 layers of 3, 6, and 10 oz fabric. Epoxy resing was used throughout. Again, more expensive than polyester or vynilester resin, but it won't shrink or creep with time, and it is stronger.

[Adam C.]

These snorkels also have inner duct work that is made separately and then glued in. We promised the original owner that we would not damage the original part, so we made a silicone plug of the inner duct. No waxing necessary as this stuff won’t stick to anything. This inner plug was actually poured into the snorkel before the outer mold shown above was made. This provided a handy flat surface for the front piece of that mold. It also assured that the inner and outer pieces would fit together perfectly.

[Adam C.]

We made a new jig to divide the inner duct into two pieces, and added a small cup on the duct inlet to give some material to aid in bonding the duct to the snorkel.

[Adam C.]

We then painted it with the epoxy tooling gel coat and followed the rest of the laminating schedule used on the snorkel outer.

[Adam C.]

We then made the other side of the duct inner.

Now the real pain in the butt is that the inner duct needs to be smooth on the inside surface. If we were to use these two molds, then it would be the outer surface of the duct that would be smooth. So we had to use these molds as master molds, and then pull molds off of them. We needed the negatives of the negatives of the positive plug... er, wait a minute... I'm confused.

Anyway, in the end there were 7 mold sections made, 5 of which are necessary to make a complete snorkel. Below is a picture of the important 5, all sanded, polished, waxed, and ready to go.

[Adam C.]

So I know that the guys out there who know a thing or two about fiberglass are wondering why we waisted all that material and time making those HUGE flanges on the molds. Well this is where my engineering mind got the best of me. VACUUM BAG.

While at school I was exposed to the Formula SAE team. Ohio State consistently makes some of the most expensive, if not best performing cars in the competition year after year. Their tub is fabricated at the great Ian Algae’s (forgive me if I spelled it wrong?) composite shop, where they do nothing but carbon pre-preg work. Their chassis are all carbon and sandwich core construction. Ian is a perfectionist, and so everyone there pretty much laughs at wet lay up, fiberglass, gel coats, etc. Unfortunately I don’t have an autoclave, and so am restricted to some type of wet lay up. So we did the best we could and vacuum bagged the parts.
While at school I was exposed to the Formula SAE team. Ohio State consistently

Vacuum bag layups begin just like any regular fiberglass part, but there is no gel coat.

[Adam C.]

While the resin is still wet, a nylon fabric (peel ply) is used to cover the entire piece. It allows the resin to flow though it, and prevents anything else (vacuum bag, bleeder cloth) from being bonded to the part. A cotton bleeder cloth is added next. It soaks up the excess resin that will be squeezed out of the part under vacuum. This is then followed by the vacuum bag which is attached with some of the most amazing “tape” that I have ever encountered.

[Adam C.]

With everything in place, a vacuum is pulled on the bag. Our pump will pull 25 inHg, which is about .83 atmospheres. It puts a pretty good squeeze on the part, compressing the layers, removing voids, and squeezing out the excess resing. This makes a very light and strong part. We keep the vacuum on until the part is cured.

In the end the cotton bleeder cloth becomes it's own second-rate composit, with nearly the weight of the part itself.

The peel ply is fun as hell to pull off, and leaves a nice textured finish for secondary bonding.

[Adam C.]

The process is also perfomed on the inner pieces but I'll save you the tedium of looking at them under vacuum (actually I don't have any pictures).

In the end we have three very nice parts that are somewhat ugly because they don't have gel coat on them. In this picture you can see that there was one bad spot that we had to grind out and repair. This was due to the bag bridging slightly in this area. The vacuum bag does not stretch and so if there is not enough extra material to conform to the part shape, it will not put pressure on the composit. This was our first part. Practice practice!

[Adam C.]

We then trimmed our parts and glued them together using a 2 part epoxy bonding paste.

[Adam C.]

We then primed and sanded them.

[Adam C.]

And now here one sits next to me as I type, on my ironing board. For that high performance laundry cooling!

[Adam C.]

In the end, these parts weigh about 1015 grams (2.24 lbs) whereas the original wet layup parts weight about 1700 grams (3.75 lbs). OK only 1.51 lbs., but that is about 40%! Just think if we could save 40% on the whole body! In addition, we could save another 30% off of our parts by switching to carbon. Whey would then only weigh in at 710.5 grams a pair.

This was a fun project, but it cost a lot more, and took a lot longer than we anticipated. Still, I wouldn't trade the experience for anything.

[Brett James-McCall]

Adam,
Brilliant, educating me again [img]/ubbthreads/images/graemlins/grin.gif[/img]

Brett

[Keith Hardy]

Fantastic work Adam! I agree the exercise must have been well worth it and the quality speaks for itself. I have one question though, if the MKII was being designed today (and for the sake of argument, lets say that the body and motor are exactly the same as the original) would these type scoops be the most efficient way of getting air to the brakes? Or, would there be a better way with less drag for example, NACA ducts? The scoops are very Shelby in fact they're almost a trademark of his, but was it (is it still) the best way (notwithstanding the desire to maintain originality in appearence).?

[Mike Dozier]

What did it cost in $$$ and man hours?

MikeD