foam core snowmobile chassis

General Composites Discussions Engineering Talk
#1

I rebuild my snowmachine every year and modify it to make it lighter, faster, stronger ect. I was going to build a full 4130 chromoly tube chassis this summer but have decided I want to build a foam core carbon fiber chassis instead. A few companies already build carbon fiber products but, but no one has done a foam core yet.

My main objectives are lightweight, and unparalleled strength. I plan on using 1/2" Divinycell foam for the entire structure. This includes the tunnel, bulkhead, subframe, gas tank, seat frame, ect. I plan on vacuum bagging everything in stages and bonding it together.

We have a local supplier(I’m in Alaska) for Divinycell which is why it’s main choice. From what I’ve read it has high strength/stiffness, resistance to fatigue, and high impact strength. Snowmobile go through an incredible amount of abuse. Jumping, rolling down mountains, sharp impacts from trees, ect. I just have a few questions.

  1. Since I’m using a foam core, how many layers of cloth would be recommended? Would one on each side be sufficient?

  2. My exhaust runs under the tunnel and I assume gets hotter than 180* which I’ve read is the breakdown point for Divinycell. Would extra layers of carbon protect against this or would I need an aluminum heat plate?

  3. Will the foam be stiff enough to bolt engine mounts too? I’ll also have a chaincase that will need to be mounted.

Here’s a picture of my sled

Here are a couple more I got off the internet that are not foam core

#2

1 - depends massively on the specific part. from the brief description you gave, probably not, particularly if you are looking for impact resistance. Also, depends on the thickness of the core. But, in a word, no
2 - Heat shield is by far the simplest option here. It would be worth testing your current configuration to see the surface temperature of the surrounding surface at present. You can get resins to cope with such temperatures, but I would just go with a heat shield.
3 - Yes if you make the carbon thick enough around the bolted area. Also depends if you are using inserts or not. you can also locally remove the core and fill with adhesive / alu core, etc.

#3

The entire core would be a 1/2" thick. My dad had suggested drilling larger holes and using inserts set in epoxy wherever I had to bolt to the chassis. I was also thinking I could even use balsa wood instead of foam where I had high compression areas such as the chaincase, motor mounts, and front a-arm mounts.

I do plan to build a couple small sections of tunnel with various layers of carbon to do some destructive testing in my garage. I think for now I’m just going to build a couple tunnels and test it them through next winter to see how they hold up and what i need to change. I built the current tunnel from .080 5052 aluminum and with all the bracing weighs about 35 lbs. A carbon tunnel, even heavily reinforced should weigh less than 15 lbs so there’s significant weight savings to be had.

Here’s a drawing(don’t have CAD) of the CF tunnel. I took the running boards all the way up to the back to increase strength the red triangle is an added support. How do you determine how much strength the foam core adds? I haven’t been able to find any kind of charts aside from what DIAB has on their website. I’m wondering if I could get away with just using 1/4" Divinycell for the core.

Here’s a pic of a new hood design I’m making. As soon as the resin dries I’m going to sand it down and pull the mold off. The finished products will also be CF.

#4

1: same as findhan. All depends on what it is. One layer on each side will not be strong at all, but if it’s ONLY for a covering, you won’t need core at all.
2: yeah, heat shield would be the way to go, unless you can build up layers of glass on the exhaust side with a high temp resin. Glass should insulate more than carbon. You can also just find some high temp foam for that one area. To be safe, you might be able to add vents to allow airflow into that area to purge out hot air.
3: NO. Never bolt through foam core. Always use an insert, be it metal, or just extra layers of fiber to replace the foam thickness. An engine mount would need good hardpoints to mount to.

#5

For determining how a core adds stiffness:
If you want to look at things simply, there is a simple way to calculate the effect of adding a core material. See the attached image. If you look at the first panel, it represents a couple of carbon plies on their own. The stiffness of this panel is proportional to its second moment of area for a given lay-up and you can see the formula
Now look at panel 2 with the core material (Nomex in this case!). For simplicity, assume that the core has absolutely minimal strength and serves only to hold the two skins apart. Holding these skins apart effectively makes the panel act like an I-beam and you can see the formula will produce a much higher result. Stiffness therefore increases massively with core thickness and not just linearly as you can see from the formula.
The limiting factor in the thickness of core is generally its shear strength btw.
But, if you apply a real world scenario, the panel should be a lot stronger as the core itself does contribute to strength. So, ½” core would add a lot of stiffness to the arrangement!

BTW, if you’re chassis plans are really as simple as that drawing (not to insult your drawing in any way) with all straight lines, I would forget moulding it and try cutting and folding. For your first attempt, I think it would get you a hell of a distance with much less work.

#6

I use FR4 of same thickness as the surrounding foam as inserts. Consider making these larger then need be so the pressure is more distributed into the panel. You can use differnt thickness of foam through out the part to put the strength where you need it.

For heat shielding I usually use this stuff - http://www.designengineering.com/catalog/design-engineering-inc/heat-sound-barrier - I’ve run the second one in some crazy applications, the fiberglass mat in it makes a huge difference compared to competing products.

#7

It is as simple as the drawing. I want to use the wood so I can vacuum bag everything against it and keep the tunnel straight and square. My rear suspension will also be bolted to the tunnel in two spots. Would it be sufficient to replace the foam with balsa wood in these areas?

Here’s a side view. The Red dots respresent the suspension mounting bolts which will be high stress areas. The green squares I was thinking could be balsa wood if I needed the added rigidity.

I got this picture from evansmarine.com. He builds carbon fiber tunnels on the side. This is basically what I’ll be building.

#8

Also it can be stronger to use a corrugated core system. This is stronger. Even lighter and minimizes water entrapment. In areas where your fasteners attach you can located core adjacent to the holes so stress will be applied longitudinally to the core. You can still use divinicell in strips. The carbon will wrap the strips and create corrougated ribs in the parts. Super duper strong. A thin stainless steel sheet bonded should reflect the heat just fine. When you start breaking test plate do not be surprised how strong a 3 layer 4 layer laminate is.
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Also this is very important do not allow aluminum to come in contact with the graphite. Vibration and water will accelerate corrosion. Titanium and stainless are a must for all inserts and associated hardware. When drilling fastener holes the holes should be l fit (loose) with a wet install with a sealant. You could use rtv. Always try to allow as much surface area to contact the Panel under the bolt head by using flat washers. Bonding them is preferred.

This is an awesome project and could definitely be worthwhile for you and the sport as a whole.

#9

It would be a bad idea to allow water to enter the core via an insert having the understanding that snow mobiles get exposed to water and that water then freezes. Having that entrapped inside a core would be devastating structurally. Allow reliefs for your bolt holes.

#10

One core you probably do not want to use is balsa. if for some reason a crack forms in the laminate the balsa will absorb moisture and saturate inside the laminate.

#11

I second those ideas. honeycomb might have water get in, freeze, expand, delaminate…like concrete, etc. Balsa also uptakes water (though, 1000’s of boats use it for some reason, and they get by…)
Also, stay away from balsa and ANY core for the mounting areas. A hardwood might be ok, but I would stick with extra carbon/FR4 block/metal for bolt locations.

#12

G10 or fr4 is a proven hard point. That is what i use.

#13

I never thought about using G10 for hard mounting points. Excellent idea. That stuff could more than likely be drilled, tapped and a metal threaded insert installed.

where do you get your G10 from? I saw once mcmaster carr or similar had some… just wondering who else stocks it.

#14

Lots of boat manufactures have been switching to materials like http://coosacomposites.com/ instead of wood and now are offering lifetime hull warranties because of it.

#15

I buy it from mcmaster carr. they stock all sorts of sizes and the prices are decent. Yes, you could probably tap it. I have installed inserts in it too. Mostly I through bolt it with countersunk screws.

#16

Thanks for all the advice. I’ve been reading up on the suggested products and methods, trying to do as much research as I can before starting. I’m currently in the middle of a house/shop remodel and once I get organized and my life back together I’ll start on the wooden mock up.

#17

It’s been awhile. I ended up putting the carbon fiber chassis on hold and made one instead out of a combination of 4130 tube and 7075 aluminum. It had a dry weight of 449 lbs with a turbo, over 120 lbs lighter than a factory Yamaha Nytro snowmachine.

Fast forward to 2013, and I want build the lightest, strongest, turboed snowmachine on snow. I started on a miniature tunnel side tonight to use for a first test piece. The next couple months will just be testing different routing depths, layers of cloth, vacuum bagging techniques, ect. All the test pieces will be hand routered so ignore the sloppy cuts. The finished product, which is way down the road, will use a CNC template for the router and rounded edges everywhere.

The miniature tunnel side in the pic is 1/2" Divinycell foam with 5/16" removed. This one will receive two layers of CF cloth on each side for testing.

#18

Why are you routing the foam?

Something else to add, I noticed some posters stating that the foam will add strength. That is not really the case. It will add stiffness to the panel but stiffness is not really strength.

#19

I was under the assumption that creating ribs in the foam would make the panel even stiffer. I’m trying to minimize the amount of 4130 I’ll need for reinforcement.

#20

Routing out in that manner is fine, its just a design typically for a metallic structure. Just easier to manufacture and a lot less hassle having it a solid core; fair enough it will be a little heavier but I would go with the simpler to manufacture route