Bumper Material Testing

[ElliottB]

Early on in 45’s design process this season we realized that parts of our bumpers needed to be very strong to protect our intake so we set out to figure out what the hardest wood we could easily get our hands on was. We settled on Red Oak. (I believe the strongest wood we could find was from Australia?)

But how could we test it and compare it against the typical plywood that is often used? Our students came up with a pretty good test just that. We clamped the wood to a couple of cinder blocks and suspended a 25 Ib weight 38" high using a ladder as a pulley for some rope. The test was to see how many hits it took to break the wood.

Results: Turns out the plywood only barely took two hits before snapping. On the other hand, the weight just bounced off the red oak several times. It only began to crack along the grain after the 3rd hit. I was quite impressed with the students plus it made for some great fun to get our Saturday meeting started.

Hopefully, this information could be helpful.

Plywood vs. Red Oak

[Skyehawk]

As you experienced, red oak will crack along the grain. fun to test though, good for student learning about materials, excellent chance to do some material testing.

If you can get your hands on ash or hickory ( Sold at Menards, maybe Lowes) that should be more tough (albeit not quite as hard. ) Still prone to splitting along the grain if not supported well, but should be better than oak.

The best is probably furniture grade Baltic birch plywood - maximize the plys for strength. regular construction grade sheathing can be as little as 4 (maybe even 3?) on some of the stuff around 1/2". Baltic birch is something like 7 to 13 ply.

As an aside you can always mount a piece of steel angle along the back of the bumper as long as it is used for mounting purposes (and meets weight requirements). Bumper rules are actually pretty lax and have been for years. This year is the first major change in a while.

[dtengineering]

I’ll second the recommendation for “Baltic Birch”… it is reasonably available and definitely more durable than “regular” plywood. Much nicer to work with, too and actually looks good when properly finished.

I am also considering your concerns in light of the requirement that the bumper be backed by hard parts of the frame (R410)… “regular” plywood has been shown to be more than adequate for properly built, legal bumpers because you are only allowed short (7”) sections of unsupported wood. You would likely see no meaningful difference in your test if the cinder blocks were only 7” apart.

Kudos to the team for coming up with a cool material testing jig and a great way to get things kicked off with a bang.

[IndySam]

It is important to note that the birch plywood you can purchase from big box stores like HD and Lowes is not “Baltic birch.”

[Strategic]

There are two things I see with this that you really want to consider:

1. Do you really need to go with Red Oak? There are other hardwoods that will do just as well and are both cheaper and easier to work with. Oak is hard, but it also has a tendency to split along the grain (as it does in your testing) which could be problematic. Impact could cause corner joints and fittings to split the wood and cause failure. I’d suggest something like Poplar, an excellent hardwood for applications like this. It’s tough and has a much tighter grain, but is also easier to machine as you need. It joins very nicely with finger joints at the corners to make extremely strong bumpers that don’t have a tendency to crack under impact. It’s what our team has used for years now and we’ve never had a bumper break, even with extreme force like falling from the traversal bar in 2022 directly onto the bumper.
2. You need to think about the weight of your bumpers. Red Oak is relatively heavy wood. A 120" set of bumper boards should weight in the range of 12lbs all by themselves, without fabric, pool noodle, fasteners, or fittings. Keeping your bumpers under the 15lb weight limit will be a challenge. Poplar is about 4lbs lighter for the same size, and even Baltic Birch plywood is only about 11lbs.

While it’s tempting to want to go with something like Red Oak, you might want to consider other woods that are just as easily available and give good performance.

[DaveSimons]

One of the main factors that keeps us coming back to Birch plywood is that it is far less prone to warping after being cut either along or against the grain. Standard pine plywood, when cut in the 5-ish-or-so widths, just gets weird in unpredictable ways, regardless of grade.

Note that you will generally NOT see “Baltic” Birch plywood, as the Baltic moniker is typically reserved for hardwood boards and project panels that are MUCH more expensive than birch plywood.

Now, let’s talk about plywood grading. For bumpers in general, I recommend purchasing the hardest wood you can afford in the highest grade you can find, which will minimize imperfections and knots, and reduce the chances of warping. NOTE that plywood grading is a superficial grading, and inner construction is dependent on the core (see below).

Plywood grades are (best to worst) A, B, C and D for face or front face. The back grades are (best to worst) 1, 2, 3, and 4. Plywood grading standards are published by the Hardwood Plywood & Veneer Association.

Plywood grades of A1, A2, B1, B2 are a good two-sided product and would be used where both sides are going to be seen. The back side (number grade) does have a few more blemishes than the front.

A3, B3, C3 are for plywood that will be seen from one side, but the back side must be clean, although there may be some stain or discoloration on the back, spliced veneer, and so on. One might use this material for the sides of a dresser (who looks inside?), for drawer sides, and shelving.

A4, B4, C4 are a one-side-good product. The back may have open knots, filled knots, bad veneer joints or seams. This material will would work well for drawer bottoms, and similar.

Now, onto CORE grades!

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:

Here is a description of the core choices that are available:

Core Types: Decorative hardwood plywood cores are mostly veneer (66 percent of the panels sold in North America), but may also be MDF (16 percent), particleboard (13 percent), or agrifiber and other materials.

Ÿ Veneer core is made from multiple plies of veneer peeled from quality logs with limited aesthetic characteristics, and glued together with the grain direction alternating at right angles. Compared to MDF (medium density fiberboard) and particleboard, its advantages include higher strength, lighter weight (typically 70 pounds at ¾-inch thick), and excellent screw-holding properties. Disadvantages include potential variation in panel thickness; core textures may telegraph through the surface layer; and the edges must be covered.

Ÿ MDF core are wood chips steamed and refined to fiber bundles, bonded, formed into a mat, pressed and then sanded to a desired thickness. Advantages include a smooth surface, uniform thickness, and reasonable screw-holding ability. Disadvantages would be that it’s heavy (100 pounds at ¾-inch thick), not as strong as veneer core, and the perception that it is not real wood.

Ÿ Particleboard Particleboard cores are made from wood chips ground into small particles, bonded, mat formed, pressed and sanded to a desired thickness. Advantages include a smooth, uniform thickness and reasonable screw-holding ability. Cons would be that it’s heavy (100 pounds for ¾-inch panel), not as strong as veneer core or MDF, and again, the perception that it is not real wood

Ÿ Combination core plywood consists of two types of construction. The first has a center veneer layered with a crossband veneer on both sides and then followed by a layer of MDF, particleboard or hardboard, plus the face and back veneers. The second type of construction has a wafer-board center with a veneer crossband on both sides, plus the face and back veneers. Combination core offers consistent flatness and good screw-hold ability in a lighter weight than MDF or particleboard.

Ÿ Lumber core plywood is constructed from strips of solid lumber. Veneer is attached once the core has been surfaced. Lumber core plywood has good bounce-back, which makes it a good choice for constructing long or wide shelves in cabinets and closets. A more expensive plywood, it is diminishing in popularity.

It’s really no wonder why a lot of engineers hate wood as a medium, and I’m still kind of surprised FIRST still uses it as a fundamental construction material.

[Patrick3357]

And this only covers a few of the most common grades of plywood.

Baltic Birch follows a Russian standard, it is veneer core, but uses single piece, birch, core veneers, most plywoods use a cheaper softwood core even if the face veneers are hardwood, and layers can be made up of multiple veneers. The hardwood single piece core veneers are why Baltic Birch is so strong. Baltic Birch grades are largely aesthetic and, similar to the standard grades you listed, the common ones are (best to worst) B/B, B/BB, and BB/BB with BB/CP, CP/CP, and C/C being rarer, structural grades which allow high numbers of patches.

Then there my is favorite grade of plywood, BS-1088 which is a British standard of marine plywood very similar to Baltic Birch in that the core veneers are single piece and made from the same species as the faces, except that the species is generally a tropical hardwood which is even stronger than birch, though the species can vary. Note that marine plywood is not pressure treated outdoor plywood, they have almost nothing in common. Marine plywood has no chemical treatment to prevent rot but is very strong and the glue is very water resistant. Treated outdoor plywood is what you can get at a big box store and it will resist rot fairly well but it is no stronger than a typical construction plywood. BS-1088 also has a stamp from an independent certifier on every sheet. I don’t know where prices are right now, but when we bought wood 6 months after the a start of the Ukraine war the BS-1088 was cheaper than Baltic Birch due to Russian sanctions.

Even the typical American plywood grades get more complicated. Add an X to the grade and you get stronger “exterior” glue (still no rot resistance). The letter and number grades only apply to hardwood faced plywood, softwood plywood’s use letters to grade both the face and back veneers, hence CDX is one of the most common construction grades, but A1 might be used for cherry cabinets. Then there’s OSB, MDF, LDF, hard board etc which shouldn’t see bumper use ever, and a host of other plywood type products that manufactures produce to their own inhouse standards for various industries.

There are also aircraft grades which are fairly strong but lightweight, and usually made from spruce. I don’t know much about them.

For the purposes of FRC bumpers, teams should look for a high veneer count, zero core voids, and if you want a lot of strength, a hardwood core. Don’t buy hardwood plywood from Home Depot or Lowes, it will be an extremely thin hardwood veneer over a soft core. If you’re buying wood at a big box store, buy the highest grade of softwood plywood they have. If you actually want a strong bumper wood, find a dedicated lumberyard that carries BS-1088 or Baltic Birch. I don’t think solid wood is a great idea for bumpers, because plywood of the same species will hold screws much better split due to fastener holes or odd forces.

[gellnick]

If only a tree would grow 6000 series aluminum… then we wouldn’t have to worry about plywood grades, just which type of bumper backing can be milled and which is 6063.