Question about Bumpers...

[Razer0901]

Hello Everyone,

I first want to apologize for another bumper question. This time it is about the material and wood specifically.

In the rule book it states that we have to use 3/4 nominal by 5 inch +-1/2. We didn't read carefully enough the first time and purchased wood exactly 3/4 thick (1 inch nominal), 5 inch tall (cut from 6 in since 5 inch wasn't exact) wood. I am not worried about the "5 inch tall specification," but instead the thickness. Is our wood that is exactly 3/4 inches legal or do we have to purchase 3/4 inch nominal wood? (We can't return the wood because it was cut already...)

Thanks!

-Razer0901
P.S. I am sorry if this is in the wrong place. I really should post this in a one question thread if CD has one, but I couldn't find it.

[Richard Wallace]

This question is a good one for the official Q&A.

3/4 inch thick (nominal) plywood really is very close to 3/4 inch thick.

3/4 inch thick (nominal) solid, robust wood may be thinner, as you pointed out.

My understanding is that the actual thickness of the plywood or solid, robust wood must be 3/4 inch, or very close; e.g., 23/32".

As a side note, good quality 3/4 inch plywood is more impact resistant than many kinds of solid wood.

[David Brinza]

R21 (Bumper construction) states:

A. be backed by ¾ in. (nominal) thick by 5 in. (± ½ in) tall plywood or solid, robust wood

Unless you are using balsa wood, you should be just fine.

[jgerstein]

Your wood is fine.

The 3/4" nominal refers to 3/4" plywood - it's specified that way to make it clear that 3/4" plywood is okay, even if it's very slightly off due to sanding or other manufacturing processes.

[Razer0901]

Thanks you for your replies!

Hopefully we won't have to remake bumpers at our first competition...

-Razer0901

[Al Skierkiewicz]

It seems that the OP is stating they bought 1"x6" boards, that measure 3/4" thick and ripped them down to 5" wide. That still fits the requirement.

[philso]

If you purchased "regular" boards and cut them to 5" wide, I would suggest replacing them with 3/4" BC sanded plywood (23/32" is equivalent). I understand that the rules allow "solid, robust wood" but our experience has been that it is hard to make good bumpers with anything other than plywood.

The regular boards have grain only in one direction. Depending on how you are constructing your bumpers, the grain only going in one direction may lead to splitting if you are driving screws into the wood near the ends. Impacts may also cause the wood to split where a screw was driven in near the ends. Once the wood split, we found it very difficult to repair effectively.

I specifically recommended the BC sanded (both sides) plywood over the cheaper sheathing grades (unsanded both sides) because it seems to be made from more layers of better quality wood and tends to have fewer voids. This seems to make it much less likely to split when screws are driven in near the edges and screws seem to hold better (don't strip the hole so easily).

Lowes sells half sheets (4' x 4') of 23/32" BC plywood for just over $20. The half sheet should be more than enough to make two sets of bumpers and can fit in most mid-sized SUV's. Home Depot used to sell it too but I have not been able to find any that carry it in the last couple of weeks.

[MrBasse]

Originally Posted by rtfgnow

FYI 24/32" is equivalent to 3/4". If you need an exception due to availability (such as teams in metric dominant areas) you should email FRC team support.

An exception for what? 1/32 of an inch is where the term "nominal" comes in. One of the only ways to get actual 3/4" ply is to buy the ridiculously expensive finish plywoods. It would be nice to be rolling around with a fine oak or maple veneer on our bumpers though... but at $50 a sheet I'll pass.