Me and a friend are trying to build a trailer using principles found here ...


They are using 25 mm aluminum square tubing with a 2mm thickness and 6.5mm holes.

At the bottom of the docs is a bill of materials listing the materials.

It appears they are just drilling holes and than inserting a bolt and nut to connect the pieces.

Is this what they are doing?

More specifically does their need to be a washer on both sides?

  • A question about a plan on this bike building site has already been answered extensively on stackexchange. It was about building a tadpole trike with square Al tubing.
    – Carel
    Jan 10 '20 at 19:50
  • Can you link to the tadpole trike please ?
    – jennifer
    Jan 12 '20 at 13:50
  • The question is gone. It may have been closed.
    – Carel
    Jan 12 '20 at 16:15

Here is a nice clear picture of the basic joint used in constructing things from their website.

enter image description here

There is a washer on both sides.
Their bill of materials calls for stainless bolts, stainless washers and stainless self-locking nuts:

B O L T S & N U T S
M6 SELFLOCKING NUTS, stainless (DIN985) - ca. 150 pieces (TWOSEATER: 200 pieces)
M8 SELFLOCKING NUTS, stainless (DIN985) - ca. 5 pieces (TWOSEATER: identical)
M8 NUTS NORMAL, stainless - ca. 5 pieces (TWOSEATER: identical)
M6 and M8 BOLTS with Hexagon head, stainless, (DIN931):
6mm x 140 mm - ca. 2 pieces (TWOSEATER: 3 pieces)
6mm x 90 mm - ca. 15 pieces (TWOSEATER: 20 pieces)
6mm x 90 mm - ca. 5 pieces in DIN 933 (full thread) (TWOSEATER: identical)
6mm x 60 mm - ca. 60 pieces (TWOSEATER: 120 pieces)
6mm x 65 mm - ca. 10 pieces (TWOSEATER: identical)
6mm x 40 mm - ca. 25 pieces (TWOSEATER: identical)

8mm x 120mm, 2 pieces (TWOSEATER: identical)
8mm x 100mm, 2 pieces (TWOSEATER: identical)
M6 WASHERS - ca. 300 pieces (TWOSEATER: 400 pieces)
M8 WASHERS - ca. 10 pieces (TWOSEATER: identical)

Concerning galvanic corrosion:

Stainless steel fasteners in aluminium plates or sheets are normally considered safe, whereas aluminium rivets or bolts holding stainless steel parts together is an unwise combination, as there is a practical risk of corrosion.

An example of the safe use of stainless steel and aluminium together is where stainless steel fasteners and hold down bolts are used to secure aluminium roadway or bridge parapet guards. Even with no insulation between the metals, there should be little risk of corrosion.

A drill press will make life much better for a project like this.

  • thanks. I'm joining a makerspace in Austin to do this. Have you made any of these xyz space frames before?
    – jennifer
    Jan 9 '20 at 21:41
  • Even with stainless bolts washers and every thing, eventually there will be galvanic corrosion. On my motorbike I have a GPS bracket made from Al and stainless bolts. The bolts rust, the washer and the nuts don't. A metallurgist explained that to avoid corrosion all the stainless stuff and all the Al must be of twice the same alloy. Any differences in composition generate electric current, hence corrosion.
    – Carel
    Jan 9 '20 at 21:42
  • @Carel If there is any voltage source then the galvanic corrosion will be amplified. Interestingly it’s the stainless part that dissolves. I’ve experienced this first hand.
    – Eric S
    Jan 9 '20 at 22:00
  • @EricShain : the combination of Fe and Al is the voltage source. In fact it's a short-circuited battery.
    – Carel
    Jan 10 '20 at 12:18
  • Can the M6 washers be made out of plastic to at least isolate part of the connection?
    – jennifer
    Jan 10 '20 at 18:09

Yes, it looks like they're just bolting one piece of box-section aluminum to another. I would use a washer on both sides. They're using steel screws and (presumably) steel washers. To avoid galvanic corrosion, I'd use plastic washers to isolate the aluminum from the steel.

Also note that in the trike design, they're using a couple of pieces of plate at the front to mount the bottom bracket. This will also help prevent the tubes from racking, which will be important for your project.

You could join your corners using prefab corner plates; the bigger the plates and the more widely set the screw holes, the better they'll be at resisting racking.

  • Are there any m6 plastic washers you recommend particularly for this app. Strange they did not use plastic given these answers?
    – jennifer
    Jan 10 '20 at 18:10
  • Looking at McMaster.com, I see they have plain-old nylon washers, polycarbonate washers, plus others that are chemical/electrical/heat resistant. I don't think you need anything fancy. I'm pretty sure that for the project I worked on where we were joining Al parts with steel fasteners, we used plain nylon washers.
    – Adam Rice
    Jan 10 '20 at 23:17

Yes - there should be a washer on each side on the outside - Aluminium is a soft metal and a bolt head/nut will sink into it over time, effectively loosening your bolt.

I'd also expect there to be a small crush tube around the bolt, down inside the tube's square cross section.

This would be a good idea even if they don't mention it on the plan - will stop the tension of the bolt from making your square cross section into a figure-8 under pressure.

There would be 6 crush tubes per joint, two per bolt, in David's excellent image.

Also will help to reduce fatigue from vibration.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.