What CrMo tubing size(s) for racks?

[satanas]

I'm going out to the local racetrack later this week to buy some rack tubing from these guys: http://racetechsteel.com.au/4130-chromoly/round-tube butam not sure whether to get 6.4mm tubing or 7.95mm, or what wall thickness. The racks will be brazed, and most likely painted (plating would be a hassle). I'm mainly interested in making a small rack for a handlebar bag, but the place is hard to get to so I'm thinking I will stock up.

What do you guys think would be the most useful sizes? I'm thinking 6.4x0.7 or 0.9 and 7.95x0.9 should do the job, but suggestions are welcome!

Thanks,
Stephen

[Thomas]

Stocking up... does this imply other sizes of racks, other functions?

I see the 7.95mm tubes thinnest wall is listed as .09mm, IMO,
more/thicker than you'd need for small rack/handlebar size... the
6.4mm X .07mm could be O.K. for light duty... If I went that
direction for rack material, I like the 9.5mm X .07mm

However, you could also go here:
http://www.aircraftspruce.com/catalog/mepages/4130tubing_un1.php
and get some similar sized 3/8ths cro-mo @ .058, or .049, or .035 for
just over $3/foot

Thomas
www.kokoPedli.com

[M-gineering]

On 11/24/2014 5:36 AM, Thomas wrote:
> Stocking up... does this imply other sizes of racks, other functions?
>
> I see the 7.95mm tubes thinnest wall is listed as .09mm, IMO,
> more/thicker than you'd need for small rack/handlebar size...

Unless you know what you're doing I would stay away from tubing with a
diameter /wallthickness ratio greater than 50:1. This tubing runs at
135, so it will be impossible to bend, very prone to beercanning and
razor sharp to boot.

Stick to imperial sizes ;)




--
mvg

Marten Gerritsen
Kiel Windeweer
Netherlands

[M-gineering]

Look at your tooling first, the greater the wall thickness / diameter,
the more likely you will be able to bend it without kinking.
1:20 is nice from a structural view, but not if you don't have a
dedicated mandrel bender, much better to choose a less efficient tube to
get around your fabricating problem

[Thomas]

Thanks, Marten! (You always seem to have knowledgeable/good things to
say, I appreciate your comments very much!)
I thought/knew something felt wrong about what I was saying, but I
forgot just what it was!!
The listed tube is 7.95 X .9mm! not .09mm !!! (so it is closer
to a 9:1)
~ T

[Thomas]

Now I'm ALL confused!! I think their web-site
http://racetechsteel.com.au/4130-chromoly/round-tube
has a mis"print" in/on it...
I'll shut-up now before I confuse me or you any more
I'm ignorant here ...
T

[Michael Jenkins]

For handlebar bags, 1/4 x 0.028" works well enough. 3/16 dia. roll pins, rod or tube is useful for butt joints giving about the right clearance for brazing.

Mike Jenkins

Barrington, IL

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Mike Jenkins
Barrington, IL

[Jim G]

I use 5/16"x0.035" tubing for racks.  You can use 1/4" tubing for a small handlebar bag, or 3/8" tubing for a stout porteur rack.  5/16" is a good, all-around size.

Alex W. has written a great blog post on tubing selection for rack fabrication -- see http://alexwetmore.org/?p=501

Do you already have a tubing bender?  If so, what size tubing does it work with?

-Jim G

[Alex Wetmore]

​Just to translate 6.4x0.7 is 1/4" x 0.028".  That is plenty strong for a handlebar bag and allows you to build a light rack (300g is easy, <200g is completely do-able).  The only downside is that it is harder to get nice bends in 028 (0.7mm) than 035 (0.9mm).

You could use 0.9mm for the rack deck, and 0.7mm for the legs and anything else not being bent.

alex

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From: frameb...@googlegroups.com <frameb...@googlegroups.com> on behalf of Jim G <yoj...@gmail.com>
Sent: Monday, November 24, 2014 11:08 AM
To: frameb...@googlegroups.com
Subject: [Frame] Re: What CrMo tubing size(s) for racks?

 

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[Hahn Rossman]

Stephan-

I know you are planning on doing PBP. Unless you have some special rack in mind it's really hard to beat a Nitto rack. I thought they could be improved in some ways, so now Nitto is making some special models for Compass/ Grand Bois. You can go look at them on the Compasscycles website. We will also have one that fits Mafac raid centerpull brakes( or the new Compass brakes)! 

Let me know if you have any specific questions.

Hahn Rossman

[satanas]

First, thanks to everyone for responding.  :-)

I'll respond to the comments made below:

1. I do not have a bender to suit, but have made racks before and bent the parts using either coaster brake or three speed hub shells as formers - crude but effective! I've generally used 0.9mm/.035" wall tubing before as that's what was available. FWIW, bending 1/4" x .5mm/0.020" SS tubing using a hub shell was okay, but the tubing suffered from deformation over time where the rack hooks attached (due to creep?).

2. With 531, tubing used to be available in 1/8" increments, so 1/4"(?), 3/8, 1/2, etc, all with a 0.9mm/0.035" wall. 3/8 or 1/2 x 0.9" is basically bombproof, and super rigid, but not necessarily super light. My experience with mild steel is that while it's stiff enough, it bends way too easily, for instance if the bike falls over.

3. Hahn, I'm aware of the Nitto racks, but the crown I ordered has mounting points on the top (4 point fixing) and I'm not so sure any of the Nitto racks will suit. I could buy another crown, but would prefer not to. And really, the only problem with making a front rack is time, and that I have. I'll have another look at the Compass (and Riv) websites but it might be fun to make something anyway.

4. It's much less trouble for me to source CrMo tubing locally here in Sydney than to have it posted from overseas, though I had considered doing that in the past.

Thanks again,

Stephen