Lightweight tubing for 650b?

[Nate H]

Hey all, sorry about the cross post with iBob... I'm looking into a new custom, integrated low trail, 650b randonneur and from reading the reviews of bikes in Bicycle Quarterly, I've become interested in lightweight tubing and what the BQ folks call planing. I weigh between 160 and 170 depending on the season and am more of a spinner. My question is if folks seem to think that lightweight tube specs such as standard sized .7/.4/.7 tubes and Kasei fork blades would be too light of a spec for a rider like me on that type of a bike? The bike will typically carry a 10-15 lb loads with the occasional 30-40 more pounds on the fork on lowriders for the weekend camping trip, light touring, etc. The lightest tubeset I've ridden prior to this was .9/.6/.9 Raleigh Competition I converted to 650b and it rode very well, but this new bike will be locked up on occasion which makes me think I should maybe split the difference and go a bit heavier than .7/.4/.7 with standard .8/.5/.8. Any insights y'all can offer—framebuilders and riders of lightweight tubed bikes especially, would be greatly appreciated!


Cheers!
Nate H
Denver CO

[David Parsons]

The first machine I made had a 7/4/7 DT (and maybe TT?) and ran for somewhere between 5500 and 6500 miles before the seat tube failed at the BB shell.   I'm 185#, and never had any trouble with the tubing (I managed to dent the TT during cleanup, and that dent never changed during the lifetime of the bike.)

The replacement machine is 8/5/8, because the first one failed AFTER Henry James ran out of 7/4/7 tubing, and I don't really notice any difference between how it rides and how I remember the first one riding.

I think the Kasei fork blades are .9 wall  (at least they are down at the tips of the blades), which is pretty standard.  They're a skinny oval, so they might have a little more side-to-side flexiness, but that shouldn't matter if the heavy loads are lowridered.

-david parsons

[Mark Bulgier]

David Parsons wrote:

>I think the Kasei fork blades are .9 wall  

Kaisei blades (note spelling) come in "Continental" wide-oval shape as well as the skinny oval, so you have to specify.

I got some "Toei Special" blades from Compass a year or more ago, haven't used them yet, so they may have changed the spec since then.  But mine are 1.0 mm thick.  That's still a good bit thinner than the classic Reynolds blades, which were 1.2 mm.

The Kaisei are impressively thin at the bottom, which isn't easy.  Tapering the outside diameter thickens them, so to end up thin at the bottom, they have to start as "taper gauge" (Reynolds's term) which is a kind of butting.  Very thin at the end that gets tapered down, so they end up hopefully somewhere around the same as at the top.  Reynolds was not perfect at that -- their taper gauge blades do still thicken up a little at the bottom, just much less than on cheap blades that are not butted before tapering.

That the Kaisei blades appear to not thicken at all at the bottom, despite being tapered down even smaller than most Reynolds blades, is a feat to be proud of.  17% thinner at the top than Reynolds, and thinner by an even larger percentage at the bottom.  Add to that the smaller diameter where they are raked, so that should make these noticeably more resilient in the raked part.  Everywhere else too, but it's the extra flex in the lower portion that really sets these apart.

I have heard that forks made with them will not pass the European bike industry crash-simulation test, but to me, that is one more reason to like them.  Forks should bend in a crash well before the frame bends.  So a frame with lightweight DT & TT should have a light (or at least weak!) fork too.  I think the Euro test is designed on purpose to fail high-quality steel forks, to make the cheap-ass forks the big manu's sell (and carpet-fiber forks) look good.  It is a ridiculous test, which is trying to make a virtue out of those forks' inability to bend. I guess you could say their greatest weakness is being too strong.  Crabon forks do snap off sometimes, but they can't really bend.

 

This poor schmuck was off the front solo and looking likely to win a big pro Classic, the kind of thing that can completely make your career.  Then his fork blades snapped off.  He's under contract and not allowed to say, but there are reports that the fork broke and then he crashed.  The blades snapping caused the crash, not vice-versa.

Those forks pass the Euro test.  Which would you rather have, those, or a custom made by a seasoned artisan using the Kaisei blades, that don't pass the test?

I know the hatin' on carbon is a well-worn and tiresome trope around here, I just bring it up as explanation for why the Euro test is garbage, and safe to ignore.  Don't let it stop you from getting a nice resilient steel fork.

Mark Bulgier

Seattle

[John Hawrylak]

From the Compass site for the Kasei Toei special fork blades

"Diameter at tip: OD 13 mm; ID 11 mm"

The wall at the tip would be 1.0mm thick

John Hawrylak

Woodstown NJ

[Mark Bulgier]

Not necessarily.  You just added an extra significant digit.

I would agree with you of the specs were 13.0 and 11.0 respectively.  But without the extra digit of precision, the Compass spec would remain the same if the wall thickness was really 0.9 mm.

But the fact that the pair I have in hand measure 1.0 mm thick does add credence to your theory.  ;)

-Mark

[Evan Estern]

I weigh between 170 and 175 and ride a 7/4/7 (top tube) Rawland Stag with a Kaisai Toei Special fork by Jeff Lyon.  I love the flex and suspension the thin tubing provides and for me at least there is no downside.  No shimmy and the bike is very stable on high speed descents.  On climbs and sprints my lungs give out well before my legs, which I take as a sign that the bike is giving back in all the right ways.  I say go for the thinner spec stuff.  I've never put more than 10 pounds or so in the handlebar bag, though I do sometimes ride with a heavily loaded messenger bag.  Zero issues there, but I can't speak to your low rider question.  If I ever get a custom bike (hope to someday) I'm sticking with this tubing spec.  YMMV of course. 

[John Hawrylak]

Mark

The extra digit is provided, X.X.  The correct term would be the specified accuracy of the OD and ID, e.g. if the ID had an accuracy of +/-0.1, then a 0.9mm wall is possible.  It is good to hear the actual, measured wall is 1.0mm

John Hawrylak

On Saturday, October 7, 2017 at 8:18:55 PM UTC-4, Mark Bulgier wrote:

[Mark Bulgier]

Yes that's right, Compass doesn't specify a tolerance.

The actual tubes will vary a bit, and my sample size of one is not enough to say anything definite, but I think it's safe to say they are "around 1 mm thick".  That "around" may well stretch to include 0.9 mm, since that was the measurement David Parsons got.

That range of variation would not make a noticeable difference though.  Human perception is usually not good enough to pick that difference out from the noise.  There are always a few outliers, "savants" if you will, who can reliable detect a difference that small, but for the vast majority*, more precise measurements over a larger sample size are not needed.  I trust Kaisei to keep the tolerances fairly tight, as bike tubing goes.

*many more people will think they can feel it than actually can.

-Mark