Relative Flexibilities for a Variety of Top and Down Tubes

[John Clay]

Forgive the cross post but I think this information might be useful to some of the list readers.

I generated a spreadsheet analysis (attached) in order to determine the relative flexibilities in the range of steel top and down tubes that have been used in the past 50 or 60 years. The tables that appear below contain the summary information.

I used torsion alone as the proxy for general flexibility. The butts were modeled as steps located at mid-taper. Thats reasonably close to reality and it made the calculations simple & quick, and the results good enough for the purpose. Most of the tubing dimensions came from True Temper. For the top tubes the lavender shaded columns are comparisons wrt 25.4/969 tubing. Aqua columns are relative to 25.4/747 tubing. Same drill for the DT section that follows except it's against 28.6 tubing.

I seem to recall, but haven't been able to find again, Jan mentioning that for he & his crew planing occurs at higher power levels; something in the range of 500 or 600 watts. And the frames that plane for them are often made of 747 conventional diameter tubing. That suggests to me that planing might not be available to those of us who operate at far more modest power output levels though I've never built or ridden a frame from thin wall tubing.

Irrespective of the subject of planing and fatigue reduction this information should allow folks to make informed decisions when attempting to deviate from a known frame tubing selection.    

Many thanks to Jamie Swan and Alistair Spence for their reviews of the work.

			Assembly Weight	Weight Change (g) wrt 969 Conventional	% Weight Change wrt 969 Conventional	% Weight Change wrt 747 Conventional	Angular Displacement (deg)	Relative Increase in Torsional Flexibility wrt 969 Conventional	Relative Increase in Torsional Flexibility wrt 747 Conventional
25.4/747 TT			213	-74	-26%		8.0	39%
25.4/858 TT			250	-37	-13%	17%	6.6	16%	-17%
25.4/969 TT			287			35%	5.7		-28%
28.6/747 TT			241	-46	-16%	13%	5.5	-3%	-30%
28.6/757 TT			265	-22	-8%	24%	4.8	-16%	-40%
28.6/858 TT			283	-4	-2%	33%	4.6	-19%	-42%
28.6/969 TT			324	84	13%	52%	4.0	-31%	-50%

Down tube, relative stiffness calculation results.

			Assembly Weight	Weight Change (g) wrt 969 Conventional	% Weight Change wrt 969 Conventional	% Weight Change wrt 747 Conventional	Angular Displacement (deg)	Relative Increase in Torsional Flexibility wrt 969 Conventional	Relative Increase in Torsional Flexibility wrt 747 Conventional
28.6/747 DT			241	-84	-26%		5.5	40%
28.6/747 DT Compass			226	-98	-30%	-6%	5.9	48%	6%
28.6/858 DT			283	-42	-13%	17%	4.6	16%	-17%
28.6/969 DT			324			35%	4.0		-28%
31.8/747 DT			268	-56	-17%	11%	4.0	1%	-28%
31.8/757 DT			295	-29	-9%	23%	3.5	-12%	-37%
31.8/757 DT Compass			286	-38	-12%	19%	3.6	-10%	-36%
31.8/858 DT			315	-9	-3%	31%	3.3	-16%	-40%
31.8/969 DT			361	93	11%	50%	2.9	-28%	-48%

John Clay
Tallahassee, Florida
USA

[John Clay]

I posted an update over here: https://groups.google.com/g/framebuilders/c/QaOg8c-17eE

It is my attempt at ranking the relative effects on frame flexibility by various combinations of top tube and down tube.

Something that I haven't seen discussed, and that I haven't added to my spreadsheet, are evaluations of and contributions that could be made by double butted seat tubes and 86 wall single butted.

John Clay

Tallahassee, Florida

[Murray Love]

John, as a curiosity, have you compared the flexibility of butted tubes with plain-gauge tubes of the butted tube's belly diameter (that is, say, 969 vs 0.6mm plain-gauge)? I've always worked on the assumption that the differences would be relatively small, due to a butted tube effectively acting like springs in series, but have never bothered to confirm, so I may be way out.

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[Jared Hardinger]

Is torsion as relevant as pure bending? I have some free-time coming up, I might throw this into ANSYS and run some FEA on a typically-sized main triangle. 

[Steve Chan]

   Thanks for posting this, its a very interesting summary.

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[John Clay]

Hi Murray - No, I haven't but I did roll my own specification ("JMC Columbus Idea", in the summary, below).

I revised the spreadsheet to reflect the butting and length dimensions of various tubes after sizing them to fit a frame for me (rather than using uncut dimensions as I did before), adding a seat tube sheet and then including my hopefully only crude (as opposed to outright incorrect) attempt at gauging the result of several combinations if worked into a main triangle, all with respect to a 969 frame of the same size; I didn't use center to center dimensions either but rather the in/in distances from OD of lug to OD of lug. The new file is attached and the main triangle summary appears below. The seat tubes appear short simply because the portion containing the seat post doesn't flex appreciably and so a portion for that was left out.

I was curious to know if there might be some combinations of existing tubes by makers other than RH/Kaisei that would yield similar results, and how close some with 0.5mm center sections might get to the RH superlight performance. It's clear that the 747 RH conventional diameter superlight tubeset sets the standard for most flexible (amongst my limited survey) but there is a good spread of existing alternatives that populate the spectrum between conventional 969 and the RH superlight. With all three main tubes included, as opposed to my previous two tube evaluation (TT & DT only), finished front triangle flexibility in the RH Mule and OS ranges look feasible from existing Columbus offerings (and probably others for which I had no dimensional data on hand) and with a little more apparent balance in contribution to the whole, by each tube; I don't know if that matters but I my hunch is that it would aid longevity by keeping the peak stress of each tube nearer the average stress of the three. Thin wall, double butted STs are necessary to approach the RH superlight flexibility but it's interesting to me that with minor changes it looks like one could get fairly close with conventional diameters, butts on the short side of what's common and 0.45mm center sections; something that appeals from longevity, dent resistance and balanced stress perspectives; in the carbon age it's probably not worth a manufacturers $$ to do so.

Jared - That would be very interesting! Give it a go if you can. In some versions I used plain gauge models of each tube, at an average wall thickness, to see the order of precedence changed; but it was a seriously crude approach. Structural analysis was not part of my engineering career and I haven't the mojo to wade through the material that would be required to do a proper job of that. This was an attempt at determining the order of various tubes or combinations and I think that torsion alone is adequate to the task; a structural analysis by someone expert in the field would be interesting so I encourage that.

Estimated tre tubi flexibility increase of various combinations wrt Columbus Cromor 969
				% Increase In TreTubi Flexibility wrt Col Cromor Std 969
	TT	DT	ST
RH Superlight	43%	36%	41%	40%
RH Mule	43%	-11%	41%	25%
Columbus Mixture	22%	20%	38%	26%
	SL	Kerin	Spirit
Columbus SL	22%	15%	2%	13%
RH OS	1%	-11%	41%	10%
JMC Columbus Idea	35%	33%	26%	31%
	JMC	JMC	JMC

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John

[John Clay]

The tapers are now modelled as 1/3 of their length being the thick end wall thickness, the remaining length being the center section thickness.

John Clay

Tallahassee, Florida

[John Clay]

I made an additional mix from Reynolds offerings at Torch and File, results are on the last row of the summary chart. It looks like another currently available alternative for a superlight, super flexible, frame.

John Clay

Estimated tre tubi flexibility increase of various combinations wrt Columbus Cromor 969
				% Increase In TreTubi Flexibility wrt Col Cromor Std 969
	TT	DT	ST
RH Superlight	43%	36%	41%	40%
RH Mule	43%	-11%	41%	25%

Columbus Mix

22%

20%

38%

26%

Reynolds Mix	50%	23%	36%	36%
	AX2000	BX5049	CX2210

[John Clay]

Let's try that again:

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John