Downtube size affect on stiffness / comfort

[Deluz]

Does the size (diameter) of a downtube affect frame stiffness or comfort in a meaningful way?
I have a custom steel bike with 853 OS tubing that has a 1.25" diameter downtube.
I am going to have another frame made and it was suggested that going to a 1.375" downtube would make the bike more responsive.
I am 140 lbs and 67 years old and ride mostly at moderate pace so I don't generate a lot of power but do like climbing out of the saddle.
In reading David Kirk's excellent article he suggests downtube size has little effect on drive train stiffness and it is the chainstays that matter most.
I am a little concerned that going too large on the tubing would take a away the feel of steel that I love.
Thanks for any feedback.

[claritycycler]

Does the size (diameter) of a downtube affect frame stiffness or comfort in a meaningful way?

....In reading David Kirk's excellent article he suggests downtube size has little effect on drive train stiffness and it is the chainstays that matter most.

Unfamiliar with the article but does he mean downtube size has no effect on frame stiffness or just that its drive train is not effected by dt size. There is a difference between frame stiffness and chain stay stiffness, right? At your age/weight and probable power output I don't think it matters much if the frame has at least decent size chainstays(this coming from a 77yo "not a strong rider" big guy perspective)

[Deluz]

Unfamiliar with the article but does he mean downtube size has no effect on frame stiffness or just that its drive train is not effected by dt size. There is a difference between frame stiffness and chain stay stiffness, right? At your age/weight and probable power output I don't think it matters much if the frame has at least decent size chainstays(this coming from a 77yo "not a strong rider" big guy perspective)

I believe he is saying DT has little effect on drivetrain stiffness. It most likely has an effect on over all frame stiffness.
Here is the article: http://kirkframeworks.com/resources/...tay-stiffness/

[TonyP]

I don't know how much the down tube alone affects the ride, but I do know that at 70 years of age I'm done with oversized tubing. I've got a coupla bikes built with oversized tubes, a Boulder All road and a Della Santa, and as much as I enjoy those bikes I much prefer the ride of standard sized (1" TT-1 1/8" DT) tubing. In fact I'd rather lighter gauge also, 8-5-8 rather than 9-6-9. In fact I'd like to try 7-4-7 tubes but I've been building my bikes and don't trust my skills enough to try brazing the 7-4-7 tubes.

My thoughts on the subject are stronger riders benefit from stiffer frames than do I do. When I was younger and stronger those oversized tubed frames were the cats ass, but as I've aged they are now almost counter productive. It seems, to me, that without the strength I had the stiffer frames aren't as lively as they once were.

Can you find a bike to try before plunking your money down on a custom job? You know what you like and changing that up may, or may not, work to your advantage.

Just my thoughts. Like I said I'm 70yrs old, also 6ft and 200lbs and think that the advantage is with standard tubing.

[Andrew R Stewart]

I would choose the 1.25" DT for myself (and have even on my touring bike) unless you were planning to carry sizeable loads routinely.

I feel there's way too much energy spent talking about frame stiffness so I won't go there.

I do question what definition the word "responsive" is being used in this discussion. My take on that word has to do with ability to change direction or accelerate more rapidly than otherwise. Nothing to do with any claimed stiffness, handling response is more about geometry and wheel weight IMO. Andy

[Deluz]

I would choose the 1.25" DT for myself (and have even on my touring bike) unless you were planning to carry sizeable loads routinely.

I feel there's way too much energy spent talking about frame stiffness so I won't go there.

I do question what definition the word "responsive" is being used in this discussion. My take on that word has to do with ability to change direction or accelerate more rapidly than otherwise. Nothing to do with any claimed stiffness, handling response is more about geometry and wheel weight IMO. Andy

The way I think of responsive is that it instantly responds to power input being applied to the wheels. I know this sounds a little vague and is not the only meaning for responsive.
For now I am sticking to 1.25” DT as I don’t see a clear reason to go larger.
Thanks for all the responses so far.

[TRuth]

Does the size (diameter) of a downtube affect frame stiffness or comfort in a meaningful way?
I have a custom steel bike with 853 OS tubing that has a 1.25" diameter downtube.
I am going to have another frame made and it was suggested that going to a 1.375" downtube would make the bike more responsive.
I am 140 lbs and 67 years old and ride mostly at moderate pace so I don't generate a lot of power but do like climbing out of the saddle.
In reading David Kirk's excellent article he suggests downtube size has little effect on drive train stiffness and it is the chainstays that matter most.
I am a little concerned that going too large on the tubing would take a away the feel of steel that I love.
Thanks for any feedback.

If I weighed 140 and rode mostly at a moderate pace I'd be thinking of downsizing rather than upsizing the DT. I'd drink all the Jan Heine Kool-Aid and get a frame made out of standard sized, thin tubing.

[steve garro]

Define "Responsive."

It WILL make it stiffer and more resistant to torsion by the pure fact that it's of larger diameter, it really ramps up.

If you called me I'd likely think you could run the same walls you have been but down to 7/4/7 = 140 lbs is light !

- Garro.

[Deluz]

I believe this is the down tube on my current bike:

REYNOLDS BX2103 DOWN TUBE "853 PRO TEAM" 31.75X0.65/0.45/0.65X650

The new frame will be using Spirit / Life. The closest one to above is 0.7 / 0.45 / 0.7 (SLFM12630)

One thing I noticed is on my current bike the chain stays are crimped quite a lot. They start at 22mm at the BB and are crimped to about 14mm for the rear tire clearance.
From what I have read this compromises the drive train stiffness. I had no idea about this when I ordered the frame so I will be paying more attention to this on the new frame.

[shand]

I see this kind of question come up quite a lot and I always think it's only half a question without context. As Steve says a bigger tube will make the frame stiffer*. It just will. That's stiff in power-transfer and also stiff in 'comfort'. Whether you will notice any of that is dependent on a lot of other factors, that's what I mean by context. Put 4" knobby tyres at 18psi on there and I doubt you could tell the difference. 19mm tyres at 110ps? Different story. Wheel stiffness will also play a huge part. Crank stiffness will also play a big part in power transfer/loss. My feeling is that a thru-axle frame vs QR will have more of an impact in rear-end stiffness than tube diameter.

Another element I find interesting is wondering about where that energy 'loss' of a flexible rear triangle/drivetrain goes. A long, long time ago, I remember running tests on some very thinwall, lightweight tubing (think it was Prestige Ultralight or something like that). We found that although those tubes did deflect a significant amount under load due to the thin walls, they did seem to return to original position very quickly. This led us to try and think we could quantify the characteristics of some of those early steel Prestige frames feeling springy or lively, a feeling that a lot of people talked about. So although the frame deflected under pedal load, it rebounded quickly enough to contribute to the next pedal stroke on the opposite side. I don't think we were actually able to prove or measure that characteristic but it sounded cool to talk about it!

*Also remember that often the point of increasing diameter is to be able to decrease wall thickness and so decrease weight but keep stiffness the same. So unless wall thickness is consistent, more stiffness is not guaranteed my a bigger diamter.

[steve garro]

The new frame will be using Spirit / Life. The closest one to above is 0.7 / 0.45 / 0.7 (SLFM12630)

If your builder is qualified to use the thinnest tubes you can buy on the planet, I'd trust their take on tubing, but a 140lb person is right in the crosshairs of what that tube was designed for.

- Garro.

[Scheisserad61]

Another thng to keep in mind is that the downtube the O.P. is referencing (reynolds .65 x .45mm ) is a specification that often does not come close to meeting reality. In my experience tubing manufacturers sometimes have a hard time keeping to plus/minus .1mm, and that last 0.05 mm is utterly laughable imaginary horseshit. Maybe, just maybe, Tange (when they were still in Japan) could have hit those numbers semi-reliably when they were making the original Prestige Superlight tubes. Other companies ? No way . . .

All the tubing companies state (supposed) wall thicknesses but they conveniently seem to neglect any sort of plus/minus specs that are always a part of drawn tubing.
Don't forget that tubing also sometimes doesn't have the actual lengths of butting that the specs claim when you ordered it. The catalog or the distributor lists the wrong thing and the postage / trouble that it takes to return things
makes it unpleasant to try and fix.

We're not talking aerospace quality-control and testing, fer chrissakes. Remember, this is the BICYCLE industry. Bicycle. Small potatoes in many ways and ask yourself how much the ultra-thin steel tubing market is going to expand in the coming ten years or so.

I've heard many riders claim that their bike rides a certain way because it has "X" thickness downtube or toptube or whatever. IMHO they couldn't tell the difference between a piece of Home Depot conduit and
whatever nonsense they think their frame is made of. This is especially laughable when riders quote inaccurate and misleading catalogs from 35 or more years ago.

Sometimes you have to back up and realize that the frame is a CONNECTED series of elements, that it doesn't make a whole lot of sense to attribute how an entire frame or bike will change from just ONE piece.
Hopefully the framebuilder you choose will balance the design you request to meet the perceived needs, along with what is realistic to actually manufacture without having durability questions.

Finally, good luck with whatever you end up with and keep riding ! The real joy is in the activity, not so much the equipment ! (within reason, of course . . .)