's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

Frame Geometry(7 posts)

Frame Geometryjagreenwald
Dec 31, 2002 11:26 PM
I have been trying to learn more about how frame geometry affects ride (and of course frame material). Anyone have some info on books or (preferrably) a web-sites that have info on this?
Jan 1, 2003 7:10 AM
Here are some sites that primarily discuss steering geometry. The Anvil website goes a bit further.

Fitting info is available on a large number of web sites, including:

Unfortunately, few generalizations can be made regarding the effect of frame material on "ride". Any material can be made into a very stiff and harsh riding bike. Aluminum will usually be the harshest riding material because the tubes must be oversized and shaped to reduce deflection that would result in early fatigue failure.

As for the effect of "geometry" on ride, the narrow range of seat and head tube angles used on bike frames isn't going to make much difference in a bike's ride. Longer chainstays (that you only see on touring bikes) can soften the ride a bit. Exceptionally short chainstays that require a curved seat tube will will make the ride harsher.

Oversized, ovalized and bladed downtubes in any material also contribute to a harsh ride.
re: Frame Geometrygtx
Jan 1, 2003 10:47 AM
I think it's pretty much all about geometry and has little to do with frame materials. Check out Sheldon Brown's bus analogy below.
re: Frame GeometryCaseysdad
Jan 2, 2003 8:52 AM
Very interesting and helpful article! Thanks for the reference.

Not to go digress far off topic, but I found it interesting to see that Mr. Brown is a proponent of the theory that no energy is actually lost due to bottom bracket flex because the pedaling energy is returned when the frame "un-flexes".

I'm still not entirely sure that I buy this... (Wasn't there a fairly lengthy technical discussion regarding this very point in this forum just a few weeks ago?) I'm no physicist, but it seems to me that any energy that goes into twisting the frame laterally is energy that isn't being transferred into the actual drivetrain to produce foreward motion. In order for that energy not to be lost/wasted, wouldn't the "returned force" created by the frame's un-flexing have to be somehow transferred to the chain/wheel and contribute to the bike's forward motion?
re: Frame Geometrygtx
Jan 2, 2003 8:58 AM
Ok, now you asked for it! Keith Bontrager wrote a huge article on this subject. Enjoy!
re: Frame GeometryCaseysdad
Jan 3, 2003 1:17 PM
Very interesting article. It does a better than average job of addressing technical issues in very accessible layman's terms. I was glad to see him (finally) get around to directly addressing this specific frame flex issue on page five.

If I correctly understand what he's saying, he essentially agrees that BB flex is a bad thing insofar as it takes pedaling forces that would ideally be applied to making the pedals go 'round (creating foreward movement of the bike) and "wastes" them by applying them to creating side-to-side flex in the frame. Granted, the forces that go into creating that flex are mostly returned when the frame flexes back, but since these lateral "return-flex" forces aren't contributing any energy to the forward/circular pedal stroke, the energy involved in BB flexing truly is wasted.

However, Keith took the argument a step further and introduced a consideration that I must admit I hadn't though of before. He points out that riders who are pedaling hard enough to produce BB flex are probably also weaving the bike from side to side - particularly if they're standing/climbing aggressively - so that they can place more of their weight/downward force directly over the pedal on the downstroke. This tilting results in a stronger downstroke, but it also means that the pedaling forces are being applied to the crankarms at a greater angle (as opposed to the crankarms being essentially perpendicular to the ground.) Naturally, this reduces pedaling efficiency slightly. Keith's argument is that the BB flex actually counteracts this inefficiency slightly by offsetting/reducing the angle created by this side-to-side tilting. Kind of interesting...

Of course, I think the bottom line is that we're ultimately talking about very small amounts of energy being diverted into any of these supposed inefficiencies and I agree with Keith that people's expectations and subjective perceptions affect things much more than the actual physics do. Which is good news for me, because now I can go ahead and buy that new ti frame that I've been lusting after all year without having to worry too much about it being too flexy and inefficient!
re: Frame Geometryeflayer
Jan 1, 2003 2:12 PM
Grant Peterson at Rivendell Bicycles has an interesting perspective on geometry.