At the risk of getting myself in trouble, I'll share my ideas on why frames do make a difference in ride quality. Be advised that the last time I wrote on this subject it seemed to cause a little horror amongst the tech wizards. I'd imagine that anybody who follows cycling tech talk these days has read or heard about how little vertical compliance can be found in ANY double diamond configuration frame, regardless of material, and I believe that to be true. But, and it's a big BUT, I don't think it's a frame's vertical compliance that is the issue. We all realize that the rider feels road feedback through the points of contact on the bike, namely, the bars, the saddle, and the pedals, and it's what is happening at the pedals that I believe is the key.
I'm going to try to keep this short as I can, so I may gloss over some points here or just as likely to leave out something critical, but will fill in the blanks should there be any and if there is interest...note that I'm talking here in terms that would compare two bikes of equal geometry, their stiffness being different and not their wheelbase, chainstay length, angles, etc.
As I see it, the problem with citing the lack of vertical flex in a frame as an argument that materials or the "frame" doesn't matter is that a bicycle in motion is never truly vertical, it is always oscillating from side to side. Compound this with the fact that the frame and fork spans the gap between the two points of contact with the ground and that frames that are generally classified as "comfortable" also tend to be laterally flexible, i.e., see significant lateral cyclic movement at the bottom bracket under load. Again, as a rider sits on a bike, he carries his weight on his hands, his bum, and his feet. You carry significant weight on your feet, more so under heavy effort. Remove your feet from the pedals while under way and you'll note a significant increase in saddle pressure. As a rider applies pressure to the pedals he unweights his body a like amount. The pedal is essentially a lever attached to the crankarm (another lever) which loads the bottom bracket, which will deflect with lateral movement (side to side) but it is important to note that the pedal will also realize downward movement not associated with the rotation of the cranks. Perhaps an easier way of explaining is to simply sit upon a bike as you would under way, hold the brakes, and then apply moderate pressue to a pedal. You will note that the bottom bracket will move laterally and the pedal will drop relative to the BB movement (movement not associated with taking up slack in the drivetrain).
Again, you carry a significant load on your feet while cycling and when you are under way, the pressure you put into the pedals causes the BB to deflect (I'm ignoring other aspects here) but the bottom bracket ALSO flexes in response to impacts from road while the pedal is under load. The stiffer the frame, the more you're going to feel that feedback through the pedals, the more flexible the frame, the more the frame will deflect to disperse the shock. Yes, the BB is going to spring back once the load is removed, but I think the process reduces the amplitude of the shock and while the difference may not be huge, I believe they are cumulative. In effect, the lateral flex of the frame provides some suspension effect, isolating the rider from some amount of the road shock and I think makes the difference between what folks would call a "comfortable" frame versus a "stiff" or "harsh" frame. I thought I should put some gratuitous classic content right about here, but Dale would see through it anyway, and besides, all the above applies to classic bikes.
Cheers! Don Ferris Littleton, Colorado
-----Original Message----- From: firstname.lastname@example.org [mailto:email@example.com]On Behalf Of robert perkins Sent: Tuesday, May 21, 2002 8:48 PM To: firstname.lastname@example.org Subject: Re: [CR] Ride Quality
Nut in zis contry! The German TOUR magazine did a very interesting test a
few years ago. Read my translation on Google Groups. Bottom line- Going
from the noodliest ultralight steel frame to the stiffest aluminum frame had
the same effect as changing your tire pressure 10 psi. They concluded this
by measuring impact on a rider with sensors.
If the URL doesn't work search for "Frames Do Influence Comfort, According
to TOUR tests"
> Message: 11
> Date: Tue, 21 May 2002 09:17:59 -0700
> From: cnighbor <email@example.com>
> To: "firstname.lastname@example.org" <email@example.com>
> Organization: Pacific Bell Internet Services
> Subject: [CR]Ride quality
> We talk about the ride quality of steel versus aluminum versus Ti versus
> whatever. My question is this has anyone every seen or done a
> quantifiable study on how air pressure or a change of tires or tire
> width affects ride quality. I know form experience that just lowering
> the air pressure 5 psi can turn a rough ride into a smooth ride and
> going from say a 25c width to 28c width does the same. And with the
> contact area of the tire being nearly equal for both widths does it
> matter if one go to a wider tire.. So how does evaluate a frame ride
> quality when tires and pressure affect the decision process so much.
> Charles Nighbor
> Walnut Creek, CA