|A good reply to the ride quality debate............||Dave Hickey|
Jun 26, 2002 11:12 AM
|I borrowed this from rec.bicycles.tech. A poster asked if changing to a carbon seatpost would improve his ride quality. Mark Hickey(no relation) had the following reply.
"Here's how to determine if a part (including frame) is going to make
your ride more comfortable...
Isolate it (i.e. take the wheels off if you're talking about a frame).
Push down on it with all your body weight.
Measure the deflection.
You'll see that the tires are the big "mover". The stem and bars,
saddle come next.
You'll also notice that you can push like crazy on the frame, seat
post, and wheels and not get any "give". That's 'cuz they don't HAVE
any to give... don't try to "fix the ride" by replacing anything that
doesn't have "give"... since the replacement won't either (suspension
seat posts and frames nonwithstanding).
You might also notice that those "compliant" wheels don't "comply"
much when you take the tires off. Just see how much weight it takes
to compress 'em 1mm (1/25"). Let's hope that doesn't happen while
|I agree, for the most part||Nessism|
Jun 26, 2002 11:35 AM
|The frame flexes very little in the vertical direction, especially in the rear triangle. The front end does move around some although not nearly as much as the fork. In my experience, the fork does make a noticable difference in ride quality.
And how much the bars and stem flex is dependant on how highly they are loaded. If the rider has a good riding position with the elbows flexed, there shouldn't be all that much weight on the front end, thus not that much flex.
|Misses a key point||czardonic|
Jun 26, 2002 11:40 AM
|As I understand it, carbon fiber isn't prized for ride quality because it has "give" (i.e. it is flexy). It is prized becuase it damps vibration better than aluminum, but is lighter than steel.|
Jun 26, 2002 12:20 PM
|So are you saying................||Dave Hickey|
Jun 26, 2002 12:55 PM
|That a carbon seat post is going to make a difference in ride quality? I agree carbon frames dampen vibration better than aluminum. I have a real hard time believing carbon seatpost will help someone who is complaining about a stiff ride. I have a carbon frame and have tried both aluminum and carbon seatposts. I cannot tell the difference in ride quality.
The original post from rec.bicyles.tech was from a rider who's bike beat him up after 50 miles. He wanted to do a double century and he wanted to know if a carbon post would make his bike ride better.
I still think Mark Hickey's reply was right on. If you want to improve ride quality, tires, saddles and bars are going to have the biggest impact.
|Maybe, maybe not. . .||czardonic|
Jun 26, 2002 1:44 PM
|It really depends on the amount of vibration that 350 mm of CF can absorb.
I don't disagree that tires, sadles and bars are probably a better way to reduce bumps and vibration. What I am saying is that the rec.bicyles.tech poster's method of testing the effectiviness of a CF post does not account for the way that CF is designed to work.
In other words, he's right for the wrong reason.
|agree with the spirit of the post, but(t)...||lonefrontranger|
Jun 26, 2002 1:59 PM
|On my Morgul Bismark Aero Al there **is** quite a difference between the new Easton carbon post versus the Al Thompson I had on there. The Aero Al is an unrelentingly stiff Easton 7005 TT/crit specific frameset. This is not imaginary; over a 50-mile road race, using the same wheelset (Zipp 303s with Vittoria CX tubs), it means the difference between having a raw saddlesore bum and stiff lower back, or not.
I only have about 4" of post showing (extended seat tube, non-compact frame).
|Engineers.... is there a difference between statics & dynamics?||PdxMark|
Jun 26, 2002 12:29 PM
|It's becoming often-stated that frames don't/can't affect the feel of a bike because they don't have anywhere near the static vertical compliance of tires. So, maybe we all just think different bikes feel differently, like my Ti bike and my Al bike with the same tires/wheels. It seems that we need a dynamic test set-up. How does this sound for a dynamic test set-up?
Put pressure sensor on the top of the handlebars with a slight load (1-2 pounds?) hanging from it in a rigid bar linkage. (Don't want a chain-type linkage because you'd what to keep contact between the load and the sensor) Go for a ride over a rough, broken up section of road with a computer recording loads over time. Switch to a different bike & repeat.
Would this work? Any ideas for improvement? Any suggestions for where I get a sensor like this?
It seems that the dynamic feel of a bike is what we all seem to discuss, rather than the feel of sitting on it in the driveway. Is there an engineering basis for the static/dynamic distinction, or is it all just smoke & mirrors?
Jun 26, 2002 12:53 PM
|Ok, I'll try my best to explain in normal words. Tire pressure affects the static load. It also will absorb the force of a bump. However, where frame/wheels/seatpost come in is in the dynamic part of the ride. Wherever you ride, you are recieve vibrations from where the bike meets the road. Vibrations travel in the form of waves. Now, this is where material selection plays an important role. Basically, carbon fiber and titanium "soak up" alot of road vibration. That means that it will chop the peaks off of the vibration waves, and leave you recieving waves that have less displacement (less noticable). While other materials will either amplify or just transmit these vibration waves directly to you. Still another factor is obviously design. Cannondale is a super example. Compare their 2.8 frame to the CAAD 5. Both are made of aluminum, even though different alloys. But the designs are worlds apart, as are the rides. So another factor is how the frame builder chooses to deal with the transmitted vibrations. Yes, you can measure vibration. So, the "push on it" test works for somethings. And tires are a huge help in taking the edge off of bumps. However, the "road buzz" is helped out by material selection and design. Of course, another way to help with road buzz is contact points i.e bar tape, saddle. So to be very long winded, you could test for vibrations from the road, making the frame a variable and everything else constant. Contact points included.|
Jun 26, 2002 4:35 PM
|Your proposed system won't really work. Essentially you're adding another mass/spring/damper system with it's own frequency response to an existing one. Essentially you're describing an accelerometer. Now knowing this you could use this simple subsystem to characterize the nature of the true system, but you're calculating and not measuring directly. Ideally you want to take measurements in such a way that the taking of the measurement doesn't have ANY affect on the system being studied. This is where things like strain gauges, lasers and optical methods come into play. It can be deceptively tricky to measure even simple mechanical systems and not skew the results. A light weight bicycle and a rider is not a simple system even though it may appear so in realtion to something like the Space Shuttle or even a car. I'm not saying it can't be done, it can and is, it's just that you need more technology than you might think to get any meaningful results. |
Now for the second part of your question. Statics and dynamics are related but different areas of study. Statics is typically applying a force and determining the deflection and limits of design based on stress/strain and material properties. It is by it's very name "static". If you push so hard here how much deflection will you get and when will it fail. Dynamics as you might guess is concerned with motion and movement - if a force is applied how does something respond. Bike wheels are really gyroscopes and you get all wrapped up in Euler's Equations when you go to quantify what happens. What you're really asking about is System Dynamics which builds on both statics and dynamics but adds in the concept fo frequency response to a forcing function.
Perhaps a good way (you decide) to characterize things would be to start with a diving board: standing very still on the end of diving board and noting the deflection is "statics". Taking that same ding board and hurling it through space (so we don't have to open the aerodynamics subject) and predicting the resulting motion would be "dynamics". Jumping on the smae diving board and noticng how high the diver goes and how fast/long/far the board vibrates would be "system dynamics."
So what is it that we're trying to capture about a bicycle? The "feel" - probably you'd be best served trying to keep everything identical except the component you're interested (the frame) and come up with a test and a way to measure it. You'd probably want to get an accelerometer and some data logging equipment and have a track where you could really characterize the driving forces being applied to the bike. then you'd need to spend a bunch of time sifting through the data to try and make sense of it all and draw conclussions based only on what the data proved so as to not bias it with your own notions.
What you're talking about here is the type of work that mechanical engineers can spend a good part of their career on. You could also go by the "seat of your pants" ride lots of different bikes and try to remain objective and open minded. Realize that frame building is still part science and part art and that if it were all so easy to quantify and characterize all of our bikes would start to look and ride the same. Fortunately it's still an art and people are willing to challenge long held beliefs by the majority and come up with some surprising results.
Well that's my opinion anyways and it's worth at least what you paid for it.
|Nice explanation. Although even that is incomplete,||djg|
Jun 27, 2002 6:35 AM
|if you're serious about getting to the "feel" part, as you'll need to tack on rather a lot from perceptual physiology (and good damn luck finding cookbook formulae or ots apparatus for the sorts of things we'd like to know here).
And that's where the "art" part comes in (ponder, ponder, aw hell, I dunno, let's go by guess and feel...)
|Engineers.... is there a difference between statics & dynamics?||pina|
Jun 26, 2002 8:29 PM
|If you want to test for bike ride quality you need to use accelerometers and look at frequency response at different points on the bike. If one wanted to design a bike from a dynamics point of view they would also do what engineers call a "modal survey" or "ground vibration test" of the system and individual parts (a la frame alone and complete bike). This would answer alot of questions and allow one to truly tune the bike instead of asking the question after the fact. Ride quality can be low and high frequency. The definition of high and low frequency can be somewhat subjective. The buzz that everyone talks of would be high frequency and low frequency would be closer to the static values people like to refer to. I think in general the low frequency shapes of the bike would account for the majority of the vibration amplitudes people feel.|
Jun 26, 2002 1:39 PM
try moving all those parts across to another frame and see if it is different. Your argument is that the frame will play the smallest part.
Measuring them independently is arbitrary, it needs to be tested as a system.
Simply moving the riders center of balance fore or aft on the same will change the ride quality, as more weight will be born by the sit bones or the shoulders/arms/hands. The geometry of the bike will dictate the amount of fore/aft change that can be made. A bike that will not allow you to get a proper COB cannot be made comfortable by changing anything other than the frame.
A bike is greater than the sum of its parts.
|A bike is greater than the sum of its parts...Interesting but...||bic|
Jun 26, 2002 4:19 PM
|One must also include the human body and how it relates to all the info given it during each ride. Each ride will be different and how our body responds will be different. Even during a ride our feeling of how the road feels can change. Are we tired, did we not see that bump, stiff or flexed. yada yada yada. Very scientific huh.|
|I have heard how great...||koala|
Jun 26, 2002 4:19 PM
|my colnago master extra lite was supposed to ride but I found it harsh and I am not light at 5-10 and 165. I took all the components off and put them on my new foco frame and the difference was amazing. The newest steels are awesome riding.|
|Thank you for someone actually listening to his arse.||Leisure|
Jun 27, 2002 3:38 AM
|I've gone over it before, and I'm not the only one. While there's nothing wrong about the theories themselves people are applying to whether or not you can measure a frame's comfort, the application obviously can't be complete because people are feeling consistent differences. I love it when a certain number of people (seemingly with little actual experience themselves, otherwise they should know better) use a couple of oversimplified models to argue that people should ignore their own perception. "You're not really feeling/hearing/smelling/seeing any difference, it's just your imagination, because I have arbitrarily decided the human body doesn't have the ability to tell."
And my coworker that had to get rid his CAAD frame because his back would hurt too much is just imagining things.
Trust your own feelings before listening to someone else's rationalizations about what they want to see you buy. There's my passionate rant for the day.
That said, I have to admit that I don't know how much you would really feel a carbon post change your ride. If there's much effect I would imagine it to be quite diminished compared to changing frames.
|Yeah, but a first quality rant - I absolutely agree. (nm)||Crankist|
Jun 27, 2002 8:57 AM