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  1. #1
    jur
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    Downtube Mini suspension upgrades

    As you may know, I have been experimenting with elastomers instead of the stock spring in my Mini to cure bounciness. I have had quite good results but not a complete cure.

    Further experiments have made me suspect that perhaps the seatpost is to blame for a large portion of the bounciness. The most recent experiment was to put in the hardest elastomer to check the effect on bounciness.

    Yesterday I decided to ride my Downtube Mini to work. I found that it still bounced badly at a particular cadence. So the seatpost is now a prime suspect.

    At one point there is a steep climb which I usually do out of the saddle. As I was climbing I noticed a clunky feeling coming through the cranks at every revolution. I looked down and what the... am I imagining it or is the tail end fishtailing...? So I stopped and pushed with my foot sideways against the suspension joint, and yes it has a fair amount of play. I did notice the play before but hadn't noticed it before during pedalling out of the saddle. I noticed it as an instability when taking corners at speed. A bit disconcerting, that.

    Since I am at the moment scrutinising the Mini with a view to making it perfect, I decided to dis-assemble the suspension joint. It is a very simple and inexpensive setup, what you'd expect of a bike at this price point. It consists of a plastic bush inside the short tube which is at the front of the rear triangle, actually 2, one from each side, and a steel spindle which rotates inside the plastic bush (again, 2 actually). The spindles insert from the sides of the brackets which are welded to the bottom bracket shell. A screw holds the spindles together inside the bushes.

    What I found was that especially one of the spindles has a lot of slop inside the bush. Also evident were wear marks from rotation of the spindles against the bushes, and no lubrication. Again, all par for the course for this bike I suppose.

    So I made 2 new bushes in which the existing spindles fit snugly/tightly. Free rotation is not needed, in fact some binding is OK since it would provide damping, as long as it is not too much. I used Delrin which is easy to machine and has a low friction coefficient. I will be including grease between the spindles and bushes anyway, to limit wear.

    The joint is not dustproof or waterproof, unfortunately. I did come up with a design for bushes which would be water/dustproof, but maybe for later. Right now I want as little as possible fishtailing to see the effect on handling.

    Perhaps Delrin is not suitable for this job, so I also came up with a design using sealed ball bearings. If the Delrin bushes wear out anytime soon, I'll be getting some sealed ball bearings.

    I also made another suspension 'elastomer' from nylon, effectively locking the suspension out. I want to see if the bobbing comes from the seat post as I now suspect.

    I'll keep you posted and try to snap a few pics before assembling it all tonight.
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  2. #2
    Eschew Obfuscation SesameCrunch's Avatar
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    Thanks for writing this up. We would greatly appreciate pictures!

    Maybe I'll adapt mine also, as I also feel the pogo-ing at a particular cadence.

  3. #3
    jur
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    Here are 2 pics of the parts in above description:

    This one shows how the assembly goes together.


    Here are my new bushes next to the originals.
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    Wheelsuck Fat Boy's Avatar
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    That's the same pivot used on the FS IX. I felt the same thing on my bike as well. The stiction is a particular form of damping that most make an attempt to stay away from. The problem is that it damps the system at exactly the time you don't want it to. Tightening that bush will make the bike track straight again.

    I'm curious about the bobbing coming from the seat post. I can see that being the case. If so, increase the stiffness of the seat post 'spring' and see if it goes away. Play with your pedaling cadence and see if that is the issue. Pedal at a point where it is bobbing, and then increase your speed. See how much faster you have to go before you stop bouncing. 10 RPM would probably be enough to smooth things out.

    Let us know what you come up with, Jur.

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    rhm
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    Jur, I look forward to hearing your conclusions about the seatpost. As I mentioned somewhere, I tried disabling the suspension on my Mini by binding it down with nylon twine; this worked quite well and I found no bobbing after that, so I don't think the seatpost was responsible. I did find, however, that there was lateral sloppiness in the frame, which came from the suspension, and which they nylon twine did nothing to stop. May have contributed to it, though I don't see how. At any rate I cut the nylon and did not repeat that experiment.

    Later I found that the bolt holding the pivot together is completely bottomed out; it is all the way tight and cannot be made tighter. So that's where the slop is coming from. What I need todo, I think, is take it all apart and find a washer of the right size so the bolt doesn't bottom out.

    But... I look forward to hearing your results.

    Rudi

  6. #6
    jur
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    Quote Originally Posted by Fat Boy View Post
    The stiction is a particular form of damping that most make an attempt to stay away from. The problem is that it damps the system at exactly the time you don't want it to. Tightening that bush will make the bike track straight again.
    Good point, I wasn't even thinking of stiction. I assembled the joint with the new bushes, with grease and as expected there is no slop whatsoever any more. I could choose the amount of drag by the tightness of the bolt. Since I wasn't thinking of stiction, I wasn't looking for it, but from what I remember the joint felt very smooth, very little stiction. There was no sticking and sudden release, just smoothness with drag. It is probably a combination of the low friction of Delrin plus the grease. So I happy on that score.

    (I am not sure what you mean by "Tightening that bush will make the bike track straight again" because the steel spindle was sloppy in the old plastic bush, no amount of tightening would cure that.)
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    jur
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    I rode the Mini to work again, with the Nylon bush installed instead of an elastomer, and this time there is no doubt: The seat post is doing the bobbing. I estimate the cadence with worst bobbing to be about 100rpm.

    There was a difference to having an elastomer in there, though. It seemed to me as if the bobbing was less prone to happen. I think the suspension probably sets it off, so with it locked out it happened less often.

    I can understand why many people have no problem at all: I have the post extended very far (long legs apparently) so the spring effect becomes worse.

    Another observation: I have the tyres pumped to about 60psi and though that's pretty hard for this application, size and bike, I didn't experience any harshness due to supposed small wheel effect. I think that is a vastly over-hyped issue, only valid for very particular size uneveness. For the sort of roads I'm riding (some gravel, some bitumen, lots of tree root bumps) there is no real issue. It feels just the same as any other bike with no suspension, and it feels a lot less harsh than a road bike with 110psi tyres. So I conclude that unless you are running with very narrow tyres pumped to over 100psi, suspension is simply unecessary. Softer tyres are the first line of defense.

    Having said all that, I can feel the difference between my medium elastomer and the nylon, most definitely. But that same difference would probably be given by some Big Apples (which would probably be more expensive from Yan's POV).

    The medium elastomer feels the same to locked out suspension when going over gradual type bumps such as caused by tree roots pushing the bitumen up, but riding over sharp corners in pavement, the elastomer takes away the edge of that sharp corner.
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    Wheelsuck Fat Boy's Avatar
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    Quote Originally Posted by jur View Post
    (I am not sure what you mean by "Tightening that bush will make the bike track straight again" because the steel spindle was sloppy in the old plastic bush, no amount of tightening would cure that.)
    My fault. I meant tightening up the lash in the area where the old plastic bush was (by making a new one) will make the bike track straight again. I was just commenting on what you were already doing.

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    jur
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    OK, so on to the quest of improving the seat post. The only thing I can think of is to take a carbon fibre post which I understand would be stiffer and maybe have some better self-damping. So I am going to make a telescoping seatpost, with the top portion a 27.2x400mm long CF post, and the bottom an aluminium sleeve with QR.

    Are there other suggestions for making the post stiffer?
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    Wheelsuck Fat Boy's Avatar
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    So if the bobbing sensation was less prone to happening with the elastomer in the suspension (as opposed to the nylon spacer), then possibly the next approach would be to install a very soft elastomer in the Mini's suspension. This would probably bottom out occasionally, but it might might reduce the bobbing on a smooth road.

    Essentially, you have three mechanical systems (tire spring, suspension spring, seat post spring) and you are trying to get the oscillations to cancel each other as opposed to amplifying themselves. It's a tricky thing, but with enough playing around, I bet you can put the resonant above your preferred pedaling cadence.

    I wonder, do people have trouble with the springy Brooks saddles giving pedal bounce?

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    Wheelsuck Fat Boy's Avatar
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    Another thought. When you are on the saddle, there are 3 springs (tires, suspension, and post), but when out of the saddle, there are only 2 (tires and suspension). It's going to be really tough to get it to work properly in both situations.

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    Wheelsuck Fat Boy's Avatar
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    Quote Originally Posted by jur View Post
    OK, so on to the quest of improving the seat post. The only thing I can think of is to take a carbon fibre post which I understand would be stiffer and maybe have some better self-damping. So I am going to make a telescoping seatpost, with the top portion a 27.2x400mm long CF post, and the bottom an aluminium sleeve with QR.

    Are there other suggestions for making the post stiffer?
    I'm a little skeptical of people talking about the 'damping' qualities of carbon fiber. I just don't see it. Damping means that you are converting kinetic energy to heat. I have yet to see a bike frame (or racecar for that matter), get warm because of the damping that is going on.

    When people say that an aluminum bike transmits more 'road buzz' to their hands than a carbon bike, then I understand that. The aluminum frame is stiffer (higher natural resonant freq. in that particular mode) and therefore transmits higher frequency inputs better than the carbon (or steel or Ti) frame.

    My guess is that the biggest change a carbon seatpost would make is in your wallet.

  13. #13
    jur
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    Quote Originally Posted by Fat Boy View Post
    So if the bobbing sensation was less prone to happening with the elastomer in the suspension (as opposed to the nylon spacer), then possibly the next approach would be to install a very soft elastomer in the Mini's suspension. This would probably bottom out occasionally, but it might might reduce the bobbing on a smooth road.
    I actually did this last week, with a result that I was half expecting: The soft elastomer ruptured. Unfortunately I chose to ride a particularly rough, steep piece of singletrack for this experiment, as it was the front wheel was lifting, I couldn't unclip fast enough and it was touch and go or I had a stack. So unfortunately I destroyed the elastomer before properly experimenting with it.
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    jur
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    Quote Originally Posted by Fat Boy View Post
    I'm a little skeptical of people talking about the 'damping' qualities of carbon fiber. I just don't see it. Damping means that you are converting kinetic energy to heat. I have yet to see a bike frame (or racecar for that matter), get warm because of the damping that is going on.

    When people say that an aluminum bike transmits more 'road buzz' to their hands than a carbon bike, then I understand that. The aluminum frame is stiffer (higher natural resonant freq. in that particular mode) and therefore transmits higher frequency inputs better than the carbon (or steel or Ti) frame.

    My guess is that the biggest change a carbon seatpost would make is in your wallet.
    I'm with you in all that. There's a lot of hype going around and very little proper engineering knowledge.

    I had a CF seatpost on my Swift and it was plenty stiff, stiffer than the Mini's post, but not as stiff as the original Swift post. So it is only for stiffness that I am thinking of CF, any self-damping must be in the high frequency range, not 100rpm.
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  15. #15
    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by jur View Post
    OK, so on to the quest of improving the seat post. The only thing I can think of is to take a carbon fibre post which I understand would be stiffer and maybe have some better self-damping. So I am going to make a telescoping seatpost, with the top portion a 27.2x400mm long CF post, and the bottom an aluminium sleeve with QR.

    Are there other suggestions for making the post stiffer?
    Were you close to the max extension?

    I made a telescoping seat post and do not experience the bobbing. However, neither of the two components of the seat post are close to their minimum insertion point.

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    jur
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    Quote Originally Posted by invisiblehand View Post
    Were you close to the max extension?

    I made a telescoping seat post and do not experience the bobbing. However, neither of the two components of the seat post are close to their minimum insertion point.
    I'm actually about an inch beyond max extension. I checked the end of the post where it sits with respect to the frame joints, and it is still below the top tube (or is that downtube?). So I don't think I will damage the frame. The post is another matter. That may bend. I have a piece of very tight-fitting 1" ally tube inserted inside the seatpost for added strength and stiffness, but obviously it is not stiff enough. I think the bounciness is therefore caused by the springiness of the post, not by minimum insertion issue. The post's srping plus my upper body mass gives a particular resonant frequency coinciding with 100rpm. A stiffer post would shift that frequency up, say to 120rpm, and since I don't pedal there, that would solve the problem.

    A telescoping post by itself may not do much for me, since most of the flexing happens near the point where the post leaves the frame. It is in that region that I have to increase stiffness more than any other. Which is why I think a CF post (MUST be 400mm to work) and a telescoping setup may be an improvement: The CF post's bottom end would be below the seat tube's top, and so form a homogenous portion with the alloy sleeve in the critical region. It would combine its stiffness with that of the sleeve, since both would be clamped down by the seat tube's QR.
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  17. #17
    Eschew Obfuscation SesameCrunch's Avatar
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    Quote Originally Posted by jur View Post
    I'm actually about an inch beyond max extension. I checked the end of the post where it sits with respect to the frame joints, and it is still below the top tube (or is that downtube?). So I don't think I will damage the frame. The post is another matter. That may bend. I have a piece of very tight-fitting 1" ally tube inserted inside the seatpost for added strength and stiffness, but obviously it is not stiff enough. I think the bounciness is therefore caused by the springiness of the post, not by minimum insertion issue. The post's srping plus my upper body mass gives a particular resonant frequency coinciding with 100rpm. A stiffer post would shift that frequency up, say to 120rpm, and since I don't pedal there, that would solve the problem.
    Jur:

    I also have my seatpost extended about 1" beyond max. As you know, I used a wood dowel (pine) to reinforce the seatpost. I have not found seatpost flex to be an issue for me. The wood does not allow for the kind of deflection you're describing. (I weigh 188 lbs.) It's not as glamorous a solution as a carbon seatpost, but it's certainly cheaper. I would recommend it as something you could try.

  18. #18
    jur
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    Quote Originally Posted by rhm View Post
    Later I found that the bolt holding the pivot together is completely bottomed out; it is all the way tight and cannot be made tighter. So that's where the slop is coming from. What I need todo, I think, is take it all apart and find a washer of the right size so the bolt doesn't bottom out.
    I think there may be some misunderstanding in the forum on what exactly the bolt which tightens the suspension spring, does.

    First, the lateral play/slop does not come from this bolt; it must come from the suspension hinge. In my case, the 2 spindles were of different size, 19.92mm and 19.97mm, and the bushes being molded were both 20.00mm. So one of the spindles rattled around in its bush, and this cause the lateral play in the hinge.

    Getting back to the bolt at the suspension spring, this bolt only sets the end-stop position. It does nothing to the spring tension.

    To understand how it works, if the rider sits on the bike, the spring compresses a certain amount, and the bolt head moves away from the stop. So any tightening or loosening of the bolt will do nothing while sitting on the bike.

    However, now picture going over a bump. The spring will compress more and also relax some from its normal riding position. When it relaxes, the bolt head moves back in the direction of the stop position. If the bolt was loose, its head wouldn't hit the stop. If it was very far in, the head would hit the stop.

    This is exactly what I got. I got the bike new, went for a ride, decided I didn't like the bounciness, tightened the bolt, found that it was already fully tight, and then inserted another washer under the bolt head to compress the spring more. Then I went for a ride. What then happened, at every bump the bolt head+washers would hit the stop with a jar instead of moving freely as it did before the washers. But the bounciness was just the same. That's when I figured out exactly what was going on, and why I needed an elastomer.
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    jur
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    Quote Originally Posted by SesameCrunch View Post
    Jur:

    I also have my seatpost extended about 1" beyond max. As you know, I used a wood dowel (pine) to reinforce the seatpost. I have not found seatpost flex to be an issue for me. The wood does not allow for the kind of deflection you're describing. (I weigh 188 lbs.) It's not as glamorous a solution as a carbon seatpost, but it's certainly cheaper. I would recommend it as something you could try.
    I will try it, but it's not so easy as I inserted a tight-fitting 1" ally tube and that will be a lot of effort to remove. I have to remove it as it didn't go in very far before getting TOTALLY stuck. So if I just put in a dowel as is, it wouldn't stiffen up the crucial portion of the post.
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    Wheelsuck Fat Boy's Avatar
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    Jur,

    I think the ultimate problem is that you've done more suspension testing than Yan has and you're finding a lot of the limitations of such a simple design. The suspension on the Mini is for 2 things. The most important is marketing. The secondary reason is that it takes the edge off of bumps for a small tire bike.

    You probably ride your Mini further than anyone else on the planet and over the most varied terrains. Ultimately, it wasn't ever really intended to do what you do with it. Not that this is that bad of an issue, but I think you're such an avid user that you are outside the normal design criteria.

    So the question now is how to make it work under these conditions. Well, it's going to take some experimentation.

    Judging from your tests so far here's what I've concluded:

    1. The suspension alone has a certain amount of 'bob' in it.
    2. The Thudbuster seatpost amplifies the problem.
    3. An elastomer in the suspension has less bob than the standard spring.
    4. Eliminating the suspension spring makes for more pedal bob.
    5. There seems to be a certain amount of pedal bob in the 'post' part of the seatpost.
    6. A softer suspension elastomer destroyed itself.

    I don't know if you have access to different elastomers, but I recommend giving http://www.mcmaster.com/ a look under 'polyurethane elastomers'. They've got quite a variety and it's good quality.

    Let's keep chipping away at this. Suspensions are my thing, and I'm sure we can cure this problem one way or another.

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    jur
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    Quote Originally Posted by Fat Boy View Post
    You probably ride your Mini further than anyone else on the planet and over the most varied terrains. Ultimately, it wasn't ever really intended to do what you do with it. Not that this is that bad of an issue, but I think you're such an avid user that you are outside the normal design criteria.
    Agreed; I have actually no problem with the Mini as it wasn't meant to be a performance bike. Out of the box it is a great offering. So no criticism to the original design per se.

    My next step is to put a dowel inside the post at least for increasing stiffness to observe the effect. I have the difficult job to extract the 1" tube that I stuffed in there. I think the lathe might just work with the saddle clamp poking out the back end of the lathe head.
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    jur
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    Quote Originally Posted by Fat Boy View Post
    Judging from your tests so far here's what I've concluded:

    1. The suspension alone has a certain amount of 'bob' in it.
    2. The Thudbuster seatpost amplifies the problem.
    3. An elastomer in the suspension has less bob than the standard spring.
    4. Eliminating the suspension spring makes for more pedal bob.
    5. There seems to be a certain amount of pedal bob in the 'post' part of the seatpost.
    6. A softer suspension elastomer destroyed itself.
    # 2: I don't have a Thudbuster on it, just the original kalloy post.
    # 4: Not entirely sure what you mean, perhaps you meant less bob?
    # 5: See 2

    The Thudbuster is installed on my R20. That other thread of suspension vs thudbuster is probably what caused confusion.

    I purposely bought a 27.2mm version Thudbuster with shim for the R20, so I can use it on this or any other bike if I wanted to.
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    Wheelsuck Fat Boy's Avatar
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    #2: So a lot of what I was saying had to be confusing. I thought you have a Thud on it.

    #4: I rode the Mini to work again, with the Nylon bush installed instead of an elastomer, and this time there is no doubt: The seat post is doing the bobbing. From this I thought that the nylon bush instead of the elastomer caused more pedal bob. Was I wrong on this?

    #5: I was trying to seperate the Thudbuster from the actual seat post. Since the Thudbuster is non-existent, this is a ridiculous statement.

    Basically, what I was trying to do is lay out the known information, not tell you what the known information was. I was screwed up on the Thudbuster portion, so it was good to get that sorted.

    Ultimately, it comes back to stiffening the seatpost. I doubt if there is going to be anything that you can put on the inside of the post that will greatly affect it. Have you thought about trying to sleeve the outside of the post? If you could machine an aluminum tube for an interference fit and then stick the seat post in the freezer and the tube in the oven, then you could probably drop it on and end up with a super tight fit.

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    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by jur View Post
    I'm actually about an inch beyond max extension. I checked the end of the post where it sits with respect to the frame joints, and it is still below the top tube (or is that downtube?). So I don't think I will damage the frame. The post is another matter. That may bend. I have a piece of very tight-fitting 1" ally tube inserted inside the seatpost for added strength and stiffness, but obviously it is not stiff enough. I think the bounciness is therefore caused by the springiness of the post, not by minimum insertion issue. The post's srping plus my upper body mass gives a particular resonant frequency coinciding with 100rpm. A stiffer post would shift that frequency up, say to 120rpm, and since I don't pedal there, that would solve the problem.

    A telescoping post by itself may not do much for me, since most of the flexing happens near the point where the post leaves the frame. It is in that region that I have to increase stiffness more than any other. Which is why I think a CF post (MUST be 400mm to work) and a telescoping setup may be an improvement: The CF post's bottom end would be below the seat tube's top, and so form a homogenous portion with the alloy sleeve in the critical region. It would combine its stiffness with that of the sleeve, since both would be clamped down by the seat tube's QR.
    Hmmmm, I am a little lost here. But then again, this is a little out of my realm of comfort.

    So your saying that given the material of the seatpost that what is important is the distance between your butt and the clamp at the bottom -- not how much material is below that clamp -- since that is what determines the resonant frequency ... sort of like a guitar string. No matter how long the string is, if I clamp the string at the 5th fret, the same note is created.

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    Part-time epistemologist invisiblehand's Avatar
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    Quote Originally Posted by jur View Post
    I will try it, but it's not so easy as I inserted a tight-fitting 1" ally tube and that will be a lot of effort to remove. I have to remove it as it didn't go in very far before getting TOTALLY stuck. So if I just put in a dowel as is, it wouldn't stiffen up the crucial portion of the post.
    It might be easier to purchase another seat post ...

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