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Basic Folding Bike Design Question

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Basic Folding Bike Design Question

Old 11-30-18, 09:51 PM
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Basic Folding Bike Design Question



Sorry if this has already been asked and answered elsewhere. What is the general difference performance-wise (stiffness, weight, ability to carry a larger rider) between a seat post arrangement like the one in the first picture compared to the second? My guess is that the Bike Friday, with more seat tube--even if it is folding--would be stiffer and therefor able to accommodate a larger, heavier rider. Is this right, or is it more complicated than that? I know that Brompton makes a telescoping seat post for larger riders, but if these work adequately, then why make any folding bike frame any other way? Any information you'd be willing to share would be appreciated.
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Old 11-30-18, 10:35 PM
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There's more to it than seatpost/seat tube design. Where and how the frame pivots to fold also makes a difference, as does the material (steel, aluminum, titanium, carbon fiber) and geometry of the frame, as well as the quality of the wheels and tires.

Bromptons are optimized for quick folding while also enclosing the drivetrain so that your clothes don't risk being dirtied by a greasy chain. Other brands make different choices.
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Old 12-01-18, 06:49 AM
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Folding bikes are a compromise. Some bike manufacturers emphasize the portability and foldability of a bike, which can impair rideability. Other manufacturers emphasize the rideability while making a bike that is less portable.

My folding bike is of the later, not very portable and folds down to a pretty large package. But it rides almost like a full size bike. But mine has a very tall telescoping seatpost. When I first got it, I could feel the flexing of the seatpost and the saddle could move fore and aft slightly whereas on a full size bike there was no flex like that. Initially that bothered me a lot, but after more riding and more distance covered, I started to not be bothered by the seatpost flex. This past spring I did a trip of over 300 miles on the bike and the flex in the seatpost did not bother me at all.

I do not have a lot of experience with different kinds of folding bikes, so I can't say which I would prefer if I had tried a number of them. But for my needs the portability is less important than the ride as I do not need to carry it into buildings very often, do not take it on mass transit, etc. But when I ride that bike I usually ride it for over 20 miles at a time, so I do not want to compromise on the ride.

And there are other manufacturers that make a bike that folds badly and rides bad too, but they rarely stay in business long.
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Old 12-01-18, 07:41 AM
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Telescoping seat post can be handy for compactness but on my Dahon it's more PITA to make sure both QR clamps are tight enough - they're also plastic lined between metal tube surfaces so it needs cranking down even harder to stop them slipping.

This week played with a folder whose seat tube is behind the BB and isn't stopped by the BB. The longer seatpost can drop right thru to the bottom & it then becomes a leg/support for when it's folded. Neat idea!
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Old 12-01-18, 11:19 AM
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what you show

Originally Posted by Headpost View Post


Sorry if this has already been asked and answered elsewhere. What is the general difference performance-wise (stiffness, weight, ability to carry a larger rider) between a seat post arrangement like the one in the first picture compared to the second? My guess is that the Bike Friday, with more seat tube--even if it is folding--would be stiffer and therefor able to accommodate a larger, heavier rider. Is this right, or is it more complicated than that? I know that Brompton makes a telescoping seat post for larger riders, but if these work adequately, then why make any folding bike frame any other way? Any information you'd be willing to share would be appreciated.
Bike Friday, in picture is a travel bike , seat mast folds down .. seat post stays fixed.. height.
Order option you remove it with another QR.. for a suitcase pack size knockdown..

I have a BiFri & a Brompton.. B's extended post is same as stock , just a bit longer.
tube bottom hits the ground, before saddle fully down , from top..

don't like plastic.. ? ..



you can use machined aluminum seat post sizing shims, in a DIY cut seat post-add a 2nd QR scheme
your tube ID, and for seat posts 1" , or 27.2..

the alloy post in like basic Kalloy line is not polished smooth, so surface not so slick..
____
There are a couple double clamp band clamps 31.8mm bottom, 27.2 top..
they are fixed.. more a slipping seat post fix..
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Old 12-02-18, 06:29 AM
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Whether the larger diameter seat post or the sleeved one could carry more weight would mostly depend upon the materials, diameters, lengths and wall thickness. Both are viable concepts. The devil is in the details.
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Old 12-02-18, 01:24 PM
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Bike Friday makes a heavy rider option with a diamond frame, if you exceed the 220 lb limit (I think) for their regular bikes. In any case, the ride is stiffer if there is no joint/hinge in the main tube. No matter how well designed, hinges eventually develop some flex.
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Old 12-02-18, 01:26 PM
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Originally Posted by Tamiya View Post
Telescoping seat post can be handy for compactness but on my Dahon it's more PITA to make sure both QR clamps are tight enough - they're also plastic lined between metal tube surfaces so it needs cranking down even harder to stop them slipping.

This week played with a folder whose seat tube is behind the BB and isn't stopped by the BB. The longer seatpost can drop right thru to the bottom & it then becomes a leg/support for when it's folded. Neat idea!
That's how my Bike Friday Pakit works, I love it!
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Old 12-02-18, 01:38 PM
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My Brompton, the Extended seat post offering adds 30 % to the tube wall thickness of that
Vs the shorter standard one .. @210 # , I doubled the QR, 2nd one,
directly around the seat post,
sitting directly above the frame Q R, solving slipping seat-post issues..
both get opened at once when folding the bike ..
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Old 12-02-18, 05:49 PM
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Thanks for the responses. Aside from all the specific information, I am taking away from this that it IS more complicated than I originally thought, which most things turn out to be, and that either design can work fine on a well-designed, well-built, bicycle.

Last edited by Headpost; 12-02-18 at 05:59 PM.
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