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Folding Bikes Discuss the unique features and issues of folding bikes. Also a great place to learn what folding bike will work best for your needs.

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Old 11-04-06, 09:09 PM   #1
SesameCrunch
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Why does a 20" wheel ride rougher?

Sorry for the newbie question. I am new to folding bikes. Just ordered my DownTube IX and am anxiously waiting for it to arrive next week.

In the meantime, can some resolve my curiousity? From reading the posts here, I gather that the smaller the tires, the more vibration is transmitted to the frame. Somehow, that is not intuitively obvious to me. Smaller tires tend to have lower psi, so it should be smoother. Is there a smaller volume of air to absorb the vibrations?

Please enlighten me. Thanks!
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Old 11-04-06, 10:06 PM   #2
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There is a smaller volume of air. Imagine the cross section of a tube, where the air resides. It's roughly the same between smaller wheels and larger wheels, but the larger wheels extend that cross-section through a much larger circumference.

Also, smaller tires do not necessarily have lower psi's. Assuming that your DownTube comes with tires similar to the Kendas that the '05 models did, I think they'll run at around 45-65 psi. I personally use 100psi tires.

Here's a question-and-answer from Alex Moulton Co., perhaps the most recognized pioneer of the small-tires-for-adult-bikes market:
http://alexmoulton.co.uk/frames.asp?id=questions

Also, picture a wheel, as seen by the side. Imagine a slight depression, hole or pothole that the wheel is going to sink into. A smaller wheel will sink further down into that hole than a larger one, which would be more likely to roll over the hole. These up-and-down motions get translated to the frame, which is one reason why you feel the vibration more.

This feeling of vibration is compounded by the easy steering of the wheel. Smaller, less mass, it turns right and left more easily than the larger wheels that most of us are used to..... which is why it'll probably be a few days before you feel at ease riding on it.
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Old 11-05-06, 10:22 AM   #3
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If a small diameter rigid wheel goes over a 1" object, for example, the wheel climbs up faster than a large dia wheel. This results in a sharper jolt to the fork and rider than a larger dia wheel, even though they are climbing over the same object at the same speed.

It is also obvious that the wheel will drop further into a hole than a larger dia wheel, and perhaps get caught easier.


Small dia wheels are stronger and stiffer than large dia wheels, they transmit most vibrations better.

It is also possible that to make the bike fold compactly trade offs in performance vs folding were made.
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Last edited by geo8rge; 11-05-06 at 10:32 AM.
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Old 11-05-06, 03:12 PM   #4
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Great points. Thanks for making me smarter :-)
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