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Optimal headtube angle, rake and trail for 20” wheels?

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Old 05-11-18, 02:24 PM
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johnce881
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Optimal headtube angle, rake and trail for 20” wheels?

Hi all, I’m held up at the design stage and thought you guys might have an answer to a little theory question... (if you feel the folding bikes sub-forum would be better, please report me to get this thread moved!)

is there any sort of consensus on front end geometry for small wheeled bikes? With big wheels, lots of major brands publish full geometry and it’s easy to compare (eg Shand: shandcycles.com/bikes/stoater/) and they all seem to agree, give or take a few mm or degrees. However, I can find no small wheel brands which publish theirs/their rationale?

it strikes me that the geometry should be subtly different?

many thanks in advance and, er, hi
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Old 05-11-18, 02:45 PM
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No expert her with 20" tires but when I began to look at "24"' tires I contacted Georgena Terry and roughly followed her 81* castor angle designs (this was from Bill Boston IIRC). I might look at the Bike Friday designs as a start. Andy
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Old 05-11-18, 03:30 PM
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Originally Posted by Andrew R Stewart View Post
No expert her with 20" tires but when I began to look at "24"' tires I contacted Georgena Terry and roughly followed her 81* castor angle designs (this was from Bill Boston IIRC). I might look at the Bike Friday designs as a start. Andy
thanks Andy, I’ll continue digging esp. with the Georgena Terry tip
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Old 05-11-18, 03:40 PM
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I should add that I found with the lower rotational inertia that small wheels have (compared to 700c) the same geometry will feel quicker then one might initially think. Example was that I would soften back the head angle by a degree from a 700c one to get some of the same feel with the 520ISO wheels I've built around. Andy
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Old 05-13-18, 09:14 AM
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Perhaps go to a shop selling folding bikes and take measurements?

One thing that should be obvious is since the 2 lines that make trail length , down on the ground

cross above the ground plane the trail is shorter .. for example, I read, Brompton trail is 35mm..

short, but handles the front bag loading nicely ..
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Old 05-16-18, 01:44 PM
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I would ask Yan (@downtube). He designs folding bikes with 20" wheels. I've ridden several different folding bikes, and the ones Yan designs handle quite well (which is something I can't say about most of the others I've ridden).

Also, you could dig through old threads about the venerable Raleigh Twenty. Bicycle theoretician John Forester did some experiments with one, and found the stock fork had too much offset; when he straightened the fork a little, the handling became much more predictable. The threads I'm referring to are ten or more years old, and I don't know how to find them.

Bear in mind that there are two different "twenty inch" wheel sizes, and that a surprising variety of tires are available for the smaller of those sizes. The tolerances, between what geometry works and what doesn't, become pretty small with smaller wheels.
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Old 05-18-18, 11:26 AM
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Mike Burrows Built a long wheel base recumbent head tube steep , maybe plumb, trail was the offset behind it

https://www.bing.com/images/search?q=Mike+Burrows+Bicycle&FORM=RESTAB

book on design.. : https://www.tredz.co.uk/prodimg/21814_1_Zoom.jpg
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