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Optimal headtube angle, rake and trail for 20 wheels?

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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, Im 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 its 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, Ill 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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Old 06-19-18, 04:17 AM
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Originally Posted by Abu Mahendra View Post
Most all name small wheel bikes will ride fine. It's not rocket science, or an obscure, arcane art.
Why so defensive? There is no information about geometry in your post except an assertion that a builder can do what they want and the rider will adapt. This is the framebuilding forum, so we talk about the geometry that we might use in a bike we intend to build. Most people want to build a bike that has desirable handling characteristics, not just, "ah, humans are adaptable." As you say, people can ride just about any geometry, no matter how awful. And if they paid money for the bike, they will defend the geometry, even if it is bad. What is the geometry of those 4 bikes?
The last one looks like it might have a steep headtube angle and relatively long rake. The other ones look like they have fairly low rake and slack headtube angles. It would be interesting to know what their geometry actually is.

It seems obvious that the ranges of trail that we are used to on 700c bikes are not really practical for a 20" wheel bike, it's going to have less trail. It would be interesting to know if the desired handling characteristics of a larger wheeled bikes translate to this different geometry. Do the handling characteristics translate in a proportional relationship?

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Old 06-19-18, 05:26 PM
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Originally Posted by johnce881 View Post
Hi all, Im 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 its 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
You understand the component parts of Trail measurements? start there..
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Old 06-19-18, 06:22 PM
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Originally Posted by Abu Mahendra View Post
What folding bikes specifically--make and model--did you ride that did not handle well?
I have a Novara folding bike that I especially dislike. I don't know the model. Also a Kent ultra-lightweight one (magnesium frame) that handles poorly, strange seat tube angle. I've also ridden several of the older ones, Raleigh Twenty, Raleigh RSW 16, and was not impressed. Moulton F-frame (though not a folding bike) also very twitchy. Of course those designs are very old and obsolete.

I've ridden a couple of Dahons that rode well enough, but I didn't like the way they fold, and their proprietary parts made it difficult or impossible to change the way they fold. I take my folding bike on a commuter train on which there is a very specific place where I can store the folded bike conveniently, and if a bike doesn't fit there, I have to modify it until it does. The Dahons I looked at did not give me any options to modify anything

Re your comments about Yan / Downtube: is this speculation, or do you have first hand information? I have neither, am merely reporting what I was told.

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Old 06-19-18, 06:32 PM
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Originally Posted by rhm View Post
I have a Novara folding bike that I especially dislike. Also a Kent ultra-lightweight one (magnesium frame) that handles poorly. r've also ridden several of the older ones, Raleigh Twenty, Raleigh RSW 16, and was not impressed. Moulton F-frame (though not a folding bike) also very twitchy.

Re your comments about Yan / Downtube: is this speculation, or do you have first hand information? I have neither, am merely reporting what I was told.
Novara, Kent and Raleigh. Not exactly the paragons of or the makes that come to mind in popular, contemporary folding bikes. If we talk mainstream modern, we'll be talking about Dahon, Tern, Brompton, Bike Friday, Pacific Cycles, Tyrell, FSIR, Crius, FnHon, etc. Let's talk mainstream, let's talk modern.

In public and in private Yann has never characterized himself as a bike designer. Chess champion, math professor, bike brand owner, delivery boy, yogi, yes. Bike designer, no. Downtube seeks to fill a particular niche and price point in the market. That price point and the likely volume he moves probably means that he does not design the bikes from scratch. Most likely, as I said before, he picks from a long catalog of existing designs held by large bike manufacturing companies in China. No need to re-invent the wheel, really. That would drive up cost, and he prides himself in keeping costs low. So, it is neither first-hand information, nor speculation, but rather a thoughtful analysis.
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Old 06-19-18, 06:47 PM
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Originally Posted by Abu Mahendra View Post
Novara, Kent and Raleigh. Not exactly the paragons of or the makes that come to mind in popular, contemporary folding bikes. If we talk mainstream modern, we'll be talking about Dahon, Tern, Brompton, Bike Friday, Pacific Cycles, Tyrell, FSIR, Crius, FnHon, etc. Let's talk mainstream, let's talk modern.

In public and in private Yann has never characterized himself as a bike designer. Chess champion, math professor, bike brand owner, delivery boy, yogi, yes. Bike designer, no. Downtube seeks to fill a particular niche and price point in the market. That price point and the likely volume he moves probably means that he does not design the bikes from scratch. Most likely, as I said before, he picks from a long catalog of existing designs held by large bike manufacturing companies in China. No need to re-invent the wheel, really. That would drive up cost, and he prides himself in keeping costs low. So, it is neither first-hand information, nor speculation, but rather a thoughtful analysis.
Ah, well, for some reason it comes across as speculation.

You consider Brompton modern? I know people who have them and love them, but I just don't get it. And yes, I have ridden one or two.

As for the others, I have a perfectly good folding bike now, and am not in the market for another, so until this one dies I will not be researching the matter further.
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Old 06-19-18, 06:58 PM
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Originally Posted by rhm View Post
...

You consider Brompton modern? I know people who have them and love them, but I just don't get it. And yes, I have ridden one or two.

...
Brompton, don't get me started. I consider it modern in the sense of a mature geometry, a stable platform. When it comes to some of its parts and materials, I consider it rather Neanderthal. I am no fan of Brompton. The brand-cultism and smug superiority of some/many of its owners only add to my distaste. I'll leave it at that.
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Old 06-20-18, 05:36 AM
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Originally Posted by Abu Mahendra View Post
I asserted or said no such thing. I am not so much defensive as exasperated that in 2018 the design and geometry of small-wheel bikes is still treated as it were a big mistery, an arcane art, an impenetrable curio. It think it betrays the great ignorance and bias about small wheel bikes that afflicts this forum. That's all.
This defensiveness is your own bias, you will see problems everywhere until you recognize that they are coming from you. People in this thread are interested in understanding geometry that makes a good riding small wheel bike, you haven't really helped. This is far from common knowledge in the framebuilding community. It has only been relatively recently that people explored these questions for 700c/650b bikes. I doubt people really have explored it for small wheel bikes, they just stick with what has worked for others for the most part.


Originally Posted by Abu Mahendra View Post
Those head-tubes are all 73 degrees, and the seat tubes are all about 71 degrees. The top tubes on the two bikes with classic diamond-shape geometry are 53cm and 55.5cm, and the chain stays 42cm. Nothing unusual or arcane on all accounts. The measurements that you'll need to pay particular attention to are trail and bottom bracket height. Obviously the smaller the wheel, the higher the BB height needs to be.
Do you know the respective rake (or trail) on each of these bikes?
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