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Bike Geometry & 20" wheels

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Bike Geometry & 20" wheels

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Old 01-13-19, 01:46 AM
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laffin_boy
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Bike Geometry & 20" wheels

Background:
I recently bought a Bike Friday Pocket Rocket Pro with 451 wheels (& Panaracer Minits 28-451 tires). After riding it enough so that I was over the “talking myself into it” phase and getting the feel of the bike it’s clear that the handling is much more “nervous” and “twitchy” than what I want & need in a bike. Understand that i’m not claiming that there’s anything wrong with the bike - only that on the bike handling spectrum from criterium bikes on one end to downhill MTB’s with 65mm of trail on the other end the Pocket Rocket is all the way on the left end of the spectrum. Sure it’s rideable - if you’re experienced and have good reflexes. Unfortunately i’m older than dirt and have some neurological issues that mean I need a super stable bike that any fool can ride no handed.

When it became clear to me that the PRP wasn’t going to work for me as it was set up I measured the trail of the bike and came up with approximately 35mm - which is about 20 mm less that the normal range for full sized road bikes. I wrote to Bike Friday and asked them (a) what the geometry figures were for my bike and (b) why they used so little trail and (c) did they have any practical suggestions as to how I could modify the bike to get the stability I need. I exchanged several emails with people in a position to answer questions like these and their answer to (c) was “no” and their answer to (a) & (b) was resounding silence. In other words they don’t want to talk about the geometry of their bikes - period. In fact they have, so far, refused to even disclose what the geometry figures are for my bike. (This is a side issue but my entire experience of buying the PRP leaves me with the strong impression of a poorly run business that can’t even spell “Customer Service”)

After that dead end I did a lot of searching online about bike geometry and found a lot of good info but it was virtually all on full sized bikes with big wheels. So this (finally!) brings me to my Questions:

Is there any inherent reason why a bike with 20” wheels can’t ride as “stably” as one with 700 / 650 / 26” wheels?

What is a “normal” trial figure for other bikes with 20” wheels?

Do any of you have bikes with 20” wheels that ride as stably as a “full sized” bike?
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Old 01-13-19, 10:23 AM
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I don't know what the "normal" trail figure for bikes with 20" wheels would be, but, from my experience riding a variety of them, there's a wide disparity in handling characteristics (as there is for other wheel sizes). Some I definitely found to be too twitchy for my tastes. I just built up a mini velo frame using the same tire size you have (451 x 28) and, according to an online trail calculator, that bike's trail is 49mm. Head angle is a relatively sharp 70.5 and wheelbase is about 1050mm. I would say these are more "stable" numbers than most 20-inchers and I went with this frame partly because of that.
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Old 01-13-19, 10:31 AM
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Originally Posted by laffin_boy View Post
Background:
I recently bought a Bike Friday Pocket Rocket Pro with 451 wheels (& Panaracer Minits 28-451 tires). After riding it enough so that I was over the “talking myself into it” phase and getting the feel of the bike it’s clear that the handling is much more “nervous” and “twitchy” than what I want & need in a bike. Understand that i’m not claiming that there’s anything wrong with the bike - only that on the bike handling spectrum from criterium bikes on one end to downhill MTB’s with 65mm of trail on the other end the Pocket Rocket is all the way on the left end of the spectrum. Sure it’s rideable - if you’re experienced and have good reflexes. Unfortunately i’m older than dirt and have some neurological issues that mean I need a super stable bike that any fool can ride no handed.



When it became clear to me that the PRP wasn’t going to work for me as it was set up I measured the trail of the bike and came up with approximately 35mm - which is about 20 mm less that the normal range for full sized road bikes. I wrote to Bike Friday and asked them (a) what the geometry figures were for my bike and (b) why they used so little trail and (c) did they have any practical suggestions as to how I could modify the bike to get the stability I need. I exchanged several emails with people in a position to answer questions like these and their answer to (c) was “no” and their answer to (a) & (b) was resounding silence. In other words they don’t want to talk about the geometry of their bikes - period. In fact they have, so far, refused to even disclose what the geometry figures are for my bike. (This is a side issue but my entire experience of buying the PRP leaves me with the strong impression of a poorly run business that can’t even spell “Customer Service”)

After that dead end I did a lot of searching online about bike geometry and found a lot of good info but it was virtually all on full sized bikes with big wheels. So this (finally!) brings me to my Questions:

Is there any inherent reason why a bike with 20” wheels can’t ride as “stably” as one with 700 / 650 / 26” wheels?

What is a “normal” trial figure for other bikes with 20” wheels?

Do any of you have bikes with 20” wheels that ride as stably as a “full sized” bike?
Put some weight over the front wheel. Get a lowrider rack for the front and a trunk bag or panniers. Trust me, the twitch will be gone. When I switched from a 700c to a Bike Friday, I felt the same way at first. After a few weeks of daily riding it was better and after a month or so I did not notice it at all. In fact, I felt safer on it due to the lower center of gravity in case of falls. I'm 69 so no spring chicken, either. Now I ride a 16" wheel Bike Friday (which seemed super twitch at first coming from the 20") as well as a Dahon 20". So, seriously, throw some weight on the front end and you will be amazed at the difference (until you get used to it, then you can take it off if you want). I fly around on my bikes now with more lean, more speed, more agility, than I ever did on my 700c Trek (although it was a great bike).
As to riding "no handed" I'm not sure you'll find 20" bikes suitable to that end at all. At least not folders. Maybe a mini velo but I'm not sure about that. But, frankly, a bike with smaller tires is going to respond to road bumps a lot more so it's a good idea to keep a hand on the bars at all times.

Last edited by linberl; 01-13-19 at 10:39 AM.
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Old 01-14-19, 09:57 PM
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Thanks for your reply.

Originally Posted by Trocadile View Post
I don't know what the "normal" trail figure for bikes with 20" wheels would be, but, from my experience riding a variety of them, there's a wide disparity in handling characteristics....Some I definitely found to be too twitchy for my tastes.
Might be a good subject for a survey - don't you think? Handling characteristics of known folding bikes compared to each other and also full-sized bikes?
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Old 01-14-19, 09:58 PM
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Originally Posted by linberl View Post
Put some weight over the front wheel.
Thanks for your suggestion
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Old 01-14-19, 11:32 PM
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Originally Posted by laffin_boy View Post
Thanks for your suggestion
Sure. If you really do want to sell it, post it on the Bike Friday YAK https://groups.google.com/a/bikefrid...um/#!forum/yak and Community page https://www.facebook.com/groups/bike...d=177147215204
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Old 01-15-19, 09:15 AM
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I'm very interested in this subject since I tend to think that e.g. a Brompton which has near zero trail would ride much better with some tweaking to the geometry. Having so little trail might have been an oversight which is now hard to rectify without massively complicating their production line, it might be an aesthetic decision to keep the pleasing lines of the bike. Or there may be other factors. For example, I imagine that slackening the head-tube angle would be a good solution, but at the same time I wonder whether the long stem design that folders tend to have and the weight of this part would then be more inclined to lean to either side... probably not... but really I'd have to test these adjustments to see how the ride quality was affected. But surely the brompton geometry is not yet optimized despite the company having existed for so long.
Having less rake on the fork is also an obvious option... perhaps it would reduce the natural suspension of the fork but more likely it would look a little less elegant aesthetically.
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Old 01-15-19, 11:38 AM
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Its reportedly 35mm,, a goal for big wheel Porteur bikes is about that ..

If you build a stayer* fork, raked backwards you have oodles of trail ..

* a bike built for motorpaced racing on the velodrome.. (not done much anymore)

but then the low speed maneuverability suffers .. a City commuting bike would be better with low speed maneuverability , doncha think?

want something other than the factory offers, mass produced ? you are free to find a Builder to make a one off..



The John Howard, Fred Rompelberg , Denise Mueller-Korenek, highest speed on a motor paced bicycle
would be using a longer trail steering , geometry for their speeds of 150 + mph..





..
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Old 01-15-19, 11:53 AM
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Yes, Mass helps

I have front pannier racks on my Bike Friday Pocket Llama , and 'Season Tikit' .. and use the front bag support foe luggage on my Brompton M3L...



FWIW a Porteur bike was a bike designed especially around quickly getting bundles of the latest edition of French newspapers to the news Agent's Kiosk

around the cities like Paris..

Having a lower trail helped the bike handle well and quickly with a front rack carrying several heavy bundles of newspapers..






....
Basically since the 2 lines that make up trail , on the ground, cross each other above the ground, If the 2 input factors remain the same..
Head tube axis and fork offset, axle plumb line ..
with a smaller diameter wheel, that crossing point is closer to the ground and so trail must be less..





...

Last edited by fietsbob; 01-15-19 at 12:11 PM.
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Old 01-15-19, 11:38 PM
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Originally Posted by fietsbob View Post
Basically since the 2 lines that make up trail , on the ground, cross each other above the ground, If the 2 input factors remain the same..
Head tube axis and fork offset, axle plumb line ..
with a smaller diameter wheel, that crossing point is closer to the ground and so trail must be less..
...
Sure, if you were to take a bike designed for 24" wheels and put 20" wheels on it the originally intended amount of trail would be way off. But hopefully that's not what we're talking about. Hopefully we're talking about fully competent bike designers adjusting the head tube angle & fork rake to the wheel size being used to give the optimum balance of stability and responsiveness.

And my question, for which i'm still searching for an answer, is whether that ideal trail figure for a bike with 20" wheels needs to be different from that of a "full-sized" 700c bike with it's "normal" 55-60mm trail. And if it needs to be different then why?
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Old 01-16-19, 10:09 AM
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Originally Posted by laffin_boy View Post
And my question, for which i'm still searching for an answer, is whether that ideal trail figure for a bike with 20" wheels needs to be different from that of a "full-sized" 700c bike with it's "normal" 55-60mm trail. And if it needs to be different then why?
It may not need to be different but there are clear factors tending to make it less. As has already been pointed out, all other geometry being the same, smaller wheels will have shorter trail. And this being the folding bike forum - most 20" bikes under discussion will also be folders. Folder designers want to reduce the folded size and this will encourage steeper head angles, reducing trail and wheelbase, while increasing cockpit size. I would assume that folder designers want to increase the cockpit to wheelbase ratio for the best compromise of cockpit room and fold. There may also be the factor of the shorter head tubes in most folder designs (including BF). While I'm not sure about this (perhaps an engineer could comment), I would think a short head tube would be stressed more by a slacker head angle.
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Old 01-16-19, 10:48 AM
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And if it needs to be different then why?
as said, low speed maneuverability .. as Multi Mode commuting is their intended purpose.
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Old 01-16-19, 10:52 AM
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Linberl hit the nail on the head...practice riding it for a while. Folders are twitchier to different degrees than full sized bikes but they track straight and true once you are used to them. I have found the low top tube to be nice when riding on sketchier surfaces since it is so easy to get your feet down.
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Old 01-17-19, 03:46 AM
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Wider handlebars is the first thing I would do. Get them as wide as you can. I need help with this one but isn't there a headset stabilization device by Hope. Please someone help me out with this one . Also the widest tires you can find would help a little making the bike slower.
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Old 01-17-19, 03:56 AM
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I just remembered the stabilization device is by Hopey, I think. On my Micro Black scooter I added a 115mm stem and wider handlebars and now it rides much more stable. Try the longest stem you can ride comfortably.
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Old 01-17-19, 11:50 AM
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But you don't have a 451 wheel Bike Friday pocket rocket which the OP asked about , it is a road style mini velo. you can pack in a suitcase ..

stem length is about fit to the bike ..

The company in Oregon has a sizing stem , adjusted to the riders satisfaction, then shipped back, and one with a nice curve is supplied with those dimensions..

A nice feature on the Pocket Rocket..
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Old 01-18-19, 10:05 PM
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I know nothing about the mathematics involved in bike geometry but it looks to me like my Tryell CSI is pretty similar to a 700cc bike in most measurements:

A Wheelbase: 1,034 mm
B Seat tube length (C – T): 405 mm
C Virtual top tube length (horizontal): 545 mm
D Head tube angle: 71.0 °
E Seat tube angle: 73.5 °
F Chain stay length: 425 mm
G Bottom bracket height: 272 mm
H Total length: 1,545 mm

Even still there is no way I would ride that with no hands. My old full size bikes I could ride all day no hands, even turn corners. But both my 406 Dahon Mu and 451 Tyrell are way too unstable. Which I personally like, I love the agility, but if stability is what you want then I am not sure you can find it with smaller wheels.
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Old 01-21-19, 06:30 AM
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Much to my surprise, I can ride a Tern Verge S8i with no hands quite easily.
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Old 01-22-19, 10:22 PM
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I started this thread because I was puzzled by the fact that my folder has dramatically less trail than full sized bikes as well as hearing that other folders also seem to have much lower trail figures than FSB's and I wondered why that was. I tried the question in the Framebuilder's Forum and did finally get an answer to my question. If any of you have wondered the same thing then there's a new-to-me geometry quality called Castor Angle (which, in spite of it's name, is NOT the same as trail) that creates a "Stability Index" figure from the ratio of wheel radius to trail. Therefore, assuming all other factors are equal (which they never are), the "correct" amount of trail for "neutral handling" on a given bike is directly proportional to it's wheel radius. So, yes, bikes with 20" wheels need to have proportionately less trail.

AND if you understand this concept you can compare the trail of your bike with what it ideally should have for "neutral handling". For example my BF Pocket Rocket Pro appears to have ~ 35mm of trail but for the tire size i'm using it should have been 43mm for "neutral handling" and closer to 48 - 50 for my particular limitations. So it's not surprising that the bike doesn't work for me.

Much more into at the Framebuilder's Forum > "Geometry gurus ?" Thread. (Sorry, i'm not allowed to post links yet)
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Old 01-23-19, 01:17 AM
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Geometry gurus ?

Here's your link. Interesting discussion over in the Framebuilders forum.
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Old 01-31-19, 01:01 PM
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This is an interesting subject to me. Over the last 2 decades, I have made 18 experimental 20” wheel bikes for myself, along with at least 2 dozen forks. They are more like “take-apart” bikes that do not dictate limits on head angles or fork rakes. I saw no point in trying different head angles, and stuck to 70 or 71 degrees for my “all-road” sort of riding. But fork rake (with the resulting trail numbers) seems to be the strongest determinant of how the bike handles. I tried rakes from 35mm to 13mm. Turns out, after about 100K miles of riding my small wheelers of varying geometries, I like plenty of trail. I am very happy with my most recent bike, a 71 degree head angle, 15mm fork rake, 42mm tire (406 wheel), which yields 70mm trail. I find nothing nervous or squirrelly about it, and it rides easily hands free.

There are good suggestions in this thread to tame a nervous handling small wheeler: add weight on front, bigger tires, longer stem/wider handlebars. Less fork rake is what has worked for me.
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Old 01-31-19, 05:04 PM
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A shorter top tube puts more body mass on the front wheel leaning over it, then compensate for the shorter top tube with a longer reach stem

For more trail the fork could be reversed ,

see 'stayer bikes',These are for high speeds , Velodrome, track, motor paced, so higher trail for stability.. note how long that stem is ...
so more body weight on the bars and front wheel

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