laffin_boy

I'm posting my question here in hopes of finding someone who actually understands bike geometry from both a theoretical & practical perspective.

My question has to do with a bike handling quality i'm going to call "stability" and my test for that quality is how easy it is to ride the bike no hands. And, specifically, my question is about road bikes with smaller wheels. In this case let's say 20" wheels. I have experience with only one bike with 20" wheels - which I find extremely "nervous" & "twitchy" - but my conversations with others who have experience with 20" wheeled bikes is that this quality seems to apply to most or all bikes with these smaller wheels. And these bikes all seem to have trail figures that are significantly smaller than most 700c bikes - often having trail figures half or less than a "full-sized" bike. I understand that trail is not 100% of what makes up the quality i'm calling stability but it does seem to be a pretty good proxy for that quality.

So my question: Is there a bike handling reason (as opposed to other specific use consideration reasons) that bikes with 20" wheels seem to have been set up deliberately with very little trail and very "nervous" handling qualities. IE: if a 20" wheeled bike were given, say, 58mm of trail would that introduce other handling problems that are worse than "twitchy" handling?

dsaul

More trail causes a bike to be more difficult to ride with no hands. When you lean a bike with longer trail, the trail causes the front wheel to steer into the lean. There are plenty of scientific explanations of trail on the internet, so I won't attempt to explain it further.

My road bike with 60mm of trail is easy to ride with no hands. My MTB with close to 100mm of trail is near impossible to ride with no hands.

calstar 

​​​​​​Hmm.... Definitely not a "geometry guru" but less trail = "twitchy" or fast handling, touring bikes usually are designed with a low trail but are stabilized with a front end load(panniers), not twitchy anymore. More trail = a more stable bike(should be easier to ride with no hands), quite a few new mtn bikes, especially downhill bikes, are being designed with more trail than just a few years ago for more stability at speed. Perhaps your MTB rides as it does due to other factors.

regards, Brian

unterhausen

I think he has it right, his bike has a lot of trail and it also has a lot of flop. Thus it's hard to ride no-hands. There are other advantages to slack geometry and long travel forks on an mtb. The geometry isn't constant, and sagged geometry is fairly important.

I don't have much experience with small wheeled bikes, but I suspect high trail makes the steering too heavy.

dsaul

"speed" is the key word here. A bike with longer trail becomes more stable at higher speeds. At low speed, the front end tends to wander with even the small lean input that comes from pedaling.

I have intentionally designed my off road bikes(MTB and Gravel) with high trail numbers to give me a bike that inspires confidence on high speed descents. I don't generally ride off road with no hands, so the low speed wander of the front end doesn't bother me.

Andrew R Stewart 

My experience with small wheels are with nominally 24" wheels (actually closer to 22.5", the 520 rim ISO family and 25/28mm profile tires). What I have found is that with their lesser rotating inertia they feel "quicker" in directional change. I equate this as being roughly "one step" different then a 700c wheel with the same rim/tire widths. So a 700c steering angle of 72.5 might be like that of a 71.5 on a 24" wheeled bike (everything else being the same. This is a seat of the pants and many year opinion from selling dozens of Terry bikes and making more then a few frames for the wife with 24" ft wheels.

The 650c wheeled bikes I've serviced, sold and made (only two frames at this point) seem to follow this opinion but about half way between the 700c and the 24" handling feel.

I certainly agree that trail isn't the only main factor at play in defining handling. The steering geometry isn't the complete picture either. Weight distribution as defined by front and rear (chain stay) centers as well as seat set back with bar reach/height are also strong factors. It is in these dimensions that my skill set begins to diminish As the small wheeled frames I've built are all (but for one) the same rider I had a pretty good sense what she liked for these numbers. I employed the time tested method of copying a known fit with only minor tweaks. Since one main reason to use a small ft wheel for many riders is to shorten the reach to the bars (and keep toe overlap from becoming an issue) the reduction of ft center has been followed by a bit of rear center reduction. But I tend to run longer chain stays then is trendy so this bit is not as much a change then some might read into my comments. In no way do I see a small rear wheel as a reason to have way short stays.

As to steering geometry in general I'll pass along what both Bill Boston and Georgena Terry have followed and add my opinion/numbers. There's a dimension that is not usually talked about called by BB and GT "castor angle" This isn't what car geometry calls castor, is in this case it is the angle with the ground that a line that passes through the steering axis contact point on the ground and also through the front axle. See attached diagram. BB (and GT) mention that a castor angle of about 81* gives "neutral" handling (whatever that means). When I talked with GT many years ago (and more recently in designing a couple of 26"/559 wheeled bikes) she says she follows this geometry guideline.

I have found that I prefer slightly longer trails then what are considered "neutral" handling ones so my frames have tended to have slightly lower castor angles. I made a chart of some of my more recent frames and their data (tire radius, head angle, rake, trail and resulting castor). See attached chart. You'll see that most of my frames have slightly less castor, between 78.4* and 80.5*. (The tire radiuses are 700c=336/342, 26"=315/318 there are no 24" wheeled frames listed because my second wife doesn't need that short a reach so uses 26" or 700c ft wheels).

What this means for 20" wheels? I'm not sure but if I had zero foundation I would look at the castor of 80*-81* and see where that puts rake/trail then look at published data and if at all possible test ride bikes that mimic known data. Andy

laffin_boy

Thanks for going to the trouble of copying your notes and your helpful remarks!

Yes I think weight dist is huge. On the specific 20" wheeled bike i'm unhappy with my handlebars are level with my saddle - which is a comfortable position but I think adds to my feelings of instability. Jan Heine, Editor, Bicycle Quarterly says: "More important is your riding position. If you sit more upright, you have less weight on the handlebars, thus less wheel flop, and so you should have more trail."

Thanks. This is a new use of castor angle to me. I get the sense that you use it mostly to double check your other settings to make sure it's going in the direction you want it to go. I calculated the castor angle on the bike in question and got 82.1° - well on the wrong side of "neutral".

Unfortunately most companies building bikes with 20" wheels (most of which are folding) don't publish their geometry figures and so what we're left with is subjective impressions and mythology.

Tom

laffin_boy

Thanks for replying. I thought I had mentioned the actual trail figure on the bike in question but, apparently, I didn't. It's 35mm with 451 x 28 tires. So if it were given a "normal" trail for a "neutral handling" 700c bike of 57mm are you suggesting that it would then feel "heavy" & "sluggish"?

My very narrow question remains: Are there handling issues for for small wheeled bikes that more or less require using much less trail than a full sized bike and, if so, what are they?

Tom

unterhausen

The way that 700c bikes virtually all have 45mm rake forks leads me to believe that there is not a lot of thought going into this. You will occasionally find a 55mm fork on smaller bikes, probably for toe clearance. They are all copying each other. I would guess that is doubly so with the small wheeled bike manufacturers. There are only a couple of innovators, and my guess is that trail isn't on their radar. But I think that people that are used to low trail probably would miss it. I think it makes a bike feel livelier, even though I know it isn't.

I wouldn't discount the importance of feel in this sort of question. If I was really interested in this, I would build a fork and see what it feels like. A friend had a 700c bike with an aftermarket fork that gave the bike high trail. It was scary riding around in a parking lot, I thought I was going to run into things because the steering was so slow.

Andrew R Stewart 

Eric hits on what I have considered doing for years but haven't yet done. Build a fork with adjustable rake but maintains head angle through the adjustment range. I have read, and seen photos, of others doing this. For a bike category (20" wheels) that has so little published data I think this experiment is a worthy consideration.

LB- How serious are you about your questions? I would consider making you a fork that you can experiment with. Of course we'd need to work through the dimensional needs and adjustment range as well as a few other aspects If you're interested let me know. Andy

Doug Fattic 

I made myself a travel take apart bicycle with 20” wheels in 2001. My goal was to be able to put it into a normal suitcase. It consisted of 3 sections. I tig welded lug like steel sleeves onto a long head tube and a separate rear triangle including the seat tube. Removable titanium top and down tubes connected the front and rear together. The top tube was held in place on both ends with a binder bolt on the lug/sleeves like a seat post into a seat tube. The down tube towards the head tube had the same kind of connection while near the bottom bracket shell I used a sleeve with binders on each end that slide over the 2 tube ends to hold it all together. I mostly just did this for fun to see if the concept would work. It had a decent ride. I was expecting some compromise in ride quality in exchange for its packing and traveling convenience. To my surprise it wasn’t much different than riding my normal bicycles.

We used to do these 1-week bicycle rides in Ukraine to raise money for our charity project. While paved roads in the countryside were reasonably smooth the ones in the city could be dirt (opposite of what one might think). For this reason I started out using wider BMX style of tires. That was a mistake because it made the bicycle ride sluggish and feel heavy. When I switched to 20” X 1 3/8” Continental tires the bicycle rode very similar to my normal bicycles. The key point I’m making here is that in this smaller tire size the width made more difference than I expected and should be a major consideration when designing the frame.

I don’t remember exactly the geometry I choose but I think it had 72º parallel head and seat angles and somewhere between 30 to 35mm of rake. Unfortunately I was rushing to get it done before I left for our trip and I sold it soon after I came back home so I didn’t document what I did. For reference I like my handlebars to be nearly the same height off the ground as my saddle.

Doug Fattic
Niles, Michigan

laffin_boy

Thanks. It may come to that and I may take you up on your offer.

Tom

laffin_boy

Andy:

I woke up this AM with the answer to my opening question. It amazes me how often i'm able to finally "see" an answer when i'm fast asleep that baffles me when i'm (allegedly) awake. I stared at the geometry diagram that you kindly posted until I thought I understood it. I was even able to calculate "castor angle" figures for my bike which I quoted to you. But it wasn't until this AM that the "duh" moment really arrived. The Boston-Terry Castor Angle (which as you pointed out is not really the castor angle of the bike) is a Stability Index figure that's the result of the relationship between the radius of the wheel and the trail figure. Period. So as wheel diameter varies then, necessarily, the trail figure must vary by the same proportion for a constant BTCA (Boston-Terry Castor Angle).

If my opening question had been: "Does the "correct" trail figure vary with wheel size or is it a fixed constant?" then I probably could have answered it myself. IE: it takes less force to turn a canoe than an ocean liner. But this BTCA is a very useful tool if you deal with different wheel diameters - and, in fact, it's exactly what i've been looking for the last month. IE: I know what the desired or "normal" trail "should" be for a bike with 622 x 28 tires (700c). Therefore that same figure on a bike with 451 x 28 tires (20") is:

(451 x 28 radius / 622 x 28 radius) X "desired trail for 622 x 28 wheels"
or
(253.5 / 339) X 58 = 43.3mm

(you can get wheel diameters here )

(to solve for BCTA with radius & trail I use this app on my android phone)

So this BCTA formula told me that the folding bike that I bought doesn't have anywhere enough trail AND that the folding bike that i'm considering replacing it with has exactly the amount of trail that should work for me. So for that i'm in your debt. Need someone to cut your grass or take out the trash? Call me.

Tom

Andrew R Stewart 

Tom- Yes. you have the gist of what Bill B called castor angle, a stability index of sorts. I haven't done the math but I wonder how it relates to Jan Heine's wheel flop factor. Back in the day when I was far more motivated to follow the math I came to understand that much of this effort is deciding which language you want to use to describe the same thing. Is it trail/steering angle or Castor or wheel flop or...?

One aspect not reflected by mere math on paper is the rotation inertia and how it changes due to wheel diameter (and maybe tire width). I alluded to this when I described that the smaller wheel seemed to shift the responsive feel from similar geometry (as defined by one's "language") with a larger wheel. I used to warn my potential Terry customers that the Classic (the Terry touring bike for years) handled more like a racing bike, with a 700C wheel, that they had before.

I wish we had hooked up may 2017 when I was in Portland to start my sort of cross country bike tour. Andy

rhm

I'm not a frame builder, but I have some experience with small wheeled bikes. I'll offer a few observations.... mostly anecdotes.

With the 'standard' size wheel (26-29 inches) you have a certain amount of "wiggle room", in which you can vary both head tube angle and offset a little one way or another for subtle changes in handling, while always remaining in the 'acceptable' range. The point I want to emphasize is that as the wheel size decreases, the "wiggle room" decreases. The tolerances are a lot smaller.

Alex Moulton was the principal prophet of small wheeled bikes starting about 60 years ago. I can't find it now, after a short google image search, but somewhere on the internet is a photo of an experimental Moulton bike from about 60 years ago, that had adjustable fork rake just as was described above. Based on what he learned, he introduced the F-Frame bikes in the early 60's. These had 16" wheels (specifically 16 x 1 3/8, 349 mm bead seat diameter). I have had a couple of those over the years, and I found both to be extremely twitchy. I could not ride either one with no-hands. I never liked the way they handled. I cannot now recall whether I tried riding either one no-hands with a heavy load on the front rack (they had an excellent frame-mounted front rack; so they were definitely designed for a front load). I found it odd that Moulton had considered this factor carefully, and arrived at a result I don't like... can't please everyone, I guess.

Raleigh answered the Moulton challenge first with the RSW-16 and then with the Raleigh Twenty. The former had 16x2 wheels (305 mm bead seat diameter, but overall diameter about the same as Moulton's wheels). My wife has one, and I find the ride to be very twitchy; I can't ride it no-hands (but it is much too small for me anyway, and this is a factor). She doesn't ride any bike no-hands. I've also ridden a Raleigh Twenty, with the same result: too twitchy for my taste.

I remember reading, presumably somewhere on Bikeforums (presumably in the folding bike subforum) a post (from about ten years ago, I'll guess) by John Forester, in which he described straightening the fork of a Raleigh Twenty, reducing offset slightly, with the result that the handling was improved. But as I say, I can't find this post anymore. I did try straightening the fork on my Twenty, and it helped.

I used to ride a Counterpoint Presto! recumbent, with 20 x 1 3/8 wheels (451 mm bead seat diameter) and that, too, was too twitchy for my taste.

Then again, when my daughter was an immortal and unbreakable ten year old she had a piece of junk 20" bike that I pulled out of a dumpster for her, and she could ride that thing all over the neighborhood with her hands in the pockets of her jacket. It was perfectly stable, and she could steer around corners etc by balance. My current folding bike (a Downtube 8H from 2015, I think) is stable and rides fine no-hands. Both of those bikes have 20" (406 mm bead seat diameter) wheels.

And for several years I rode a folding bike with 16" wheels, the same size rims as the Raleigh RSW-16 (mine was a Downtube Mini from 2007). It was designed for 16 x 1.75 tires, but in that configuration the handling was a bit twitchy, and I could not ride it no-hands. I found by putting the narrowest available tire on the rear (1.25") and the fattest one on front (2") I shifted the head tube angle enough that I could ride it no-hands without any difficulty.

Andrew R Stewart 

I would be interested in the geometry numbers of these bikes. Both the ones that don't feel stable and those that do. I have (on long term loan so not available for me to measure) a Raleigh Twenty (as opposed to the folding DL-20) and don't remember it's no hands stability. Although I have had far less challenges riding various bikes no handed, including on rollers, then many people I know have. So my opinion about this aspect might not be in the center of the bell curve.

The comments about slacking the head angle would support my view of small wheels need greater inherent stability then many might think.

As an aside- The Moulton front racks are different then the current trend of front racks. Today (and for decades on front bags on touring bikes) we see a lot of larger front loads that steer with the ft wheel. If fact I don't know of any current bikes, excepting Moultons, that don't have their ft racks mounted onto the fork (but I haven't done any searching about this). But the Moultons have their ft racks mounted to the frame, the rack remains stationary with the frame. So when turning the rack weight doesn't remain centered over the tire's contact patch or ft axle. Again I have not done any research about this load/rack difference. I've just known of it for decades and sometimes think about it. Andy

rhm

The Worksman LGB (with a 20" front wheel) has the cargo basket mounted to the frame, Moulton style.

I still have some of the bikes I mentioned, so if you want me to measure, please tell me exactly what to do. I'm not confident of my ability to measure accurately and precisely.

laffin_boy

Yes, you're correct, - smaller wheels have a proportionally smaller range of trail which will produce desirable handling characteristics but I don't think that's why many folding bikes handle so poorly. (any reputable bike manufacturing facility should be able to produce forks with a rake that's + or - 0.5mm and that's sufficient to hold the trail figure within acceptable limits) It seems more likely that there's 2 possible reasons for the epidemic of twitchy folding bikes. One might be that steeper angles / shorter wheelbases make the bike able to be folded more compactly and in the folding bike arms race they're willing to sacrifice good handling for smaller luggage. The other is that they simply don't know what they're doing. Of course I wouldn't be foolish enough to post these blasphemous opinions on the folding bike subforum or i'd likely be hunted down and have my folder confiscated with extreme prejudice.

Tom

laffin_boy

Well wheel flop is directly proportional to trail. More trail = more flop. I use this online trail calculator to figure trail and it gives you flop too. I view flop mostly as an annoyance since, as a practical matter, we only see it's exaggerated effects at ultra low speed. (BTW I think the wheel size he uses in that calculator are wrong. I use the figures from this chart to get the wheel diameter and then just play with the tire width figure in the trail calculator till it agrees with the desired diameter)

Yeah, wheel inertia is definitely a big part of the stability problem with small wheeled bikes since I=mr2 meaning that the inertia varies as the square of wheel radius. And weight weenies like me make it worse by using the lightest tire that we can get and therefore reduce the rotating mass on the rim of the bike where it has the maximum effect. Fortunately Jan Heine has rescued me from the mythology that skinny tires are faster and so my new folder will have the lightest 451 x 41 tires I can get.

Tom

rhm

I don't think I've ever seen a 451 tire with a nominal width over 32mm or an actual width over 28 mm.

laffin_boy

These are available in 1.6" (41mm) & 1.85" (47mm) for BMX racers. The 1.6" (even though it has more "tread" than I want) is only 290g!

Tom

Andrew R Stewart 

One can always sand off unwanted tread cap thickness. Car companies have done this to produce better handling production tires and fool the consumer mags Andy

Andrew R Stewart 

I should have remembered about Worksman, having sold and serviced more then a few many years ago...

I would be very interested in the small wheeled bikes steering geometry that you have. There's a few ways to measure and all have some chance for error. I find that the best way to minimize errors is to do two different measuring methods and average them.

The absolute best way is to use the end points of the center lines that make up the steering axis angle, the axle to ground contact, the steering axis to axle (rake) and a direct tire diameter. Using center points reduces drift and increases the length of measuring so to also reduce line/angle errors. Trig then produces the angles.

The bikes must be secured upright, balanced side to side evenly, the front wheel held straight ahead (more on this later), kept from rolling fore or aft and on level ground/floor. A carpenter's square (24"x12"), a yard/meter stick/rule, masking tape, fine point marker, a couple of thin stiff somethings (like popcycle sticks or the same rulers) and pen and paper are needed to measure with.

A point along the side of the head tube at it's top and bottom need to be marked. Masking tape and the marker helps here. One trick is after placing the tape on the headtube is to take the yard stick and with one end close to the seta tube and BB and the other end on the top/bottom of the head tube rub it against the tape. This will mark really close to the side's center of the head tube. Mark these with the pen/marker. For each mark you want to do a couple of things. The first is to drop a plumb line to the ground and measure up. The second is to hold the yardstick along the headtunbe so it runs parallel to the steering axis best you can and touches the floor. Placing a strip of tape from just in front of the tire's contact patch and back to well under the top of the head tube serves as the marking surface for the plumb lines from the head tube and the steering axis center line. The steering axis center line will need both the bottom and top of the head tube measurements (By doing two of these rise and hypotenuse measurements you have two different calculations of the steering angle to average).

Next is the axle to floor to establish the tire's contact point. The first way is to use the carpenter's square or plumb line and center the axle WRT the ground, then mark that spot on the tape on the ground/floor. The other way is to take those two stiff and this strips. slide each under the front and rear of the tire with even pressure and at right angles to the bike's length. This gives two "lines" to measure half way between to get the tire's contact center point. mark this on that ground/floor tape. (From this point to the steering axis's point is trail).

Rake is the last bit (although with trail, steering angle and tire diameter rake can be calculated, but direct measurement is the double check). This is the hardest for most to measure as the steering axis is a virtual line and the rake is measured at a right angle to that. One can use the carpenter's square and/or yard stick to establish the steering axis and then using a piece of paper (with a square corner) measure/mark the axle's distance from the axis. Having a second person can help as more then two hands is needed usually. One can lay the bike on it's side, remove the ft wheel and do two measurements. One with the rake/fork pointing forward and one with the fork/rake turned 180* backwards to vary the measuring and provide mote averaging data.

The tire diameter is pretty straightforward to measure.

One can use the digital angle protractors to double check steering angle. Running through all the lining ups and markings a second time will give more averaging. Working with another while doing this can help with extra hands and allow for a second set of eyes looking at line and points. How to brace the bike completely upright is your issue, I find that trapping the back wheel between chairs/boxes works well. Using a strap (toe clip strap for us oldies) around a wheel and the frame will steady the roll fore and aft.

With these dimensions I (you) can calculate the castor angle and wheel flop. If you do this please report with your findings. Andy

rhm

Oy! Well, no wonder I get different results every time I measure a frame: sounds like I've been doing a halfassed job.

Many thanks for the detailed instructions; at this reading I'm not sure I quite follow, but when I actually put this into practice I'll figure it out (or post followup questions).

I will get to this one bike at a time and, with luck, will give you some results.