Theoretical Framebuilding Part 1- Geometry

Theoretical Framebuilding Part 1- Geometry


Technology alone is a poor substitute for experience.

– Richard Sachs

In my days working in science I had a supervisor that always said – do the experiment on paper FIRST and that means putting down the 5 minutes it takes you to walk to the spectrometer with the samples as well as anything else. He was right. If it didn’t work ‘on paper,’ it was NEVER going to work as you intended in practice aka it would fail miserably. This is the first of three part series on why and what makes a custom fit bicycle frame, different (and better) than ‘off the peg’ one. The posts are intended to shed light behing ‘hand picked, custom butted, special shaped tubing mix’ and all the terms you see and hear mentioned about custom bikes. This article is meant to educate and represents a successful experiment ‘on paper’ by an aspiring framebuilder (me). The science, metallurgy, physics and engineering are all researched, the opionions (as harsh as they may be) are all personal. The articles are ranked by order of importance. Part 1 discusses probably the most important aspect – geometry. Part 2 talks about tube forming and butting and it’s effect on “ride feel.” Part 3 delves in metallurgy and what heat does to metals and explores terms like, heat treatment, alloying, material stiffness and strength.

You might wonder what a thermonuclear weapon or an H-bomb in the header image have to do with custom bicycle framebuilding. Well to this day there is no OFFICIAL and CONFIRMED information on how the weapons work it is all speculation, though in November 1979 the author of the above article in The Progressive was able to deduce pretty much how it all functioned by just looking at the type of parts etc. ordered by army contractors, in a way he reverse engineered it all. Granted welding or brazing metal tubes in 3 triangles is in a different alley than thermonuclear weapons, Custom Bicycle Framebuilding just like cycling has not been spared by the many myths and legends and the extremely contradicitng information that is prevalent in pretty much everything in our daily lives. This is in a way refreshing since it pretty much points you towards the right direction – see what everybody else is doing and just do the opposite an you will be fine.

The curious individual that I am, I do like riding my bike quite a bit and over time I discovered that what is available out ther in terms of equipment out ther offered for somebody above average proportions height is highly dysfuncitonal , and that is putting it midly and abstainnign from nouns describing pure excrement. I was not able to find out why, information was just repeating some old dogmas “that everybody (and/or the PROs) have been doing up to now, so it can’t be wrong.” Needless to say this is what I refer to as a micro$oft/window$ type answer – correct but useless.

Therefore in my search for the most efficient position to make a bike feel like an extension of me, I came across Steve Hogg’s bike fitting website. If you have not seen it, go there after you finish this article, you will not be disappointed. If there ever was a methodical and clearly explained way on how us asymmetrical humans can function efficiently on a symmetrical apparatus that is a bicycle, this is the one stop place.

Through another article getting into the details on what the geometry does to bike handling. As I mentioned earlier I had a bike that I could fit on, though it rode interesting… Brand is not important, though suffice to say, it is one that has been ridden to great victory quite some times by the PROs. It was twitchy and extremely scary to ride at speed, leaned a LOT in corners, regardless of speed, oversteered like crazy, and on anything slightly technical would leave me trailing everybody else. Therefore as much it is not about the bike, the cogs in my head with the information from the above article pointed me that I might be able to really get something out of a custom bike. Here is the place to say that while 10% of people NEED a custom frame due to body proportions/injuries, everybody can benefit from one since off-the-peg items in generally are designed to fit no individual in particular and as many people in general.

Off-the-peg items in generally are designed to fit no individual in particular and as many people in general.

‘On paper’ I came up with a geometry that was supposed to cure all of the above problems. After some going back and forth with Steve Hogg himself, there were some touches here and there and I placed my order. The frame that I receved rode like it was supposed to and oh SO MUCH MORE! I will post a detailed full review on it soon.

Therefore with the next 3 series of articles it aim to dissect what makes a custom built frame special, different and perfectly suited for one person – you, no matter how (different from) average you are.

Bike Geometry Explained

I do send a warning the what I am about to describe are PRINCIPLES what might work for you and/or your clients will most likely be different from my examples here and second, all of the variables work together in combination and cannot be isolated.

Bicycle Frame Geometry. Source: Wikimedia Commons

Bicycle Frame Geometry. Source: Wikimedia Commons

How your body weight related to the wheels of the bike both horizontally (front to rear) and vertically (up and down) is the DEFINING variable that determines how a bike would behave according to its intended purpose (road, mountain, track, etc). To give you an exmaple a 140cm (5’2″) woman, your average Joe rider 175cm/75kg (5’11″/165lbs), a tall lanky person like me 200cm/82kg (6’7″/185lb) and a tall musuclar 100+ kg (220+lbs) frame snapping individual (they exist, I have seen them). Each of those examples has UNIQUE weight distribution when seated on a bicycle, with the short and tall people being the two extremes of the spectrum.

That weight distribution is a direct result of body proportions AND functionality.

Body Proportions, Weight and Stability

According to bike fitter, Colby Pearce, bikes exist on a spectrum on how the rider’s position relates to the machine. On one extreme end you have time trial bikes and on the other end you have downhill mountain bikes. Priority is aerodynamics and power production with almost no rider weight changes on one side versus super low seat heights giving the the ability to drastically shift bodyweight to maneuver (at speed) over extremely varied terrain. In between you find the road bicycle and I don’t necessarily mean a racing one. Since the principles outlined below apply across the board to all types of bikes, for simplicity I will focus on the road bike since a lot of riding such as cx/gravel, touring, etc. originates from there.

The more weight you have towards the rear of the bike, it will result is a bike that that wants to ‘wheelie’ when climbing and in off road situations (MTB) one that you cannot steer on loose surface. Conversely too much weight at the front can cause real wheel slip and unresponsive/sluggish steering. You want balance that is achieved either by shifting back and forth, up and down as is the case of downhill MTBs or by having your center of gravity at the right spot. Road bikes achieve that balance by placing 45/55% of rider weight front/rear. Off road/MTB requires dynamic weight shifting to match the terrain, i.e. more weight/grip at the front tyre, etc.

Center of Gravity

This is not a new idea. The seat tube angle is probably the most important measurement on a bike since this puts the rider at a balance point from where other positions originate – out of the saddle climbing and sprinting for example. The idea is that since the bottom bracket and pedals don’t change position, out of saddle efforts would always place the rider at similar position relative to the BB, however if you are sitting too far back too far forward, out of saddle positions require a massive shift of your center of gravity. Anything from 72-74 degrees is reasonable , going into extremely like 69 (super slack) and 77 (super steep) creatives massive compromises i.e. hitting the bars with your knees when out of the saddle and/or having ridiculously long reach to the handlebars.

Here we come to the balance again.

All movement is a balance between two muscle systems – phasic and postural. In short, there must be a stable platform for movement to occur. Postural muscle provide that platform for the phasic muscles. Since all movement starts with commands from the brain, it is no wonder evolutionary the postural muscles ALWAYS get priority. What does that imply for pedaling a bike?

In short (full article by Steve Hogg) in order to pedal a bike efficiently you should be in a stable enough position to be able to cantilever your torso forward WITHOUT the use of upper body muscles for support; your arms should NOT be holding you upright. Why?

Remember postural muscles get priority so energy is NOT going into pedaling, but trying to keep you upright. In addition a good number of the upper body postural muscles are involved in breathing – you don’t want to be limiting that. Under load the position should be self-supporting. Steve Hogg refers to this as the balance test. Warm up and pedal at about 80-90rpm at 80-90% effort (hard but not maximum). At the right saddle setback you should be able to swing your arms backwards. Also bike use/power produces plays a role into it (racing vs touring for example)

Balance Test

Balance Test

Ideally a custom frame should be build that your saddle of choice at the right setback sits in more or less the middle of the rails on your seat post of choice. Hence we get back again to center of gravity and seat tube angle.

The balance test is where it gets tricky. A lot of ideas on position are based on this body part over that frame bit and/or the flavor/pro of the week (slammed stem, my handlebar drop is bigger than yours, etc) coupled with the fact that most people spend 40+h of the week sitting at a desk, creating massive amounts of muscle imbalances and overall dysfunctions (i.e. the runner that has a gait with the butt backwards and upper body leaned forward is one example).

While we all “love” the interview question: where do you see yourself in 5 years, it gives the first clue to how the frame should be built. If you are flexible and can functionally!!! ride an aggressive position, it’s an easy answer, if however, you have some limitations and lack range of motion/can’t bend well at the hips, do you see yourself improving down the road? The answer to the last question can make today’s made to measure frame, tomorrow’s poor fitting one. My advise to all aspiring frame builders and also towards cyclists getting a made to measure frame is – Don’t compromise based on the latest fads. There is no substitute for experience. My father told me on multiple occasions that all compromises are like a boomerang that comes back and hits you in the head when you don’t expect it or eloquently put:

Folks don’t come to you to get your version of what is sold at the mall. That wasn’t even the case when steel was ubiquitous. Clients call and arrive at your doorstep because you have abilities and experiences that enable you to translate their information and construct a frame of higher quality and fine design, material is NOT part of the equation.

-Richard Sachs

The takeaway point here is that by going with body dimensions and stuff like knee over pedal spindle (KOPS) is an extremely limited approach that results in (sometimes quite significant) compromises which became super obvious for small and large riders, if you fall in the middle of the bell curve, you usually ‘learn to live with them,’ since in a way you don’t know any better. The methods mentioned above require time and some understanding of functional anatomy. Experience cannot be taught so it takes some getting used to it and to be able to translate somebody fit dimensions into a frame geometry. There are ‘standard sizes’ out there, however, you need understand what changing each of the numbers would do to the bigger picture, most importantly why pick one number over the other.

Some random examples:

  • If you need 140mm stem to be in a functional position the frame is too short for you
  • If you need an upright stem and lots of spacers, the frame is too low for you.

Obviously bike components have quite some adjustment built in them, however below are some aspects that can be directly influenced by the builder and comprise geometry.

Three Riders on a Bike

Below you see how a bicycle frame grows. The images are to scale representing a 48, 55 and 60 cm frames (S-black, M-red, XL-green). It’s not a perfect representation, though it serves a purpose.

While it may not be obvious to you, when frames grow, it is mostly the front triangle and wheels do not change in size.

How Bicycle Sizes 'Grow'

How Bicycle Sizes ‘Grow’

Two points stick out right away. Large frames put more weight towards the rear axle and small frames, if built with standard 700c wheels cause massive toe overlap and/or too long reach relative for the size. I am repeating myself that compromises become blatantly obvious at the two ends of the scale while the middle range ‘learns to deal with them.’

What do I mean by compromises?

How Bicycles Steer and Stay Upright

Almost everyone can ride a bicycle, yet apparently no one knows how they do it. I believe that the apparent sim- plicity and ease of the trick conceals much unrecognized sub- tlety, and I have spent some time and effort trying to discover the reasons for the bicycle’s stability.

-David E. H. Jones

It is not just the wheels acting as gyroscopes. David HE Jones went on to construct bicycles that were unrideable and he found it more difficult that he expected. In the end, coupled with the super computer of the time he made calculations and came up with the Jones stability index. Basically anything between 1 and 3 was rideable with 2 being the ideal. I am not posting the formula (you can read it for yourself) for a reason. This is only one part of what influences stability (head tube angle and fork rake) and cannot be viewed in isolation. Stable bike = bike that holds a straight line and takes more input in order to deflect it (corner) as compared to a less stable one. And no, unstable bikes are not the same as agile since agility involves coordinated-like movement, rather than twitchiness.

Therefore below are the elements of frame geometry that can be directly influenced by the frame builder that influence stability.

Obviously things need to be balanced and tailored for the purpose of the bike – extremely low bottom bracket on a bike to be used on a banked velodrome would cause pedal strikes, etc, etc. As usual keep in mind the big picture/purpose of the bike frame.

Chainstay Length

Longer chainstays = greater stability

Sometimes referred to as rear center, the longer chainstays shift more of the rider/bike weight to the FRONT axle relative to the wheelbase of the bike. Too much weight over one axle is not a good thing since it leads to an almost binary grip or no grip with no slip in between (and in my personal experience why I used to corner like a scared puppy on ice…). Riders who can get into a more aggressive position on the bike would need shorter chain stays then more upright ones for example. The length has also technical implications on the angle of the chain. Shimano’s 135mm spaced 10 speed (and above) group sets (mountain and cyclocross) have a recommended minimum length of 425mm, and I believe road ones (130 mm rear spacing) have minimum of 415 mm, yet most bikes fall extremely short of the mark. Why?

This is the place for a historical nugget of information or what I call “we have always done things this way, so it can’t be wrong” In and old edition of cycling textbook (Cycle Sport by Peter Konopka) I remember a picture of a “racing bike’ that had a curved seattube and the caption that was done to make for a stiff rear triangle (shorter tube is stiffer), and that was in the era of 19mm tyres….. Back in the day steel and bicycle tubing was a far away cry from even the low end materials of the last 2-3 decades and long tubes can be made extremely stiff without a weight penalty. Yet the old wisdom remains that the shorter the better. I can personally attest that a longer rear triangle makes a bike that rides (corners) like on rails rather than being plain scary and close to unrideable in anything other than straight line at speed. Granted someone of a smaller stature might not need 430mm like I do, 410-415mm on the SMALL sizes (48-52) and 420-425mm on the MEDIUM (54-58) ones is a good starting point (based on 28in 700c wheels). How is being able to corner FASTER with more confidence not racy?!???!?

Bottom Bracket Drop

Custom Bicycle Framebuilding Bottom Bracket Drop

75 mm Bottom Bracket Drop

Lower bottom bracket = greater stability

Back in the day cyclists used toe clips and even the first iterations of clipless pedals were quite bulky. As such pedal strike could be a real issue, especially in criteriums when pedaling in corners (whether that is the best way to do stuff is another question….). Simple physics points that the lower the center of gravity of bike+rider the more stable. Therefore the lowest bottom bracket drop you can get away WITHOUT hitting the pedals in a corner, is the ideal one for YOUR bike. In my opinion 70-75mm for road riding is ok. Smaller sizes that use shorter cranks can go lower. Bikes designed to be ridden with bigger tyres (touring, cyclocross/gravel) should have even lower bottom bracket drop – 80mm – since the tyres raise the overall center of gravty. Last, this is my personal experience, take it for what it’s worth – a more stable bike leans LESS at the same speed as compared a more unstable one, so the chance of pedal strike decreases even further. Speedplay users also have a distinct advantage here. Also bike companies have liabilities so avoiding tricky situations like wiping out 20 guys in a high speed corner is a good reason to avoid ‘bad reputation.’

Head Tube Angle, Fork Rake and Trail

Custom Bicycle Framebuilding Geometry Trail

72.5 degrees HT angle; 46mm rake; 58mm trail

More trail = greater stability

This is what David Jones used in his calculations. All of those three are interrelated. Headtube angle combined with the fork offset/rake create trail. Think of it as front tyre contact patch. The more trail the more the bike wants to ride in a straight line and stay upright and subsequently the easier it is to ride no hands. It takes MORE effort to deflect it away from center. A number to throw out there is 56mm or what is referred to as neutral steering, though as I mentioned above, your neutral might not be the same somebody like me. Most big brands keep only one type of fork rake and that leads to compromises in trail as the frames change size. In the ferrous times of old (ie pre carbon), a custom frame was coupled together with a custom steel fork, and it wasn’t solely for aesthetics.

The relationship goes as follows.

  • More fork rake = less trail with the SAME headtube angle.
  • Less (more slack) head tube angle = more trail with the SAME fork rake.

Front Center/Toe Overlap

With the proper weight distribution between the axles as per the bike’s intended use, the wheels end up where they end up. While not directly influencing stability per se, too much front center causes a light front end which is too twitchy to control (at speed) with the tendency to wash-out under the rider on loose terrain. To overlap can be a byproduct of front center and  it might be annoying in stop and go traffic and/or in techical hairpin turns in cyclocross or on bikes with fenders.

Do you even front center, bro?

Toe overlap can be used, and unfortunately it is also abused by manufacturers. Big sizes (for riders with large shoes) suffer the most since increasing the headtube angle (less slack/higher number=LESS front center), in order to keep weight distribution somewhat balanced between front and rear axles (more weight towards the front of the bike; same principle as chain stay length, and not get a version of the green bike above). This seems all fine since at first glance, yes you can live with some toe overlap, though in order to keep inventory low, fork rakes come in limited sizes (or ONE size in some cases for ALL frame sizes) and the end result is LESS trail (~53mm so below neutral or what I call crash inducingly twitchy…ask me how I know). As you saw above less stability if you are already more unstable due to higher rider+bike weight (large sizes = taller and heavier riders), is NOT a good thing (coupled with too much weight on the rear axle).

Small sizes suffer from the opposite problem (slack head tube angle/lower number) in order to make toe overlap reasonable (i.e. not half your shoe), resulting in MASSIVE trail figures (62+mm), combined with the smaller overall rider weight, causing the front of the bike to be a bear to steer (or a version the green bike above). Hence why 26in/650c wheels are a viable option for small riders. As far as toe overlap, having ridden bikes both with or without, I lean towards one without. Hey, it’s custom so why not?

Stem Length and Height

Longer and/or lower stem = greater stability

This can be changed and is not directly influenced by the frame builder, however, it is a part of the whole bike stability thing (and extremely long/short stems point at frame that is the wrong size….).

Just like chain stays influence how much weight is on the rear axle you can load the front axle more by either having the stem/handlebars more forward or lower. This is important when climbing in the saddle so that the bike stays planted rather than trying to ‘wheelie.’ Also a light front end is not confidence inspiring at (high) speed cornering and causes upper body fatigue from having to constantly ‘point’ the bike forward as it veers due to road irregularities.


Bike geometry is probably the most important aspect when it comes to a “made-to-measure” bicycle frame. There is much more than just tube lengths. The frame geometry is a result of the rider straddlign and pedaling it and both should work in harmony to achieve good handling. It starts with body dimensions/proportions and continues with the rider’s functionality and finishes with the intended use of the bicycle. All you have to do in the end is connect the dots.

Theoretical Framebuilding Series.

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