Third (last) part about geometry. How we sit on bicycle (bikefit)

6 October 2023
Although the fit is the most important thing, I write about it last. To grasp this one, you just need to be in touch with a particular person and her physiology.

But it’s also difficult to fully understand the previous parts without realising that all the dimensions are zeroing in on the rider.

There are three areas where a person makes contact with the bike:
• feet-pedals,
• butt-saddle,
• hands-bars.
MINIMAP | Feet-Pedals — Butt-SaddleHands-BarsConclusions |

1. Feet-pedals area

As we remember from part two, it’s good to have a lower bottom bracket — which also means the preference of:
1) a narrower q-factor,
2) shorter cranks,
3) more compact pedals.

Of course, there are many “buts”.
1) We can squeeze a gravel tyre in between the road q-factor (145–150 mm), for the benefit of those who are fine with a narrower foot position, but some people, on the contrary, need a wider one.

In the latter case we can move the clipless cleats inside the soles (or rather — extend the shoes over the cleats).

I must say, cleats are a topic on their own. A particular person may even need asymmetry there: one cleat to be moved more to the side, or back/forth, or downwards with a spacer — depending on the physio features of the human.
Over the internet, I’ll limit myself to just a couple of points:
• many bikefitters like to move the cleats backwards rather than the other way round,
• you should never use zero-float cleats without professional supervision.

To put your feet even wider without changing the q-factor, there are pedals with extended axles. Alternatively, you can screw the pedals in with washers (but you can’t use much of them before running out of the safe margin of thread) or adapters (but they add a lot of width at once).
2) Short cranks (160–165 mm) not only reduce the overlap, but also allow you to sit more aerodynamically — in the upper pedal position, the hip does not rise as high and, speaking in simple terms, leaves more room for the torso to drop down.
For some reason, the manufacturers tend to use too long cranks on their complete bikes. Here are some formulas. If any of them returns a number shorter than your cranks, you might want to consider shorter ones.

Maximum crankarm length:
= your height × 0.095
= your inseam × 0.2
= inseam × 0.125 + 65 mm
= tibial length × 0.41

The inseam is measured here from the floor to the backbone of the book, which should be tight up against the crotch and perpendicular to the wall. How to measure the tibia — I suggest you browse the internets.
Wide-spread is opinion that there is no such thing as too short cranks. Anyway, note that with shorter cranks:
• all the gears get heavier,
• it’s harder to spin up a heavy gear from a standstill,
• the pedalling cadence tends to get higher.

If your physiology requires a wide pedal stance (see above), shorter cranks mitigate the risk of the pedals striking the ground.
3) Pedals have different thicknesses — called the stack of a pedal or, more precisely, of a pedal body + cleat. This one is really simple: the smaller the better. Same also applies to the thickness of the (in)soles.

In addition to pedals with longer axles, there are also those with shorter ones. If they fit you physiologically, there’s no reason not to choose them.

2. Butt-saddle area
MINIMAP | Feet-Pedals — Butt-Saddle — Hands-BarsConclusions |

The main figure of the bikefit is the saddle height. It is measured down to the bb, plus the length of the crank, minus the pedal stack. That’s why we first deal with the legs and then set the saddle.

Secondary measures are the saddle setback from the bb and the tilt. As for the setback, please forget about the kops (knee over pedal spindle) rule right away, it’s unscientific rubbish.

By default, the saddle is set without tilt — so that its front two-thirds are parallel to the ground. To tilt the saddle, you need a weighty (yep!) reason.
The exact dimensions only apply to a specific shape and size of a saddle. That’s why the seat tube angle is a relative-ish parameter (I touched upon this in part two).

The only criterion here is whether a particular saddle can be adjusted so that the butt is at a certain distance (and with a certain setback) from the bb/pedals. That depends also on how much of the adjustment is in the saddle rails, the seatpost offset, etc.
How to understand what distance and setback those are — is a separate topic, which we will not go into now. I do, however, have a few rules of thumb for choosing the saddle itself.

A road/gravel saddle should be:
• stiff enough,
• as flat as possible (not arch-shaped in cross-section),
• suitable in width (~ 1.5 cm wider than the sit bones — you should measure them with a special device),
• if you ever feel numbness in perineum — there must be a cutout (and preferably a wider one).
MINIMAP | Feet-PedalsButt-Saddle — Hands-Bars — Conclusions |

3. Hands-bars area

Once the saddle is fixed, you can move on to the hands. The drop bar has three basic grips:
A) on the uppers/tops,
B) on the bottoms/drops,
C) on the duals/hoods.
A) The tops are the easiest, so let’s start with them. We hold on to them mostly on climbs. Or when resting on long rides.

The position of the tops depends on three parameters:
• reach — the horizontal distance from the point of contact between the stem and the headset cap to the plumb line above the bottom bracket,
• stack — vertical distance from the same place (stem bottom / headset top) to the height of the bb,
• stem length and angle.
Here I nod to the first part: what we really want to do is to first define a handlebar position that suits our anthropometry and then “move” from it: to the back along the stem (of sufficient length), down the head tube angle (to approach the desired front-centre from the rear of the stem) and forward again with the rake (to get the desired trail at a given head angle — and arrive precisely to the front-centre).
The bar tops should be far enough away from the pedals so that you don’t knock your knees against them when riding out of the saddle. But close enough to it so that you don’t have to stretch forwards, which can cause back pain and other aches.

(Remember that the saddle is already fixated against the bottom bracket, so the distance from the handlebars to the saddle is the same thing as the distance from the handlebars to the bb, only with the added constant of saddle setback).
B) Next — the drops. We use them mainly on descents, or for the sake of better aerodynamics at long-ish speedy segments or in a headwind. (For short-timed aero gains, it is better to just bend your elbows more, which is uncomfortable, but faster.)

The position of the drops relative to the tops depends on the shape of the bar and the angle of its installation. As a general rule, the drop bar should be set so that the pointed out segments in the picture do not look upwards (but parallel to the ground or downwards).
Nowadays they often make handlebars that are wider at the bottom. This is logical: at the drops we want more control, and also not to hit our elbows with our knees, but to separate them by different width.
C) Hands on the hoods — is the default position. Unlike on the tops here we have access to the brake levers, albeit with less leverage and a less firm grip than on the drops.
The shifter hoods are aligned so that the hand is in a natural position relative to the forearms. As a rule of thumb (hah!), the pointed out section in the picture should face slightly upwards.
It is the shifters that need to be moved — and only after the handlebar has been set at the correct angle. A common mistake is to try to adjust the hoods comfortably by turning the bars towards yourself. After that it becomes impossible to get a good position on the drops.
Often the hoods are tilted inwards — this happens to be physiologically substantiated, i.e. comfortable.
If there is such an adjustment on your shifters, you can move the brake lever closer to the bars so that you can reach it with your index finger from the drops. If there is no adjustment, you may have to find a compromise between a comfortable wrist angle on the hoods and good brake accessibility on the drops.
The width indicated on the handlebars is usually measured just at the point where the shifters are attached. Look at yourself in the mirror from the front — if your arms are not parallel, you could try a different handlebar width. Or sacrifice comfort for aerodynamics and get the narrowest one :)
And that is how we finally get to the conclusion that the basic position of the arms is determined by: the stack, reach, length and angle of the stem; the shape, width and angle of the handlebars; the shape of the shifters and their installation. And the actual stack and reach (as opposed to the ones advertised for a frame) also fluctuate depending on the number of spacers under the stem. That’s bikefit, baby!
4. General considerations

So, the points of contact with the bike (legs, butt, arms) are not really points, but zones — with quite a lot going on in each — and yes, there are some “tolerances” within which the fit can be considered correct.

The smallest amount of the leeway is in the saddle height — it’s no coincidence that the saddle has such a large adjustment range. Even if the bike’s sizing is not ideal, the saddle must be in the right place.
The next most important thing is the bars position. And here the industry is getting all doomer and gloomer (complained boomer to zoomer, lol). For the sake of beauty (and a couple of watts of savings on aerodynamics), expensive bikes are fitted with integrated barstems with internal wiring — and while in the past you could buy a bunch of cheapo stems and spend a minute to swap them over in search for an optimal fit, now it’s cheaper to go straight to a bikefitter before buying a new bike (and then anyway become more flexible, stretch your posture — and still spend hours of mechanic’s or your own time replacing the handlebars that cost as much as an entry-level bike).
While different stem lengths can be used to change the reach to bars, the foundation is the frame size. There are three main parameters that determine the size of a bicycle:
• one’s height,
• inseam,
• conditional flexibility.

Height is often cited by bike manufacturers and sellers as the only criterion, but this is a flawed practice. People of the same height can have various inseams. But — one could argue — isn’t that’s why the saddle is so adjustable. Well, yes but what also depends on the ratio of height and inseam is torso length — for the same height, it is the shorter the longer the legs are. And the torso length affects how far the handlebars should be from the saddle (i.e. frame reach and stem length).

Therefore, if you have relatively long legs (& short torso), you should look for a frame of a smaller size than the manufacturer recommends for your height, to avoid a too-short stem. And vice versa — if you have relatively short legs (& long torso), a larger frame is more likely to be suitable for you.
This is where the third parameter — flexibility — complicates things. It determines how low you can afford the bars to be in relation to the saddle. And a smaller frame (with a lower stack/bars) with the seatpost extended for a long-legged person requires of her much more flexibility than a larger frame (putting bars higher up) with the seatpost recessed for a short-legged person.

Therefore, long-legged people with short torsos should look towards endurance road bikes and “comfort” gravels (which have less reach with the same stack — or more stack with the same reach), and short-legged people with long torsos should rejoice: they are better suited to more sporty (long) frames, and enjoy the benefit of having better aerodynamics and a lower centre of gravity.
The average inseam-to-height ratio is 2.1 to one. Flexibility, alas, cannot be measured. Even a bikefit allows for determining it only approximately, after which you have to ride a lot, try, adjust, repeat.

The best advice I can give here is that if you’re not completely sure you have your fit really dialed-in, buy a bike with a separate stem and handlebars, no fancy hidden wiring and a standardised round seatpost. (Heads-up: even if you are sure your fit is final — well, it’s not).
MINIMAP | Feet-PedalsButt-SaddleHands-Bars — Conclusions |

Traditional tl/dr:

• first deal with cleats & pedals, then proceed with the saddle alignment, and then move on to the handlebars position; once all those zones are fixed, you can be choosing the frame size by the seat tube angle, stack and reach (with certain allowances);

• Long-legged and flexible riders go for a race frame smaller than the recommended size for their height; yet-inflexible riders look for an endurance/comfort frame;

• Long-torsoed riders are more fortunate — flexible ones can combine the recommended size with a longer stem; the flexible-to-be go with a larger size; both get better aerodynamics and weight distribution as a non-insignificant bonus;

• Integrated handlebarstems, hidden wiring, and proprietary seatposts are the enemies of bikefit. If your fit hasn’t been cemented over the years of riding, don’t fall for it. Especially since the aerodynamic benefits there are so marginal for non-racers that all practical benefit comes down to the ephemeral “look like a pro” thing;

Thanks again to the Made show photographers that we get to see so many beautiful and diverse “not like pro” bikes :)