Progressive geometry (that’s lost the “forward geometry” pun)

5 October 2023
Ever seen pictures like this? Let’s dissect this mess.
bicycle drawing with overwhelming measurements denoted
Those measurements can be classified into two types:
• depending on our physio (bikefit) & affecting how we interact with the bike,
• affecting how it is handled per se, and behaves in general.

Progressive geometry is all about the latter.

The most important figure there, I’d say, is Trail. That’s like the offset in a shopping cart or an office chair wheel, but a bit more complicated.
coaster wheels of shopping cart and office chair
(Did you know that a chair can be equipped with racing wheels — even various ones, depending on surface — but I digress).
coaster wheels of office chair resembling racecar wheel and rollerblade wheel
MINIMAP | Trail — Front-CentreStem LengthConclusions |

So, Trail. It is formed by two other parameters:
• head angle (of the steering axle), and
• fork offset aka Rake. What is being offset — from the axle — is the front hub.

I marked Rake orange. Note: Rake’s only purpose is to decrease Trail where otherwise it would be too high due to a slack head tube.
drawing showing dependency of trail on head angle and fork offset
Trail — marked green — is the “on ground” measurement between the plumb line from the hub and the projection of the steering axle.

(More precise parameter is the mechanical trail, which is measured from the ground underneath the hub perpendicular to the steering axle — but we will put it aside for now).

Apparently, Trail is quite a modest measurement — normally within 10 centimeters. So, why it is that important?

You won’t believe it, but it is still a scientific mystery why a bicycle gravitates to staying upright. In short, there’re two theories:
• due to the gyroscope effect,
• because of the caster effect — or Trail.

A further explanation is here:

Anyhow, Trail affects how the bike handles. Same as a caster is straightened when a shopping cart or an office chair is pushed forward, the bicycle front wheel is stabilised straight when it is pushed from the rear.

And the larger the Trail, the more stable the wheel — at a given speed: a bicycle is normally moving fast enough to be stable at more or less any positive Trail. But, as I will explain, there are nuances.

Okay, less words & more pictures already. And right away — the wild card :)
bicycle with steep head angle and no fork offset
You remember that Rake serves the purpose of decreasing Trail. But, as we can see here, there may be no Rake at all — while Trail would still be in check. That’s possible if the steering axle is really steep.

Note: Trail is also affected by the wheel radius. The track bike above has wheels as small as 24" in diameter (as on BMX). If the wheel was bigger, the trail would increase too.
bicycle wit slack head angle and no fork offset
Here is another weird bike without Rake and with small wheels. Though they are even smaller here (16 inches), Trail, on the contrary, happens to be higher — because of the slack steerer.
bicycle with fork offset and low trail
One more iconic folding bike with 16-inch wheels — and this one, vice versa, has a very low Trail. To achieve that, Rake has been introduced, which clearly shows due to the curved fork.

But why on earth such similarly-purposed bikes would have Trail so different — if that is the “most important” measurement? Well, let’s not burry ourselves with the circus bicycles, and finally move on to normal ones.
bicycle with classical racing look and curved fork
Take road racing bike. This classical model again demonstrates the curvature in the fork, but Rake could be formed by any different shape of it — as far as it ends in the right place.
bicycle with modern racing look and straight-leg fork
After a hundred years of racing bikes evolution, they landed on a narrow spectrum of Trail — 55–60 mm. As far as not-too-weird bicycles are concerned, that is practically the minimum (with a notable exception — but we’ll hold on it for now). So, when we talk road racing bikes, we normally mean a low Trail.
bicycle with progressive geometry
And here above is a gravel bike with progressive geometry: slack head tube, long front triangle of the frame — and, consequently, a short stem — and a high Trail. Here, in particular, it equals 92 mm, which is a lot.
bicycle Santa Cruz Stigmata with progressive geometry
A more glamorous example of such a gravel bike is the latest-and-greatest 4-gen Stigmata, the progressive geometry of which has been buzzed about from every corner lately. No wonder this bike is made by the company who’s famous for mtb: progressive geometry comes to the world of gravel from that direction.

Anyway, let’s make things clear by placing one of those bikes to how it should really be positioned.
bicycle with progressive geometry tilted down with reduced trail
Now it shreds down a 20 % grade. But wait — where has the Trail gone? The plumb line didn’t care and stayed vertical, and so Trail is almost gone. Well, if you look closely, just a tiny bit of it is still there, so all good.

Although such grades are more common on gravel roads, let’s put a road bike, with inherently low Trail, in same situation.
bicycle with road geometry tilted down with negative trail
And now its Trail has become negative. Which would cause the wheel to try and steer away from the straight, making the bicycle unstable.

Btw, here I’ve taken the bike with a very short front triangle — so you could imagine yourself riding it, and grasp the feeling of how the progressive gravel bike looks (and is) balanced on the slope, thanks to the front wheel shifted forward, while the road bike is eager to tumble over the bars.

So, progressive geometry lets us ride down grades so sharp that a road bike would rather call for a walk. And that is achieved by both a good margin of Trail, and a stretched front — the distance between the bottom bracket (and pedals) and the front wheel (its hub). It is called Front-Centre and we will look into it.

But first, let’s get back to the mentioned exception with a very low Trail — even lower than road bikes’.
bicycle with randonneuring geometry with low trail and bag in front
This is a classical brevet bike. Randonneurs ride these at audaxes — rides spanning from 200 km to eternity (okay, usually up to 1500 km).

It has a super-low Trail for a reason — there is a load in front of the steering axle, which, combined with a regular Trail, would make the bike difficult to control.
bicycle with fork offset and low trail with bag in front
Finally, we have the answer to the above question of why the folding Brompton has such a low Trail compared to the folding Strida with the equally-sized wheels. Brompton owners just love to carry stuff like that.

Now — to Front-Centre.
drawing of bicycle demonstrating front-centre measurement
MINIMAP | Trail — Front-Centre — Stem LengthConclusions |

As we understand, the farther the front wheel from the bb, the more balanced the bike on descends — which is a good thing.

But why than road racers prefer to have the wheel closer instead: even choosing a size or two smaller racing frames, which are short in the first place, in combination with a long stem?

Most often races are won either at climbing segments, or at finish sprints. In both instances an athlete needs to stand up from the seat and really step it on the pedals while pushing & pulling against the bars. And it is much better to do that if the wheel is directly under one’s arms, and not somewhere in front of them. Thus, a compact Front-Centre and a long stem.
drawing of bicycle demonstrating possible geometry change with green line
MINIMAP | TrailFront-Centre — Stem Length — Conclusions |

But this is where a geometrically-cautious reader would ask: but why not just elongate the front triangle, shifting the steering axis forward (to the green line) — and get the same reach to the bars, same Front-Centre, and same Trail, but with zero-length Rake and stem? Why we even need them? Well, there’s one-and-a-half reason.

The longer the stem, the more distance hands need to move to turn the bars. In that sense, a long stem adds some steering stability. However, that is just half the reason for a long stem, as even a short one would be just fine, if to add a long Trail and/or wider bars to the mix (as proven by any modern mtb).

Let’s take a look at probably the most progressive gravel bike in existence — Evil Chamois Hagar.
bicycle Evil Chamois Hagar with progressive geometry
A very short stem + a very long Trail = sufficient stability.

The real reason behind racers choosing long stems is how much of the bar drops is behind the steering axle. If to try and sprint from those “backward” drops, one would in fact get a negative-stem effect — which means no stability; inability to confidently leverage the drops.

Btw, try and use your spatial imagination to see how far in front of the steering axle the hands would be on the hoods — even with such a short stem-per-se, the “effective stem” would be some 12–15 cm. So, the above formula happens to need a correction:

A very short stem + a very long Trail = sufficient stability — on the hoods.

But not on the drops. And hence why the road/gravel racers require long stems.

Remember the Stigmata, the one that is proclaimed to be the marvel of progressive geometry? Let’s now see it as a complete build for Keegan Swenson — seemingly the strongest gravel racer right here and now. In fact, the build that he rode to the victory in the most prestigious gravel race of 2023, the Unbound 200.
bicycle Santa Cruz Stigmata set-up for Keegan Swenson
Pardon the distracting numbers, I couldn’t find a better picture. What really interests us here is the 120 mm stem, compared to the standard 70 mm, put on the smallest-in-range size S frame, ridden by a 178 cm athlete. The progressive geometry out of the window — at least for the competition setup.

That way Swenson gets both a long stem and a high Trail. This brings unnecessary margin of stability (there is more than enough anyway at the speeds he rides at), as well as two other factors, that are not even relevant for gravel racing — as they only play at very low speeds — but concern us nevertheless.
bicycle with toe overlap with low trail and rack in front
1. The lack of a toe overlap between the front of the shoe and the wheel when it is turned. Here on the picture we have a massive overlap — not just the shoe, but even the pedal is almost striking the tyre.

(As a bonus, once again we can see a low Trail for the front-cargo functionality.)

The overlap is not bothersome when riding at speed, but when maneuvering the bike at a pedestrian pace, it can be frustrating or even cause a fall.
bicycle with mechanical trail demonstrated with negative stem
2. A significant wheel-flop — that is the reverse effect of a long Trail, directly proportional to it. Now is a good time to recall the notion of the mechanical trail, which differs from the “ground trail” in that the former is measured perpendicular to the steering axis (as on the picture above).

The mechanical trail acts as a lever between the tyre contact patch and the steering axle. This creates a power-steering effect at a low speed — if one hand pushes against the bar stronger than the other, the wheel actively tries to flop and turn. The longer the mechanical trail (lever), the sharper the wheel will be steered away even with a small difference in the pressure of the hands.

Thus, a high Trail, on one hand, makes the wheel stable — but on the other, causes instability to the steering at a low speed. This may be a bit distracting when climbing a steep grade, especially so on a loose terrain.

Also, if you’ve ever noticed how sprinters are swinging their wheel all over the place when dashing forward, it is easy to understand why they prefer a low Trail with a minimal flop.

(Take a chance to look at the negative-length stem on the bike above — be sure this one is not intended for sprinting :)
bicycle serving as another example of interesting geometry
MINIMAP | TrailFront-CentreStem Length — Conclusions |

Seems we have the front geometry all sorted out. Later on, I will get to the backside as well. And now — some categorical conclusions from the above:

1) the most important geometrical figures, that affect how a bike handles, in the front of it, are Trail and Front-Centre;

• the main advantage of a high Trail — added margin of stability, particularly on steep descends where Trail turns from long to normal (instead of turning from normal to a too-small or even negative);

• also, I have a theory that the shift of Trail to negative on sharp and fast descends is what causes the wobbling — a dangerous and poorly understood phenomenon;

• the main advantage of a low Trail — maneuverability of the bike, especially when climbing steep grades, where Trail turns from low to normal, but at least not too-high (in practice, this is mostly relevant when maneuvering between competitors in a race);

• a secondary advantage of a low Trail — the lack of a prominent wheel-flop, which is particularly inconvenient on steep uphills (due to both the geometrical increase of Trail, and a low speed);
bicycle with progressive geometry tilted up with enlarged trail

• the main advantage of a long Front-Centre — balanced distribution of weight between the wheels on sharp downhills, which allows to keep steering and braking normally, as well as eases up the endo angle (after which one tumbles over the bars);

• an additional plus of the long Front-Centre — the lack of the toe overlap between the shoes toe area and the backside of the wheel turning towards them;

• the main advantage of a short Front-Centre — stability when riding off-saddle with your weight shifted forwards (when the wheel is directly under the arms pressing against the bars, and not in front of them, which would exaggerate the flop);
bicycle with unfinished frame without saddle
2) the head tube angle and Rake — although they are usually advertised in geometry charts — are in fact merely technicalities: they influence both Trail and Front-Centre, but have little effect on their own; in other words, those are the parameters for the bike builder, but not for the rider;
cyclist sprinting out of saddle shifted forward on bicycle
3) in theory, the only valid purpose of choosing a small-sized frame with a long “like a pro” stem — is to improve stability when sprinting on the drops. There is also a vicious idea that a long stem helps compliance (ask your bank clerk), but you should really decide: either a stiff stem for sprinting, or a noodly one for comfort — but hey, there are wider tyres for the latter.

In the real world, racers also choose the combo of a small frame and a long stem to achieve a lower, more aerodynamic, position. At that, nothing prevents the manufacturers from designing a lower frame in the first place — apart from the fact that normal people would never be able to fit to it, and thus no one would ever buy the thing :)
bicycle serving as decoration with random lines on top of it instead of demonstrating trail
Final tl/dr:

The progressive geometry for gravel bikes has quite a few advantages for amateur cyclists — and few negatives for racers. The latter, however, can be mitigated by the well-proven method: choosing a smaller-sized frame with a respectively longer stem.