Rest of bike handling geometry — backside, its balance with front

6 October 2023
drawing demonstrating chain stay length measurement
MINIMAP | Chainstay Length — Bottom Bracket DropConclusions |

With the front sorted out, the main parameter at the back is chain stay length. It can be measured directly along the chain stay or horizontally (and in parallel to the plane of the wheels — see picture after next). This is not a fundamental difference, and I will operate the former.
drawing demonstrating horizontal chain stay length measurement
When taken on its own — and as applied to speedy bikes — the shorter the better. Shorter chain stays allow for more accurate weight placement on the rear wheel when pedalling out of the saddle, which gives more grip on steep climbs and better “bike feel” in sprints.
drawing demonstrating auxiliary chain stay length measurement
Actually, the progressive geometry means a combination of long front and short chain stays. And there are a number of implications to it.
track bicycle with short chain stays and split seat tube
For starters, the minimum length of the chain stays is limited by the size of the wheel. For example, on the tracklocross bike above, the seat tube had to be split in two to prevent the wheel from colliding with the seat tube — for the sake of very short chain stays.
road bicycle with short chain stays and split seat tube
It’s not a new trick: here’s a road bike from the 80s with the same feature. But with gravel bikes it’s more complicated. Wheels with thick tyres are just bigger, both in diameter and width, and if you bifurcate the seat tube, there’s no room for the seatpost.
close-up of road bicycle with short chain stays and split seat tube
That’s given that gravel bikes tend to have a lower top tube: firstly, to have a lower standover height (leaving space between the frame and the crotch so you don’t smack your parts when jumping off the pedals offroad), and secondly, to have a more extended seatpost so it flexes over bumps or is a dropper.
bicycle with dropper seat post
In an attempt to solve the problem of insufficient space, manufacturers can: (A) make the seat tube thinner — but this marginal gain of space requires a custom seatpost with a smaller diameter. Or completely integrated one.
bicycle with thin seat tube and integrated seat post
(B) bend the seat tube — but then the more you extend the seatpost, the more it pushes the saddle to the rear — and at some point it’s too far back from a bikefit perspective.
bicycle with bent and slack seat tube
(C) on the contrary, make the seat tube more vertical. In this case, vice versa, it is not always possible to move the saddle backwards enough.
bicycle with steep seat tube
A completely different approach is to use a smaller wheel. The standard size originating from road bikes is 700c (which is also called 28" on road and gravel tyres, and 29" on mtb rubber, although it’s the same rim size; and may even be the same tyre size — for example, 50 mm and 2.0" respectively :)

For a standard wheel, practically the minimum chain stay length after which a gravel tyre (give or take 40 mm wide) will start rubbing the seat tube is 420 mm. And taking into account the mud clearance, it’s more like 430 mm. At the same time, road bikes can have 400 mm chain stays, which is a big difference.

This is partly why smaller diameter wheels — 650b (27.5") — have become quite common. The diameter of such rim is 584 mm, while the 700c has a diameter of 622 mm (makes sense, doesn’t it). With the 19 mm lesser radius, such wheels can be comfortably placed behind a seat tube with the road-length chain stays — even with thicker (and therefore taller) rubber.
bicycle with 650b wheels and wide tyres
Smaller diameter wheels affect the way the bike behaves — both directly and indirectly through changes in geometry.

A smaller wheel has less weight, which is also distributed closer to the axle, reducing the gyroscopic effect. This makes it easier to spin up the wheel, and the bike accelerates faster. On the other hand, when the bike has already accelerated to a speed, the larger wheel not only retains inertia better, but also rides over bumps more smoothly. This, in turn, reduces both the “stopping factor” and cyclist fatigue.

The “stopping factor” is easy to understand. Imagine riding into a kerb on a bike with tiny wheels (like a Strida or Brompton from the first part in this series). You’re going to stop against it. If you do this on a full-sized bike, it will probably hop onto the kerb, although the speed will be reduced and you will be jolted. The bigger the wheel is, relative to the same obstacle — whether it’s a kerb or just a bump in the tarmac — the less the wheel will lose speed when you pass it.

And you will feel less of a shake. It’s not just a matter of comfort — there are studies that clearly show that shaking causes the cyclist’s muscles to do extra work to absorb it, which increases heart rate instantaneously — and fatigue over time. That’s why wide tyres are faster than the narrow ;)
drawing demonstrating bottom bracket drop measurement
MINIMAP | Chainstay Length — Bottom Bracket Drop — Conclusions |

In addition to directly affecting the behaviour of the bike, a smaller wheel does not allow the bottom bracket to be placed as low as with standard wheels. This is called the bottom bracket drop. The lower the bb, the more planted and stable the bike is — but if it’s too low, the pedals start striking ground in corners.

The pedal strike is deemed to be the most common cause of cyclists falling in corners. It’s not hard to imagine that with a smaller wheel diameter this will happen more often, unless you move the bb higher, which most framebuilders try to avoid, same as too long chain stays (but sometimes motorbike companies take their shot :)
bicycle with very long chain stays and no bottom bracket drop
Smaller wheels also don’t solve another problem: if you look at the bb area from above, it becomes clear that the limited width of the bike is shared simultaneously by a wide tyre, the right chain stay, the mud gap between them, one or two chainrings (and in the latter — a front derailleur battery above them) and the crank, which in the rear position must not be too close to the chain stay (as parts can bend toward each other under load). And the closer we move the wheel towards the bb, the less room there is for all of that.
drawing demonstrating chainline and q-factor and stance width measurements
In mtb, where tyres are even wider, this problem had to be solved by widening the pedals — increasing the q-factor — to leave more space between cranks for all of the above.

But this, firstly, leads to a too wide foot position for many people bikefit-wise (on mtb, where there is no long continuous pedalling in the saddle, this is less critical than on road and gravel bikes), and secondly, that results in the need to raise the bb — again, in order not to strike the wide-stance pedals in a corner.

In addition, the chainring is moved more to the side, and as a result, in order to keep the chainline straight (so that there is no too much of crosschaining between the chainrings and sprockets), mtb hubs are getting wider and wider (taking up more space between the rear dropouts): first we had boost standard, and then the super boost.

As a result, the chain stays are spread wider — and closer to the cranks again. Because of this vicious circle, the q-factor needs to be changed significantly — the mtb has an average of two centimetres more than road bikes. Apart from the fit issues, this is also considerably less aerodynamic (again, not that mtb crowd even care, but we, the gravellers, shoud).
bicycle for purposes of demonstrating mtb geometry
Fortunately, by the time gravel bikes became popularised, framebuilders had 3D printing and CNC milling at their disposal to make the chain stay as thin and intricate as possible. Combined with the elimination of the front derailleur and second chainring up front, this allows just the right amount of space to fit a gravel wheel between the cranks with a road q-factor.
close-up of bicycle chain stay with 3d-printed element
close-up of bicycle chain stay with cnc-machined element
Another approach is to “drop” the chain stay so that instead of a tyre-stay-ring sandwich layout, we have three almost independent pairs: tyre-stay, stay-ring and tyre-ring. This makes it easier to fit everything, while leaving generous mud gaps between all components — so that any sticky stuff carried over by a threaded wheel doesn’t clog the bb area, which can lead to a complete halt and subsequent digging out the mud with roadside objects (tip: it also helps to spray water from a biddon).
Okay, now it seems we have sorted out all the issues with short chain stays — but actually no. In fact, the progressive geometry has an inherent flaw. The combination of a long front and short chain stays causes the total weight of the rider-bike system to shift to the rear wheel.

Normally, we want no more than about 55 % of the weight to be on the rear wheel and the remaining 45+ % on the front wheel. The reason for this is a specific balance scenario that requires some explanation.

We want to take corners at high speed, but we don’t want to fall in them because we didn’t have enough tyre grip.

That said, front wheel grip is much more important — if the rear wheel starts to slide sideways, a skilled rider can catch it and stabilise the bike. If the front wheel loses traction on the tarmac, it’s almost certainly an instant crash.

(I’ll take this opportunity to point out: this is why you should never use the front brake in corners of any noticeable steepness. Never.)

So, with proper cornering technique, the wheel with the least weight on it slips first. Since the really fast corners — where we lean the bike as much as possible and test the limit of tyre grip — are downhill, the weight is shifted a little bit to the front wheel, and the original 55/45 % rear-front balance becomes 49/51+ %.

In this case, the rear wheel will slide first, meaning we’d have a room for mistake. Moreover, skillful road riders sometimes “feather” the rear wheel a little bit in a corner to feel the moment of loosing traction and estimate the tyre grip in the particular conditions they are in.

If less than half of the weight in such a situation will be on the front wheel — and with the progressive geometry it is quite likely — well, you get the point.
Surprisingly, manufacturers of bikes with the progressive geometry don’t seem to have figured out one simple way to solve this very serious problem.

As a side note, the progressive geometry comes from mtb, where the problem of imbalance is not as dangerous: unlike on asphalt, slipping of the front wheel on dirt is a common thing, and threaded tyres allow you to catch the grip again on a loose surface, rather than immediately fall onto a hard road.

The grip of a tyre is the better the wider it is. So, if you make the front tyre significantly wider than the rear one, even with the weight distribution imbalance on the progressive geometry, the rear wheel will tend to slip out sooner, which is what we’re aiming for.

Luckily, there are none of the fitment problems described above as far as the front wheel is concerned — and there’s nothing stopping us from putting a really wide tyre up front — as long as the fork chosen for the bike allows for it.

An idea for manufacturers, but also every owner of a progressive bike can make quite practical conclusions for themselves.
MINIMAP | Chainstay LengthBottom Bracket Drop — Conclusions |

Progressive tl/dr:

• shorter chain stays are preferable — 420–430 mm for a gravel bike or 400–405 mm for a road one;

• the lower bottom bracket is better — but until the pedals start striking the ground; in that sense, goods things are: narrow q-factor, shorter crankarms, more compact pedals;

• a sloping top tube gives you a smaller standover height and a more flexy seatpost (or a room for a dropper); you can also sit lower in the supertuck position;

• if you have a progressive bike — consider putting a wider tyre up front so you avoid falling in high-speed corners; and never brake in a corner with the front brake (on any bike).
Almost all of the photos above are from Made bike show. The pics are hi-res, so you can take a good look at the details. And if you hadn’t enough, there’s a whole bunch more there:

I borrowed a couple of photos from Escape Collective, which has dedicated several posts to Made. One is available before the paywall (the pictures here are therefrom — legal stuff :) and then you have to pay, which is worth it: