Explaining the effects of frame geometries

Question

I asked about buying a bike here, and someone asked on my behalf about test-riding a bike before buying.

My question is, what should I know, or should have known, about the sizing or 'geometry' of a bike?

In reply to my question about buying, people talked about the wheel size etc.; but not about the frame size, or the fork angle.

In the question about test-riding, people talked about aspects of the 'fit' which I think can be adjusted (i.e. the height and front/back position of the seat, and the height of the handle-bar) ... and which are documented elsewhere ... and which are therefore maybe superficial and less important for the purposes of this question.

But, what about the geometry which can't be adjusted: the distance between the wheels, for example?

On a bike which I've just bought, I've noticed for example (since buying it) that the front wheel is rather closer to my foot than I am used to; I expect that's just one example (of a geometry/metric which as a novice it hadn't occured to me to think about).

This Wikipedia article "Frame geometry" introduces the vocabulary but without much detail. Googling reveals mostly short articles too, more ad-bait than detailed/informative.

Given that I'm asking for details, a kind of 'science' question, this question is probably too long to answer here. What I might be asking for then is a link to a suitable introduction/reference (online, or a book...).
Alternatively you might (I don't know) be able to give a short/summary answer along of, "The most important measurements to look at are A, B, and C, which have the following effects... Taken together these few most important measurements account for 90% of the effects of geometry on fit/performance/handling/comfort."

Or, is this a topic that people don't learn about by reading, and which can only come from extensive experience of riding multiple bikes? How did you come by your understanding of geometry? Can you look at two bikes of a similar type and say, "Oh, those have different shapes/measurements" and understand what that (difference in measurements) means or implies?

PS.: interested in 'ordinary' frames: not frames with a suspension (so, not frames for off-road/all-terrain/jumping); not recumbent; not track/racing; not exotic/expensive materials.

freiheit · Accepted Answer

I see frame geometry having 3 primary affects

Fitting the rider; which you're already addressing and I won't talk about here...  But a lot of geometry stuff comes down to making the other stuff work with fitting riders on the bikes. It's very important.
Fitting stuff on the bike
Handling characteristics.

Since you asked, I'm talking about your basic on-road diamond-frame bike. Cruiser, road, commuter, urban, touring, racer...
Fitting Stuff On the Bike

Wheel clearance affects how big the tires can be and how easy it is to mount fenders. For the rear wheel you need to look at the chainstay bridge, seatstay bridge and seat tube. For the front wheel, look at the fork crown and downtube. Often either not measured or simply given to you as a maximum tire size measurement.
"Toe overlap" -- how close the front wheel gets to the front of the pedals. More of an issue with fenders, and not really an issue once you're going any kind of speed. I never see this on the geometry specs from manufacturers, but definitely worth checking. You can get used to moderate toe overlap, though.
Longer chainstay length gives you more heel clearance for loading panniers on the rear rack. I know from personal experience that the most likely place to kick a pannier off the rack is in an intersection, so if you plan to carry panniers this can be very important. Short chainstay length can be compensated for by getting a rack that allows panniers to be mounted further back (longer top, top further back, however they do it).

Handling characteristics
First off, the way a bike handles, "stability" and "maneuverability" are essentially opposites. It's a tradeoff. Whether you want stable or maneuverable depends on what you're going to do with the bike, how fast you'll be going, your riding experience, etc. And there's some more complicated harder-to-understand secondary effects.
I really think that for most of the handling stuff you're best off just test-riding bikes that fit you. Experiment with the handling. Try going slow. Try going fast. Try making a sharp turn. Try making a subtle turn. Try a dodge/weave. Try those turns at different speeds. Try a fast start. Try a fast stop. Try all that in all the handlebar positions. Unless you're designing a bike, a lot of this is all very theoretical, heavily interrelated (can't really change one variable without changing the others), and likely to be very small differences when comparing actual bikes.

Bottom Bracket (BB) height (sometimes you can find this out, sometimes you have to make a guess based on wheel+tire radius minus BB drop).  Higher bottom bracket makes it easier to go over stuff. Not really an issue if you stick to roads, but can be a factor when trying to pedal through sharp turns. Higher BB also lets you use longer cranks which some people want. Compare a Cyclocross to a Road bike and the CX bike will have a higher BB.

Wheelbase length (distance between hubs/contact patches). A longer wheelbase will make the bike more longitudinally stable. In other words, less prone to wheelies and less prone to flipping you over the handlebars.  This could theoretically translate to better ability to stop faster. Note that one way to give you a longer wheelbase is longer chainstays. The position of your center of gravity makes a big difference, too.

Headtube angle (steering axis angle). A shallower/slacker (more pointed forward) angle gives you more trail. A shallower angle also increases how much the weight can make the wheel flop on its own and negatively affect stability at low speed. This is usually measured from horizontal, so a steep angle will be closer to 90° and a shallow angle will be a lower number.

Fork offset/rake (how far the front hub is in front of the steering axis). More offset gives you less trail. More offset also tends to give you more toe clearance.

Trail is how far the front contact patch is behind the steering axis. Note that while the trail is the important measurement, the headtube angle and fork offset (and wheel size) are what determines the trail. A bicycle with no trail would be unrideable (but many people like having very little trail).
There's different ways of measuring the trail (along the ground, or perpendicular to the steering axis) that make comparisons more intuitive, but more trail means more stable, and less trail means more maneuverable. Or to put it another way: less trail means more twitch and more trail means less responsive.
The stability from trail increases as the bicycle goes faster. If you look at some cruiser bikes they'll tend to have a shallow headtube angle and a lot of trail: they're stable and easy to balance at low speeds but probably hard to steer at high speed. If you look at a road/racing bike, the headtube angle is usually much closer to vertical and the trail is fairly small: they may feel unstable and wobbly (hard to balance) at low speeds, but are still easy to steer (responsive) at high speeds.
Note also that the handlebar setup and rider preferences/experience make a huge difference whether your prefer maneuverable/twitchy/responsive or stable/unsteerable/unresponsive.

References

http://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

Jay Bazuzi · Answer

A local bike shop owner pointed out to me that getting the right frame is the most important thing on a bike. If it's not right, then any other money or effort you put in to the bike is wasted. So, good topic!
Grant Petersen wrote this article about geometry on Rivendell's site: http://www.rivbike.com/kb_results.asp?ID=34. It's not a complete treatment of the subject, and it's in Grant's typical rambling style, but I like reading his stuff for an "alternative viewpoint" vs. the mainstream trends.
Some excerpts:

Chainstay length....too short, bike is too jumpy.
Tire and fender clearance....too little, can't run a big fun tire or fenders
Seat tube angle....too steep, can't put seat back far enough
Head tube angle and fork rake: Combine to influence how the bike responds.
Fork blade length: Affects front wheel clearance
BB drop: Affects ground clearance, standover height, and bike "feel"

superfly · Answer

the below link offered some good insight on the topic of frame geometry numbers and how they influence riding characteristics. I do agree and should stress that rider preference and experience is an equal factor and when looking for a new frame, being informed and identifying this balance should be taken in consideration.

http://cyclingtips.com.au/2011/02/the-geometry-of-bike-handling/

Videoboy Matt · Answer

I have been racing mountain bikes since the mid 90's and later playing around with BMX. If you ride enough bikes, you will notice the frame geometry can make a huge difference on how it performs off road. For a while many mountain bike companies were attempting to do a seatstay that curved around the wheel (S shaped) with a shorter chainstay..this brought the back wheel closer to being under the seating position, often by 2-3 inches. What effect it had was it was easy to wheelie the bike, or climb a hill..it was terrible when it came to riding flat on a street, as the bike was jumpy, and the front end always wanted to pull up, especially with a suspension fork every time you hit a bump. I recently rode a poorly designed Trek (A company known for selling bikes more for components and less for geometry) and the head tube angle was too shallow with little trail..and it was one of the most unstable bikes I have ever ridden. At slow speeds the bike wobbled, and for a Cross Country Hard Tail..this is a terrible characteristic (especially in the $1200+ price range). I later bought a Kona and it has a rather long trail with the fork brought forward by about an inch from the headtube that made it very stable. It has a very slanted top tube, that makes it much easier to maneuver and shift your weight without hitting the frame..something you want on a mountain bike.

Answered by Videoboy Matt on March 22, 2021

Weiwen Ng · Answer

I think the question deserves some additional treatment as to frame sizing. I realize the OP may have downplayed aspects of frame sizing in his question, but that's arguably the wrong approach. It benefits the end user to know how to select a bike size, and the parameters I describe may not have been in common use in 2011, when the question was asked. They are now in very common use. Thus, although the approach I describe may be a bit difficult for totally new riders to just pick up and use de novo, I believe the question is worthy of a new answer.
Basics of frame sizing: stack and reach
The image below is from a Road.cc article. It shows the two most important parameters in sizing a bike, stack and reach. Both are measured from the center of the bottom bracket to the center of the top of the head tube.

Reach is the bike's x-dimension, and stack is the y-dimension. Different types of bicycle, e.g. touring bike, hybrid, endurance road bike, road racing bike, will have different ratios of stack to reach. For example, given the same overall size, the average endurance road bike has higher stack and shorter reach than the average performance road bikes (here, I mean bikes oriented to road racing, rather than endurance riding), and the average touring bike will have higher stack and shorter reach than the average endurance road bike.
However, within a certain sub-category (e.g. touring, endurance road, cross country MTB, enduro MTB), manufacturers may differ. Some may make longer and lower frames than other manufacturers within the same category. To further complicate things, the relative fit may change across bike sizes. For example, considering endurance road bikes, which are designed to cover long distances comfortably, and not necessarily to race. Bike Insights, which collects and analyzes bicycle frame geometry, rates the 2020 Trek Domane in its larger sizes as somewhat upright, and very upright in the largest size. In contrast, Ribble's (a major UK online bike shop with a house brand of bikes) 2020 Endurance Ti Disc is rated as very aggressive, meaning longer and lower, across all its sizes.
As discussed in the road.cc article and this Cyclingtips article, the frame stack and reach don't include the headset and spacers. External headsets cups can add quite a bit of stack height, and riders can further modify the actual riding position using spacers. You modify the reach by selecting a stem length, and secondarily a stem angle. I think that -17 degree stems were common among classic road bikes, but they're rare now. More common angles are 10 and 6 degrees; you can flip the stem for added stack height (although now your reach changes). Additionally, you can select handlebars with greater or lesser forward reach, although handlebar selection also needs to consider your hand size. These items control the adjustable portion of your riding position, and many riders may make small adjustments as they go by based on other riders' feedback and personal intuition.
Once you know your current stack and reach, and your preferred handlebar position (i.e. total height of spacers under stem, stem length, handlebar reach), this makes it easy to estimate if a new road bike will fit you, and it enables you to guess what parts might need to change. For example, when I bought a gravel bike last year, I compared the frame's stack and reach to my own, and I was able to specify that I wanted 10mm of spacers under the stem and ask to change from a 100mm to a 120mm stem.
To my recollection, stack and reach figures have become commonplace within at least the last 5 years. When I started riding in the early 2000s, I don't recall them being discussed at all. You would size a bike by its top tube and seat tube lengths, and perhaps the seat angle. My custom road bike was built in 2007, and its original CAD diagram didn't discuss stack or reach at all. At the time the original question was asked in 2011, I can't recall how commonly stack and reach were used, but they may not have been that common.
This may be too much for a totally new rider, but riders with unusual physical proportions might throw the Bike Insights categorization off. My custom road bike is rated as very aggressive for an endurance bike (it's a private entry, so other users won't be able to see it on the site). However, I have proportionately short legs and a long torso. Thus, my actual riding position is, I think, moderately aggressive at most.
Saddle height and setback, seat tube angle
Stack and reach describe the bike's sizing from forward of the bottom bracket. One other set of measures needs to be considered related to your preferred saddle position. Before you set your actual fore-aft riding position, you need to get your saddle's height and fore-aft position set first. You would move the seatpost up or down to adjust height, and you slide the saddle's rails in the seatpost to adjust the latter. If you need a further aft position than your rails allow, you may need to find a seatpost with more setback. To my knowledge, seatposts with setback figures ranging from 0 to 20mm are quite common. Set forward seatposts may be available but they are rare; triathletes may have used them at one point to help modify the riding position on standard road bikes. Seatposts with over 20mm of setback should be rare, and I can't name any off the top of my head.
I suspect most riders will be able to raise or lower their seatposts enough to get their correct saddle position on a bike that's roughly their size. Very occasionally, some bicycles might have features that interfere with this, but I suspect this doesn't affect the majority of riders. For example, Trek's higher-end bikes have seat masts, and they specify a certain minimum and maximum saddle height for each frame size. To my recollection, I would otherwise prefer to ride their second-smallest frame if not for the seat mast restriction. I believe this stems from my proportionately short legs. I have far less seat post showing on all of my bicycles than any peer riders who aren't on level top tube frames (i.e. more classic styled road frames).

The bike's seat tube angle, pic courtesy of Ride Far, may be a consideration. A steeper seat angle will put your saddle further forward if you leave it centered on the rails. The bike's seat angle will limit the extremes of the position you are able to put your saddle in. That said, I suspect most riders may not need to worry about seat tube angles, although some may prefer slacker or steeper angles.
I am an edge case as to seat angle as well. I prefer a very steep seat angle. Many stock bikes in my size have 74 or 74.5 degree seat angles. My custom road bike has a 76 degree seat angle. I tend to rule out bikes with 73 degree seat angles, because there is not enough adjustment available on the saddle rails with many common seatposts.

Seatposts come in different amount of setback, which enables you to further adjust your saddle position. The picture above shows a zero, standard, and extreme setback post, although note that this is for a triathlon bike with a proprietary seatpost. On road bikes, setback from 0 to 25mm is common.

Explaining the effects of frame geometries

5 Answers

Fitting Stuff On the Bike

Handling characteristics

References

Basics of frame sizing: stack and reach

Saddle height and setback, seat tube angle

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