16 ms·

Bicycle Geometry Terms

174 points by thanhhaimai 6y ago

TheAlchemist 6y ago
This site is fantastic ! The "compare" part is really great - with a little scatterplot price vs estimated spec level - I've used it massively when choosing my bike.
- playa1 6y ago
  Very useful site, the geometry comparison amazing!
seesawtron 6y ago
Do you think people who fix bikes or are bike enthusiasts know which of these components affect which aspect of riding? I can imagine knowing all the specific names for the parts is difficult but their affect on biking experience seems very relevant. Something that was not so obvious to me.
- FridgeSeal 6y ago
  Anecdote: My friends and I are avid mountain bikers, and semi-frequent road bikers and most of us have a pretty decent grasp on the effect of the main factors that will affect how a bike rides and handles and discussions are often had on the validity and suitability of different bikes geo choices for different kinds of riding. None of us are experts and we couldn’t tell you exactly how a bike would feel just by looking at the geometry, but you get a decent mental model after a while.
- notatoad 6y ago
  yes. most people probably haven't read a guide like this, but if you've ridden a good variety of bikes you get a sense for how the bb height affects stability, or how the head tube angle and the trail affect your steering. some bikes even offer adjustable geometries by moving a bolt to a different position, which really makes it easy to understand how a change in geometry affects the handling characteristics.
  - ehnto 6y ago
    I got a rapid lesson in how changing the length of the seat stay to slacken the head tube angle leads to a reduced bottom bracket height. A rock I would normally clear snagged a pedal and I went careening over the bars. A good example of how all of these variables influence each other and how bike fit and geometry is always a balancing act.
- matsemann 6y ago
  There are professional "bike fitters" that know how to adjust all these things (those that can be adjusted) and more in such a way you get everything set up the best given your body and goals. For instance, given a bike and a rider, one can adjust seat height, seat position, stem length, handlebarhheight, crank arm length and possible more things to get the correct reach and stem height. However, all that also has to be adjusted to account for body size (limb lengths), mobility etc, in order to level the hips, get the knees correctly over the pedals, aero position, correct weight on the hands, no chafing in the butt etc. It's really an art to adjust everything at once, as fixing one thing affects the others. So while a bikefit can cost about $300, it's money well spent if one is serious about the biking (and one has then probably spent ~20x that on bike+gear anyways.. ). I'm about to head out for a ~120km ride today, so I've found a bike fit invaluable.
  - TopHand 6y ago
    I agree with you a 100%. Your bike should help you maintain good riding posture even when you are becoming road tired. Just be careful about who you pay to fit the bike. Most shops claim to be experts, but I have had bad experiences with some shops. It doesn't show until you've ridden a great many miles.
  - usrusr 6y ago
    Money well spent if the fitter is actually good. I've seen some rather unconvincing results.
  - loeg 6y ago
    Professional bike fitting is sort of a sub-discipline of physical therapy, at least in the US. (That might not be obvious to people who haven't had a professional bike fit; it wasn't to me before I did.) To the extent your health insurance covers PT, it covers PT bike fits.
- billsmithaustin 6y ago
  An example: if you want to carry saddlebags (panniers) attached to a rack over the rear wheel, it helps to have a bike with long chain stays so that your heels do not bump into your saddlebags when you pedal.
- barrkel 6y ago
  Many of these have equivalents on motorcycles, and even if you don't know the terms, you feel the effects, and if you have a bit of knowledge, you'll know that adjusting suspension preload (or, in extremis, raising or lowering the forks in the clamps) affects rake and trail which then affects steering. http://cycle-ergo.com/ http://cycle-ergo.com/ is a site some motorcyclists use to figure out which bike suits their body and preferred riding position.
- ehnto 6y ago
  In my experience knowing and caring about bike geometry to this level of detail is definitely for the advanced enthusiast. Just knowing it doesn't mean you can apply it either. I knew the terms in the article, I even knew what I personally prefer and how a bike might feel by reading the numbers, but I won't be offering to do anyone's bike fit any time soon!
- analog31 6y ago
  For your second bike, yes. ;-) Granted, different people are enthusiasts for different reasons, for instance I rely on my bike for getting around town, but will never be a competitive rider. So, the interest in geometry ranges from just being safe and comfortable to ride, up to performance aspects of activities such as mountain biking, racing, or touring. One hassle is that different makers sometimes supply a different set of measurements, especially inexpensive or older bikes, so you have to figure things out a bit. Also, a short test ride might not tell you how happy you will be after 100 miles. If there are things that you are happy or unhappy about on your existing bike, I suggest measuring it and try to figure some things out that way. When shopping for a bike (or hunting through piles of used bikes, as I've done), bring a tape measure.
- zbtaylor1 6y ago
  Definitely! I've been riding road bikes for ~5 years now and have "built" the last 3 bikes I've owned. I've become increasingly interested in frame geometry as it relates to bike fit (as opposed to handling characteristics). After riding a few frames and comparing their geometries I've narrowed down an ideal range of measurements for myself based on how I got along with each frame. I can browse a geo chart and be pretty certain whether or not a frame will work for me by checking a few measurements.
- loeg 6y ago
  Broadly, yes. Mechanics almost certainly. Enthusiasts can get away with not knowing some things to a point, but I think you either lose the enthusiasm or pick it up eventually.
cranekam 6y ago
Great explanations! It’s really silly that seat tube length is used as the single measure of frame size. Much more important is reach and standover height, since those can’t easily be changed. A longer or shorter stem will adjust reach at the expense of changing how responsive the steering is, whereas a short seat tube can be fixed with a longer seat post. Seat tube length is also measured differently by different manufacturers — sometimes it’s center to center (of the tubes), sometimes center to top, etc. Seat tube length is convenient but it’s not what people should focus on when trying to find a bike that fits.
- danburbridge 6y ago
  There is a slow movement to using stack and reach for frame sizing, which are the two most critical measurements for bike fit.
- jsinai 6y ago
  My thoughts exactly. Being on the shorter side what matters is the stand over height - too high and it greatly limits on/off/standing manoeuvrability and agility. I was surprised to learn that the smallest adult bikes tend to have a stand over height of 68-70cm. For some bikes, it’s even higher. For the lower tail of the adult height distribution, this is too tall to use comfortably.
  - monsieurgaufre 6y ago
    Look into bikes with 650b wheels for this.
    - loeg 6y ago
      Pay attention to BB height, specifically. That'll dictate ground clearance and comfortable saddle height. 650B wheels are often fitted with bigger tires, and can have the same effective diameter as 700C wheels with smaller tires (depending on specific tire size).
      - monsieurgaufre 6y ago
audiometry 6y ago
If you are interested in messing around with these settings and many more, look at the great parametric bicycle cad system “Bikecad”. Brett curry maintains and updates it for eons now. And crazy licensing — one time payment gets lifetime of updates.
- smartbit 6y ago
  Nice! https://www.bikecad.ca/ https://www.bikecad.ca/ Costs CAD500 is a one time fee. Runs on Linux, Mac and Windows. A Free online version is also available. It takes >5min to load, just be patient.
  - unicornporn 6y ago
    https://www.bikegeocalc.com https://www.bikegeocalc.com if you want something more simple. Use it all the time.
  - app4soft 6y ago
    There is also rattleCAD app developed by Manfred Rosenberger.[0] rattleCAD <= 3.x was fully free & open-source, but actually rattleCAD 4.0 is proprietary with freeware/demo version & few subscription modes.[1] [0] http://www.rattlecad.com/ http://www.rattlecad.com/ [1] http://www.rattlecad.com/download.html http://www.rattlecad.com/download.html
- loeg 6y ago
  If you want to compare a bunch of bikes, https://geometrygeeks.bike/ https://geometrygeeks.bike/ has a solid database of geometry data.
- zbtaylor1 6y ago
  For anyone interested in tracking their own bike fit at home, I've been building a web app based on the park tools road positioning chart (https://www.parktool.com/blog/repair-help/road-positioning-chart https://www.parktool.com/blog/repair-help/road-positioning-c...). This is different than frame geometry, which is what is being discussed in this 99spokes article, but it's quite useful for anyone that likes to play with their fit or wants to transfer their current fit to a new frame. It's a work in progress, but the app can be found here: http://app.mybike.fit http://app.mybike.fit.
Lio 6y ago
This is great. One thing I do notice missing is the affect of stem length on steering, particularly for road bikes. It’s not uncommon for road bikes to have both a steep head tube angle and a longer stem e.g. > 110mm to increase stability at speed. Narrower and lower handlebars give you better aerodynamics but also affect both comfort and steering.
orthoxerox 6y ago
What is missing is a way to convert your body's geometry to the bike's geometry. Like, you give it your inseam length (sole to taint) and the highest pull-up bar you can grab without jumping and the site tells you what bike geometry parameters determine the correct bike size for you.
- matsemann 6y ago
  I wrote a lengthy comment here about bike fitters. They can also help with this. It's however often more complicated than just a few measurements. They often have a special bike where literally everything can be adjusted (google retül bike), and then when the fit is done (~1h) they got lots of parameters that can be put into a DB to spit out bikeframes+gear that will work for the rider. However, my experience is that the system still needs a human expert to verify and tweak the results. However, for a basic fit your method works. For instance when buying a Canyon they ask for your measurements for some things to recommend frame size. But this of course if very rudimentary and already assumes you want that model.
- petre 6y ago
  Go see a bike fitter before you buy if possible. Totally worth it. Saves you time and money and potential injuries due to improper fit.
fit2rule 6y ago
I love this site - it is very informative. I wonder if anyone has done something similar for music - i.e. "Musical Geometry terms" ..
- ehnto 6y ago
  Are you able to elaborate on what you'd hope for from a "Musical Geometry" site? I have a side project on the backburner that tries to understand the building blocks of music in order to answer questions like "What could I call this chord?" and "What notes are in the key of C?". Hence my interest!
  - fit2rule 6y ago
    I've seen lots of 'music explained' types of sites, but if someone were to use their mad graphics skills to explain the geometric/arithmetic relationship between various components of music, I'd pay for it. There are sites that show you the mathematical differences in scales and harmonic series - to represent this with visual geometry would be divine.
jibolso 6y ago
Great explanations!
thomasfl 6y ago
There are so many opportunities for new businesses that want to provide people with high quality well maintained bikes. In my hometown Oslo, Norway, you can see super expensive e-bikes and carbon frame roadbikes everywhere. If your bike needs repairment, you have to wait at least one month.
deleted 6y ago
[deleted]
Jemm 6y ago
Odd how it does not give the weight of bicycles in the specs.
- lgvld 6y ago
  This also the first thing I checked. For many it does, actually, e.g.: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 But I’m in no way a bike specialist -- maybe the weight of a bike is a feature that can be overlooked for gear of this level/price (expensive, I mean). Maybe a {1,2,...},000$ non-electric bike will always weight less than 10 kg (carbon and stuff) ? It depends on the bike purpose, of course. I’m currently looking for a bike, but more in the 200$ range... ;) This is the cheapest bike listed: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4
- bryanrsmith 6y ago
  We rely on spec data published by manufacturers, and unfortunately many do not specify the weight. It's also possible we're just failing to match it. If you find a bike without a weight that the manufacturer does have on their site you can use the feedback form & we'll get it fixed up.
- loeg 6y ago
  Weight can vary quite a bit for the same frame based on easily changed components like (1) wheels, (2) groupset (gears, pedals/cranks, brake/shift levers, that kind of thing), (3) handlebar/stem/seatpost material. I recently bought a medium-spec build of a lightweight frame bike, and could shave 2+ lbs with upgrades, if I wanted to. Not paying for those upgrades out-of-box saved me ~4800 euro up front, and the bike is 17.9 lbs instead of 16.2 lbs. Meanwhile, I could stand to lose 15 lbs myself. :-)
SeanLuke 6y ago
The description of trail could be a lot better, given how important it is. It's effectively the degree to which the front wheel acts as a caster. When you push a shopping cart the caster wheels automatically straighten out and trail behind the steering axis in a straight line. The front wheel is doing the same: when moved forward it resists being steered and wants to go straight. The degree of trail effects how much it wants to go straight. Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds. High-trail bikes can also be ridden no-hands because the wheel is effectively on autopilot. Any time you need more nimbleness and precision in steering -- perhaps a mountain bike? -- you want to lower the trail. Porteur bikes -- bikes that carry big boxes of flowers over the front wheel say -- also need very low trail because they're already hard to steer due to all that inertia. And with small wheeled bikes it is very difficult to achieve significant trail without a very shallow head tube (steering) angle, so many folding bikes by necessity have a very "live" steering. One of the dumber bike decisions I can think of is Brompton's inclusion of a rake on the front wheel, thus making its steering worse.
- loeg 6y ago
  Trail on road bikes, racing and not, seems to be universally in the range of 55-60mm (across frame sizes, too). I suppose it depends on what kind of racing you're doing, but some kinds of bicycle racing involve cornering agility / nimbleness, and you want a shorter trail for that.
  - petre 6y ago
    Randonneur bikes (also road but with fatter 650Bx42 tires) typically have longer trail (typically in the seventies) because they carry front loads.
    - loeg 6y ago
      Right — I was commenting specifically on "Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds."
    - 6y ago
loeg 6y ago
As a bicyclist, I'm amused this made it on to the HN front page. I'm not sure I agree with the description of stack. Cockpit height can be adjusted with riser stems and different handlebar shapes; it's not solely a function of axle-crown and headtube length. Without getting into it too much, some other descriptions are not great either. The general overview is ok but they make some overly strong claims. I'm not sure I've seen wheelbase used much in a bicycle context, although it makes sense. You see it used for cars a lot. Edit: The go-to source for bicycle information I'd point others at is Sheldon Brown's webpage, https://www.sheldonbrown.com/ https://www.sheldonbrown.com/ .
- xenocyon 6y ago
  Wheelbase is important if you're using a bike for transportation. A longer wheelbase makes for more stability and helps with carrying groceries, children, or other cargo. It does make the bike feel less nimble so may not be optimal for purely recreational riders.
  - petre 6y ago
    Not only. Modern trail and all mountain bikes also have longer wheelbases so they're more stable at descending slopes at full speed.
- WD-42 6y ago
  Wheelbase is one of the most important measurements for mountain bikes. It determines stability at the cost of the ability to turn quickly.
  - loeg 6y ago
    It's definitely also important for road / track bikes! My observation is just that it usually isn't described by that name (or even necessarily that specific measurement). (In my experience, YMMV, etc.) For example, Look publishes Chainstay (sometimes called "Rear Center") and "Front Center" (BB to front axle) measurements, separately, for their frames, but does not publish axle-to-axle / wheelbase measurement. Sometimes it is described sort of qualitatively as "cramped" (on the short side).
bill1306an 6y ago
Although I'm an avid reader, this is my first ever comment. I built this site https://comparemtb.com/ https://comparemtb.com/ to give people a way to visually compare the geometries of many different mountain bikes. I'd be interested in any feedback. Thanks.
- loeg 6y ago
  Recently I've used https://geometrygeeks.bike/ https://geometrygeeks.bike/ to compare geometry numerically. Having a database like that is pretty useful.
- johnmaguire2013 6y ago
  As a motorcycle rider, this site was invaluable to me when considering new bikes to buy: https://cycle-ergo.com/ https://cycle-ergo.com/ Now that I'm getting into bicycles, I'd love something like that. Have you considered adding bikes other than MTB?
- eitland 6y ago
  Thanks for finally posting! > I'd be interested in any feedback. I'll start with pointing out that there seems to be no instructions or hints on the landing page.
- petre 6y ago
  Also https://bikeinsights.com/ https://bikeinsights.com/
  - bill1306an 6y ago
    CompareMTB guy here. Thanks for sharing your site. Didn't know you existed. Nice work.
    - deleted 6y ago
      [deleted]
    - petre 6y ago
      It's not mine, I'm merely a user. It was featured on the Path Less Pedaled YT channel.
vondur 6y ago
It’s been an interesting few years in relation to MTB geometry. Low bottom brackets with slack head angles and steep seat tube angles have really taken off. My last MTB purchase was in 2018 and it’s geometry seems pretty conservative to newer bikes.
discreteevent 6y ago
If you are choosing a bike based on stack and reach then note that they are related. The reach and stack are measured from the same point (join of top tube and head tube). So if you see a bike that has 20mm more stack and 10mm more reach than another then it actually may have 16mm more reach at the same stack height from the bottom bracket as the smaller bike, (depending on head tube angle) so it's longer than you think.
jpm_sd 6y ago
For a deep dive into all things bicycle, I always recommend Sheldon Brown's delightful knowledgebase, which started in the late 90s: https://www.sheldonbrown.com/ https://www.sheldonbrown.com/ (sadly Mr. Brown is no longer with us, but his web archive lives on) And there's always Bicycling Science for the serious nerds among us https://www.amazon.com/Bicycling-Science-Press-Gordon-Wilson/dp/0262731541 https://www.amazon.com/Bicycling-Science-Press-Gordon-Wilson...
anewdirection 6y ago
I dont ecen know where to begin. So much of this is alightly incorrect its maddening. As someone who has setup race motorcycles please look elsewhere for correct terms and physics.

174 points by thanhhaimai 6y ago

This site is fantastic ! The "compare" part is really great - with a little scatterplot price vs estimated spec level - I've used it massively when choosing my bike.

playa1 6y ago
Very useful site, the geometry comparison amazing!

seesawtron 6y ago

Do you think people who fix bikes or are bike enthusiasts know which of these components affect which aspect of riding? I can imagine knowing all the specific names for the parts is difficult but their affect on biking experience seems very relevant. Something that was not so obvious to me.

FridgeSeal 6y ago
Anecdote: My friends and I are avid mountain bikers, and semi-frequent road bikers and most of us have a pretty decent grasp on the effect of the main factors that will affect how a bike rides and handles and discussions are often had on the validity and suitability of different bikes geo choices for different kinds of riding. None of us are experts and we couldn’t tell you exactly how a bike would feel just by looking at the geometry, but you get a decent mental model after a while.
notatoad 6y ago
yes. most people probably haven't read a guide like this, but if you've ridden a good variety of bikes you get a sense for how the bb height affects stability, or how the head tube angle and the trail affect your steering. some bikes even offer adjustable geometries by moving a bolt to a different position, which really makes it easy to understand how a change in geometry affects the handling characteristics.
- ehnto 6y ago
  I got a rapid lesson in how changing the length of the seat stay to slacken the head tube angle leads to a reduced bottom bracket height. A rock I would normally clear snagged a pedal and I went careening over the bars. A good example of how all of these variables influence each other and how bike fit and geometry is always a balancing act.
matsemann 6y ago
There are professional "bike fitters" that know how to adjust all these things (those that can be adjusted) and more in such a way you get everything set up the best given your body and goals. For instance, given a bike and a rider, one can adjust seat height, seat position, stem length, handlebarhheight, crank arm length and possible more things to get the correct reach and stem height. However, all that also has to be adjusted to account for body size (limb lengths), mobility etc, in order to level the hips, get the knees correctly over the pedals, aero position, correct weight on the hands, no chafing in the butt etc. It's really an art to adjust everything at once, as fixing one thing affects the others. So while a bikefit can cost about $300, it's money well spent if one is serious about the biking (and one has then probably spent ~20x that on bike+gear anyways.. ). I'm about to head out for a ~120km ride today, so I've found a bike fit invaluable.
- TopHand 6y ago
  I agree with you a 100%. Your bike should help you maintain good riding posture even when you are becoming road tired. Just be careful about who you pay to fit the bike. Most shops claim to be experts, but I have had bad experiences with some shops. It doesn't show until you've ridden a great many miles.
- usrusr 6y ago
  Money well spent if the fitter is actually good. I've seen some rather unconvincing results.
- loeg 6y ago
  Professional bike fitting is sort of a sub-discipline of physical therapy, at least in the US. (That might not be obvious to people who haven't had a professional bike fit; it wasn't to me before I did.) To the extent your health insurance covers PT, it covers PT bike fits.
billsmithaustin 6y ago
An example: if you want to carry saddlebags (panniers) attached to a rack over the rear wheel, it helps to have a bike with long chain stays so that your heels do not bump into your saddlebags when you pedal.
barrkel 6y ago
Many of these have equivalents on motorcycles, and even if you don't know the terms, you feel the effects, and if you have a bit of knowledge, you'll know that adjusting suspension preload (or, in extremis, raising or lowering the forks in the clamps) affects rake and trail which then affects steering. http://cycle-ergo.com/ http://cycle-ergo.com/ is a site some motorcyclists use to figure out which bike suits their body and preferred riding position.
ehnto 6y ago
In my experience knowing and caring about bike geometry to this level of detail is definitely for the advanced enthusiast. Just knowing it doesn't mean you can apply it either. I knew the terms in the article, I even knew what I personally prefer and how a bike might feel by reading the numbers, but I won't be offering to do anyone's bike fit any time soon!
analog31 6y ago
For your second bike, yes. ;-) Granted, different people are enthusiasts for different reasons, for instance I rely on my bike for getting around town, but will never be a competitive rider. So, the interest in geometry ranges from just being safe and comfortable to ride, up to performance aspects of activities such as mountain biking, racing, or touring. One hassle is that different makers sometimes supply a different set of measurements, especially inexpensive or older bikes, so you have to figure things out a bit. Also, a short test ride might not tell you how happy you will be after 100 miles. If there are things that you are happy or unhappy about on your existing bike, I suggest measuring it and try to figure some things out that way. When shopping for a bike (or hunting through piles of used bikes, as I've done), bring a tape measure.
zbtaylor1 6y ago
Definitely! I've been riding road bikes for ~5 years now and have "built" the last 3 bikes I've owned. I've become increasingly interested in frame geometry as it relates to bike fit (as opposed to handling characteristics). After riding a few frames and comparing their geometries I've narrowed down an ideal range of measurements for myself based on how I got along with each frame. I can browse a geo chart and be pretty certain whether or not a frame will work for me by checking a few measurements.
loeg 6y ago
Broadly, yes. Mechanics almost certainly. Enthusiasts can get away with not knowing some things to a point, but I think you either lose the enthusiasm or pick it up eventually.

cranekam 6y ago

Great explanations! It’s really silly that seat tube length is used as the single measure of frame size. Much more important is reach and standover height, since those can’t easily be changed. A longer or shorter stem will adjust reach at the expense of changing how responsive the steering is, whereas a short seat tube can be fixed with a longer seat post. Seat tube length is also measured differently by different manufacturers — sometimes it’s center to center (of the tubes), sometimes center to top, etc. Seat tube length is convenient but it’s not what people should focus on when trying to find a bike that fits.

danburbridge 6y ago
There is a slow movement to using stack and reach for frame sizing, which are the two most critical measurements for bike fit.
jsinai 6y ago
My thoughts exactly. Being on the shorter side what matters is the stand over height - too high and it greatly limits on/off/standing manoeuvrability and agility. I was surprised to learn that the smallest adult bikes tend to have a stand over height of 68-70cm. For some bikes, it’s even higher. For the lower tail of the adult height distribution, this is too tall to use comfortably.
- monsieurgaufre 6y ago
  Look into bikes with 650b wheels for this.
  - loeg 6y ago
    Pay attention to BB height, specifically. That'll dictate ground clearance and comfortable saddle height. 650B wheels are often fitted with bigger tires, and can have the same effective diameter as 700C wheels with smaller tires (depending on specific tire size).
    - monsieurgaufre 6y ago

audiometry 6y ago

If you are interested in messing around with these settings and many more, look at the great parametric bicycle cad system “Bikecad”. Brett curry maintains and updates it for eons now. And crazy licensing — one time payment gets lifetime of updates.

smartbit 6y ago
Nice! https://www.bikecad.ca/ https://www.bikecad.ca/ Costs CAD500 is a one time fee. Runs on Linux, Mac and Windows. A Free online version is also available. It takes >5min to load, just be patient.
- unicornporn 6y ago
  https://www.bikegeocalc.com https://www.bikegeocalc.com if you want something more simple. Use it all the time.
- app4soft 6y ago
  There is also rattleCAD app developed by Manfred Rosenberger.[0] rattleCAD <= 3.x was fully free & open-source, but actually rattleCAD 4.0 is proprietary with freeware/demo version & few subscription modes.[1] [0] http://www.rattlecad.com/ http://www.rattlecad.com/ [1] http://www.rattlecad.com/download.html http://www.rattlecad.com/download.html
loeg 6y ago
If you want to compare a bunch of bikes, https://geometrygeeks.bike/ https://geometrygeeks.bike/ has a solid database of geometry data.
zbtaylor1 6y ago
For anyone interested in tracking their own bike fit at home, I've been building a web app based on the park tools road positioning chart (https://www.parktool.com/blog/repair-help/road-positioning-chart https://www.parktool.com/blog/repair-help/road-positioning-c...). This is different than frame geometry, which is what is being discussed in this 99spokes article, but it's quite useful for anyone that likes to play with their fit or wants to transfer their current fit to a new frame. It's a work in progress, but the app can be found here: http://app.mybike.fit http://app.mybike.fit.

Lio 6y ago

This is great. One thing I do notice missing is the affect of stem length on steering, particularly for road bikes. It’s not uncommon for road bikes to have both a steep head tube angle and a longer stem e.g. > 110mm to increase stability at speed. Narrower and lower handlebars give you better aerodynamics but also affect both comfort and steering.

orthoxerox 6y ago

What is missing is a way to convert your body's geometry to the bike's geometry. Like, you give it your inseam length (sole to taint) and the highest pull-up bar you can grab without jumping and the site tells you what bike geometry parameters determine the correct bike size for you.

matsemann 6y ago
I wrote a lengthy comment here about bike fitters. They can also help with this. It's however often more complicated than just a few measurements. They often have a special bike where literally everything can be adjusted (google retül bike), and then when the fit is done (~1h) they got lots of parameters that can be put into a DB to spit out bikeframes+gear that will work for the rider. However, my experience is that the system still needs a human expert to verify and tweak the results. However, for a basic fit your method works. For instance when buying a Canyon they ask for your measurements for some things to recommend frame size. But this of course if very rudimentary and already assumes you want that model.
petre 6y ago
Go see a bike fitter before you buy if possible. Totally worth it. Saves you time and money and potential injuries due to improper fit.

fit2rule 6y ago

I love this site - it is very informative. I wonder if anyone has done something similar for music - i.e. "Musical Geometry terms" ..

ehnto 6y ago
Are you able to elaborate on what you'd hope for from a "Musical Geometry" site? I have a side project on the backburner that tries to understand the building blocks of music in order to answer questions like "What could I call this chord?" and "What notes are in the key of C?". Hence my interest!
- fit2rule 6y ago
  I've seen lots of 'music explained' types of sites, but if someone were to use their mad graphics skills to explain the geometric/arithmetic relationship between various components of music, I'd pay for it. There are sites that show you the mathematical differences in scales and harmonic series - to represent this with visual geometry would be divine.

jibolso 6y ago

Great explanations!

thomasfl 6y ago

There are so many opportunities for new businesses that want to provide people with high quality well maintained bikes. In my hometown Oslo, Norway, you can see super expensive e-bikes and carbon frame roadbikes everywhere. If your bike needs repairment, you have to wait at least one month.

deleted 6y ago

[deleted]

Jemm 6y ago

Odd how it does not give the weight of bicycles in the specs.

lgvld 6y ago
This also the first thing I checked. For many it does, actually, e.g.: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 But I’m in no way a bike specialist -- maybe the weight of a bike is a feature that can be overlooked for gear of this level/price (expensive, I mean). Maybe a {1,2,...},000$ non-electric bike will always weight less than 10 kg (carbon and stuff) ? It depends on the bike purpose, of course. I’m currently looking for a bike, but more in the 200$ range... ;) This is the cheapest bike listed: https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4 https://99spokes.com/bikes/trek/2021/fx-sport-carbon-4
bryanrsmith 6y ago
We rely on spec data published by manufacturers, and unfortunately many do not specify the weight. It's also possible we're just failing to match it. If you find a bike without a weight that the manufacturer does have on their site you can use the feedback form & we'll get it fixed up.
loeg 6y ago
Weight can vary quite a bit for the same frame based on easily changed components like (1) wheels, (2) groupset (gears, pedals/cranks, brake/shift levers, that kind of thing), (3) handlebar/stem/seatpost material. I recently bought a medium-spec build of a lightweight frame bike, and could shave 2+ lbs with upgrades, if I wanted to. Not paying for those upgrades out-of-box saved me ~4800 euro up front, and the bike is 17.9 lbs instead of 16.2 lbs. Meanwhile, I could stand to lose 15 lbs myself. :-)

SeanLuke 6y ago

The description of trail could be a lot better, given how important it is. It's effectively the degree to which the front wheel acts as a caster. When you push a shopping cart the caster wheels automatically straighten out and trail behind the steering axis in a straight line. The front wheel is doing the same: when moved forward it resists being steered and wants to go straight. The degree of trail effects how much it wants to go straight. Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds. High-trail bikes can also be ridden no-hands because the wheel is effectively on autopilot. Any time you need more nimbleness and precision in steering -- perhaps a mountain bike? -- you want to lower the trail. Porteur bikes -- bikes that carry big boxes of flowers over the front wheel say -- also need very low trail because they're already hard to steer due to all that inertia. And with small wheeled bikes it is very difficult to achieve significant trail without a very shallow head tube (steering) angle, so many folding bikes by necessity have a very "live" steering. One of the dumber bike decisions I can think of is Brompton's inclusion of a rake on the front wheel, thus making its steering worse.

loeg 6y ago
Trail on road bikes, racing and not, seems to be universally in the range of 55-60mm (across frame sizes, too). I suppose it depends on what kind of racing you're doing, but some kinds of bicycle racing involve cornering agility / nimbleness, and you want a shorter trail for that.
- petre 6y ago
  Randonneur bikes (also road but with fatter 650Bx42 tires) typically have longer trail (typically in the seventies) because they carry front loads.
  - loeg 6y ago
    Right — I was commenting specifically on "Racing bikes etc. are high-trail bikes because by going straight the steering is stabler at high speeds."
  - 6y ago

loeg 6y ago

As a bicyclist, I'm amused this made it on to the HN front page. I'm not sure I agree with the description of stack. Cockpit height can be adjusted with riser stems and different handlebar shapes; it's not solely a function of axle-crown and headtube length. Without getting into it too much, some other descriptions are not great either. The general overview is ok but they make some overly strong claims. I'm not sure I've seen wheelbase used much in a bicycle context, although it makes sense. You see it used for cars a lot. Edit: The go-to source for bicycle information I'd point others at is Sheldon Brown's webpage, https://www.sheldonbrown.com/ https://www.sheldonbrown.com/ .

xenocyon 6y ago
Wheelbase is important if you're using a bike for transportation. A longer wheelbase makes for more stability and helps with carrying groceries, children, or other cargo. It does make the bike feel less nimble so may not be optimal for purely recreational riders.
- petre 6y ago
  Not only. Modern trail and all mountain bikes also have longer wheelbases so they're more stable at descending slopes at full speed.
WD-42 6y ago
Wheelbase is one of the most important measurements for mountain bikes. It determines stability at the cost of the ability to turn quickly.
- loeg 6y ago
  It's definitely also important for road / track bikes! My observation is just that it usually isn't described by that name (or even necessarily that specific measurement). (In my experience, YMMV, etc.) For example, Look publishes Chainstay (sometimes called "Rear Center") and "Front Center" (BB to front axle) measurements, separately, for their frames, but does not publish axle-to-axle / wheelbase measurement. Sometimes it is described sort of qualitatively as "cramped" (on the short side).

bill1306an 6y ago

Although I'm an avid reader, this is my first ever comment. I built this site https://comparemtb.com/ https://comparemtb.com/ to give people a way to visually compare the geometries of many different mountain bikes. I'd be interested in any feedback. Thanks.

loeg 6y ago
Recently I've used https://geometrygeeks.bike/ https://geometrygeeks.bike/ to compare geometry numerically. Having a database like that is pretty useful.
johnmaguire2013 6y ago
As a motorcycle rider, this site was invaluable to me when considering new bikes to buy: https://cycle-ergo.com/ https://cycle-ergo.com/ Now that I'm getting into bicycles, I'd love something like that. Have you considered adding bikes other than MTB?
eitland 6y ago
Thanks for finally posting! > I'd be interested in any feedback. I'll start with pointing out that there seems to be no instructions or hints on the landing page.
petre 6y ago
Also https://bikeinsights.com/ https://bikeinsights.com/
- bill1306an 6y ago
  CompareMTB guy here. Thanks for sharing your site. Didn't know you existed. Nice work.
  - deleted 6y ago
    [deleted]
  - petre 6y ago
    It's not mine, I'm merely a user. It was featured on the Path Less Pedaled YT channel.

vondur 6y ago

It’s been an interesting few years in relation to MTB geometry. Low bottom brackets with slack head angles and steep seat tube angles have really taken off. My last MTB purchase was in 2018 and it’s geometry seems pretty conservative to newer bikes.

discreteevent 6y ago

If you are choosing a bike based on stack and reach then note that they are related. The reach and stack are measured from the same point (join of top tube and head tube). So if you see a bike that has 20mm more stack and 10mm more reach than another then it actually may have 16mm more reach at the same stack height from the bottom bracket as the smaller bike, (depending on head tube angle) so it's longer than you think.

jpm_sd 6y ago

For a deep dive into all things bicycle, I always recommend Sheldon Brown's delightful knowledgebase, which started in the late 90s: https://www.sheldonbrown.com/ https://www.sheldonbrown.com/ (sadly Mr. Brown is no longer with us, but his web archive lives on) And there's always Bicycling Science for the serious nerds among us https://www.amazon.com/Bicycling-Science-Press-Gordon-Wilson/dp/0262731541 https://www.amazon.com/Bicycling-Science-Press-Gordon-Wilson...

anewdirection 6y ago

I dont ecen know where to begin. So much of this is alightly incorrect its maddening. As someone who has setup race motorcycles please look elsewhere for correct terms and physics.