
Bicycle Geometry Terms - thanhhaimai
https://99spokes.com/bicycle-geometry-terms
======
SeanLuke
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
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
Randonneur bikes (also road but with fatter 650Bx42 tires) typically have
longer trail (typically in the seventies) because they carry front loads.

~~~
et-al
Long meandering post on trail.. after rim brakes vs disc brakes and wheel
size, trail is probably the next most contested subject in bikenerdom.

SeanLuke provides a good example with the caster. I also recommend reading
Dave Molton’s blog (who’s built frames longer than most people here have been
writing bloated JS SPAs) for more:

 _Trail, fork rake, and a little bit of history_ \-
[https://davesbikeblog.blogspot.com/2007/05/trail-fork-
rake-a...](https://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-
little-bit-of.html)

Now porteur bikes _and_ typical randonneuring bikes often have _lower_ trail.
People are often focused on the trail numbers, but the simple reason is to
position the front load (handlebar bag or stack of newspapers) on or right
behind the front axle. Framebuilders do this by increasing the rake (fork
offset) to stretch out the front axle, which results in decreased trail. The
reason is that you want the weight at the fulcrum to reduce any "inertia" when
steering--so the bike remains easy to steer with the additional weight.

If you’re not intending on riding your bike with a front load most of the
time, then you probably do not want a low trail bike as they are more
sensitive to rider input on the handlebars when there is no weight. Also,
randonneuring is just an activity, so you can ride any bike with any geometry
you want. But in America, especially on the West Coast, there is a bit of
bike-cosplay in the randonnerding scene, so folks like to ride bikes built in
the old French Constructeur style.

Lastly, SeanLuke is probably not familiar with mountain bikes, but modern
geometry for XC and enduro bikes is towards slacker headtube angles (resulting
in _higher_ trail) for stability when descending.

For anyone interested in geometry, I highly recommend browsing Bike Insight's
category section:
[https://bikeinsights.com/categories](https://bikeinsights.com/categories)

~~~
SeanLuke
> Lastly, SeanLuke is probably not familiar with mountain bikes, but modern
> geometry for XC and enduro bikes is towards slacker headtube angles
> (resulting in higher trail) for stability when descending.

I'm certainly not familiar with modern mountain bikes. Very interesting that
some have slacker headtubes for downhill, but it definitely makes sense.

------
loeg
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/) .

~~~
paulcole
Everyone should think about this comment the next time they’re wow’ed by some
link here.

When you’re knowledgeable about something and read an article/blog/whatever
about that topic, it’s shocking how much is either somewhat off-base or just
plain _wrong_.

The articles that aren’t about your areas of expertise are no different.

~~~
jcheng
I agree with your general point but I think OP is being a bit picky about this
particular page. The descriptions are pretty good and wheelbase is indeed a
common bicycling term (I see it all the time in reviews and it’s listed on the
product pages for the last three road bikes I bought—I just checked).

This website overall is a great resource for bike nerds, specifically their
bike database; there’s nothing else like it AFAIK.

~~~
loeg
> I think OP is being a bit picky about this particular page.

That's fair. I didn't look at the other pages on the site, and in general the
descriptions are solid.

To be clear, I was amused that it made the front page not because of
inaccuracy, but because I don't think of HN as a community very interested in
cycling.

------
cranekam
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.

~~~
jsinai
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
Look into bikes with 650b wheels for this.

~~~
loeg
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
This is correct. I was in a hurry and didn't explain at length as i should
have.

------
bill1306an
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.

~~~
petre
Also [https://bikeinsights.com/](https://bikeinsights.com/)

~~~
bill1306an
CompareMTB guy here. Thanks for sharing your site. Didn't know you existed.
Nice work.

~~~
petre
It's not mine, I'm merely a user. It was featured on the Path Less Pedaled YT
channel.

------
audiometry
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
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.

~~~
app4soft
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)

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TheAlchemist
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
Very useful site, the geometry comparison amazing!

------
jpm_sd
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...](https://www.amazon.com/Bicycling-Science-Press-Gordon-
Wilson/dp/0262731541)

------
Lio
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.

------
thomasfl
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.

------
seesawtron
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.

~~~
matsemann
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.

~~~
loeg
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.

------
orthoxerox
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
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.

------
discreteevent
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.

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

~~~
loeg
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. :-)

------
vondur
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.

------
anewdirection
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.

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fit2rule
I love this site - it is very informative.

I wonder if anyone has done something similar for music - i.e. "Musical
Geometry terms" ..

~~~
ehnto
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
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.

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jibolso
Great explanations!

