geometry chat

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Larry Winget
Larry Winget's picture
geometry chat

Low trail porteur 29er discussion optional.

Thu, 06/12/2014 - 23:04
VT regularbike
VT regularbike's picture

[img][/img]

Miguel wrote:
i mean as long as we're spending money, lets just set the wallet on fire ok

Thu, 06/12/2014 - 23:18
Larry Winget
Larry Winget's picture

Here is what I've been messing around with.

The design goals are:

28mm tires w/fenders possible (57mm calipers), but not much bigger (want short-ish chainstays). Would want to run 28+ tires most of the time
works with Columbus Hiver fork (45 rake 378 a-to-c).
neutral handling in front (weight back due to setback/slacker STA made up partially by longer stem
slightly more upright but not too faster backwards position (fits now and stem can be flipped up when I'm even more of a geezer)

I don't know shit about shit. Tell what I'm doing wrong.

The Pitbull of Personal Development®

Fri, 06/13/2014 - 01:43
catdrew f

my cx bike has 425 chainstays and fits a fender or 35s with space

you should be fine

i have no idea why you are doing htis outside wanting to build your own bike

let someone else stress it

Fri, 06/13/2014 - 03:38
Larry Winget
Larry Winget's picture
(Reply to #4)

catdrew f wrote:
i have no idea why you are doing htis outside wanting to build your own bike

let someone else stress it

I might do that (well farm it out somewhere to have it built), but really I'm just trying to understand how this shit works.

As far as I can tell this is pretty much right in the middle of two stock geometries I've mocked up - either of which I think would work fine once I figure out how far down the fred scale I'd want the bike to be.

The Pitbull of Personal Development®

Fri, 06/13/2014 - 09:33
cookietruck
cookietruck's picture
(Reply to #5)

tarckeemoon wrote:
Here is what I've been messing around with.

The design goals are:

28mm tires w/fenders possible (57mm calipers), but not much bigger (want short-ish chainstays). Would want to run 28+ tires most of the time
works with Columbus Hiver fork (45 rake 378 a-to-c).
neutral handling in front (weight back due to setback/slacker STA made up partially by longer stem
slightly more upright but not too faster backwards position (fits now and stem can be flipped up when I'm even more of a geezer)

I don't know shit about shit. Tell what I'm doing wrong.

not sure what the front center is on this
but this is very close to what my davidson is
but with 15mm shorter chainstays and a few mm shorter frame reach
i think this would probably ride just fine and ~425mm chainstays is what i would want on my davidson if i didn't use a small rear pannier

but yeah, i wouldn't worry about this if you weren't building yr own frame...

Fri, 06/13/2014 - 09:49
rauce
rauce's picture

My surly pacer fits 28's and fenders or 35 slicks without and it has 415mm chainstays.

That thing is probably within a couple mm of 60cm of front centre based on the difference in reach and rake vs. my cervelo.

Fri, 06/13/2014 - 11:25
cookietruck
cookietruck's picture
(Reply to #7)

rauce wrote:
My surly pacer fits 28's and fenders or 35 slicks without and it has 415mm chainstays.

That thing is probably within a couple mm of 60cm of front centre based on the difference in reach and rake vs. my cervelo.

yeah sounds right
the davidson is 602mm front center and like i said, 420-425mm chainstays would be what i would want if i didn't need heel clearance for panniers

Fri, 06/13/2014 - 11:37
asterisk
asterisk's picture

Just curious, what does your Davidson have?

Fri, 06/13/2014 - 11:39
cookietruck
cookietruck's picture

cs are 435mm
front center 602
ht and sta are 73*
tt is 575
fork rake is 43mm
i forget bb drop, i think it's 75mm and wheelbase is 1012mm or so...would have to look at the sheet on those

sport tourer/all arounder

edit: oh no, the bbdrop is 72mm and wheelbase 1027mm

Fri, 06/13/2014 - 11:52
halbritt
halbritt's picture

Most bike manufacturers would run a 43mm rake fork with a 73HTA giving a little more trail. Makes not a lot of difference. I personally prefer mid-50s in any case, especially if you're going to run larger tires which increase mechanical trail.

In short, if you get this built and the 42cm stays actually work, then it should handle really nicely.

What's the BB drop?

...shift like jesus making one set of footprints in the sand in your time of need

Fri, 06/13/2014 - 11:58
jimmythefly
jimmythefly's picture

seventy

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Fri, 06/13/2014 - 12:57
halbritt
halbritt's picture

Needs more

...shift like jesus making one set of footprints in the sand in your time of need

Fri, 06/13/2014 - 14:01
Larry Winget
Larry Winget's picture

How much more? My Charge is 70 and handles pretty well with 28s and I tend to prefer "on the bike" over "in the bike" by a little bit.

The Pitbull of Personal Development®

Fri, 06/13/2014 - 20:21
catdrew f

i talked to david kirk abotu that exact topic today

it doesnt matter by itself, don't ask for hte bb drop

either the frame works or it doesnt; ive ridden road bikes with 63mm bb drop to 80mm

all will work if everything else works in sync

erichie didnt lie when he said the frame is the frame

focusing on a singular value is the fasted way to turdrocket imho

Fri, 06/13/2014 - 21:46
halbritt
halbritt's picture

That's asinine.

I'm saying that whole frame with all those angles might scratch some people's itch with a little more BB drop. However...

tarckeemoon wrote:
How much more? My Charge is 70 and handles pretty well with 28s and I tend to prefer "on the bike" over "in the bike" by a little bit.

If that's the case, then keep it at 70. I felt the same way about the in the bike vs. on the bike until I rode a 700c bike with fat ass tires and 75mm of BB drop. Not like a few more mm of drop will make much difference in any case.

read this:

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

...shift like jesus making one set of footprints in the sand in your time of need

Sat, 06/14/2014 - 01:40
catdrew f

dude what are you talking about

bb drop in itself doesnt do shit

you can make a fast handling bike with a low bb, high bb, and mid bb

saying a bike needs more drop doesnt make any sense

the only addition a lower bb has given me, ignoring all other factors is the frame feels stiffer in the bb. as teh stupid baum article stated, you need to work a design parameter in with everything else.

Sat, 06/14/2014 - 01:55
Larry Winget
Larry Winget's picture

I think the stuff about chainstay length is largely bullshit. But yeah, it all has to work together.

The Pitbull of Personal Development®

Sat, 06/14/2014 - 01:58
catdrew f

only time id stress it is being super tall

i run a really high seatpost (86cm) and a ton of setback (~15cm)

I rode a bike with sub40cm chainstays, and it was not the rear center that made me dislike the bike.

friggan long chainstays feel a bit weird, but that is just hte overall wheelbase getting out of hand

420 (to me) seems wholly reasonable for what you are wanting to use the bike for

Sat, 06/14/2014 - 02:07
Endpoint
Endpoint's picture

7 is the magic number for a race bikes and racelike bikes atmo atmo atmo. i don't see any reason to go outside that by more than 5mm for a 700c wheeled bike unless we are talking trail bikes. even for a touring or randomachine. If you like to ride fast you probably like to pedal through corners regardless of what you happen to be doing that day.

but yeah... LOW TRAIL!
HIGH TRAIL!!!
TASTES GREAT!
LESS FILLING!!

Tue, 06/17/2014 - 21:06
halbritt
halbritt's picture

Depends on the fatness of the tires, really. In the end, BB height should be pretty much the same. Maybe a little higher for crit bikes and a little lower for touring bikes.

...shift like jesus making one set of footprints in the sand in your time of need

Tue, 06/17/2014 - 22:02
NOVELTYNAME
NOVELTYNAME's picture

Nvm

"Folks want options!"

Tue, 06/17/2014 - 23:10
circuithero
circuithero's picture

Chainstay length only makes a difference when cornering under power. And that might still be a question of wheelbase and setback like catdrew said. Doesn't every number mean something?

Wed, 06/18/2014 - 01:54
JUGE FREDD
JUGE FREDD's picture
(Reply to #23)

Endpoint wrote:
If you like to ride fast you probably like to pedal through corners regardless of what you happen to be doing that day

I pedal through fucking everything balls out

on a bike with ~73mm drop, 180mm cranks, and tires that drop the BB another 15mm relative to a CX tire

I almost never strike pedals in a corner, it's only when way offcamber or rawdogging it around 40mph

I do strike the tips of my crankarms on stuff when getting rad

pedal width and girth counts for 20x more than the bb height or arm length -- put bmx platforms on any of my low bikes and I will drag them *everywhere*

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Wed, 06/18/2014 - 02:14
JUGE FREDD
JUGE FREDD's picture
(Reply to #24)

circuithero wrote:
Chainstay length only makes a difference when cornering under power. And that might still be a question of wheelbase and setback like catdrew said.

yep it's entirely about balance

a 3% tube length difference has fuckall to do with stiffness, unless you squash it extra for clearances (which can easily fuck you)

circuithero wrote:
Doesn't every number mean something?

except for top tube length (effective or otherwise), also downtube length is equally meaningless

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Wed, 06/18/2014 - 02:20
Larry Winget
Larry Winget's picture

Size comes into play with how weight is distributed, which then has a effect on handling.

The Pitbull of Personal Development®

Wed, 06/18/2014 - 02:39
halbritt
halbritt's picture

Right. Assuming all the same contact points, shorter tubes are going to be somewhat stiffer. If you're chasing grams, they're also going to be somewhat lighter.

...shift like jesus making one set of footprints in the sand in your time of need

Wed, 06/18/2014 - 10:46
jimmythefly
jimmythefly's picture

Interesting.

Looking at a "traditional" frame with parallel geometry. If you keep the A-C, seat tube length, and clearance from rear rim to seat tube constant, then dropping the BB will:

Lengthen the chainstays and down tube, but shorten the seat stays, head tube, steerer tube.
Gives a bit more room between rear tire and BB shell, which can help with how much chainstay may need to be squashed for tire/chainring clearance.
Moves the rider's center of gravity forward and down a bit (at the distance and angle the BB moved of course), which also moves the bars lower and more over the front tire.

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Wed, 06/18/2014 - 11:58
halbritt
halbritt's picture

Good summary.

...shift like jesus making one set of footprints in the sand in your time of need

Wed, 06/18/2014 - 14:11
JUGE FREDD
JUGE FREDD's picture

there are two different measurements, and they do actually matter independently

it gets much clearer when you're messing with it on wildly different wheel sizes

BB height off the ground directly influences pure handling ability (lower being better in almost all cornering), plus clearances for pedal strike below and taint strike above

BB drop from the axles does a whole bunch of crazy stuff on its own. Real low with the "in the bike" feeling is awesome at higher speed, super high (esp. way negative) is awesome for low speed technical trials shit. It's also a huge influence over how the whole chainstay assembly comes together.

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Wed, 06/18/2014 - 14:34
Larry Winget
Larry Winget's picture

How well are the major manufacturers able to model this stuff?

Or do they bother?

The Pitbull of Personal Development®

Wed, 06/18/2014 - 20:13
Gary Fissure
(Reply to #31)

jimmythefly wrote:
Interesting.

Looking at a "traditional" frame with parallel geometry. If you keep the A-C, seat tube length, and clearance from rear rim to seat tube constant, then dropping the BB will:

Lengthen the chainstays and down tube, but shorten the seat stays, head tube, steerer tube.
Gives a bit more room between rear tire and BB shell, which can help with how much chainstay may need to be squashed for tire/chainring clearance.
Moves the rider's center of gravity forward and down a bit (at the distance and angle the BB moved of course), which also moves the bars lower and more over the front tire.

I'll ask some dumb questions here
-why would seat tube length stay constant? (except to demonstrate seat stay lengths would be shorter, all other things being equal)
-why does HT shorten?
-is this (indirectly) why people talk about front center instead of HT length/TT length?

mander wrote:
And what if the part is, like, just a nut? *huge bong rip*

Wed, 06/18/2014 - 20:21
Larry Winget
Larry Winget's picture

I think it may be time for a gif.

I get the head tube thing if you're trying to maintain the same stack with a constant a-to-c and dropping the BB down.

The Pitbull of Personal Development®

Wed, 06/18/2014 - 20:30
Endpoint
Endpoint's picture
(Reply to #33)

JUGE FREDD wrote:
Endpoint wrote:
If you like to ride fast you probably like to pedal through corners regardless of what you happen to be doing that day

I pedal through fucking everything balls out

on a bike with ~73mm drop, 180mm cranks, and tires that drop the BB another 15mm relative to a CX tire

I almost never strike pedals in a corner, it's only when way offcamber or rawdogging it around 40mph

I do strike the tips of my crankarms on stuff when getting rad

pedal width and girth counts for 20x more than the bb height or arm length -- put bmx platforms on any of my low bikes and I will drag them *everywhere*

I rode a bike for a while with the bb that low (650x38 tires most of the time). Was annoying enough to never go that route again. Pedal strike with mtb q factor cranks and atacs was a little too frequent and to hell with pedaling through any of the single track around here with that. Different riding styles I guess. If I leaned that bike the way I lean my crit bike I would have eaten shit on the regular. When I switched between that and a 700x30 bike with 7cm drop I never felt any less stable on the higher bike. Then again you have several inches of height over me and I'm sure that any additional drop is going to benefit you more than a shorter rider.

Wed, 06/18/2014 - 20:52
chenghiz
chenghiz's picture
(Reply to #34)

Sneaky Viking wrote:
[
-why would seat tube length stay constant? (except to demonstrate seat stay lengths would be shorter, all other things being equal)

Assuming it's because the seat tube determines how far your hips are from the bb (assuming no seatpost change), which shouldn't change.

Wed, 06/18/2014 - 20:50
JUGE FREDD
JUGE FREDD's picture
(Reply to #35)

Endpoint wrote:
Pedal strike with mtb q factor cranks and atacs

total width matters so much more than anything else it's silly, followed by pedal girth

a cm of bb height or crank length is comparatively meaningless

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Wed, 06/18/2014 - 21:57
VT regularbike
VT regularbike's picture

Lulz totally saw that coming

Miguel wrote:
i mean as long as we're spending money, lets just set the wallet on fire ok

Wed, 06/18/2014 - 22:16
bobbythehawk
bobbythehawk's picture
(Reply to #37)

JUGE FREDD wrote:
Endpoint wrote:
Pedal strike with mtb q factor cranks and atacs

total width matters so much more than anything else it's silly, followed by pedal girth

a cm of bb height or crank length is comparatively meaningless

i've been trying to beat this into my polo "customers" heads (with their BMX cranks)

Wed, 06/18/2014 - 22:22
JUGE FREDD
JUGE FREDD's picture
(Reply to #38)

VT regularbike wrote:
Lulz totally saw that coming

assume it was intentional

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Wed, 06/18/2014 - 23:54
Endpoint
Endpoint's picture

True indeed on cornering (although issue was there, albeit lessened when I had road cranks on the same setup)... the issue with smacking pedals on shit riding off road had zero to do with q though. Even with 170s that was annoying. Current FTW rig has almost none of that with the higher bb and still corners like a champ.

I'm of the opinion that any small gains to be had going below the overall bb height achieved with a 7-7.5cm of drop on a 700x28 setup are ultimately outweighed by the slightly more significant drawbacks. Except of course in the case of specific bikes but I don't think your average homermobile fits into that category.

Thu, 06/19/2014 - 09:27
jimmythefly
jimmythefly's picture
(Reply to #40)

Sneaky Viking wrote:
jimmythefly wrote:
Interesting.

Looking at a "traditional" frame with parallel geometry. If you keep the A-C, seat tube length, and clearance from rear rim to seat tube constant, then dropping the BB will:

Lengthen the chainstays and down tube, but shorten the seat stays, head tube, steerer tube.
Gives a bit more room between rear tire and BB shell, which can help with how much chainstay may need to be squashed for tire/chainring clearance.
Moves the rider's center of gravity forward and down a bit (at the distance and angle the BB moved of course), which also moves the bars lower and more over the front tire.

I'll ask some dumb questions here
-why would seat tube length stay constant? (except to demonstrate seat stay lengths would be shorter, all other things being equal)
-why does HT shorten?
-is this (indirectly) why people talk about front center instead of HT length/TT length?

-Yes seat tube stays constant because I limited it to "traditional" horizontal-top-tube geometry, where the frame size is seat tube size.
-HT shortens because if you keep the seat tube the same length and connect it to a horizontal top tube, then when you lower the BB/seat tube the top tube must also necessarily get lower. As the TT gets lower, the height of the head tube gets lower, BUT the bottom of the head tube stays where it is because it is locked in place by the fork A-C.

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Thu, 06/19/2014 - 14:58
jimmythefly
jimmythefly's picture

I wasn't sure what to do exactly because lots of things could be altered but what the hell I made some drawings.

All these variables can be changed so if you want something specific give me a holler.

First set. These maintain a constant frame (BB drop) and wheel size, so most applicable to thinking about what you can do with a frame that already exists.

Lean angle change depending on M520 vs MX30 pedals.

Lean angle change 166Q vs 146 Q cranks.

Lean angle change 700x32 vs 700x52 tires

Second set. These are more applicable to geometrychat/framedesignchat. They show how for a given lean angle, BB height can be adjusted depending on which components are used.

Same lean angle from either 166Q cranks + 70mm drop, or 146Q cranks + 75.4mm drop.

Same lean angle from either MX30 pedals +48.3mm drop, or M520 pedals + 70mm drop.

Same lean angle from either 700x32 + 70mm drop, or 700x52 + 92.9mm drop.

edit: I added in the "Horizontal distance to saddle" to help compare how center of gravity might change.
edit2: should probably add a BB height dimension.
Pedals are M520 spds vs. MX30 BMX flats.

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Thu, 06/19/2014 - 15:37
JUGE FREDD
JUGE FREDD's picture
(Reply to #42)

Endpoint wrote:
the issue with smacking pedals on shit riding off road had zero to do with q though. Even with 170s that was annoying. Current FTW rig has almost none of that with the higher bb and still corners like a champ.

that kind of smacking has more to do with the trail itself

a lot of trails through get buffed out to a very specific height just through use, and it depends on what gets ridden there

some trails on my homercar I do smack crankarms, but then that crabon fork mod that raises the BB ~8mm eliminates it *entirely*

 
and then there's trails where the bike doesn't matter, you're just gonna strike all the time

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Thu, 06/19/2014 - 15:27
Endpoint
Endpoint's picture

So based on first set reducing q factor from mtb to road standard is about the same as raising bb 5mm at same cornering angle? Or bringing the pedal in 10mm via pedal width or q factor equates to raising bb 5mm?

Thu, 06/19/2014 - 15:32
jimmythefly
jimmythefly's picture

That's this one:

jimmythefly wrote:
Same lean angle from either 166Q cranks + 70mm drop, or 146Q cranks + 75.4mm drop.

But I'd be really careful about making a larger generalization ("reduce outside pedal width by 20mm (10 per side) is the same as raising the BB 5mm"). I'm pretty sure it will not be a nice linear relationship as tire, wheel size, crank arm length change.

Which reminds me -I didn't do any comparisons of crank arm length change, nor of wheel diameter change.

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Thu, 06/19/2014 - 15:56
Andrew_Squirrel
Andrew_Squirrel's picture

Nice diagrams Jimmy, you have 90% of a BQ article, you should submit to Jan.

Thu, 06/19/2014 - 16:03
Endpoint
Endpoint's picture
(Reply to #46)

JUGE FREDD wrote:
some trails on my homercar I do smack crankarms, but then that crabon fork mod that raises the BB ~8mm eliminates it *entirely*

Yeah.. that.... getting closer to that magic 6.5-7.5 zone.

Thu, 06/19/2014 - 16:19
NOVELTYNAME
NOVELTYNAME's picture

so will JTF be fine with his sol-esque remixdonee with (29x1.8?2.0?)
because im curious about that now

"Folks want options!"

Thu, 06/19/2014 - 16:19
JUGE FREDD
JUGE FREDD's picture
(Reply to #48)

Endpoint wrote:
JUGE FREDD wrote:
some trails on my homercar I do smack crankarms, but then that crabon fork mod that raises the BB ~8mm eliminates it *entirely*

Yeah.. that.... getting closer to that magic 6.5-7.5 zone.

it started off at ~72, and got raised to near 60 (new fork 2cm taller)

and that experiment was with 584x54mm tires, same exact radius as 622x35

It's a struggle, but you cut out his tongue, and his last words are "atmo atmo Atmo ATMO ATMOOOOOGORIHGGHRSHGGRLMGGMMGMgrrglegurglegrr....."
– akasnowmaaan

Thu, 06/19/2014 - 16:24
jimmythefly
jimmythefly's picture

I shall surely perish.

Na, I'll be OK. That bike will get 700x42 tires, which will mean that other than low-trail fork, geometrically it will be just the same as it has always been.

I put the 700x48s on confirm they still fit, but will probably never get ridden with them. Not much mud room, not much advantage over 700x42. And if I do feel I want more than 700x42, I'll go straight to my 29er. Or maybe get really homercar and borrow some 650B wheels from someone to see if they fit. Or maybe go 26er.

deadforkinglast wrote:
But honestly, I have no idea how I am going to follow through on that plan and I already have a pretty rad bike. I think I just like fucking with my bikes.

Thu, 06/19/2014 - 16:30
Endpoint
Endpoint's picture
(Reply to #50)

JUGE FREDD wrote:
Endpoint wrote:
JUGE FREDD wrote:
some trails on my homercar I do smack crankarms, but then that crabon fork mod that raises the BB ~8mm eliminates it *entirely*

Yeah.. that.... getting closer to that magic 6.5-7.5 zone.

it started off at ~72, and got raised to near 60 (new fork 2cm taller)

and that experiment was with 584x54mm tires, same exact radius as 622x35

You right... I meant relative height on that bike, not drop.

Thu, 06/19/2014 - 16:32

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