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Old 11-09-08, 07:52 PM   #1
SaiKaiTai
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* Bike geometry is a whole other world

And, honestly, I never gave it a lot of thought.
I'd sit on the bike... ride it around and it felt good or not.
But now that I'm even remotely considering building up a Soma, I've taken an interest in what all the numbers are and what they mean.
Does a point five change in head tube or seat tube angle really make a difference?
Wow... kind of mind boggling but it does make you think about these bikes of ours in a whole different way.
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Old 11-09-08, 08:12 PM   #2
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Originally Posted by SaiKaiTai View Post
And, honestly, I never gave it a lot of thought.
I'd sit on the bike... ride it around and it felt good or not.
But now that I'm even remotely considering building up a Soma, I've taken an interest in what all the numbers are and what they mean.
Does a point five change in head tube or seat tube angle really make a difference?
Wow... kind of mind boggling but it does make you think about these bikes of ours in a whole different way.

Have you taken the "eyeball" test?

If you don't score under 4, you need to reconsider thinking about "geometry!"
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Old 11-09-08, 08:18 PM   #3
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Originally Posted by SaiKaiTai View Post
And, honestly, I never gave it a lot of thought.
I'd sit on the bike... ride it around and it felt good or not.
But now that I'm even remotely considering building up a Soma, I've taken an interest in what all the numbers are and what they mean.
Does a point five change in head tube or seat tube angle really make a difference?
Wow... kind of mind boggling but it does make you think about these bikes of ours in a whole different way.
Yes.
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Old 11-09-08, 08:26 PM   #4
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Yes.
+1

Little differences in geometry can make important differences in the ride and handling characteristics of a bike.
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Old 11-09-08, 08:49 PM   #5
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And I would think that steeper angles, a more upright seat and head tube, mean a "racier" performance?
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Old 11-09-08, 08:58 PM   #6
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Research, baby, Google is your friend. (but yes, generally, especially the head angle)
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Old 11-09-08, 08:59 PM   #7
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You have your work cut out for you, but it sounds like a fun project. I hope you pics of your progress.
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Old 11-09-08, 09:25 PM   #8
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A very slight difference in seat tube angle on its own means nothing, as you can correct for it by sliding your seat forward or back or getting a seat post with a setback. However, it may be accompanied by a slightly different rear triangle geometry and change in wheelbase, so that can affect the handling and feel of the bike. A change in head tube angle would likely have more obvious impact on the feel of the bike.
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Old 11-09-08, 09:32 PM   #9
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Wait until you start trying to understand fork rake and trail...you will have to pick a fork for the Soma, right?
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Old 11-10-08, 05:42 AM   #10
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Just imagine adding frame tube thickness to the mix. Chain stay frame tubes could be made out of Ti pipes that are this thick or made out of pipes that are that thick. Start figuring out what changes those choices in thicknesses of material make to the ride quality beyond the geometry will give you a heck of a headache.
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Old 11-10-08, 06:46 AM   #11
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Add head tube length into the mix.
Start adding center/forward or center/rear lengths.
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Old 11-10-08, 07:00 AM   #12
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Is there anyone else out there with my attitude toward this? My take is that I do not have time to get into all this. I like to ride, and that takes up most of my free time, if I really knew what free time meant. Then add in time the wife wants for all types of projects, other family duties with the kids, that are supposed to be grown, and community activities. Then the 800 pound gorrilla in the corner that goes by the names of work, career, etc. I let the bike manufacturers work out the numbers.
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Old 11-10-08, 08:45 AM   #13
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Is there anyone else out there with my attitude toward this? My take is that I do not have time to get into all this. I like to ride, and that takes up most of my free time, if I really knew what free time meant. Then add in time the wife wants for all types of projects, other family duties with the kids, that are supposed to be grown, and community activities. Then the 800 pound gorrilla in the corner that goes by the names of work, career, etc. I let the bike manufacturers work out the numbers.
I take your approach to this....sort of. I'm not really interested in "learning" all there is to know about frame geometry. Rather, when I have a bike the feels really good and performs as I want it to perform, I make sure I keep a record of important details. This way I can replace the bike with the same geometry. Currently my favorite ride is:
Seat tube length - 57.3 cm
Seat tube angle - 73.5 cm
Head tube length - 144 cm
Head tube angle - 73.5 cm
Bottom bracket height - 26.7 cm
Top tube length - 56.0 cm
Stand over height - 80.6 cm
Chain stay length - 40.5 cm
Wheel base - 98.1 cm
Drop out width - 130 cm.
Fork Rake - 42

I suppose there are other things that I "should know", but I'm more interested in riding.
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Old 11-10-08, 08:50 AM   #14
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All these things make differences, but the reality is that you can ride bikes with differences and enjoy them regardless. I have a crit bike, a touring bike, a road race type bike, and a sport-tour type bike. They all have their own feel, and handle different ways, but they can all be used for anything I would do. The big difference is the harsh ride of the crit type frame, but this is partly due to the stiffness of the oversize heavy gage steel frameset. The upside is the thing turns better than any bike or motorcycle I've ridden.
SKT, you will be happy with any Soma you get, if you're looking for a steel framed bike.
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Old 11-10-08, 08:59 AM   #15
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It only matters if you are purchasing a frame set or building a custom bike for which you want to control some of the performance parameters. Many if not most folks will just purchase what's available.

On the other hand, some wish to know just for the heck of it.
Some actually do build their own bikes.
Some want additional information to assist in fitting their next bike.

For additional comparisons, see:

Sterio enthusiast
Computer enthusiast
Automobile enthusiast
Tool enthusiast
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Almost any woman who is shopping for clot...........anything.
Skiers
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actually anyone who is an enthusiast about anything.

Or Tom.

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Old 11-10-08, 09:03 AM   #16
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If you just ride and don't pay attention to how the bike feels or reacts, that's fine. However once you start noticing differences between bikes, it opens up a lot of new things. At first, the differences are subtle. But the more you notice, the variations become larger,

Seat tube angle, by itself, isn't much. The seat position can compensate for a 0.5 degree. But the seat tube angle also affects the rear triangle (seat stays and chain stays). The chain tube length affects how "snappy" the bike accelerates or how comfortable and smooth a ride is. It also affects the top tube length. TT length might mean a different size stem. But you get the idea.

The two often overlooked things that impact the ride of a bike are geometry and wheels. Often they are more important than frame material.
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Old 11-10-08, 09:16 AM   #17
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Or Tom.




I'm in the camp with those who don't get into the mathematics much. Although I do consider myself to be a pretty good mechanic, (in spite of DnvrFox's geometry test ).

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Old 11-10-08, 09:37 AM   #18
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For some reason, there are only two bike-frame geometry numbers that I can remember without looking stuff up: chainstay length and wheelbase.

- A wheelbase over 41 inches will mean the bike is more suitable for relaxed riding and touring
- A chainstay length over 17 inches long suggests the same thing (more relaxed wheelbase, chainstays long enough that you can put panniers on the bike and avoid striking your heels).

The wheelbase number is the product of a lot of different decisions - chainstay length, frame angles, top tube, fork rake, etc. I've never ridden enough similar bikes back to back to understand whether I can feel each of those differences independently (I'm sure I couldn't)....but there is a *huge* difference in the rides between a 39-inch and a 41-inch wheelbase bike. At 41 inches it means the bike maker has made lots of smaller decisions that will add up to a bike that feels more relaxed than one at 40 or 39 inches.

I take a tape measure with me when I look at used bikes and always measure the wheelbase.

The chainstay length is a good number to remember when looking at bikes that are pitched as "all rounders," like 'cross bikes that can be used for light touring or commuting. Some of these bikes have chainstay lengths just under 17 inches (say 16.5), and some are just over. The shorter ones would likely ride a bit snappier but not have enough chainstay for mounting large panniers.
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Old 11-10-08, 09:45 AM   #19
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Unless you are planning to design or build your own frame, you really don't need to know this stuff. If you are looking to buy a new bike, simply ride different bikes and choose the one that fits and handles in a way that you like. Most manufacturers have already separated their lineups into categories based on their intended use and the geometries they chose to accomplish that.

But some of us are just more curious than others about how things work and why they work like they do. For example, Specialized has the Tarmac line of bikes with traditional racing geometry for quick handling and the Roubaix line with slacker geometry for a more relaxed and comfortable ride. Look at their geometry charts to see how they use different head tube angles and chainstay lengths on bikes the same size between those two lines.

When looking at new potential frames for future bike builds, I like to compare the geometry of the new bike to the geometry of my best fitting and handling bike to get an idea how it might handle. Frames that come closest to my gold standard (Ribby) with 575mm effective top tube, 73.5 degree head angle, 73 degree seat angle and 410mm chainstay length are the ones I would expect to match my idea of a sweet handling road bike.
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Old 11-10-08, 10:55 AM   #20
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Tom's rides form a valuable data base for this forum. Much of his information is priceless.
Blues Dawgs bikes have pushed the design parameters as well. (finding out what works and what doesn't takes time and money.....its always nice to be able to use someone elses ideas.)

I became a geometry obsessive last year when I began the construction of a TT bike. They are so different that little of my prior experience with conventional designs was of help. Especially since it couldn't be a TT for the typical rider. (there are no designs for the average 60+ first time TT'er who hasn't ridden him/herself into TT shape) Off to school (the hard way by digging it out on my own). In the process of learning about TT fit, it was necessary to absorbe lots of info on the fit of normal bikes. Very little of this info is in the form of firm hardcore facts. There is a world of opinion as to how many degrees of this and how many centemeters of that should be combined with 1 eye of newt to produce a bike that handles just so.

The end result was a TT bike that works pretty good for me.......at this time.........on my roads.......at this level of fitness........with enough adjustability to possible work next year and the year after that. Even at that, my learning curve helped only just enough to get the frame right. I still have 3 sets of bars, 5 stems, 2 sets of brake handles and 2 forks invested in the bike, mostly so that I could put real performance changes together with theory.

As a spin off though, I began construction of "Dormouse" at the same time. I wanted an MTB based bike that would handle most terrain but still give the handling feel of the TT so that I could get some training benifit from most of my flatter offroad rides. I managed to get that also, but you couldn't buy that bike in any LBS because it flys in the face of what is presently being sold.

That's when understanding bicycle geometry becomes valuable. That point where you know more about what you want the bike to do for you than someone else does. It's one thing to copy what already is. Its something else to head off in a new direction but with some idea of what your going to do and how.
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Old 11-10-08, 11:00 AM   #21
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My head hurts

edit: I'd imagine, then, that this would be a pretty "snappy" performer?

SIZE 55cm
CENTER of BB to TOP of TT 21.69/551
EFFECTIVE TT LENGTH 22.44/570
HT ANGLE 72
ST ANGLE 73

CHAINSTAY 16.73/425
WHEELBASE 40.33/1024
FORK RAKE 1.69/43
BB HEIGHT 11.02/280
HEADTUBE 5.31/135
STANDOVER 32.36/822

inches/mm

Last edited by SaiKaiTai; 11-10-08 at 11:21 AM.
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Old 11-10-08, 12:00 PM   #22
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If you think bike geometry is hard, try bike algebra...or worse yet, bike trigonometry. Egads!
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Old 11-10-08, 01:27 PM   #23
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My head hurts

edit: I'd imagine, then, that this would be a pretty "snappy" performer?

SIZE 55cm
CENTER of BB to TOP of TT 21.69/551
EFFECTIVE TT LENGTH 22.44/570
HT ANGLE 72
ST ANGLE 73

CHAINSTAY 16.73/425
WHEELBASE 40.33/1024
FORK RAKE 1.69/43
BB HEIGHT 11.02/280
HEADTUBE 5.31/135
STANDOVER 32.36/822

inches/mm
Looks like a cyclocross bike to me. The head angle is pretty slack for a typical road bike. The chainstays and wheelbase are long for a road bike but short for a tourer.
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Old 11-10-08, 01:40 PM   #24
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Looks like a cyclocross bike to me. The head angle is pretty slack for a typical road bike. The chainstays and wheelbase are long for a road bike but short for a tourer.
Hm. OK... this gives me a little clue anyway.
These are the specs of the Jamis Aurora Elite, which Jamis considers a touring bike
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Old 11-10-08, 03:04 PM   #25
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Hm. OK... this gives me a little clue anyway.
These are the specs of the Jamis Aurora Elite, which Jamis considers a touring bike
As you are thinking about N+1- look at the type of riding you want to do. Your OCR is a comfort geometry bike. Ideal for the type of riding you currently do. Now if Jamis consider the Aurora as a Touring bike- Will it really fit in with the type of riding that you do?

Think you and I do the same type of riding. We just go out and ride. You are beginning to get your distance up- so is a touring bike the ideal for you? I have two bikes- both have performance geometry and they both suit my style of riding. They are set up a bit differently though and IF I am out for more than a 60 mile ride- then I take the TCR Bars just a bit higher and bar reach about 1 cm longer. Boreas is great for riding but is not quite as comfortable on the longer rides.


I would suggest that you try out the "Performance" geometry bikes aswell. In fact check out as many bikes on test rides as you can. I would hate it for you to get a bike that is not as good as the OCR and it rarely gets taken out.

And as to the geometry-It will not matter as long as it rides well for you. My experience is on Mountain bikes and I found that there are some bikes that suit me and others that don't. The only way I found out was to borrow them from my mates- or arrange test rides.
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