# Analyzing bike geometry



## dgeesaman (Jun 9, 2010)

OK, so I'm supposedly fitted to my bike. So far as I know I have no discomfort related to fit so I can't disagree.

It's been said (here) many times that various models and brands do not have consistent sizing methods, and that a prospective buyer should compare geometry to determine the closest fit. I've read a bit on the 'net about fitting but what I can find relates to the process. It seems to me that a "fit" is basically a particular body position. My question is how to take this fit and determine how to achieve this on other bike models/brands. I suspect the info is out there but I haven't been able to weed it out of google yet.

I'm comfortable working in CAD and other geometry related things so I'd appreciate a link or a brief description of how to analyse my current bike fit and determine how to move that to another bike frame geometry.

David


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## dgeesaman (Jun 9, 2010)

Ha, as soon as I say I can't find something on google, there it is:
http://www.slowtwitch.com/Bike_Fit/Geometry_and_Handling/Bike_geometry_225.html

Of course this article is tri-specific, so I'm still open to discussion about how roadie fit is done compared to tri bikes.


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## PoorCyclist (Oct 14, 2010)

In an ideal world, you get fitted first -- on a size cycle, 
figure out what frame geometry will result in your comfortable position (or whatever your goal is)... then send in the angles and lengths for a custom build frame. At least that's how I would do it if I have enough to spend 


But not everyone can afford that so you buy a bike then tweak it to work for you best you can.. or you look for a bike with similar geometry to your ideal frame from step 1.


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## PJ352 (Dec 5, 2007)

If I'm reading your question correctly, I think to answer reliably we'd need to know your goal. 

Are you trying to duplicate (as near as possible) your current riding position on a bike with similar geo? Are you interested in a particular brand/ model and are looking for a way to see if it's possible to duplicate your current position?

Regarding available software, the Gunnar site has a pretty basic tool, but not knowing the particulars I'm not sure how useful it'll be.

http://gunnarbikes.com/site/my-gunnar/gunnar-fit-tool/


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## dgeesaman (Jun 9, 2010)

I have a bike and I don't have any issues with the fit. It's a 58cm 2010 Trek 2.1. I bought it to ride distances from 20-100mi and go as fast as possible.  In a year or two I might upgrade to a nicer version of the same thing.

I'm mostly curious to learn and understand bike sizing better. Others regularly state that they fit a 57 in this brand, 54 in that brand, 56 ...etc and frankly I'm lost on how they arrive at this. I'd like to understand this more.


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## PJ352 (Dec 5, 2007)

dgeesaman said:


> I have a bike and I don't have any issues with the fit. I bought it to ride distances from 20-100mi and go as fast as possible.  In a year or two I might upgrade to a nicer version of the same thing.
> 
> I'm mostly curious to learn and understand bike sizing better. Others regularly state that they fit a 57 in this brand, 54 in that brand, 56 ...etc and frankly I'm lost on how they arrive at this. *I'd like to understand this more*.


For cyclists to reliably state that they take different sizes in different brands, they'd need to know and understand their sizing requirements, but the primary reason for this variation in frame sizes is because in the bike industry there are no standards for measuring to determine them, so a 54 cm Trek Madone might be comparable to an XS in a Ridley - or a 52cm in a Specialized Tarmac. 

Essentially, that sticker on the frame tells you little about how well the frames actual measurements will accomodate a given rider. Thus the need for most to work with a reputable fitter, because after they pin down the riders sizing requirements, they'll interpret the geo numbers (not the frame size) and get a best match to the riders reach, drop saddle height and standover requirements (among other factors). Once sizing is determined, fit follows, and appropriate saddle and stem adjustments made. 

I know you're familiar with the fit process because you've already been through it at your LBS, but did any of what I've offered help clear some of the confusion?


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## dgeesaman (Jun 9, 2010)

Perhaps you've covered this by stating "Thus the need for most to work with a reputable fitter,..., they'll interpret the geo numbers"

I'm aiming to take the geometry of my current bike and find near matches in another bike. Bike makers post the geometry specs of their bikes, how exactly is that info used when bike shopping?

I think this bit from the Empfield article is the type of info I'm after:
"These two dimensions—the length of the effective top tube and the length of the head tube—are the only dimensions you need when determining your proper anatomical bike size." (There's more than those but within the scope of that article this is a reasonable starting point)

All of this though got me reading a bit more about the fitting process. I was fitted to a Trek in my price range. Who's to say the geo of that bike are anywhere near optimal for me? Therefore, when I buy another bike in the future I'll investigate options such as a Serotta fit bike and motion analysis. When it comes to adding $400 to the investment in a mid- to high-end bike this seems like a no-brainer.


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## PJ352 (Dec 5, 2007)

dgeesaman said:


> Perhaps you've covered this by stating "Thus the need for most to work with a reputable fitter,..., they'll interpret the geo numbers"
> 
> I'm aiming to take the geometry of my current bike and find near matches in another bike. Bike makers post the geometry specs of their bikes, how exactly is that info used when bike shopping?
> 
> ...


To answer your first question, once you have a baseline (your _seemingly_ well fitting Trek) you can use the numbers of that bike (touched on in the Empfield article) to compare to your bike(s) of interest. To do this, you'll need to understand exactly what the numbers represent. 

For example, your Trek has an effective top tube of 573mm, but that doesn't mean that a bike with the same ETT will fit the same. You also have to look at the seat tube angles of the bikes to estimate differences in reach. Your bike has a STA of 73*, so if you were to find a bike with the same ETT but having a STA of 72.5, reach would be shortened by about 5mm's. With a STA of 72.5 to maintain the Trek's reach, you'd need an ETT of about 578mm's. A simple way to think of these variations is that for every degree of STA change, reach changes by about 1cm.

Also, assuming you want to maintain your current setback/ KOPS, keep in mind that changing STA will also change your saddle position, so going to a 72.5 STA will move your saddle back on the rails (but up towards the bars). 

Next consideration would be saddle to bar drop. Your current HT length is 19cm's. Because I don't know your stem angle or spacers used, I can't give you specifics, but if you were to find a bike with a HTL of 17, for you to maintain the same drop you'd need to add 20mm's to your current (total) height. This is a general statement, because other factors like fork length and what's called frame stack enter into the equation.

Beyond this, there are obvious other considerations like saddle height, standover and tweaks to fit with adjustments to stem length/ angles. Also, the bikes general geo (wheelbase, trail) which will dictate handling (and to some extent, ride) thus the need to decide on intended purposes before getting to the point of crunching (geo) numbers.

Lastly, while I definitely think becoming educated on such matters is worthwhile, there does come a point where a fitters tools and experience should be depended on. So (IMO) a balance of knowledge, taking a pro-active role in the sizing/ fitting process and having a level of faith/ confidence in the fitters ability, should all be part of the picture.


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## Dave Hickey (Jan 27, 2002)

Just a few other comments....

1. If you are buying a complete bike, frame geometry is not enough. Stem length and handle bar reach vary... Bar reach( hood position on the bar) can vary up to 3cm depending on the bars used..

2. I have 10 bikes that a regularly ride....only 2 of the bikes are set up the same. the other 8 vary in saddle/bar drop, reach, position, etc...... I don't get crazy making sure a bike is exactly like another...I have no issues switching between bikes.

3. All of my bikes fall into the 49cm-53cm seat tube range. The top tubes vary from 51.5cm to 54cm.... I adjust with stems and bars to get _close_ in fit but not exact...

4. You can drive yourself crazy analyzing geometry charts.. I look for head tube height and top tube length as the primary dimensions..head tube height will help determine saddle to bar drop and top tube length is the starting point for reach.......seat tube angle is looked at but as long as it's close to normal, I don't worry about it.....


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## perttime (Jun 27, 2005)

PJ352 said:


> ... Your bike has a STA of 73*, so if you were to find a bike with the same ETT but having a STA of 72.5, reach would be shortened by about 5mm's....
> ...
> ...Also, assuming you want to maintain your current setback/ KOPS, keep in mind that changing STA will also change your saddle position, so going to a 72.5 STA will move your saddle back on the rails (but up towards the bars).


I got a bit lost there, so would like to paraphrase....

If you keep the same ETT length, but go for a slacker seat tube, you'll have to move the seat forward on the rails, if you want maintain KOPS, or whatever BB to seat relationship you have.... so with the same ETT you now have a shorter cockpit.

Some bike brands give the Reach measurement: horizontal distance from BB to Head tube. I find that a very useful number, together with a seat tube angle that lets me put the seat where I want it.


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## spade2you (May 12, 2009)

This thread reminds me why I just stick with my 1 brand.


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## Matador-IV (Aug 2, 2010)

IMO,

Learn to calculate/evaluate "reach & stack" measurments.

I rode a bike locally that was very comfortable, but way out of my price range. I used the geometry diagram to calculate reach & stack. I then used all the data to compare online bike geometries to the bike I had ridden locally.

It is possible to have 2 bikes with similiar HT & TT lengths, but drastically different reach & stack.
.


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## rx-79g (Sep 14, 2010)

dgeesaman said:


> OK, so I'm supposedly fitted to my bike. So far as I know I have no discomfort related to fit so I can't disagree.
> 
> It's been said (here) many times that various models and brands do not have consistent sizing methods, and that a prospective buyer should compare geometry to determine the closest fit. I've read a bit on the 'net about fitting but what I can find relates to the process. It seems to me that a "fit" is basically a particular body position. My question is how to take this fit and determine how to achieve this on other bike models/brands. I suspect the info is out there but I haven't been able to weed it out of google yet.
> 
> ...


Hi David,

It is easy to get buried under terminology and the different ways of looking at geometry. Let me try to put this simply:

Your "fit" is a body position - a profile - that you can choose to apply to different bicycles, regardless of specific geometry differences.

That position consists of three major contact points - the pedals (centered around the bottom bracket), the saddle and the handlebars. Since saddles and bars can be made to move up/down and fore/aft, the bottom bracket is the fixed point that your "position" is firmly attached to. The saddle is set in reference to the BB, and the handlebar as well.

A simplistic (but not inaccurate) way of determining if you'll fit on another frame is to start with the seat tube angle. If it is a degree different than your current seat angle, then that will affect how your saddle is positioned fore/aft. If it is a greater number (74 vs. your bike's 73), this means your seat will have to be mounted 1cm further aft on the seatpost per degree of added steepness of the seat tube.

Next, you compare top tube length. Use the "level" or "virtual" top tube length. If you had to move your saddle aft in the previous step, add that distance to the top tube length, or subtract if you had to move it forward. Now compare this adjusted top tube length with your current one. How close is it?

Look at your current stem length and add it to your current top tube length. Now subtract the adjusted top tube length of the new frame from that number. The result should fall within the normal stem range of 9 to 13cm. If it doesn't, recheck your math. If it's right, and a normal stem won't work, then the frame is probably completely wrong. 

Last, compare the headtube height. (Keep in mind that integrated headsets will have a higher number, but actually compare to a smaller traditional headset.) Subtract your current headtube from the new one, and look at your headset spacers and stem angle. Does it look like the same handlebar height could be created on the new frame by using more or less spacers or a different angle stem?

The above method will only tell you if your current position could be comfortably duplicated on the new frame. It assumes that your current position works well with your current frame and no outlandish parts were used to achieve it (like a 15cm stem). It also assumes you're using a similar shaped saddle and handlebar.

That's the "fit" portion of geometry. Geometry is also a description of how a bike handles, distributes your weight, climbs, etc. You can have the identical fit on two frames and a totally different riding experience depending on steering angle, wheelbase, frame material, etc. Generally, if your fit on any frame involves using a seatpost with a normal amount of setback, a stem that is close to the middle range and a moderate amount of headset spacers, the frame should work with you well, distributing your weight to the wheels appropriately.

Example:
Yours bike New bike
Seat tube angle 73 degrees 74 degrees
Top tube 55cm  Virtual top tube 53cm
150mm head tube 175mm headtube
30mm spacers 
110mm stem

New bike has a steeper head tube, so your seat will have to be 1cm further aft, which increases the comparable top tube length from 53cm to 54cm. Your bike has a 55cm top tube plus a 11cm stem = 66cm. Subtract 54cm of New bike from 66 = 12cm. 12cm is a decent stem size. Your bike has 150mm headtube + 30mm spacers = 180mm. The New bike has an integrated headset, so subract (say) 25mm = 150mm, which is the same as your current bike, so add 30mm of spacers.


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## PJ352 (Dec 5, 2007)

perttime said:


> *I got a bit lost there*, so would like to paraphrase....
> 
> If you keep the same ETT length, but go for a slacker seat tube, you'll have to move the seat forward on the rails, if you want maintain KOPS, or whatever BB to seat relationship you have.... so with the same ETT you now have a shorter cockpit.
> 
> Some bike brands give the Reach measurement: horizontal distance from BB to Head tube. I find that a very useful number, together with a seat tube angle that lets me put the seat where I want it.


That's ok, I got a bit lost writing it. After I read your paraphrase a couple of times, I've decided I'm in agreement.  

I agree that it including reach/ stack measurements in geo charts is very helpful.


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## dgeesaman (Jun 9, 2010)

All very interesting and I pretty much follow it. Thank you.

I guess it boils down to a basic concept: each bike places the seat, pedals, and handlebars in a particular place. When going from one bike to another the basic idea is to keep those points in the same exact place. I'll try laying this out in CAD and see what happens.

David


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## AndrwSwitch (May 28, 2009)

Don't forget bottom bracket drop/height. It's the measurement either from the line defined by the hubs down to the bottom bracket or up from the ground with some particular tire. A lower bottom bracket lets you ride a shorter head tube with the same riding position - it basically just moves your entire riding position vertically downward. A higher bottom bracket does the opposite. Within racing/training-oriented road bikes sized in the middle-50s, bottom bracket heights don't usually vary by much, making the head tube length alone a fairly useful number for comparing one bike to another. But bikes of different classes can be quite different - many mountain and 'cross bikes have less drop to the bottom bracket, although the wheel size and choice of tire used to measure can play havoc with any reasonable comparison of height, and some touring bikes have more drop.

As an example, the current year's model of my 'cross bike is listed on its manufacturer's web site with a 285mm bottom bracket height. Their raciest bike has a 264mm bottom bracket height. Not a huge difference, but certainly enough to make you use a differently-angled stem or put a giant stack of spacers on one bike.

If you're shopping for a fairly conventional road frame to match a fairly conventional road frame, you might glance at the BB height/drop, see that it's within a few millimeters, and move on, but if you're looking for something different, make sure you know what you're getting into. (What are you shopping for?)


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## rx-79g (Sep 14, 2010)

AndrwSwitch said:


> Don't forget bottom bracket drop/height. It's the measurement either from the line defined by the hubs down to the bottom bracket or up from the ground with some particular tire. A lower bottom bracket lets you ride a shorter head tube with the same riding position - it basically just moves your entire riding position vertically downward. A higher bottom bracket does the opposite. Within racing/training-oriented road bikes sized in the middle-50s, bottom bracket heights don't usually vary by much, making the head tube length alone a fairly useful number for comparing one bike to another. But bikes of different classes can be quite different - many mountain and 'cross bikes have less drop to the bottom bracket, although the wheel size and choice of tire used to measure can play havoc with any reasonable comparison of height, and some touring bikes have more drop.
> 
> As an example, the current year's model of my 'cross bike is listed on its manufacturer's web site with a 285mm bottom bracket height. Their raciest bike has a 264mm bottom bracket height. Not a huge difference, but certainly enough to make you use a differently-angled stem or put a giant stack of spacers on one bike.
> 
> If you're shopping for a fairly conventional road frame to match a fairly conventional road frame, you might glance at the BB height/drop, see that it's within a few millimeters, and move on, but if you're looking for something different, make sure you know what you're getting into. (What are you shopping for?)


Actually, BB height is only an issue if you are comparing geometries between two very different bikes. But the slight differences between two road bikes is pretty small.

And when you go to set up a cross bike (or anything else with a different BB height) you'll find that the BB raises everything evenly and won't really impact position - especially if both bikes have level top tubes.


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## AndrwSwitch (May 28, 2009)

rx-79g said:


> Actually, BB height is only an issue if you are comparing geometries between two very different bikes. But the slight differences between two road bikes is pretty small.
> 
> And when you go to set up a cross bike (or anything else with a different BB height) you'll find that the BB raises everything evenly and won't really impact position - especially if both bikes have level top tubes.


That was kind of my point. Except that raising the bottom bracket doesn't necessarily lengthen the head tube. On frames with high bottom brackets and short head tubes (Surly Cross-check, I'm looking at you) people often have to have pretty epic spacer stacks and high-angled stems to get the handlebars to the right place.


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## rx-79g (Sep 14, 2010)

AndrwSwitch said:


> That was kind of my point. Except that raising the bottom bracket doesn't necessarily lengthen the head tube. On frames with high bottom brackets and short head tubes (Surly Cross-check, I'm looking at you) people often have to have pretty epic spacer stacks and high-angled stems to get the handlebars to the right place.


You could say that the Cross-check has a short head tube, but you could also say it has a long top tube, or a short seat tube. Depends which feature you are using to gauge the frame size. But for a bike with a stated size of 54cm, the Cross-check has a very long top tube at 56cm. If it had a more typical top tube length for a cross bike built for someone who usually rides a 54cm road bike, the head tube length wouldn't be an issue.


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