General Discussion Triathlon Talk » Importance of bike geometry Rss Feed  
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2006-10-27 11:10 PM

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Subject: Importance of bike geometry
How are the seat tube length and the top tube length important if you can: use longer cranks and raise your seat, and get a longer stem/aerobars to get the desired length. I would think the most important part of the geometry would be the seat and head tube angles. Please enlighten me.


2006-10-27 11:52 PM
in reply to: #581758

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Subject: RE: Importance of bike geometry

hosayif - 2006-10-27 11:10 PM

How are the seat tube length and the top tube length important if you can: use longer cranks and raise your seat, and get a longer stem/aerobars to get the desired length.

 

I think, emphasize think, that it has to do with center of gravity. Having a longer seat tube increases the height of your center of mass; likewise with a longer stem, putting more of your weight forward of where it should be. Putting your weight outside of what the bike is designed for is going to cause handling problems and may, in the long run, cause comfort issues as well.

 

Tolerances, though, can be pushed. For example, I bought a 2005 QR Tequilo on crazy discount from All3Sports in Atlanta back in March that was just a bit too big for my torso. When I was fitted, they swapped out a stem and aerobars to bring the cockpit closer to me, but only because I've got crazy-long femurs and wouldn't fit on anything smaller. In that case, I would think that bringing weight in, decreasing the center of mass would be just fine. It's expanding that center that causes the problems.

 

My $0.02

 

SR



Edited by sranney 2006-10-27 11:53 PM
2006-10-28 12:02 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
Are you trying to rationalize the purchase of a bike that is too small?  Yes, you can fine tune a bike fit by doing those things, but you should start with one that is the correct size. 
2006-10-28 12:03 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
I'm looking at building a bike and don't really know which size frame to get. I have never been properly fitted on a bike, so I don't know what size to get.
2006-10-28 8:14 AM
in reply to: #581773

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Subject: RE: Importance of bike geometry

hosayif - 2006-10-28 12:03 AM

I'm looking at building a bike and don't really know which size frame to get. I have never been properly fitted on a bike, so I don't know what size to get.

 

The easy response to that is to go get fitted.  Either that or find someone who has the frame(s) you're looking to build up and ride their bikes. 

Bike sizes are not universal though: a Trek 56cm frame doesn't have the exact same geometries as a GT 56cm frame which doesn't have the exact same geometries as the frames from manufacturer c, d, ... z. 

 

2006-10-28 8:45 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry

IMHO, the seat tube length is irrelevant as long as you can stand over the top tube and get your seat to the right height. Having an ideal length top tube allows you to use the frame as the geometry was designed. Putting too short or long of a stem will change the handling of the bike. Moving the seat to the extreme front or back of it's range will change the effective seat angle.

Here is a fit calculator which should do a good enough job of predicting frame sizing based on your measurements.



2006-10-28 8:46 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
Keeping in mind that people come in all different shapes and sizes even if they are the same height. For my height, everything written says I should be riding "A" size bike when in fact after being fitted AND what I felt most comfortable on has always been size "B". Torso length, arm length, etc have a big effect. I am not an expert but rather a plain ol rider that ran into this. One other thing to keep in mind is to find somebody that has as many bikes and sizes as possible. Most bike shops that I have run into are honest and will do what ever it takes, but there are a few that want to sell what bikes they have even if they have to modify it (what you said above)and one day you will see that you are riding a bike that really is too big, too small, or too uncomfortable even though you got a great deal or the bike looks cool. One last thing is that the odds of ANY bike being perfect just the way it is are slim, THATS why you can adjust the seat, crank arms, etc. That stuff is for fine tuning and not there to "make it fit".

I will tell you about the wet suit I bought one time that was a little too small some other day................ :-)
2006-10-28 9:50 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
If your seat tube is too long you won't be able to stand over the bike

If your seat tube is too short you can fix the distance from the cranks to the saddle with a longer seat post but the verticle drop from the saddle to the bars will still be too long. You can adjust the bar height with spacers and/or a stem with rise but there is a limit to how much you can raise the bars - and a lot of spacers also just look ugly
2006-10-28 11:53 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
I was reading an article at slowtwitch and he said that the only two important measurements for fit are top tube and head tube length and that all the other measurements were for handling purposes. (anyone care to explain that?) Also, I used that fit calculator and it gave me a certain top tube length for the competitive fit. Is this length equivalent to the length of what a top tube should be on a tri bike? I would think it would be different since we're in such a different position than a road cyclist is in.
2006-10-28 12:44 PM
in reply to: #581758

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Subject: RE: Importance of bike geometry

Without getting professionally fitted there is no way to exactly know the perfect geometry for your body.   That being said, it is pretty easy to get within a few percent of ideal by riding a few bikes, taking note of their geometry and how it felt to you.  Then compare those numbers to new frames you are looking at.  If you have a bike now that is a good fit and comfortable, base it on that bike.  

If you plan on higher level racing, the exact fit is crucial to get every last bit of performance.  For an amateur or casual racer like most of us, getting really close is good enough.   Not to mention most of us don't have the cash to go get pro fitted every year. 

2006-10-28 10:56 PM
in reply to: #581891

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Subject: RE: Importance of bike geometry
As others have suggested, getting properly fitted by an experienced fitter is probably the best first step.

Before you worry about any of the other measurements, you have to decide between a "slack" and "steep" seat angle. A traditional road bike will typically have a fairly slack seat angle, maybe around 73 degrees. A TT or tri-bike, will typically have a "steeper" angle, maybe around 78 degrees. The seat angle is important because it has a major impact on how you position yourself on the bike (i.e., on a road bike, you usually ride in a more upright position, but on a tri-bike, you usually ride in a flatter position). It doesn't tell you what frame size you need, but you have to know how you're going to be positioning yourself on the bike before you can start figuring out top tube lengths, etc. As you guessed, you really can't use the same measurements from a road bike fit (slack geometry) to select a tri-bike (steep geometry).

Once you have settled in on a seat angle, I think that top tube and head tube length are the critical measurements in choosing a frame that fits you. It is easy to get wrapped up in the seat tube length because that is typically how frames are sized, but there is so much variation that it is somewhat arbitrary. For example, some manufactures measure the seat tube length from the bottom bracket to the center of the top tube; others measure it to the top of the actual seat tube, even if it is 3 cm above the top tube; still others measure it to where the top tube would intersect if the top tube were horizontal (this is typical with compact frames).

It is important to remember that "bike fit" is about determining the optimal spacial relationship between the three points of contact you have with the bike: your hands, your feet, and your butt. By "optimal" I mean best suited for your abilities, goals, riding style, etc. to balance comfort, aerodynamics, and power output. The optimal fit for identical twins may well be different if one is more flexible than the other, or if one is focusing on sprints while the other is focusing on IMs.

The top tube length is important because it (along with your stem length and the setback of your saddle) determines your cockpit length. You can fine tune the saddle to handlebar distance with longer/shorter stems (e.g., a 56cm top tube with a 110mm stem would give you the same cockpit length as a 58cm top tube with a 90mm stem). What changes is your weight distribution relative to the front wheel, which will affect your handling.

Similarly, the head tube length is important because it (along with your saddle height, stem angle, and spacer stack) determines your saddle to handlebar drop.

Ultimately, your seat tube length is fairly important because you can always use a longer/shorter seat post. However, bikes with longer seat tubes (from the same manufacturer) generally have longer top tubes and head tubes.

The "other" measurements, such as chain stay length, wheelbase, and head angle, affect the way the bike handles but don't have much impact on fitting because they help determine how your weight is distributed relative to the wheels, but don't affect the spacial relationship of your three points of contact.

Edited by MCH13 2006-10-28 11:07 PM


2006-10-29 10:14 AM
in reply to: #581758

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Subject: RE: Importance of bike geometry
Haven't read the other replies, so I may just be repeating what others have already said, but...

The most important thing is how you fit on the bike -- as in, will the bike allow you to be in your best position.

BUT...

If, by putting you in that position, the bike becomes unbalanced, it will become a miserable thing to ride. Sure, you can put a long stem on a bike with a short top tube, and it'll "fit", but you'll be so far out over the front wheel, with so much of your weight on it, that the bike will be a nightmare to control. And yes, you can put an extremely down-sloping stem to get low enough on a bike that otherwise "fits", but then you've got a lot more frame than you need, and it'll be heavier than it could be (maybe not a big concern).

Weight balance, steering "quickness" or stability... These are the aspects that you tend to lose when you get towards the extremes of making a bike "fit".
2006-10-30 12:39 AM
in reply to: #582089

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Subject: RE: Importance of bike geometry
mch13s reply is great advice.

Remember those 3 points of contact he describes. If you were to connect the 3 points into a triangle, that triangle can be rotated forward or backwards in space, moved up or down, or moved fore and aft...all of those transpositions of the triangle will affect the handling, but it is the triangle itself that determines teh fundamental "fit".

2006-10-30 2:01 AM
in reply to: #582089

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Subject: RE: Importance of bike geometry
Oops...just realized that this should have said that ultimately, your seat tube length is fairly UNimportant.

Ultimately, your seat tube length is fairly important because you can always use a longer/shorter seat post.
2006-10-30 1:16 PM
in reply to: #581891

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Subject: RE: Importance of bike geometry
hosayif - 2006-10-28 11:53 AM

I was reading an article at slowtwitch and he said that the only two important measurements for fit are top tube and head tube length and that all the other measurements were for handling purposes. (anyone care to explain that?) Also, I used that fit calculator and it gave me a certain top tube length for the competitive fit. Is this length equivalent to the length of what a top tube should be on a tri bike? I would think it would be different since we're in such a different position than a road cyclist is in.


Top tube measurement is important because it determines the cockpit distance. You can adjust this distance somewhat by getting a longer or shorter stem but you only have a little leeway because if you go to either extreme your handling will be negatively affected. The seat tube length doesn't matter because you can get longer seat posts. Remember though that the seat post is at an angle so a taller the seat will make the effective length of your top tube longer. Also moving the seat forward or back to adjust the effective seat tube angle will lengthen or shorten the effective tope tube length.

Head tube length is important because it determines how high your aero bars are and thus how "aero" you can go. This too can be adjusted with different stems and different aero bars. You just want to make sure the head tube is not too long. To short is probably ok becasue it is not too hard to add height.

I don't think the fit calculator will work for tri bikes because the they are different. Top tubes on tribikes are generally shorter. I was confused by this at first because the aero position would seem to require a longer cockpit. But what you have to consider is the short head tube. A shorter head tube will mean that the top tube meets the seat tube down lower. Because the seat tube is at an angle it makes sense that the lower the top tube is the shorter it would be.

If you want to compare apples to apples then you have to measure from the front of the saddle to the center of the headset. This will allow you to compare the cockpit on any bike of any configuration.
2006-10-30 2:37 PM
in reply to: #581891

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Subject: RE: Importance of bike geometry
Read Dan's articles on Slowtwitch (Geometry Experiment). You'll get the full education on how front center, head angle, and fork rake, as well as chainstay length, affect the handling of the bike.


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