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Old 04-10-06, 05:49 AM   #1
tansc
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VERY confused by frame size!

Hello

I'm looking at buying a road bike online ebay, and I'm VERY confused by the numbering.

What is a good guide for the length from the crank to the top tube that corresponds to the rider's height?

My inseam length is 78cm, which according to some websites mean i should have a 520 mm roadie seat post length (c to c). But most bikes I find are around 560 - 580mm (c to c), and some of the riders of those bikes are shorter than me!

Seems strange that it's so difficult to find a good frame size given I'm average height (1.74m)...Am I calculating something wrongly?

Cheers

Sing

Last edited by tansc; 04-10-06 at 06:49 AM.
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Old 04-10-06, 06:45 AM   #2
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First off spend a few bucks and get fitted properly since you don't seem to know your correct size. That being said, sizing among manufacturers and also between compact/standard frame type do vary. Since an eBay sale is unlikely the option of returning it for a different size you should be dead on sure before proceeding.
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Old 04-10-06, 06:57 AM   #3
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I agree, spend a couple of extra bucks on a fitting and then spend a couple more extra bucks and buy a new bike from a bike shop. You have no idea how many people I see that buy bikes off ebay that they are unhappy with because the quality is not what they expected, they don't like the ride, its the wrong size etc, etc.

EBAY is great if you know exactly what you're looking for but if you're a beginner go buy something from a local shop and get advice from people who work with the stuff everyday. Even if you get fit and buy the proper frame size there is no guarantee you will like the ride of the frame (different frames do ride differently). The chances of you actually getting the proper fit and enjoying the ride will go up exponentially if you buy from a shop vs buying on EBAY. After you have some experience with road bikes and have tried different frames and sizes you will have an idea of what your looking for when you shop on EBAY.

Just an opinion from someone who deals with this issue almost everyday.
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Old 04-10-06, 08:21 AM   #4
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See my other post. My trouser inseam is 76cm and 55cm (C-T) is my perfect-fit frame size in a traditional road bicycle.
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Old 04-10-06, 08:32 AM   #5
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I agree with the advice to visit a "brick and mortar" bike shop for both frame size recommendations and to purchase your first bike. E-bay is suited to experienced purchasers who know exactly what they need and what they are getting.
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Old 04-10-06, 09:56 AM   #6
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There are wide variations in the way manufacturers measure their bikes. Top tube length measurements are at least as important as height measurements. There are also wide variations in top tube lengths between manufacturers, even in equivalent height bikes. Best to compare geometry charts with your relative leg and torso lengths before going in for a fit. Find a frame that best fits your leg to torso ratio.

Al
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Old 04-10-06, 11:00 AM   #7
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If you do buy a bike on EBay, make absolutley sure of what the seller is quoting for frame size, i.e. from where to where. Help out by suggesting how to measure. A lot of sellers have no idea of what frame size means.
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Old 04-10-06, 11:29 AM   #8
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Hi im new to this, what is the 'Top Tube' thanks...
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Old 04-10-06, 11:41 AM   #9
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Quote:
Originally Posted by tansc
My inseam length is 78cm, which according to some websites mean i should have a 520 mm roadie seat post length (c to c). But most bikes I find are around 560 - 580mm (c to c), and some of the riders of those bikes are shorter than me!

Seems strange that it's so difficult to find a good frame size given I'm average height (1.74m)...Am I calculating something wrongly?
According to your inseam and overall height I would say you are close in your estimation of frame size 52cm. C to C. There is a frame sizing chart on my website you may find helpful.
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Old 04-10-06, 11:55 AM   #10
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Originally Posted by callumchapman91
Hi im new to this, what is the 'Top Tube' thanks...
For anyone new there is a good description of the various frame parts here on Wikipedia.
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Old 04-10-06, 11:57 AM   #11
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Quote:
Originally Posted by Dave Moulton
For anyone new there is a good description of the various frame parts here on Wikipedia.
Thanks alot mate!
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Old 04-10-06, 10:42 PM   #12
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Be aware of the completely different sizing of frames depending on whether they have a top tube that is parallel to the road or slants down from the front to the rear of the bike. Slant top tubes are more forgiving of sizing errors or put another way fit a larger range of riders per size.
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Old 04-11-06, 05:56 AM   #13
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Originally Posted by sch
Be aware of the completely different sizing of frames depending on whether they have a top tube that is parallel to the road or slants down from the front to the rear of the bike. Slant top tubes are more forgiving of sizing errors or put another way fit a larger range of riders per size.
Steve
Steve makes a valid point when it comes to compact frames with a sloping top tube. I posted something similar to this a year and a half ago, so I hope those who have read it before will forgive because I feel it may be helpful to those new to the sport.

After measuring and studying hundreds if not thousands of customer’s measurements over the years as a custom frame builder. I came to the conclusion that although human bodies are all different; they do follow certain rules of nature.

Choosing frame size on inseam alone does not work across the board because for example it is common for a 6 foot man to have an inseam as short as 30 inches; and you can’t put a 6 foot man on a 51 cm. frame which is what his inseam suggests.

Tall people are not scaled up models of short people. Most of the height difference is in the legs; body length differs by a lesser proportion. If you have long legs then you also have long arms. Short legs; short arms. This makes sense since most animals are four legged; why should we be any different?

Leg length is a combination of the inside leg measurement plus the length of the foot. Length of the foot is important because when pedaling the toe is pointed downward at the bottom of the pedal stroke; so the foot becomes an extension of the leg.

People, who have a short inside leg measurement for their height, generally have longer feet. (Bigger shoe size.) It is as if they were designed as a much taller person, but their heel got turned further up their leg; making a short leg, long foot.

Imagine two people both six feet tall standing side by side; one has a 34 inch inside leg, the other a 32 inch leg measurement. Because they are the same height it follows the one with the shorter leg has a body 2 inches longer; he also has longer feet and shorter arms than the other guy. They can both fit on the same size frame, (59 cm. center to top i.e. 57 center to center.)

They will both have close to the same seat height, because the short leg guy has a longer foot so his seat needs to go higher than his inseam would suggest. They can also use the same top tube length and handlebar stem, because one has short body long arms; the other long body, short arms; making their reach the same. Minor adjustments in seat height and stem length may be called for.

It has occurred to me that with these compact frames on the market now and only available in small, medium, and large; customers are only ball parking frame size anyway. It has long been my opinion that frame size is linked to the overall height of the rider more than any other measurement because of the rules of nature I have just spoken of.

I have formulated this based on my own frame sizing chart. If you are 5’ 3” to 5’ 5” frame size equals Height divide by 3.3. For people 5’ 6” to 5’ 10” frame size = Height divide by 3.2 and if you are 5’ 11” to 6’ 4” frame size = Height divide by 3.1

An example would be someone 6’ 2” = 74” divide this by 3.1 = 23.87 in. (61 cm. measured center to top. i.e. 59cm. center to center. A person 5’ 7” = 67” divide this by 3.2 = 20.93 in. (53cm. center to top. i.e. 51cm. center to center.) The easiest way to convert from inches to centimeters is to get a tape measure with both metric and inches on and simply read across. If you don’t have a tape measure the formula is “inches x 2.54”

The above gives you a frame size for a traditional frame with a level top tube. I have heard it said that with a compact frame, reach, which is a measurement from the seat post to the handlebars (C to C horizontally), is more important than seat tube length; I am inclined to agree. Reach would be estimated frame size center to top plus 10cm. Example frame size 56cm. = reach 66cm. This does not mean everyone uses a 10cm. stem because the effective top tube length may shorter that 56cm. so the stem length needs to be correspondently longer.

If you want something more accurate go to the chart on my website. The chart was derived not by any mathematical formula but by records of custom frames built over many years. Most people find it pretty accurate. What I have put forward here is an attempt to come up with a simple formula that comes close to this. If you are an experienced rider don’t change your position based on this alone. It is intended as a place to start for a newcomer to the sport.
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Old 04-11-06, 09:04 AM   #14
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At 1.74m a 52 (c-c) sounds pretty close. I am at 1.81m and use a 57cm bike but I have long legs and a short torso. An average built of my size would probably use a 56 or, may be, a 55. Keep in mind that top tube length is probably the most important measurement. For the same seat tube length some manufacturers have longer top tubes than others.
As recommended above get yourself fitted or do the work yourself using one of the following links on the web:
http://www.competitivecyclist.com/za...LCULATOR_INTRO
http://www.wrenchscience.com/WS1/Sec...ing/Height.asp
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Old 04-11-06, 01:27 PM   #15
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Quote:
Originally Posted by Dave Moulton
... For people 5’ 6” to 5’ 10” frame size = Height divide by 3.2. ...
5'8" / 3.2 = 21.25" = 54cm

If this is interpreted as C-C, then it matches my stated preference of 55cm C-T. If the top tube and seat post are long enough, as on my Peugeot UO-8, a 54cm C-T frame does indeed work for me.
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