hermesnz
new member
Reged: 02/14/04
Posts: 14
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Hello
I was wondering what formula people are using to size their frame, and what length of seatpost forum members have on their bikes (classic not sloping frames).
I am a novice and was sized by a local bike shop who are headed by a former pro road cyclist. They said that my frame size was 59cm in 'classic terms' but that nowadays that would be seen as way too big and they would put me on something like a 56 or 57 cm frame.
Any input would be appreciated.
Thanks
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PsyDoc
friend
Reged: 01/14/04
Posts: 37
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The following information should be used as a good starting point for frame sizing. Frames differ not only in their size, but also in the angles that are used which may influence your choice of frame size.
A typical solution is to take your inseam measurement from your pubic bone to the floor in bare feet about 6 inches apart. Place a book (or a piece of 2x4) with a 1 1/2" or so binding between your legs and pull up until you cannot pull up any further. In other words, you want to measure to the pubic bone so pull up hard. Make sure the book is level. Have a friend measure from the floor to the top of the book binding.
Then, take that number and convert it to "cm" by multiplying it by 2.54. Next, take the converted measurement and multiply by .67. That will give you a good estimate of the frame size (measured center of bottom bracket to the top of the top tube; commonly referred to as c-t) you would need to get. But, someone commented that this could put riders over 6-feet on a frame that is too small (c-t) and they may be best served by subtracting 27-28cm from their inseam length for a c-t frame size.
In order to find the frame size you would need from a manufacture that measures frame size from the center of bottom bracket to the center of the top tube (commonly referred to as c-c), you would multiply your inseam by .65 or you can subtract 1 - 1.5cm from the c-t value.
Once you have a good starting point, then you really need to go out and ride a few different sizes to see which one fits you most comfortably. I am 5'-9" and have a 33 1/4" inseam (or 84.45 cm). I ride a 57 cm (c-t) bike.
I found that the more I knew about frame sizing, the more confused I became. Here are a few links on sizing information that will benefit you.
http://www.bsn.com/Cycling/
Note: click on "Ergobike: Competition Bicycle Size/ Proportions Analysis" and at the bottom of the new page that opens, you can choose your inseam measurement. The program then calculates all the other measurements that are "average" for someone with your inseam. You will get a plethora of information back. BUT, keep in mind that this analysis is for a “Competition Bicycle Size” and the size you feel more comfortable on may be a size larger or smaller.
http://www.coloradocyclist.com/BikeFit/index.cfm
http://www.huberbikes.com/production_bike_fit.htm
Chris Huber advocates finding your frame by taking your inseam measurement x .66 for your center-to-center frame size. You would need to add about 1.5 – 1.8cm to this value
to get your center-to-top frame size.
http://www.wrenchscience.com
http://www.rivendellbicycles.com/frameinfo/Frame_Sizing.htm
http://www.sheldonbrown.com/frame_sizing.html
http://www.PeterWhiteCycles.com/fitting.htm
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hermesnz
new member
Reged: 02/14/04
Posts: 14
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Hi there
Thank you. That is useful.
Has the formula for determining the size of frame changed over the past twenty five or so years?
In looking at older photographs of the Tour de France etc I see guys riding bikes with a lot less seatpost showing than today (even on frames with traditional geometry).
Thanks again.
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PsyDoc
friend
Reged: 01/14/04
Posts: 37
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I have no idea whether the "formula" for sizing has changed much, but other things have. For example, the older TdF photographs show cyclists that are riding bikes that by today's standard appear too big. In my opinion, most of the cyclists riding bikes today are riding bikes that are 1-2 sizes too small for them. Why? One possible explanation has to do with the size of the bikes the pros are riding. Why do pros ride bikes that might be a size or two smaller? Might be due to: weight, stiffness, aerodynamics, etc. that might give them an edge in racing. However, at the non-pro level, most of these are a non-issue for the average cyclist.
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Krash
new member
Reged: 09/04/05
Posts: 17
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Back in the day, steel frames came in two flavors - Columbus or Reynolds - and were measured from the center of the bottom bracket to the center of the top tube (center to center) or to the top of the top tube. I used to reliably say "I ride a 58 cm, center to center frame."
Now that top tubes slope, frames are compact or standard, materials are made out of everything under the sun, and tube diameters are also all over the place, it's tougher to figure out.
Short answer: yes, frame sizing has changed a bunch.
My 1980's Columbus SL frame does indeed show much less of the seat post than my 2002 carbon even though both bikes have identical top tube lengths and fit great. Styles have changed. Personally, I can't get used to the compact frames with a mile or so of seat post sticking out but some find that ideal.
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Allan
journeyman
Reged: 05/04/04
Posts: 198
Loc: Bds,W I
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Realistically speaking a compact frame does little or nothing to improve the way the frame rides or weighs, its basically a fad thats taken over the frame building world. In the smaller sizes the sloping tube gives the bike a 'go fast' look, and in some instances when frames are down to 40 or 50s cm sizes, it can help with creating some extra stand over height for these size of riders.
In the bigger sizes it looks plain weird, mostly because the seatpost is soooooo long, and with all that post showing it flexes back and forth quite a lot when pedalling on inclines or in high ratio gears, so much so to the point it can be felt.
Then there is the issue of how much the materials can take of this repeated flexing before something finally breaks, usually right by the seatpost clamp.
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Its time to ride.
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Insightdriver
captain
Reged: 03/07/04
Posts: 472
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The benefits of a compact frame do not have anything to do with ride feel or weight savings. There is a mechanical advantage to making the main triangle smaller and it is easy to make a seat tube stiff enough because the forces there when riding a bicycle are much easier to deal with than the forces in the bottom bracket, for example. I also agree that a large rider should be on the correct frame size and many manufacturers just don't make sizes big enough. Many bike shops, though, because they can, will put huge seat tubes on bikes so they can sell them to large riders.
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Lon
sage
Reged: 12/20/03
Posts: 595
Loc: Pittsburgh, PA
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Insight...Allan is completely right...it is all marketing. Orginally it was as a means to reduce inventory cost. Giant introduced it to get more dealers. Their "sales" hook was you only had to stock 4 sizes and a bunch of stems to fit everyone. This of course is not geared to the super road store fit approach but rather the volume bike store approach. Please don't join what some have done when I pointed this out is to tell me all the industry spin after the fact. I used to get Bicycle Retailer delivered right to my home. I read the article when Giant announced the compact frame and why they were doing it.
The first spin was it is lighter...however the astute soon pointed out those 4 foot seatposts weigh a lot more. The next spin was it is stiffer...we don't all ride steel any longer...who needs anything stiffer than what a normal design can be built to be? Then they came up with the "improved" look...purely personal...many I think look plain old dumb and Allan gave a perfect example.
One statement since Giant's initial introduction that is true is stand over. They didn't start out with that as a reason but it was an unexpected outcome. I have relatively short legs and a compact would give me more stand over. HOWEVER the bottom line is I have about a cm or a little more and what use is any more stand over? I don't hit and even if I drop off over it I don't hit so who cares?
Take care. 
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Dave_Thompson
prophet
Reged: 12/19/03
Posts: 717
Loc: Spokane, Washington
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Quote:
Insightdriver wrote:
The benefits of a compact frame do not have anything to do with ride feel or weight savings. There is a mechanical advantage to making the main triangle smaller and it is easy to make a seat tube stiff enough because the forces there when riding a bicycle are much easier to deal with than the forces in the bottom bracket, for example. I also agree that a large rider should be on the correct frame size and many manufacturers just don't make sizes big enough. Many bike shops, though, because they can, will put huge seat tubes on bikes so they can sell them to large riders.
Insight, I gotta disagree with you. There is not any mechanical advantage to a compact frame as there is no 'main triangle'. It is in fact a(often)parallelogram or an rectangle with (usually) unequal sides. A compact will usually have a different 'ride feel' because of the resonant differences in the tubing lengths of the various parts of the frame, thereby giving different frequencies to the road noise and there are usually minor weight savings with a compact frame, but not much.
A much smaller compact frame will be stiffer than the same frame that is larger, but stiffness, as marketed by makers today, is not the do-all-to-end-all. Like "efficient power transfer", it is another marketing phrase that has captured the attention of the public and repeated by self-serving publications (Bicycling comes to mind) until it has become a mantra.
Because the so-called youth market that everyone, including the bicycle sellers, had been brought up on very short fads, fashions and attention spans, the bicycle industry is forced to have a "new/best" thing to tout more frequently that ever. The compact frame is one of them.
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Steel lover, but then I like Ti with carbon too.
Licensed bike geek.
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Insightdriver
captain
Reged: 03/07/04
Posts: 472
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IMOHO
We are all correct on different points. My statement that a smaller triangle is stiffer than a larger one is factually true, not necessarily true in practise, though. I do agree that Giant came out with it because they could make only four sizes since standover now was moot. Oh, yeah, and I agree that standover is one of the most over-hyped and least-needed sizing methods for a bike.
I also agree that the resonances of the various tubes combine to make up a, "ride characteristic." I also understand that from a physics standpoint that a flexible frame is just as efficient at power transfer as a stiff frame. What you store in energy in flex is released as energy when you switch to your other leg.
I'm not so disgusted with marketing. I guess I see it in all advertising that I'm numb to it any more. I just try to discern the meat among the chaff. I also must admit that I've bought into marketing myths and must re-educate myself as I learn more.
Making four sizes of something is cheaper than having to make 10 sizes for a lot of reasons. If it really reduces cost, then the consumer benefits.
Oh, here is another thought: Those of us who post on this board are a minority in knowing how important proper fit is, and knowing the details that go into making fit proper. Although fit is paramount, most people who get a bicyle are, by a huge margin, simple recrational riders. They ride short rides at easy paces and don't see the point in proper fit. The majority of people who go into a bike shop to buy a bike choke at the multi-thousand dollar tags on the high-end bikes.
Getting my Calfee changed me. I remember once, years ago, someone making a comment that any bike under three thousand dollars was a low-end bike. I strongly disagreed because for most of my life I never spent over one thousand dollars for a bike and was satisfied (being in my clueless state I did not know what I needed nor did I care, as far as fit is concerned).
I find now I'm looking for a replacement bike and for me, personally, knowing what I had and what I want, any bike under three thousand dollars just isn't what I want. On the Cycling Forums site there is a long-standing thread on, "Don't buy a $1000 bike to start with." The majority who have posted in that thread have a bike under $1000 and most of them are highly experienced cyclists, albeit, not into racing. Ironically now I'm shopping in that stratospheric league of people who buy a bike that costs more than many cars!
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Allan
journeyman
Reged: 05/04/04
Posts: 198
Loc: Bds,W I
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I decided to add another reply to this forum posting on the sloping design frames, since its a subject thats had quite a lot of cyclists remarking on the pros and cons of it. I will add this bit to my previous posting on some reasons about what has been said recently as well as some findings of my own.
All the sizing issues as stated were done to reduce the amount of frames the manufacturer had to make, like what Giant started with, and this is really a cost cutting factor, nothing more.
Its like a footwear manufacturer saying, shoes will now come in only three sizes, so if your feet are out of this range we WILL make them fit somehow, OUCH.
The topic of reduced weight is totally redundant, as the longer post now needed, adds back most, if not more of what was removed by the slope to the shorter seattube.
The reduced size main 'triangle' is no more stronger/stiffer by taking off an inch or two from one corner. If you took off at least four to six inches, i would say "sure" to stiffer, but not such an insignificant amount can enhance the frame stiffness to some "wonder" value.
Finally the way how the sloping framed bike rides is subject to what you THINK in terms of performance.
I personally have tried a few sloping frames in my size which is 60cm or 61cm CtoC, and found that i had almost twelve inches of post showing on this sized frame, so visually it was a nighmare to start with just looking at it.
The ride was an interesting bobbing swaying feel of the seat, and a glance down at the seatpost showed it flexing quite a bit, mostly near the clamp area.
Needless to say i was not impressed one bit and would never buy a sloping frame in my size since this type of situation is surely going to fail at sometime.
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Its time to ride.
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