's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

frame sizing question(24 posts)

frame sizing questionMaverick
Dec 23, 2001 9:51 PM
i have an inseam of 76.7cm which is equivalent to 51.4cm(center to center). what difference does it make if i get a frameset of 49cm (CC) compared to a frameset of 51cm (CC)?? will the smaller frame be a little stiffer?? what about the handling characteristic of the bike? i'm currently looking at the LOOK KG241 (team Kelme) with classic geometry (49cm CC) on sale for US$530.00
your opinion pls.
Dec 23, 2001 10:12 PM
a pic of the KG241 Team Kelme
Top tubes are nearly identical . . .DCW
Dec 24, 2001 5:09 AM
only 1 mm difference, so that isn't an issue. Larger frame is probably a few grams heavier, also immaterial. There shouldn't be a discernible difference in stiffness.

One consideration is the saddle to bars height differential. All other things being equal, that will be most of an inch greater with the smaller frame. If you want, you can eliminate or reduce (or increase) that by your stem angle choice.

I'd go with the larger frame. It might also be easier to sell later on down the road.

By the way, a lot of folks have been saying very nice things recently about Look frames on this and other boards. The 241 is apparently a very good frame and a very good buy at that price. (The best deal I've seen on a 281 is about twice that.)
Not Much Difference... nmjagiger
Dec 24, 2001 1:48 AM
get the right size....C-40
Dec 24, 2001 5:32 AM
If your inseam is 76.7cm, the common formula for frame size (.67 x inseam) would suggest a 51.4cm measured center to top, not center to center. To get center to center size, subtract 1.5cm. This would suggest a 49cm to be the appropriate size.

The standover height of a 51cm frame should be about 75.1cm. This would give you only 1.6cm of standover clearance, which is less than I would recommend, but still useable. The 49 would have a 73.2cm standover. LOOK frames are made in 1cm increments. A 50cm might be a good compromise.

I always recommend double checking your inseam using an existing bike. Block the wheels up until the top tube is high enough to produce firm crotch pressure when standing over it in bare feet, wearing cycling shorts or just underwear (this looks pretty stupid, but works). Measure this top tube height. This method may yield a larger inseam dimension than you have previously measured, and change your opinion on the appropriate frame size. I would maintain clearance in the 2-4cm range.

Top tube length is also a consideration. LOOK maintains the same 53.8cm TT length in the 49-51 size range. The slack 72.5cm seat tube angle effectively shortens the TT length by 1.7cm, compared to a frame with a 74 degree STA. Keep this in mind if you are comparing this frame to other brands. You may find the need for a stem that is one size longer with this frame.

LOOK head tubes run on the short side. Measure the head tube length of you current ride. If you will be using a threadless stem, getting enough head tube length is important to avoid the need for lots of spacers or a high rise stem.
C-40, I think I need you to draw me a picture . . .DCW
Dec 24, 2001 6:30 AM
I've been reading your various posts about the effective top tube length of when the STA is relaxed, and either it's just not sinking in or I disagree with a premise or two.

Holding the HTA constant and relaxing the STA yields a larger bottom bracket angle. That gives a longer top tube. I think you agree with that.

The next step baffles me. It seems to have something to do with knee-over-pedal spindle (KOPS) position. I think you say that to get the proper KOPS, you have to move your saddle way forward, which changes both the effective STA and the effective tt length. Do I have any of that right?

If I don't, that's where I will need a picture. If I do, great, but I think I disagree with one premise --that KOPS is that important. I understand the slack STA folks to argue against the tyranny of KOPS. I had thought Lemond, among others, to advocate a stretched out position that comes from a slack STA and a longer top tube. If the knee ends up back aways because of a short femur, it's no big deal. It's more of a power position.

By the way, speaking of tyranny, I also wonder about the tyranny of standover clearance. Except for my very first 10-speed bike purchase (1971), it has never been the least been relevant in any of my 10 frame selections. Back then it was a rough proxy for proper measurement, but only accidentally correct.

Since then, I have found that if I fit the seat tube and top tube correctly, the standover height has been fine (at least it never became an issue). In 1971, the arguments for (barefoot) standover clearance were two: (1) if you came off your pedals and off your saddle you wouldn't want your tender parts to absord the impact on your top tube; and (2) when standing at the beginning of a race or at a stop light, etc., you would want to be able to stand flat-footed comfortably.

As to (1), I have never in 30 years of riding and racing come off my pedals and saddle in a way that would have me land on my feet with my bike upright. In fact, I can't see how that could happen on a modern bike. That might have happened with my old 3-speed Raleigh, but not with a bike with clips or clipless pedals. As to (2), two things are true: my cleats and shoes raise me off the ground a bit, and I usually stand with one foot clipped in or with my bike at a slight angle, anyway. That's not because of standover clearance, but rather because of supporting all my weight on one (usually left)leg.

Finally, I guess I've been approximating frame seat tube sizing wrong for all these years, too. I have used .65(inseam) for c-c measurement. I'm of average proportions, and it's been a good starting place. For Maverick, it would yield a 51 cm frame, I think.

(Last question: I assume by your handle that you ride a C-40. My son has a new Dream Plus on the way. His inseam is 84 cm and he rides frames of 54 and 55 (c-c) comfortably. He ordered a 56 in Colnago sizing, based on the c-t (near clamp) measurement and top tube length. Did he do wrong?)
some answers perhapsCT1
Dec 24, 2001 6:52 AM
Your sons 33" inseam would indicate something like a 56-58cm Dream+. The 56cm will be a tad small but that shouldn't be a problem as long as he ordered the Flash fork with the steel steerer tube. If he is getting a full carbon he may have some problems unless he like lots of saddle/bar drop.

C40 is CORRECT! Take a close look at a standard geo chart and you will see that the "effective" TT length is indeed "shortened" when the ST angle is "relaxed". If if helps just take a fixed TT length # and then mentally relax the angle WAY-WAY back and you will see that the TT gets "pulled" toward the back of the frame. OK..... that's the best this engineer can do. ;)

FWIW.... I think describing tech over the net is near impossible. I could probably "prove" that 1+1=3 hahaha.

BTW, I've got a LOOK KG 281 and it's critical that you DON'T GET A SMALLISH frame in the LOOK if you are getting an all carbon fork. The steerer tubes on the full carbon 1" steerer tubes are quite flexy, so a too small frame will cause all kinds of problems. The LOOK frames feel small for a given size!!!!

I'm 5' 8" with a 31.5" inseam and I have a 52cm LOOK frame and a 0.5cm stack under the stem. I've got 2.6" of saddle to bar drop with this setup. I could possible use a 53cm frame with zero stack spacers and a 70 degree stem. That would be the ABSOLUTE ideal setup with this frame. BTW: The LOOK frames ROCK big time!!! :)

good luck
some answers perhapsDave Hickey
Dec 24, 2001 7:27 AM
John, I'm not disagreeing with you, but isn't your assumption based on the head tube being the same angle as the seat tube? If that's the case, I agree. If the head tube is not the same angle than that might not be the case.
some answers perhapsCT1
Dec 24, 2001 8:55 AM
No assumption about the head tube angle at all. In fact, the head tube angle has NO impact on this issue.

Draw a vertical line through the BB to the TT. The distance from this line to the bars will get smaller as the seat tube angle gets more relaxed. That's why the "effective" tt length gets shorter as the st angle gets smaller.

Hope that helps. Again, this is hard to describe over the net.

some answers perhapsDave Hickey
Dec 24, 2001 9:31 AM
Assuming the headtube is going back the same amount. Let's say the headtube stays in a fixed position, as you relax your seattube your TT gets longer correct? Your vertical line works great and helps understand what your saying, but aren't you assuming as your seat tube relaxes, so does your headtube? If that's the case, then yes your "effective" TT does get shorter. John, I've always respected your opinions on the board, I'm not disagreeing with you. I'm just missing something. If you don't want to carry this discussion on the board, please email me at
some answers perhapsCT1
Dec 24, 2001 9:52 AM
OK, I think I see where we are diverging..... :)

In my previous remarks I've been assuming the head tube angle stays FIXED. This assumption means that the front triangle gets smaller as the st angle gets slacker. I.e. using an unrealistic example of a 45 degree st angle the front triangle would be tiny-tiny.

If you look carefully at a LOOK carbon frame you will notice that the front triangle is indeed a bit smaller than a frame with a steep st angle. I'm not sure how much of this is due to the very slack seat tube angle on the LOOK and how much is due to the angle of the down tube.... but nonetheless, the front triangle IS indeed smaller on the LOOK frames as compared with say a Colnago which has a steep st angle.

The bottom line here is that my Colnago which has a "REPORTED" short TT actually has a LONGER effective TT than my KG 281. That's why it is SOOOO important to look at st angle and tt length when selecting frame size.

Thanks for the follow up comments and clarification of the "disagreement". :)

ride on
I totally agree. Thanks(nm)Dave Hickey
Dec 24, 2001 10:17 AM
Get the pictureKerry Irons
Dec 24, 2001 7:24 AM
Imagine two frames, same top tube length. One has a 74 degree STA, the other a 72. For proper fit, your saddle must be in the same relationship to the BB, regardless of geometry and tube lengths. Therefore, with the 74 degree frame, your saddle will be much farther back relative to the seat post bolt - picture the seat tube rotating forward with the steeper STA, leaving your saddle in the same point in space. This "effectively" pushes the handlebars forward from your saddle. The steeper STA results in a longer "effective" TT length. With the slack STA, your saddle is roughly 2 cm farther forward compared to the seat post bolt, and therefore your handlebars are 2 cm closer to the saddle. A shorter "effective" top tube length.

PS - there is no "tyranny of KOPS" unless you want there to be. KOPS is a place to start, not the final destination. Individual physiology and preference determines the final saddle to BB relationship. Regardless of whether your knee is ahead of, at, or behind the pedal spindle, the concept of effective TT length applies.

PPS - the term "effective TT length" has also been applied to figuring out how a sloping top tube bike compares to a bike with a horizontal TT. That is not what we are talking about here, though it certainly can add to the confusion.
well put nmgtx
Dec 24, 2001 11:51 AM
DCW's Son's Dream Plusdjg
Dec 24, 2001 10:39 AM
I'd guess that the 56 will be fine for your son, but I'd point out that the Colnago 56 is actually a 55 c-t (that is, it is sized to the top of the seat tube, not to the top of the top tube). The top tube should be about 55 cm too (54.8). I'm getting this off their charts, and by measuring my own CT1 (also a "56"). So he's probably getting something like a 53.5 c-c measurement. If he's generally comfortable on a 54, I wouldn't worry much about going down 1/2 cm (unless he's growing), especially with the 73.5 degree seat angle and the 55 tt.
don't totally agree....C-40
Dec 24, 2001 1:23 PM
Colnago measures from the center of the bottom bracket to the bottom of the seat post clamp, at least on the C-40 model. For all practical purposes this is standard center to top measurement. If you compare head tube lengths, C-40s are equal to or a bit longer than other frames of identical size, measured center to top.

My 55cm C-40 measures 56.2cm to the top of the seat tube (actually seat tube clamp). The head tube length is 134mm and the TT length is 54.3. The standover height is 79.5cm.

If your CT1 measures 56cm to the top of the seat post clamp, it sound like it's a 55cm. Is your head tube length closer to 134mm or 143mm? This dimension will tell you immediately which size it is.

See the geometry chart at Perhaps Colnago doesn't totally abide by this chart for all models, but it's accurate for the C-40.
OK.... here's the scoop (or is that cr@p)CT1
Dec 24, 2001 3:47 PM
***** Colnago measures their frames (with ST clamps) from the center of the BB to the BOTTOM of the seat tube clamp! On the Dream+ that is actually just over 1cm ABOVE the top tube. The bottom of the clamp on C40's and CT1's are a hair under 1cm above the TT. Sooooooooo, a 55cm Dream+ will actually have about a 54cm CT (of the TT) meausurement. A 55cm MXL however, WILL have a 55cm CT measurement because these frames are measured to near the top of the seat tube lug. I.e. there is no seat tube clamp on the MXL.

Note: The top of the head tube is exactly at the same height as the bottom of the clamps! Soooo, ALL of the differnet Colnago frames (or a given size) have the same measurements relative to the top of the head tube.

The only REAL sizing difference between the different Colnago frames (for a given frame size) is standover height. Wheeeee....... now isn't that fun! ;)

happy trails, .... hmmmmm, I guess that should be roads, eh.

don't totally agree....sort of...djg
Dec 26, 2001 11:09 AM
My mistake, partly. That is, it's 56 to the bottom of the clamp, not the top. But it's still about 55 cm in a traditional c-t measurement. And yes, the top tube is about 55 cm (I've seen one chart that says 55, and one that says 54.8; 54.8 looks right to me, but I'm not sure I can sight things well enough to be sure about a millimeter or two).
frame sizing...C-40
Dec 24, 2001 10:42 AM
Regarding your frame size question the 56cm Colango should be just right for an 84cm inseam. Mine is 83cm and I ride a 55cm, which fits perfectly.

My comparisons of TT length and STA assume that the KOP is being maintained the same on both frames. The vast majority of builders do believe that KOP is relevant. I've always found that the further back I place the knee the lower my (comfortable) cadence becomes. To produce the same amount of power, more torque must be applied. The trick is to find a KOP that optimizes cadence and torque. Everyone is different.

You're right that standover clearance is not real important, but purchasing a frame that is too small vertically also produces a large saddle to bar height difference and requires excessive amount of head tube spacers or a high rise stem to correct. I judge frame (vertical) frame size as much by head tube length as anything.
more on frame sizing...C-40
Dec 24, 2001 1:46 PM
To further elaborate on vertical frame sizing, I should add that the exact size selection depends on the desired fit of the rider. Recreational and touring riders tend to select a frame on the large side, with very little standover clearance. Nothing wrong with this, if you want a more upright position.

Racers tend to select frames that are smaller than the formula would recommend, because they want a low aerodynamic position and have the fitness to tolerate a large saddle to bar height difference. What looks dumb (to me anyway) is to buy a small frame and then use lots of steering tube spacers or a high rise stem to raise the bars to the same height that they would be if the correct frame (and head tube length) had been chosen.

I consider my selection of a 55cm frame for my 83cm inseam to be optimum for my needs. I have a 10cm bar to saddle height difference with an 80 degree stem (110mm) and no steering tube spacers. Some riders would find this too extreme.
Dec 24, 2001 4:16 PM
Yikesss, that's an aggressive setup! Especially considering your relatively normal inseam length. I like your zero stack setup.

I've got an 80.8cm (31 7/8") inseam and went with a 54cm Colnago frame. I'm using 7.6cm (3") of drop which is quite a bit considering my old worn out back. I had actually toyed with the idea of getting a 55cm which would have given me a zero stack height setup. I finally decided that I could live with a little stack height as the fork has a steel steerer tube.

BTW, did you see the team time trial in the Tour this year. Notice that the poor saps on the ONCE team had their bikes set up with the bars SO low that none of them got down on the extended part of the horns! The whole team rode the TTT on the upper part of the bars. Those TT bikes looked VERY un-comfie with 20-30cm ??? drop.

happy trails
There's more to life than leg lengthKerry Irons
Dec 24, 2001 7:06 AM
Use one of these sizing calculators to give you some insight into all of the dimensions of the bike:

And don't forget that two bikes with the same top tube length but different seat tube angles don't have the same effective TT length.
Putting this whole debate in perspectiveDave Hickey
Dec 24, 2001 8:06 AM
There has been a debate the last week regarding effective top tube length. While C-40 and CT-1 make valid points, people are just asking for advise on frame size. From what I see, Maverick would be fine on either a 49,50 or 51 KG241. The top tube difference between a 49 and a 51 is 1mm( 53.8cm vs 53.9cm)

Every saddle sold has at least 2-3cm of adjustment
Seat posts can vary 5-6cm in setback
Stems can range from 6-14.5cm
Handle Bar reach can by as much as another 2-3cm

Taking the above measurements to the extreme, you can vary your reach by as much as 19.5cm with the same top tube length

The only argument against getting slightly smaller frame is stem height with a carbon steerer. But even that can be corrected by flipping over most stems. Please don't take offense, but if I asked for sizing advise on this site, I'd never buy a frame because of fear it wouldn't fit.
thanks to everybodyMaverick
Dec 24, 2001 8:18 AM
you guys has done a great job in helping me. good to have you all around.