# Bicycle Mechanics - longer top tube + shorter stem = shorter top tube + longer stem?

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adam_mac84
12-18-09, 09:00 PM
as in the thread topic, is this similar. If i were to get a bike with a 10-15mm longer top tube, can i just swap in a shorter stem and have an equivalent length?

for instance is there any reason that 528mm top tube and 100mm stem would be different than 548 and 80mm stem?

I understand the concept of top tube measurement, are 'fit calculators' that are available online and at bike shops with 'generic fittings', spitting out a relatively plastic measurement assuming that most stems are a standard length?

operator
12-18-09, 09:06 PM
No.

Don't buy a frame that's too big for you and think a shorter stem will compensate for that extra length. Get a properly fitted bike.

adam_mac84
12-18-09, 09:08 PM
i am a bit curious why different shops will fit me to different sized bikes when they are nearly the exact same frame geometry. (the numbers were arbitrary...) i ahve been fit anywhere from a 58 to a 62cm frame

adam_mac84
12-18-09, 09:14 PM
well, if these were some measurements (these are for my wife, were both getting bikes this winter, for her im looking at an allez circa 2004) she is right between 52cm and 54cm, so there has to be an adjustment somewhere in stem/toptube, only one of these are not a fixed part of the bike.

http://www.specialized.com/us/en/bc/SBCBkModel.jsp?arc=2004&spid=21068&menuItemId=0
geometry specs

her 'fit calculator'
Seat tube range c-c: 53.4 - 53.9
Seat tube range c-t: 55.0 - 55.5
Top tube length: 53.3 - 53.7
Stem Length: 9.6 - 10.2
BB-Saddle Position: 63.1 - 65.1
Saddle-Handlebar: 52.5 - 53.1
Saddle Setback: 5.2 - 5.6

HillRider
12-18-09, 10:00 PM
Assuming the same seat tube angle so the toptube measurements are comparable, yes, within a relatively narrow range, stem length can be adjusted to compensate for toptube length. Extensive changes will alter the bike's weight distribution and affect handling.

operator
12-18-09, 10:39 PM
i am a bit curious why different shops will fit me to different sized bikes when they are nearly the exact same frame geometry. (the numbers were arbitrary...) i ahve been fit anywhere from a 58 to a 62cm frame

Because most shops will have no idea wtf they're doing. Go to a shop with a good reputation for fitting people.

bigvegan
12-19-09, 02:50 AM
"i am a bit curious why different shops will fit me to different sized bikes when they are nearly the exact same frame geometry. (the numbers were arbitrary...) i ahve been fit anywhere from a 58 to a 62cm frame."

Because:

a) It depends on what the bike shops have in stock,
b) some salespeople have a preference for larger or smaller frames, and steer their customers accordingly, and
c) the geometry between manufacturers and sizes does differ.

"Because most shops will have no idea wtf they're doing. Go to a shop with a good reputation for fitting people."

This is true. If you're spending more than \$700-800 on your bike, it might be worth shelling out \$150 or so to get a professional fitting done.

Retro Grouch
12-19-09, 07:13 AM
The deeper you probe into the topic, the more there is to learn. Everything works together.

The larger frame will often have a taller head tube. Racers tend to like a lower handlebar position, non-racers often prefer the opposite.

Toe clip clearance frequently becomes an issue with small frame sizes. Sometimes manufacturers diddle with head tube and seat tube angles to improve toe clip clearance while maintaining a shorter top tube.

I'd mention the seat vs. bottom bracket relationship but that might start another whole discussion.

adam_mac84
12-19-09, 07:27 AM
where can i read more about all of these relationships? is there a good book? I would like to learn more

DaveSSS
12-19-09, 09:53 AM
There are only three dimensions needed to determine the fit of a frame - the TT length, the seat tube angle and the headtube length, with the headset. From these, you can tell how one frame differs from another and know exactly what type of stem setup that frame will require, compared to another.

The only other dimension that may be relevant for some people with short legs is the standover height. While it does not affect the frame's fit, you don't want to crush the family jewels when you stop the bike and put a foot down.

The modern concept for bike fitting uses the terms reach and stack to compare frames. At present, only a few companies, like Cervelo, Trek and C'dale publish these values, but they are valuable information. The only small drawback to the use of reach is that it must be take at the same stack height to properly predict the stem length difference between frames. A simple correction of 3mm for each 10mm of stack height difference will adequately correct that problem.

When comparing frames that do not list the stack and reach, the alternative requires the TT length, the STA and the HTL (with headset). If the frames have different STAs, then the frame with the steeper STA should have 8-10mm per degree added to the TT length before taking the difference in order to correctly predict the stem length difference. There is a good drawing that explains reach and stack at this site:

http://www.cervelo.com/bikes.aspx?bike=R32010#G

One of my admirers will beat me for saying this, but it is true. Different brands do not always have the same frame dimensions in a given size, since there are different ways to measure "size". You may find one brand in a certain size has nearly identical dimensions to one that is 2-3cm larger or smaller in another brand. I recently compared a BMC to a LOOK 585 and found that the 49cm BMC was nearly identical to a 51cm LOOK 585.

JTGraphics
12-19-09, 10:11 AM
well, if these were some measurements (these are for my wife, were both getting bikes this winter, for her im looking at an allez circa 2004) she is right between 52cm and 54cm, so there has to be an adjustment somewhere in stem/toptube, only one of these are not a fixed part of the bike.

http://www.specialized.com/us/en/bc/SBCBkModel.jsp?arc=2004&spid=21068&menuItemId=0
geometry specs

her 'fit calculator'
Seat tube range c-c: 53.4 - 53.9
Seat tube range c-t: 55.0 - 55.5
Top tube length: 53.3 - 53.7
Stem Length: 9.6 - 10.2
BB-Saddle Position: 63.1 - 65.1
Saddle-Handlebar: 52.5 - 53.1
Saddle Setback: 5.2 - 5.6

Based on this calculator fit I'd go with a 52 over a 54 if those are your choices in sizes but you still need to check fit by riding it.

DaveSSS
12-19-09, 10:58 AM
I would not put any value on the results of a fit calculator. A TT length without a STA to go with it has no meaning at all.

The Specialized bikes you mentioned have an 11mm difference in the TT length, but with the correction for the difference in the STA, the reach difference is only 6-7mm. That's less than one stem size different. The head tubes differ in length by a much larger 20mm.

I notice that the BB-saddle position (saddle height?) from the fit calculator is a very short 63 to 65cm. That would normally suggest a cycling inseam of 73-75cm. If that's the case, then she could not stand over either bike and the suggested seat tube length makes no sense. Both sizes would be too large.

cyccommute
12-19-09, 11:01 AM
There are only three dimensions that I mistakenly think are important for the fit of a frame are the TT length, the seat tube angle and the headtube length, with the headset. From these, you can't tell much about how one frame differs from another and you don't violate my stem esthetics or my delicate sensibilities.

The only other dimension that may be relevant for some people with any dangly bits they want to preserve is the standover height. While it does not affect the frame's fit, you don't want to crush the family jewels when you stop the bike and put a foot down.

Fix it for you.

cyccommute
12-19-09, 11:41 AM
as in the thread topic, is this similar. If i were to get a bike with a 10-15mm longer top tube, can i just swap in a shorter stem and have an equivalent length?

for instance is there any reason that 528mm top tube and 100mm stem would be different than 548 and 80mm stem?

I understand the concept of top tube measurement, are 'fit calculators' that are available online and at bike shops with 'generic fittings', spitting out a relatively plastic measurement assuming that most stems are a standard length?

well, if these were some measurements (these are for my wife, were both getting bikes this winter, for her im looking at an allez circa 2004) she is right between 52cm and 54cm, so there has to be an adjustment somewhere in stem/toptube, only one of these are not a fixed part of the bike.

http://www.specialized.com/us/en/bc/SBCBkModel.jsp?arc=2004&spid=21068&menuItemId=0
geometry specs

her 'fit calculator'
Seat tube range c-c: 53.4 - 53.9
Seat tube range c-t: 55.0 - 55.5
Top tube length: 53.3 - 53.7
Stem Length: 9.6 - 10.2
BB-Saddle Position: 63.1 - 65.1
Saddle-Handlebar: 52.5 - 53.1
Saddle Setback: 5.2 - 5.6

Firstly, your comparison in your original post wasn't between a 52 and a 54. A 528mm top tube is from a 50 cm Allez and a 548mm top tube is for a 54 cm Allez. The 52 cm Allez splits the difference at 537mm. If you are really comparing a 50cm Allez to a 54 cm Allez there is something seriously wrong here.

Looking at the standover heights, the 50cm has a standover of 722 mm while the 54 cm has a standover of 777mm. That a difference of 55 mm or about 2 inches. It may not seem like much but if your wife really fits a 50 cm (probably not), trying to ride a bike that has 2 inches less clearance could result in some damage to...um...delicate areas:eek:

I'd suggest riding both if possible. If you can't ride the different sizes in a 2004 model, today's Allez have a similar geometry to the 2004 models so you can get an idea of how the two different sizes might fit. In the end, riding the bike is proof of how it's going to fit. You can run all the numbers you want but the proof of the pudding is when the rubber hits the road.;)

DaveSSS
12-19-09, 03:10 PM
I'm not nearly as smart as DaveSSS and don't know squat about geometry, so I'll give you the simpleton's explanation. If you need accurate information send a PM to DaveSSS - he's my hero! I should have paid attention during high school geometry.

Firstly, your comparison in your original post wasn't between a 52 and a 54. A 528mm top tube is from a 50 cm Allez and a 548mm top tube is for a 54 cm Allez. The 52 cm Allez splits the difference at 537mm. If you are really comparing a 50cm Allez to a 54 cm Allez there is something seriously wrong here.

Looking at the standover heights, the 50cm has a standover of 722 mm while the 54 cm has a standover of 777mm. That a difference of 55 mm or about 2 inches. It may not seem like much but if your wife really fits a 50 cm (probably not), trying to ride a bike that has 2 inches less clearance could result in some damage to...um...delicate areas:eek:

I'd suggest riding both if possible. If you can't ride the different sizes in a 2004 model, today's Allez have a similar geometry to the 2004 models so you can get an idea of how the two different sizes might fit. In the end, riding the bike is proof of how it's going to fit. You can run all the numbers you want but the proof of the pudding is when the rubber hits the road.;)

I new you were my biggest fan.

cyccommute
12-19-09, 07:19 PM
I new you were my biggest fan.

Must be an engineer:rolleyes:

There are only three dimensions needed to determine the fit of a frame - the TT length, the seat tube angle and the headtube length, with the headset.

You made this claim, now back it up. Here are a set of your three measurements for 10 bikes. You tell me the rider height with a +/- 4" range that would fit each.

Head tube Top tube seat angle
19 59 72.5
12 52.9 73
12 56 74.5
16.4 57 73
19 57.2 73.1
11.5 49.4 75.5
17 55 72.5
18.2 57 73.2
16 56.5 73
18.5 55.8 73.7

I will tell you that the bikes are within \$500 of \$2000. None are women's models.

Put up or shut up.

operator
12-19-09, 07:30 PM
I new you

Lullz.

DaveSSS
12-20-09, 09:44 AM
Yes I am a mechanical engineer, but the math needed to understand bike frames only requires high school level geometry and trigonometry.

Anyone who would suggest fitting a bike based only on a rider's height isn't too sharp. As an example, I have the same 83cm cycling inseam that Cyccocommute has posted, but he's 5-6 inches taller, IIRC. I'm only 5'-6.5" or 169cm tall. My bikes would fit his legs, but not his much longer torso.

The three key dimensions of TTL, STA and HTL (with the headset) or better yet - the frame reach and stack - define the frame's possible range of bar height and saddle to bar reach and permit the comparison of the fit among any number of other frames. They do NOT magically fit the frame to a particular rider. If a rider has a fit that he can define, then I can match it with another brand and model.

When someone posts that they ride a particular size and model of bike and wants to buy a different brand that fits the same is when those dimensions are applied. If a person has never ridden a bike and knows nothing about what type of fit they want, I always recommend a fitting by a professional - not some guy whose main job in a bike shop is selling bikes and may have only a rudimentary understanding of bike fitting.

If a person posts only his height and cycling inseam, I always ask if he has some cycling experience and some idea of his preferred saddle height. Two riders can have the same inseam but if one pedals with a horizontal foot, his saddle height may be 2-3cm lower than a rider who naturally raises his heel. Neither is right or wrong, but it needs to be considered when selecting the best fitting frame. All you can do for a person with no expereince is steer them away from a frame choice that is likely to be much too large or small.

cyccommute
12-20-09, 11:55 AM
[FONT=Tahoma]Anyone who would suggest fitting a bike based only on a rider's height isn't too sharp. As an example, I have the same 83cm cycling inseam that Cyccocommute has posted, but he's 5-6 inches taller, IIRC. I'm only 5'-6.5" or 169cm tall. My bikes would fit his legs, but not his much longer torso.

The three key dimensions of TTL, STA and HTL (with the headset) or better yet - the frame reach and stack - define the frame's possible range of bar height and saddle to bar reach and permit the comparison of the fit among any number of other frames. They do NOT magically fit the frame to a particular rider. If a rider has a fit that he can define, then I can match it with another brand and model.

When someone posts that they ride a particular size and model of bike and wants to buy a different brand that fits the same is when those dimensions are applied. If a person has never ridden a bike and knows nothing about what type of fit they want, I always recommend a fitting by a professional - not some guy whose main job in a bike shop is selling bikes and may have only a rudimentary understanding of bike fitting.

If a person posts only his height and cycling inseam, I always ask if he has some cycling experience and some idea of his preferred saddle height. Two riders can have the same inseam but if one pedals with a horizontal foot, his saddle height may be 2-3cm lower than a rider who naturally raises his heel. Neither is right or wrong, but it needs to be considered when selecting the best fitting frame. All you can do for a person with no expereince is steer them away from a frame choice that is likely to be much too large or small.

All hat. No cattle.

I'll remind everyone, again, what you said.

There are only three dimensions needed to determine the fit of a frame - the TT length, the seat tube angle and the headtube length, with the headset.

Bike fit is based on body size. Height usually determines factors like reach and inseam. If your three dimensions...up from one...head tube:rolleyes:... or is it 5 now, it's so hard to keep track as you move the target... are indeed all that is needed to determine fit, then you should easily be able to tell me what size body fits on each of those 10 bikes. I made it easy for you because all I asked for was height with a 4" range.

Maybe you need it harder. Since you can't tell me a height, tell me the torso size, the reach and the inseam of a rider for each one. Let's make it even harder and only give you a 1/2" range on each of those measurements. If you have the torso size and the inseam, you can tell me how tall the rider is.

I can just as easily tell someone who has a bike and wants a different model which one they should start with based on size. As previously demonstrated to you with 7 different brands at the same level of components and the same price range, the geometries don't vary that much between brands. A 58 cm bike from one brand is going to compare very similarly to a 58 cm bike in another brand...at the same price and for the same usage.

Will the fit be perfect when the person rides it for the first time? Maybe, maybe not. The ride has to be dialed in but that's why we have different stems and adjustable saddles and seatposts.

If someone is new to riding, I look at their height and inseam and suggest a size based on that measure. Will the fit be perfect? Again, maybe or maybe not. But that's why we have different stems and adjustable saddles and seatposts. Do I need a professional fit to get a newbie out on a bike? No. A professional fit might help as they progress in ability but if the rider is just looking to get started, they probably don't need...or want... to spend an extra \$90 to \$150 for a bike fitting session.

Longfemur
12-20-09, 12:25 PM
It ends up the same for the length of your cockpit, but it's not the same in terms of overall fit of the bike and how the bike handles. It depends on the kind of bike we're talking about. A modern racing bike will not work as well with a longer wheelbase but shorter stem, but a touring bike or any "french fit" type of bike will.

Also important to keep in mind that stem length should be proportional to frame size. A shorter frame will use say an 80 cm stem whereas a taller frame might need 120 cm.

JohnDThompson
12-20-09, 01:23 PM
A short top tube with a long stem will put more of your weight over the front wheel. If you're doing loaded touring this may not be a bad thing. If you're going to be racing in hilly terrain it may not be so good.

Bottom line: get a frame that fits you, designed for the type of riding you intend to do.

DaveSSS
12-20-09, 02:20 PM
Cyccommute...

You are still suffering from that reading comprehension problem. Since you seems obsessed with one of my statements and believe it is incorrect, I'll repeat the whole thing so it's not taken out of context and misconstrued:

There are only three dimensions needed to determine the fit of a frame - the TT length, the seat tube angle and the headtube length, with the headset. From these, you can tell how one frame differs from another and know exactly what type of stem setup that frame will require, compared to another.

If you read the sentences above carefully; note that I mention the fit of the FRAME, but absolutely nothing about any relationship to the rider of that frame. Those dimensions tell me nothing about a potential rider or what type of fit he may want. You're just reading ideas into my statements that are simply not there.

The only thing I should add is that I also look at the BB drop, but the vast majority of road bikes these days use 70mm or very close to it. If someone asks about a Serotta, that often has an 80mm drop or a cross frame with a small BB drop, then I take that into account.

I have to laugh at your statement that height determines inseam, when my inseam is as long as yours, but you are much taller. You've proven yourself to be wrong. I've read numerous posts over the years from 6 foot tall riders who have legs no longer than mine. Then consider the fact that some riders want a very upright touring/recreational fit and some other rider wants a long and low racing fit. Two riders with the exact same height and inseam might ride frames that are quite different.

DMF
12-20-09, 04:29 PM
Well, I'd consider trail and wheelbase to be important frame dimensions, but perhaps you don't consider them under "fit".

In any case, let's stop this pissing contest before it starts.

cyccommute
12-20-09, 07:16 PM
Cyccommute...

You are still suffering from that reading comprehension problem. Since you seems obsessed with one of my statements and believe it is incorrect, I'll repeat the whole thing so it's not taken out of context and misconstrued:

If you read the sentences above carefully; note that I mention the fit of the FRAME, but absolutely nothing about any relationship to the rider of that frame. Those dimensions tell me nothing about a potential rider or what type of fit he may want. You're just reading ideas into my statements that are simply not there.

The only thing I should add is that I also look at the BB drop, but the vast majority of road bikes these days use 70mm or very close to it. If someone asks about a Serotta, that often has an 80mm drop or a cross frame with a small BB drop, then I take that into account.

I have to laugh at your statement that height determines inseam, when my inseam is as long as yours, but you are much taller. You've proven yourself to be wrong. I've read numerous posts over the years from 6 foot tall riders who have legs no longer than mine. Then consider the fact that some riders want a very upright touring/recreational fit and some other rider wants a long and low racing fit. Two riders with the exact same height and inseam might ride frames that are quite different.

Who are you fitting to the frame...ducks? Fit of the the frame is the fit of the rider to that frame. It can't be anything else. If you want to talk about how the stem fits on the bike, don't talk about about frame fit. As usual you are running away from your statement while being insulting.

You need to check your own reading comprehension. I said height usually determines reach and inseam. Don't you know anything about outliers? You at 5' 6" tall with a 33" inseam certainly don't fit in the 'normal' range for male human body styles. The average 6' tall human male is going to have an inseam closer to mine than proportions like yours. I don't know a whole lot of 6' tall guys with 36" inseams.