# Formula for ideal top tube length?



## mwilcko2 (May 1, 2004)

Anyone know of any sites (or formulas) that have a free ideal top tube length calculator?

Trying to decide between a 55 and 57. I'm 5' 10.5, but have a 32" inseam, which seems to be a little shorter than others at my height. I'm assuming I would have a slightly longer torso, so I would rather go by top tube than the old inseam x .67.

Thanks!


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## FatTireFred (Jan 31, 2005)

can't look at TT without STA... but there is so much more to consider, e.g., rider flexibility and desire bar height, etc.


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## C-40 (Feb 4, 2004)

*no such thing...*

The problem, even with a sophiscated computer program, is errors in taking exact body measurements. Most don't seem to be able to figure the saddle fore/aft position, which can have a big effect on the stem length and reach. I would not trust any of them.

TT length, by itself, does not determine the frame reach, which is the distance in front of the BB centerline. If your 32 inch (81cm) inseam is an accurate cycling inseam, to saddle like crotch contact in bare feet, then you do have short legs and a long torso. I've got the opposite proportions with an 83cm inseam at 5'-6" tall.

When looking at frames you also need to consider the seat tube angle. Steeper STAs increase the reach my about 1cm per degree. You shouldn't have the other problem that many people do - insufficient head tube length. Even frames with 55cm TTs should have more than enough HTL for your saddle height.


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## cogswell23 (Aug 15, 2007)

C-40 said:


> TT length, by itself, does not determine the frame reach, which is the distance in front of the BB centerline. If your 32 inch (81cm) inseam is an accurate cycling inseam, to saddle like crotch contact in bare feet, then you do have short legs and a long torso. I've got the opposite proportions with an 83cm inseam at 5'-6" tall.
> 
> When looking at frames you also need to consider the seat tube angle. Steeper STAs increase the reach my about 1cm per degree. You shouldn't have the other problem that many people do - insufficient head tube length. Even frames with 55cm TTs should have more than enough HTL for your saddle height.


Pardon my ignorance, but wouldn't a slacker STA increase reach, and a steeper STA decrease length? Hypothetically, if one had a 90degree STA, the ST would just stick straight up from the BB shell, so the saddle would be situated straight up above the bb. But a slacker angle would move the saddle back more, increasing reach. Wouldn't the saddle be positioned farther back on a frame with 70* sta than a 72.5* sta, all other things being equal?

Is there something I'm not understanding?


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## FatTireFred (Jan 31, 2005)

cogswell23 said:


> Pardon my ignorance, but wouldn't a slacker STA increase reach, and a steeper STA decrease length? Hypothetically, if one had a 90degree STA, the ST would just stick straight up from the BB shell, so the saddle would be situated straight up above the bb. But a slacker angle would move the saddle back more, increasing reach. Wouldn't the saddle be positioned farther back on a frame with 70* sta than a 72.5* sta, all other things being equal?
> 
> Is there something I'm not understanding?




a slacker angle DEcreases 'effective' TT length- provided the TT lengths are the same- and therefore 'reach' is also decreased... your saddle position relative to the BB should stay the same


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## cogswell23 (Aug 15, 2007)

FatTireFred said:


> a slacker angle DEcreases 'effective' TT length- provided the TT lengths are the same- and therefore 'reach' is also decreased... your saddle position relative to the BB should stay the same


Okay, sorry to be dense, but I'm just not picturing this.

Let's say Ive got a bike with a 55cm TT and a 73* STA. Let's say it's a funny bike with an adlustable STA. I decide one day to decrease my STA, which I do by angling my seat tube backwards to make a more acute angle with the ground, right? Now there's actually a gap between my 55cm TT and my seattube where they met. I would need to have a telescoping TT on this make believe bike, so that I could reconnect. The TT would now be maybe 56cm, and my reach would be longer. Why wouldn't it be?

I'm not trying to argue here--I believe you guys. I just can't quite put it together in my head. If you could give me a more detailed explanation I'd appreciate it.


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## Bianchi67 (Oct 16, 2005)

In your make believe example your reach would be exactly the same ... after you moved your saddle 1cm forward to maintain the same knee over pedal. 

This is where people get confused and think a slacker STA would increase the reach when the opposite is true. Your saddle location needs to remain at the same point in reference to the bottom bracket (or at less it should).


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## The Flash (May 6, 2002)

I'm 5'11 with a 30" inseam...almost all torso with short arms to boot....I ride a 55TT with a 100mm stem on a bike with a 74STA....You can always add a longer stem and bars with a bit more reach, but making it shorter is quite difficult....


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## C-40 (Feb 4, 2004)

*explanation..*

What you're missing is that frames should always be compared with the saddle in the same location, relative to the BB. A slack STA does NOT force the saddle to be further back. To compensate for a slack STA, the saddle must be pushed forward. When this is done, the reach is reduced. Reach is the horizontal distance from the center of the BB to the HT/TT intersection point and what determines the difference in the stem length required.

Reach is reduced by about 1cm per degree, depending on the exact frame size. I've owned two frames, one with a 72.5 STA and 54cm TT and another with a 74.5 STA and 52.5cm TT. The exact difference in the reach for the 51cm frame size is 51 x (cos72.5-cos74.5) = 1.7cm. Even though the first frame had a 1.5cm longer TT, the reach was 2mm shorter.


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## cogswell23 (Aug 15, 2007)

Thanks a lot. I see what you mean.

Thanks for taking the time to explain.


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## CoLiKe20 (Jan 30, 2006)

wow.. this is complicated.


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## jamesau (Apr 22, 2002)

mwilcko2 said:


> Anyone know of any sites (or formulas) that have a free ideal top tube length calculator?
> 
> Trying to decide between a 55 and 57. I'm 5' 10.5, but have a 32" inseam, which seems to be a little shorter than others at my height. I'm assuming I would have a slightly longer torso, so I would rather go by top tube than the old inseam x .67.
> 
> Thanks!


The 'LeMond formulas' provide for reach (TT + stem) and should put you in the ballpark.
http://www.cyclemetrics.com/Pages/Docs/6-BikeFitting/LemondSystem/fit_formulas.htm


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## lamazion (Sep 11, 2004)

Does that formula really put people "in the ball park"? I'm just over 6 ft., with relatively long legs. This formula give me a reach of 64.5. My current ride is 67.5 and that's a little shorter than in the past. This formula would have me riding a 55 or 56 cm frame to get the reach with a 90mm stem. On most frames, that would give me have a bar drop around 15cm (my fitting recommended 5-6 cm).


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## rogerstg (Aug 1, 2007)

Look at the Wrenchscience.com site. It is pretty comprehensive.


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## tarwheel2 (Jul 7, 2005)

Good points about the relationship between effective top tube length and seat tube angle. Here's a real life example. I've got an Merckx and De Bernardi road bikes, both size 57 center-center seat tubes. The Merckx has a 57 top tube with 72.5 seat tube angle. The De Bernardi has a 56 top tube with 74 STA. Both bikes fit me about the same using a stem with a 10 cm reach/ 90 degree angle, but the saddle is pushed forward more on the Merckx and back further on the DeBe.


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## cmg (Oct 27, 2004)

the problem with formulas is that they don't suggest the proper seat tube angle for rider balance. It's assumed that the saddle fore and aft will accomplish it. That's why you see riders with short stems less than 10cm.


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## jamesau (Apr 22, 2002)

lamazion said:


> Does that formula really put people "in the ball park"? I'm just over 6 ft., with relatively long legs. This formula give me a reach of 64.5. My current ride is 67.5 and that's a little shorter than in the past. This formula would have me riding a 55 or 56 cm frame to get the reach with a 90mm stem. On most frames, that would give me have a bar drop around 15cm (my fitting recommended 5-6 cm).


It's what the OP asked for. Maybe I misspoke when I said it puts you in the ballpark. 
It's real close with me (6'1", 88cm inseam, 69.5cm calculated reach).

Here's my revised blanket statement: Provided the measurements are good, and the subject has reasonably average proportions and flexibility, the formulas may give you a decent starting point. But they may not.


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## abqhudson (Sep 8, 2004)

OK. I'll admit my error. I just received a new fat-tire frame built with the same top tube length as my old skinny-tire frame. Same seat tube length also. But the new frame built for all-day rail trail rides has much more relaxed angles. My mistakes: in anticipation of the new frame, I bought a seatpost just like my favorite one on my skinny-tire frame and a stem exactly like the one on my skinny tire frame. NEITHER will work on the new fat tire frame: The seat post will not allow the seat to move far enough forward (I've discovered that I now need a zero setback post) and the stem is 2 cm too short. Same toptube length, but 2 cm difference in stem is required to get to the same position on the bike! Now I'm waiting on a new stem and seatpost. Live and learn - it's not at all obvious.


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## aju (Jul 16, 2007)

How does bar reach factor into this then? Is a certain standard read assumed when looking at TT+Stem length, or should this be part of this overall reach calculation?


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## abqhudson (Sep 8, 2004)

I don't know if this will help or not - I'm trying to duplicate the fit of my skinny tire bike frame on my new frame. Make the seat height measured: c of bottom bracket to top of seat (along the seat post) the same; Make the saddle nose sit at the same setback from the center of the bottom bracket; and, make the "reach" (measurement) of the nose of the saddle to the handle bar the same, and make the handlebar height in relation to the seat height the same. I did get a professional fit on my skinny tire bike, so I'm trying to duplicate that position on the new bike. I realize that none of this may be helping the OP. Seat position, TopTube length, stem length, handlebar height and headtube angle will all impact reach. At least that's the way I see it.


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## samh (May 5, 2004)

*long vs short forearms*



The Flash said:


> I'm 5'11 with a 30" inseam...almost all torso with short arms to boot....I ride a 55TT with a 100mm stem on a bike with a 74STA....You can always add a longer stem and bars with a bit more reach, but making it shorter is quite difficult....


Does it matter if 2 people (same inseam, chest size) have same arm length, but one has shorter forearm, one has longer forearm?

Also: lennard zinn's book has a top tube calculator I think, but like fit kit just a general guideline.


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