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Does size matter?

Old 01-24-17, 12:54 PM
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WizardOfBoz
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Does size matter?

In trying to figure out the proper size for a steel frame bike I am contemplating building, the difference is bother me between what is recommended now and what seemed proper to me many moons ago when I was a (not so) wee lad.

When I was in HS and worked in the bike shop, my bikes were in the 25-26 (63.5-66cm) inch range. I was 6'2.5" and my inseam was about 35". So with my 26" paramount, with a 175mm crank, the seat tube extended about 3 inches. There's a little uncertainty here as my original frame was custom (steep angles), and after the drunk hit it it was replaced by a standard 26" frame (which was too long). In fact, in the bike shop I worked at, we sold a 26" paramount to Jim Chones (the 6'11" NBA player). So, I grant that a 26" is probably too big, but I've just got a shorter Cinelli stem to see if I can get some use out of it.

Fast forward 35 years (more, actually! ) and I went into the bike shop and got sized for a Trek. 58cm fit great, but the frame was too long! I replaced the 110mm (IIRC) stem with an 75 or 80mm (IIRC). Anyway, the bike fits like a glove.

But this difference in size between two bikes I very much liked suggests a question: what is the optimal size? I mean, you could just start with stand-over height, and adjust the seatpost and stem length to accommodate a wide range of frame sizes, right? Which one is best? Or maybe I should ask this: Why do we size the seat tube based upon some "optimal" fraction of inseam? Would a smaller frame with a longer seat tube be lighter? Or less stiff and efficient? Would a larger frame be much heavier and stiff, or would the stiffness give you a rough ride?

Do most folks here just use the standard formula? Are there folks out there that prefer a smaller frame with a super long seat post, or a larger frame with the short seatpost? If you deviate from the norm on frame size, why?
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Old 01-24-17, 01:13 PM
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I am 5' 11" with a fairly long torso and a 58cm era e fits me great. We can't possibly be measuring the same way. I prefer a larger frame so I don't crane my neck so much (arthritis). Standover height is not a big deal to me.
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Old 01-24-17, 01:20 PM
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I would have thought not, but when I check around (for example on the felt sizing page, Bike Sizing - Felt Bicycles) I'm almost exactly a 58cm. And I have a long body, too, but on my Domane, I had to get the shorter stem!?

But theoretically, does anyone know why a larger frame with a shorter seat post extension is better/worse than a smaller frame and a longer seat post? That's what I'm getting at.

Thx.
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Old 01-24-17, 01:28 PM
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6 FT with a 34 inch Cycling Inseam.

My 60 cm Felt Fit me like no other bike has.

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Old 01-24-17, 01:32 PM
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if you are 6' 2.5" and had to shorten the stem on a 58 Domane, it sounds like you are riding sitting straight up. I am 3" shorter than you and had no such issue.
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Old 01-24-17, 01:37 PM
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Fit is extremely personalized. We collapse a bunch of biometric sizes down to one or two (height and maybe inseam), then collapse a bunch of frame variables (including top tube, stem, and downtube length) into a frame size, and get a magic answer which ignores or downplays what a rider prefers (how upright a ride they prefer, how often they ride, how far they ride). Then, the majority of the time, the answer is good enough.

Here's a calculator that attempts to match you to a variety of bikes.
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Old 01-24-17, 01:52 PM
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If you're building your own frame, start with a bike that feels "right" to you. As you noted, modern frames tend to be smaller vertically than classic frames, but may have longer top tubes to accommodate a taller rider. If you know the measurements of your old Paramount, you could copy that and make adjustments if needed. Otherwise, if the modern Trek felt good apart from the top tube length, copy that, but shave a couple centimeters off the top tube. Don't be afraid to experiment; it's really quite hard to build an unrideable bike.
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Old 01-24-17, 02:05 PM
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Depending on body proportions (arm, leg, torso length) and your posture on the bike, two people with the same height may prefer frame sizes that are several CM different. For taller people, top tube length and handlebar height are usually more important than seat tube length.
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Old 01-24-17, 02:13 PM
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6'1 and feel good on a 58. At one time I had a 57 and it was the best fit like a glove feel ever. Sadly that frame was discontinued after mine broke.

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Old 01-24-17, 02:22 PM
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With my Bike Friday the frame size is Length of the top/only tube, the seat and handlebar masts are separate considerations. and the company can make new ones for used bikes.

M is a 56 top tube .. my other, DF bikes, are close to that 55~57 ...

Road bike , level top tube , TT 565, ST 56... I dont have a lot of seat post out, so for those who play on the Web
competitive cycle "French Fit"

though I sorted things out long before the internet..





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Old 01-24-17, 02:27 PM
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I ride a 58cm Specialized Tarmac SL Pro, and I'm 6'4, sure some might consider a little small for me, but I always preferred the feel of 58cm over 61cm. My reason for doing this is because I wanted the slightly smaller frame, because it will way less, and its easily accommodated by raising the seat a little higher than normal. Plus I used to own a tri-bike so I enjoy a more leaned over aero position.
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Old 01-25-17, 08:36 PM
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Originally Posted by obed7 View Post
if you are 6' 2.5" and had to shorten the stem on a 58 Domane, it sounds like you are riding sitting straight up. I am 3" shorter than you and had no such issue.
Not so. I went into the shop, and they said "you should probably be on a 60cm, but let's try the 58. It will be too small by a bit." I rode it a bit and told them the fit seemed pretty good. After I got my Domane, I then had them set up the bike (I could do it myself, but holding the goniometer or the plumb line while riding on the street) and the natural point that I grabbed the tops near the brake was about 2cm back from the brifter. So going from (IIRC a 110mm) to a shorter (80mm IIRC) gave me a perfect fit while riding on the tops.

I replaced the Bontrager handlebars with a Ritchey Superlogic (the BT bars didn't have enough tubing length for my taste). The thing fits great.

That said, my 26" paramount fits great too!
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Old 01-25-17, 08:43 PM
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The personal anecdotes help answer the questions (thanks), but I think I've not made clear exactly waht I want. Let me restate the question:

If I had a bike of a give size, what technical or theoretical factors would drive me to get a larger(smaller) frame with a shorter(longer) stem, set up with the seat tube less(more) elevated?

As a specific example, why would a 61cm be better for me than a 58cm, given that I can adjust stem length and seat elevation to get just about the same distance between seat, pedals, and handlebars?

What I'm thinking is that to get a certain stiffness and efficiency one wants a larger frame and shorter seat tube extension, while to get a comfortable more compliant ride and lower weight one wants a smaller frame. Does this make sense?
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Old 01-26-17, 07:28 AM
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Originally Posted by WizardOfBoz View Post
The personal anecdotes help answer the questions (thanks), but I think I've not made clear exactly waht I want. Let me restate the question:

If I had a bike of a give size, what technical or theoretical factors would drive me to get a larger(smaller) frame with a shorter(longer) stem, set up with the seat tube less(more) elevated?

As a specific example, why would a 61cm be better for me than a 58cm, given that I can adjust stem length and seat elevation to get just about the same distance between seat, pedals, and handlebars?

What I'm thinking is that to get a certain stiffness and efficiency one wants a larger frame and shorter seat tube extension, while to get a comfortable more compliant ride and lower weight one wants a smaller frame. Does this make sense?

If one were of a the height that they were deciding between a large/smaller frame, AND they realized that by changing seat position, seat height, and stem length/height they could get their 3 contact points (feet, butt, hands) in the same spot...WHAT ELSE COULD MOTIVATE PICKING ONE SIZE OVER THE OTHER?

Here's What I understand:

LARGER FRAME:
-slightly more compliant frame, but less seat post exposed, so less seat post to flex, which might lead to a stiffer ride for your seat.
-Shorter stem means "more control" over steering, but also means "more twitchy".
-less stand-over height means more risk to the nether-regions, but with modern sloping top tubes, this is rarely an issue...but with older (or custom) horizontal top tubes it is something to consider.
-Can't get the handle-bars as low, so keeps you in a slightly more endurance position...though if you wanted to get more LONG, a longer frame could allow you to do that.

SMALLER FRAME:
-less weight for the frame
-MORE frame stiffness over-all, but more seat-post exposed means if you have a compliant seat-post, it can soften your ride.
-more ability to get the handle-bars lower than the seat for a more aero position.
-longer stem can mean less twitchy at high speeds.

I was choosing between a 54 & 56 frame, and went with the 54 mostly for lighter weight, more responsiveness when riding. I'm happy with my decision.
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Old 01-26-17, 07:51 AM
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Many years ago I had heard that you had the right size and stem length when riding the hoods your view down at the wheel obscured the front axel. Don't know if that is accurate but it does work on my 58.5CM Specialized Allez with a 110 mm stem. This 1985 bike was sometimes referred to as a compact frame and I am not sure what that means, steep angles? This bike is more responsive in turns than any others I have had.
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Old 01-26-17, 07:53 AM
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12string, this is exactly the discussion I was after. I see a lot of the sizing programs that tell me that with a 34.25 inseam, I want a 57cm bike. Hard to rationalize when I've ridden that 26" (66cm) all these years. I may just try a 58 steel frame, and see how that works.

Thanks!
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Old 01-26-17, 08:01 AM
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Originally Posted by churnman View Post
This 1985 bike was sometimes referred to as a compact frame and I am not sure what that means, steep angles? This bike is more responsive in turns than any others I have had.
I didn't know either. So I went to the source: the Oracle of Harris Cyclery (Sheldon Brown). Note my edit(deletion) of some garbled editing (where the [] is).

Compact Frame Traditional "road" frames have been built with level top tubes since the early 1900s. The influence of mountain and BMX bike design has led to the increasing popularity of frames with sloping top tubes, higher at the front. "Compact" road frames have sloping top tubes, and are intended to be used with a long seatpost. Compact frames are a little bit lighter than traditional ones, but this is partially offset by the weight of the longer seatpost. Some riders believe these frames are stiffer.
When lines extending forward from the down tube and top tube intersect directly above the front axle, as is usual with a road frame, weight loading only tensions [] the down tube and compresses the top tube, imposing no bending loads on them. When the intersection is behind the axle, he frame is better at resisting braking loads. When the intersection is ahead of the axle, the bending loads on these tubes require the front end of the frame to be of stronger construction.
Manufacturers like compact frames because they are more versatile in terms of fit. Usually 3 or 4 sizes are enough to fit 98% of customers. This saves a lot of money for a manufacturer who doesn't need to deal with so many different sizes.
Sheldon Brown's Bicycle Glossary Cn - Cz
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Old 01-26-17, 08:02 AM
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I think that you are almost sure to do it wrong.

I can't think of anything that I've done in my life that I didn't think I could do better the second time. That's a good thing because it proves that I'm capable of learning. Experience matters. I'd find a custom frame builder and work out a deal to tap into his knowledge.

I have a personal friend who is 6'7". He obviously needs a bigger bike frame than is common. I put him in touch with Leonard Zinn who has made a nitch market of building frames for tall riders. Leonard makes frames with relatively short seat tubes but taller head tubes. That's because, for a large size frame, you would lose the triangle benefit otherwise. That's just one example that I, for one, would have never thought about.
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