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Seat tube measure in classic bike

Old 02-01-16, 07:07 AM
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Seat tube measure in classic bike

Hi there,
So I've been looking for another classic steel frame to change my actual one.
I'm a little bit more than 5'8", about 32" leg inseam. My actual bike has a 52 seat tube (which I find comfortable) and a 51.5 top tube (which I find to be small). So I'm looking for a frame with a longer top tube.

So far, I've found two that I like. One has a 54 seat tube and a 53 top tube, and the other one has a 50 seat tube and a 53 top tube. Same top tube, ideal for me, but huge difference in the seat tube.

Personally, I prefer a smaller seat tube (I like it to be a space so as to stand away from the top tube), but maybe 50 is too small? About this, I've read that Sheldon says the seat tube measure actually is the less important, as I could solve this with a longer seat post. Would this be uncomfortable to ride?

I've had a 54x54 bike in the past, and I didn't like a lot that 54 seat tube.

Any recommendations?

Thanks a lot, as usual!
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Old 02-01-16, 07:23 AM
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Go with whatever makes your anatomy happy., which would appear to be the shorter 50cm frame.
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Old 02-01-16, 07:33 AM
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Much more information is needed. I suggest going to Colorado Cyclery's web site, getting someone to help you, and doing extensive measurements.
A seat tube shouldn't be that important, but the distance from the center of the bottom bracket to where your pelvic bones touch the saddle is.

Your top tube measurement is important, but we have no idea if you're using a set-back seat post, how long your stem is, etc, or even the geometry of the bike.

Also, how much of a drop do you ride from the saddle to the top of the bars, or to the drops?


I'm 5'6" with a 30" trouser inseam, and ride a 56x56. So you see what works for one obviously does not work for another.

Originally Posted by longi
Hi there,
So I've been looking for another classic steel frame to change my actual one.
I'm a little bit more than 5'8", about 32" leg inseam. My actual bike has a 52 seat tube (which I find comfortable) and a 51.5 top tube (which I find to be small). So I'm looking for a frame with a longer top tube.

So far, I've found two that I like. One has a 54 seat tube and a 53 top tube, and the other one has a 50 seat tube and a 53 top tube. Same top tube, ideal for me, but huge difference in the seat tube.

Personally, I prefer a smaller seat tube (I like it to be a space so as to stand away from the top tube), but maybe 50 is too small? About this, I've read that Sheldon says the seat tube measure actually is the less important, as I could solve this with a longer seat post. Would this be uncomfortable to ride?

I've had a 54x54 bike in the past, and I didn't like a lot that 54 seat tube.

Any recommendations?

Thanks a lot, as usual!
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Old 02-01-16, 09:38 AM
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Originally Posted by oddjob2
Go with whatever makes your anatomy happy., which would appear to be the shorter 50cm frame.
+1.

Bear in mind that at different times, builders and designers have favored different approaches to bicycle fit. I used to have a 1936 track bike that had a 53 cm seat tube and a 60 cm top tube. 60 is not as excessive as it sounds, since the frame had a very relaxed seat tube angle, and to compensate for that they mounted the seat farther forward than normal on a modern bike, but even so, it was a long top tube.

Look at different bikes, and especially look at bikes from different eras and different regions. They vary. You'll find something that fits the way you want it to.
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Old 02-01-16, 09:59 AM
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All the bike fitting experts reside in General and Road cycling.
Trust them.
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Old 02-02-16, 04:26 AM
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Thanks to everyone for the recommendations!
I'll think about it.

Cheers!
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Old 02-02-16, 05:17 AM
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Originally Posted by Wildwood
All the bike fitting experts reside in General and Road cycling.
Trust them.
Trust me.... I KNOW WHAT'S RIGHT FOR YOU!



I read this cover to cover several times back in the early 70's before I realized that it was written for budding Italian adolescents to form them into Olympians!

Around the age of 18 to 22 the human body starts to loose the ability to adapt to unnatural positions like ballet and bike racing! I'd just turned 30!

ONE SIZE DOES NOT FIT ALL!

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Old 02-02-16, 05:37 AM
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Originally Posted by verktyg
ONE SIZE DOES NOT FIT ALL!
And sometimes many sizes fit one. All my bikes feel different but after half a mile they all feel great. With appropriate tweaking of saddle position on the seat post, stem length, bar height etc. you can make the bike fit you. Within reason of course.
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Old 02-03-16, 04:41 AM
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Originally Posted by jimmuller
And sometimes many sizes fit one. All my bikes feel different but after half a mile they all feel great. With appropriate tweaking of saddle position on the seat post, stem length, bar height etc. you can make the bike fit you. Within reason of course.
I was mainly referring to seat tube length... (Plus chiding fit experts)

I can ride 54cm to 57cm size frames but I like short top tubes - 53cm to 56cm. I use short stems on many of my bikes because I have a short bull neck and like to look through my glasses instead of over the top.

Short stems Flickr album:

https://www.flickr.com/photos/282672...57626832237811

In the 70's I rode in the drops. Now I prefer a more upright position.

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Old 02-03-16, 05:59 AM
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Originally Posted by verktyg
(Plus chiding fit experts)
As was I . Also offering some advice to the OP about dealing with the ST length.

The spec I find the most entertaining is ST angle. You could take two bikes with different angles and lengths, set your seat post height appropriately, and move the saddle forward or back on the rails to get your "ideal" saddle-crank relative position, and end up with the exact same fit. Sure the rear triangle stiffness might be different but that's much more subtle than fit. It's sort like cutting off one end of a blanket and sewing it on the other end, then sliding the blanket toward the cut end.
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Old 02-03-16, 06:35 AM
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Originally Posted by jimmuller
It's sort like cutting off one end of a blanket and sewing it on the other end, then sliding the blanket toward the cut end.
********** I'm going back to bed to think about that one a while.

I seem to have found a fit that is real comfortable - all day 106 mile comfortable - so I keep using that on each bike. I made a 'story stick' to capture two main dimensions. A wooden dowel sticks out one end. The dowel rests on the sit bones contact point of the saddle and the other end of the stick rests on the right pedal axle. That sets the seat height. There is also a mark on the stick that I use to gauge the reach from the sit bones contact point to the brake hoods. I use this story stick to set seat height and set brake hood location when I build a new bike (like my recent salt bike build) or to reset the seat height if I need to pull the post for some reason.

I have a second stick for seat fore-aft positioning. Simple stick that rests across the saddle at the sit bones contact point and has a plumb-bob hanging off each side. I can use that to measure the saddle position relative to the BB spindle. I really don't worry about that one much but I can measure it easily.
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Old 02-03-16, 07:37 PM
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Originally Posted by jimmuller
As was I . Also offering some advice to the OP about dealing with the ST length.

The spec I find the most entertaining is ST angle. You could take two bikes with different angles and lengths, set your seat post height appropriately, and move the saddle forward or back on the rails to get your "ideal" saddle-crank relative position, and end up with the exact same fit. Sure the rear triangle stiffness might be different but that's much more subtle than fit. It's sort like cutting off one end of a blanket and sewing it on the other end, then sliding the blanket toward the cut end.
I actually have to partially disagree with you on this....I have 3 modern bikes that all have 74+deg seat tube angles due to their smaller sizes, and I find that even with the saddle slammed all the way back, I've still sometimes wished for more setback. Two of the bikes have the Velo Orange extra-long setback seatposts...partly to compensate for saddles with the adjustable part of the rails being closer to the back of the saddle, and partly to compensate for the steep seat tube angle. On the other hand, I have a vintage frame that I believe has a 72deg STA, and using a saddle that's basically identical to one that I have on one of the other bikes, I don't have to slam it back so far...and that's actually with a zero-setback seatpost.
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Old 02-03-16, 08:32 PM
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Originally Posted by agmetal
I actually have to partially disagree with you on this....
It sounds like the relative position between saddle and BB (or pedals) must be very different between those bikes. Perhaps your preferred riding position on each is also dictated by the bar position

Regardless, if you have hit the limit of any particular movement then the other pertinent dimension will matter.

My point was this - for a 57cm ST, the difference between 72deg and 74deg ST angle is only 1.9cm fore and aft. That is within the adjustment range of where many of us keep our saddles. As long as you don't use up all the range before you get it where you want it then you can always move the saddle and make both 72 and 74deg bikes give you the same position over the crank.
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Old 02-04-16, 12:49 AM
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Originally Posted by verktyg
I read this cover to cover several times back in the early 70's before I realized that it was written for budding Italian adolescents to form them into Olympians!

Around the age of 18 to 22 the human body starts to loose the ability to adapt to unnatural positions like ballet and bike racing! I'd just turned 30!
I think there is some truth to this. I started riding really young, going on 20 mile rides when I was 10 or 11, so I think my body may have been shaped by cycling to some degree. Not that I became an olympian. (though I have ridden with a couple).

Back to the OP, the classic sizing method was to stand over a bike, and be able to lift it between 1 and 1.5 inches from the ground when pulled up (both wheels) till it was stopped by your crotch. The older school 70s rule was more like 1", and in the 80s it became fashionable to ride a smaller frame, so it was more like 1.5 to 2", for racing at least.

With sloping top tubes, it's all different obviously.
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Old 02-04-16, 11:29 AM
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Unless one has neck issues, I feel that it is always worth exploring what the upper limit is in terms of the forward reach to the bars, with saddle moved forward as needed for a comfortable reach distance while riding seated, in the drops.

I went through a coupe of iterations of fitting to arrive at my preferred, more-forward positioning, as my upper body musculature adapted to more "aggressive" or "aero" positioning, and I feel that it makes it much easier and more comfortable for me to ride at a faster pace.
It was worth my time and effort to overdo it even, just to see at what point that my ride time might be cut short due to having more weight bearing on my arms. Again though, if one rides frequently enough, the adaptation that occurs in the arms/shoulders is sustained between rides!

The two additional benefits to a more-forward positioning are 1) that a more-aero position is maintained with less bending at the rider's waist, which assists with comfortably sustaining power, and 2) that one can more effortlessly transition between seated and standing positioning with less pause of and wasted power while heaving oneself out of the saddle, and with much less stress on the knees while doing so. The more-frequent transitions enhance comfort of the entire body, and really quicken the power response to changing terrain. I can even use a taller low gear since leaning further forward while out of the saddle allows my legs to straighten more fully with resultant reduced bending angle of the knees.

That's my belief on how a road bike's configuration can best be exploited to the benefit of the rider, and I've learned this only in my 50's as I was starting to slow down.
Note that I've had to alter my saddle tilt to match the effective forward rotation of my whole body about the bottom bracket. Also, that I position the bar tops only about 3" below the saddle top even after my forward readjustment, which has me favoring somewhat larger frame sizes.

Lastly, if one is adapting their existing bike to such a forward repositioning, a longer stem length will have the bike becoming more stable at speed, and a zero-offset seatpost might start making sense as well. Bikes with very shallow headtube angles do not handle so well with very long stem extensions such as greater than 10cm, so may require a wider handlebar to control steering while riding off of the saddle.

The 59cm criterium bike below (that I picked up off Craigslist as a "financial opportunity" purchase) seemed quite unlikely to provide a comfortable fit for this 5'9" rider, but even with the same stem that it came with, I did a 3-hours-long group ride on it yesterday, over undulating and uneven foothills terrain, in complete comfort. Really surprised me, as the longer stem even made for a stable steering feel at all times, even with a 75-degree headtube angle!
Note the adjustment configuration as I discussed above, and that the saddle used has tall, soft padding in the nose area:


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Old 02-05-16, 06:17 PM
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Originally Posted by jimmuller
As was I . Also offering some advice to the OP about dealing with the ST length.

The spec I find the most entertaining is ST angle. You could take two bikes with different angles and lengths, set your seat post height appropriately, and move the saddle forward or back on the rails to get your "ideal" saddle-crank relative position, and end up with the exact same fit. Sure the rear triangle stiffness might be different but that's much more subtle than fit. It's sort like cutting off one end of a blanket and sewing it on the other end, then sliding the blanket toward the cut end.
I agree its amusing unless you like or need a lot of saddle setback. Then you have a trade off between very long seatpost setback and very long saddle rails versus a very laid-back seat tube (70 to 72 degrees?). The saddle is much more sturdily held in such a position with a laid back seat tube than with a 74 to 75.5 degree seat tube. And the rider reach to the handlebars in all those cases with a (for example) 54 cm TT will be very different.

Frame specification based on stack, reach, and seat tube angle really makes a lot of sense to me, even for '70s road racers.

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Old 02-05-16, 08:51 PM
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The only problem that I have with published "reach" dimensions (aside from my unfamiliarity with the numbers) is that the reach changes depending on the reference height at which "reach" is measured.

So, when comparing different frames that happen to have different "reference" heights (above the bottom bracket), the reach measurements aren't really directly comparable with much accuracy.

So it's back to looking at toptube length and seattube angle, with consideration of how the frame's headtube angle will affect how long of a stem extension length that will give good, neutral handling when riding both in, and out, of the saddle.
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