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Discussion Starter · #1 ·
I found a 1980's Bianchi Randonneur in the largest size 25", that I am building up. It is strange that it has such a short top-tube (57cm) for the 63" seat tube. I would have assumed it would have a 60cm top-tube in that size; especially since the smaller size 23" frame has the exact same 57cm top-tube.


In order to get it to fit me I plan on putting on a 130mm stem instead of my typical 100mm stem. Does anyone know whether this will adversely effect the handling?

Mine has Tange stickers on the front fork (that has the B stamp), but none on the downtube (it's peeled off). Also it is dark green, which is the only one of that color that I could find online.


From the 1983 catalog: bulgier.net - /pics/bike/Catalogs/
 

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You first need to determine where the saddle will go with respect to the BB when properly fit, then determine what stem length will be best. A 130mm stem will add a bit more tiller effect, but not consequentially so.
 

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You first need to determine where the saddle will go with respect to the BB when properly fit, then determine what stem length will be best. A 130mm stem will add a bit more tiller effect, but not consequentially so.
^ This. are you doing 130 with a 0 degree offset seat post? Could try something with 25mm offset and a 110mm stem.
 

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Wait until you can sit on it before you worry about stem length. Note that the seat tube angle is 72 thus the seat tube places the seat further back than bikes you may be used to (I'm guessing). So while the top tube measurement is long it might not be effectively that long depending where you place the seat.
 

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This ^.

The chart you posted doesn't show the 25" frame you mention (typo?), so this is just a general statement to confirm Jay's post: You can't make a judgement about top tube length without knowing seat tube angle and the resulting frame offset. With the saddle in the same position over the bottom bracket, shallow seat tube angles reduce reach; steep seat tube angles lengthen reach.
 

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If you're currently riding another bike that fits you, measure the reach on that, top tube + stem. That will give you an idea of what you need for the new bike.

I am weirdly constructed with a long trunk compared to my legs, so most of my bikes have 130 mm stems and one with a 140 mm. The handling is fine.
 

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As said, until you get your saddle setback set, and that may require a different seatpost, then stem size is not a discussion.
 

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The more I think about it, the more I'm of the opinion that a 63x57 is way off anything with a flat top tube I have seen so far, specially when the smaller size at 58x57 is where one would expect it to be.
I would wonder what HT and ST angles a 63x57 would have before I looked at the longer stem reach.
 

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Set your contact points using whichever stem you need and take it for a spin. However the stem length affects the handling will disappear after a ride or two, as you grow comfortable on the bike.

Are you going to set it up as a rando bike, with a front bag? If so you may not mind a shorter stem. In fact, if you don't have the longer stem in your inventory you could try a shorter stem that you already have and go from there. You can always change out the stem and that's easier if you don't have to buy the trial stem. Less stretched out for comfort, and all that.

Anyway, what about the bike? 700c or 27" wheels? Probably not 650b, or is it? Is it low trail and does it have canti brakes? You gonna mount fenders and lights?
 

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Discussion Starter · #10 ·
Yeah, that is what is strange. If they kept the same seat-tube angle and head-tube angle (as in the 23" geometry diagram), but extended the seat-tupe length from 23" to 25" than the top-tube would need to be longer. But it measures exactly 57cm center to center on my 25" frame.

Dark basement, in-progress, photo - with the 130mm stem:
 

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Discussion Starter · #11 ·
Set your contact points using whichever stem you need and take it for a spin. However the stem length affects the handling will disappear after a ride or two, as you grow comfortable on the bike.

Are you going to set it up as a rando bike, with a front bag? If so you may not mind a shorter stem. In fact, if you don't have the longer stem in your inventory you could try a shorter stem that you already have and go from there. You can always change out the stem and that's easier if you don't have to buy the trial stem. Less stretched out for comfort, and all that.

Anyway, what about the bike? 700c or 27" wheels? Probably not 650b, or is it? Is it low trail and does it have canti brakes? You gonna mount fenders and lights?
I will probably use a rear rack and fenders. It will be a commuter bike and a gravel ride bike for 25-75 mile rides.
 

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I will probably use a rear rack and fenders. It will be a commuter bike and a gravel ride bike for 25-75 mile rides.
I just saw the pic of the bike that you posted, and it looks like it was built to be sat bolt up-right. It should work out fine for that use, enough room for larger volume tires?
 

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The slack 72 degree seat tube makes for a shorter effective reach than a steeper seat tube, because the saddle will be moved back less to get the same position relative to the BB.

It's not unusual for very tall bike sizes to have top tubes that are barely longer than the next smaller size, because tall men have proportionally longer legs.
 

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Discussion Starter · #14 · (Edited)
Looks like the head tube also increased in length along with the seat tube. Since the HA and SA are the same and the TT stays parallel with the ground then the TT length would not change for different seat tube lengths.
Thanks, now that makes sense: MATH!
 
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