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Top Tube, Stem Length, Fork Offset & Handling

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Top Tube, Stem Length, Fork Offset & Handling

Old 08-16-22, 04:13 AM
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branko_76 
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Top Tube, Stem Length, Fork Offset & Handling

Several years ago I found a 1980 Miyata 912 that had been highly modified by the previous owner, the best way to describe it would be that he had "middle-aged" it. I bought it to restore it back to its original light-weight, fast-paced and nimble handling characteristics. The last time I had ridden a 912 was in 1990. After restoring it and riding it for a couple of seasons, I now prefer it to my older Falcon that had been my main bike for almost 20 years.

The Miyata is faster and much more nimble than the Falcon. It has a tighter wheelbase, beefier seat stays and less fork offset. Despite the stiffer frame geometry, I find the Miyata more comfortable to ride. After taking several measurements to find out why this is, I think the main reason is the length of the top tube which is a full inch shorter than the Falcon.

Before I undo the cloth handle bar tape I meticulously installed on the Falcon to swap out the stem for a shorter one, I thought I'd ask for opinions on the pro's and con's of :

"short top tube / long stem"
vs
"long top tube / short stem"

also, is there an optimal relationship between stem length and fork offset?


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Old 08-16-22, 06:39 AM
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Changing stem length changes the moment arm between hand position and the tyre contact patch; so A+B may equal C+D and yet feel quite different.

However, the only way to know is to get the spanners out and ride it and decide. It's an ergonomics question you are asking at the atomic level; you're the atom.
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Old 08-16-22, 07:20 AM
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Richard Sachs at one point wrote the target was an 11 cm stem. That is not very specific as it leaves out handlebar shape which can vary quite a bit, some bar shapes have more forward “throw’.

Falcons were known to have long top tubes, 22.5” ctc or more would not surprise me.
Ernie Clements had definite views on things.

to get a best idea, align the saddle to the crank position, not just by measure of the saddles (unless the same make / model) , with an assistant compare at 9 o’clock crank position a plumb bob dropped from you knee with a line that passes by the pedal, when those are the same, it will allow you to know you are matched. Then measure pelvis to bar center and to hand position with you “on the hoods”.

in general though, an 11- 11.5 cm stem seems to yield comfortable control of the steering. Not all can achieve this and still have a good position.

you might also compare the trail numbers of the two bikes. A chance that the falcon has less trail, even with a more relaxed steering angle on the Falcon.
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Old 08-16-22, 07:26 AM
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There are all kinds of rules of thumb you can use to determine approx. stem length. I never measured my TT length--the frame fits me according to the seat tube length, so since it's not a custom frame, the TT length is sort of out of my control. I prefer to be a little stretched out, so I have a 120 mm stem.
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Old 08-16-22, 09:16 AM
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I Can't help you climb out of your own private rabbit hole but when I slip back into mine I use this method to measure my bikes:
Back the rear tire against a wall and measure the horizontal distance to the wall of the ST-TT intersection, saddle center, BB center, Front axle, HT-TT intersection, Handle bar tops, Hoods.
Also measure the vertical distance of these same parts.
To compare bikes, use the same wheels and tires.
Have Fun.
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Old 08-16-22, 09:21 AM
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Originally Posted by smd4 View Post
There are all kinds of rules of thumb you can use to determine approx. stem length. I never measured my TT length--the frame fits me according to the seat tube length, so since it's not a custom frame, the TT length is sort of out of my control. I prefer to be a little stretched out, so I have a 120 mm stem.
setback and top tube length do suggest a position on the bike. To ignore them is weird.
you are correct "as built " is just that, you do not have control, but it should influence a decision to buy, hold or sell.
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Old 08-16-22, 09:26 AM
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Originally Posted by repechage View Post
setback and top tube length do suggest a position on the bike. To ignore them is weird.
you are correct "as built " is just that, you do not have control, but it should influence a decision to buy, hold or sell.
I don't recall ever considering TT length in sizing a bike back in any shop I ever worked for. Stem length was used to compensate for any perceived shortness or longness. Weird for you. Works perfectly for me.

Be that as it may, the bike has been bought, being held, and most definitely not being sold.
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Old 08-16-22, 09:33 AM
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Middle aged it... I resemble that comment. As a guy who owns a 1980 912 and a straight guage falcon I feel compelled to give you my totally uninformed comments. First I was kind of puzzled by how short the top tube is on the 912, but it is comfortable. Of course I often run porteur bars so everything is comfortable to me. The thing that really surprised me about those two bikes is that I preferred the falcon. Both climbed well, both took pretty well to 650b when I tried that but with 700c the falcon was the clear winner for it's greater room for tires. The Miyata always built up a touch lighter and that short top tube was nice on my arthritic neck. Now back to your original question.... I have no idea...
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Old 08-16-22, 10:09 AM
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Originally Posted by 52telecaster View Post
Middle aged it... I resemble that comment. As a guy who owns a 1980 912 and a straight guage falcon I feel compelled to give you my totally uninformed comments. First I was kind of puzzled by how short the top tube is on the 912, but it is comfortable. Of course I often run porteur bars so everything is comfortable to me. The thing that really surprised me about those two bikes is that I preferred the falcon. Both climbed well, both took pretty well to 650b when I tried that but with 700c the falcon was the clear winner for it's greater room for tires. The Miyata always built up a touch lighter and that short top tube was nice on my arthritic neck. Now back to your original question.... I have no idea...
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Old 08-17-22, 10:28 AM
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Maybe I'm wrong, but isn't "hoods directly over the front axle" considered a good rule of thumb for relatively neutral handling? That is, neither too twitchy or dead for a given bike's front end geometry.

I'm curious about this thread. A recent acquisition made me realize that I prefer bikes with a saddle to hoods reach 2 inches shorter than I usually ride. To change my other bike to match would require an 80mm stem and short reach handlebars, a decrease of 50mm or about 2 inches. I would like to plunk down the cash but am worried about twitchy handling...perhaps it would be cheaper and healthier simply to stretch and strengthen my core/back. But there's only so much you can force your body into!
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Old 08-17-22, 10:33 AM
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Originally Posted by Piff View Post
Maybe I'm wrong, but isn't "hoods directly over the front axle" considered a good rule of thumb for relatively neutral handling? That is, neither too twitchy or dead for a given bike's front end geometry.

I'm curious about this thread. A recent acquisition made me realize that I prefer bikes with a saddle to hoods reach 2 inches shorter than I usually ride. To change my other bike to match would require an 80mm stem and short reach handlebars, a decrease of 50mm or about 2 inches. I would like to plunk down the cash but am worried about twitchy handling...perhaps it would be cheaper and healthier simply to stretch and strengthen my core/back. But there's only so much you can force your body into!
Being old and fairly inflexible I can tell you the twitch is no big deal. 10 miles and you won't even know it's twitchy. It's amazing what you get used too. It just starts to feel really good.
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Old 08-17-22, 12:07 PM
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It can be quite a rabbit hole. I'm pretty non-scientific.

If I had a bike that I'd ridden for 20 years pain free, especially knees, I duplicate the saddle position on the new bike. Personally I use the back of the saddle and the BB with a level at 90 degrees. I will double check back of the saddle to pedal at furthest point of rotation.

Then it is just about measuring to the center of the handlebar tops. That basically gets you to the difference to make up in part or whole.

I would think that there is a range of stem lengths, where the change is hardly, or not even, perceptible to most people, but can be used to get you in a comfortable position.

The stem length/bar width can be baffling sometimes, and I tend to think weight distribution might be a bigger factor. I put a flat bar on a road bike that was always a little sluggish and now it zips around. In theory going from 40cm to a 60cm bar would slow things down. I ride more more upright on that bike. And handlebars don't turn that much to corner.

John
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Old 08-17-22, 01:04 PM
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I'm well beyond "middle-age" (unless you know anyone 124 years old...), but I know of the kind of bike of which the OP speaks. I refer to such bikes as "geezer-fied", and alas, it seems to be an epidemic in my area. My "WTF am I gonna do with this" pile is overflowing with upright bars/stems removed from once very nimble roadies, now waiting to be set free once again.

As for long+short vs short+long, the elephant in the room is body proportion. Neither is "better", but one is "better for you", based on which one favors your proportion of leg length vs. torso length. There are, of course, many other nuances/factors, such as level of flexibility, arm length, etc., but overall, leg/torso proportion is a good place to start when thinking of ST/TT proportion.
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Old 08-17-22, 01:14 PM
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Originally Posted by madpogue View Post
I'm well beyond "middle-age" (unless you know anyone 124 years old...), but I know of the kind of bike of which the OP speaks. I refer to such bikes as "geezer-fied", and alas, it seems to be an epidemic in my area. My "WTF am I gonna do with this" pile is overflowing with upright bars/stems removed from once very nimble roadies, now waiting to be set free once again.

As for long+short vs short+long, the elephant in the room is body proportion. Neither is "better", but one is "better for you", based on which one favors your proportion of leg length vs. torso length. There are, of course, many other nuances/factors, such as level of flexibility, arm length, etc., but overall, leg/torso proportion is a good place to start when thinking of ST/TT proportion.
Any tall stems u don't need I will gladly take.
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