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Top tube length and stem length: Does the proportion matter?

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Top tube length and stem length: Does the proportion matter?

Old 09-28-06, 01:20 PM
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Top tube length and stem length: Does the proportion matter?

So, just for giggles, I was using Competitive Cyclist's online fit calculator to determine what size my next frame would be, and I came across some interesting findings. Everything, interestingly, was spot-on with my current fit, except that it says my saddle should be 5mm back from where it is now. Easy enough to try, I guess...

Here's the interesting part. CC's fit calculator says I should have a 57.0-57.4 cm top tube and a 12.1-12.8cm stem. My current fit is with a 59.5cm top tube and a 100mm stem.

So, at the risk of throwing an inane question out there, does that make a difference? When you add top tube and stem length together you get almost the same measurement, but is there any advantage to a shorter top tube and a longer stem?
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Old 09-28-06, 01:30 PM
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Originally Posted by FarHorizon
Technically, yes. As I understand it, what fit gives the best power transfer is the one that positions your body best in relation to the bottom bracket. After that, the stem length fits your torso to the bike.

If your body isn't in the proper position in relation to the pedals, then having the correct "reach" from your seat to the bars is irrelavant. From what you describe with your current fit, you've got the opposite - your reach is right, but your position to the pedals is off.

Try the modification on seat position, live with the slight difference in reach, and see how it feels.
Other measurements, though, like the saddle to BB setback, were also OK, other than moving the saddle back.

I'm just curious what difference I might notice if I go to a frame with a shorter top tube/longer stem for my next bike.
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Old 09-28-06, 01:46 PM
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With most frames, I believe the smaller size will place the head tube lower, relative to your saddle. Therefore, you will either need more spacers or have to put up with a bigger drop to the top of the handlebars.
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Old 09-28-06, 01:55 PM
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Originally Posted by botto
only thing i can think of is 1. you can go for a smaller/lighter frame and 2. a longer/more responsive (aka twitchy) stem length.

i'm sure there's someone out there who actually knows the ins & outs of bike fit MUCH more than i do.
I always thought shorter stems were theoretically more twitchy. The shorter the stem, the less movement of the handlebar it takes to move the wheel a given degree, correct? The longer lever takes less force to move, but has to move farther. the shorter lever takes more force to move but doesn't have to move as far for the same effect.
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Old 09-28-06, 02:02 PM
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The other difference it will make to handling is that it'll move your center of gravity fore/aft relative to the bike's wheelbase.
i.e. with a shorter TT and longer stem your weight will be more over the front wheel.

i'll leave it up to someone more knowledgeable to tell you what this actually means
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Old 09-28-06, 02:02 PM
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It's interesting. Right now, my head tube length is 18cm and I have one 10mm spacer under my stem, which is flipped down.

The BMC Pro Machine, for example, in a 57, has a 57.5cm top tube and a slightly longer head tube, 18.7cm. So I could keep the same saddle-to-bar height and go with no spacers... woohoo!

Guess I'll just have to take the plunge at some point...
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Old 09-28-06, 02:06 PM
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On a practical level, I just changed my stem from 120mm to 100mm. (In hindsight I probably would have gone to 110) I really didn't notice much difference in handling. perhaps just a touch quicker when I first got on it. but within a few miles I wasn't aware of any difference. I don't thinkit effects handling significantly unless you're talking ultrashort. Biggest downside is the bike looked better with a longer stem.
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Old 09-28-06, 02:09 PM
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Originally Posted by DrPete
It's interesting. Right now, my head tube length is 18cm and I have one 10mm spacer under my stem, which is flipped down.

The BMC Pro Machine, for example, in a 57, has a 57.5cm top tube and a slightly longer head tube, 18.7cm. So I could keep the same saddle-to-bar height and go with no spacers... woohoo!

Guess I'll just have to take the plunge at some point...

don't forget you also have to take into account the stack height of your headset.
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Old 09-28-06, 02:20 PM
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Originally Posted by baxtefer
don't forget you also have to take into account the stack height of your headset.
All the frames I'm looking at have FSA internal headsets.
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Old 09-28-06, 02:30 PM
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I'm going crazy trying to get my new bike dial in. My main problem is my Cannondale was fitted and I never had any problems. Now I'm reviewing all the fit info, my Cannondale had the seat all the way forward and a 140mm stem. The fitting was long enough ago I forget most of the rational, but I'm a fair bit forward of KOPS. I'm also trying to work with a 12 degree stem, but get too much drop if I flip it, so I guess I'll lose some OCP points. I might have been able to go one size larger, but I think I just have some freaky measurements. I'm doing CC fit tonight after I get everything measured.

Sorry not much help, but needed to vent on the fit game a little...

John
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Old 09-28-06, 02:38 PM
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Pete,

When I look at TT length vs stem length, I try to pick a combo that will put the hoods (my fav climbing position while standing) vertically above, and in line with the front hub. When I deviate from this spot, it makes the bike feel like it's trying to steer itself when climbing. You can try it by trying to climb out of the saddle while holding onto the tops. If you try to steer, it'll feel like you're steering a yacht. But it'll steer fine from the hoods because you're closer to that plumb line over the hub.

I know what my overall reach should be, and if I can figure out what length stem puts my hands over the hub while on the hoods (for that HT angle and fork rake combo) then I can figure out my TT length.

Hope that made some sense.
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Old 09-28-06, 02:38 PM
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Originally Posted by R900
Sorry not much help, but needed to vent on the fit game a little...

John
I'm right there with ya. It's totally frustrating.
Within the next month I'm hoping to actually have the funds to buy my dream bike, and I've already sort of resigned myself to the fact that I'll probably have to buy it sight unseen and ride unridden.

Since I'm looking at frames that really aren't stocked anywhere locally (Pro Machine, Orca, Paris Carbon, Soloist) it's a tough process.

At least places like CC have a 10% restocking fee, which is less than new bike depreciation... Either way, if I actually have the cash to go for it I'll be quite happy, because deciding between the frames I listed is a hell of a great "problem" to have.

I'm keeping my fingers crossed...
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Old 09-28-06, 03:04 PM
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xx

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Old 09-28-06, 03:51 PM
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Originally Posted by cydewaze
Pete,

When I look at TT length vs stem length, I try to pick a combo that will put the hoods (my fav climbing position while standing) vertically above, and in line with the front hub. When I deviate from this spot, it makes the bike feel like it's trying to steer itself when climbing. You can try it by trying to climb out of the saddle while holding onto the tops. If you try to steer, it'll feel like you're steering a yacht. But it'll steer fine from the hoods because you're closer to that plumb line over the hub.


Hope that made some sense.
+1

This is my understanding of the situation. I'm using a short stem myself and my handlebars are just slightly behind the ideal spot and its perfectly rideable but you end up swaying a little out of the saddle. I've recently fitted a stem 10 mm longer and it has stabilised the bike slightly when out of the saddle.

Regards, Anthony
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Old 09-28-06, 04:16 PM
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Originally Posted by AnthonyG
+1

This is my understanding of the situation. I'm using a short stem myself and my handlebars are just slightly behind the ideal spot and its perfectly rideable but you end up swaying a little out of the saddle. I've recently fitted a stem 10 mm longer and it has stabilised the bike slightly when out of the saddle.

Regards, Anthony
Huh... interesting. I'd never even thought about that!

Armed with this info, I decided to see where my hoods are on my Flyte right now, and the leading edge of the bar along the curve of the drop is directly above my front hub. All in all, I've been happy with the handling/cornering of my Flyte.

I'm interested in the whole concept of positioning the center of gravity relative to the wheelbase and what effect that has on the ride. Guess I'll just have to try to find bikes with similar geometry and give them a shot. I was hoping that my LBS down the street (an Orbea dealer) would get some Orcas in stock, but they said they're too expensive to just have around (low demand), so even if I order an 07 Orca from them it would be a special order. Could probably find a similar-geometry Orbea to test ride, though.

And meanwhile I'm praying for the finances to work out right... I like to think I would have earned it. I'm learning that the capability for rational thought completely leaves the bride about 10-14 days before the wedding day, and that any problems related to the wedding then become a result of the groom's incompetence/unwillingness to help/etc.

Dammit, Jim, I'm a surgeon, not a psychiatrist!
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Old 09-28-06, 04:21 PM
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Originally Posted by DrPete
Huh... interesting. I'd never even thought about that!

Armed with this info, I decided to see where my hoods are on my Flyte right now, and the leading edge of the bar along the curve of the drop is directly above my front hub. All in all, I've been happy with the handling/cornering of my Flyte.

I'm interested in the whole concept of positioning the center of gravity relative to the wheelbase and what effect that has on the ride. Guess I'll just have to try to find bikes with similar geometry and give them a shot. I was hoping that my LBS down the street (an Orbea dealer) would get some Orcas in stock, but they said they're too expensive to just have around (low demand), so even if I order an 07 Orca from them it would be a special order. Could probably find a similar-geometry Orbea to test ride, though.

And meanwhile I'm praying for the finances to work out right... I like to think I would have earned it. I'm learning that the capability for rational thought completely leaves the bride about 10-14 days before the wedding day, and that any problems related to the wedding then become a result of the groom's incompetence/unwillingness to help/etc.

Dammit, Jim, I'm a surgeon, not a psychiatrist!
Everything else aside, I think you've totally earned whatever bike you decide to pick up.
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Old 09-28-06, 04:37 PM
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The bigger the bike, the longer the stem. The smaller the bike, the shorter the stem. The below is taken from Hampsten cycles:

small (100-105mm, for frames in the 50-54cm range), medium (110-120mm, for 54.5-57cm range), and large (120-130mm, for 57.5cm and up range).
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Old 09-28-06, 04:58 PM
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I'd say it makes no difference, up to a certain point. e. g., 55cm TT + 110 stem = 54cm TT + 120 stem. But 55 cm TT + 90 stem is not equal to 50 cm TT + 140 stem. The whole frame geometry would change too much.
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Old 09-28-06, 05:13 PM
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I've read that some bikes such as Colnagos are designed to be fitted to the rider with a shorter TT but longer stem, so that the center of gravity is correct on the wheelbase. I've never ridden one myself though. Not sure how this applies to other makes/brands. I expect one would have to test ride them, or get specific fitting info for that brand if one cannot test ride them.
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Old 09-28-06, 05:21 PM
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Originally Posted by DrPete
All the frames I'm looking at have FSA internal headsets.
FSA Orbit headsets can come with different cone spacers on the top. Stock FSA headset that came with my bike had a 15mm spacer. I swapped it out for another FSA with better bearings which came with an 8mm spacer. Might not make a huge difference for you, but something to keep in mind because some bikes (the Roubaix comes to mind) with a huge 20mm cone spacer.

Have you considered the Canyon F10? Stiffer than the Cervelo R3 and the frameset can be had for about 999 Euro (about $1400 US) to your door from Germany with a valid racing license. They're also clearing out '06 models, so you could call or email them and see what sizes they have left.

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Old 09-28-06, 05:50 PM
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100mm stem is pretty short, I have a 110 on the same size frame (59) and I am considering a shorter top tube and longer stem

I think a 130 on a 58 would be ideal for me, but I still have some flexibility to work on--so it's not happening too soon

I wouldn't sweat it, but it's something to keep in mind when your next purchase comes around
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Old 09-28-06, 05:57 PM
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Originally Posted by baxtefer
The other difference it will make to handling is that it'll move your center of gravity fore/aft relative to the bike's wheelbase.
i.e. with a shorter TT and longer stem your weight will be more over the front wheel.

...

^^^what the wicked retahhded guy said
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Old 09-28-06, 06:00 PM
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I was told by a mail-order supplier that 90-100 mm stems were the two most frequently purchased sizes for persons that are dialing in their bikes for proper fit, so, I don't see that sizing as unusual. I think that the only time you'd have a problem is with a teeeeny stem length combined with an extremely steep fork tube angle and forks with no rake, e.g., a tricycle doing about 10 mph, because just small movements at the grips would translate into much greate turning angles.
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Old 09-28-06, 06:04 PM
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Originally Posted by DrPete
I'm learning that the capability for rational thought completely leaves the bride about 10-14 days before the wedding day, and that any problems related to the wedding then become a result of the groom's incompetence/unwillingness to help/etc.
Just wait! Talk to your OB/GYN buds about what happens during childbirth.
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Old 09-28-06, 06:15 PM
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Originally Posted by wagathon
I was told by a mail-order supplier that 90-100 mm stems were the two most frequently purchased sizes...
and this proves what?

btw, the sign outside mcdonalds says "100 million hamburgers" sold


Originally Posted by wagathon
I think that the only time you'd have a problem is with a teeeeny stem length combined with an extremely steep fork tube angle and forks with no rake, e.g., a tricycle doing about 10 mph, because just small movements at the grips would translate into much greate turning angles.
you're talking about handling, a long stem has to do with weight distribution/"center-of-gravity" (and yes, I do realize a SHORT stem can produce twitchy handling), but the OP hasn't mentioned this--he wanted to know why a shorter top tube with a longer stem was suggested
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