Paultso

So I currently ride a 54 Caad10 and it fits almost perfect. Top tube c-c is 54.5 and seat tube is 54

I'm considering purchasing an older steel frame that has a 56.5 top tube c-c and 56 seat tube

Any idea how things will shake out or am I missing too many inputs for this calculus to work out...

Thanks guys.

Oh yeah, I don't plan on training on this bike, plan to just make it a nice weather, tool-around town machine

Thanks in advance guys!

pdedes

So assuming equal angles, you'll have 2cm less seatpost showing and require a 2 cm shorter stem. Don't know your handlebar drop, but I'm sure you can figure it out.

CenturionIM

Is your 54cm CAAD10 small for you? If it fit great, then you might want to rethink buying this bike. 2 cm in top tube is a sizable change. While it might work, trying to ride a frame that's simply too large is a formula for long term pain.

seat tube fit is easy, just lower your seat post.
To get the top tube fit right, you can do three things (individually or in combination):
1. get shorter stem.
2. move saddle forward
3. Raise handle bar

Nagrom_

You can't change your saddle setback to resolve reach issues. It's mitigating a problem, by starting a new one.

woodcraft

Not that much of an expert, but if it's an older bike with a threaded headset, most of them have pretty long stems, and a short enough stem might be a hassle to come by.

OTOH, if you're putting few miles on it, the fit is not as critical.

marqueemoon

How long a stem do you run on the CAAD10?

LesterOfPuppets

80mm 1" quills with 26mm clamp are actually still widely available. from $12 Kalloys to $61 Nittos. There are a few 25.4 clamps out there too.

If you get one with a 26.4mm bar then you pretty much have to go eBay these days, however.

I'm lucky enough to live near good bike shop territory so scored a used 90mm cinelli with 26.4mm clamp for $20.

I pretty much have a 5cm range of bike sizes I can work with, generally.

thisisbenji

This.

& This.

If you run a 110mm+ stem on the CAAD I think you'd be okay.

If you have to push your saddle forward you could cause knee pain.

fiataccompli

Keep in mind older "classic" geometry typically called for much less seat tube than modern bikes. So, in a *very* general sense, I would not find 56cm in a classic frame terribly shocking compared to 54cm in a modern frame. I can usually find a reasonable fit for myself with bikes ranging from 53-57cm, but as someone said, there are variables that you can not adjust away, so none of that (for me) is true in all cases. If it's cheap enough, try it out & if it still doesn't work, flip it for one that does.

Paultso

thanks for putting it into perspective

Paultso

Currently running a 90mm stem, its short since my stem is slammed

Paultso

exactly, i almost want to give it a shot since this is a mid 90's and don't think the geometry is as aggressive as my caad

thisisbenji

I would disregard seat tube length, I would only consider horizontal top tube length. My Specialized Crux has a 52cm seat tube, and my Allez has a 49cm seat tube. Yet both have 53.7cm top tubes and fit perfect. If your already running a 90mm stem on a 54.5cm top tube I wouldn't get something with anything longer.

ultraman6970

56.5 is too long for the guy. My best guess is that you will end up with a 8 or 7 stem to make it work, if the bikes have the same angles. IF the steel bike has shallower angles then the reach might be the same or close and maybe the 9 stem will work, but IMO like 3 cm in length is too much to make it work. Unless you really dont care at all if the fit is tiny off.

pdedes

If your current ride has you on a 90 mm stem, the shorter stem (70mm) you would need on the bigger bike will compromise handling. I prefer stems in the 110-130 mm range with normal trail for neutral handling.

hamster

This is NOT correct. Or at least it wouldn't be correct if the bikes were from the same family. You're right about the seatpost, wrong about the stem.

You are forgetting that the seat post is not vertical. When you go from 54 to 56 seat tube, the point where the seat tube meets the top tube is moved up and backwards. Assuming equal angles, the switch from 54 to 56 would move the junction up 19 mm and back 6 mm. There's a good chance that angles aren't equal either: bigger bikes generally have more off-vertical seat tubes. Going from 54/74° to 56/73.5° would move the junction up 18 mm and back 10 mm.

Since the top tube is measured from that junction to the top of the head tube, this backwards move "eats" some of the extra top tube length. So, instead of 20 mm you only have to deal with 10-14 mm.

Generally speaking, TT length is a bad metric. It's easy to measure, but it's too confusing and easily misunderstood, even if the top tube is horizontal. It's almost totally worthless when the top tube is slanted. There is a better metric called "reach": horizontal distance between BB and the top of the head tube. It varies a lot less than TT length. For example, on a Trek Madone 5.2, the difference in ETTs between 62 cm and 52 cm frames is 64 mm. The difference in reach is only 19 mm: the rest is eaten by the move of the junction point.

LesterOfPuppets

I second pdede's most recent post. If you're running 90mm stem and have a good fit then you're pretty much at the big end of your useful frame size range.

hamster

You'll have 2cm less seatpost showing, 1-1.5 cm more distance from the tip of the saddle to the base of the stem (for which you compensate by getting a shorter stem), and less clearance for the family jewels. None of these are showstoppers, unless your seatpost is already most of the way in on the 54.

The biggest potential problem is that the 56 will have a higher head tube: the top of the head tube will be 2-2.5 cm higher from the ground than it is on the 54. The best way to compensate for that is to use fewer spacers below the stem. If you don't have any spacers below the stem on the 54, it's probably not a good idea to get the 56, because you'll not be able to get into the same posture (you'll be forced to be slightly more upright.)

LesterOfPuppets

Of course an old bike is likely to have a -17 stem whereas his CAAD10 is probably +/-10 degree. Even with a stubby 90mm stem that 7 degree diff would put the -17 about a cm lower than the -10, assuming CAAD stem is flipped.

hamster

Good point. Possibly even more. Nowadays most stems are +/-6 deg. It's less of a problem, then.

thisisbenji

That's what I was getting at. I ride a 53.5cm top tube with a 110mm stem. My bike actually has a longer reach than his 54.5cm and a 90mm stem. There's no way I'd ride something with a 56.5cm top tube.

Paultso

sigh, ok i guess it just wasn't meant to be. More money in the bank!