shoota 

Current bike: 55.5cm ETT, 100mm stem, fits fine.

Frame on the way: 56.5cm ETT

I just assumed I could get a 90mm stem to make up for the 1cm longer TT. However, I was just told that the new frame is too big for me and that I could put down more power on the size down which is 55 ETT.

So what do you guys think?

RollCNY

IMO, frame size has everything to do with comfort and little to do with power output.

pdedes

I'd pick the smaller frame.

shoota 

damn..too late for picking a different frame, this one is already on its way.

I should mention that the old frame HT angle is 73.5 and the new one is 73 so I think that means the reach of the new one should be 5mm more.

fa63

No, the head tube angle doesn't make that much difference in reach. Plus if the new frame has a shallower HT angle, then the reach would be less, not more.

Can you tell us what the old and the new frame are?

fa63

By the way, you won't put out more power on a smaller frame. You might go faster if you can get yourself positioned in a more aerodynamic way on the smaller frame, but on the other hand, you might actually put out less power if you are unable to sustain the low position comfortably for any extended period.

dalava

If you are OCD like me, you are totally screwed... this idea is already in your head and no matter what the frame will feel too big for you

shoota 

Actually the old frame (Cannondale six13) has a shorter headtube I believe than my new frame which is a 2012 Scott Foil.

Ah don't say that. I think I could have made either a 54 or a 56 work. With my short arms the 54 would probably be nice but I guess on the 56 I'll just have to get a 90mm stem and slam it

Jed19

Yeah, but being comfortable on your frame is a sine qua non for laying down power consistently.

Jed19

OP, you can make the new bike work. I have done exactly what you are contemplating with one of my bikes. I started out using a 90mm stem to get my preferred fit, but after a while (meanwhile constantly working on my flexibility), I put on a 100mm stem on there, and no issues to report.

pdedes

interesting that you pose the question after you've already made your purchase.

BigJeff

Foils are fast and pretty.

Great choice. I once tried to buy one on eBay but it was a scam and the seller faked shipping the bike. It took weeks to get my money back through PayPal.

Drew Eckhardt 

As long as you can get the same position it doesn't matter - you'll make the same power either way. In most cases either of two consecutive sizes will work fine.

If you need a very high position, the shorter head tube on small frame might preclude using enough spacers.

If you need a very low position like professional cyclists, the large frame may limit stem options due to the extreme negative rise needed.

Seat tubes get steeper on smaller frames; too steep plus short saddle rails can make it more difficult to find a seat post allowing enough setback.

rpenmanparker 

Actually the larger frame is better for you, because most stems only come in whole sizes. So the 90 mm stem will fix everything. You would have been 5 mm off either high or low with the shorter top tube.

Of course this is all hypothetical. If the frames are not the same brand and model, the numbers just give you an approximate fit idea. You don't know exactly how the dimensions were measured. Don't worry about it until you get it all put together, then do what you need to do to make it right. Be sure to have full measurements from your old bike before you start. Make six measurements with the bike perpendicular to a wall, perfectly upright and the inflated rear tire touching the wall: (A) wall to center of the bottom bracket horizontally, (B) floor to center of the bottom bracket vertically, (C) wall to front tip of the saddle horizontally, (D) floor to bottom of the front tip of the saddle vertically, (E) wall to back of the bars horizontally and (F) floor to top of the bars vertically. With these measurements you can get a near perfect match between the two bikes in saddle setback (C-A), saddle height from the BB (D-B), bar height (F-B), saddle to bar drop (f-D), etc. If you want the saddle top height, substitute that for the saddle front bottom. But everything is relative, so it really doesn't make much difference, and the saddle tip bottom is easier to measure precisely and accurately.

DOS

I would be more concerned about power loss on a too small frame. Assuming same ST angle, your set up on the new frame should work fine.

shoota 

Yeah... well I wasn't seconding guessing it at all until my buddy told me it was too big. But I still think this size is a fine choice.

Fortunately I fall between "high" position and "pro" position so I'm guessing a slammed stem will be in order. And I don't mind riding in the drops either.

Very good notes, I appreciate that.

I think so too. My current frame just barely worked for me because the head tube is so short that I had to bring the stem all way up to the top of the steerer tube. I'm thinking with the Foil's taller head tube this won't be an issue.

WheresWaldo

There is not as much difference in these two frames as the individual tube sizes would suggest. I see the Foil Stack height is 1 cm lower at the top of the head tube and the Reach is about 4 mm shorter than an old chart on the Six13. So it would appear to me at least, that a 56 cm Scott Foil has a slightly smaller frame than a 56 cm Six13. It is only millimeters here but it really shouldn't be an issue. I am wondering why you thought you would need a smaller stem with the Scott, was it based on the less than well informed opinion of some teen or twenty-something bike shop employee?

Instead of taking all these tube measurements individually, you should focus on Stack & Reach, which are nearly identical (within a few mm) between these two frames. Also note that the BB Drop of a Foil is less than a Six13 it's only 2 mm but that means 2 mm higher COG.

shoota 

Great post thank you. Ok couple points here.
My Cannondale is actutally 55.5. That year had a ton of different sizes and among them was 55.5 and 56. Don't know if that changes anything.
Nope my decision was not based on any bike shop info. (Other than when I got into cycing the trek shop told me 58cm was my size and while it worked ok I much prefer my ~56cm frames.
I wish I understood stack and reach better but admittedly I don't. What is COG?

WheresWaldo

COG = Center of Gravity
Stack & Reach = Stack & Reach Primer: Chapter One - Slowtwitch.com

To know the difference between a 56 and a 55.5 you need to know a bunch of different measurements. Stack and Reach can be either measured, if you physically have the bike, or calculated. If you have your Six13 then follow the diagram posted in the Slowtwitch article to measure Stack & Reach.

In a nutshell, there are 3 contact points on a bike; feet, hands, arse. They can be moved on a bike, but you are who you are, and those dimensions are fixed by physiology. You know where the BB is on a bike, then you know how high the saddle has to be then you also know where your hands will fall, there is no changing that. Now if the Six13 fits well any bike frame with the same or similar Stack and Reach will fit when fitted with the same components. If the reach is longer, a correspondingly shorter stem will put you in exactly the same position. Physical human size vs bike sizing, the human always wins. Same with Stack (for the most part, some anal retentive souls will argue that head tube angle screws this all up, the issue is marginal at best), if the stack of the new bike is 5 mm shorter one more 5 mm spacer will fix it and visa-versa.

This sizing question comes up every six months or so on BF, the answer is always easy: Stack & Reach. But easy is not the BF way, we need a few pages of half truths and junk theory before you get any useful information.

shoota 

Thanks for the info. I've been wanting to learn how to use stack and reach so I might as well use this opportunity to do that. I'm going to read that link and then go measure the six13. Be back in a bit.

shoota 

Ok got it. Those aren't the easiest measurements to take but here they are.

six13:
stack - 560mm
reach - 400mm

Foil (L):
stack - 569mm
reach - 394mm

Those numbers look very promising to me.

rpenmanparker 

That is why I gave you the method using the rear tire against the wall. It allows you to make correlated measurements that yield stack and reach by subtraction. I don't bother calculating frame stack and reach, because I am working with the built up bike, but the methodology is very similar. Just measure from the wall to the center of the top of the head tube and also from the floor to the center of the top of the head tube. When you subtract the BB to wall and BB to floor from those, respectively, you get stack and reach.

WheresWaldo

As they should, so just as a preliminary fit the Foil will be a <1 cm steerer tube spacer taller than the Six13, but I believe you said that the Six13 was at the top of the steerer anyway, so the Foil is a bit taller and would work, IIRC. Six mm shorter reach would mean going from a 100 mm stem to a 105 - 110 mm. That should put you at about the same handlebar position you are now. Again this is with all other things being equal (crank length, handlebar reach and drop, same pedals, same saddle, you get the picture).

shoota 

Thanks man, I really appreciate you taking the time to help out here.

shoota 

Ok so here she is in all her 15.2lbs, harsh riding, stiffness glory.




Took her out for the first real ride, our local shop ride that I'm very familiar with. I felt comfortable but also felt like I didn't quite have the punch that I'm used to. It felt labored at times. It could have been that it was 40F but I didn't feel all that cold. Some of the guys said it's because the geometry is different but if my hands, butt, and feet are all in the same place how could the geo makes that much difference? Is there something I can do fit-wise to simulate the more traditional geo of the Cannondale six13 that I'm used to?