Geometry Comparisons
 

I am in the market for a new road bike right now and I am shopping around to see what I want to buy. After a few test rides and thinking that something with a mor relaxed geometry would be more my speed. The Cannondale Synapse is a well known example and I have been on one. A few weeks ago Competitive Cyclist had the BMC Road Racer with the Di2 Ultegra group on sale for $2500 and the price really caught my eye. Since then, I've been mulling a bit over whether should change my mind about what I'm looking for and then I really examined the geometry chart on the BMC. Now I have a question that I hope someone here can help me with.

I have been measured and I look to be a good fit on a 56cm Cannondale.

Here is the geometry of the Cannondale:
http://www.cannondale.com/2013/bikes...mpact-crankset

Here is the geometry on the BMC:
http://www.competitivecyclist.com/fr...012-10666.html

The 57cm version of the BMC seems to have a similar set of measurements to the Synapse although there are some differences. How much of a difference would there be between the fit of both bikes? In my totally amateur intepretation, they look to be similar enough that a shorter stem could almost make them ride the same. Am I way off base? If so, what am I missing?

The reach on the 57cm is close to the 58cm cannondale. BMC's are known for being a size larger than you think too fyi.

This is where I was thinking that a shorter stem might bring the reach back in but still keep the taller head tube that the 57cm size affords. The 54cm BMC geometry seems a touch more aggressive than what I think I'm looking for. That is why I was wondering if the 57cm size with a stem swap to shorten the reach would make the ride similar to a more comfort oriented frame setup. I really like the looks on the BMC and the price for the Di2 model is hard to beat.

How tall are you? I think a 57cm if you are shorter than 6'2" on that bmc is going to be awkward to make fit. On a 54cm you could always get a longer stem and keep it unflipped to make it fit better.

Consider whether you would like the BMC with a huge stack of spacers under a stumpy little stem. Also, though - when you were fit to the c'dale, what was going on with the stem? Was it flipped & slammed, or pointing up and at the top of a spacer stack? If the former, the BMC might work, if the latter, it's probably won't.

Good luck with your search!

Would it be that much different though? The head tube on the C'dale is 18cm compared to 17.5cm on the BMC. Am I underestimating the impact of a 5mm spacer? The difference in reach measurements is less than 1cm as well. Would 10mm less stem look that much different? All the rest of the measurements are within 5mm which is why I started thinking this in the first place. They look to be virtually the same. Do the 5mm discrepancies here and there really make them that different?

You are looking at numbers as the same but all those little differences add up to quite a bit different frame. The BMC fits big and stretches you out a lot. If you aren't flexibile and top tall you are going to have a bad time. I know the di2 is attractive but I think you are chasing parts instead of a frame that fits. If you could fit the 54cm bmc with a longer stem and seat set back a little I'd consider it, but that 57cm is big.

FWIW the BMC is available in 54 if you are really interested. The guy's/Gals above gave great advice. I know this can be complicated with numbers but one thing to consider. If you go with the 54 and it feels a little cramped you can get a longer stem and open it up. If you go with the 57 and it's way to big, then it's way to big. The good thing is CC has free shipping and lifetime returns.

It isn't so much that I'm chasing parts as I could get the BMC with the Di2 in the 54cm size too but the head tube ends up being 3cm shorter in that frame size which was spooking me a bit. The 54cm frame gets smaller than the C'dale in every measurement except reach which made me wonder if it would end up being too smallish since the 56cm Synapse did feel right on a test ride.

It is easier to make a bike bigger and raise the stem then to make a too big of a bike fit. 3cm too short can easily be added on a stem by not flipping it and leaving spacers in it.

Here are the numbers you need to compare - forget head tube and ETT, those are confounded by head/seat angles and BB height.

BMC
Reach 39.5
Stack 57.4

c'dale
Reach 38.6
stack 58.2

so the BMC is both 1.1 cm longer and 0.8 cm lower.

You didn't answer my question about the canondale setup. If the canondale was set up tall (lots of spacers), you are going to need super-lots-of-spacers to get that position. Luckily, raising the stem does make it a little shorter in the total reach to the bars. So, it might work, but if the c'dale was already topped out on the spacer stack, it might just be too many spacers for it to work. Or maybe you are flexible enough to adjust to a lower position than how you were originally fit. Sounds like you were not really fit anyway, though, you just tried it out.

So if it was me, I would probably get a real fit first, and then decide. Or you could get the BMC and then return it if it doesn't fit, which would be kinda lame for CC, but they do offer that return policy, so that's their problem.

what road bike do you have now, and are you happy with how it fits? Normal amounts of spacers & stem, normal saddle position? That would be another data point.

First road bike actually coming from mountain bikes that I ended up logging quite a few road miles on last year hence the switch to road discipline. The Synapse I rode on wasn't slammed on the headset by any means but it wasn't a tower either.

Sounds like the 54cm size is going to be the better choice if I decide to go for the BMC. The worst case scenario is that it comes in and I don't like it and it gets sent back. For the savings it would be worth the trouble if it came down to that. Thanks for the input all.

To compare the reach of two frames, you should correct the reach so both are compared at the SAME stack height. This is simple to do. Just subtract 3mm of reach for each 10mm of stack height difference, from the frame with the shorter stack height. With an 8mm spacer on the BMC, to make the stack height the same, that's makes the reach about 39.2, so it's still about 8mm longer and one size shorter stem would be needed.

While this example is trivial, many people compare frames with a 2cm difference in the stack and don't make the reach correction of 6mm. That's enough to be noticeable.

For those wondering about the amount of correction, it's just the cosine of the HTA times the stack height difference.

I would suggest that you test ride both bikes a couple times, and side-by-side if you can, before you decide. And don't just go by the specs on a datasheet. I very much agree with Christobevii3 that "all those little differences add up to quite a bit different frame", and what will be required on each frame to achieve a proper fit for you can vary greatly as well.

My Synapse is my "relaxed-fit" road bike that I have equipped with a 34t/50t crankset, an 11t/28t cassette, and a bit more upright positioning, and I use it for longer more relaxed rides like centuries or day-touring, and my Salsa Campeon is my "race-fit" road bike that I equipped with a 39t/53t crankset, an 11t/24t cassette, and a bit more aggressive forward-leaning position, and I use it for shorter, faster group rides. On paper they don't look too different, but on the road they are night and day.

Awesome info, thank you!

Excellent info indeed Dave.

Now with all things taken into account, if an 8mm spacer and one size smaller stem is all that is needed to bring the 57cm BMC closer to geometric equilibrium with the C'dale, would the 54cm frame be the better choice after all? There is nearly a 40mm difference in stack height between the 54cm BMC and the 56cm C'dale resulting in an comparative reach measurement for the 54cm BMC of 37.7cm. In order to emulate the fit of the C'dale on the 54cm BMC frame it would take nearly 40mm of stem spacing and a size longer stem. To boot, I would be extending far more seat post to get the proper saddle height as the seat tube of the 54cm frame loses 4cm as well. Given that my goal was to achieve the relaxed fit of the C'dale on the BMC frame, would the 54cm be the one to try it on?

*disclaimer*
I understand the importance of test riding and proper fitting for ultimate fit guarantee. I am more intrigued by the mathematical side of this conversation now more than anything.

Does the BMC have a carbon steerer tube? If yes, you will face a limit to what you can do for height increase. If you look at the pictures of differenct BMC builds on CC, some start with 30mm stack, others 10mm. I would make sure you know what it is going to be cut to before you start.

To make the two overlap, nothing says that you couldn't swap in a 17° stem, or even short reach bars to pull the length in. How much do you want to do to get to this? BMC also has a steeper HTA, so as you go playing with raised and shortened hand position, it will most likely handle differently than intended. How will that compare to the C'Dale?

If all you want is the fit to match (hands, bottom, and feet in same spots), you probably can. If you want the rides to be identical, I would say it is less likely.

And from personal experience with an SL02 (same frame but 105), they fit much longer than the numbers say. I owned one for 30 days (thanks CC!) and I could not make it short enough.

Good point on the handling. This would be a function of wheelbase correct? I can't seem to find a measurement for wheelbase or even the rake on the fork of the BMC anywhere on the net.

Wheelbase (some) and trail (mostly). Trail is based on HTA and rake. None of this info is published for either model (except maybe wheelbase).

Wheelbase and trail is there for the Synapse but not the BMC. Does a stem adjustment of less than 1cm threaten these measurements that much though? Referring back to DaveSSS's post earlier, the adjustments would be 8mm worth of spacer and 10mm less stem length. If that little of a change would affect handling that drastically then why can buy a 3T stem in sizes ranging from 70mm all the way to 130mm?

Stem length won't impact trail, wheelbase or rake (offset) at all. But it changes the amount of hand movement on the bars to degrees of steering input. The BMC has a steeper HTA, so it will have tighter steering if both have the same fork rake (offset), which I would wager that they do. So if you use a very short stem, it means more degrees of fork turn for the same hand input, on a frame that is probably already twitchier than the C'dale.

Realize that one man's responsive is another's twitchy, so it may all be meaningless.

They make stems in all sizes because people come in all sizes. Short stems tend to be used on small frames, and long stems on larger frames, and I would assume this is because statistically normal people of each height fall into measurements that meet a lengthening top tube, seat tube, and stem for each increase in frame size. I'm probably not saying that well. I ride 58's or 60's (L or XL). Every one I have ever looked at started with a stem of 120 or 130 as stock. If I tweaked one, it was by a cm, or a few degrees. No frame I tried required dramatic move from what it was originally built and specced as.

One of the problems with this comparison is it's apples and oranges. The C'dale is a relaxed fit bike with a 20mm taller head tube and the BMC is a bike with a racing fit and normal head tube length, for a given size.

Which is best depends on your preferences. If you want or need a small saddle to bar drop, then a frame like the synapse is the better chaoice. With the BMC, you would be buying a too-big frame to get the desired bar height. Either that or you use a +6 stem instead of a -6 to gain about 20mm of bar height.

I started thinking about this route after go through the exercise of CC's fit calculator. They refer to three types of bike fit one of which being the French Fit. Are you familiar with the logic? After reading about this I started to notice the measurements on the larger sized racing geometry frame weren't too far removed from the typically "proper" sizes of the relaxed fit geometry bikes anyway. Hence, my conversation here. CC defines French Fit as:



Is there any science to this or is it salesman voodoo?

The problem with the French fit idea, is that it was developed back in the days when all frames had horizontal top tubes and head tubes were rarely, if ever, extended in height. Back then, the only way to get a tall head tube was to buy an overly large frame. The problem is that the overly large frame would have more reach and probably require a stubby stem. The weight balance would also be poor, with too little weight on the front. The overall idea is too have a small saddle to bar drop, with more reach. That made more sense in the days when the shifters were mounted on the down tube and deep drop bars were common. These days, most bike use brake/shift levers, so it's more common to use a larger saddle to bar drop and spend more time with your hands on the brake hoods.

Today, frame desingers concentrate on stack and reach, although the reach values must be properly corrected to the same stack height, when making comparisons.

Whether one of the new relaxed fit frames is the best choice depends on several things, including your body proportions. If you have long legs and a short torso, you might need the 20mm of additional head tube length, to avoid using too much spacer or a high rise stem. A person with short legs might find a racing-fit bike with a short head tube to be too tall, with a frame having enough reach.

I have long legs for my height, with an 83cm cycling inseam, 73cm saddle height and 169cm height. I like a lot of saddle to bar drop (9-11cm) so I look for a 120-125mm head tube, to get a 140mm total length, with the headset. I use no spacers and either a -6 stem for a 9cm drop or a -17 for an 11cm drop. I use a 110mm stem length, even with my short torso and a lot of saddle setback.

It's all about deciding what saddle to bar drop you need or want, then getting right stack height to produce it, without resorting to a goofy stem/spacer combination.