Hello everyone,

I'm considering a BMC Roadmachine ONE

I am 184 cm / 6 feet
My inseam is 90cm / 34 inches
and my arm is 71 cm / 28 inches
I made the chart below comparing 56 to 58
Difference in size (to me) appears minimal
But I realize a cm can also affect fit.
Opinions?
Thank you

Just pick one and be happy. If you are in the overlap they'll both be good. What you might want to look at are whether crank lengths, handlebar width and any other component changes that might matter more to you.

The smaller frame will give you a more livelier ride. Some like that and others don't.

Rather than an inseam measurement, measure your actual saddle height to pick the best stack height to produce your desired saddle to bar drop, without a lot of spacers or a high rise stem. There's a big difference between a properly measured cycling inseam and a pants inseam. I never know which might be used.

With a 73cm saddle height, I look for a stack height in the 520-530 range to produce a maximum saddle to bar drop of 10cm. Changing from a -17 stem to a -6 will raise the bars by about 2cm.

The front of the bars around the hoods or in the drops are already behind the front wheel contact area and well behind the front axle, and you probably don't want to ride that bike with an excessively short stem since that would bring it back even further. I'd go with the next size down. I test rode the RM02 in 56 and it felt a bit big for me at 6'0. I ended up buying a Cervelo C3 in 54, but liked the RM02 and it would have been my second choice, also in 54. It's easier to add a little reach, especially to an endurance steering geometry, than it is to try to remove it. But I was also in the market for a bike to spend all day on, which is a little different from something spent an hour or two on - for the latter it almost doesn't matter, at least to me.

I really don't know what gives you the idea that you are "between sizes." I am 6'4" and with a 34" inseam. My bikes are:
1: A 60 cm Trek Emonda,
2. A 60 cm Trek Madone
3. A 58 cm Colnago CLX3.0
4. A 58 cm Lemond Zurich
5. A 64 cm Backroads Touring bike.

Now if you are a professional racer, it is pretty important that you have exactly the correct size that would center you perfectly over the center of mass of the bike. The precise measurements is very important because these guys literally live on their bikes so changing the position on the bike can cause them muscle damage. So this is so critical that they now use digital readout measuring tools for everything from the seat height to the damn tilt of the saddle.

But this doesn't apply to people who put in less than 5 or 10,000 miles per year. Saddle height is important only insofar that if you are off more than a couple of percent you get knee pain. If the reach is incorrect you get shoulder, arm and hand pains. None of this is serious and you just play around with it until it is correct. Even crank arm length from 170-175 mm is not important in any way other than saddle height since this is measure from crank BDC to to saddle surface.

If you intend on being a pro racer by all means get a custom measurement and follow them to the letter. Otherwise there are very few measurements that make much difference in anything other than your comfort. Saddle position is adjustable, stems are changeable. Handlebar width is changeable. Old steel frames I used to ride a 62 cm

If you are in between sizes you should most likely find an other model. Compromising one ore more measurements just isnt wort it in a market with hundreds of models to choose from.

All other things being equal, the smaller one will leave more room in the shed for another bike.

The 58 has a taller headtube (206mm compared to 181 mm) and a 1cm longer top tube. Do you like to get a little lower and have no neck or back issues? If yes, the 56 might be a bit quicker. If you need the extra headtube height for comfort then the 58 might make riding the drops a bit more comfort oriented for you. In that case you would also need to consider what stem length or handlebar drop you want.

56

can make a smaller bike bigger cant make a bigger bike smaller

Thank you and thats why I picked 56 over 58.
It, naturally, felt more nimble.

Thank you for your input.

I used a measured inseam in my OP.

Thank you for your contribution.

If you're 6'0 and prefer a 54 - don't you (we) end up in a fetal/prawn position?

Assuming proportionate legs/torso/arms.

The chart I posted is directly from BMC on the Roadmachine.
You can see that there is overlap between 56 and 58 - in rider height.

heh heh heh

good one

I don't have a very flexible neck. Meaning, the more stretched I am over the top tube (tank in a motorcycle), the harder it is for me to flex/pivot my neck upwards.

This is why I went with the 56. I was less draped across the top tube.

Thank you for your input.

Very briefly, how so?

Thank you.

You can put on a longer stem and raise the seat to make a smaller frame accomodate a larger rider.

and I imagine the reverse is not so compatible (shorter stem and lowered seat on a larger bike), why?

The reach difference between the two frames is a trivial 4mm, but the stack difference is 24mm. The smaller frame can be matched to the larger one with spacers and/or more stem angle. That's why it's important to start with saddle height and know how much saddle to bar drop you want. If you're sure that the minimum stack on the larger frame is OK, then choose it, but be aware of lowering limitations.

In my case, I have a 527mm stack and it works with only the 15mm headset top, no spacers and a -17 stem. Once the steerer is cut for no spacers, only stem angle can be used to raise the bars. If I ever need more bar height, I'll just go back to a -6 or -8 stem.

I fit on 60s and 58s and have moved to 58s with longer stems/posts. The 58s just feel better. That's hard to quantify but, in the end, what mattered for me.

As a bonus - if you sell your old bikes, there's a MUCH bigger market for the smaller frames, e.g. 54, 56

If the 58 stack is 24mm more (higher), wouldn't that mean a more comfortable ride?
Since my torso is more upright? And tube top length is only 4mm more?

A taller stack height does not produce a more comfortable ride. It does change the lowest possible bar height. I'm comfortable with my 10cm saddle to bar drop, but if you only want less, then plan your setup accordingly. There are riders who only have a 2cm drop, but that requires a tall head tube, maybe 20mm of spacer and a high rise stem.

Do you have a current road bike? If so, and you're comfortable with its fit, I would take measurements from that and compare it to the bikes you're considering. Even better if you can compare measurements in person to the actual bike.

Even better if there is a fitter you can consult. Bike fit isn't straight forward, and it's worth spending some time and money with a fitter to confirm you're getting the right size.

Alas , no current road bike.
Most fitters here want to size me for slow-selling stock in the brand they carry.

I've settled on a 56 Roadmachine
Rode out today

Phenomenal