pamaguahiker 

soma grand rando and it's 650b wheels compared with 700c

I had this posted to touring, maybe being more of a fit question, it will be better here. Any answers or advice are welcome.

I have been wrestling with the idea of turning my Bianchi cross bike back to simply cross and building up a touring/rando type using some of my parts that I know work best for me when touring and rando type rides. I am presently using my Bianchi cross specific bicycle for touring type rides. I do note that with a load, it still rides nicely but feels sluggish at times and front heavy which is why I am steering towards a more rando specific frameset. I especially have been looking at several models and the new Soma Grand Rando has my attention. Of course, the factor coming up is the tire size of 650B which is the only option for the soma grand rando.

I haven't tried a 650B at all for any amount of riding time. But I stood over one that was 'my size' as stated by someone who was an enthusiast for the 650b. And It just felt low and small. I rode it only briefly, so i cannot give it a fair review. It might just be that way because of what I am used to riding but I was wondering, at 6' tall (inseam about 34), will the 650b feel too small for me? Or will I be just as well to continue to run 700c wheelset.

When I did ride the Bianchi as simple cross, it was a fast, sweet ride and I think it would make a awesome crosser again and also use for the 'New' catchphrase niche gravel grinder type bike. Which, BTW, I feel that my cross would handle just about any of that terrain, just as it did years ago before those words became a new type of cycling.

thank you

ThermionicScott 

Perhaps that person's idea of the right frame size is different from yours. I don't think this has anything to do with the size of the wheels.

When I built up my 650A "rando" bike, I used the same frame size as I had with 700x28s, knowing that I'd lose a little height. Came out perfect -- I spend very little time with my crotch on the top tube anyway.

pamaguahiker 

Thank you for the feedback!

Road Fan

The overall wheel diameter for a fat 650b, using a 42 mm Hetre, is about 670 mm. The same dimension for a road 700x28c, such as a Gatorskin, is about 680 mm. These are really similar, resulting in a radius difference of only 5 mm. You could get the same or more "reduction" with a super-skinny 20 mm tubular wheel. They are often 665 mm or less.

It's not likely to be the wheel that made a big difference, unless you went from a marginally tall frame to one that really does have just a little more clearance. It's much more likely the 650 b you swung your leg over really is a smaller frame.

So the logical question is, what size should you look for and how can you tell the size of the frame you are looking at? How do you measure your inseam? If you use pubic bone height (PBH), the center-center size can be calculated real easy just by multiplying by 0.66. That would be the c-c seat tube length for a traditional design frame with a horizontal top tube, and is probably a minimum length for the top tube.

pamaguahiker 

I use the PBH and after calculations are finished. i get around 58cm. I have always used a 59cm c-t so that calculation is precise. And after going to google and doing some searches for comparisons, i had managed to come across several pictures and diagrams. There are some out there that overlay different wheels with tires and the difference is indeed minimal. It gives me a good goal direction to work with ad helps guid me a bit. The general consensus is, don't worry too much as the difference is minimal.

Thank you for the feedback!

ThermionicScott 

You bet. Any idea what the frame size was of that 650B bike you tried?

pamaguahiker 

It was a 57cm c-t and i normally ride a 59 c-t.

fietsbob

top tube varies with the size/seat tube length and TT slope..

then theres the angle of the seat tube generally the builders have a narrow range

but not all agree to what is "best" [... 70 to 74 degrees... ]

need to get a lot more Data.

650b is Just the wheel size .. it says zero about the frame size ..