i'm 5'8" with about a 30" inseam, and i've heard people tell me i need everything from a 52" seat tube to a 55. this is confusing for me, especially since i am looking at the lower end bianchis, which all jump from 52" to 55" with nothing in between. so in everyone's experience, is it better to err on the size of smallness or bigness? what are the advantages/disadvantages of each? thanks in advice for your help.

p.s. i know i should just get what 'feels right' but being new to road biking, it all feels somewhat awkward to me, in all honesty

I'm also 5'8" with a 3o inseam and I have a 54 cm bike. So I guess the 55 cm would be closer to what you need. BY the way you did mean cm not inches, right?

If the top tube is way too long, you will be stretched out too much. Try to figure out where you want the bars. It would be good if you could get help from a fit expert.

Whether to err on small or large -- assuming you are within a range of frame sizes that fit you (the $million question), then I'd say the smaller bike within that range will give you better control and comfort. Like, I have a 56, and a new 55. The new 55 is fitted very much like the 56 (in terms of measurements between pedals, bars, and seat), but instead of my seat being pushed far forward as on my 56, its back slightly past the 1/2-way point. So my weight is more spread out fore/aft on the bike. IMO.

You are not expected to know what feels right, because you don't know what to feel for. It's like the first time having sex -- you'll just get excited knowing that, technically, you are "doing it," (and by doing it, I mean riding the bike) but only hindsight and experience will tell you how good it was or what you needed to work on (in terms of fit and set-up, of course).

Who's telling you what size you need?

Seat tube length was never a very good way to size bicycles. If you think of it, seat height is the easiest adjustment to make on a bicycle. Why would you use that measurement as the do-all end-all determining factor? Making it even more complicated are the relatively new compact frames that result in smaller seat tube measurements and make stand over estimates useless.

My advice is to shop for a shop first. You need to find a bike shop guy whose experienced in bike fitting. Once you find the right guy, everything else will fall into place.

You need adaquate standover clearance, at least 1" or 3" for compact style sloping frames. Given that clearance, the more critical dimension is length. You need the bars in a comfortable position which you get with a combination of top tube (TT) and stem length. Excessively long or short stems affect handling. 10cm is about the std length and you can usually alter it +-2cm with no ill effects. Pick a bike with a TT length that allows you to use a 10cm(ish) stem where the bars feel good, not too extended or cramped.
Take a tape measure to the shop for comparisons. The dimension you need is from the nose of the saddle to the grip part of the brake hoods.

Many fitting guides make assumptions about how you want to ride. This one doesnt.

I would say the 52 cm. would be closest to your needs. Go to the Bike Tech page on my website; there is a frame size chart you may find useful and details on how to transfer this information to a compact frame.

That's why you need to find a LBS with a good reputation for fit, as others have said. Then, testing bikes that are at least roughly set up for you, you can get a better sense of what feels right. It may be that those lower-end Bianchis with the 3 cm gap between sizes may not be right for you. A friend had a bike that was too big for her, and the shop that sold it to her had compensated by putting on a tiny stem--it looked almost like the bars were attached directly to the top of the fork itself!

well see, that's the problem. i have been to several different reputable lbs' and they've all told me something different. actually though, most have told me i'm a 53 or a 54, which seems about right. i guess i'm just wondering if i go for a 52 or a 55 am i going to regret it later. it's so hard to tell by just riding down the block and back again.

I'd err on the side of smallness... you end up with a slightly lighter and tighter frame (front wheel tucked in a little more, better weight balance), and there really isn't any downside. Put your seat at least as far back as KOPS, then position the bars for comfort... which may take a few tries. If your seat is far enough back then you'll have little weight on your arms, and a long reach should feel comfortable. If you like a high bar then get a 17 deg stem and flip it up.

+1 for the 52.

There is a tradeoff which ever way you err...if erring is even an appropriate term because sizing conventions vary. Competitive Cyclist's fit formula puts that somewhat into perspective with their different sizing ranges.. Depends if you race versus tour for example and whether your priority is comfort versus aerodynamics. It is no coincidence that all ballon tired bikes have high handlebars. They are designed for comfort not speed. The tradeoff when choosing a smaller bike is a larger fall from seat to handlebars because smaller bikes have shorter head tubes. That is incidentally why bikes have been sized historically by seat tube because seat tube for the old convention of horizontal top tube bikes was proportional in length to head tube which shared approximately the same converging angle to the ground. The other thing with a small bike is toe overlap. If you have big feet and longish cranks at 175mm or so and you are on a small bike and turn the wheel sharply your foot can hit the front wheel. The next thing is wheelbase. Longer wheelbase bikes tend to be more stable at speed. So there are many tradeoffs to consider. My belief is you should err on the side of your so called ideal size based upon your condition and riding preference. Bigger if you want to tour or ride recreationally and smaller if you want to race.
HTH,
George
P.S. Thanks Dave for sharing your website and expertise. I ride a bike just a hint above your recommendation more out of choice and handlebar height then convention.

Thank you George; you are welcome. My frame size chart and recommendations on my web site are for an efficient racing position. Rather than give people multiple choices which I think are confusing, I am saying here is a place to start; this will be your smallest size frame and your longest handlebar stem. From there, do as you have done, and choose a slightly larger frame and maybe a shorter stem if your goals are comfort over absolute efficiency.

If your choices are limited like the original poster of this thread who has a choice between a 52 cm. and a 55 cm. This person appears to be right in the middle of these two sizes; in which case I would suggest choosing the smaller size because it offers more scope for adjustment.

Dave, its great to have a person such as yourself participate on this board. You have such a wealth of information based upon the hundreds of cyclists you have built frames for over the years. Your website is quite informative. In particular I like your commentary about saddle setback relative to femur length...how setback in actual fact is more a function of overall inseam versus just femur length...good perspective. That is percisely the path I chose....slightly taller frame with a bit more setback with slightly shorter stem in deference to my riding style and relatively long inseam relative to torso length.
Dave...a final question please based upon your experience. If not an out and out racer, what do you think is the premier choice for frame material today?...perhaps with a CF fork?
Thanks,
George

I really can’t comment on any material other than steel because I have had no experience there; either building or riding. I always felt that a good steel frame is like a very strong spring in that it has just the right amount of give when a rider makes a sudden effort and it transfers that energy directly and efficiently to the rear wheel.

Aluminum does not have these spring like qualities, and CF it would depend on how the frame is constructed. Like I say; I don’t have the experience to give an honest answer.

Thanks Dave.

thanks, guys. all great insights. i guess the question for me is: is it acceptable for me to get a frame that is somewhat on the small side but which is otherwise exactly what i am looking for in a bike (in terms of price, material, etc.), or am i better off trying to find something that is exactly in my size?

You opened this thread to get advice and you have got it from various sources. As far as I can see there has been no bad advice given; so now it’s time to take all these opinions and from that form your own opinion. It’s decision time.

To verify the suggestions why don't you try out the competitive cyclist fit calculator, I can't see this mentioned in the thread previously. If it has I apologise, here it is.

https://www.competitivecyclist.com/za...LCULATOR_INTRO