Frame size tolerance
 

I know this is a "how long is a piece of string" question, but how far can you go past your "ideal" frame size?

I am 5'6" with in inseam of about 30". (I'm not quite sure about the inseam, but I used the ruler and tape measure jammed into the crotch method.)

I've never had a bike fitted before, but the guy at the LBS looked at me and said that I was about a 52cm. I think I complained to him how most of the previous bikes I bought second hand were too big for me, so I'm not sure whether he may have bumped me down a size to be on the safe side.

The thing is that height is a bell curve, so there are far more 54cm frames than 52. As I am going for second hand bikes, it gives me a far wider choice if I include 54cm frames in my search.

I tried a 52cm and a 54 cm frame in the shop IIRC. Both womens, as not 52 mens were in stock. The 52 fit me perfectly. The did feel very slightly longer, but I was still comfortable on it. But I'm not sure if I just was lucky that time.

Can I also just get a stem that is 2cm shorter than the existing one? Or does that mess up the geometry?

It's a personal choice but I'm also 5'8" with 32.8" inseam. I have long legs and a shorter torso so I tried a 50cm in a store with 52.5cm top tube and was surprised. I believe it may do. (Unless I'm incorrect about my neck being a little pressured while being in the drops. Haven't tried a road bike in a while.)

A shorter stem = more sensitive steering which I find a little more dangerous going downhill. Like I said, I might be incorrect. I may try other bikes to see but still strongly considering a 50cm. (That goes against what a lot of people say though.)

There is lots of frame size tolerance, like probably two sizes on either side of estimated/ideal (when talking in 2cm measurements; T-shirt sizing is more like one size either way), especially when going smaller, because sizing up is practically unlimited, whereas one hits limitations to downsizing a too large frame more quickly.

I'm also one who believes there is no point to riding a larger frame when a smaller one will do. The smallest frame that provides you the fit clearances you need (e.g. toe overlap, reach, pannier clearance...whatever) will give you the widest range of fine tuning, handle more crisply, weigh less, and a longer stem and more seatpost extension allow selecting those components for material and design qualities that impact ride quality (i.e. a longer Ti seat post will damp vibration more than a short extension Alu one).

Regarding the stem question, yes, shorter stems are easy and cheap to swap; whether you can drop 20mm will depend on starting size, as finding stems shorter than 80 or 90mm is not only problematic, but questionable in terms of handling (as hybrdbkrdr notes). In general, going from 110mm to 90mm will not have deleterious effects, I think.

Lastly, given your height, I'd guess 50-52cm frame size for you, too, however those sizing numbers really don't mean much because they're usually seat tube length measurements, which are no indicator at all for fit. Sometimes, those frame "sizes" don't even measure out to any tube on the bike at all! To get a solid sizing guess, you need at least an effective top tube measurement, and it gets more complex from there (e.g. front-center, stack, etc.).

You're doing the right thing, though; go to the shops, get opinions, and see what feels good. But no, don't be afraid to move a couple sizes either way of the reccos if you think it's the right fit.

First off you should get the measurements of the effective top tube length as that is more accurate to fitting a bike than the down tube length, which is probably the length you were told. If you are buying a used bike I would bring a tape measure to measure the top tube. Given your height a 50-54cm effective top tube length is appropriate and the exact size depends on the geometry of the bike.

I am 5'6" with a 32.13" inseam, which results in a 48.7% inseam:height ratio. The average male is 46.5-47.5%. Knowing this is helpful since most frames are built for an average individual and I am not average. For my inseam the online calculators recommend a 54cm effective top tube, but when I tried one the reach was too long. The 52cm effective top tube fit perfectly. By the way, my bike has a 90mm stem.

Thanks for the info guys.

I never thought about dropping to 50cm. It makes sense as it is easier to extend things like seat post and stem than make them shorter. Unfortunately there are almost no 50cm frames out there so it kinda makes it moot.

Define the purpose you have in mind.

Go fast racer .. with really low handlebars? or pootling along tourer with higher bars..

you havent even tapped into the Travel and folding bike sector, like Oregon's Bike Friday
there the size is top tube length , a torso~arm reach - length determination.

And the height of Bars and seats are on masts that are made at the time you order the bike,
as is the whole bike, but if you change your mind, they can be replaced.

Thanks for the tip about the top tube length Doc V, it really helps to narrow down my search.

fietsbob, I basically looking for a entry level second hand road bike for casual cycling and for use as a bike around town. As I am going second hand, I can't really go for customisations.

Two fairly good guides for finding the proper top tube length are: 1) when your hands are on the hoods the front axle should be obscured by the handlebars or the axle should be in front of the handlebars and 2) when your hands are on the hoods your elbows should have a slight bend. I find that my face is over the head tube on a properly sized frame.

You've complained that your previous bikes were too big.

The salesman has suggested 52cm.

You've stated the 52 felt perfect.

You're considering buying another too big bike for convenience's sake. And, expecting us to tell you this is fine.

Be patient for your right bike. If it fits you well, you might be inclined to keep it a while.

And, yes, you can change stem lengths by a couple centimeters. But, it's best to start with a frame that is as close to ideal as possible and then only make minor corrections for fit.

I don't want to hi jack the thread but I am facing the same Ideas. 5'7" and Looking to replace a 53.5 top tube bike and everyone is telling me that they can make the new one fit. what is the ultimate stem size? I would think the one on the bike originally is best. The fits I have had the stem keeps get shorter and the seat moves forward. Is my assumption correct that I need a smaller bike?

You haven't given enough relevant info for anyone to comment meaningfully, but since you're better off starting a thread of your own anyway, you can also supply more critical info.

Yes, if the frame allows you to achieve a good saddle height and the top tube is not hitting hard in the crotch, you can lengthen and shorted the stem to get the best possible reach. It is possible that a given top tube will not let you get correct reach with any stem, as well.

I haven't seen stem changes messing up handling, but others may have had that happen. I haven't seen it.

I WOULD say that if your neck feels constricted when in the drops, that you need a longer reach to the handlebars, to allow your body to stretch out appropriately. You might want to look into what can really be done, and what reach you really need. Trial and error can in principle get you to a correct fitting, but it can also keep you going in circles.

So there is tolerance in a sense, that many different frames may result in a very good fit. But there are also hard limits the may prevent a frame from becoming suitable for you.

To give you an idea... I'm 5'10 with a 32" inseam and ride a 54cm. If anything you would be bordering on riding a 50cm, so if you have to "broaden" your range you should consider 50cm and 52cm bikes.

If you were to get fitted a good fitter will give you a R.O.R. which is range of rideability... my road bikes fall into this at 53 cm square and 55.5 square and the taller bike has a stem that is 20mm shorter and the smaller bike has a more aggressive fit and saddle to bar drop with a longer stem.

The top tube measurement is the important one as this determines your riding position to ensure you are not too cramped or too stretched out.

I ended up with a 51cm Cannondale Synape. Fits me perfectly. The compact bars also make for a very comfortable drops position.

Back when I was 5'10 with a 32.5 inseam I also rode a 54 steel frame. These days I am down to 5'9" thanks to age and neck injuries but with a new sloping top tube framed bike I'm riding a "virtual" 57 cm. I'm pretty comfortable but when I get on a slightly smaller frame it has that "fits like a glove" feeling. As has been said there is a range of sizes you can fit but I'd look for the smallest frame that fits well.

I have short legs and a long torso, so it really depends on how I want to set up a bike.

If I want a higher handlebar, I'd fit by reach/top tube like some advise. If I want any kind of reasonable saddle to bar drop, which will still be less than 3" with a slammed -17 stem, I have to pick based on stack height like many pros do.

pretty far

I am just a hair over 5'6. My usual bike is a 54, with a 9cm stem, flipped and slammed. I could make a 10cm work if I was in better shape.
My commuter is a 55. I have been comfortable on a 51 and 52 also.

You can make a bigger frame work by tilting your pelvis to lengthen your torso, as per Cobb's video on A style back position here http://www.cobbcycling.com/content/positioningvideos

This takes some work, but is worth it. It would be easier to fit you on a 51-52, but a 54-55 can work.

Not brand specific.. .. size is another name for seat tube length..
and if the top tube slopes downward, you have a much wider range of saddle to Pedal adjustment downward

than one with a horizontal top tube (sorry if I state a known fact, again)

If I got the essence of what your saying right, that rider flexibility affects positioning, then yes, I agree with that. However, I'm not so sure that it affects frame sizing all that dramatically, or that Cobb would consider A and B style backs 'positions'.

My basic thought is that flexibility and pelvic positioning allow riders to get into lower, more comfortable and aero positions, but that frame size is still determined by a complex recipe of things like riding style, intended use, body type, frame geometry, etc. Within that range of frames that work, then, flexibility and pelvis angle help achieve the most more aero position. Rider ability to go lower might, in some cases, suggest a smaller frame (lower stack), but I don't think simply being flexible would recommend a larger frame.

To that point, Cobb's A & B back thing refers to rider body types, and how much back flexibility they have. Recognizing the styles informs him on how he should attack fitment for the max aero and comfort benefits for a given rider on a given bike. I don't think Cobb would suggest B backs should aspire or practice to be As, and I wonder if he'd think it's even possible. Similarly, Fizik's Chameleon/Snake/Bull thing recognizes flexibility and how that impacts riders' likely pelvic position on the saddle; it's a fit adjustment tool, not an aspirational training goal.

I'm a Cobb believer, and pelvic rolling worked for me in getting lower and more comfy. But rather than suggesting I go longer, it actually shortened my overall reach and allowed me to keep my weight centered on the bike for excellent handling feel and control.

Anyway, I guess the point that I wanted to make was that while flexibility and pelvic tilt give greater range of fit setups, I think it's a very wrong approach to try to train in flexibility in order to make a larger frame work; I just don't see the point, and think that for sports road riding, the smallest fitting frame is preferable in all ways.