LeSexyFishorse

How substantial is a few mm when talking about road bike fit? I have an opportunity to get a nice Road bike for cheap but the frame measurement from BB to top is 51mm vs what is 56mm+ which is recommended for me. How negative effects will you get from that kind of a difference? Are they something you can overcome or does it make the bike a "hard no" on the buy?

DaveSSS 

I assume you're referring to the stack height? If so, those values would have to be in centimeters, not millimeters. 5cm is a big difference, since the most common change between frame sizes is 2cm. The reach dimension will not be the same either. I just bought two new frames that are the smallest I've ever owned, but I know exactly what stack and reach l want, so I knew exactly what my new setup would be. I went from a 527mm stack to a 509mm stack. I used 15mm of spacer under the stem and the same -17 degree stem, in a 10mm longer length, to correct for a shorter reach. The stack is only 3mm lower than my old frames.

wphamilton

A few mm is trivial, representing a small change in the positioning of your torso and arms.

Our tolerance for these slight variations appears to vary among individuals. I wouldn't think twice about a 5mm higher handlebar, nor a couple of mm reach either way. For others it's a deal killer.

If you mean frame size, 51 vs 55 CM as Dave suggested, that's out of the question. The bike won't ever fit and you'd regret it.

jadocs

Sounds like cm to me and not mm.

Also sounds like you are referring to seat tube height. Meaning this is technically a 51cm frame and you should be on a 56cm frame.

Is this a standard geometry frame?

Moisture

No. Don't buy it

LeSexyFishorse

I mean the distance from center of the BB to the top of the seat tube. Sorry if that was not clear. I am also referring to a classic road bike type frame if that helps. Sorry I think the mm was a translation error, should be cm I think. They recommended a 56 for me when I went for a bike fit which I understand is a Medium frame?

jadocs

If a fitter told you that you need a 56cm frame, a 51cm frame will be too small for you. It's not just about how high the seat tube is. Smaller frames have less reach and you will be cramped.

Barry2 

For the past 14 months I've been riding a 61cm Cervelo. I had to replace the seatpost with a zero setback and replace the stem with a shorter one to get a more reasonable reach.
With these changes, I've been able to comfortably get 2,000+ miles in 2020.

However, I've recently replaced the bike with a 58cm Cervelo. I had to in the end.

You can modify a larger bike to get close to your needed dimensions (assuming you can standover the top tube), but it is really not the best solution.

Barry

alcjphil

The difference between 51 cm and 56 cm is 5 cm which translates to just about 2 inches. That is a huge difference in frame size. However, other frame dimensions are equally important. Top tube length, for example is also very important. A 56 cm frame with a 57 cm top tube will fit very differently than a 56 cm frame with a 55 cm top tube

chaadster

Let me point out to the OP that the frame size recommendation given by their fitter is almost certainly a center-to-center measurement (c-c), i.e. from center of the bottom bracket to center of the top tube. Center-to-top (of seat tube) is pretty meaningless unless talking about sizes of frames all made in the same way, as in the same model from the same make.

As has been pointed out, notably by alcjphil , there are much more relevant measurements to use when shopping across brands (and even models). I’d suggest talking to this “fitter” and getting something useful, like stack and reach measurements, if in fact they can even provide them. I suspect that no good fitter would send a client out in search of a 56cm frame these days; the variations in frame design are far to great to give such a simple number any helpful significance.

LeSexyFishorse

So if I understand correctly, too small a frame will make me feel cramped when riding? Is this something I can remedy safely? Could I not extend the handlebars and raise the seat to get a better feel? Again talking about the 51 vs 56cm limit here. How bad of a compromise would it be? I guess what I am trying to see is if I can live with a smaller frame for a bike I really really like or if I should just live with the heartbreak and let her go. 2 Inches doesn't seem like much to me but I suppose I don't have the experience to properly quantify the effect.

LeSexyFishorse

I think they might have been referring to center to top of seat tube since I told them I was looking for a vintage road bike frame.

jon c. 

That's too large a difference. I ride a 54 and wouldn't consider trying to make a 51 fit.

Moisture

You need to understand more about what stack and reach/ top tube length is right for you in order to answer your question re living with a smaller frame

Barry2 

For a trip to the shops, 5cm don’t matter.
But for 2,000 miles a year I purchased one 3cm smaller...... yes 1 1/4” smaller.

So you will need to make the decision for your age, intended use, and wallet size.


Barry

LeSexyFishorse

Unfortunately, I dont know if I can get stack and reach numbers since its an auction site.

LeSexyFishorse

How did you do with your 3cm smaller frame? I am in my 30's, physically fit, I intend to do some fast riding for exercise but no serious rides allowed at the moment due to the lockdowns. Unlikely I will go further than 20-30 miles per ride I suppose. Trying to get this one cheap which is why I am buying at auction. This makes the frames that I like for this price even rarer at auction. They do happen and I dont mind waiting if the downsides of a small frame of this degree are significant. What do you think?

Bigbus

Hey Lesexy, no matter how high you raise the seat and slide it back plus push out the handlebars, you're going to feel like you're all over that bike and not in a good way. The human body can adapt to a lot, but I doubt if that bike will ever feel right for you. With that said, I know how you're feeling and you really want to make it work. Good luck with your decision,

veganbikes

Get a bike that fits. Bikes that don't fit are not fun and not worth it. Yes you can find stories of people doing and see them but it just isn't comfortable and your fitter wouldn't be happy seeing you on a bike that is so ill fitting.

Pro riders will sometimes opt for a smaller frame for aerodynamic gains but they are pro riders who train hard and ride a lot, a lot and dedicate themselves to getting the best position to win and also to handle the bike as well as you can. Plus they have doctors, masseuses, fitters and mechanics who can get it all done and make it work. We are mere mortals and don't quite have all of that nor should we strive for that unless we want to be professionals. Other than that I wouldn't want to work that hard to bike to not win, I would want to enjoy more.

A good deal is only a good deal if it is a good deal. I could buy a bike to resell or take parts off of or something like that but buying a bike that doesn't fit with the intention of riding it, isn't a good deal for sure. I have seen a lot of bikes in my time that are awesome bikes at decent prices but if it doesn't fit I don't buy it as it is not a good deal for me. I don't need another wall hanger that I didn't ride and can't really ride with any sort of comfort, I want bikes I can ride and enjoy.

chaadster

The mid-90s were 25 years ago, and by that time, the adoption of TIG welding, new tubing shapes, and the compact geometry revolution were already happening and changing how frames were designed and fit, so depending on what the definition of vintage is, I nonetheless think it’s rather ridiculous that a serious fitter would spit out a frame size recco based on seat tube C-T. I still think you should be putting this question to them, and getting a proper explanation for their sizing suggestion.

But, if we assume you y were talking about racing bikes from the 60s, 70s, and 80s, then yeah, dropping to a 51cm from a 56cm proper fit is gonna be very small, cramped, and a generally poor selection. I’d advise against it, for sure.

LeSexyFishorse

Yeah I have decided to take more experienced advice and pass on the 51. Its a shame because I really wanted it but I suppose if I would just end up hating the feel because it doesnt fit it would be a waste anyway. I guess I will just be patient. Thank you for all your insights guys, really appreciate it.

chaadster

I understand your frustration, but your patience will be rewarded! There are good supplies of vintage bikes, covering at least three decades of production, on eBay, Facebook Marketplace, and Craiglistlists across the country, so undoubtedly you’ll find exactly what you want and need.

Sy Reene

You also have to be wary that whoever's selling the '51cm' measured frame, whether they just measured the actual distance from the BB to the top of the seat tube. Of course a bike with a sloping top tube will have a much shorter actual seat tube, while you're fitter was likely referencing a size based on a horizontal top tube. IOW, there is a possibility that a sloping top tubed bike might have an actual measurement of 51cm if the seller is just measuring with a yardstick, but be equivalent to eg. a nominally labelled 56cm bike.

Camilo

And a shorter head tube which will - stem height being equal - cause a larger saddle to bar drop because of that and the fact that because of the need to extend your seat post higher because of the shorter seat tube. You might be able to make up the difference in reach (shorter top tube) and handlebar drop with a taller and longer stem, but maybe not.

Like others have said. Without knowing the other geometry factors, its much more than likely that a 51 cm frame is quite a bit too small for you.

Study up on frame geometry and make some fit measurements on your current bike to get familiar with this stuff. Buy a frame that is within normal stem height and reach of being able to mimic your current fit (assuming you're happy with it).

Moisture

Should be able to find stack and reach dimensions regarding most bikes if you search up the name of bike and look for a geomtery chart.

You being 5'11, should not go any smaller than 56cm. Unless your legsnare very short you can go 58.