54cm or 56cm

lukea-d

At 5'10", most size charts for road bikes say I could fit either a 54cm or 56cm. Assuming I get set up properly on each, what difference in ride would I feel between the two sizes?

diamonddog

You could feel stretched on the 56 or cramped up on the 54 depends on effective top tube length.
Best to sit on them and try them.

Grill

When in doubt go small.

bobones

54 will have more saddle to bar drop because it will have a shorter head tube, but that can be negated with spacers or slamming stem on the 56. The shorter wheelbase might make the 54 have marginally quicker steering. You might have more toe overlap with the 54. If you keep the same stem (both likely come with 100mm) then you'll be more stretched out on the 56. It's easier to make a small bike fit than a bike that's too big so unless you have disproportionately long legs and don't want a lot of saddle to bar drop then go for the smaller size. However, bike geometries vary and one brand's 56 might be more suitable than their 54.

trek_dan

Same height and both my bikes are 54's. My first road bike was a 56 and fitted ok (bought it off sizing charts without trying first) but looked a bit odd with a short stem and seat quite far forward on the rails.

bernithebiker

54's look cooler than 56's.....

mfin

Can we have a sticky thread on Sizing... to cover things like people shouldn't put saddles forward on rails to compensate for sizing, fore/aft is a fit thing that should be done along with saddle height first... all about basic bike fit, cos all this stuff gets repeated and repeated every day or so it seems!

Grill

mfin wrote:

Can we have a sticky thread on Sizing... to cover things like people shouldn't put saddles forward on rails to compensate for sizing, fore/aft is a fit thing that should be done along with saddle height first... all about basic bike fit, cos all this stuff gets repeated and repeated every day or so it seems!

Except that this isn't always the case. Many people aren't sensitive to KOPS and as it isn't a de facto metric then it will have exceptions like everything else.

rolf_f

Pointless anyone guessing (because that is all it is) without knowing at least how long the OPs legs are.

giant_man

Yup. it's all down to dimensions and geometries of both sizes.

bobones

The question was about the difference in ride between a 54 and 56 if they were set up to fit correctly.

rolf_f

bobones wrote:

The question was about the difference in ride between a 54 and 56 if they were set up to fit correctly.

Good point. Though really you're hardly likely to get both to fit equally correctly. If you are exactly half way between two sizes, surely you should be looking at a different frame altogether.

Grill

Rolf F wrote:

bobones wrote:

The question was about the difference in ride between a 54 and 56 if they were set up to fit correctly.

Good point. Though really you're hardly likely to get both to fit equally correctly. If you are exactly half way between two sizes, surely you should be looking at a different frame altogether.

Err... no as the most you're likely to be off on any important key fit area is around 10mm. One of the many reasons that components come in different sizes...

rolf_f

Grill wrote:

Rolf F wrote:

bobones wrote:

The question was about the difference in ride between a 54 and 56 if they were set up to fit correctly.

Good point. Though really you're hardly likely to get both to fit equally correctly. If you are exactly half way between two sizes, surely you should be looking at a different frame altogether.

Err... no as the most you're likely to be off on any important key fit area is around 10mm. One of the many reasons that components come in different sizes...

I daresay you are right though the impression I get on these forums is that people always seem to know if their frame is the wrong size and that attempts to correct that without changing the frame always tend to seem to be compromised. But then in my case fit is awkward anyway and there tends not to be any doubt over which size is correct - the problem is the geometry.

Grill

I agree to a certain extent although I think that's because most people buy frames that are too big. Geometry is definitely the area in which people need to take more notice.

kajjal

Grill wrote:

I agree to a certain extent although I think that's because most people buy frames that are too big. Geometry is definitely the area in which people need to take more notice.

Very true and where a good local bike shop should help and advise. Buying a bike without a test ride is very risky.

Bozman

Kajjal wrote:

Grill wrote:

I agree to a certain extent although I think that's because most people buy frames that are too big. Geometry is definitely the area in which people need to take more notice.

Very true and where a good local bike shop should help and advise. Buying a bike without a test ride is very risky.

20 years of buying bikes/frames and I've yet to have a test ride, plus, how could you have a test ride when you build a bike up?
The only issue I've had size wise was with one of my first bikes, the LBS told me that I needed a 56 which turned out to be poor advice, I've not bought a bike from an LBS since. I took my mate to buy a Giant TCR from a large bike shop in the Sheffield area and I've never heard so much crap about sizes, if I hadn't been there he would of ended up with the wrong sized frame, that makes me wonder just how many folk are out there on the wrong sized frame.
I'm roughly 5'10" and I know that I need a 53/54 TT, I'll watch out for head tube length because this 45 yr can't hack a really aggressive riding position, but sizing up a frame isn't rocket science.

sazzaa

A few years ago my local bike shop (Edinburgh Co-op) advised me to fork out £500 for a mtb that was too small for me (I'm 5ft 7 and the bike was a 15"), so with my new bike I'm happier trusting Google and spec sheets/size guides, and buying online from a decent bike shop.

rolf_f

Bozman wrote:

20 years of buying bikes/frames and I've yet to have a test ride, plus, how could you have a test ride when you build a bike up?

20 years doesn't really make any difference to one year! Unless you change size, you only need to get it right once and then, at least for fit purposes, you don't really need a test ride. My first road bike is my most expensive bike - I would never have dreamed of spending that kind of money on a bike unless I knew for sure it was the right size. So I rode one for about 30 miles. As it happens, it was the next size down but it had been built up to fit me though it was clearly on the limits of set back, stem length, spacer piles etc. Thinking about it I'm probably contradicting myself a bit from earlier but the smaller frame really was a force fit. Anyway, having proved the fit, I know now that I can effectively buy any bike with a similar geometry and get a spot on fit with it.

bristolpete

In a previous life I spent three years working as a BG fitter with Specialized and the things worth noting are that 99.9% of the buying public can ride one of two sizes. The key factor is comfort and performance and as others have said looking at the type of riding the buyer wants to attain. Racing maybe size down and go with a longer cockpit. Bimbling then perhaps buy the bigger frame and enjoy the slightly taller head tube. The Symantecs of fit and the stress people face are actually exaggerated in the media now as in reality the difference from let's say a 54cm Cannondale to a 56cm Cannondale are 545 top tube to 560 top tube which is 1.5cm. Now, the head tube grows from 140 to 155 bewteen 54/56 and on a bike 10mm up is 3mm back, 10mm out is 3mm down, so the actually bars are only mm different in position.

The key component is the rear of the bike, we all know this but still bike brands sell bikes with OEM kit and more often than not the not ideal cranks. So this brings me to stand over and leg length and overall body shape and reach. I am 5'9 but my inseam is akin to a rider 5'7 so I am all body. According to Cannondale I should ride a 52cm bike due to leg length, but this is rendered impossible by my upper body size and tiny head tube. It would look like BMX. Personally having ridden for years and worked out what I like I would likely buy a 56cm Cannondale, slap on 165 /170 cranks for a good kops reference point and lift the seat by 2.5mm if going 170 or 7.2mm if going 165mm over the stock 172.5 cranks and enjoy a nice slammed stem. Stand over would not be ideal or what people perceive to be ideal but I ride on the saddle not the top tube and the original awkward stand over with no seat post is negated by the rise of the seat post by fitting shorter cranks. In essence it would look right.

Steve Hogg whilst bombastic wrote a great piece on fit and reach based on effective torso and wrote of the kops theory as bunkem. Cervelo hit the magic formula with stack and reach based geo. Also as others have said no two brands have mirroring geo and its worth looking at wether compact or traditional geo work for your shape. Short legs long body seem to favour compact frames akin to specialized, giant, Cervelo etal whist guys with long inseam offset by short reach work well and look proportional on cannondale, Pinarello etc.

kajjal

Bozman wrote:

Kajjal wrote:

Grill wrote:

I agree to a certain extent although I think that's because most people buy frames that are too big. Geometry is definitely the area in which people need to take more notice.

Very true and where a good local bike shop should help and advise. Buying a bike without a test ride is very risky.

20 years of buying bikes/frames and I've yet to have a test ride, plus, how could you have a test ride when you build a bike up?
The only issue I've had size wise was with one of my first bikes, the LBS told me that I needed a 56 which turned out to be poor advice, I've not bought a bike from an LBS since. I took my mate to buy a Giant TCR from a large bike shop in the Sheffield area and I've never heard so much crap about sizes, if I hadn't been there he would have ended up with the wrong sized frame, that makes me wonder just how many folk are out there on the wrong sized frame.
I'm roughly 5'10" and I know that I need a 53/54 TT, I'll watch out for head tube length because this 45 yr can't hack a really aggressive riding position, but sizing up a frame isn't rocket science.

This is the beginners section of the forum hence the advice given and I did say go to a Good LBS not some random bike shop.

Bozman

Rolf F wrote:

Bozman wrote:

20 years of buying bikes/frames and I've yet to have a test ride, plus, how could you have a test ride when you build a bike up?

20 years doesn't really make any difference to one year! Unless you change size, you only need to get it right once and then, at least for fit purposes, you don't really need a test ride. My first road bike is my most expensive bike - I would never have dreamed of spending that kind of money on a bike unless I knew for sure it was the right size. So I rode one for about 30 miles. As it happens, it was the next size down but it had been built up to fit me though it was clearly on the limits of set back, stem length, spacer piles etc. Thinking about it I'm probably contradicting myself a bit from earlier but the smaller frame really was a force fit. Anyway, having proved the fit, I know now that I can effectively buy any bike with a similar geometry and get a spot on fit with it.

It does when I was initially sold the wrong sized frame but didn't realise at the time, plus I've bounced around plenty of brands within that time and as you know a 54 in one make will have a completely different geometry to a 54 in another, you do have to know what to look for because a 25mm layback on a seat post and a longer stem might not help, but when you do know what you're looking for you can pretty much buy blind.

Trying isn't always an option, when I bought my Look there was no option to try for size because they didn't sell complete bikes. How many folk have just purchased a Scott Cr1 without trying for size, I know they're cheaper but you're not going to throw £500 away are you.

wishitwasallflat

great answer Bristol pete but re

bristolpete wrote:

I would likely buy a 56cm Cannondale, slap on 165 /170 cranks for a good kops reference point and lift the seat by 2.5mm if going 170 or 7.2mm if going 165mm over the stock 172.5 cranks and enjoy a nice slammed stem.

Apologies if its thick of me - but if the crank gets shorter why do you lift the seat? Surely if the seat height is set correctly for your knee flex with for example a 170mm crank and you then fit a 165mm you need to lower the seat by 5mm or you will be reaching an extra 5mm when pedalling and knee flex/extension will be changed?

bristolpete

wishitwasallflat wrote:

great answer Bristol pete but re

bristolpete wrote:

I would likely buy a 56cm Cannondale, slap on 165 /170 cranks for a good kops reference point and lift the seat by 2.5mm if going 170 or 7.2mm if going 165mm over the stock 172.5 cranks and enjoy a nice slammed stem.

Apologies if its thick of me - but if the crank gets shorter why do you lift the seat? Surely if the seat height is set correctly for your knee flex with for example a 170mm crank and you then fit a 165mm you need to lower the seat by 5mm or you will be reaching an extra 5mm when pedalling and knee flex/extension will be changed?

An oversight by many. The seat raises by the amount reduced or lowered by the length added as it is the pedal at the bottom of the stroke remember, not at the top. The top of the pedal stroke will follow the bottom. Hope that helps.

I am a big advocate of bike fit, but I tend to think and see people get a little more obsessed with fit after a couple of years on a bike. There are a lot of happy bike riders out there getting around on stock cranks, wrong frames and enjoying the bike simply as they have not got into or considered the options. In some cases ignorance is bliss I think but learning simple things like the above movement of the saddle can be worth the price of admission alone.

wishitwasallflat

bristolpete wrote:

wishitwasallflat wrote:

great answer Bristol pete but re

bristolpete wrote:

I would likely buy a 56cm Cannondale, slap on 165 /170 cranks for a good kops reference point and lift the seat by 2.5mm if going 170 or 7.2mm if going 165mm over the stock 172.5 cranks and enjoy a nice slammed stem.

Apologies if its thick of me - but if the crank gets shorter why do you lift the seat? Surely if the seat height is set correctly for your knee flex with for example a 170mm crank and you then fit a 165mm you need to lower the seat by 5mm or you will be reaching an extra 5mm when pedalling and knee flex/extension will be changed?

An oversight by many. The seat raises by the amount reduced or lowered by the length added as it is the pedal at the bottom of the stroke remember, not at the top. The top of the pedal stroke will follow the bottom. Hope that helps

Thank you that's clear now - I was mixed up thinking about the top of the stroke cause I've just been advised to try shorter cranks cause of a meniscus problem but your explanation makes perfect sense.

rolf_f

Bozman wrote:

Rolf F wrote:

Bozman wrote:

20 years of buying bikes/frames and I've yet to have a test ride, plus, how could you have a test ride when you build a bike up?

20 years doesn't really make any difference to one year! Unless you change size, you only need to get it right once and then, at least for fit purposes, you don't really need a test ride. My first road bike is my most expensive bike - I would never have dreamed of spending that kind of money on a bike unless I knew for sure it was the right size. So I rode one for about 30 miles. As it happens, it was the next size down but it had been built up to fit me though it was clearly on the limits of set back, stem length, spacer piles etc. Thinking about it I'm probably contradicting myself a bit from earlier but the smaller frame really was a force fit. Anyway, having proved the fit, I know now that I can effectively buy any bike with a similar geometry and get a spot on fit with it.

It does when I was initially sold the wrong sized frame but didn't realise at the time, plus I've bounced around plenty of brands within that time and as you know a 54 in one make will have a completely different geometry to a 54 in another, you do have to know what to look for because a 25mm layback on a seat post and a longer stem might not help, but when you do know what you're looking for you can pretty much buy blind.

Trying isn't always an option, when I bought my Look there was no option to try for size because they didn't sell complete bikes. How many folk have just purchased a Scott Cr1 without trying for size, I know they're cheaper but you're not going to throw £500 away are you.

Point stands - you still only need to get it right once - and then as you say you can pretty easily work out what the significance of the geometric differences is. I suppose it depends on how paranoid you are to start with to not be quick to trust a salesman who doesn't know as much as he should; there was no way I was going to be sold the wrong size frame!

As for buying a Look - there is an option to try for size. It just depends on where you go to buy the bike. The bike I was talking about was a Look 585 and the shop Epic. Sometimes it is worth making a long journey (I did - three times!). Coincidentally, I do also have a CR1 SL gathering dust in the spare room. The CR1 was the other bike I tested at Epic so I knew that would fit as well! I've often looked at bargain priced frames flagged up on here and given them no consideration at all no matter how cheap just because I knew they wouldn't fit me in any size.

Kajjal wrote:

This is the beginners section of the forum hence the advice given and I did say go to a Good LBS not some random bike shop.

No, it's actually the 'Buying advice' section - beginners is elsewhere!

neeb

Frame size numbers are almost meaningless - actually it's worse than that, because they do often correspond more or less between brands, so that people get lulled into a false sense of accuracy and then assume that a certain frame size number will represent the same fit on every bike.

The problem is that until recently, there were no easy reference measurements you could use to see whether one frame would fit the same as another. Effective top tube length is sensitive to seat tube angle in terms of what it means for reach to the bars, and head tube length is sensitive to bottom bracket drop and fork length in terms of what it means for the height of the bars / spacers required.

The good news is that there are now standard measurements that will tell you pretty much exactly how a frame will fit compared to any other one, but the bad news is that half of the manufacturers still don't publish these. These measurements are (frame) STACK and (frame) REACH.



Note that the bottom bracket (not the dropouts or the seat tube) is the starting point for both these measurements. You can calculate stack and reach if they are not provided in geometry charts, although sometimes the manufacturer's charts don't even give you enough information to do that. It's geometrically quite complicated, but someone has written a very useful routine to do it:

http://bb2stem.blogspot.fi/2011/10/stac ... -v100.html

As well as calculating frame stack and reach, this will calculate total stack and reach including stem and spacers etc (the window scrolls down). All of the information you need to fill in should be available from geometry charts, except possibly fork length and offset, which you might need to search around for. It's also useful that if a geometry chart doesn't give fork length and offset, you can calculate these (fork offset perhaps only approximately) if it does give stack and reach.

The only input that might cause some confusion is this one:

SPACERS + HEADSET + STEM STACK*0.5

This means total height of spacers, plus total height of headset, plus half of the total stem stack height. So if you have 1cm of spacers, the headset is 15mm tall and the stem clamp is 40mm tall, the number to enter would be 10 + 15 + 20, = 45mm.

Just make sure you take your time getting all of the numbers correct, and double check it twice...

rolf_f

neeb wrote:

The good news is that there are now standard measurements that will tell you pretty much exactly how a frame will fit compared to any other one, but the bad news is that half of the manufacturers still don't publish these. These measurements are (frame) STACK and (frame) REACH.

The only trouble is that stack and reach, as far as I can work out, don't really tell you much more than head tube height and virtual top tube length.

Stack is obviously a little better than head tube height as it takes into account BB height though I think (please correct if wrong) that BB height variations can probably be accommodated with a not silly amount of spacers.

Reach is no better than virtual top tube length - neither tell the full story without the seat tube angle. Reach doesn't account for it at all whereas vtt does but not in a useable way as you still need to know what the angle is.

What reach and stack need for the whole picture is something eg called setback - the extension of the reach line to the seatpost -that would take the frame angle into account. What is nice about this is that it might help discourage people from meddling with the saddle position to fit a wrong sized frame because the setback is clearly aligned to leg dimensions and the reach to torso. The alternative is to use reach in conjunction with VTT as that does account for the angle - ie the difference between the two is the setback.

neeb

Rolf F wrote:

The only trouble is that stack and reach, as far as I can work out, don't really tell you much more than head tube height and virtual top tube length.

No, they are definitely more general than those figures (I assume by virtual top tube length you mean effective top tube length).

Rolf F wrote:

Stack is obviously a little better than head tube height as it takes into account BB height though I think (please correct if wrong) that BB height variations can probably be accommodated with a not silly amount of spacers.

It also takes into account fork length and offset (or, for the purposes of calculating stack, the vertical fork height which you can get from these figures). This can vary by as much as 1cm even between race orientated road frames, e.g. based on the stack height quoted in the geometry charts, the Scott Foil has a longer fork than most, so a 140mm head tube length on that frame actually works out about the same as 150mm head tube length on many other frames.

Rolf F wrote:

Reach is no better than virtual top tube length - neither tell the full story without the seat tube angle. Reach doesn't account for it at all whereas vtt does but not in a useable way as you still need to know what the angle is.

Reach does account for seat tube angle, in the sense that two frames with the same reach (but different effective top tube lengths and seat tube angles) will have exactly the same reach to the top of the head tube once the saddle is placed in the same position relative to the BB. You should always be setting the saddle position relative to the BB rather than using it to change the reach.

The only thing this doesn't take into account is the amount of setback you need on the seatpost to get the saddle in the same position relative to the BB on frames with different seat tube angles. Unfortunately, seat mast offset is one of those other variables that manufacturers often refuse to tell anyone..

neeb

P.S. I can see how you might think from the diagram that stack doesn't take into account fork height - but remember that the geometry figures assume that the front dropout is going to be the same height from the ground as the rear dropout, so the figure for stack actually factors in the vertical height of the fork (which is a product of its length and offset).

rolf_f

neeb wrote:

Reach does account for seat tube angle, in the sense that two frames with the same reach (but different effective top tube lengths and seat tube angles) will have exactly the same reach to the top of the head tube once the saddle is placed in the same position relative to the BB. You should always be setting the saddle position relative to the BB rather than using it to change the reach..

That's exactly the point though. How do you set the saddle position relative to the BB without accounting for the seat tube angle? You need that setback measurement, or the seatpost angle (and, frame angles are never a straightforward measurement without a bit of trig capability) to know what adjustment to the saddle is needed. Eg, for two frames with the same reach but different seat tube angles, the relative saddle positions are immediately obvious given the setback measurement - ie move the saddle by the same value as the difference between setback measurements. It makes it actually very simple - you just need that third measurement - either as a virtual top tube length (yes - same as effective tt - both are standard terms and equally valid!) or setback from the BB.

The whole thing can be defined by two triangles (I think!) - anything else (eg wheelbase etc) is more about handling than fit.



I take your point about fork offset etc