On the cusp of frame sizes

[Jules59]

I find I'm often on the cusp between medium and large frames when sizing bikes on-line using the websites' guides.
If only height is required then its often a Large (56) frame that is suggested for me, but only just.
But if the site also takes into consideration in-seam and arm length then I'm well into the medium (54).

Generally, is it better to go for a smaller frame or a larger frame if one is on the cusp so to speak ? Or are there too many factors for that question to be answered.

[Jamesh]

I ride with a mate same heigh inside leg etc. 5'10"
However he rides a 52 where as I ride a 55cm!!!!

I've ridden his bikes (it's a tradition that if one gets a new bike we offer each other a ride!) And it's like riding a BMX or kids bike! Scrunched up like anything.

[NATURAL ANKLING]

Hi,
It might be better to go on Virtual Top Tube length aka Effective top tube.
https://geometrygeeks.bike/understanding-bike-geometry

"Top Tube Length
aka: Effective Top Tube, Horizontal Top Tube, Virtual Top Tube
Top tube is a good indicator of overall size of the bike.

It's measured 'effectively' horizontally from head tube axis to seat tube axis.

Older bikes frequently had horizontal top tubes, but now sloping tubes are much more common. Where the top tube is horizontal the Effective Top Tube and Actual Top Tube will be identical.

A longer top tube has you more stretched out on the bike, all other things being equal. This will give you a racier, more aerodynamic position on a road bike - possibly at the expense of all-day comfort.

Actual Top Tube Length
Top tube length as measured from head tube axis to seat tube axis, along the tube itself.
This is not particularly useful for bike fit.

Seat Tube Length
"Centre to Top" or C-T is the length from bottom bracket centre to top of seat tube. Useful because you can measure it and helps you work out how much seatpost you need.

"Centre to Centre" or C-C is the length from bottom bracket centre to the middle of where the top tube meets the seat tube. Was a bit more useful when bikes were made without sloping top tubes.

"Effective" is the length from the bottom bracket centre to the point where a horizontal line from the top of the head tube meets the seat tube axis. Less commonly used.

One of these is often used as an overall measure of a bike's size."

[Jules59]

Agreed. I use effective TT length too - otherwise its very difficult to compare sizes.

I'm now 5'10" (I've shrunk an inch over the years due to loss of vertebral disc space). My old Boardman is a Large (they only did 3 sizes at the time back in 2013 and the Medium felt too small) and has an effective TT of 570. I had to shorten the reach a lot to get comfortable but still suffered with pain between the shoulder blades on long rides.
After much deliberation I decided on a smaller framed bike with an effective TT of 550 (even though many website still put me in the large eTT 570 range). I haven't had it long enough to know if it will help my shoulder pain - time will tell.

[iandusud]

Likewise I use the effective top tube measurement but also the stack height, mostly affected by the length of the head tube but also by mudguard clearance and other factors. This latter measurement will determine how high you can set the bars. So for example I like a bike with a 570mm top tube but if I rode a pure racing bike I would find the front end too low. There are ways around this such as stems that have a lot of rise on them or fork steerer extensions but I feel it's better to get a frame geometry that is suited to the use of the rider. Of course some bikes, particularly touring bikes from good builders come with an extra long fork steerer and spacers to allow good adjustment of the bar height.

[GrahamJ]

I recommend bikeinsights https://bikeinsights.com

[Jdsk]

Generally, is it better to go for a smaller frame or a larger frame if one is on the cusp so to speak ? Or are there too many factors for that question to be answered.

Interesting discussion, as it always is.

Generally the smaller.

Jonathan

[Benz3ne]

I'm the same, 5' 10" (and a half...). I've gone down the medium route as my thinking is, if a large is too big, I can't adjust it sufficiently. If a medium is too small, it's quite easy to add extra reach or height.
It's worked for me so far, I think!

[531colin]

For 2 sizes of the same bike the trade-off is always the same.......
The bigger bike gets you the benefit of higher handlebars, at the cost of longer reach.

....of course, if you buy a bike designed by an old touring cyclist, you can get a sloping top tube and an extended head tube so that you can get the bars level with the saddle even with a carbon steerer

_DSC0829 by 531colin, on Flickr

[foxyrider]

I sold bikes for over 20 years and if there was any doubt it was always go smaller - you can raise saddles, move bar position etc to make it bigger, your options if you go large are much reduced.

For example, my CX bike is sized as small (52cm), when i tried medium (56cm) in the same bike, the saddle was scrunched to the top tube and it was like trying to ride an Orangutan! This from someone who normally rides frames 56-60cm and i have long arms. I did change the bars and stem to what i use on other bikes and the saddle is fairly high but not at the max.

[Jules59]

Another question then.
Its often said that as an aid to bike fitting, the front axle should be obscured by the handlebars as viewed when riding comfortably. But is this when you are "on the hoods", "on the bars" or "in the drops" ?
I've always assumed that it was "on the hoods". But I may be wrong.

[Vorpal]

I think it depends on your proportions. I have long legs & a short body. This generally means that as long as I can put the saddle back enough, I should size down to get the right reach. So, for example, going by Trek's sizing chart, I should take a 54 cm Domane, I would actually buy a 52 (and a layback seat pin). Similarly, they would put me on a M/L (18.5" frame) women's MTB, but I would probably be more comfortable on a M.

That said, some bikes are much easier to adapt one way or another than others. And things like adjustable stems can make that much easier.

[TheBomber]

Its often said that as an aid to bike fitting, the front axle should be obscured by the handlebars as viewed when riding comfortably.

There are two parts to bike fitting - fitting the bike to you and fitting the bike to the road/trail. The position of the front hub is irrelevant when considering the fit of the bike to you, but does matter when you consider how it handles. Although that old front hub mantra is often repeated, that doesn't in itself make it true. Does anybody offer up any evidence of why it should be so?

[mattsccm]

I would be ignoring any manufactures size anyway. They all have their own version of medium or what ever. It's just about good enough to tell you where to start and stuff all else. Look at the numbers and match them as best yiou can to as bike that does fit.

[foxyrider]

Another question then.
Its often said that as an aid to bike fitting, the front axle should be obscured by the handlebars as viewed when riding comfortably. But is this when you are "on the hoods", "on the bars" or "in the drops" ?
I've always assumed that it was "on the hoods". But I may be wrong.

I don't set up my bikes to achieve that, however, all my full size bikes pretty much are set up so that the bars obscure the front hub when i'm in a relaxed riding position, ie tops/hoods. Of course it doesn't work out that way very often with straight bars! Given all the variables, i'm not sure its all that relevant, particularly if you favour a more 'upright' 'relaxed' riding position, all my bikes have a sizeable saddle/bar offset which tends to put the line of sight through the bars more readily.

In other 'old advice', my grandfather used a hand around the seatpost as a height indicator - if you couldn't grip the seatpost with your full hand the frame was too big (think old skool 'square' steel frames here. I think this tended to put people on larger frames than were perhaps ideal, even the component mfrs seemed to buy in to this, old seatposts, pre 1980, were rarely longer than 6"/10cm above the insert line. several of my modern bikes use @ twice that much exposed pillar.

Then we've got the old nuggest of elbow to fingertip as a sizer for the ride cockpit and then............