Bike Sizing

[Banjo]

I am 5ft 9 and 31 inside leg ,I sit happilly on a 56 cm Scott that everyone including the LBS says is big for me.

Most other peoples bikes seem to have more seat post extended and ride smaller frames.

I was pleased to see I am not completely alone on this. Lance Armstrong at 5ft 10 rides a 58 cm Trek Madone 6.9
Does he know anything about bikes ?

 

[Steve H]

Hi Banjo - I'm pretty new to cycling, so may not know any better...

... but I'd guess this would cause you to sit up in a more upright position when you were riding. If you had a smaller frame and a higher set saddle you'd be able to get more of an aerodynamic position - if you wanted it??

Nice looking bike!

 

[jimboalee]

I am 5ft 9 and 31 inside leg ,I sit happilly on a 56 cm Scott that everyone including the LBS says is big for me.

Most other peoples bikes seem to have more seat post extended and ride smaller frames.

I was pleased to see I am not completely alone on this. Lance Armstrong at 5ft 10 rides a 58 cm Trek Madone 6.9
Does he know anything about bikes ?

How are you measuring your 'inside leg'?

 

[jimboalee]

For sizing a bicycle,

Measure your Standing Height, and take from it, THIS :-

The result is your 'Crotch height'' or 'inside leg' as many would call it.

The frame size you need is 2/3 of your 'Crotch height'.


EXAMPLE: Me.

Height 1725mm
Sitting height 905mm

Crotch height = Height - Sitting height = 820
2/3 x 815 = 547.

I ride a 54cm SWorks.

 

[GrasB]

There are 2 ways to fit a road bike, neither are incorrect but the result is there are 2 body positions.

Short & high - this is the more usual bike fit for road racers. This means the top tube is quite short, typically this means a low top tube so exposes a lot of the seat post, also the handle bars are quite low compared to the saddle hight to give a good aero position on the drops. This is better for climbing but does require a fair amount of flexibility.

Low & long, this is generally more preferred for touring or long distance riding. The top tube is quite long for the rider this also means that there's not as much seat post exposed for the same relative geom of frame & the handle bars are higher compared to the saddle (often shallow drop handle bars are used as there's less scope for getting onto the drops). For general riding it's usually a little more aero but on the drops the torso is usually a little higher. This is better for people who are doing a lot of riding on the hoods without steep climbing & also requires less flexibility.

Though I'm talking about 2 positions there are varying degrees of these positions with a grey area in the middle so it's not as clear cut as this. However when most people look at you they are judging the bike based on a short & high riding position rather than a low & long.

 

[jimboalee]

There are FOUR ways to size a bike frame to a customer.

The FRENCH fit,, not used very often lately,
The EDDY fit,
The LEMOND SYSTEM fit
&
The CROWBAR fit.

Don't laugh, the Crowbar fit was used by Crowther and Barker.

The Crowbar fit still survives in parts of South East Birmingham and Solihull.

 

[jimboalee]

There are 2 ways to fit a road bike, neither are incorrect but the result is there are 2 body positions.

Short & high - this is the more usual bike fit for road racers. This means the top tube is quite short, typically this means a low top tube so exposes a lot of the seat post, also the handle bars are quite low compared to the saddle hight to give a good aero position on the drops. This is better for climbing but does require a fair amount of flexibility.

Low & long, this is generally more preferred for touring or long distance riding. The top tube is quite long for the rider this also means that there's not as much seat post exposed for the same relative geom of frame & the handle bars are higher compared to the saddle (often shallow drop handle bars are used as there's less scope for getting onto the drops). For general riding it's usually a little more aero but on the drops the torso is usually a little higher. This is better for people who are doing a lot of riding on the hoods without steep climbing & also requires less flexibility.

Though I'm talking about 2 positions there are varying degrees of these positions with a grey area in the middle so it's not as clear cut as this. However when most people look at you they are judging the bike based on a short & high riding position rather than a low & long.

There are four methods for a road bike, and also several methods for a touring bike.

 

[HLaB]

I cant recall what size of inside leg I am but I'm 5 foot 9 and 1/4 inch and the bike fit I had done in a shop and the one I done online both say a 56cm frame is apt for me, having said that I don't actually ride a 56cm, my Bianchi is a 55cm, the Kinesis is a 54cm and the Sirrus is a effective 54cm

 

[jimboalee]

Seat angle is determined by the ratio of upper and lower leg lengths.

I would advise using the leg length ratio method to decide on the seat angle on both road and tour bikes, but on tour bikes, set the saddle back 3/4" to simulate a shallower seat angle.

Case in point. I rode ( or attempted to until the accident ) the LEJOG on my SWorks. A roadrace bike pure and simple. I set the seat back 3/4" from theory and it became a tourer geometry bike.

The important measurements are the hip ball centre to the BB axle centre in two dimensions, fore/aft and height.

Crank length is determined by hip ball centre height above the ground when barefoot.

 

[Banjo]

For sizing a bicycle,

Measure your Standing Height, and take from it, THIS :-

The result is your 'Crotch height'' or 'inside leg' as many would call it.

The frame size you need is 2/3 of your 'Crotch height'.


EXAMPLE: Me.

Height 1725mm
Sitting height 905mm

Crotch height = Height - Sitting height = 820
2/3 x 815 = 547.

I ride a 54cm SWorks.

OK done that and my crotch height is in fact close to 33" two thirds of 33" = 22 " which is 56cm so I am on the right frame height bike. Yet almost everyother road bike rider I see seems to have way more seatpost showing than I do.

The lbs looked at all kinds of measurements and wanted me to have a 54 ,as soon as I sat on a 56 I knew it was right for me though I did opt for a 110 stem instead of the 120 standard one so maybe they were adding torso/arm length into the equation.The way I see it is I can adjust my reach by leaning forward more or fitting a shorter stem or moving the seat but I have no control over the length of my legs so the frame height HAS to be right IMHO.

This would suggest to me that from my observations most road riders are on smaller frames than they should be?? This cant be the case surely?.

This is my bike,I have done several days with over 8 hrs in the saddle with no ill efect so I think I can safely say that it works for me.

While on the bike fit subject if I can pick your brains a bit more Jimbo or anyone else if they have any thoughts.

At the moment I can reach the brakes in the drops but only just.I am thinking of rolling the bars down slightly to try to improve that, I intend leaving the stem flipped as it is now at least for the time being.I am reluctant to do anything drastic as the bike is comfortable now.

 

[Banjo]

Hi Banjo - I'm pretty new to cycling, so may not know any better...

... but I'd guess this would cause you to sit up in a more upright position when you were riding. If you had a smaller frame and a higher set saddle you'd be able to get more of an aerodynamic position - if you wanted it??

Nice looking bike!

Hi Steve The black and yellow Trek is Lance Armstrongs not mine unfortunately

Re being more /less sttrrtched out,I think it is the length of the bike has a lot of effect in this regard as on standard bikes the length increases with the height. In an ideal world we would all get measured then have the perfect bike built for us. Maybe one day if I keep doing the lottery....

 

[GrasB]

jimboalee, those aren't fundamental positions based on the geometry of the human body, you're just naming certain compromises between the extremes of short + high & low + long.

 

[jimboalee]

The OP put up a photo of Lance Armstrong's race bike, so I naturally assumed OP was talking about roadrace bikes.

The method of determining a person's frame size I described is adequate for anyone who hasn't got a severe physical deformity.
Mass producers of bike frames will size the frame around a nominal average dimensioned skeleton starting at the 'Crotch height'.

Mike Burrows professed that every rider should be able to be accomodated on one of three frame sizes, although this expanded to five when the very short and very tall started to complain.

Trek have eight frame sizes for their roadbike range. In 2cm steps.

If OP finds his 'crotch height', and multiplies it by 0.666666666, and chooses the frame size smaller than his result, he won't go far wrong.

BTW, if OP's 31" IS his 'Crotch height' and not his trouser leg measurement, he should be looking at a 52cm frame size for a roadrace bike.

 

[GrasB]

Of course, but there's a difference between what is comfortable & what is preferred. I tend to step down a frame size (as long as the effective TT isn't less than 515mm) compared to what I'd expect for someone my size & have a lot of exposed seat post, sure I'm comfortable on the frame size you'd expect but I just prefer to be on a slightly 'undersized' frame.

 

[jimboalee]

Of course, but there's a difference between what is comfortable & what is preferred. I tend to step down a frame size (as long as the effective TT isn't less than 515mm) compared to what I'd expect for someone my size & have a lot of exposed seat post, sure I'm comfortable on the frame size you'd expect but I just prefer to be on a slightly 'undersized' frame.

We're agreed.

"chooses the frame size smaller than his result".