Reckon I'd notice a 3cm frame size difference?

[Bigtallfatbloke]

My bike has a 64cm frame (thats BIG!)...I hav ejust fallen in Lurve with a bianchi over on the evans site which has a max frame size of 61cm...think I'd notice the difference?

 

[Steve Austin]

yes. but its worth checking the Toptube length as it might not be the 3cm difference that the frame size is.

 

[mickle]

Yeah what Steve says. Frame sizes as described by the manufacturers are actually irrelevant. The critical dimension is the top tube (measured horizontally between the centre of the seat tube and the centre of the head tube). The manufacturers use the length of the seat tube to describe thir frame sizes but as we know there is up to 8" of adjustment available for this dimension by raising or lowering the saddle. In the days when frames were 'square' (ie the top tube length matched the seat tube length) this method of describing a frame was relevant, now it just confuses. To make matters more complex, individual manufacturers each have their preferred method of describing frame sizes and of course 'compact' frames add another dimension.

For example, Bianchi measure their compact road frames from the centre of the BB to a point on the seat POST which is level with the top of the head tube. Ferchrissakes.

Its all bollocks. Forget about seat tube dimensions. Establish what top tube dimension you require (TT + stem = 'reach') and go from there.

 

[Bigtallfatbloke]

Forget about seat tube dimensions. Establish what top tube dimension you require (TT + stem = 'reach') and go from there.

ok I will try, Thanks. But being the dork I am I am confused a little as to which measurements you mean here (sorry).

Ok so I get out the tape measure...then what...which bit do i measure? If reach is the issue then shouldn't i be measuring myself? if so which bits...arms from armpit to wrist...palm?? Or have I got it all wrong?.....

I have been to LBS's but all they do is take one look at me and try to sell me the largest framesize they have of whatever model they happen to have in stock that week.

Is there a website that explains exactly what I can do for myself to get the right size along the formula you have given?

Th eonly site I have seen that offers a bike fit service of any quality was the paul hewitt site...but it's to far away for me to travel at the moment.

 

[pieinthesky]

What Mickle says is correct. You must also bear in mind the drop in height from your saddle to the handle bars.

A Smaller frame size might have the correct top tube length, but if the head tube is too short you will not be able to get the handlebars high enough. (Or low enough if it is too long).

So I would say check the top tube length and the head tube height.

If you do a search on the web you will find all sorts of formula and calculators for determinig your frame size. There is one on the Enigma site for example. Be careful of the results however, handlebar height for example is a personal thing and seems to depend more on riding style and comfort than body measurements and exact science.

 

[mickle]

Measure top tube (known as 'effective' top tube on sloping frames) length by measuring horizontally between the centre of the head tube and the centre of the seat tube (or seat post if its a sloping frame).

This measurement combined with the stem length gives you your 'reach', ie the distance from the centre of the seat post to the centre of the bars.

Have a go at measuring your own frame, remembering to keep the bike and the tape absolutely level.

If youre happy with the set-up on your current bike use its dimensions to inform your purchasing decisions.

Seat tube length is irrelevent.

 

[GrahamG]

I go up a size for my bikes as I have short arms and long legs so need longer seat-tube and then just fit a very short stem to bring the reach within the comfort zone.

The mechanic in my LBS reckoned there was no way I needed a 23" frame - I got one, albeit with a slightly shorter TT and it's the most comfortable bike I've sat on.