frame size...which?

Si78
Si78 Posts: 963
edited April 2009 in MTB beginners
hi all,
newbie who is waiting to buy his first good mtb thru cycle 2 work scheme.

my question , and im sure its probably a stupid one, what size frame should i choose? when i went down to halfords they didnt have the boardman sport, gt aggressor or kona fire mountain i was looking at in all the sizes available, and i didnt get chance to ride any.
quite difficult anyway because the bike bit is upstairs, which i think is a bit daft
so,
im 6 ft tall (183cm),

arm span is 5ft 11in (180cm), so according to one internet sizing guide i looked at im a 'negative ape' because my arm span is less than my height... apparently this means i should go down a size from the 'ideal frame size'

distance between the floor and my 'BARSE' (in trainers) is 34" (86cm).

sizing guides say you take 3" off the inside leg measurement to allow for cross bar clearence (sloping top bars on modern mtb's makes that fairly meaningless i reckon), and then take off another 10" for the distance between the ground and the bottom bracket.

that puts my 'ideal' frame size at 21", which i think is too big. ive ridden my mates GT, which is an 18" frame (he is a couple of inches shorter than me), and it felt ok...seat just needed to be raised a bit.

basically, after boring you all with all this waffle!, i want to know what size frame all you good people (of a similar size to me ) use...

ive always thought that having a smaller frame would make a bike more nimble. is this true?
and lighter too i guess!

are there disadvantages to buying a slightly smaller bike? how much does the length of the bike / wheelbase etc change in relation to the size of the frame?

most of my riding during week will be riding to work, with some off roading / xc at the weekends.

thanks in advance for any views offered, it will be really appreciated!
si
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