have i got the wrong size?

iijili

New Member
sorry if this has already been covered. i bought a mtb last week for 15 yr old son but not sure i got the right size. heis 166.5cm (5ft 5.5"), inside leg 85cm (33.5") and the bike we bought is cube aim sl 29er, 17" frame. we're just starting out so didnt really know anything about how to choose the right size. we've been to several shops including specialist bike shops. the advice was very poor, based on height only, not even asking about inside leg, or doing any kind of fitting, and only one shop let us ride the bikes. most only had the bike he liked in one size so hard to make comparisons. i hadnt realised inside leg was important until visiting a bike sizing website - this asked about bike type, height and inside leg (but after entering his height wouldnt let us enter his actual inside leg - max was 83cm) and didnt ask for wheel size. it suggested 18.5" frame (i've bought 17" - shop recommendation based on height only). so i'm wondering whether the size we got is too small - how can we tell? this bike comes in 5 frame sizes - 15" 17" 19" 21" 23" and as we've chosen towards the lower end of the range thats another thing that makes me think maybe its too small. can anyone offer any advice please?

also is it usual when buying bikes for teens to allow for growth, or get something thats right for them now? going by the height of the rest of us in the family i expect he'll grow to about 5ft10".
thanks for any advice.
 

Drago

Flouncing Nobber
Location
Poshshire
It's hard to say for sure, but he's certainly at the lower end of rider size to fit on a 29er.
 

summerdays

Cycling in the sun
Location
Bristol
I'm not sure what I say will be of any use, but here goes. I'm female - and exactly the same height though my inside leg is shorter, which must mean he has quite a short torso at the moment. How much of a growth spurt has he had so far? I have both a 17" and a 19", though I almost feel too stretched out on the 19" as I grow older and less supple. I would try some different bikes - even if they aren't the ones that he wants just to get a feel of a different size of bike. How far out is the seat post and is there much left to put it up if he does grow? (In puberty do you grow in the legs first then the torso or both at the same time?) Is he just wanting a mountain bike style or is he actually wanting to do serious romps through the woods when I think you wouldn't want an oversized bike.
 
OP
I

iijili

New Member
thanks, whats youve said is really helpful. yes he is quite short in the torso relative to his height (as am i), and thats something that i feel none of the shops we visited took into account - they only asked about height. i think youre right that leg growth comes first, at least in his case, going by his current inside leg (33"/85cm). we should have tried different bikes but were misled by shops telling us 17" is the right size. he is happy with the size of the bike but i feel a bigger frame may have been better still, and he probably wouldnt grow out of it. we only found 2 shops selling this bike, and in only the one size. yes there is still quite a bit that the seat post could be raised. he might want to do serious romps in the woods in the future, its not a priority, but he does like the bike to be very manoeuvrable, and wasnt even too keen on a 29er at first.

is it possible to tell from looking at his posture on the bike whether its the right size, eg how far forward he has to reach to the handlebars, how much space his knees have when riding etc?
 

SpokeyDokey

64 and a little bit.
Moderator
thanks, whats youve said is really helpful. yes he is quite short in the torso relative to his height (as am i), and thats something that i feel none of the shops we visited took into account - they only asked about height. i think youre right that leg growth comes first, at least in his case, going by his current inside leg (33"/85cm). we should have tried different bikes but were misled by shops telling us 17" is the right size. he is happy with the size of the bike but i feel a bigger frame may have been better still, and he probably wouldnt grow out of it. we only found 2 shops selling this bike, and in only the one size. yes there is still quite a bit that the seat post could be raised. he might want to do serious romps in the woods in the future, its not a priority, but he does like the bike to be very manoeuvrable, and wasnt even too keen on a 29er at first.

is it possible to tell from looking at his posture on the bike whether its the right size, eg how far forward he has to reach to the handlebars, how much space his knees have when riding etc?
Post a picture up is best.
 

sreten

Well-Known Member
Location
Brighton, UK
Hi,

If you assume bikes are sized for average height and build, then for a height
and a short torso you go a size smaller, and for long torso a size higher.

Might sound wrong, but isn't, saddle height can fix pedal distance, but
there is no easy way to adjust the seat to bar distances, and this
determines the frame fit more than anything else, your torso.

So most likely if your son has a short torso and long legs then
a standard bike for his height will be too big, front to back.

So I wouldn't worry too much. There will be little point dropping
the bars in the limited adjustment range, given seat height setting.

If the atypical torso / leg length ratio persists then you need to learn
the typical tweaks you make to a "typical" bike to suit you well.

rgds, sreten.

FWIW such bikes only need 3 frame sizes and various different
bits like stem length and height adjusters to fit all sizes, but that
turned out to be a commercial no-no, you need 5 frame sizes
on offer for the punter, even though they are not strictly needed.
 
Last edited:
OP
I

iijili

New Member
Hi,

If you assume bikes are sized for average height and build, then for a height
and a short torso you go a size smaller, and for long torso a size higher.

Might sound wrong, but isn't, saddle height can fix pedal distance, but
there is no easy way to adjust the seat to bar distances, and this
determines the frame fit more than anything else, your torso.

So most likely if your son has a short torso and long legs then
a standard bike for his height will be too big, front to back.

So I wouldn't worry too much. There will be little point dropping
the bars in the limited adjustment range, given seat height setting.

If the atypical torso / leg length ratio persists then you need to learn
the typical tweaks you make to a "typical" bike to suit you well.

rgds, sreten.

FWIW such bikes only need 3 frame sizes and various different
bits like stem length and height adjusters to fit all sizes, but that
turned out to be a commercial no-no, you need 5 frame sizes
on offer for the punter, even though they are not strictly needed.

thanks, thats actually very reassuring. and as you say, the opposite to what i thought. i expect the torso/leglength ratio wont change much, going by my own ratio.
 
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