Modern Mountain bike sizing ??

[BADGER.BRAD]

Hello all,
In the past the few new Mountain bikes I have had I have always sized in the same way as a road bike with the starting point being a 21 inch frame then adjust to suit and maybe change a few components, With mountain bike frames being somewhat different now days with a low cross bar/top tube I am a bit confused as to what I should be going for. At present I'm using my lads bike for rougher of road excursions but intend to buy a second hand full suspension bike so cannot be too fussy on finding a bike with all the correct dimensions but would like to have a rough idea as to what would roughly equal a 21 inch frame in today's frames. My main use will be Cross Country cycling and I am 5' 10.

Thanks all

 

[Drago]

Cross country is an MTB racing discipline. Ask for an XC bike and you'll get something quite inappropriate.

Do you mean general trail riding, ie, light off road on unpaved surfaces?

 

[N0bodyOfTheGoat]

Modern trends would put you on something like a 475mm reach frame.

 

[fossyant]

Did you have a 21 inch road bike and the same MTB. That's not right, if you had a 21 inch road bike then you should have been looking for a 15-17 inch MTB. You'd be looking for plenty of room (lots of seatpost) to fit a dropper at some point.

 

[Drago]

A dropper might be OTT if it is indeed it is trail riding the OP is after. Indeed, a gravel or CGR machine may even be optimum. We need him to confirm the true nature of the terrain and how hard he intends to go at it before we can give sensible advice.

 

[ktmbiker58]

The days of selecting MTB frames by inches has long gone - most manufacturers supply a table of rider height ranges for frames usually classified as XS to XL as a starting point.

Regarding type of bike this is my very rough guideline , other opinions are available and just as valid !! :-

For trail riding - bridleways, hardpack tracks, restricted byways - a hardtail with 100 or 120mm forks is fine - I actually run a rigid fatbike on that type of terrain and it's a great ride, and perfectly rideable on the trail park blue runs. For red runs I run a full suspension bike but still just 130mm travel. For black runs I've got a full suspension bike with 160mm travel and a dropper post - dropper posts are a PITA - I get through an average of 1 a year (actually 1 every 366 days so out of warranty except in a leap year ) - as a very general rule I reckon droppers and body armour go hand in hand, if you don't need body armour you don't really need a dropper.

 

[wafter]

Investigate reach and stack - this is the current sizing standard and while not perfect is infinitely more useful than attempting to represent a range of different shapes and sizes simply by one arbitrary frame measurement or wildly variable size labels.

You can measure the reach and stack of your existing bike with a tape measure and straight, vertical wall next to a flat, horizontal floor.

Bear in mind that MTBs have been getting longer and taller, while typically for a comparable fit the frame reach will be in the region of 50-70mm longer on an MTB compared with a drop-bar bike.

Drop bar bikes tend to run stems in the 100mm vicinity while MTB stems are usually about half this, while of course the bars themselves add a considerable amount of reach - however road applications tend to favour a longer, lower posture compared to more upright positions for MTBs.

For reference my two modern drop-bar bikes (M and L sizing, for what that's worth) have reach values of 375-385mm, while my "M" hardtail's frame has about 440mm of reach. Fit on the drop bar bikes is perfect after many hours of fine-tuning; the MTB much less-so but it feels OK and it's probably a bit less critical because you're less likely to be stuck in the same position for long periods of time..