i usually take a 21 inch size in mountain bike, but I want to buy a road bike. I'm 6'2'' and have a 34 inch inner leg. How do the sizes translate from mountain bike to road bike? Thanks
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There does not seem to be a universal standard for bike sizing. Due to the variety of designs and geometry size will vary between manufacturers and intended use, touring, racing, commuting are all slightly different. You often hear about the importance of fit and it's relation to performance and comfort and this is where a good relationship with your LBS is key. Most brands will have a guideline to get you in a range. One brand may place you solidly on a 58cm frame a different brand might place you between a 58 and a 60. You really have to test ride several sizes and brands to see what fits. A proper fitting will take into account your geometry and the bike geometry and try to get a match. It may involve swapping stems, adding headset spacers adjusting saddle setback. These adjustments make the frame feel bigger or smaller to get you comfortable.
When I got my bike the shop did a really good fit, and when I went back complaining of pain here and there, they changed things around to make sure I was ok.
My brother didn't have such good service as he bought his bike from a bigger and busier bike store in London. He rode it for more than six months before getting a professional bikefit for 120 pounds. Not cheap but he said the improvement was immediately obvious almost like he was a different (and better) bike.
This is especially useful if you have non standard body geometry. You can also adjust the fit to your specifications, racing, endurance, comfort etc.
I cannot specifically recommend one, but a Google search revealed a number of options:
The old traditional scheme for sizing a road frame was "standover height". You'd stand with your feet flat on the ground, straddling the bike. If you had "comfortable" (for a male) clearance of the (traditionally-positioned) top tube then the bike was the right size.
I've found in practice that this scheme maybe leads one to select a bike that is slightly too large, but it's a good first approximation.
Of course, these days many bikes are not traditional diamond frames, so you kind of have to extrapolate to apply the technique. And there are other aspects of fit, such as "reach", that usually follow overall fit, but not always.