Road sizing vs mtb sizing
 

So I'm a bit confused on this subject and wondering if someone can help clear things up.


So I ride 55cm road bike around 22in) but my mtb I have(it's the suggested size for me) is an 18in, which is significantly smaller than my road bike, which fits me fine. I was just wondering why do mountain bikes get sized down so much compared to road bikes. Would it be that bad if I road say a 19' or 20' mtb considering that my road bike fits fine?

Your question is good. I have the same queastion like you but i cant found answer online.

I bought my MTB first based on information I had found online, a 24". It fits and is comfortable but too big to be really nimble and flick around on tight or technical sections like switchbacks. My next bike was a road bike, size 60CM compact. The frame feels very small but still roomy, with the top tube down close to my knees but still long enough in the top tube.

So, I feel that I sort of ended up with it backwards. I just finished my new bike last weekend and as of today had logged 105 miles on it. It closer matches the size of my 24" MTB and is very comfortable. I'm undecided on my next bike, it's between a vintage steel or a better MTB. If I do a MTB I will be looking for a frame that feels more like my first road bike.

Go to a fit calculator:
Bike Fit Calculator | Find Your Bike Size | Competitive Cyclist
and once through pick road bike, the next pick MTB.

different frame geometry used for handling reasons

Actually, road bikes are specified larger than all other types of bike. The more interesting question is why that is.

Higher speed stability vrs lower speed stability is my official answer

Another reason of course is MTBs have long top tubes and wider tires than a road bike so it makes sense to shrink them to fit.

That's the way its always been.

First, bike sizing, especially mountain bike sizing, is pretty much virtual now. You can get by just paying attention to the XS, S, M, L, XL, XXL designations.

But if you compared the measurements of your road bike to a serious mountain bike that fits you (no Trek 820s, etc.) you'd find the virtual top tube a lot longer, the seat tube (actual and virtual) a lot shorter, and the bottom bracket a lot higher. The bottom bracket height of most road bikes is 10.5 - 10.8 inches. If you were that low on a trail bike you'd be dragging the cranks over rocks all day, so bottom bracket height is 13-14 inches, before sag, depending on the discipline. Now imagine straddling a 55 cm frame with the bottom bracket 3-4 inches higher. Can you do it?

Now let's look at the components of reach. On a road bike they are the top tube, the stem, and the handlebar reach that goes beyond the stem. The top tube is short to keep the bike nimble and laterally stiff (and increase toe overlap :)), and the stem tends to get long to compensate for the shortness of the top tube and help the rider weight the front wheel to improve high speed handling. On the mountain bike, you have the top tube and the stem. For the most part, the bar is behind the stem, especially on 29ers. The stem is short to improve low speed maneuverability. The top tube is long to maintain that athletic, forward leaning posture over the bike, but not so much as on a road bike. The athletic posture is still best for getting power to the rear wheel, steering, especially on descents, maintaining balance, and keeping both wheels on the ground while climbing.