bike size

Question
QUESTION: Hello

Have a small size mountain bike 17" that bought years ago from bike store. I think I should have had medium - 18".  I like the bike but only now I discovered that it may be slightly too small for me.  If I raise the seat so that my legs are almost fully extended, than I am too high from the ground.  Can ride like this maybe on asphalt but not on a trail.  On the trail, I thing that my legs are too bent.  I am 5, 10" inseam is 31".  Is there any corrections/parts etc. that I can make?  

Thank you,     Gabe

ANSWER: Gabe,

Not being able to see you on the bike, I really can only guess at what is going on.  For someone 5'10" with a 31' inseam a 17 may be a bit small.  However, if you can get the seat up high enough for your legs to extend so that your knees straighten with your feet flat at the bottom of the pedal stroke, the bike may be big enough.  The main problem you will have with your seat high enough is that the handlebar is too close and you: a) will be too upright for comfort as you ride; and, b) your knees will hit the handlebar on a steep uphill.

Now, in terms of seat height, it is normal that your feet are not be able to touch the ground when you are on the seat.  The seat should be adjusted so that your leg extends nearly to straight at the bottom of the pedal stroke (at the 6 o'clock position) with your foot flat (not heel-down or toes pointed).  During each pedal stroke your leg should bend and then straighten in order to provide the greatest comfort and protection from injury.  Your knees should not be bent the entire pedal stroke.  In terms of coming to a stop, then, you have two options with your seat at the proper height:

1) slide forward off the seat before completely stopped, put your feet on the ground and stand over the top tube (between the steerer and the seat tube).  The top tube should have plenty of clearance for you on a 17" mountain bike

2) stay in the seat,  rotate the non-standing-side pedal to the bottom (6 o'clock) as you are coming to a stop and lean the bike to the opposite (standing) side in order to put your foot on the ground. This is what most experienced riders do when they come to a stoplight or stop sign.

The main point of this is that your seat should be adjusted to the proper height for riding.  After that, you should practice ways to get your foot (feet) on the ground after the bike is properly adjusted.  Above are two ways, but I am sure you can figure out others.  Frankly, I clip into my pedals when mountain biking, so in order not to have to clip out when I stop, I pull up to a tree slowly on the trail and reach out to lean against the tree while I am stopped with both feet still in my pedals.

Good luck, feel free to ask further if I you don't understand my answer.

Graeme



---------- FOLLOW-UP ----------

QUESTION: Thanks again for such a detailed answer!  Well, comfortable on the asphalt when the seat is high,definitely ( or I think so) seat is too high if I leave it like that when on the trail.  Maybe the question would be - how far from the ground the pedal itself should be (when it is at 6 clock position)  ? Or, maybe it doesn't matter now that I think of it again - not sure..  
Also, is there any site or diagram that will show exactly the maximum angel of the leg when bent ?  

What irritates is that I bought it from a bike shop in Ca. (now I am in AZ) and the owner should guide me to the right size.. but it's been 10 years.. didn't' ride consistently but now feel the the urge..  

I am 61 years young

Anyhow, thanks again,    Gabe

Answer
Gabe,

You really can't easily change the distance from the pedal to the ground at 6 o'clock - that is determined by the length of your cranks, the bike geometry and the size of the tires.

Here's how to put yourself in the accepted optimal biomechanical position on a bike.

1.  Position the seat at a height that allows your leg to extend properly (as explained above).
2.  Sitting on the seat and with your left foot on the pedal and leg at the 9 o'clock position (left crankarm horizontal), the front of your kneecap should extend no further forward than the pedal spindle (the axle the pedal spins around).  You can change fore/aft position of your seat usually by   You can check this with a plumb line or makeshift plumb line dropped down from your kneecap.

Once you have found the technically correct position for the seat, tighten down the seat bolts and ride in that position for awhile.  If after some riding you want to change the seat position either up/down or backward/forward, do so only in very small increments (1/4 inch or less), riding awhile after each change.

Two other suggestions might be helpful now that you are taking up riding more seriously.  First, to feel what proper setup is, go to a Spinning class.  Spinning instructors are required to learn how to set riders up properly on the stationary bikes and the same principles will apply to you on a regular bike.  Second, join a bicycling club.  Such clubs are composed of tons of knowledgeable people who are more than willing to help you set up your bike.

As for the size problem, I am sorry to hear you may have been sold a bike that was too small.  That happened to me at a store more than 20 years ago and may continue to happen as bike shops succumb to pressure to sell their inventory.  The fact is that if you are so inclined, you can sell the bike used and likely get another used one for not much more.  Frustrating, yes, but if you are really interested in enjoying riding, you should be riding a decent, appropriately-sized bicycle.

Take care,

Graeme