Compared to my inseam (31 inches), what size bike should I have?

Question

Sorry. I'm looking at road bikes.

Answer 1

Whether road or mountain, there's one thing in common. Sizes differ a great deal between different models, even of the same brand and even if advertised the same size.

One big difference on road bikes is fork height and whether or not its effect is listed on geometry charts, which varies.
While Scott Speedster seems to use the same comfortable and speedy ergonomics as the economical Schwinn Super Sport, fork height and tire clearance is minimial on the Scott, thus making for a less comfortable time-trial ride that is also slower from an ergonomic standpoint.

For the most modern designs, you should probably ride a size medium bike.
However you should test drive many different bikes until you find the one that you absolutely refuse to leave at the store. You know the one that you've already test driven five times? Settle for nothing less than the bike that makes you want to ride.

Sorry that your question is un-answerable, but road bikes are extremely narrow in fit tolerances, size figures are meaningless because of design varieties, and road bikes cannot usually be re-fitted after purchase to create the correct size from one that wasn't. So, avoid E-bay except for small parts.

Size also varies with application. If you want a faster, more comfortable ride, sometimes a smaller bike will do it, as long as the handlebars are in reach. If you want one heck of a workout, a larger bike is good for that (a trainer). Smaller bikes are good for shorter arms because the top tube is shorter and you can always jack the handlebars with a booster device. Larger bikes are more traditional in fit, and do have the handlebars up taller, which is very nice, yet their top tubes are much longer, usually more than cancelling any perceived benefits in height.

Notice that very few Tour de France riders were on what we call an appropriate size bike. There are varying reasons, but #1 is that it is possible to change the stem for a boost or other adjustment, but impossible to shorten the top tube for a reach. Only very few had the big "breakaway" bikes. The majority were the fun little sprinters.

The big breakaway has one unique trick. Get it going with a tailwind or slight downhill with some 11 on 53 gearing and it will pull ahead of the rest at nearly car speeds. So will the XC mountain variety known as "steamroller." Both have the same extreme penanty to pay on the uphill, often resulting in in lower average speeds and usually resulting in totally ineffective sprinting due to heavier weight and very poor leverage of its longer top tube. The thrill on the downhill as the speedometer passes 45 (MPH) is rarely worth struggling to barely keep up the rest of the time. Thus, traditional sizing is no more.

Modern bike sizing and engineering will put a twist to any sizing scheme, making it all fail at some point, relating to some bike.

With the modern bike, frame size and leg accomodation becomes far less important than the effect of reach on stamina.

There are lots of surprises and lately, the trend has been towards both inexpensive and very nice surprises. The end of the mandatory view of the front tire, along with "technology trickle down" where a less expensive bike has a similar or same nice frame as the better models (remaining effective differences being wheels and crankset--easy to change), are all recent developments that are sure to please.

Test drive!!

Answer 2

The general rule is that the frame size should be 0.67 times your inseam.

In centimeters, your inseam is 78.7 (2.54 x 31). So your ideal frame size is 53 (78.7 x .67 = 52.7).

But this is only a start. As suggested by others, you really need to ride the bike. The geometry of the frame--particularly the length of the top tube--can make a big difference. And to further confuse things, a few manufacturers measure frame size slightly differently than others.

Answer 3

well the clearance under the top rail is not the best factor IE: compact frames. if you have a 31" inseam you prob are close to me in size 5' 10" A 53-55cm frame would be a good place to start. consider a bike fit at your dealer.

Answer 4

A medium frame (17") will probably work best for you, but it depends on the type of bike you're buying. Your best bet is to go to a bike shop and do a test ride. Typically, you want about 2 inches of clearance when you straddle the top tube with feet flat on the ground.

Answer 5

get thee to your local bike shop and get fitted... it is worth the extra time and effort.