Knowing what size bike to look for [duplicate]

Question

I am presently riding a hybrid that apparently is a bit too small for me, and I'd like to replace it with a road bike. Having ridden this improperly-sized bike for as long as I have (about 8 months, including one MS150), I am interested in making sure that the next bike I buy is properly sized.

The sizing numbers published for different bike models are only somewhat helpful, because a 54cm bike, even of the same type, will fit different people depending on who made the bike. It would have been more helpful if the sizing was listed as a range of heights for riders (such as, "best fits riders from 175 to 185 cm").

Somewhat complicating the issue is that (as I have mentioned in another question) I am chronically short of funds, and there is a high chance that I'll be buying something off of Craig's List or some other such place. I don't want to waste the seller's time by trying the bike if there's any chance that I won't buy it. I'd much prefer to know when I show up to make the purchase that the bike is manufactured for a rider of my height.

The same goes for mail-order. If I go this route, I want a reasonable certainty that the bike is made for someone my size. I don't want to deal with the hassle of returning a bike that doesn't hit my own personal Goldilocks zone.

I'd also rather not trouble bike shop personnel to let me try a bike to see if the sizing is right unless I'm ready to buy from that shop (possible but by all means not certain); I like to think that I'm a golden rule sort of person and using their time in this way seems too much like taking advantage of them. However, if this is considered by them to be a cost of doing business, then it's not a show-stopper.

That all being said, what's the best way to go about finding what size bike will fit me best? (If it helps to be more specific, I'm mostly interested in a road bike.)

Answer 1

While Michaels answer is the more correct one, I found ETT or effective top tube length quite useful for me when buying second hand. It is easier for the seller to measure, and was more useful to me than the seat post height since that is trivially adjustable and my upper body is smaller than it should be for my height..

Edit: The effective top tube length is the horizontal distance from the top tube/head tube junction to the seat tube. This measurement has become significant as more and more bikes are designed with sloping top tubes. (from https://www.bikecad.ca/effective_top_tube_length)

Answer 2

As you’ve already noticed, comparing “sizes” or calculating a certain size for your body measurements doesn’t really work. Generally you shouldn’t compare size numbers but the stack and reach values. They are comparable across different brands and models and the only measurements which really matter.

Once you’ve ridden a bike you’ve felt comfortable on, be sure to find out its stack and reach values. This is usually easiest by looking up the specific model on the internet. If you don’t need/want an aggressive seating position (i.e. low handlebars) the stack value is not that important since it’s almost always possible to go higher by adding spacers or a raised stem. Reach can only be adjusted by changing the stem length. Handling is usually okay-ish from around 80mm to around 140mm stem length on a road bike (most are sold with 90 – 110mm). This limits the reach adjustability to a few centimeters.

As you’ve probably noticed, stack and reach completely disregard the saddle position. That’s because the saddle can usually be adjusted in such a wide range that it’s not an issue.

Answer 3

As you say, bike sizes (either nominal seat tube length or small-medium-large etc) are only an approximation. In addition to that, the sizes do not tell you anything about the riding geometry of the bike - whether is has an aggressive or relaxed riding position. .

The stack and reach (describes in another answer) measurements are much better because they are absolute and define the riding geometry. The trouble is it's difficult to know what stack and reach you prefer if you don't have a bike that fits you already or have not tried out several bikes.

The general advice repeated on this site is that when buying you need to try bikes out for size a test ride them. Don't be afraid to ask for a few test rides at some local bike stores. Offering test rides is a cost of doing business for them and they are not just trying to sell you a bike, they are also trying to give the best impression so you'll come back for accessories and bike repair services. Also, it's worth checking out local bike stores for discounted models or deals they are offering.

Don't be afraid to ask to view and test bikes offered on Craigslist. As long as you have a good idea that a bike is in your size range it's perfectly reasonable to check it out. Just be polite and work with sellers to set up appointments to view bikes that are convenient for them. Also don't be afraid to ask reasonable questions before hand. A decent seller will be happy to answer.

Answer 4

Generally it is better to err one size too small than one size too big on the frame. Overall fit really comes down to a triangle drawn between bottom bracket(pedals), seat, and grips. This triangle can be stretched or rotated.

• The length of the pedal to seat is pretty easy to fit.
• Seat to grip length is still pretty straight forward and not too critical.(determined by effective top tube plus steering stem plus handle bar choice)
• bottom bracket to grip length then determines your bend angle at the hip, adjusted with steer stem.
• The rotation of the whole triangle is a bit more of a personal choice, it is determined first by seat tube angle and fine tuned with the seat clamp(most clamps allow the seat to slide a bit fore and aft.) and steer stem angle.
• Crank length is determined by femur length and determines knee flex. (most folks cranks are too long, use gears to get leverage not your knees) But nobody seems to make multiple lengths these days, other than the most expensive models and even there it is a trivial selection.
• Fore aft weight balance is a consideration but somewhat trivial unless you have an abnormal leg to torso ratio or find a rare bike frame that was intended for a special purpose. Affected by seat tube angle, chain stay length(moves rear wheel), sliding seat fore-aft, top tube and head tube angle(moves front wheel), and steer stem length.

And so I would judge a frame in this order:

1. consider seat tube angle fits what you want in general riding position and that it has your required features eg. clearance for fenders or braze-ons for racks, and of course overall quality/value.
2. effective top tube length is in a reasonable range so your steer stem isn't at a min or max length.(allows some future adjustments)
3. double check the seat be adjusted high enough or low enough.(plus a bit extra for changes in cranks or a new seat) usually not an issue for average bikes with average people if the top tube length is good.
4. parts that can be changed; cranks, steer stem, handle bars, seat...