Member Questions: Getting a Good Bike Fit

author : FitWerx

comments : 0

We are often asked the question, "What size frame should I buy" and the answer is “it depends”. I know that’s not the answer you want to hear, but hopefully the following will shed some more light.

Member Questions from drcherrybomb21, Gaarryy and Gregkl
"I got measured at a LBS to buy a road bike. They measured me at a 52. I ended up buying a Specialized Allez comp in a 52. I have been stagnating with my biking and have recently been told that my bike is too big for me. I was then measured at a 48. I went back to that same LBS where I was again measured at a 52. I'm so confused." - drcherrybomb21

"Why can I go to different bike shops or even the same shop but different people and be told I'm a different size? It makes me think that all the talk about being fitted is really not that important or more of an imperfect science where you just need to be in the ballpark and comfortable when I can be a "perfect" fit on different size bikes. I've been told for the exact same bike that I should be a 56 - 61.  This different information is very frustrating." - Gaarryy

"Other than coming out to Fitwerx for a bicycle fit, how can I know I am getting a good fit? " - Gregkl

Answer by Dean Phillips

Lead Fitter FitWerx2

We are often asked the question, “What size frame should I buy” and the answer is “it depends”. I know that’s not the answer you want to hear, but hopefully the following will shed some more light.

Many shops will determine your best frame size by taking your inseam measurement and doing a quick calculation, but while that will give you an approximate answer it’s not always the best answer. Other shops may simply base your frame size on your height using the assumption that as you get taller you need a larger frame size. While these methods sometimes result in the right frame size, they can often result in the wrong frame size and very often a frame that’s not even close to working well for you. We always recommend to our readers and customers that they seek out a reputable bike fitter that will invest the time necessary to determine both the best matching frame for you and also your best position on that frame.

Let’s look at this another way. I am 5’7” and you are 5’11”. Our inseams measure the same, but your torso is longer than mine. Based on our inseams it’s determined that we should both be on a size 54, when in reality you should be on a size 56 when you consider your longer torso and need for a longer top tube. If you tried to set your position up on that size 54 you’d likely need a longer stem, and perhaps a greater amount of spacers underneath the stem in order to achieve your position on that bike. I, on the other hand, wouldn’t fit on the size 56 since I’d likely need a stem so short that the bike would be unsafe and unstable.

We’ll always recommend working with a bike fitter that will position the rider on a size cycle to determine that rider’s best position before making a bike recommendation. The size cycle looks like a bike, but is extremely adjustable. The fitter can adjust the top tube, head tube and stem lengths and heights without limitation. If you just use a bike that is already set up then your optimized position may never have been determined because of the limitations the bike presents. Once your position is optimized the experienced fitter will take the measurements from the size cycle and make the correct recommendation based on your position and not on which bikes are on the sales floor or stored out back.

Your actual position could work on two different size bikes, which can make things even more confusing. How is that possible? Depending on the top tube and head tube lengths an experienced fitter can achieve the same position on two different size bikes by changing the size/height of the spacers under stem and the length and rise of the stem to achieve identical positions on two different size bikes. Try to picture the final position and then “remove” the bike. Then take that identical position and “add” a different size bike. The position would be the same, but to accommodate the position, you would have to make changes to the bikes. Those changes could include stem length and rise, spacer stack under the stem, aerobar selection and more.

Our recommendation is to go with the best fitting bike that will also allow for future adjustability as you develop as a rider. Other factors that must be considered when choosing your bike include your riding history, goals, planned use, flexibility, actual pelvis position and rotation on saddle, injury history and more. A knowledgeable fitter will use this information as they work to optimize your position and when helping you decide on which bike to purchase.

We often see athletes every year or two because they want to tweak their position. Perhaps they want to get more aggressive or more relaxed, but if they bought a bike that didn’t allow for adjustability we would be limited in making any changes to the bike without changing how the bike handles, the safety and more. It’s also important that the bike is setup up within the bike manufacturer’s recommendations for stem length, stem angle, spacers underneath the stem, and saddle position. The bike is designed to ride and handle well for a range of positions and if you stray outside this range the handling and stability of the bike can go downhill.

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date: December 12, 2009