Determining Bike Size

[David_Zen]

I don't yet have a road bike but the mountain bike I ride is a size 17 inches. I am about 5 ft 7 inches tall. I know someone who is selling a Trek road bike that is a size 50 cm. How do I determine if this bike is a good size for me? I know it's not too big. The person is selling at a good price and I would like to buy but the size issue is the only thing that has me hesitating.

 

[jhuskey]

Originally posted by David_Zen
I don't yet have a road bike but the mountain bike I ride is a size 17 inches. I am about 5 ft 7 inches tall. I know someone who is selling a Trek road bike that is a size 50 cm. How do I determine if this bike is a good size for me? I know it's not too big. The person is selling at a good price and I would like to buy but the size issue is the only thing that has me hesitating.

Just as a guess I would say a 52- 53 cm would fit you better and there are a lot of factors.People vary in anatomy one persons legs may indicate a 31 inch standover and another a 33 inch and they could be the same height and then there is the stem length, seat position,bar width,the type of riding you do and so on. Without going to a bike shop for sizing I suggest riding the bike and if it feels right it probably is ok.

 

[David_Zen]

Thanks, I'll probably go see the bike and take it for a test ride.

 

[ed073]

Originally posted by David_Zen
I don't yet have a road bike but the mountain bike I ride is a size 17 inches. I am about 5 ft 7 inches tall. I know someone who is selling a Trek road bike that is a size 50 cm. How do I determine if this bike is a good size for me? I know it's not too big. The person is selling at a good price and I would like to buy but the size issue is the only thing that has me hesitating.

Frame size (centre-to-centre) can be calculated by:

Inseam length in socks, feet about 15cm apart
x
0.65

Saddle height (top of saddle to centre of BB) should be the same inseam measurement x 0.885....but use this as a starting point only. Make small changes until you feel most comfortable

Good enough for Greg LeMond.

 

[Cipher]

Originally posted by David_Zen
The person is selling at a good price and I would like to buy but the size issue is the only thing that has me hesitating.

You might want to read this by Sheldon Brown as well as other links on the subject of proper bike fit at the end of his article.

http://sheldonbrown.com/frame-sizing.html

 

[tafi]

Originally posted by ed073
Frame size (centre-to-centre) can be calculated by:

Inseam length in socks, feet about 15cm apart
x
0.65

Saddle height (top of saddle to centre of BB) should be the same inseam measurement x 0.885....but use this as a starting point only. Make small changes until you feel most comfortable

Good enough for Greg LeMond.

Unfortunately not good enough for a lot of others. Anyone who is not "average" (what ever that means) in proportion will find it difficult to fit a bike sized on their inseam.
I take myself as an example. Depending on which way you look at it I have either a long pair of legs or a short torso for my height. My legs are long enough (according to Lemond's formula) to fit a bike with a 57cm seat tube. However based on my own experience I need somewhere around a 52~54cm top tube. If I rode a bike built by Lemond with the right seat tube I would be riding a 57 or 58 cm top tube way too long. similarly the 58cm trek that I'd also fit would be too long aswell. You can shorten the stem but I wouldn't go any shorter than 10cm (particularly on a steep American bike) because it will get more twitchy to handle.

I would mbe more inclined to size a bike based on Top tube measurement.
I curently ride a 54cm Trek with a MTB seatpost and a 10cm stem with a lot of stack height to get close to the posi I need.

A good formula for top tube (cm) is approximately given by (very approximately):
TT=(torso length+arm length)/2 + 4 - (stem lenghth)

For me this estimated a little on the short side but it can vary a bit based on what seat tube angle you decide to use and how far back your saddle has to be. The ultimate test is getting on the bike.